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ADVECTION FOG Active condensation formed when warm moist air moves laterally over land surfaces, causing lower layers of air to be chilled to the dew point temperatures. ALTITUDE Angular distance between the horizon (horizontal planes) and the sun. ALTOSTRATUS Middle clouds that appear bluish-grey, never white due to high amoun t of condensed water vapour in the cloud. ANEMOMETER Device that measure wind velocity. ANTICYCLONE Areas of high pressure with descending and divergin air flows that rotate clockwise in the northern hemisphere, and counter clockwise in the south hemisphere. ASPECT Angle of the mid-day sun; direct exposure. BAROMETER Instrument measuring air pressure. CHINOOK Warm, dry, downslope of air flow; characteristic of the rain shadow region on the leeward side of mountain. CIRRUS Wispy, filamentous, ice-crystal cloud, feathery hair-like, highest visible clouds CUMULONIMBUS Towering, percipitation-producing cumulus cloud that vertically develops across altitudes associated with other clouds, lightening and thunder produced CUMULUS Vertical clouds, often white in color with dark flat base. RIsing air currents produce cauliflower appearance. May be transformed into cumulonimbus with extreme heating in atmosphere. CYCLONES Areas of low atmospheric pressure. FRONTAL/CYCLONIC PERCIPITATION Occurs when 2 air masses meet; warmer air will be forced to rise and clouds and rain will result. HURRICANES Develop over warm oceans due to great amount of water vapour, usually associated with equatorial air masses. MONSOONS Wind system that develops as a result of existence of high pressure systems over large land masses in winter, and low pressure systems in summer. (Indian, Asian continents) NIMBOSTRATUS Rain producing low cloud, dark, grey clouds characterized by light drizzle. OCCLUDED FRONT Overrunning of a surface warm front by a cold front and the subsequent lifting of the warm air wedge off the ground; moderate- heavy percipitation. RADIATION FOG Formed by radioactive cooling of a land surface, especially on clear nights in areas of moist ground; occurs when air layer above surface is chilled to the dew point temperature, producing saturated conditions. RAINSHADOW Area on the leeward slope of mountain range where percipitation is greatly reduced compared to the wind ward slope on the other side. TORNADOES Develop around intense low pressure, associated with massive thunder storms that develop along fronts between warm tropical and cool polar air masses. PRESSURE GRADIENT Change in air pressure as one moves from one location to another. HEAT ISLAND Increased temperatures found in the center of large urban areas compared with surrounding of rural suburban areas. CONTINENTAL POLAR Very cold, stable, and dry air mass, dense so air mass stays in contact with ground, clear sunny skies and high pressure MARITIME POLAR Cool, moist, unstable air mass MARITIME TROPICAL Warmer and more stable than maritaim polar, sunny skies and winter percipitation CONTINENTAL TROPICAL Very hot and dry conditions