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Some interesting questions about rain and weather
(brought to you by the Tropical Rainfall Measuring Mission):

(last update: April 28, 2004)

What is "latent heat"?
What are the "greenhouse effect" and "global warming"?
What is "atmospheric circulation"?
How does rain affect the global circulation?
Is there more or less rain during an El Niño event?
Is the "Southern Oscillation" related to "El Niño" or "La Niña"?
What is the monsoon?
What is "convection"?
Where a weather map shows a "low", would a barometer on the ground also show low pressure?
Are vertical winds stronger than horizontal winds?
What is the difference between a tornado and a hurricane?
In which direction do hurricanes and tropical cyclones travel? In which direction do fronts travel?
What is a "marine layer"?
What is an "inversion layer"?
What happens when wind pushes a moist air mass against a mountain?
Why do jet aircraft release trailing white "contrails" at high altitudes?
Does it always rain more in winter than in summer?
How wide an area can a rain storm cover?
Do winds blow clouds along their path or do clouds move independently?
What is the "dew point temperature"?
What are clouds made of? Are they more likely to form in polluted air or in pristine air?
Is the temperature inside a cloud higher or lower than in the surrounding air?
Are low clouds warmer than high-altitude clouds?
When rain stops falling, is the air aloft cooler or warmer than before the storm?
Is rain formed by the condensation of water vapor or by the melting of ice?
Can ice and hail occur in the tropics?
How big can hail get?
What is cloud seeding and (how) does it work?
How much rain can fall in an hour?
Are raindrops really shaped like teardrops?
How big can a rain drop get?
How fast do raindrops fall?
When it is raining on the ground, how far up into the atmosphere does this rain extend?
How does rainfall over the sea affect the sea surface?
How are the rain maps on the evening news created?
How do airliners detect storms? How do pilots decide whether to fly over or avoid a storm?
... and here are the answers:

  1. What is "latent heat"?
    When water boils, it absorbs heat to evaporate. When the reverse process takes place, namely when vapor condenses, it must release heat. This heat associated with the phase change of water in the atmosphere is called "latent", as opposed to the "sensible" heat supplied directly by Earth's surface and by the sun. Scientists have found that the sun's direct contribution explains only about 1/4 of the energy used by current global atmospheric dynamics. The other 3/4 are transferred to the atmosphere by water which used heat from the ocean to evaporate: when this vapor condenses into clouds and/or rain, it releases "latent" heat in the atmosphere. The latter plays a major role not only in cloud formation (see question 22) and in storm development (see question 24) but also in driving air motion in the tropics and indeed around the globe (see question 4).

    By measuring the profile of rain as a function of altitude, TRMM is providing the first reliable global latent heating estimates ever made.

  2. What are the "greenhouse effect" and "global warming"?
    The greenhouse effect is a warming process of the earth. When the sun's (shortwave) rays reach the earth, some of their energy is reflected back to space and the rest is absorbed. The absorbed energy warms Earth's surface, which then emits heat back toward space as longwave radiation. A significant part of this out-going radiation is absorbed in the lower atmosphere by greenhouse gases such as water vapor and carbon dioxide (because these gases absorb longwave radiation particularly well), and this trapped energy is then re-radiated, thus warming Earth's surface and lower atmosphere. Without these greenhouse gases, Earth's average surface temperatures would be about 30°C cooler.

    The burning of fossil fuels, industrial emissions and deforestation have markedly increased the concentrations of some greenhouse gases in the atmosphere. Scientists have determined that higher greenhouse gas concentrations are "enhancing" the greenhouse effect and thus leading to global climate change: because the greenhouse gases trap heat, the enhanced effect has been called global "warming", but the true nature of the change is multi-faceted and quite complex.

    Indeed, our understanding of the mechanisms behind long-term climate change is still woefully lacking. As far as rain is concerned, we do know that the ultimate condensation of water vapor into falling rain 1) decreases the main greenhouse gas (water vapor) in the atmosphere; 2) alters the warming budget of the atmosphere through cloud formation; and 3) through the latent heat released, drives the global circulation in the atmosphere and therefore plays a major role in shaping global weather patterns. However, we are only beginning to understand how these mechanisms interact with one another. The TRMM instruments are allowing us to track and study, for the first time, the small-scale atmospheric changes due to rain, thereby helping improve our models of global circulation, and hence of the evolution of our weather and of the planet's climate.

