1. Understanding Weather

2. Weather Factors
Weather – refers to the state of the atmosphere at a specific time and place. It occurs in the troposphere.

3. 5 Factors that affect Weather
Air Temperature
Wind
Humidity
Relative Humidity
Dew Point

4. Sun Provides almost all of Earth’s energy Causes evaporation
It is a source of heat energy
Earth’s surface absorbs the Sun’s heat
The Sun provides the energy that drives Earth’s weather

5. Air Temperature High Temp. – air molecules move fast and it feels warm
Temperature is a measure of the average amount of motion of molecules.
Low Temp. – air molecules move slower and it feels cold

6. Warm Air
Low Pressure
Cold Air
High Pressure

7. Thermometer – instrument used to measure temperature

8. Wind Air moving in a specific direction
Wind results because air moves from areas of high pressure( cold air) to areas of low pressure(warm air).

9. Instruments to Measure Wind
Wind Direction is measured with a wind vane
Wind Speed is measured with an anemometer
Wind Vane
Anemometer

10. Humidity The amount of water vapor in the air
More water vapor can be present in warmer temperatures
Less water vapor is present in cooler temperatures
When enough water vapor is present in air for condensation to take place, the air is Saturated

11. Relative Humidity
Is a measure of the amount of water vapor present in the air compared to the amount needed for saturation at a specific temperature.
Weather forecasters report the amount of moisture in the air as Relative Humidity
If you hear the forecaster say that the relative humidity is 50%, it means that the air contains 50% of the water needed for the air to be saturated.

12. Psychrometer – instrument to measure humidity
Psychrometer – instrument to measure humidity. Uses the difference in readings between two thermometers, one having a wet bulb and the other having a dry bulb, to measure the moisture content or relative humidity of air

13. Dew Point
The temperature at which air is saturated and condensations forms.
Dew point changes with the amount of water vapor in the air.
Air near the ground cools to its dew point, water vapor condenses and forms dew,
Frost may form when temperatures are near 0⁰ C (32⁰ F)

14. Clouds Form as warm air is forced upward, expands, and cools
When relative humidity reaches 100% tiny water droplets begin to form(condensation)
Billions of droplets attach to particles in the air to form a cloud
3 main types of clouds: Stratus, Cumulus, and Cirrus.

15. Stratus Forms at low altitudes
Clouds of layers, or smooth, even sheets
Stratus clouds near the ground are called FOG
Associated with fair weather or rain or snow

16. Cumulus Clouds are masses of puffy, white clouds, often flat bases
Associated with fair weather or thunderstorms

17. Cumulonimbus = Thunderstorms

18. Cirrus Cirrus clouds are the highest clouds.
They usually mean fair weather.
They look white and feathery and made of ice

19. Precipitation Water falling from the clouds
4 different forms: Rain, Snow, Sleet, and Hail
Air temperatures determine which it will be.

20. Rain and Snow Rain drops falling in Temps Above Freezing. Snow forms
When the
Temps are
So cold that
Water vapor
Turns into
A solid.

21. Sleet Sleet forms when Raindrops pass through A layer of freezing
Air near the surface.

22. Hail
Hailstones are pellets
Of ice that form inside
A cloud.

23. The relative humidity of air helps determine whether a location will have a dry day or experience some form of precipitation.