Meteorology Chapter 10 - Thunderstorms and Tornadoes

Radar

Radio Detection and Ranging. Transmits and receives microwave radiation. Power that comes back depends on the size of the target, the composition of the target, and the number of targets.

Radar reflectivity

A measure of the power returned to the radar. The more that comes back, the higher the reflectivity.

Bright spots on radar

Heavier precipitation

Green spots on radar

Not precipitation; bugs, debris, etc. up-drafted by the storm.

What do doppler radars detect?

Velocity of precipitation and speed of wind. Only radial velocity.

Radial velocity

Velocity towards and away from the doppler radar.

Weather radar limitations

Beam of radiation spreads with range.
Limited range because of the earth's curvature.

Thunderstorm

The process by which unstable parcels rapidly rise, condense, and produce precipitation and lightning. The most obvious manifestation is cumulonimbus clouds.

Ingredients for thunderstorms

Warm air in low levels, moist air in low levels, cool air in upper levels.

Peak time for thunderstorms

4:00PM. When the Earth is warmest.

Severe thunderstorm

Winds gusting greater than 50 knots, hail greater than quarter sized, and tornadoes.

Vertical wind shear

Wind speed and direction that changes with height.

Air mass thunderstorms

Thunderstorms that develop away from front that rarely become severe. Triggered by localized convective liftings or orographic lifting.

Stages of development of air mass thunderstorms

Cumulus cloud
Updraft through the cumulus cloud
Becomes cumulonimbus cloud with updraft and downdraft

Downdraft

Cold air descends to ground

Updraft

Warm air ascends through clouds

Gust front

Cold air produced by thunderstorms. Leading edge of precipitation.

Reason for thunderstorms

Redistribution of heat and wind

Squall line

Narrow bands of thunderstorms. Can form fronts. Can form as disorganized thunderstorms agglomerate into an organized line of thunderstorms. Can produce hail or tornadoes. Biggest hard is straight line winds. Composed of multiple closely-spaced thunderstorms along the line. Can produce severe weather. Long lifetimes that constantly regenerate along the gust front.

Supercell

Highly organized, generally isolated thunderstorms that is characterized by a storm-scale vortex called a mesocyclone. Produce strong and violent tornadoes.

Mesocyclone

Rotating updraft

Lightning

Atmospheric spark. 5 times hotter than the surface of the sun. Deals with the imbalance of charges.

Thunder

Expansion of air caused by lightning increasing pressure.

Tornadoes

Violently rotating vortex in contact with both the parent cumulonimbus cloud and the ground. Comes out of a mesocyclone. Winds can exceed 300 mph. Strong winds create significantly reduced pressure because of the pressure gradient force.

Funnel cloud

A vortex not in contact with the ground

Condensation funnel

The region of the tornado manifested by condensation

Enhanced Fujita Scale

Tornado intensity is rated according to the damage produced, not by the wind speeds. Dependent on the severity of damage and on the structure that is damaged.

EF-0

65-85mph. Light damage. Some damage to siding and shingles.

EF-1

86-110mph. Moderate damage. Considerable roof damage. Winds can uproot trees and overturn single-wide mobile homes. Flagpoles bend.

EF-2

111-135mph. Considerable damage. Most single-wide mobile homes destroyed. Permanent homes can shift off foundations. Flagpoles collapse. Softwood trees debarked.

EF-3

136-165mph. Severe damage. Hardwood trees debarked. All but small portions of houses destroyed.

EF-4

166-200mph. Devastating damage. Complete destruction of well-built residences, large sections of school buildings.

EF-5

>200mph. Incredible damage. Significant structural deformation of mid- and high-rise buildings.

Tornado watch

Conditions are supportive of tornadoes. Covers a large area. Valid for 6+ hours.

Tornado warning

A tornado has been spotted on the ground. Covers an area about the size of a county. Valid for ~1 hour.