Wonder what a cold front is? How high and low pressure works? Just read on...

• What are the Hs and Ls on the maps?
• If the air is leaving a high pressure area, how does the pressure stay high?
• If the air warms in a high pressure area, why do we get cold Arctic high pressure areas in the winter?
• What does L mean on the maps?
• If air spirals into a low pressure area, why doesn't the atmospheric pressure rise and thus destroy the low pressure area?
• What are the fronts on the weather map?
• What is a cold front?
• What is a warm front?
• What is a stationary front?

What are the Hs and Ls on the map?

H marks the center of a high pressure area. The barometric pressure is higher there than at surrounding places. The force of this pressure causes air to move out and away from the high pressure center. Because of the rotation of the Earth, the flow turns to the right in the northern hemisphere. We wind up with air appearing to spiral out and away from the high pressure center.

If the air is leaving a high pressure area, how does the pressure stay high?

In order for the pressure to stay high, more air must be drawn into the high pressure area at somehigher altitude. This air then circulates down through the high pressure area and spirals out at the bottom. As air moves downward, it is compressed and becomes warmer. As air warms, it is able to accomodate more water as an invisible vapor. If no new water is added, the air will become drier and drier in comparison with how much water could be allowed. Another way of saying this is to point out the relative humidity drops. If the air warms and becomes drier as it sinks through the high pressure area, that

If the air warms in a high pressure area, why do we get cold Arctic high pressure areas in the winter?

It's realtive. If the air starts out at -25° Fahrenheit, and then warms up to 0° Fahrenheit through sinking and warming, we can conclude that warming has occurred, but it is still mighty cold.

What does L mean on the maps?

An L marks a place where the atmospheric pressure is lower than at surrounding places. Because of the pressure difference and the rotation of the Earth, air will spiral into the low pressure center. THink of the low pressure center as being a kind of vaccum cleaner. Low pressure areas tend to form along the boundaries separating air masses with different temperatures. it turns out that the jet stream is usually found over areas that have the biggest temperature changes, so low pressure areas are generally located under the jet stream. Because of this, the direction of the jet stream can be used to predict which way the low pressure area will travel.

If air spirals into a low pressure area, why doesn't the atmospheric pressure rise and thus destroy the low pressure area?

This would happen if the air is not removed from the low pressure area at some other altitude. In a strengthening low pressure area, air is being removed at the top of the storm faster than it is drawn in at the bottom. As the air rises through the storm system, is cools and loses the capacity for allowing water to exist as a vapor. Soon, the water vapor condenses to form clouds, and then rain or snow. THe air rises towards where the pressure is falling the most. This will be in the area toward which the storm is heading. This explains why most of the precipitation with a storm occurrs as the storm approaches you, and why there is often clearing very soon after the pressure starts to rise.

What are the fronts on the weather map?

A front marks the starting point for a new airmass. When a front passes your location, the temperature changes. The AccuWeather maps show cold fronts, warm fronts, and stationary fronts.

What is a cold front?

A cold front marks the leading edge of a change to colder weather. The movement of any front is determined by the movement of the air on the cold side of the front. That's because cold air is more dense than warm air. As cold air plows into a warm air mass, it pushes the warm air up and out of the way. The rapid lift is often sufficient to trigger showers and thunderstorms. Some people think that when it's very cold outside, it must be a cold front. Actually, that's not true; the cold front is just the boundary line. The coldest air may be hundreds of miles of behind the front.

What is a warm front?

If cold air retreats and allows warm air to advance, we have a warm front. All of the transition from cold to warm occurs on the cold side of the front. This means once the warm front goes by, it is immediately about as warm as it is going to get. As warm air glides over retreating cold air, the lifting results in saturation and often large sheets of clouds with extensive precipitation.

What is a stationary front?

It's a front that is not moving. The air on the cold side of the front is neither advancing nor retreating. If warm air is trying to advance, it will be forced to rise over the stubborn cold air. This can lead to extensive areas of clouds and precipitation, just as we have with warm fronts.