Since rising air cools, clouds form wherever air is lifted far enough to cool to its dew point. So the question "where will clouds form?" becomes "what lifts the air?" There are four main answers.
Key idea
Air is lifted by convection (the ground heats a bubble of air, which rises), by mountains (wind forces air up a slope: orographic lifting), at fronts (a wedge of dense cold air pushes under warmer air) and by convergence (winds crowd together near the ground, and the air has nowhere to go but up).
Why it works
Each one gives air a reason to go up. In convection, a patch of sunny ground heats the air above it, and the warmer, less dense bubble floats upward, like the warm air in a hot-air balloon. Over mountains the wind has to go somewhere, and the slope forces it up. At a front, the denser cold air stays low and slides under the warm air, lifting it. Where winds flow together, as when sea breezes from two coasts meet over Florida on summer afternoons, air piles up and is squeezed upward.
- Convection: puffy cumulus over sunny fields on summer afternoons; thunderstorms when it is strong.
- Orographic lifting: clouds and rain on the windward slopes of mountains.
- Frontal lifting: bands of cloud and rain along cold and warm fronts.
- Convergence: lines of storms where winds meet, and the rising air at the center of low-pressure systems.
Worked example: Forced over a ridge
Wind pushes dry air at 18 °C up a ridge 1.5 km high. If it stays unsaturated, how cold is it at the top? (Use 10 °C per km.)
- Cooling: $1.5 \times 10 = 15$ °C.
- $18 - 15 = 3$ °C at the top.
Answer: 3 °C
Worked example: Name the lift
On a hot, still July afternoon, puffy clouds pop up one by one over a plowed field, but not over the lake next to it. What lifted the air?
- There is no wind to push air up a slope, and no front is mentioned.
- The dark field heats in the sun much faster than the lake, warming the air above it.
- Warm bubbles rise from the field: convection.
Answer: Convection
Common mistake
Thinking clouds are fed by water evaporating straight up into them, like steam rising off a pot. Vapor is in the air everywhere, mostly invisible; clouds form only where air is lifted and cools to its dew point. Check: a cloud base is flat and level across the sky. That flat base is the height where rising air reaches its dew point, the subject of the next lesson.