Thursday, December 2, 2010

Winter Solstice

Great post by Bill Steffans describing the winter solstice.

Winter Solstice

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Tornado Recap

Tornadoes are characterized by the very low pressure in the center of the twister and tornadoes have the largest pressure differences (inside of tornado vs. surrounding air) of any weather phenomena. Tornadoes occur during the warmer months in Michigan and typically in late afternoon/ early evening.

Weather 2 Review

When you are standing on the ground the atmosphere above you is exerting pressure on you. This pressure is exerted in all directions ( remember demo with pop bottle and collapsing pop can). The pressure map (above right) shows the isobars, lines that connect points of equal pressure (std pressure 1013mb), This difference in pressure generates winds as the air moves horizontally from an area of high pressure to an area of low pressure. The closer these isobars are to each other, steep pressure gradient, the stronger the wind. remember these differing areas of pressure are caused by the unequal heating of the Earth's surface by the Sun. In the northern hemisphere low pressure systems blow counterclockwise toward the center- rising air. High pressure systems blow out from the center and rotate clockwise.

Winds are influenced by friction with the Earth's surface and the Coriolis effect, above left. TheCoriolis effect influences the direction of the wind. In the northern hemisphere all free-flowing objects are deflected to the right and in the southern hemisphere to the left.

If you recall from our daily weather reports low pressure systems, mid-latitude cyclones, move from west to east across the United States. Low pressures, also known as cyclones, typically produce precipitation. High pressure systems, anticyclones, typically produce fair-weather. Also, remember from our daily weather reports that these systems are also steered by the jet stream- a high altitude, high velocity wind.

Global and Local Winds,

Global winds are produced by the global pressure zones which encircle the Earth. These pressure zones are associated with certain world climate- such as the rain forest in the equatorial low and the world's great deserts in the sub-tropical high. Refer to your diagrams. These global winds are prevailing winds in that they constistantly blow from the same direction. Ex, the prevailing westerilies blow from the west.
Global winds distribute heat and moisture around the globe. Warm air will move towards the pole and cold air moves towards the equator.

Local Winds
These are small scale winds such as mountain, land and sea breezes which are found throughout the world on in certain areas. These winds develop because of heating of land and water and weak pressure differences are produces creating these breezes. Be sure to look at your diagrams.

An  anemometer is used to measure windspeed, a barometer is used to measure pressure.

El Nino, La Nina
Check this link for information on Pacific Ocean surface temperature changes. These surface water changes impact the climate, fishing and farming practices in South America and to a lesser extent in the US.

Mid-latitude Wave/Cyclone

Let's look at some of the features of a mid-latitude cyclone
  • fronts are the boundries between cold and warm air masses.
  • cold fronts are represented by triangles/ warm fronts semicircles.
  • review stages in the development of a mid-lititude cyclone
  • warm air is always forced upwards by cold air, along the cold front
  • this upward movement of air along a cold front causes heavy short lived precipitation
  • these cyclones create a great number of thunderstorms.

Tornadoes and Hurricanes

Hurricanes - above left
  • hurricanes are very large low pressure systems
  • most hurricanes that strike the US track along the East Coast or Gulf of Mexico basin
  • check this link for more info
  • hurricanes need warm moist air for fuel
  • when hurricanes strike land the lack of warm moist air and friction causes it to lose energy
    interesting to note the "eye" has the warmest temperatures
  • the heavy rains and fastest winds are found in the NE quadrant of a hurricane.
    storm surge is the flooding that is associated with a hurricane.
Tornadoes - above right
  • very high wind speeds, Fujita scale
  • has the greatest range of pressure
  • tornadoes are very intense low pressure areas
  • check this link for more info

Air Masses

Air Masses- are immense bodies of air that have similar properties at a given altitude. Air masses take on the properties over which they form, of which there are two- temperature and moisture. The diagram above shows the air masses which impact the continental United States. Air masses are identified by a combination of two letters, the first letter is lower case  the second letter is upper case.

  • m - maritime, forms over water will contain moisture
  • c - continental, forms over land will be drier.
  • T - tropical, forms in warmer regions
  • P - polar, forms in colder regions
  • A - arctic, forms in very cold regions.
For example an air mass that forms over land in Canada would be labeled cP. The two air masses that have the greatest effect on the US are cP and mT.


Picture above illustrates the rain shadow effect in mountain areas. Remember our discussion on the "adiabatic" process. The change in temperature due to a change of pressure. Also the leeward side of mountain ranges in the US is typically the east side.

Greenhouse gases - the burning of fossil fuels releases carbon dioxide. CO2 is an important gas as it absorbs radiation that is emitted by the Earth.