Lesson 3 of 6 · Clouds and precipitation
From invisible vapor to falling water
9 minute lesson · 3 checks along the way
Your goal for this lessonDistinguish water vapor, cloud particles, and precipitation, and explain why observing a cloud does not directly measure rainfall at the ground.
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A cloud is a population of particles
Water vapor is a gas and is invisible. The visible body of a cloud consists of small liquid droplets, ice particles, or both, scattering and absorbing radiation. Droplets commonly form on tiny airborne particles called . The availability of those particles matters, but adding dust alone does not guarantee a cloud: temperature, water content, and the surrounding flow determine whether water condenses or evaporates.
Rising air encounters lower surrounding and expands. An unsaturated parcel that exchanges little heat with its surroundings cools approximately adiabatically. If it becomes sufficiently saturated, can form droplets, releasing latent heat and changing the rate of further cooling. Mountains, fronts, and buoyant convection can all provide lift. The cloud boundary continually changes as droplets form, mix with drier air, and evaporate; it is not a solid container of water.
Growing particles take different routes
A cloud can remain visible without producing rain at the ground. Its particles must grow and fall through the surrounding air. In warm clouds, droplets with different fall speeds can collide and merge. In mixed-phase clouds, ice can grow while supercooled liquid droplets evaporate, because the saturation vapor pressure over ice differs from that over liquid water. Aggregation and collection add further ways for frozen particles to grow.
The layers below a cloud then affect what reaches the surface. A snowflake can melt into rain; falling can also evaporate into dry air before reaching the ground. Vertical motion changes particle paths and residence times. Consequently, a dark cloud, a cold cloud top, and a wet rain gauge describe different parts of the process. None can simply replace the others when investigating how much water a landscape receives.
Check 1 of 3
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Work through a decision
Compare a concentration with a rainfall total
Consider an invented uniform cloud layer containing 0.4 grams of liquid water per cubic meter, extending through a depth of 500 meters. Above one square meter, its volume is 500 cubic meters. Multiplying gives 200 grams, or 0.2 kilograms, of liquid water. If all that existing liquid fell straight down, spread evenly, and neither evaporated nor gained new water, it would make a layer 0.2 millimeters deep, using liquid-water 1,000 kilograms per cubic meter.
That is an inventory calculation, not a forecast that this cloud must deliver 0.2 millimeters. A real storm continually receives moist air, condenses additional water, moves, and loses water by evaporation and precipitation. Over its lifetime, rainfall through an area can exceed the liquid present overhead at one instant. Conversely, none of that instantaneous inventory may reach the local surface. Keeping storage separate from continuing flow resolves the apparent contradiction.
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Combine instruments across the water cycle
A gauge measures precipitation collected at one location. Radar measures returned electromagnetic signals from particles and uses additional assumptions to estimate precipitation. Satellites can observe cloud-top radiation, microwave signatures, and, for specialized instruments, vertical precipitation structure. Their coverage and sensitivities differ. Comparing these measurements requires matching location and time, while allowing for sampling gaps and changes between the cloud and the ground.
For a practical investigation, inspect a cloud image alongside a time-matched rainfall map. Find an area with visible cloud but little recorded surface rain, then ask which explanations the available evidence can distinguish. Small droplets, evaporation below cloud, or a timing mismatch are different possibilities. A sound conclusion identifies what was observed, what was inferred, and what another instrument would need to measure to separate those possibilities.
Sentinel-3A observed Hurricane Matthew on October 7, 2016. This thermal-infrared product uses assigned colors and the original temperature key to reveal temperature contrasts. Its colors are not visible cloud colors or rainfall totals.
Look closely
Use the temperature key to compare a cold cloud region with a warmer surrounding region. Could those colors alone tell you how many millimeters of rain reached the ground?
See what to notice
No. The colors encode the thermal observation, not accumulated surface rain. Rainfall inference also needs particle and cloud information, retrieval assumptions, timing, and checks against measurements such as radar and gauges.
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Put it together
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