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Topic 6
Weather
| Term | Definition |
|---|---|
| Air mass | A large portion of the troposphere with fairly uniform temperature and humidity at any given altitude. The characteristics of an air mass are determined by the region over which it forms. |
| Air pressure | The weight of the column of gases that make up the atmosphere. As air rises, the pressure becomes lower. Descending air produces high pressure. |
| Air temperature | A measurement of the average kinetic energy or the degree of heat in the atmosphere. |
| Anemometer | An instrument used to measure wind speed. |
| Anticyclone | A high pressure system. |
| Atmosphere | The mixture of gases that surround the Earth, consisting primarily of nitrogen, oxygen, argon, carbon dioxide, and water vapor. |
| Barometer | An instrument used to measure atmospheric pressure. Three basic type of barometers include mercury, aneroid, and recording |
| Barometric pressure. | Atmospheric pressure. The force produced by the weight of the column of air above a surface. |
| Barometric tendency | The direction and amount of change in the atmospheric (barometric) pressure over the previous three hours. |
| Climate | The long term characteristic weather of a region. Climates can be classified on the basis of several factors, including: temperature, precipitation, vegetation and P/Ep ratio |
| Cloud base | The base level or bottom of clouds; the elevation at which clouds begin to form. The air temperature equals the dewpoint temperature at the cloud base. |
| Cold front | The interface between a cold air mass and any air mass with a warmer temperature. Cold fronts often form when continental polar air masses collide with maritime tropical air masses. Represented on a weather map by a line with a series of triangles |
| Condensation | The change in state of matter from a vapor to a liquid. Condensation of water vapor during cloud formation releases large amounts of energy to the atmosphere. |
| Continental polar (cP) | An air mass that forms over a land surface with a relatively high latitude. Continental polar air masses generally are composed of cold, dry, dense air. |
| Continental tropical (cT) | An air mass that forms over a land surface located at a low latitude. An air mass containing warm, dry air. |
| Cyclone | A low pressure system. Large low-latitude cyclones are also called hurricanes and typhoons. |
| Dewpoint temperature | The temperature at which condensation occurs. As the air temperature approaches the dewpoint temperature, the relative humidity increases. |
| Dry-bulb temperature | Air temperature as recorded by the dry bulb of a psychrometer. |
| Evaporation | Vaporization. A phase change or change in state of matter from liquid to vapor. |
| Freezing rain | Liquid droplets that move through colder air and freeze as they strike a cold surface. |
| Front | The interface or boundary between two air masses. Fronts are designated as being cold, warm, occluded, and stationary. |
| Hail | A solid form of precipitation formed in severe thunderstorms when ice particles move up and down past the freezing isotherm and form concentric layers of ice. Hail stones can vary in size from several millimeters diameter to the size of softballs. |
| High-pressure system | An anticyclone. A portion of the atmosphere in which descending air raises the atmospheric pressure. Air will diverge and circulate in a clockwise direction around a high pressure system in the Northern Hemisphere. |
| Hurricane | An atmospheric storm in which sustained wind velocities reach 74 miles per hour; a form of tropical cyclone. |
| Isobars | Lines drawn on a weather map that connect points of equal barometric pressure. Most weather maps use a 4.0-millibar interval between successive isobars. |
| Iso-intensity line | Isoline. Lines on a field diagram or map that connect points that have equal values. |
| Isoline | Iso-intensity line. A line drawn on a field map that connects points having the same value. |
| Jet Streams | Narrow bands of winds with relatively strong velocities located in the atmosphere. |
| Lake Effect Snow | Narrow belts of heavy snowfall that form on the downwind side of large lakes. In late fall and early winter, snow storms passing over the unfrozen Great Lakes pick up moisture that is dropped in “snow belts” that are located downwind of the lakes. |
| Leeward side | The side of an object or feature, such as a mountain, that is opposite the windward side. Air generally descends the leeward side of a mountain, producing a rain shadow. |
| Low-pressure system | A cyclone, or a region of the troposphere where air converges and rises, creating a center with lower barometric pressure than the surrounding region. |
