Heat, Moisture and Clouds
Weather & Meteorology Fundamentals
Course 1 · Chapter 3 · Heat, Moisture and Clouds
Every cloud is a story in three steps: the sun supplies energy, water is lifted into the air, and the air cools until the water becomes visible. This chapter follows those steps. It explains where the heat comes from, how air holds moisture, why rising air cools and what happens when it cools enough, and how the resulting clouds are classified. By the end you should be able to look at a sky and say something sensible about what the air is doing. Figures are given from general knowledge and flagged where they should be checked.
Where the Heat Comes From
Almost all the energy that drives weather comes from the sun. About thirty per cent of the sunlight reaching the Earth is reflected straight back to space by clouds, air and bright surfaces such as ice (check the figure). The rest is absorbed, mostly by the land and sea, which warm up and then warm the air above them. The air is heated from below, not from above, which is one reason the lowest layer is the unsettled one (Chapter 2).
Radiation
Energy travelling as light and heat rays, from the sun to the ground and from the ground back out to space.
Conduction
Heat passing directly from a warm surface to the thin layer of air touching it. It is slow, and affects only the lowest metres.
Convection
Warm air rising and cooler air sinking. This is how heat is carried upward and the engine of much weather.
Latent heat
Energy absorbed when water evaporates and released when it condenses. Condensation in a cloud warms the air around it, which can help the cloud grow.
Moisture in the Air
Warm air can hold more water vapour than cold air. Meteorologists use two measures to describe how moist the air is.
| Measure | What it tells you | Note |
|---|---|---|
| Relative humidity | How full the air is, as a percentage of what it could hold at its current temperature | Changes with temperature even if the water content does not |
| Dew point | The temperature to which the air would have to cool to become saturated | A better guide to how much moisture is really present |
When the air temperature falls to the dew point, the air is saturated, and the vapour begins to condense into liquid droplets. That is why dew forms on grass on a clear night, and why a cold glass of water "sweats" on a warm day.
Why Rising Air Cools
When a parcel of air rises, it moves into lower pressure, expands, and cools. No heat needs to leave it: the cooling comes from the expansion alone (check the explanation). Dry air cools at about 10 degrees Celsius per kilometre as it rises (check the figure, usually given as 9.8). Once it is cooled to its dew point, condensation begins, which releases latent heat and slows the cooling, so saturated air cools more slowly, at a rate that varies with temperature (check). This difference decides whether a rising parcel keeps rising: if it stays warmer than its surroundings, it carries on upward and builds a tall cloud.
From Vapour to Cloud to Rain
Water vapour does not condense easily in perfectly clean air. It needs tiny particles, such as salt, dust or smoke, called condensation nuclei, to gather on (check). A cloud is a collection of billions of droplets or ice crystals, each far too small to fall. Rain forms when droplets grow large enough, by colliding and merging, or, in cold clouds, by ice crystals growing at the expense of supercooled water droplets, a process named after the scientist who described it (check the details). Both routes need time and a thick enough cloud, which is why drizzle falls from thin cloud and heavy rain from thick.
Naming the Clouds
In 1802 an English pharmacist and amateur meteorologist, Luke Howard, proposed a system of Latin names for clouds, which is the basis of the system still used (check the date and details). The main ideas are simple.
| Name part | Meaning | Example |
|---|---|---|
| Cirro- | High cloud, made mostly of ice crystals | Cirrus: thin wisps |
| Alto- | Middle-level cloud | Altocumulus: patchy puffs |
| Cumulus | Heaped, with a flat base | Fair-weather cumulus |
| Stratus | Layered, sheet-like | Low grey stratus |
| Nimbo- or -nimbus | Rain-bearing | Nimbostratus, cumulonimbus |
Exercises
Explain why air is mostly heated from below, and name the four ways heat moves in the atmosphere, with a one-line example of each.
Air at 15 degrees Celsius has a dew point of 5 degrees. Using a cooling rate of 10 degrees per kilometre for rising dry air, estimate the height at which cloud would begin to form, and say what assumption you made.
Look at the sky on three different days and name the cloud type or types you see, using the table, saying which weather each suggests.
Quick Reference
- Energy: from the sun; about a third reflected; the rest warms land and sea first
- Heat transfer: radiation, conduction, convection, latent heat
- Moisture: relative humidity and dew point
- Rising air cools: about 10 degrees per km when dry; slower once saturated
- Cloud formation: cooling to the dew point plus condensation nuclei
- Names: cirro-, alto-, cumulus, stratus, nimbus
- Fog: cloud at the ground
Next chapter: Wind and Pressure, which covers why air moves and how the Earth's spin shapes the flow.