1. Air Pressure & Humidity

2. What is Air Pressure? The force exerted onto an object by the weight of air

3. What Affects Air Pressure? Altitude Temperature Humidity

4. Altitude and Air Pressure Atmospheric pressure decreases with height Pressure decreases rapidly at first and then more slowly at higher levels

5. Measuring Air Pressure on Weather Maps Meteorologists use isobars to connect areas of equal pressure

6. High Pressure High Pressure brings typically brings fair weather with little or no cloud cover Air pressure is high so the air pushes down and creates “convergence” of air We call areas of high pressure “Anti cyclones”

7. Low Pressure Low pressure typically brings rain and clouds Air pressure is low which creates a “divergence” of air, favoring cloud formation We call areas of low pressure “Cyclones”

9. Cyclone vs. Anticyclone In high pressure systems (anti cyclones), winds move clockwise In low pressure systems (cyclones), winds move counter-clockwise

10. Highs and Lows on a weather map High pressure systems are indicated by a blue “H” on a weather map Low pressure systems are indicated by a red “L” on a weather map

12. Using Pressure to Predict Wind Patterns Besides knowing the weather conditions associated with pressure systems, meteorologists can predict wind patterns

13. Wind Direction Pressure also affects the wind direction Air moves from high to low pressure

14. Isobars Isobars are lines that connect areas of equal pressure

15. Recognizing high and low pressure with isobars Much like a topographic map, areas of equal pressure are measured with isobars. You find the low and high pressure systems by identifying enclosed isobars

17. Highs and Lows Typical low pressure: Anything under 1013mb Lowest ever recorded: 870mb Typical high pressure: Anything above 1013 mb Highest recorded: 1086mb

18. Wind Speed and Isobars Isobars that are close together represent high wind speeds Isobars that are further apart represent slwoer wind speeds

19. Humidity

20. What is Humidity? The quantity representing the amount of water vapor in the atmosphere Typically, when humidity levels are high, the air feels heavy and sticky

21. Humidity cont. Humidity can often make the air feel warmer than what it actually is – Heat Index Presence of high humidity can also indicate precipitation in the near future As altitude increases, humidity decreases – Less water molecules in the atmosphere

22. Dew Point The dew point is the temperature at which air can no longer “hold” all of the water vapor Air has to cool to the dew point in order to reach saturation Dew point is expressed as a temperature

23. Dew point and humidity When the dew point and the current temperature are the same you will have about 100% humidity If it is 100% humid, chances are it is raining Humidity = Dewpoint / Current temp

24. Dew Point cont.