  3. What is "atmospheric circulation"?
    The air in the atmosphere is never stationary. Its horizontal and vertical motion, the "atmospheric circulation", is governed by complex physical laws, of which the most pertinent are radiative forcing, and energy conservation. Radiative forcing is the action on the atmosphere of the shortwave radiation from the sun and of the longwave heating from Earth's surface. These two main energy sources are exerted unevenly on different parts of the globe, and have different effects on the various regions and the different layers of the atmosphere. Energy conservation is the basic law requiring that whatever heat or energy is present cannot vanish but must be used up through air motion or stored through evaporation or condensation or other physical processes. In addition, surface topography plays a definite role: for example mountains can impede horizontal wind; they can force air masses pushed against them to rise; etc. Finally, the coriolis force, due to the rotation of the earth about its axis, forces any existing wind flow to deviate to the right of its original heading in the northern hemisphere (to the left in the southern hemisphere).

    These are the forces which govern the atmospheric circulation. The two most important factors which generate wind flow in the first place are 1) the substantial variation in the angle of the sun's rays as one moves from the equator to the poles, which makes the effect of solar radiation strongly dependent on latitude and on the earth's attitude with respect to the sun (i.e. on the season), and 2) the significant differences between the temperature of the oceans and that of continents -- land reacts to the seasonal change in solar radiation much faster than oceans do. The variation in the solar radiation between the equator and the poles and the subsequent "meridional" (north-south) temperature gradient produce "zonally symmetric" circulation (i.e. independent of longitude), which, as one moves towards the pole, gets progressively more strongly affected by the coriolis force and develops a zonal (east-west) component, until, at higher latitudes, the circulation becomes mostly zonal. The thermal imbalance between oceans and continents, along with the topography, are responsible for zonal asymmetries in the circulation.

  4. How does rain affect the global circulation?
    The latent heat released by the condensation of water vapor causes the surrounding air to expand, thus creating lower-level low-pressure pockets (where the air has expanded and became lighter) and high-pressure pockets aloft (where the air has been pushed up by the expanding lower-level air). Away from the tropics, this effect is far less important to the short-term evolution of the weather than the meridional temperature gradient. Because the latter is not very strong in the tropics, these pressure variations due to the latent heat dominate as the major driver of the tropical circulation. Furthermore, the tropical upper-level highs crucially modify mid-latitude storm tracks: this is exactly how El Niño affects the weather around Los Angeles and indeed around the globe.

    By measuring the profile of rain as a function of altitude, TRMM is providing the first reliable latent heating estimates ever made throughout the tropics.

  5. Is there more or less rain during an El Niño event?
    Three related events govern El Niño conditions:
    • the normal higher-pressure zone over the Eastern Pacific and the usual lower-pressure zone over the Western Pacific get reversed;
    • the prevailing surface winds, the easterly trade winds, weaken;
    • the mass of warmer water usually accumulated in the tropical Western Pacific ocean moves to the Eastern Pacific.
    During El Niño, the abnormally high pressure in the West discourages any destabilizing winds while the low in the East favors storm formation; similarly, the abnormal warming of the ocean in the East feeds any weather disturbance with rain-producing evaporation while the cooling in the West reduces any moisture that is available in the air for rain to form. That is why El Niño episodes bring often dramatic droughts to Australasia while drenching coastal areas of Peru with rain.

    The TRMM estimates of the ocean surface rainfall are being used in ocean circulation models in order to improve the ability of the models to predict changes in ocean circulation and weather patterns such as El Niño events.

  6. Is the "Southern Oscillation" related to "El Niño" or "La Niña"?
    The Southern Oscillation Index (SOI) is the difference between the air pressure in Tahiti (representing the location in the "eastern" Pacific with the most complete meteorological record) and that in Darwin (representing the western Pacific). Among the different versions of how to calculate the SOI, the "Troup" SOI represents the standardized anomaly of the mean sea-level pressure (MSLP) difference between Tahiti and Darwin, and has been tabulated since 1876. It is calculated as follows:
    SOI = 10 (P - Pm) / s(P),
    • P = Tahiti MSLP - Darwin MSLP,
    • Pm = long-term mean of P for the month in question,
    • and s(P) = standard deviation of P for the month in question.

    Negative values of the SOI are characteristic of El Niño episodes. The SOI was positive during 1996 and early 1997. It first dropped below 0 (to -8.5) in march of 1997, and reached a minimum of -28.5 in march 1998. By comparison, the previous record lows were -39.3 in may 1896, -36.7 in april 1905, -34.2 in february 1983, and -30.7 in december 1940.

    Positive values of the SOI are associated with stronger trade winds and warmer sea temperatures in the western Pacific, sometimes referred to as "La Niña" episodes. Waters in the central and eastern tropical Pacific Ocean become cooler during this time. The most recent strong La Niña was in 1988-89, with an SOI of 21 in november 1988; a rather weak La Niña occurred in 1995-96, with a maximum SOI of 13.9 in june 1996. The record maxima were 32.3 in april 1917, 30.4 in november 1973, and 21.3 in september 1975.

