1. Water in the Atmosphere

2. evaporation – the process by which water molecules in liquid water escape into the air as water vapor

3. humidity – a measure of the amount of water vapor in the air
relative humidity – the percentage of water vapor in the air compared to the maximum amount the air could hold
Relative humidity can be measured with a psychrometer.

4. How Clouds Form
- Water in the Atmosphere
Clouds form when warm, moist air rises and cools. Water vapor condenses on tiny particles to form liquid water or ice crystals.

5. How Clouds Form
Clouds of all kinds form when water vapor in the air becomes liquid or ice crystals.
condensation – the process by which molecules of water vapor in the air become liquid water
dew point – the temperature at which condensation begins

6. - Water in the Atmosphere
Types of Clouds
Scientists classify clouds into three main types based on their shape: cirrus, cumulus, and stratus. Clouds are further classified by their altitude.

7. Types of Clouds Meteorologists classify clouds into three main types:
cumulus
clouds that look like fluffy, rounded piles of cotton
stratus
clouds that form in flat layers
cirrus
wispy, feathery clouds

8. Precipitation

9. Water Droplets
- Precipitation
Droplets come in many sizes. A raindrop has about one million times as much water in it as a cloud droplet.

10. precipitation – any form of water that falls from clouds and reaches Earth’s surface

11. Common types of precipitation include:
Rain – Most common type of precipitation. At least 0.5 mm in diameter. Rain made up of smaller particles of water is called drizzle.
Sleet – Ice particles smaller than 5 mm in diameter.
Freezing rain – Rain that falls through cold air near the ground and freezes when it touches a cold surface.
Hail – Round pellets of ice larger than 5 mm in diameter.
Snow – Falling ice crystals. They have an endless number of different shapes and all have 6 sides.

12. How Hail Forms
- Precipitation
Hailstones start as small pellets of ice in cumulonimbus clouds. They grow larger as they are repeatedly tossed up and down until they become so heavy they fall to the ground.

13. Measuring Precipitation
Scientists measure precipitation with various instruments, including rain gauges.