H 2 0 In The Atmosphere

1. H20 In The Atmosphere • Notes

2. Introduction • All water in the atmosphere is subject to the hydrologic cycle.

3. Transpiration from plants

4. Humidity • Humidity is a measure of how much water vapor is in the air. Warm air can hold more water vapor than cold air. • Remember that water vapor is a GAS. • Relative humidity is a percentage measurement. Relative humidity is the percentage of water vapor in the air compared to how much humidity the air can hold. • When the air can hold no more water vapor, it is said to be saturated, or have 100% relative humidity. • Relative humidity = current amount/saturated amount.

5. Water Vapor vs. Temperature So, at 30°C, the air can hold a maximum of 35g/m3 of water vapor. If a measurement is made that says that the 30°C air has all 35g/m3, then the air has a relative humidity of 100%. 35/35 = 1 or 100% If the 30°C air is measured to have only 3.5g/m3of the maximum 35, then the relative humidity is at 10%. 3.5/35 = .10 or 10% Water vapor content (g/m3)

6. Humidity • A psychrometer is an instrument that measures relative humidity. A psychrometer has two thermometers. One thermometer has a wet cloth covering it (wet-bulb thermometer), and the other doesn’t (dry-bulb thermometer). • You can tell relative humidity by comparing the two temperatures on the two thermometers.

8. How Clouds Form • Condensation is when a gas becomes a liquid. For example, water vapor condenses to form liquid water. This happens in the atmosphere when clouds form. • There are two things needed for condensation to occur in the atmosphere: • There must be cooling of the air (which happens naturally as warm air rises) • There must be condensation nuclei (tiny solid particles) in the air.

9. How Clouds Form • Cold air holds less water vapor than warm air. When air cools, water vapor condenses into either water droplets or ice crystals. • This happens when the air drops to a temperature in which it cannot hold anymore of the water vapor that was already in it. This is known as the dew point. • Tiny particles must be in the air for condensation of water to take place. Water needs something to condense onto. This is usually dirt/dust/microscopic organisms/etc.

10. Water Vapor vs. Temperature Let’s say that the air already has 10g/m3 of water vapor in it. If the temperature at the surface is 20°C, is it saturated? No As the warm air rises, it will begin to cool off. At what temperature can it no longer hold it’s 10g/m3? 10°C So when the rising air reaches 10°C, it will have reached its dew point or saturation point, the water vapor will begin to condense, and a cloud will begin to form. Water vapor content (g/m3)

12. Saturated

14. Types of Clouds • There are three basic cloud types, based on their shapes. We then define them further based on their altitudes. • Cirrus clouds are the wispy clouds that look like feathers in the sky. Cirrus clouds only form high in the sky and are always made of ice crystals.

16. Types of Clouds • Cumulus clouds look like fluffy, rounded piles of cotton. Cumulus clouds can produce thunderstorms. When they do, they are known as cumulonimbus clouds. Nimbus means rain.

18. Types of Clouds • Stratus clouds are flat layers of clouds. • The prefix/suffix -strat- always means layers. • Stratus clouds usually cover most of the sky. • Stratus clouds that produce rain are known as nimbostratus clouds. • Another special type of stratus cloud forms on the ground. You know this as fog.

20. Note the height prefixes. Cirro- for the highest Alto- for mid-level clouds