How Weather Happens

The WEATHER is the condition of the atmosphere (or changes in the atmosphere) at a particular time and place. The weather is shaped by 4 main factors:

• the air
• the sun
• water
• the earth's movement


Weather occurs in the part of the earth's atmosphere called the troposphere. This is the layer which is closest to the earth's surface, or the actual sky that we can see.

Three of conditions studied in relation to the weather are:

• Temperature: the degree of how hot or cold a substance is. It is a measure of the average speed of molecules. As the heat within a substance increases, molecules within that substance move faster and the temperature increases. If heat is removed, the molecules move slower and the temperature decreases.

 
• Air Pressure (or Atmospheric Pressure):  the force created by the weight of the air above. Assuming you are at the bottom of the atmosphere, the weight of the air above creates a force on you much like the weight or force of water on a diver at the bottom of an ocean. On the average, the atmosphere exerts a force of about 15 pounds per square inch of your body. Fortunately, air inside your body is pushing outward, which counteracts the atmospheric pressure pushing inward.

There are 2 different kinds of pressure systems:
• HIGH: High Pressure areas usually produce fair weather. Air in the atmosphere is sinking and pressure of the atmosphere is higher at the surface. Since air is not rising and cooling, thick cloud cover does not usually form within high pressure areas. Stormy areas of low pressure are the exact opposite of fair areas of high pressure.
• LOW: Low Pressure areas usually produce cloudy, stormy weather. When a low pressure area is over a region, the air in the atmosphere is rising. Because air rises from the surface of the earth, the pressure of the atmosphere is lower at the surface.

• Fronts: lines marking the leading edge of a change in air mass or temperature. Similar to the leading edge of army soldiers moving forward along the front line, the leading edge of an air mass moves forward along a weather front. When we see an approaching front, we know that the air (and temperatures) behind the front can be much different from present air mass. Fronts are usually connected to a rotating low pressure system that is producing rain or snow. Fronts also help to lift and cool the air, creating clouds and precipitation. Air rotating counterclockwise around the low brings warm air from the south ahead of the low and cooler air from the north behind the center of the low.