1. Chapter 18: The Atmosphere
Section 2:
Water and Wind
Chapter 18: The Atmosphere

2. Properties of Air
Even though it appears that air has no mass, air consists of atoms and molecules that, in fact, have mass.
Because air has mass, it also has other properties, including density and pressure.

3. Properties of Air: Pressure
Pressure—The force pushing on an area or surface.
Air Pressure—A force that is the result of the weight of a column of air pushing down on an area.
Air is pressing from all directions

4. Measuring Air Pressure
The changes in air pressure affect the weather.
Falling air pressure indicates a storm is coming, while rising air pressure usually means the weather is clearing.
Barometer—An instrument that is used to measure changes in air pressure.
Mercury Barometer
Aneroid Barometer

5. Mercury Barometer
Mercury Barometer— An instrument that measures changes in air pressure, consisting of a glass tube partially filled with mercury , with its open end resting in a dish of mercury.

6. Mercury Barometer
When the air pressure increases, it presses down more on the surface of the mercury.
Greater air pressure forces the column of mercury higher, and when air pressure decreases, the column will drop.

7. Aneroid Barometer
Aneroid Barometer— An instrument that measures changes in air pressure without using liquid.
The word aneroid means “without liquid.”

8. Aneroid Barometer
An aneroid barometer has an airtight metal chamber that is sensitive to changes in air pressure.
When air pressure increases, the walls push in, and when air pressure decreases, the walls bulge out.
The chamber is connected to a series of springs and levers, and as the chamber changes shape, the needle moves.
Aneroid barometers are smaller than mercury barometers and do not contain liquid, which allows them to be portable and used more practically.

9. What Causes Winds?
Because air is fluid, it can move easily from place to place.
The force that makes air move is caused by a difference in air pressure.
Fluids tend to move from areas of high pressure to areas of low pressure.

10. What Causes Winds?
Wind—The horizontal movement of air from an area of high pressure to an area of low pressure.
All winds are caused by differences in air pressure.
Most differences in air pressure are caused by unequal heating of the atmosphere.

11. What Causes Winds? (How Convection Currents Work)
Convection currents form when an area of Earth’s surface is heated by the sun’s rays.
Air over the heated surface expands and becomes less dense. (This causes air pressure to decrease.)
The air above an area that is less heated will be cooler and denser. (High air pressure)
The area with a higher air pressure flows under an area that has a lower air pressure, because it has a higher density.
This forces the warm air to rise.

12. Measuring Wind Winds are measured by their direction and speed.
Wind direction is determined by a wind vane.
The wind swings the wind vane so that one end points into the wind.
The “name” of the wind tells you where the wind is coming from.
North
South
East
West

13. Measuring Wind Anemometer—An instrument used to measure wind speed.
An anemometer has three or four cups mounted at the ends of spokes that spin on an axle.
The force of the wind against the cups turns the axle.
The force of the wind against the cup turns the axle.
A speedometer attached to the axle shows the wind speed.

14. Cool Breezes
A cool breeze can make one feel refreshed and uncomfortable depending on the season.
The wind blowing over one’s skin removes body heat.
The stronger the wind, the colder one feels.
Wind-Chill Factor—Increased cooling caused by the wind.
The wind-chill factor makes a normal temperature feel cooler.

15. The Coriolis Effect
Because Earth is rotating, global winds do not follow a straight path.
As the winds move, the Earth rotates from west to east underneath them, making it seem as if the winds have curved.
Coriolis Effect—The way Earth’s rotation makes winds in the Northern Hemisphere curve to the right and winds in the Southern Hemisphere curve to the left.

17. Global Wind Belts
The Coriolis effect and other factors combine to produce a pattern of calm areas and wind belts around Earth.
The major global wind belts are:
The Trade Winds
The Prevailing Westerlies
The Polar Easterlies
The calm areas include:
Doldrums
The Horse Latitudes

19. The Water Cycle
Water Cycle—The continuous movement of water between the atmosphere and Earth’s surface.
The four steps in the water cycle are:
Evaporation
Condensation
Precipitation
Runoff

20. The Water Cycle
Evaporation—The process by which water molecules in liquid water escape into the air as water vapor.
Condensation—The process by which molecules of water vapor in the air become liquid water.
Precipitation—Any form of water that falls from clouds and reaches Earth’s surface.
Runoff—Water that makes its way back to the oceans, lakes, streams, or pond.
Also known as the “collection” stage.
Any water that is not absorbed in the soil.
“groundwater”

22. Humidity 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.

23. How Clouds Form
Clouds of all kinds form when water vapor in the air becomes liquid water or ice crystals.
This happens through condensation.
Dew Point—The temperature at which condensation begins.
If the dew point is below the freezing point, the water vapor may change directly into ice crystals.

24. How Clouds Form
Clouds form whenever air is cooled to its dew point and particles are present.
If air is warmed near the ground, it becomes less dense and rises in a convection current.
When the rising air expands and becomes cooler, clouds may form.

25. Types of Clouds Meteorologists classify clouds into three main types:
Cumulus
Stratus
Cirrus
Clouds are also classified by their altitude.
Each type of cloud is associated with a different type of weather.

27. Types of Clouds
Cumulus—Clouds that form less than 2 kilometers above the ground and look like fluffy, rounded piles of cotton.
The word cumulus means “heap” or “mass.”
Cumulus clouds usually indicate fair weather.
Towering clouds with flat tops, called cumulonimbus clouds, often produce thunderstorms.
The suffix –nimbus comes from the Latin word “rain.”

28. Types of Clouds Stratus—Clouds that form in flat layers.
Strato means “spread out.”
Stratus clouds usually cover all or most of the sky.
As they thicken, stratus clouds may produce drizzle, rain, or snow.
They are then called nimbostratus clouds.

29. Types of Clouds
Cirrus—Wispy, feathery clouds made mostly of ice crystals that form at high levels, above about 6 kilometers.
Cirrocumulus clouds, which look like rows of cotton balls, often indicate that a storm is on its way.

30. Additional Information
Part of a cloud’s name may be based on its height.
The names of clouds that form between about 2 and 6 kilometers above Earth’s surface have the prefix alto-, which means “high.”
Main Types of Clouds
Altocumulus
Altostratus
Fog—Clouds that form at or near the ground.
Fog often forms when the ground cools at night after a warm, humid day.
The ground cools the air just above the ground to the air’s dew point.
The next day the heat of the morning sun “burns” the fog off as its water droplets evaporate.