Air Temperature, Pressure and The Wind

1. Air Temperature, Pressure and The Wind

2. Maximum Daily Temperature • Incoming solar radiation is maximum at noon • But incoming radiation is greater than outgoing radiation until late afternoon • So warming continues until outgoing becomes greater than incoming

3. Factors Influencing Max Temperature • Soil Type • Some types absorb more energy due to color, texture, etc. • Soil Moisture • Wet soil heats up slower • Some energy goes into evaporation • Vegetation • Evaporation again • Clouds, Humidity, Haze • Reflect radiation

4. Effect of Wind on Temperature

5. Factors Influencing Max Temperature • So ideally, to get the hottest daytime temp: • Soil which is a good absorber (sand for one) • Dry soil • No vegetation • No moisture in the air • What type of area does the above describe? • Desert areas

6. Atlanta vs Phoenix • Both at same latitude - same amount of solar radiation coming in • AtlantaPhoenix humid dry cloudy clear lots of trees less trees 87º July avg. 105º July avg.

7. Air Temps and Human Comfort • Defn. - wind chill factor - how cold we feel due to temperature and wind • Why do we feel colder when the wind blows? • Conduction warms air near the skin • When the wind blows, the warm layer of air is moved away and more heat from the body is used to warm the colder air • Fast wind = faster loss of body heat

8. Air Temps and Human Comfort • What about cold and moisture? • We feel colder when its also wet outside. Why?? • Water conducts heat better than air • Can cause hypothermia • big problem for ill-prepared hikers in the mountains

9. Measuring TemperatureMapping Isotherms

10. Pressure • Hear this term often in weather forecasts but what does it mean in the atmosphere? • It’s the weight of the air above • Pressure and Weather? • High pressure? • Usually nice weather • Low pressure? • Associated with stormy weather

11. Wind • wind and pressure are related • Wind blows due to horizontal differences in pressure

12. Atmospheric Pressure • Remember: pressure is basically the weight the air above us. Pressure at the surface is due to the weight of all air molecules in the column • IMPORTANT:Uneven heating of the earth surface creates temperature differences which create pressure differences

13. Atmospheric Pressure • Simplified model • Assumes air can’t leave the column • Columns of air have the same # of molecules and are at the same temperature • The pressure at the surface is the same • What would happen to density of air if the temperatures of the air changed? • Cool #1, Warm #2

14. Atmospheric Pressure • City 1, temp decreased so density increased • City 2, temp increased causing density to decrease • Pressure at surface stayed the same • Bottom line: It takes a shorter column of cold air to exert the same amount of pressure as a taller column of warm air

15. Atmospheric Pressure • Now let’s go up to a certain height in the atmosphere • At this level, in which column is the pressure greatest? • So, relatively speaking, the pressure is LOW at this level in column 1 and HIGH in column 2 L H

16. Atmospheric Pressure • The difference in pressure establishes a force we call the “pressure gradient force” (directed from H to L) • Now, if we remove the side barriers of the columns, air will rush from high to low pressure in order to equalize things …. • WIND!!

17. Atmospheric Pressure • Pressures at the surface will also change due to molecules moving • Pressure at City 1= • (rise or fall) • Pressure at City 2= • (rise or fall) • Conclusions • Differences in temp from place to place can cause differences in pressure resulting in the movement of air. • Cold Air Mass Pressure at Surface= High • Warm Air Mass Pressure at Surface= Low L H

18. Measuring Air Pressure • Mercury Barometer • Just a large, hollow glass tube immersed in mercury • As air pressure changes, mercury is forced up or down the tube…pretty simple right • On average, the height of the mercury would be 29.92 inches (avg. sea level pressure) • Or 1013.25 millibars

19. Measuring Air Pressure • Aneroid Barometer • Has a hollow metal “cell” which expands or contracts as pressure changes • Same type as in the 3-dial weather instruments people hang on walls

20. Measuring Air Pressure

21. Units of Measurement

22. Atmospheric Pressure • Seen something like this on TV right? • Lines are called “isobars” • These are lines of equal pressure (in millibars) • This kind of map depicts “sea-level pressure”, not surface pressure • We wouldn’t always be able to tell where high and low pressure systems were otherwise

23. Pressure and Wind • Northern Hemisphere: surface winds blow clockwise and outward from high pressure (anticyclones) • counter clockwise and inward around low pressure systems (cyclones) • Note: • Winds cross isobars slightly • Tightly packed - stronger winds • Reversed flow in Southern Hemisphere

24. High or Low Pressure?

25. Wind • 4 forces acting to influence wind speed and direction • 1) Pressure gradient force • 2) Coriolis force • 3) Friction • 4) Centripetal force

26. Pressure Gradient Force • Due to the difference in pressure over a distance • Greater pressure gradients lead to a greater PGF • like at the right • Hurricanes are a good example • Very low pressures at the center • Pressure increases rapidly as you move away from the center • Strong PGF

27. Pressure Gradient Force • ALWAYS directed from high to low pressure • Direction is at right angles to the isobars • But do winds flow in these directions around pressure systems?

