Some of the words and phrases used on this site are fairly limited to the many aspects of outdoor cooling. This page is to give you a quick, simple-to-understand reference for the more uncommon (and some of the quite common, too).
Air temperature. The degree of heat or cold of the air only -- not including humidity or wind factors. If the air temperature is high, perspiration on the skin evaporates and cools the skin. If it is too hot, our body's cooling system (perspiration) cannot keep up and we overheat, becoming subject to heat stress.
(Solar) Altitude. The measurement of the sun's vertical angle up from the horizon. Right at the horizon the sun's altitude is 0°. Directly overhead the sun's altitude is 90°.
(Solar) Azimuth. The horizontal rotation angle from Due South (in the Northern Hemisphere) or from Due North (in the Southern Hemisphere).
BTUs (British Thermal Units). measure the amount of energy needed to decrease or increase the heat of one pound of water by one degree Fahrenheit for one hour. It is a common way to measure cooling capacity of air conditioners.
Celsius. A temperature scale that sets the freezing point of water at 0 degrees and the boiling point at 100 degrees. Was referred to as "centigrade" until 1948.
CFM (Cubic Feet per Minute). The number of cubic feet per minute (CFM) that a fan can move is a measurement of a fan's power. Its metric equivalent is CMS (Cubic Meters per Second).
Climate. The average weather conditions in an area determined over a period of years.
CMS (Cubic Meters per Second). The number of cubic meters per second (CMS) that a fan can move is a measurement of a fan's power.
Direct sunlight. Sunlight that is not blocked by shade; unobstructed sunlight. Direct sunlight can increase the perceived temperature by as much as 36°F / 20°C. Even if the temperature is only 64°F / 18°C, if the sunlight is direct it might feel like 100°F / 38°C.
Evaporative Cooling. Evaporation of water combined with moving air creates a cooling effect.
Fahrenheit. A temperature scale that sets the freezing point of water at 32 degrees and the boiling point at 212 degrees. Used mostly by the United States and a few Central and South American countries.
HI (Heat Index). Describes how hot it feels when the effect of air temperature and humidity are combined.
Heat Stress. A general term to describe a number of heat-related illnesses such as heat cramps and heat stroke. See the page on preventing heat stress with outdoor cooling.
Humidity. The amount of water vapor in the air. High humidity prevents the body’s cooling evaporation process, because there is too much moisture in the air to accept more from our skin. Humidity prevents the evaporation of perspiration making it difficult to stay cool.
HVAC (Heating, Ventilating, and Air Conditioning). An acronym for these closely related fields of environmental comfort.
Indirect Sunlight. Sunlight that is blocked by shade; diffused sunlight. This varies from light shade where the shading material is sparse or when there are light-colored or shiny surfaces reflecting light into the shade, or it may be dark or deep shade when the shade material is dense and there are no reflective surfaces nearby.
Radiant Heat. The transfer of heat from a hot object. Also called "heat transfer". Heat radiating from the sun to warm the earth is an example of radiant heat.
Reflected Heat. Heat that "bounces" off of surfaces is said to "reflect" from them. A metal bench doesn't have much heat of its own, but after it's been in direct sunlight for awhile, it reflects a lot of heat.
Refrigerant. Anything used to cool. This includes air, water, and chemical refrigerants used in air conditioners. The most commonly known is freon, which is no longer used because of its harmful effect on the environment.
RH (Relative Humidity). The percentage of water vapor ("atmospheric moisture") actually in the air compared to how much water vapor the air can hold ("saturation"). Raindrops and snowflakes aren't measured in relative humidity. Cold air cannot hold as much moisture as hot air.
Solar Altitude. The measurement of the sun's vertical angle up from the horizon. Right at the horizon the sun's altitude is 0°. Directly overhead the sun's altitude is 90°.
Solar Azimuth. The horizontal rotation angle from Due South (in the Northern Hemisphere) or from Due North (in the Southern Hemisphere).
Solstice. One of two days in the year when the day (as opposed to the night) is either longest or shortest, 20 or 21 of June and 21 or 22 of December.
Ultraviolet Radiation. One of the kinds of radiation (energy) that comes from the sun. Heat and light are two other kinds of energy radiated from the sun. But ultraviolet radiation contains much more energy than the other kinds. So much in fact, that it can damage living cells.
Weather. The state of the atmosphere at a given time. Includes temperature, precipitation, humidity, pressure, winds.
Wind Chill. Wind lowers the perceived temperature of the air. Describes how cold it feels on our skin when the effect of air temperature and wind speed are combined. The lower the air temperature and the more the wind blows, the colder it feels, even though the air temperature remains the same.