Understanding Air Pressure and Humidity: Key Concepts in Physical Geography

1. Office Hours Tue: 12:30 PM to 2:30 PM Wed: 9:00 AM to 10:30 AM & 12:00 PM to 2:00 PM Thr: 9:00 AM to 10:30 AM Course Syllabus can be found at: http://www.wx4sno.com/portfolio/BSU/Fall_2011/ This lecture will be posted AFTER class at: http://www.wx4sno.com/portfolio/BSU/Fall_2011/Lectures/

2. Lesson 13 Air Pressure Hess, McKnight’s Physical Geography, 10 ed. 97-103 pp.

3. Air Pressure • Air pressure is the force exerted by the atmosphere along a surface • Can be at ground level • Or can be at any height above ground level (AGL) • Air pressure is caused by atmospheric gases being pulled toward Earth by gravity • Force can be measured, usually in either inches of mercury or millibars

4. Air Pressure, cont.

5. Factors Influencing Air Pressure • Pressure • Density • Temperature • All three are related • If one changes, the other two change as well • The following are generalizations not laws pertaining to air pressure

6. Air Pressure Generalizations • Rising air produces low pressure near a surface • Strong rising air can lead to the development of dynamic lows • Descending or “subsiding” air produces high pressure near a surface • Strong subsidence results in dynamic highs • Warm temperatures produce low pressure near a surface • Known as thermal lows • Cold temperatures produce high pressure near a surface • Known as thermal highs

7. Dynamic High & Low Pressure • Descending air leads to surface high pressure • Rising air leads to surface low pressure

8. Thermal High & Low Pressure • Warm air rises, which produces low pressure at a surface • Cold air sinks, produces high pressure at a surface High Low

9. Measuring Air Pressure • Two systems of measurement: • Inches of mercury (inHg) • Height of mercury within a vacuum column • Sea level: 29.92 inHg • Millibars (mb) • Measure of force pressing down on a surface • 1mb = 1000 dynes cm-2 • 1 dyne = force required to accelerate 1 gm-2 1 cm-2 • Sea level: 1013.25 mb • The larger the number, the higher the pressure

10. Isobars • Differences in pressure can be mapped with lines of equal pressure, known as isobars • Elongated areas of high pressure are known as ridges • Elongated areas of low pressure are known as troughs

11. Isobar Map

12. Station models • Weather observing stations (human-operated and automatic) are located around the world • These stations report temperature, pressure, and a lot more information about the current weather

13. Station models, cont. • For this lesson, we’re only concerned with temperature and pressure • Temperature is located in upper-left • Always measured in degrees Fahrenheit • Pressure is located on upper-right • Given as an abbreviated measurement…needs to be converted…

14. Station models, cont. • To read the correct pressure: • Add either a “9” or a “10” in front depending on which would bring the value closer to 1000.0. • Then add a decimal before the very last digit • In the example above, 998 is given on the station plot • Adding a “9” in front and a decimal before the last digit give us 999.8 mb • Pressures generally fall between 950.0 mb and 1050.0 mb

15. Station Model Pressure Examples • 986.5 mb • 1013.8 mb

16. Drawing Isobars • Connect station plots with equal pressure values • Some isolines will fall between stations

17. Lesson 14 Humidity Hess, McKnight’s Physical Geography, 10 ed. 134-138 pp. & A8, A-9

18. Humidity • Humidity is the amount of water vapor in a sample of air • Two important ways to measure it for this lab • Mixing ratio (g/kg) • Relative humidity (%)

19. Mixing Ratio • Mixing ratio is the actual amount of water vapor in a sample of air • Grams water vapor per kilogram dry air (g/kg) • Mixing ratio does not change as the volume of air changes • The greatest amount of water vapor a parcel of air can hold is known as the saturation mixing ratio • At this point the air is completely saturated and condensation occurs • Recall, saturation is when there is 100% water vapor in air and when that happens, water vapor goes from a gas to a liquid through condensation

20. Relative Humidity • A comparison between actual amount of water vapor in the air to the maximum amount of water vapor the air can hold at a given temperature • This is also known as capacity • Expresses the amount of saturation in a percentage (%)

21. Calculating Relative Humidity • For example, if we have a mixing ratio of 13.5 g/kg and a saturation mixing ratio of 22.5 g/kg, relative humidity would be:

22. Things to Consider • As temperature increases, water vapor capacity also increases • This means that as temperature increases, relative humidity decreases • As temperature decreases, water vapor capacity decreases • As temperature decreases, relative humidity increases

23. Dew Point • Temperature at which relative humidity is 100% • Water vapor content = water vapor capacity • Dew point temperature can never be higher than actual air temperature • When air temperature = dew point temperature, condensation occurs

24. Sling Psychrometer • Two thermometers mounted side-by-side • The bulb of one thermometer is exposed to the air, like a normal thermometer • Dry-bulb thermometer • The bulb of the second thermometer is wrapped in cloth soaked in distilled water • Wet-bulb thermometer • See page 74 for more information

25. Finally… • Omit problem 3d on page 77 • Only do Part 1 & 2 problems (S.I. Units)…omit pages 79 and 80. • You can do pages 79 and 80 for up to 1 point extra credit