1. Weather Basics Air Pressure and Winds

2. Air Pressure Air has a mass and exerts a force called atmospheric pressure Air pressure is measured in millibars in metric system Standard air pressure is 1013.25 mb

3. The Mercury Barometer The Height of mercury in barometer (1643 Torricelli) 29.92 inches or 760 mm is the standard Mercury rises and falls with changes in air pressure

4. Aneroid barometer A sealed chamber expands or collapses with changing air pressure

5. Altitude Air pressure decreases exponentially with altitude

6. Isobars Air pressure is plotted on weather maps using lines called isobars Isobars connect weather stations with equal air pressure Air moves from high to low pressure

7. Pressure gradient The spacing of the isobars indicates the pressure gradient. Steep pressure gradient or closely spaced isobars means the wind blows faster Isobars that are widely spaced mean a weak pressure gradient and a weak wind.

8. Air pressure is also affected by: Temperature Warmer air exerts less pressure Humidity Humid air exerts less pressure

9. Air pressure differences cause winds to blow Air always moves from high to low pressure Unequal heating of earth’s surface can set up pressure differences Warm air exerts less pressure than cool air Local breezes - land and sea breezes result

10. Land and Sea Breezes During the day the water heats more slowly then the land so the land becomes has lower air pressure The breeze is from the High pressure cooler water to the low pressure, warmer land At night the situation reverses as the land cools off more then the water.

11. Coriolis Effect An air parcel moving from the N. Pole to the equator goes straight across a nonrotating Earth With the Earth turning under the air mass, it is deflected/curved to the right of its original path in the Northern Hemisphere

12. This is called the Coriolis effect. It deflects wind to the left in the Southern Hemisphere. The stronger the wind the greater the deflection. Coriolis force does not determine the direction toilets flush or sinks drain. It is too weak a force for this. (another myth)

13. Coriolis Effect The deflection is strongest at the poles and least at the equator

14. The affect of friction Acts to slow air motion for air near the surface Not a factor for upper level flows

15. Geostrophic Wind When the pressure gradient and Coriolis force are balanced the flow is called Geostrophic flow Properties Parallel to isobars Low pressure on the left in the N. Hemisphere Fastest when the pressure gradient is greatest or the isobars are closest together

16. Cyclones and Anticyclones Cyclone is a low pressure cell with counterclockwise air flow around it (N. Hemisphere) Midlatitude storms, hurricanes, and tornadoes are examples of cyclones Convergence = Friction near the surface causes winds to flow toward the center of the low pressure cells and up in the middle Low pressure cells have rising air which cools to form clouds and precipitation

17. Anticyclones High pressure cells with clockwise flow of air around it Divergence = Friction causes air to move away from a high pressure center and subside or sink. This brings fair weather.

20. Global Circulation If the Earth did not rotate, air would rise near the equator and sink near the poles. Creating 4 massive convection cells The prevailing winds would be from the north in the Northern Hemisphere at the surface.

22. Global Circulation Continued The rotation of the Earth and the resulting Coriolis force causes this thermally driven circulation cell to end at about latitude 30 degrees. The Northern Hemisphere HADLEY CELL consists of rising air near the equator and flowing towards 30 degrees N and sinking

23. The surface air flow in the tropics is northeasterly. Due to the Coriolis force. Prevailing easterly winds in the tropics are called trade winds. Cold air moves down from the poles across the surface towards 60 degrees to create the polar easterlies In the middle latitudes, from about 30 to 60 degrees the prevailing winds are Westerlies.

25. Global Circulation Continued Near the equator there are no prevailing winds and the region is known as the DOLDRUMS Areas of low pressure which is rising air Remember a wind is a horizontal movement of air Air at 30 degrees north and south is called the Horse latitudes associated with high pressure or sinking air Desert regions of the world. Calm spots, no strong winds

26. Global winds The cold polar air converges or collides with the warmer mid- latitude air where Polar Easterlies meet Westerlies to create low pressure cells (polar front) These cells bring clouds and possible precipitation and storms to the middle latitudes

27. The Jet Stream. Strong upper level winds in the troposphere are Westerlies Geostrophic flow because of lack of friction The winds are strongest where North to South temperature differences are greatest Winter speeds may reach 300 mph but are typically 75-125 mph Summer speeds are much slower

28. Second Subtropical Jet Stream In winter, there is often a second subtropical jet stream This forms between tropical and subtropical air masses

29. The jet stream The position of the jet stream also shifts with the seasons Cold winter may extend as far S as 30 degree N If it is South of your latitude it pulls down cold artic air to you If it is North of your location, warmer and drier conditions are likely to exist Midsummer may be around 50 degrees N

30. Curvature of the jet stream When the jet stream is curved, it induces the formation of cylones and anticyclones When the jet stream is flat, little cyclonic activity is generated A flat jet stream has been associated with calm unchanging weather

31. Today’s Jet Stream This map show the location of the jet stream on Friday, Feb. 23rd. How did this affect our weather over the weekend?

32. Adapted from lecture notes of Professor Robert Krotkov http://www- unix.oit.umass.edu/~astro105/ http://www- unix.oit.umass.edu/~astro105/ and the Atmosphere text by Lutgens and Tarbuck