28. Coriolis Force (Effect) • Tricky subject • An “apparent” force due to the rotation and curvature of the earth • Causes wind to deflect to the right in the Northern Hemisphere • Left in SH • Force is at a right angle to the wind

29. Coriolis Force

30. Coriolis Force • Summary: • Causes objects to deflect to the right of a straight path in the NH (left in SH) • Amount of deflection depends on • 1) Rotation of the earth • 2) Latitude • 3) Speed of object (wind, airplane, etc.) • http://www.eoascientific.com/interactive/the_coriolis_effect/the_coriolis_effect.html

31. PGF - Coriolis Balance • Above the friction layer near the surface, the PGF and CF roughly balance each other • That’s why air aloft flows parallel to isobars • Wind which flows at a constant speed parallel to evenly spaced isobars is called • Geostrophic Wind

32. Wind Review • 4 forces (talked about 2 so far) • 1) Pressure gradient force • Directed from High to Low pressure • Stronger PGF = stronger wind • 2) Coriolis force • Deflects wind to right in NH • Faster wind = stronger CF • These two forces are roughly in balance above the surface • Causes upper level winds to generally flow west to east in mid-latitudes (parallel to isobars)

34. Convergence and Divergence • Remember what winds are like around high and low pressures? • Winds diverge from high pressure and converge at low pressure • If air converges at low pressure (at the surface for ex.), what must it do? • Must rise (can’t go into the ground right?)

35. Convergence and Divergence • What about air diverging from a high pressure center (again at the surface)? • Some air will have to sink from above to replace it • This explains why we have clear weather w/ highs and cloudy weather w/ lows

36. Measuring Wind • Wind directions: • Real easy, just think of a 360° circle • East wind - 90° • South wind - 180° • West wind - 270° • North wind - 360° • Again, always described in terms of direction from • ex. NW wind is out of the northwest, not toward the northwest

37. Measuring Wind • Wind vane • Simple instrument which measures direction only • Anemometers • Measures speed only • Example is of a “cup” anemometer

38. Local Wind Systems • Interesting topic because they exist almost everywhere • Coastal regions - Seabreezes • Mountains - Chinook winds, mountain/valley breezes • Often due to 1) terrain or 2) temperature differences from one place to anothe

39. Sea Breezes • Sea breezes are a good example of a thermal circulation • Form in tropical/sub-tropical regions in the warm season • People from coastal areas of FL know about these - nice at the beach • Can also have “lake breezes” and “river breezes” which form in the same way

40. Sea Breezes • water/land is causing the contrast in temp/pressure • Thermal low(why) develops over land and wind at the surface moves from water to land

41. Land Breezes • Remember from several weeks ago we said that land heats up and cools down quicker than water? • Happens at night causing a reversal of the sea breeze • Called a land breeze

42. Land Breezes • Air over land cools quickly at night • So now air over water is warmer than air over land • Winds switch to offshore • generally weaker than the sea breeze

43. Monsoon • What kind of image does that word conjure up? • Lots of heavy rain? • Floods? • Actually it means: • A seasonal shift in wind patterns • So it doesn’t directly address rain at all • occurs as a seasonal cycle - not a daily cycle

44. Asian Monsoon • Asian monsoon is the most well developed one • Weaker monsoon over the desert southwest of N. America • Very cold air over the continent during winter • Causes high pressure and flow toward the water • This also keeps moisture offshore • So, very dry over land in the winter

45. Asian Monsoon • Winds change during the summer as the land heats up and low pressure develops • Now wind moves onshore and brings with it very moist air • This region, particularly central India, gets absolutely hammered w/ rain • Some places over 400 inches a year! (most of it coming in 6-7 months!)

46. Asian Monsoon • Why so much rain (other than the obvious)? • Terrain Lots of mountains