Meteorology

1. Meteorology “Climate is what you expect. Weather is what you get.” -Mark Twain

2. Air Masses • Masses of air are named for the area over which they form. • Air masses take on the characteristics of their region, both temperature and moisture levels. • Continental tropical (cT): warm, dry • Continental polar (cP): cool, dry • Continental Arctic (cA): very cold, dry • Maritime tropical (mT): warm, humid • Maritime polar (mP): cool, humid

4. Air mass modification • Air masses change as they travel over new areas. • They start to acquire characteristics of the new surface beneath them. • Air mass modification is the exchange of heat or moisture with the surface over which an air mass travels.

5. Convection currents and the Coriolis effect

6. Several convection cells dominate the globe:

7. These convection cells (“Hadley cells”), combined with the Coriolis effect, determine major wind patterns

8. Jet streams • Narrow bands of fast, high-altitude westerly winds that separate convection cells

10. The Jet stream’s impact on weather • The position of the jet stream varies • It can split into different branches and reform • It is like a railroad track; large scale weather systems follow it like trains • The jet stream moves air of different temperatures from one region to another.

11. Fronts • Air masses with different characteristics sometimes collide • A front is a region separating two air masses of different densities • Density differences form from differences in temperature, pressure, and humidity • There are four different types of fronts

12. Cold fronts

13. Warm fronts

14. Warm vs. Cold Fronts

15. Stationary frontsLittle precipitation; if any, it is similar to warm frontsTemperature and Pressure differences are very small.

16. Occluded fronts • Sometimes, a cold front travels very quickly, and barrels into a warm front. As it forces the warm air upwards, the colder air that the warm front was gliding over is left behind. The old cold air then collides with the newly arrived cold front. The warm air is squeezed upwards, yielding precipitation on both sides of the front. • Warm and cold front symbols are on the same side of the line; they are often purple (blue + red)

17. Occluded front

18. High Pressure Systems • Air sinks (presses down) • As it reaches Earth, it spreads away from the center • Because of Coriolis effect, in the Northern hemisphere, the air is deflected to the right. • It is hard for clouds to form in these conditions; fair weather dominates.

20. Low-Pressure Systems • Air rises in cases of low pressure • The rising air is replaced by outside air, so net flow is inward toward center and then upward • Air is drawn into a low-pressure system in a counter-clockwise fashion in the northern hemisphere (opposite from a high-pressure system) • Easy for clouds to form in these conditions • Wave cyclones may also form in low-pressure systems

21. Low-Pressure System

22. Weather Instruments • Thermometers measure temperature with a liquid that expands when heated (alcohol or mercury) • Barometers measure air pressure with mercury that goes up or down with pressure, or with a vacuum of air in a metal chamber. The chamber contracts or expands with changes in air pressure. Air pressure is measured in millibars, where the standard pressure at sea level is 1,013 mb, or 1 atmosphere.

23. Weather Instruments, Cont. • Anemometer: used to measure wind speed. Like a horizontal windmill with cups on the end of the blades. • Hygrometer: measures relative humidity. One way is with a wet and dry bulb thermometer. As water evaporates from the wet bulb, cooling it, the temperature differences between the two thermometers is applied to a relative humidity chart to determine relative humidity.

24. Weather satellites • Used during the day • Take pictures of clouds, but do not register rain • Combined with Doppler radar information about rain and wind to provide information

25. Infrared imagery • Detect differences in thermal energy • Used to map cloud cover and surface temperatures • Helpful in detecting and plotting strong thunderstorms as tops of tall cumulonimbus clouds extend to great heights and are very cold.

28. Where is the severe weather?

31. Station Models

32. Another version of a station model

33. Practice interpreting this station model:What is the weather? What is the forecast?