Water in the atmosphere

1. Water in the atmosphere • Water in the atmosphere exists in all 3 forms: solid, liquid, and vapor. • Ice can change to water and then to vapor if heat is added to it. • Ice can also change directly to vapor in a process known as sublimation if it is below freezing and the air is very dry.

2. Water in the atmosphere • Water vapor can also turn to solid without becoming liquid. This is deposition and is how frost forms. • Most water in the atmosphere comes from evaporation of surface water. • Latent heat is the heat that is put into water to turn it to vapor. This heat may later be released when the vapor turns back to liquid.

3. Water in the atmosphere • Humidity is the amount of water vapor in the air. • Saturated air has the maximum amount of vapor in it. • A psychrometer is a device used to measure humidity. It is basically 2 thermometers, one of which is covered with a wet cloth. The wet cloth will cause the temperature to decline as water evaporates.

4. Water in the atmosphere • The specific humidity expresses the amount of moisture in the air in exact amounts such as 17 g/1000 g. • Dew point is the temperature that a certain air must be cooled to reach 100% humidity. • When temperatures fall overnight and reach the dew point, then dew will form on grass and plants close to the ground.

5. Water in the atmosphere • Cloud formation can only occur if there is a surface for the water molecules to stick to. • Condensation nuclei are small particles of dust, ice, or salt that water can stick to. • In order for clouds to form, the temperature also has to drop to the dew point. There are several ways this can happen.

6. Water in the atmosphere • Convective cooling occurs when air rises. The molecules under a lower pressure will move further apart and cool down. • Forceful lifting occurs when air is lifted as it passes over a mountain and cools.

7. Water in the atmosphere • A third method occurs when warm moist air and cool air mix. The warm air cools to the dew point and clouds form. • The final method is advective cooling. This occurs when warm moist air moves over cooler land or water and condenses.

8. Water in the atmosphere • Fog is clouds that are formed along the surface. • Radiation fogs found here are formed when the ground cools at night and the air close to the surface condenses. • Advection fogs forms along shorelines when moist air moves over cold ground.

9. Water in the atmosphere • Upslope fog forms when moist air rises up the side of a mountain and condenses. • Steam fog forms over lakes when cool air moves over a warm lake.

10. Homework • How does water get from the surface to the atmosphere? • What is latent heat? • Define psychrometer, dewpoint, and humidity. • What is a condensation nucleus and why is it important?

11. Precipitation • Drizzle is a very fine rain. • Rain is larger drops. • Snow may occur as tiny pellets or large flakes. • Sleet is tiny particles of ice. • Glaze ice forms when falling rain freezes on contact with the Earth. • Hail is lumps of solid ice.

12. Precipitation • Drizzle is a very fine rain. • Rain is larger drops. • Snow may occur as tiny pellets or large flakes. • Sleet is tiny particles of ice. • Glaze ice forms when falling rain freezes on contact with the Earth. • Hail is lumps of solid ice.

13. Precipitation • Tiny water particles are held in the clouds by updrafts. They do not fall until they grow large enough to overcome the updrafts. • Coalescence is the process where water particles join others to form large drops.

14. Precipitation • Supercooling means that the water vapor is below zero but doesn’t condense until it contacts a freezing nuclei. • Cloud seeding is a process where nuclei are added to a supercooled cloud to make it rain. • A rain gauge measures rainfall.