  7. What is the monsoon?
    "Monsoon" (from the Arabic "Mawsim", season) is a seasonal reversal of the direction of the prevailing winds. The word was first used for the winds over the Arabian Sea, but it is now used for similar winds in other parts of the world (south and south-east Asia, Australia, Africa, the southwestern United States and northwestern Mexico, and Chile). The primary cause of the monsoon is the seasonal temperature difference between large land areas and the neighboring oceans. In summer, land masses heat up more quickly than ocean surfaces. As a result, the air over large land areas becomes warmer and lighter than that over oceans. This causes wind flow to be directed from the ocean to the land. The oceanic air carries a great deal of moisture and produces a significant amount of rainfall over the land. This is the wet monsoon. During winter, the land masses and the overlying air become much colder than the neighboring oceans and the air over them. This leads to the reversal of the flow, forcing cold dry air over the land to flow toward the ocean. This is the dry monsoon.

    The pattern and amount of rain associated with the wet monsoon varies greatly from year to year and, in addition to affecting the general atmospheric circulation, it affects the economy of certain parts of the world very significantly (the Indian sub-continent for example). This explains the strong interest in improving the predictions of the monsoonal circulation. TRMM is, for the first time, providing accurate estimates of the rainfall over the strong-monsoon areas. This information is being fed into forecasting models to improve their accuracy.

  8. What is "convection"?
    Convection refers to the vertical motion (and related processes) associated with the rising of warm air pockets or the sinking of cool air pockets, and the resulting formation of clouds and precipitation. When the vertical motion of the air is rapid, convection can quickly lead to severe weather such as thunderstorms.

  9. Where a weather map shows a "low", would a barometer on the ground also show low pressure?
    Not necessarily. Atmospheric pressure always varies with altitude. At mid-latitudes, the mid-altitude pressure (at about 5000 m) is usually the best indicator of the short-term evolution of the weather. When the mid-altitude pressure is low, the pressure near the surface could still be high or low. Indeed, the interaction between the pressure at the different altitudes plays a determining role in the synoptic-scale weather development ("synoptic-scale" means over areas between 1000 and 3000 km in diameter). At mid-latitudes, synoptic-scale low-pressure systems generally move from west to east. When such a system is developing, the center of the system on the ground (the lowest-pressure point) is always ahead of the center of the system aloft. As the system matures, the upper-level low catches up with the location of the lower-level low.

    In the tropics, convective systems such as tropical cyclones and squall lines are low-altitude lows topped by upper-level highs resulting from the warming of the lower-level air. Otherwise the pressure in the tropical atmosphere generically does not vary with altitude as strongly as it does away from the tropics: that is why the well-understood theory for mid-latitude weather dynamics does not apply to the tropics.

    By supplying us with systematic latent-heating estimates over the tropics, TRMM is providing quantitative information about the driving force behind tropical weather. This will allow us to improve our global circulation models, and therefore improve our short- and longer-term climate predictions.

  10. Are vertical winds stronger than horizontal winds?
    Vertical winds have typical speeds of less than one meter per second, or about two miles per hour. When convection occurs, as in thunderstorms or tropical cyclones, vertical winds can reach speeds up to several tens of meters per second. The largest up-drafts are encountered in continental thunderstorms, in which small areas can experience vertical wind speeds in excess of 100 miles per hour. In contrast, horizontal surface winds in tropical cyclones often exceed 150 knots, or almost 200 miles per hour, and some tornadic winds have been estimated at close to 300 miles per hour.

    The TRMM instruments can detect convection quite accurately, especially over the oceans, where no other reliable means of localizing convection are available.

  11. What is the difference between a tornado and a hurricane?
    Tornadoes and hurricanes appear to be similar in their general structure. Both are characterized by extremely strong horizontal winds swirling around the center, strong upward motion dominating the circulation with some downward motion in the center. The tangential winds far exceed the radial inflow or the vertical motion, and can cause much damage. Hurricanes always rotate counterclockwise in the northern hemisphere (clockwise in the southern), the direction of their rotation being determined by the Earth's rotation. This is almost always true of tornadoes too, although on rare occasions "anticyclonic" tornadoes spinning in the opposite direction do occur (tornadic circulation is determined by the local winds). This is where the similarities end.

