Glossary
Tail-end Charlie
slang for the thunderstorm at the southernmost end of a squall line or other line or band of thunderstorms. Since low-level southerly inflow of warm, moist air into this storm is relatively unobstructed, this storm often has a higher chance of strengthening to severe levels than the other storms in the line.
Teleconnection
linkage between changes in atmospheric circulation that occurs in widely separated parts of the globe. It is similar to when you touch the surface of a pond--the ripples you cause move across the pond and interact with other waves. So, if you poke one area, another is affected, sometimes at a great distance from the original poke.
Temperature Gradient
the gradient of the temperature field; the rate of change of temperature over some distance in a given direction, usually horizontally or vertically
Terminal Aerodrome Forecast (TAF)
an NWS aviation product that is a short statement of the expected weather conditions at an airport during a period. TAF’s use the same weather code found in METAR weather reports.
Terminal Doppler Weather Radar (TDWR)
a specialized weather radar used to detect microbursts in the area of some large airports, run by the FAA.
Terrestrial Radiation
the total amount of infrared radiation that is emitted from the earth’s surface or atmosphere
Thermal
a relatively small-scale, rising air current produced when the Earth’s surface is heated. Thermals are a common source of low-level turbulence for aircraft.
Thermal Instability
instability due to a heavy fluid (such as cold, dense air) being over a light fluid (such as warm, moist air). This is also known as potential instability and convective instability. The greater the temperature or density difference between these two fluids, the greater the instability.
Thermistor
an electrical resistor that makes use of a semiconductor whose resistance varies sharply in a known manner with the temperature. Using this instrument, we can quickly measure temperature.
Thermocline
as one descends from the surface of the ocean, the temperature remains nearly the same as it was at the surface, but at a certain depth, temperature starts decreasing rapidly with depth. This boundary is the thermocline. This boundary slopes between the western and eastern Pacific differently depending on whether there is an El Niño, a La Niña, or if the conditions are Neutral.
Thermodynamic Chart (or Thermodynamic Diagram)
a chart containing isopleths (contours) of pressure, temperature, moisture, and potential temperature, all drawn relative to each other such that basic thermodynamic laws are satisfied (e.g., Conservation of Energy). Such a chart typically is used to plot atmospheric soundings, and to estimate potential changes in temperature, moisture, etc. if air were displaced vertically from a given level. A thermodynamic chart thus is a useful tool in diagnosing atmospheric instability.
Thermodynamics
the relationships between heat and other properties (such as temperature, pressure, density, etc.)
Thermometer
an instrument for measuring temperature that consists typically of a glass bulb attached to a fine tube of glass. It has a numbered scale and contains a liquid that rises and falls with changes of temperature
Thermosphere
a layer above the Mesosphere. Temperature increases with height in this layer because oxygen molecules absorb ultraviolet radiation. However, you would not feel hot in this layer (in fact, you would feel cold) because very few particles would hit you.
Theta-e Ridge
an axis of relatively high values of equivalent potential temperature. Severe weather and excessive rainfall often occur near or just upstream from a theta-e ridge.
Thickness (THK)
the difference in height between two pressure levels. For example, the 1000-500 mb thickness is the height difference between the 1000 mb and 500 mb levels. A higher thickness indicates warming (if you heat a gas, it expands), while a lower thickness indicates cooling (cooling a gas contracts it).
Thin Line
a line of reflectivity separating air of differing densities. Thin lines typically are located ahead of a thunderstorm, but can be associated with cold fronts and dry lines. The thin line is caused by backscattering of the radar’s pulse off a sharp contrast in air density across a short distance.
Third Law of Thermodynamics
as temperature approaches absolute zero, the entropy of a system approaches a constant. Only with a pure, perfect crystal (one in which every molecule is identical and the alignment is perfectly even in the substance) can entropy be zero. Water vapor (gas) has high entropy. As it cools and becomes a liquid, it loses some entropy (it has become more organized). When the water cools further, it becomes a solid and loses more entropy. With each step, the vibration of molecules becomes less and less.
Thunder
the sound that follows a flash of lightning and is caused by sudden expansion of the air in the path of the electrical discharge
Thunderstorm (TSTM)
a local storm produced by a cumulonimbus cloud, with lightning and thunder, and usually accompanied by strong gusts of wind, heavy rain, and sometimes hail
Tilt Sequence
radar term indicating that the radar antenna is scanning through a series of antenna elevations in order to obtain a full volume scan.
Tilted Storm (or Tilted Updraft)
a thunderstorm or cloud tower that is not purely vertical but instead is slanted or tilted; it is a sign of vertical wind shear, a favorable condition for severe thunderstorm development
Tipping-Bucket Rain Gauge
a precipitation gage where collected water is funneled into a two compartment bucket; 0.01 in, 0.1mm, or some other designed quantity of rain will fill one compartment and overbalance the bucket so that it tips, emptying into a reservoir and moving the second compartment into place beneath the funnel. As the bucket is tipped, it activates an electric circuit. The total precipitation is determined by counting the tips.
