1. Weather
Earth Science

2. What is Weather?

3. Definition Weather: The state or condition of the atmosphere at
any given location for a short period of time

4. What is the Major Cause of Weather
Constant changes in the atmosphere due to
variations in insolation which causes heat
energy to be unevenly distributed

5. Atmospheric Characteristics
Layers of the atmosphere: (ESRT p. 14)
Thermosphere
Mesosphere
Stratosphere
Troposphere

8. Weather Variables Temperature: The average kinetic energy of molecules
A) Thermometer: Liquid in a closed tube. Liquid expands when heated
B) Thermograph: Records temperature over a weeks time in a graph

9. Weather Variables Air Pressure: Pressure due to the weight of the
overlying atmosphere
pushing down on any
given area

10. Air Pressure Barometer: Instrument used to measure air
pressure. Measured in:
Inches (in)
Millibars (mb)
Barograph: Records air
pressure over a weeks
time

11. What Makes Air “Heavy” or “Light”?
Troposphere Composition: (ESRT p 1)
78% N2 Atomic Mass: 14 x 2 = 28
21% O2 Atomic Mass: 16 x 2 = 32
1% Other

12. Moist vs. Dry Air < 1% Water H20 Atomic Mass: Hydrogen = 1 x 2 = 2
Oxygen = 16
So H20 = 18
Dry air is much heavier than moist air!!!
As water vapor (gas) is added to a unit of air, you
are adding a lighter gas and a heavier N2 and O2
are pushed out.

13. Weather Variables Wind: The horizontal movement of air
parallel to the Earth’s
surface caused by
differences in air
pressure.

14. Wind Winds are named for the direction from which it comes.
Wind Vane - direction
Anemometer – speed

15. Weather Variables Moisture Two Major Sources:
1) Evaporation from the oceans
2) Transpiration from trees (plants)

16. Humidity Definition: The general term for water vapor
content in the atmosphere
Absolute Humidity – The actual amount of water vapor in a volume of air
Relative Humidity – The ratio of the absolute humidity to the maximum amount the air can hold

17. Water Vapor Capacity Warm Air = High water vapor capacity
Cold Air = Low water vapor capacity

18. How do we Measure Relative Humidity?
Psychrometer:
Measures humidity
with two thermometers
1 dry bulb
1 wet bulb

19. Determining Relative Humidity
Evaporation is a cooling process!!!!
The amount of cooling depends on the rate
of evaporation.

20. Therefore:
The greater the difference between the dry bulb and the wet bulb, the drier the air
The smaller the difference between the dry and wet bulbs, the more humid the air.

21. Dew Point Definition: The temperature at which the air
is saturated with water vapor.
Saturated: A parcel of air is holding all the
water vapor it can at a particular
temperature. RH = 100%
The closer the wet bulb and dry bulb, the more
saturated the air

22. Clouds Requirements: Water Vapor Temperature cools to dew point
Condensation Nuclei (Dust)

23. Cloud Forming Process Air Rises Expands Cools Dew Point is Reached
Water Vapor Condenses on Condensation Nuclei

24. Three Basic Types
1)Cirrus – High, thin wispy clouds of cold air ice crystals
2)Stratus – Low, layered, thick clouds
3)Cumulus – Vertically developing clouds, puffy – fair weather

26. Fog
Definition: A cloud on or just above Earth’s
surface (stratus).

27. Precipitation Definition: Liquid, or solid water falling from clouds.
Instruments:
Rain Gauge
Ruler

28. Forms of Precipitation
Rain
Freezing Rain
Sleet
Hail
Snow