    The most obvious difference between tornadoes and hurricanes is that they have drastically different scales. They form under different circumstances and have different impacts on the environment. Tornadoes are "small-scale circulations", the largest observed horizontal dimensions in the most severe cases being on the order of 1 to 1.5 miles. They most often form in association with severe thunderstorms which develop in the high wind-shear environment of the Central Plains during spring and early summer, when the large-scale wind flow provides favorable conditions for the sometimes violent clash between the moist warm air from the Gulf of Mexico with the cold dry continental air coming from the northwest. However, tornadoes can form in many different circumstances and places around the globe. Hurricane landfalls are often accompanied by multiple tornadoes. While tornadoes can cause much havoc on the ground (tornadic wind speeds have been estimated at 100 to more than 300 mph), they have very short lifetimes (on the order of minutes), and travel short distances. They have very little impact on the evolution of the surrounding storm, and basically do not affect the large-scale environment at all.

    On the other hand, tropical cyclones (the term includes hurricanes and typhoons) are large-scale circulations with horizontal dimensions from 60 to well over 1000 miles in diameter. They form at low latitudes, generally between 5 and 20 degrees, but never right at the equator. They always form over the warm waters of the tropical oceans (sea-surface temperatures must be above 26.5° C, or about 76° F) where they draw their energy. They travel thousands of miles, persist over several days, and, during their lifetime, transport significant amounts of heat from the surface to the high altitudes of the tropical atmosphere. While their sporadic occurence prevents them from systematically influencing the large-scale circulation, they still affect it in ways which must be accounted for and need to be better understood.

    The TRMM satellite is providing unique information about the structure and evolution of hurricanes. Indeed, some of the first data obtained from the satellite revealed the fine three-dimensional structure of the tropical cyclone Pam.

  12. In which direction do hurricanes and tropical cyclones travel? In which direction do fronts travel?
    Hurricanes and other weather disturbances which start in the tropics are subject to the winds which prevail in the tropics, namely the east-to-west-blowing trade winds. That is why, in general, these storms initially travel westward. Fronts and other mid-latitude weather systems, on the other hand, are carried by the winds which prevail at mid-latitudes, most of which travel from the west to the east. As hurricanes and other tropical cyclones reach the mid-latitudes, they too get picked up by the westerly winds and start changing direction there.

  13. What is a "marine layer"?
    This question is perhaps best answered with an example. Off California, when warm air is pushed by the prevailing westerly winds that dominate the circulation at mid-latitudes, it travels over the Pacific ocean and can thus accumulate quite a bit of moisture. When it reaches the California coast, it cools on contact with the cold surface current along our shore, producing haze or fog. This low-altitude layer of moist relatively-cool air is called the "marine layer". In summer, it is quickly re-warmed by the relatively hot land surface as it moves over the Los Angeles basin.

    The orbit of the TRMM satellite brings it over Los Angeles relatively frequently, but it misses all points North of Santa Barbara. The follow-on mission will cover most of the United States.

  14. What is an "inversion layer"?
    Under normal conditions, within the "troposphere" (the lowest layer of the atmosphere, where all rain and most clouds occur) the temperature of the air decreases with altitude. An "inversion" occurs when a warmer air mass aloft sits over cooler surface air. For example, this can develop during night time or during the winter months, when the air next to the ground cools rapidly due to the cooling of the earth's surface while the temperature of the air above it does not change significantly. In Southern California, inversion layers form more commonly as a result of the sinking and compressional warming of the upper level air associated with the summer-time high pressure system in the region: while the upper-level air sinks and warms up, the air near the cool ocean surface remains colder, and the inversion layer is formed. The relatively less warm air below gets trapped, and with it the smog that builds up at the surface.

  15. What happens when wind pushes a moist air mass against a mountain?
    Because atmospheric pressure decreases with altitude, as air is forced to rise against a mountain slope it expands, and therefore cools. This cooling encourages the moisture to condense into liquid drops or ice if the temperature is low enough. When the drops and/or ice particles become sufficiently large, they fall as rain or snow or even hail.

  16. Why do jet aircraft release trailing white "contrails" at high altitudes?
    Jet engine exhaust contains moisture which, as it comes into contact with the cold atmosphere, crystallizes into fine ice particles which then streak the cold sky at high altitudes. When jetliners drop to lower-altitude warmer air, the ambient temperature is typically too warm for vapor to condense.

  17. Does it always rain more in winter than in summer?
    No, quite the opposite in fact. About two thirds of global rain falls in the tropics, where, as a rule, most of it falls during the local summer (December to March in the southern hemisphere, June to September in the northern hemisphere). This is due mostly to the summer warming of the oceans, which increases evaporation and, with it, the moisture available in the air for rain to form.

    However, away from the tropics, there is indeed more rain and snow in winter. That is due mainly to "fronts". Frontal systems are the most common weather systems at mid-latitudes, and winter systems are far stronger than summer ones, mainly because the temperature difference between the equator and the poles is greatest in winter (during summer any colder air mass quickly gets warmed up by the continents).

    The TRMM instruments are helping us quantify the total rainfall over the tropics. By comparing with the measurements of identical instruments on other satellites, TRMM is also helping us improve our estimates of rain over the entire globe.

