What you will learn in Water in the Atmosphere and Precipitation

1. 1. How much water is in the air

   Four ways to say how much water a parcel of air carries, and only one of them changes when nothing but the temperature does. Seal the air in a flask so it cannot expand, warm it, and watch the "more room" explanation fail.

2. 2. Rising, cooling, condensing

   Push a parcel of air upwards and it cools by expanding — at a fixed rate until it saturates, and at a slower, changing rate after. The cloud appears exactly where it saturates, which is why cloud bases are flat.

3. 3. Four ways to make it rain

   Convectional, orographic, frontal and cyclonic rain are one mechanism with four reasons for the air to rise. The mountain case can be computed all the way through, and the air comes down the far side warmer than it went up.

4. 4. Stability, and what the sky looks like

   Lay a rising parcel against the air around it and the sky follows: layered cloud from stable air, heaped cloud from unstable air, and a thunderstorm from air that only needed a push. Two real balloon soundings from Kolkata, one winter dawn and one May afternoon.

5. 5. From droplet to raindrop

   A raindrop is a million cloud droplets, and a cloud does not rain by getting heavy. Two ways of gathering the million, and what the air on the way down turns them into: rain, snow, sleet, freezing rain or hail.