← Meteorology Today Export Options Alphabetize Word-Def Delimiter Tab Comma Custom Def-Word Delimiter New Line Semicolon Custom Data Copy and paste the text below. It is read-only. Select All Summer solstice Approximately June 21 in the Northern Hemisphere when the sun is highest in the sky and directly overhead at latitude 23 1/2 N, the Tropic of Cancer. Autumnal Equinox The equinox at which the sun approaches the Southern Hemisphere and passes directly over the equator. Occurs around September 23. Indian summer An unreasonably warm spell with clear skies near the middle of autumn. Usually follows a substantial period of cool weather. Winter solstice Approximately Dec. 21 in the Northern Hemisphere when the sun is lowest in the sky and directly overhead at latitude 23 1/2 S, the Tropic of Capricorn. Vernal equinox The equinox at which the sun approaches the Northern Hemisphere and passes directly over the equator. Occurs around March 20. Radiational cooling The process by which the earth's surface and adjacent air cool by emitting infrared radiation. Radiation Inversion An increase in temperature with height due to radiational cooling of the earth's surface. Nocturnal inversion An increase in temperature with height due to radiational cooling of the earth's surface. Thermal belts Horizontal zones of vegetation found along hillsides that are primarily the result of vertical temperature variations. Orchard heaters Oil heaters placed in orchards that generate heat and promote convective circulations to protect fruit trees from damaging low temperatures. Wind machines Fans placed in orchards for the purpose of mixing cold surface air with warmer air above. Freeze A condition occurring over a widespread area when the surface air temperature remains below freezing for a sufficient time to damage certain agricultural crops. A freeze most often occurs as cold air is advected into a region, causing freezing conditions to exist in a deep layer of surface air. Also called advection frost. Controls of temperature The main factors that cause variations in temperature from one place to another. Isotherms A line connecting points of equal temperature. Daily (diurnal) range of temperature The difference between the maximum and minimum temperatures for any given day. Mean (average) daily temperature The average of the highest and lowest temperature for a 24-hour period. Annual range of temperature The difference between the warmest and the coldest months at any given location. Mean (average) annual temperature The average temperature at any given location for the entire year. Heating degree-day A form of the degree-day used as an index for fuel consumption. Cooling degree-day A form of degree-day used in estimating the amount of energy necessary to reduce the effective temperature of warm air. A cooling degree-day is a day on which the average temperature is one-degree above a desired base temperature. Growing degree-day A form of the degree-day used as a guide for crop planting and for estimating crop maturity dates. Sensible temperature The sensation of temperature that the human body feels in contrast to the actual temperature of the environment as measured with a thermometer. Wind-chill index (WCI) The cooling effect of any combination of temperature and wind, expressed as the loss of body heat. Also known as Wind Chill. Frostbite The partial freezing of exposed parts of the body, causing injury to the skin and sometimes to deeper tissues. Hypothermia The deterioration in one's mental and physical condition brought on by a rapid lowering of human temperature. Liquid-in-glass thermometers An instrument for measuring temperature. This is the most common type of thermometer. It has a sealed glass tube attached to a glass bulb filled with liquid. Maximum thermometer A thermometer with a small constriction just above the bulb. It is designed to measure the maximum air temperature. Minimum thermometer A thermometer designed to measure the minimum air temperature during a desired time period. Electrical thermometers Thermometers that use elements that convert energy from one form to another (transducers). Common electrical thermometers include the electrical resistance thermometer, thermocouple, and thermistor. Radiometers An instrument designed to measure the intensity of infrared radiation emitted by an object. Also called an infrared sensor. Bimetallic thermometer A temperature-measuring device usually consisting of two dissimilar metals that expand and contract differentially as the temperature changes. Thermograph An instrument that measures and records air temperature. Instrument shelter A boxlike (often wooden) structure designed to protect weather instruments from direct sunshine and precipitation.