| Maritime Climate | A middle-latitude climate near a coastline. The nearness to a large body of water generally produces a moderate temperature range and high precipitation totals. |
| Maritime polar (mP) | An air mass that forms over a large body of water at a high latitude. Air masses that are characterized by containing cool, moist air. |
| Maritime tropical (mT) | An air mass that originates over a warm (tropical) body of water. The air in a maritime tropical air mass will generally be warm, |
| Millibars | The unit of barometric pressure generally used on weather maps. One-thousandth of a bar. One atmosphere of pressure equals 1013.2 millibars. |
| Northern Hemisphere | The half of the Earth located between the equator and the North Pole. |
| Occluded front | A weather front that has lifted off of the surface. Cold-type occlusions often form when a cold front catches up with a warm front. The warm air associated with the warm front is lifted completely off the ground. |
| Planetary winds | The differential heating of the Earth’s surface produces a system of large-scale atmospheric convection. The system of prevailing winds that cross the various latitudes of the Earth’s surface. The mainland United States is in the Southwesterlies. |
| Precipitation | Weather phenomena of rain, snow, sleet, and hai |
| Pressure gradient | The change in atmospheric pressure between two locations. The wind velocity is directly proportional to the pressure gradient.The greater the pressure gradient, the greater the wind velocity. |
| Prevailing Southwesterlies | The planetary wind belt that moves across most of the mainland United States. The global winds that move across the middle latitudes in the Northern Hemisphere from the southwest towards the northeast. |
| Psychrometer | A weather instrument consisting of two thermometers. The bulb of one thermometer is covered with a wet sock. The instrument used to determine the dewpoint temperature and relative humidity of the air. |
| Rain shadow zone desert | An arid region that forms on the leeward side of a mountain barrier. As warm, moist air is forced up the windward side of a mountain, adiabatic cooling results in the removal of moisture through condensation and precipitation. |
| Relative humidity | The ratio of the actual amount of water vapor in the air to the capacity of the air to hold moisture at a given temperature. Relative humidity is generally expressed as a percent (%). |
| Sleet | A solid form of precipitation that consists of small pellets of ice (< 5 mm). |
| Snow | A solid form of precipitation that generally forms complex six-sided crystals but can also occur in the form of prisms and plates. |
| Source area | The region over which an air mass forms. The source area determines the characteristic temperature and humidity of an air mass. |
| Station model | The symbols used to represent the weather conditions of a location on a weather map. |
| Stationary front | Any type of front that stops moving. The weather along a stationary front is similar to the conditions associated with a warm front. Generally, there are prolonged periods of clouds and precipitation. |
| Temperature | A measure of the degree of heat or random motion of the particles in an object or region. |
| Vector quantity | A measurement or quantity that contains both magnitude and direction. Forces, such as gravitation, magnetism and pressure are vector quantities. Velocity is also a vector quantity. |
| Warm front | The interface that forms when a warm air mass collides with any air mass of air at lower temperatures. Warm fronts often form when a maritime tropical air mass overrides a continental polar air mass |
| Water vapor | Water in the gaseous phase. |
| Weather | The combination of all atmospheric conditions at a given location for a short period of time (hours or days). |
| Wet-bulb depression | The calculated difference between the wet-bulb temperature and the dry-bulb temperature; this value is used in the calculation of relative humidity. |
| Wet-bulb temperature | The temperature recorded on the thermometer of a psychrometer that is covered with a wet "sock." The wet-bulb temperature is generally less than the dry bulb temperature because as the psychrometer swings, evaporation from the "sock" lowers the Temp |
| Wind | The horizontal movement of air. Surface winds are the result of the differential heating of the Earth. Advection. |
| Wind vane | An instrument used to determine the direction from which the wind is blowing. Simple wind vanes are shaped like arrows whose heads point into the wind. |
| Wind velocity | A vector quantity that measures the speed and direction of the wind. |
| Windward side | The side of an object or feature, such as a mountain, that generally faces into the wind. The wind forces air to rise up the windward sides of mountain |
Created by:
bhowell65
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