  18. How wide an area can a rain storm cover?
    The area covered by a rain storm can vary greatly in size, depending on the type of the storm and the main forcing responsible for generating the storm in the first place. While the smallest rain events are on the order of just a few kilometers and are associated with individual cumulus clouds, the largest rain areas can extend beyond 1000 km and are associated with large frontal systems or hurricanes. Rain-covered areas can vary anywhere between these two extremes.

    TRMM is providing important information about the size and frequency of occurence of rain storms in the tropics. This is filling a significant gap in our observations and, thus, increasing our knowledge about the water cycle and the atmospheric circulation over the globe: while small-scale rainstorms occur much more frequently in the tropics, only the large ones have a significant impact on the global circulation.

  19. Do winds blow clouds along their path or do clouds move independently?
    Both statements are to some extent true. It often seems that clouds float in the air, and get carried along by the wind. This impression is somewhat misleading. It is true for individual small clouds, with a lifetime of an hour or less. As these die, new clouds are formed where the temperature, pressure and humidity conditions are right. In that sense, the direction in which large-scale systems (cloud clusters) move is determined by two factors: the direction in which individual clouds are blown by the wind, and the possibly different direction in which new clouds form as the older ones die.

  20. What is the "dew point temperature"?
    The amount of water vapor which can exist in the air before saturation is reached and condensation and cloud formation start is a strong function of the temperature. Indeed, for every temperature there is a fixed maximum amount of water vapor which the air can hold. This "saturation" amount of water vapor is an increasing function of temperature: warm air can hold more water vapor than cold air. The implication is that if the air is not saturated and if one starts decreasing its temperature by some means (night time cooling for example), one will eventually reach a temperature for which the air will become saturated. This is the temperature at which condensation would start and dew would form, and because of that it is called the "dew point temperature". In that sense the dew point temperature is actually a measure of the humidity of the air and not a measure of how warm the air is. It can only be lower than the actual temperature, if the air is not already saturated. In the latter case, the relative humidity is 100%. The bigger the difference between the actual temperature and the dew point temperature, the lower the relative humidity (the drier the air) and the lower the cloud development potential is. The subtropical high pressure system which often sits off the coast of California in summer drives falling dry air off the continent onto the cold coastal waters. This air mass has a low dew point temperature, indicating its dryness and inability to support strong storms. This explains the lack of summertime precipitation over Southern California.

    Since moister air is more unstable and has a much greater potential for producing clouds than drier air, precise knowledge of the dew point temperature distribution is of crucial importance for accurate prediction of storm development, propagation and evolution. By telling us where rain is falling, and how much, the TRMM data will improve our knowledge of the available (remaining) moisture over the oceans.

  21. What are clouds made of? Are they more likely to form in polluted air or in pristine air?
    Depending on their type, clouds can consist of liquid water drops, ice particles or both. Low, shallow clouds are mostly made of water droplets of various sizes. Thin, upper level clouds (cirrus) are made of tiny ice particles. Deep thunderstorm clouds which can reach up to 20 km in height contain both liquid and ice in the form of cloud and rain drops, cloud ice, snow, "graupel" and hail.

    How do these particles form? First, tiny cloud droplets are born when the water vapor in the air is cooled and starts to condense around tiny "condensation nuclei" (particles so small they are invisible to the naked eye). The presence of these aerosols is crucial: without them, in absolutely clean air, condensation would not start until the relative humidity has reached several hundred percent (this suggests that the "saturation" level of 100% humidity is poorly defined; in fact, the atmosphere always contains more than enough nuclei of all sorts for condensation to start as soon as the dew point temperature is reached). The more particles there are in the atmosphere, the easier cloud droplets will be formed and the smaller they will be (since more particles will be competing for the same amount of water, so each one of them will attract less). This is why clouds over land have more droplets of smaller sizes than clouds over oceans where the air is generally much cleaner.

    The process of ice formation similarly requires the presence of nuclei. However, there are much fewer particles which make suitable ice nuclei. This is why freezing often does not start until the temperature of the air reaches -15° C (if there are no ice nuclei at all, freezing will not occur before the temperature drops to -40° C). Hence, clouds with temperatures below 0° C can still consist of water droplets called "supercooled" water. These drops freeze immediately upon contact with any surface. When they fall to the ground as freezing rain, they can form a thin layer of sleet on roadways, an almost invisible and very dangerous hazard for drivers.

