1. Section 2: Water and Wind
Preview
Key Ideas
Bellringer
The Water Cycle
Air Pressure
Wind

2. Key Ideas What happens to water in the troposphere?
What is air pressure, and by what terms is it also known?
What causes wind?

3. Bellringer
Atmospheric pressure can be measured with a barometer. The simplest barometer, illustrated below, is a glass tube containing mercury with one end sealed and the other end open in a dish of mercury. The height of the mercury in the tube is a measure of how much the atmosphere is pressing on the mercury in the dish; the greater the amount of atmospheric pressure, the higher the mercury level in the tube.

4. Bellringer, continued
Which barometer is measuring the greater atmospheric pressure? Explain your answer.
Atmospheric pressure increases as the altitude at which the pressure is measured nears sea level. If all things are equal except atmospheric pressure, which barometer is more likely to be at the top of a mountain and which is more likely to be at sea level?
Explain how you decided which barometer is more likely to be at the top of a mountain.

5. The Water Cycle What happens to water in the troposphere?
Water is continuously being moved through the troposphere.
water cycle: the continuous movement of water between the atmosphere, the land, and the oceans
Water is continuously being moved, primarily between the oceans and the continents.

6. The Water Cycle, continued
Evaporated water vapor condenses to form precipitation.
Evaporation occurs when solar energy heats water molecules, and they rise as gaseous water vapor.
transpiration: the process by which plants release water vapor into the air through their leaves
precipitation: any form of water that falls to Earth’s surface from the clouds

7. Visual Concept: Water Cycle

8. The Water Cycle, continued
Air contains varying quantities of water vapor.
humidity: the amount of water vapor in the air
Relative humidity is the actual amount of vapor in the air compared to the maximum amount the air could hold at that temperature.
Air that has a relative humidity of 100% is said to be saturated.

9. Visual Concept: Humidity

10. The Water Cycle, continued
Warmer temperatures evaporate more water.
Warm air can hold more water vapor than cold air can.
Water vapor becomes liquid at the dew point.

11. The Water Cycle, continued
dew point: the temperature at which the rate of condensation equals the rate of evaporation
Air or a gas begins to condense to a liquid.
When humidity is high, there are more molecules of water in the air and it is easier to form liquid.
The higher the humidity, the higher the dew point.

12. Visual Concept: Dew Point

13. The Water Cycle, continued
Clouds form as warm, moist air rises.
Water vapor condenses into tiny droplets of liquid as it cools.
Depending on where clouds form, they can have different shapes and characteristics.

14. The Water Cycle, continued
Cloud names describe their shape and the altitude at which they form.
Clouds are named with combinations of three root words:
cirrus
stratus
cumulus
Cirrus clouds are thin, wispy, and occur at high altitudes.
Stratus clouds are layered and look like sheets.
Cumulus clouds are white and fluffy with somewhat flat bottoms.

15. The Water Cycle, continued
Cloud names reflect combined characteristics.
Cirrostratus clouds are high, layered clouds that form a thin white veil.
Altostratus and altocumulus clouds are stratus and cumulus clouds that occur at middle altitudes.
Cumulonimbus clouds are towering rain clouds that often produce thunderstorms.
Nimbostratus clouds are large, gray clouds that often produce steady precipitation.

16. Cloud Types

17. Visual Concept: Formation of Clouds and Precipitation

18. Air Pressure What is air pressure, and by what terms is it also known?
The barometric pressure, also called atmospheric pressure or air pressure, is the pressure that results from the weight of a column of air extending from the top of the thermosphere to the point of measurement.

19. Air Pressure, continued
Changes in barometric pressure often accompany changes in the weather.
Falling pressure may indicate that a large air mass is leaving the area.
Rising air pressure may mean that an air mass is moving in.

20. Air Pressure, continued
Mercury barometers indicate air pressure by a column of mercury.
At sea level, the barometric pressure of air at 0 °C is about 760 mm of mercury.
760 mm of mercury is defined as 1 atmosphere (1 atm) of pressure.
The SI unit for pressure is the pascal (Pa).
A pascal is equal to 1 newton per square meter.
Aneroid barometers do not contain liquid.

21. Visual Concept: Atmospheric Pressure

22. Wind What causes wind? Differences in pressure create winds.
Pressure gradients cause air to move.
pressure gradient: a difference in air pressure from one place to another
The air in a pressure gradient moves from areas of high pressure to areas of low pressure.
wind: the movement of air from a high-pressure area to a low-pressure area

23. Visual Concept: Wind

24. Wind, continued Earth’s rotation affects the direction of winds.
Coriolis effect: the curving of the path of a moving object from an otherwise straight path due to Earth’s rotation
Points at different latitudes on Earth’s surface move at different speeds.
Earth goes through a full rotation in 24 hours.
Points on the equator travel the Earth’s full circumference in 24 hours.
Points closer to the poles do not travel as far.

25. Visual Concept: Coriolis Effect

26. Wind, continued Predictable air circulation forms wind patterns.
Winds in the Northern Hemisphere curve clockwise.
Winds in the Southern Hemisphere curve counterclockwise.
The resulting patterns are very regular, and have been named by meteorologists.
Polar easterlies
Westerlies
Northeast trade winds
Southeast trade winds

27. Circulation Patterns

28. Wind, continued Global wind patterns form circulation cells.
Flowing from a high-pressure area to a low-pressure area, air flows both north and south in a large loop.
Three loops of rising warm air and sinking cold air can be found in each hemisphere.
Air in each of the hemispheres completes three loops, called cells.

29. Visual Concept: Global Winds and Surface Currents