Jet Stream
An area of strong winds that are concentrated in a relatively narrow band in the upper troposphere of the middle latitudes and subtropical regions of the Northern and Southern Hemispheres. Flowing in a semi-continuous band around the globe from west to east, it is caused by the changes in air temperature where the cold polar air moving towards the equator meets the warmer equatorial air moving polarward. It is marked by a concentration of isotherms and strong vertical shear.
Wind
Air that flows in relation to the earth’s surface, generally horizontally. There are four areas of wind that are measured: direction, speed, character (gusts and squalls), and shifts. Surface winds are measured by wind vanes and anemometers, while upper level winds are detected through pilot balloons or aircraft reports.
Troposphere
The lowest layer of the atmosphere located between the earth’s surface to approximately 11 miles (17 kilometers) into the atmosphere. Characterized by clouds and weather, temperature generally decreases with increasing altitude.
Middle Latitudes
The latitude belt roughly between 35 and 65 degrees North and South. May be referred to as the temperate region.
Subtropical
The region between the tropical and temperate regions, an area between 35° and 40° North and South latitude. This is generally an area of semi-permanent high pressure that exists and is where the Azores and North Pacific Highs may be found.
Temperature
The measure of molecular motion or the degree of heat of a substance. It is measured on an arbitrary scale from absolute zero, where the molecules theoretically stop moving. It is also the degree of hotness or coldness. In surface observations, it refers primarily to the free air or ambient temperature close to the surface of the earth.
Poles / Polar
The poles are the geographic point at 90 degrees latitude North and South on the earth’s surface. They are equal distance from the equator. The polar region is considered to be that area between 60° and 90° latitude, both North and South.
Equator
The geographic circle at 0 degrees latitude on the earth’s surface. It is equal distance from the North and South Poles and divides the Northern Hemisphere from the Southern.
Isotherm
The line of equal or constant air temperature. If something is isothermal, it is of equal or constant temperature with respect to either time or space.
Shear
It is the rate of change over a short duration. In wind shear, it can refer to the frequent change in wind speed within a short distance. It can occur vertically or horizontally. Directional shear is a frequent change in direction within a short distance, which can also occur vertically or horizontally. When used in reference to Doppler radar, it describes the change in radial velocity over short distances horizontally.