  22. Is the temperature inside a cloud higher or lower than in the surrounding air?
    Clouds are warmer than their surrounding air. As moist air rises in the atmosphere it expands due to the lower pressure at higher altitudes, and cools. Eventually the moisture in the air reaches saturation and starts to condense, forming cloud droplets and rain. During the condensation, significant amounts of heat are released (for water to evaporate it requires heat -- the reverse process releases heat), and this warms the air inside the cloud. Indeed, this released heat is the main driving force behind cloud growth. The process goes as follows: when the first cloud droplets are formed, small amounts of heat are released; the air surrounding the droplets becomes slightly warmer hence lighter than the surrounding air; this pushes it further up; as it goes up, more condensation occurs, more heat is released, the air gets even warmer and lighter and goes up even faster; and so on.

    While the heat released during cloud formation plays a very important role in the development of the cloud and affects the atmosphere around it, there are no easy ways to measure it. The rain quantities and drop distributions measured by the TRMM satellite will be used in conjunction with cloud models to estimate for the first time the "latent" heat released during cloud growth and decay.

  23. Are low clouds warmer than high-altitude clouds?
    Yes. Because atmospheric pressure decreases with altitude, as air rises it expands, and therefore cools. Before TRMM, this is the main property which had allowed scientists to estimate the height of a cloud: those that appeared cold to the infrared sensors on weather satellites were classified as having high tops, while those that appeared warm were classified as shallow. Discriminating between harmless wispy high-altitude cirrus clouds and the "deep" stormy cumulus clouds required careful and sometimes tricky examination.

    The radar on board the TRMM satellite can automatically distinguish between precipitating cumulus clouds and the harmless cirrus which TRMM's infrared sensor also detects.

  24. When rain stops falling, is the air aloft cooler or warmer than before the storm?
    After a storm, the air aloft is warmer than before the storm, although the low-altitude air may occasionally be cooler. The warming is due to the fact that, before the storm, the water in the atmosphere existed as vapor, and, through the cloud-formation and precipitation phases of the storm, fell to the ground as liquid (or ice or snow): at the end of the day, the original vapor had to release heat to become liquid (or solid). This heat drives the development of the clouds until the supply of warm moist air from the surface is cut off. Indeed, as the rain falls through the drier layers under the cloud, some of it naturally starts evaporating again, thus lessening the warming near the surface.

    In convective areas, the warming prevails at all levels. However, since melting and evaporation are strongest at lower levels, their effect leads to the decrease in low-level warming and, thus, explains the observed upper-level maximum in the warming.

    In area-wide steady ("stratiform") rain which trails the convective storms, the low pressure created under the cloud by the heating draws air which, coming from outside the storm, is drier and therefore encourages the falling rain to evaporate, thus becoming cooler than the air before the storm.

    So we have the contrast between the steady warming within a convective storm and the higher-altitude-warming/lower-altitude-cooling in stratiform rain. These two "latent heating" mechanisms are crucial to understanding the contribution of an individual tropical storm to the heat budget in the atmosphere. TRMM is providing the first reliable latent heating estimates ever made globally. While this latent heating occurs on the short time-scale of an individual storm's lifetime, it is a major driver of the global circulation, thus affecting the long-term atmospheric heating budget.

  25. Is rain formed by the condensation of water vapor or by the melting of ice?
    This important question is still under investigation. Much of the rain is produced by clouds whose tops do not extend to temperatures colder than 0° C. The mechanism responsible for rain formation in these "warm" clouds is merging or "coalescence" among cloud droplets, which are first formed by vapor condensation. Coalescence is probably the dominant rain-forming mechanism in the tropics. It is also effective in some mid-latitude clouds whose tops may extend to subfreezing temperatures. However, once a cloud extends to altitudes where the temperature is colder than 0° C, ice crystals can form and "ice-phase" processes become important. In favorable conditions, ice-involving processes can initiate precipitation in half the amount of time water-only processes would need. Hence, at mid-latitudes, cumulus cloud rain is probably initiated by ice-processes and melting of ice. Observations have shown, however, that precipitation can first appear at levels warmer that 0° C, where vapor condensation and coalescence are the main rain producers. Thus, precipitation may be initiated by either process.

    The TRMM radar is quite accurate in determining when a rainy column extends up past the freezing level, i.e. when the precipitation involves ice processes. The answer so far seems to be "less often than was once thought". TRMM is thus providing qualitative information about the frequency of occurence and intensity of ice processes in the tropics, and in this manner, could help evaluate their relative importance in rain formation. However, more data will be needed to obtain a more definite answer to the question. The radar on the follow-up mission will be equipped with additional capabilities to address this issue.

  26. Can ice and hail occur in the tropics?
    Ice and hail can and indeed do occur in the tropics. In fact, they are always present in tall clouds with strong updrafts, and, over land, can sometimes fall all the way to the ground. These deep clouds can reach heights of up to 20 km. They are the means by which the heat accumulated in the equatorial regions is transported up in the atmosphere, and subsequently poleward away from the tropics. Detection of ice and hail in the tropics indicates the presence of deep strong convection and an accompanying significant re-distribution of heat.

