Lecture 11 Atmospheric Pressure & Intro to Wind

1. Lecture 11AtmosphericPressure&Intro to Wind

2. Understanding Pressure AIR PRESSURE is the pressure exerted by the weight of the air above. Is DEFINED as: TheFORCEexerted against a surface by the continuous collision of gas molecules

3. Measuring Air Pressure Unit: Newton (N) At Sea Level one “atmosphere” exerts 14.7 pounds per square inch 101,325 N per square m (N/m2) Meteorologist use millibars (mb) 1 mb = 100 N/m2 Standard Sea Level Pressure ~ 1013.25 mb* * This is a number you MUST memorize!!!!

4. Understanding Pressure • Example: Why aren’t we crushed by the weight of the air above us? 1) We developed under this pressure. 2) Pressure force of air is exerted in all directions 3) If you lower the pressure drastically the cells of our bodies would burst!! Balloon SHRINKS in all directions and dimensions equally!!

5. Understanding Pressure Example: What if pressure was only exerted in one direction? If the force is only in one direction just the weight of an aquarium on top, not equally in all dimensions will cause our balloon to pop POP!!!!

6. Measuring Air Pressure Besides mb we use “inches of mercury” or inches of Hg. 29.92 in. Hg. Barometer instrument to measure pressure Filled with liquid mercury

7. Measuring Air Pressure Aneroid barometer No fluid Aneroid cell Have at home Barograph An aneroid barometer that records a trace of the pressure as time passes

8. Pressure and Wind Horizontal differences in pressure causes winds Horizontal differences in pressure are caused by differences in heating Winds flow from regions of high pressure to regions of low pressure Horizontal differences in P lead to the PRESSURE GRADIENT FORCE

9. Changes in temperature drive winds The gas law (eq of state): p ~ r x T ***Pressure ~ density x Temperature****  Heated air expands. 1 2 H 3 L

10. Influence of Temp and Water Vapor (A) Warm Air Fast moving molecules Typically less dense LOW PRESSURE (B) Cold Air Slow moving molecules Typically more dense HIGH PRESSURE **Factors other then Temp can affect Pressure… you can have “warm” high pressure

11. Influence of Temp and Water Vapor The addition of water vapor actually makes air LIGHTER (less Dense)!!!! Molecular weights of N2 (14) and O2 (16) are greater than H2O (10) If you “substitute” some of the N2 and O2 with H20 the overall weight of air will be less! N2: 4 * 14 = 56 O2: 2 * 16 =32 H2O: 5 * 10 = 50 Total = 138 N2: 7 * 14 = 98 O2: 3 * 16 =48 Total = 146

12. Influence of Temp and Water Vapor SUMMARY Cold, dry air masses produce High Surface Pressures Cold, humid air masses are less “high” than cold, dry Warm, dry air masses are less “low” than warm, humid Warm, humid air masses produce Low Surface Pressures HIGH PRESSURE LOW PRESSURE

13. ISOBARS Contours (lines or curves) of constant PRESSURE Just like your isotherms for temperature They are corrected for altitude to equivalent Sea Level Pressure (SLP)

14. Sea Level Correction Pressure decreases by ~10 mb every 100 m.

15. ISOBARS – Let’s do an example! L L H H

16. What is Wind? Wind is nature’s attempt at balancing inequalities in pressure FACT: Unequal heating of the Earth’s surface generates these inequalities. FACT: Solar radiation is the ultimate source of energy for Wind