Watch a weather report on television and you'll likely hear the meteorologist say something about an approaching low pressure system; followed up by a prediction for a chance of rain in your area. It's not a coincidence, however, many people don’t know why this happens with regularity. There's a good explanation. Learn about how low pressure systems affect weather and you may be able to forecast the weather on your own by watching a barometer.
Rising Air Condenses
High and low pressure readings are an indicator of how much weight the atmosphere is pushing down on a particular area. When the pressure is low, the air is free to rise into the atmosphere where it cools and condenses. This condensation forms clouds made of water droplets and ice crystals around dust particles in the sky. Eventually these droplets and ice fall as rain. Without the low pressure, much of the air and the water vapor within it wouldn’t reach a high enough altitude to condense, so it wouldn't rain. This is why low pressure is often associated with rain.
Rain comes down in varying intensities, so long, steady rain isn't always what you'll see. When a long, steady rain does happen, it's because of the location of the low pressure system in relation to a warm front. In the United States it's common to see a low produce long, steady rain or snow just north of a warm front. Warm, moist air enters the area of low pressure and is pulled up and over the cool air ahead of the front. This results in longer, steadier periods of rain or snow.
When a low pressure system is just ahead of a cold front, the warmer, less stable air ahead of the front can become a thunderstorm inside the counter-clockwise rotating low pressure area. This is the situation that produces the most severe thunderstorms and shorter, heavier downpours seen commonly in the spring and summer in many areas. The lower the pressure, the higher the air is able to rise and form storm clouds. Typically, the higher the clouds the more severe the thunderstorm potential is.
Causes of Low Pressure
The main cause of the differences in pressure around the world is the sun. Because of the rotation and shape of the earth and the rising and setting of the sun, temperatures are different around the world. Difference in temperature affect the amount of pressure. The atmosphere constantly adjusts to try and equalize pressure across the planet, often without success. The constantly changing pressure adjustments due to temperature makes high and low pressure areas move around. In large rain or snow systems, the low pressure system is lowered further because of the warming that takes place as a result of the presence of water vapor heated by the sun.