Tornadic Vortex Signature (TVS)
a distinct feature in the Doppler radar radial velocity field; it indicates intense, concentrated rotation -- more so than that of a mesocyclone. Existence of a TVS strongly increases the probability of tornado occurrence, but does not guarantee it. A TVS is not a visually observable feature, other than via radar.
Tornado
a violently rotating column of air hanging from a cumulonimbus cloud and in contact with the ground; a condensation funnel does not need to reach to the ground for a tornado to be present. If a rotating debris cloud is present, even if there is no condensation funnel, it is a tornado.
Tornado Alley
a colloquial term used in reference to the area of the United States in which tornadoes are most frequent (usually the central part of the country). Tornado Alley maps often look different because tornado occurrence can be measured many ways--including by all tornadoes, strong and violent tornadoes only, etc. Keep in mind that tornadoes do happen outside of Tornado Alley all the time.
Tornado Family
a series of tornadoes produced by a single supercell, resulting in damage path segments along the same general line.
Tornado Warning (TOR)
issued by the local National Weather Service office when a tornado or funnel cloud has been sighted visually OR detected by radar; the location and direction of movement, if known, are given, and residents of the warning area should take immediate safety precautions
Tornado Watch (WW)
issued by the Storm Prediction Center in Norman, OK as a precautionary alert when conditions are favorable for the development of tornadoes in the specific area. There is no need to take shelter yet, but residents should be prepared for possible tornadoes in the area on that day.
Total-Totals Index (TT)
a measure of stability that is used as a severe weather forecast tool; equal to the temperature at 850 mb plus the dew point at 850 mb, minus twice the temperature at 500 mb. In general, a value of less than 50 indicates a low chance of severe thunderstorm development. A value greater than 55 indicates a high chance of severe storm development.
Towering Cumulus (TCU)
a large cumulus cloud with great vertical development, usually with a cauliflower-like appearance, but lacking the characteristic anvil of a cumulonimbus; same as "towering cu" (cu is pronounced "Q")
Trade Winds
the wind system, occupying most of the tropics, which is northeasterly in the Northern Hemisphere and southeasterly in the Southern Hemisphere
Training (or Back-Building) Thunderstorms
when one thunderstorm cell moves downstream, but additional cells form on the upwind side, moving directly over the path of the previous cell. Since relatively small areas under these storms receive tremendous amounts of rain, flash flooding may occur.
Transpiration
the process by which plants transfer water within themselves to water vapor in the atmosphere
Triple Point
1. the intersection point between two boundaries (dry line, outflow boundary, cold front, etc.) OR 2. a point on the gust front of a supercell, where the warm moist inflow, the rain-cooled outflow from the forward flank downdraft and the rear flank downdraft all intersect. Definition 1 is a favored location for thunderstorm development, while Definition 2 is a favored location for tornado development (or redevelopment).
Tropical Cyclone
the general term for a large low pressure system that originates over the tropical oceans; includes tropical depressions, tropical storms, and hurricanes
Tropical Depression (TD)
a tropical cyclone with winds equal to or less than 33 knots (around 38 mph)
Tropical Prediction Center (TPC)
one of 9 NCEP Centers. It includes the National Hurricane Center (NHC) and provides forecasts of the movement and strength of tropical weather systems and issues watches and warnings for the U.S. and surrounding areas. This center is located in Miami, Florida.
Tropical Storm (TS)
a tropical cyclone with winds stronger than 34 knots but less than 63 knots (39 to 73 mph)
Tropical System
a low-pressure area that has tropical characteristics (lots of moisture and warm temperatures). Tropical systems include cyclones, hurricanes, typhoons, tropical storms, and tropical depressions. They develop over large bodies of warm water and lose their strength if they move over land.
Tropics
located between 0 degrees and 30 degrees N or S, this area receives the greatest amount of solar energy (compared with the polar regions and mid-latitudes).
Tropopause (TROP)
the upper boundary of the troposphere, usually characterized by an abrupt change in how the temperature changes with height; below the tropopause, temperature generally decreases with height; above the tropopause, temperature generally increases with height (at least through the Stratosphere).
Troposphere
the portion of the atmosphere that extends outward about 10 to 20 km from the earth’s surface, and in which temperature generally decreases rapidly with altitude, clouds form, and convection is active
Trough of Warm Air Aloft (TROWAL)
an area of intense lift where heavy precipitation may form during winter weather
Turbulent Kinetic Energy (TKE)
the average kinetic energy per unit mass that is associated with eddies in turbulent flow.
