Download Weather: Storm Formation In General How does a storm develop

Weather: Storm Formation In General
How does a storm develop?
What do most people do when a storm passes through? Usually, they will head indoors if
they can, to get out of the rain, the wind, and other byproducts of the storm. As long as you pay
attention to the weather forecast every few days, most people get by fine without worrying too
much about stormy weather. When you’re at sea, particularly in a smaller, wind-powered vessel,
the weather becomes a very important part of your daily routine.
For people who live away from the coast, storms are usually associated with frontal
systems occurring throughout the year. The evening broadcast may feature a weather forecaster
stating that, “An upper-level low is passing through today into tomorrow, and its associated cold
front may bring brief showers across the region, mostly in higher elevations.”
Low pressure systems act kind of like a vacuum cleaner, sucking air in at the bottom (the
Earth’s surface) and taking it up to a higher altitude. Air will flow out of a zone of high pressure
and into a zone of low pressure; in the Northern Hemisphere, Coriolis deflection shunts the air to
the right and clockwise out of a high pressure zone, and pulls it in counterclockwise towards the
center of a low pressure zone. High pressure occurs when air is pushing down out of the upper
atmosphere (often due to air being cooled while aloft), and low pressure occurs when air is rising
up from the surface (often due to surface heating).
In the middle latitudes, jet stream activity can create areas of high and low pressure, often
referred to as a “ridge of high pressure” or a “trough of low pressure”. In the diagram below, a
high pressure ridge on the left-hand side is funneling air down to the surface, creating a zone of
high pressure at the surface. On the right, air is being drawn up by a trough of low pressure,
creating a zone of low pressure at the surface.
The uplift associated with a low pressure system, no matter how small or large it is, pulls
air into the low pressure center and sends it higher in the atmosphere. As the air rises, it cools,
and its ability to carry water is diminished. Clouds form, and if the uplift is strong enough and
there is sufficient water vapor to condense into liquid water or ice crystals, precipitation can
occur. An interesting phenomenon occurs at the surface, where the inward spiraling of air
towards the low pressure center mixes different types of air.
In North America, this mixing often pulls colder, denser air from the north and warmer,
less dense air from the south. As these different masses of air collide, the colder, denser air tends
to stay close to the surface and bulldozes its way into the warmer, less dense air. The leading
edge of the cold air is often called a cold front, and as it enters the zone of warm air, it causes
frontal uplift across its edge. This swift uplift of air often creates intense storm activity until the
front passes through an area. After the front moves on, the colder air tends to clear up and the
storms dissipate.