    The passive microwave imager on the TRMM satellite can detect hail although it cannot determine at what altitude it is occuring. The follow-on mission will have a more sophisticated radar equipped with additional channels to estimate the amount of hail and melting ice particles below the freezing level.

  27. How big can hail get?
    Hail stones vary in size. Most commonly they are 1 cm in diameter but have been observed to be as large as 10 to 15 cm. Hailstones are formed when either aggregated ice ("graupel") or large frozen raindrops grow by collecting cloud droplets with below-freezing temperatures. An important aspect of hail growth is the latent heat of fusion which is released when the collected cloud water freezes. So much liquid water is collected in the process of hail growth that the latent heat released can significantly affect the temperature of the hailstone and make it several degrees warmer than the cloud environment. As long as the temperature of the hailstone remains below 0° C, its surface remains dry and its development is called "dry growth". The heat transfer from the hailstone to the surrounding air, however, is generally too slow to keep up with the release of heat associated with the freezing of the collected cloud drops. Therefore, if a hailstone remains in a supercooled cloud long enough, its temperature can rise to 0° C. At this temperature the collected supercooled droplets no longer freeze immediately upon contact with the hailstone. Although some of the collected water may be lost to the warm hailstone by shedding, a considerable portion can remain to be incorporated into the stone forming a water-ice mesh that is called "spongy hail". This process is called "wet growth". During its lifetime, a hailstone may grow alternately by the dry and wet processes as it passes through air of varying temperature. When hailstones are sliced open, they often exhibit a layered structure, which is evidence of these alternating growth modes. Hailstones need time to grow before they become too heavy and fall to the ground. Here is an approximate empirical relation between the fall velocity V of a hailstone and its diameter:
    V = 9 D0.8 m/s,

    where D is the diameter in cm. Hence a hailstone with a diameter on the order of 15 cm will fall at 75-80 m/s (170-180 miles/hour)!! This implies that updrafts of a comparable magnitude must exist in the cloud to support the hailstones long enough for them to grow. Because of this, hail is found only in very intense thunderstorms. Therefore, hail detection in storms is a clear indicator of their severity.

    TRMM can detect the presence of hail in tropical storms but cannot determine at what height it is occuring. The radar on the follow-on mission will have specific channels which will detect frozen particles in general and hail in particular. This will provide crucial information about storm severity.

  28. What is cloud seeding and (how) does it work?
    Cloud seeding grew out of the idea that if tiny particles are placed in a very cold cloud, the water can crystallize into ice around them, and then grow big enough to fall to the ground. Initially people tried this by placing "dry ice" (actually frozen carbon dioxide) in clouds. It has since been found that silver iodide works as well, although through a slightly different mechanism: water condenses around the iodide particles into droplets which, when large enough, fall to the ground as rain. So far, however, the process has worked conclusively only for very cold shallow clouds that were nearly ripe for natural precipitation to occur anyway: in those very special cases, the seeding triggered the precipitation. This apparently minor degree of success is nevertheless significant, because in those cases seeding has helped lessen hazardous ground fog and thin out supercooled clouds which can cause ice to accumulate dangerously on an aircraft's wings.

    The TRMM instruments are not sensitive enough to identify fog or supercooled liquid clouds, but they are sensitive enough to detect minute amounts of rain, as little as 0.7 mm/hr or about one-tenth the rate at which a typical sprinkler system sprays water.

  29. How much rain can fall in an hour?
    Rain rates up to 400 millimeters per hour have been measured in particularly strong typhoons. However, even within the strongest storms, such high instantaneous rates are rarely sustained for periods longer than a minute, although a point on the ground can accumulate as much as 100 millimeters during the passage of an exceptionally strong rainstorm.

    The TRMM mission is producing estimates of (not very accurate) three-hour, (moderately accurate) five-day and (unprecedentedly accurate) monthly totals of the accumulated precipitation over areas about the size of the Los Angeles basin. The follow-on mission will aim to produce very accurate three-hourly rain maps at a resolution that is sufficiently fine to predict flash-floods.

  30. Are raindrops really shaped like teardrops?
    Not really. Raindrops start out as round cloud droplets. As they grow and start falling, they begin to experience the resistance of the air, which causes them to flatten a bit. Further growth leads to thinning in the center, until the eventual breakup of the drop.

  31. How big can a rain drop get?
    Drops vary in size from the tiny cloud droplets (measuring less than 0.1 mm in diameter) to the large drops associated with heavy rainfall, and reaching up to 6 mm in diameter. Collision among drops and surface instabilities are generally thought to impose this 6-mm size limit, although drops as large as 8 mm in diameter have been reported in shallow warm showers in Hawaii.

