Bob Doyle, Columnist
How does the weather work? Let’s start with the prevailing winds.
The sun’s rays are most intense at the equator and least intense in the polar regions. So the air is heated most at the equator, causing it to rise.
In the polar regions, the opposite is true. Here the cold air sinks, and warmer air is drawn in.
If the Earth didn’t rotate, there would two loops of air circulation, where the warm air rose at the equator, traveled both north and south and then descended at the poles.
So the upper winds would flow northward in the Northern Hemisphere and southward in the Southern Hemisphere. Then the winds close to the ground would blow in the opposite direction in both hemispheres. But the Earth rotates at a high speed at the equator, decreasing as one approaches the poles.
This causes a rightward deflection of air and objects in the Northern Hemisphere and a leftward deflection in the Southern Hemisphere.
This sideways deflection (caused by the Coriolis force) has broken the wind patterns in each hemisphere into three zones: Polar zone (90 – 60 degrees latitude), Temperate zone (60-30 degrees latitude) and Tropic zone (30 degrees latitude to equator).
The prevailing winds in each zone (direction from which the wind comes from) are: east for Polar, west for Temperate and northeast for Tropic zone. (Change north to south for the southern hemisphere cells.)
These prevailing winds are what often pushes weather changes across our region of the country.
So both good and bad weather tends to move from west to east across our area (about latitude 40 north, in Temperate zone). But our temperate zone has neither strong warming or strong cooling so it is more subject to disturbances both north of us (Polar zone) and in the Tropic zone.
The jet stream is a flow of strong winds from west to east in the upper troposphere, that occur at a boundary between cold and warm air.
There is a polar jet stream between polar air and tropical air that moves north in spring and south in fall. The jet stream placement often causes continued cold or warm weather in our area, acting as a barrier to any air masses of different temperatures.
There are five types of air masses that take on the characteristics of the regions over which they were formed: cT is for continental tropical (dry and hot), mT is for maritime tropical (wet and hot), mP is for maritime polar (wet and cold), cP is for continental polar (dry and cold) and cA is for continental artic (dry and very cold).
Fronts are areas where two different air masses meet. A cold front occurs when a cold air mass pushes under a warm air mass, acting like a wedge as the denser colder air pushes the warm air upward, leading to storms, winds and eventually cooler temperatures.
A warm front occurs when a warm air mass rides over a cold air mass, leading to a gradual change in the clouds (first high cirrus clouds appear, then medium height clouds and lastly low rain clouds).
When two fronts (warm and cold) meet and both are pushing in opposite direction, we have a stationary front. An occluded front requires three air masses: two cold and one warm. When the two colder air masses meet, the colder one burrows under the warmer cold front, and pushes the warm air mass high above the Earth’s surface.
Occluded fronts are fertile breeding grounds for mid latitude cyclones (low pressure systems). A low pressure zone has rising warm air, reducing the pressure.
This air then is pushed sideward by Coriolis force, resulting in a counterclockwise wind circulation (Northern Hemisphere).
A high pressure zone has descending cooler air, increasing the pressure. The winds in high pressure zones circulate in the clockwise direction (Northern Hemisphere).
Low pressure systems cause cloud formation and unsettled weather. High pressure systems are associated with dry, stable weather.
Weather forecasts rely on satellite detection of weather system position and movement, patterns of weather system movements in the past and models of the Earth’s atmosphere that can be fed current weather quantities to allow prediction of the weather three to five days from the present.
Now featured at the Frostburg State Planetarium is “Our Glorious Atmosphere” with public Sunday programs at 4 p.m. and 7 p.m. through month’s end.
The opening portion of the program is an informal tour of the current winter evening sky using our Planetarium star field.
The feature presentation presents the basics of the atmosphere, the atmospheres of the other planets, our prevailing winds, how humanity is altering our atmosphere and the likely outcome decades from now.
Call (301) 687-7799 for a free Planetarium bookmark to be sent to your mailing address. You can also visit the Planetarium section of the Frostburg State Web site (www.Frostburg.edu).
Tonight the crescent moon is to the right of the bright planet Jupiter low in the 6 p.m. western dusk. The evening moon will grow to half full on Friday.
This is the best lighted shape for viewing the moon’s craters and mountain ranges with binoculars (held steadily) or a telescope. Along the moon’s left (straight edge), the sun is rising, illuminating the crater rims and mountain peaks.
Each night the sunrise line on the moon advances more than a hundred miles, revealing new features and altered shadows not seen the previous night.
Bob Doyle invites reader’s comments and questions; his email is rdoyle@frostburg.edu .
