Frequently Asked Questions


Global Precipitation Measurement (GPM) is an international satellite mission to unify and advance precipitation measurements from space for scientific discovery and societal applications.

GPM will be able to measure precipitation globally; over the land and ocean, in both the tropics, mid-latitudes, and cold locations near the poles. GPM will be able to measure both light and heavy precipitation including the microphysical properties of precipitation particles. This wide range of locations and precipitation types presents a host of challenges not encountered by TRMM, which only measures moderate to heavy rainfall in the tropics.

Currently, The Tropical Rainfall Measuring Mission (TRMM) measures heavy to moderate rain over tropical and subtropical oceans.  GPM will provide advanced measurements, including coverage over medium to high latitudes, improved estimates of light rain and snowfall, advanced estimates over land and ocean, and coordination of radar and microwave retrievals to unify and refine precipitation estimates from a constellation of research and operational satellites.  GPM will also provide more frequent observations, every 2 to 4 hours.

GPM data will primarily be used by operational forecasters, but the information will also benefit numerical weather prediction models, climate prediction patterns, crop monitoring, and other research applications. In addition, scientists will use GPM data to advance our understanding of precipitation and its role in the Earth's environment.

The increased sensitivity of the Dual-frequency Precipitation Radar (DPR) and the high-frequency channels on the GPM Microwave Imager (GMI) will enable GPM to improve forecasting by estimating light rain and falling snow outside the tropics, even in the winter seasons, over land, which current satellites are unable to measure.  These advanced measurements will extend current capabilities in monitoring and predicting hurricanes and other extreme weather events, as well as improved forecasting for floods, landslides, and droughts.

General Precipitation Questions

Precipitation forms when cloud droplets or ice particles in cloudsgrow and combine to become so large that the updrafts in the clouds can no longer support them, and they fall to the ground.

A thunderstorm is formed when a combination of moisture and warm air rise in the atmosphere and condense. While over land, thunderstorms are most likely to occur at the warmest, most humid part of the day, which is usually the afternoon or evening. Over the ocean they are most likely to occur in the early hours of the morning before dawn.

Thunderstorms form when an air mass becomes unstable (when air in the lowest layers is very warm and humid, or air in the upper layers is unusually cold, or if both occur). Rising near-surface air in an unstable air mass expands and cools, making it warmer than its environment, which causes it to rise even farther. If enough water vapor is present, some of this vapor condenses into a cloud, releasing heat, which makes the air parcel even warmer, forcing it to rise yet again.  Water vapor fuels the storm.

A tropical depression forms when a low pressure area is accompanied by thunderstorms that produce a circular wind flow with maximum sustained winds below 39 mph. An upgrade to a tropical storm occurs when cyclonic circulation becomes more organized and maximum sustained winds gust between 39 mph and 73 mph.

As rising water vapor condenses and latent heat is released, surrounding air is warmed and made less dense, causing the air to rise.  The thunderstorms that make up the hurricane’s core are strengthened by this process. As air rises within the storms, pressure at the surface decreases and moister, tropical air is drawn to the center of the circulation, providing even more water vapor to fuel the hurricane.  A hurricane has sustained wind gusts of at least 74 mph.

They are different names for the same type of storm, collectively known as tropical cyclones.

What they’re called is determined by where they form.  In the Atlantic Basin and east of the International Date Line in the Pacific Ocean, they’re called hurricanes.  Typhoons form in the North Pacific Ocean, west of the date line.  The storms are called cyclones in the Indian Ocean and in the Coral Sea off northeastern Australia.

Availability of water vapor and intensity of updrafts within a cloud determine the size of a raindrop. Larger drops tend to result from the vigorous updrafts within a thunderstorm and fall faster than smaller drops. Mist or drizzle produce smaller drops that fall at lower speeds.

Most hurricanes begin in the Atlantic as a result of tropical waves that move westward off the African coast.

Hail forms when thunderstorm updrafts are strong enough to carry water droplets well above the freezing level. This freezing process forms a hailstone, which can grow as additional water freezes onto it. Eventually, the hailstone becomes too heavy for the updrafts to support it and it falls to the ground.

The tropics receive a great amount of direct solar energy, which produces more evaporation than higher latitudes. The warm, moist air rises, condenses into clouds and thunderstorms, and falls back to earth as precipitation. More evaporation results in more precipitation.

The forecast of a hurricane's path is dependent upon the accuracy of the predicted winds from computer forecast models. The speed and direction of steering winds generally vary with altitude. Weak tropical cyclones tend to be steered more by lower-level winds, while upper-level winds usually influence the paths of stronger hurricanes.

Temperatures in the US are colder than they are near the equator, so air pressure is lower than it is in the tropics. Because wind flows counterclockwise around low pressure, winds usually blow from west to east, pushing weather systems to the east.

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.

Depending on their type, clouds can consist of dry air mixed with 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 raindrops, cloud ice, snow, graupel and hail.

It is important to understand that even a cloud that looks impenetrably dark is almost entirely made of dry air. Water vapor and precipitation each make up a maximum of just a couple of percent of the mass of a cloud, except in a few very intense storms.

How do these precipitation 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.