FRONTS and STORMS

Front - Boundary separating two air masses of different type.

Types of Fronts

Cold Front - cold air moves into an area occupied by warm air. Steep frontal boundary with warm air forced up and over. Often accompanied by cumulonimbus clouds and thunderstorms. Move at speeds of 20-30 mph, followed by rapid clearing.
Warm Front - warm air moves into area occupied by cold air. Gentle slope to the front. Move at 10-20 mph. Cloud sequence cirrus, status, nimbostratus. Steady gentle rains.
Occluded Front - rapidly moving cold air overtakes warm air. Often associated with periods of prolonged heavy showers.
Stationary Front - boundary between warm and cold fronts that does not move appreciably.

Cyclonic Storms

In the northern hemisphere these are low pressure systems with counterclockwise air circulation that generally form at the polar front. At the front, cold polar air is moving eastward while warm, moist Pacific air is flowing to the west. Friction along the front causes waves as warm air overrides cold. A cyclone begins to develop. Eventually the cyclones spin off the polar front and are driven southeastward by the let stream. A typical "wave cyclone" has a life span of 7 to 10 days. As long as the frictional waves persist along the polar front a series of cyclones will be spawned.

Severe Storms

Thunderstorms - most serious form of severe weather because they kill more people each year than tornadoes or hurricanes.

Local - associated with warm moist air masses. Commonly form during the afternoon or early evening.

start when warm, moist air is forced upward
condensation occurs and clouds form (cumulus stage)
cloud grows upward until the top is below freezing and precipitation develops
as precipitation falls, downdrafts are created pulling cold air downward (mature stage)
cold air is replaced by warm air until the warm air supply is used up and the rainfall stops (dissipating stage)

Organized thunderstorms - form along fronts. Longer and more persistent than local thunderstorms. They often form along squall lines associated with the passage of cold fronts.

Lightening - a complex process

Cloud droplet freezes from the outside inward. This concentrates + charges on the outside and - charges on the inside
Core freezes last and volume increase shatters the droplet
Heavy core falls toward the base of the cloud taking the - charges with it
Lighter external particles rise with + charge
Negative charge on the base of the cloud causes a positive charge to develop on the ground
Lightening flash begins when negative particles move down from cloud (Invisible Lightening Leader)
Return strokes (visible) result in reverse flow of + charges to the cloud. Can also move cloud to cloud.

Thunder - caused by the high temperatures generated by the lightening stroke which expands the surrounding air explosively.

Tornadoes - violent wind storms produced by a spiraling column of air that extends downward from a cumulonimbus cloud. Typical wind speeds average 300mph and can reach 500mph. Also there is a severe pressure drop in the center of the funnel. This combination of high wind speed and a near vacuum can be very destructive. The U.S. averages about 800 tornadoes a year, most from April through June. The Midwest is especially vulnerable in the spring because of the clash between cold polar air and warm Gulf air.

Formation of tornadoes is poorly understood. For some reason especially violent thunderstorms cause the air which rushes inward and upward to spiral downward creating a funnel from a few hundred feet to over a mile in width. The tornado travels along the ground, usually in a northeasterly direction, for distances of a few hundred feet to tens of miles eventually dissipating.

Hurricanes - tropical storms that move independent of a recognizable frontal boundary. Have wind speeds in excess of 75mph; the upper limit is about 200mph. Friction with the ground is the limiting factor.

Hurricanes form initially by surface convergence. The warm air spirals upward. As it rises, the latent heat of condensation heats the air further causing it to rise faster and higher. This sucks more toward the center of the hurricane. Eventually an eye forms which marks the upper limit of wind velocity. Inside the eye the winds are calm. Typically, a hurricane eye is 10 to 20 miles in diameter.

Hurricanes are spawned over the ocean in latitudes just to the north and south of the equator. They feed from the warm, moist water requiring a surface T of +80 degrees F, and can not be sustained over land or cold water. Areas of the U.S. that are especially susceptible to hurricanes are the Gulf Coast and southern Atlantic Coast. Hurricane season starts in June and runs through September.