1. Weather
State Objectives 4.c, 4.e, 4.h.

2. What are some ways in which weather affects your everyday life?
Discussion
What are some ways in which weather affects your everyday life?

3. What is Weather? Atmosphere - layer of gases surrounding Earth.
Weather is the conditions of the atmosphere.
Temperature
Air Pressure
Humidity
Wind
Clouds
Precipitation
Main cause for changes in weather is energy from the sun unevenly heating the surface of Earth in the form of radiation

4. Layers of the Atmosphere
Thermosphere
Temperature increases
Mesosphere
Temperature decreases
Stratosphere
Troposphere

5. Temperature
Indicates the amount of heat (kinetic energy) in the atmosphere.
Represents the speed of the molecules.
The higher the temperature, the faster the air molecules are moving.
Warmer air rises and cooler air sinks which causes convection currents.
Measured with a thermometer.
Standard unit is Fahrenheit (°F)
SI Unit is Celsius (°C)

6. Air Pressure/Barometric Pressure
Air has weight because it has mass.
Air pressure is a measure of the force of air being exerted on a given area of Earth’s surface.
As temperature increases pressure decreases.
Cool air is more dense, which causes it to sink (high pressure).
As altitude increases air pressure decreases.

7. Weather and Air Pressure
Changes in pressure indicated a change in weather is approaching.
Low pressure systems are associated with clouds & precipitation.
High pressure systems are associated with clear skies.
Steady pressure indicates current conditions will continue.
Measured with a barometer
in inches of mercury or in millibars.

8. Humidity Amount of water vapor in the air.
Relative humidity is a percentage of the amount of water vapor in the air compared to the maximum amount of water vapor it can hold at that temperature.
The warmer the temperature the more water vapor it can hold.
Saturated means the air is holding 100% of the water vapor it can hold at that temperature.
Measured with a hygrometer or a psychrometer.

9. Dew point
Dew is the water vapor that has condensed on a surface into a liquid.
Depends on two factors:
Amount of water vapor in the air
Temperature near the surface
Dew point is the temperature at which water vapor condenses into a liquid.

10. Relative Humidity Chart

11. Wind Caused by differences in air pressure
Air moves from areas of high pressure to areas of low pressure
Wind Speed
Measure of how fast the air is moving.
Measured with an anemometer.
Wind Direction
Direction from which the wind is coming, NOT the direction it is blowing
Ex. North winds blow from N to S
Measured with a wind vane.

12. Global Wind Patterns
Blow steadily across Earth in paths that are thousands of kilometers long
Steer weather in certain directions (usually west to east in the U.S.)
Caused by thermal energy from the sun
The sun does not heat the surface evenly causing uneven heating of the atmosphere.

13. Global Winds

14. Types of Global Winds Surface winds at low altitudes:
Trade winds: blow from east to west near the equator.
Westerlies: blow from west to east in the mid-latitudes.
Coriolis Effect: Earth’s rotation causes winds to curve to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

15. Types of Global Winds
Jet streams at high altitudes are bands of strong winds (up to 350 km/h) near the top of the troposphere at the northern and southern boundaries of the prevailing westerlies.
Race from west to east

17. Clouds
Clouds form when air rises, cools, and condenses. They are classified according to their height and shape.

18. Types of Clouds
1. cumulus- a low, puffy cloud that forms on sunny days when heat from the surface causes warm air to rise.
2. stratus- a low, gray, sheet-like cloud that forms when warm, moist air moves over cooler ground. They are seen most often during the winter and may bring steady rain.

19. Cumulus and Stratus

20. Types of Clouds
3. cumulonimbus (thunderheads) - vertical clouds that may be over four miles tall. They form where cold air forces warm air to rise quickly.
4. cirrus- high, featherlike clouds. They are the highest clouds in the sky. They do not produce precipitation.

21. Cumulonimbus and Cirrus

22. Precipitation
Precipitation – occurs when drops of water or crystals of ice become too large to be suspended in a cloud and fall in the form of rain, freezing rain, sleet, snow, or hail.