Storms and Fronts

-Introduction
-Energy Sources of Storms
-Storm Fronts
-How Storms form
-Tracking a Storm

When winds begin moving great sea of cold or warn air from the regions where they form, air masses come into conflict. The arrival of a new mass means the old one has to be shoved out of the way. There is a war on.

The place where air masses clash is called a 'front'. Fronts are often the scene of the most dramatic weather changes can stay in place or move. When they move, one air mass is advancing, another retreating. When the warmer air is advancing, the boundary is a warm front. When the colder air is taking over, it is a cold front. When neither side is wining it is a stationary front.

Fronts normally are parts of larger weather systems centered on areas of low atmospheric pressure. They are parts of weather systems often called storms.

Energy Sources of Storms

Cyclones draw much of their energy from temperature contrasts created by the sun's unequal heating of the Earth.

When conditions are right, gravity pulls cold, heavier air under warmer, lighter air. Contrasts between warm and cold air have potential energy, just as a skier at the top of a slope has potential energy. When the skier pushes off and starts down the slope, her potential energy becomes kinetic energy. When something happens to start warm and cold air masses moving over and under each other, their potential energy becomes a storm's kinetic energy.

Energy is added to storms when air rises and the water vapour in it begins condensing into clouds and precipitation. When water vapour condenses or turns into ice, it releases heat called 'latent heat'. This heat adds to a storm's energy. Also, storms draw some energy from the high-speed winds of the upper atmosphere. After a storm starts, the winds can pull in more cod air and warm air, enhancing the temperature contrasts and keeping the storm going.

A storm's wind speeds depend on the differences between air pressures around the storm and in its low-pressure center. The greater the difference, the stronger the winds. Another factor also comes into play.

Think how a spinning ice-skater can speed up or slow down by pulling in her arms or holding them out. When the skater pulls in her arms, she spins faster. When she holds them out, she spins slower. Scientists call this the 'conservation of angular momentum' and it works for storms too. As winds spiral into a low-pressure area they make a smaller and smaller circle. Like the ice-skater who pulls her arms in, the winds spin faster.

Storm Fronts

Cold front

Cold air is displacing warm air. The heavier, cold air is shoving under the warm air, pushing it upward. Unless the air is extremely dry, clouds form. Often the clouds grow into thunderstorms. The slope of the front is fairly steep, especially if it is moving fast, around 25 mph for examples. About 30 miles back from the front the warm-cold boundary would be at around 3,000 feet above the ground.

Warm front

Warm air is replacing colder air. The lighter warm air slides over the heavier cold air, creating a boundary with a gentle slope. You could be around 100 miles ahead of the warm front¢wstill in the cold air¢wand the warm-cold boundary could be only 3,000 feet above you. If you are 600 or 700 miles ahead of your front, in the cold air, wispy clouds at about 30,000 feet overhead could be the first sign of the approaching warm air. As the front moves toward you¢wor if you move toward the front¢wthe clouds overhead grow thicker and lower and eventually rain, snow, sleet, or freezing rain begin falling.

Stationary front

Neither the cold nor the warm air is advancing; it is a standoff. Widespread clouds can form on both sides of the frontal boundary.

Occluded fronts

Occluded fronts, or occlusions, are created when cold, warm and cool air come in conflict, forming boundaries above the ground as well as at the surface. They are often described as being caused by a cold front catching up with a warm front, but that seldom happens. Their clouds and precipitation are a mix of typical cold-front and warm-front clouds. Cold occlusion Cold air is shoving under cool air at the Earth's surface. The cold-warm air boundary aloft is often west of the surface front. Warm occlusion Cool air rises over cold air at the surface. The warm-cold air boundary aloft is often east of the surface front.

How Storms form

Fronts are often parts of larger storms. Storms form in zones where warm and cold air are close together, sometimes along fronts.

The formation of a storm can be described in four stages:

1. Both the warm and cold masses of air are high-pressure areas with clockwise winds. The boundary does not have to be sharp enough to be called a front.

2. A low-pressure are forms on the boundary and counterclockwise winds around it begin moving the air. Warm air begins advancing on the east side¢wcreating a warm front. Cold air begins advancing on the west side¢wcreating a cold front. The fronts and low-pressure area begin stirring up clouds and precipitation.

3. The low-pressure area grows stronger. Its pressure decreases. Winds increase in speed, and clouds and precipitation spread.

4. Sometimes the cold front catches up with the warm front, forming an occluded front, but this is rare. Scientists are still working out the details of the structure of occluded fronts and how they form. Often, the formation of an occluded front is the beginning of the end of the storm.

Tracking a Storm

A few hundred miles east of an advancing warm front, high, thin cirrostratus clouds will create a halo¢wa circle of light¢waround the sun or moon. As the front comes closer, the clouds grow lower and thicker and turn the sky gray. A few hours later even lower clouds arrive with rain or snow.

What happens depends on whether you're north or south of the storm's center as it passes by. When a storm's center passes by to the north, clouds and precipitation arrive ahead of the warm front. The weather turns warmer and may clear up. More clouds, perhaps thunderstorms, arrive. Finally the weather turns colder and begins to clear.

If the storm's center is south of you, clouds will become lower and thinker with rain or snow lasting around 12 to 24 hours. The precipitation will taper off to showers or snow flurries, the air will remain cold and the sky will clear. You'll never be in the storm's warm air.

During some winters, storms seem to follow the same paths. They are being guided by jet-stream patterns that become established for long periods. Jet streams are the cores of the general west-to-east flow of winds in the upper atmosphere. These winds not only flow west to east, but also often make excursions to the north and south.

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