    The reflectivity of a drop when illuminated by radar is roughly proportional to the square of its volume. It is this property which radar meteorologists exploit to estimate the total volume of rain from the reflectivity observed. This estimation process is rather difficult because the radar-rain relation is not linear, and the range of drop sizes within a single storm can vary greatly.

    Because TRMM has passive instruments as well as the active radar, it is capable of estimating total rain volume and drop size distributions.

  32. How fast do raindrops fall?
    This question is tricky because some precipitating raindrops may not fall at all, if the surrounding wind has a sufficiently strong upward component. In still air, the terminal speed of a raindrop is an increasing function of the size of the drop, reaching a maximum of about 10 meters per second (20 knots) for the largest drops. To reach the ground from, say, 4000 meters up, such a raindrop will take at least 400 seconds, or about seven minutes.

    The TRMM radar does not have the capability to measure the fall speed of precipitating particles. Future spaceborne precipitation radars, however, will. This capability is important because it helps characterize the type of rain being measured.

  33. When it is raining on the ground, how far up into the atmosphere does this rain extend?
    Usually up to the "freezing level", where the temperature has decreased to below 0° C (over the tropics, that occurs at about 5000 meters; over Los Angeles, it fluctuates between 2000 and 4000 meters in winter, and between 4000 and 5000 meters in summer). In different types of rain, there can be frozen water such as hail mixed with the rain below the freezing level, and/or "super-cooled" liquid drops above it.

    In most storms, the TRMM radar is very good at detecting the freezing level. Unfortunately, it is not very good at discriminating between ice and liquid drops. The follow-on instrument will have separate channels to try to sort out the concentrations of frozen, melting, and liquid particles, thereby providing very detailed information about the evaporative cooling and/or condensational heating released into the atmosphere by rainstorms.

  34. How does rainfall over the sea affect the sea surface?
    Because seawater contains salt, it is denser than fresh water such as rain. That is why rain which falls over the ocean "floats", and therefore generally causes an initial local increase in the mean height of the sea surface. The rapid redistribution of the freshwater from these short-lived "lakes", along with the temperature difference between the cool rain and the warmer water, both lead to changes in the oceanic circulation. Rain storms indirectly affect the ocean in two additional ways: the winds typically associated with a storm strongly affect surface currents, and the shielding effect of storm clouds leads to a decrease of the sea surface temperature. Finally, raindrops penetrate below the sea level to depths as great as 30 cm. Their energy is converted into small-scale turbulence just under the sea surface, thereby attenuating the short wind waves and giving the sea surface the glassy appearance often noted by mariners during very intense rain.

    The TRMM estimates of the ocean surface rainfall are being used in ocean circulation models in order to improve the ability of the models to predict changes in ocean circulation and weather patterns such as El Niño events.

  35. How are the rain maps on the evening news created?
    Weather satellites have been used by the National Weather Service since the sixties to map clouds, sea surface temperatures, and vertical temperature and moisture distributions. These data are supplemented by crucial ground observations, and are subsequently input to numerical models which try to predict the short-term evolution of the weather. The results are then made available to the public, most directly in the form of composite weather maps. The rain shown on these maps comes mostly from ground weather radars.

    Unfortunately, the crucial ground observations are often simply not available over the tropics because these regions are typically inaccessible. In addition, the large-scale numerical models are still woefully inaccurate over the tropics, mainly because we do not yet have a solid understanding of the often severe dynamics which govern the circulation in the warm moist tropical atmosphere, and which have a determining effect on the weather far away from the tropics. The unprecedented precision of the TRMM measurements is replacing the missing ground measurements over the tropics, and it is helping fill the gap in our understanding of the processes which start around the equator but affect the weather all over the globe.

  36. How do airliners detect storms? How do pilots decide whether to fly over or avoid a storm?
    All commercial airliners are equipped with weather radar, the antenna being housed usually in the nose cone. Radar echoes which exceed a pre-determined threshold indicate significant storm activity ahead, thus warning the pilots about turbulence several tens of miles (i.e. several minutes) before the flight path would run into it. Because this margin that the on-board radar allows is rather short, the crew start their journey with sophisticated weather maps (produced by blending ground observations with satellite data) showing the location and dynamics of the large-scale weather systems which might affect the plane along its journey.

    The TRMM radar is revealing the extent of the sensitivity of space-borne radar to the various types of precipitation (snow, ice, hail, rain) as well as to the presence of vertical up- and down-drafts. This information will help make system design choices for future airborne weather radars.

Click here to download a postscript version of the document "Some Interesting Questions about Rain and Weather".

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