GALE
On the Beaufort Wind Scale, a wind with speeds from 28 to 55 knots (32 to 63 miles per hour). For marine interests, it can be categorized as a moderate gale (28 to 33 knots), a fresh gale (34 to 40 knots), a strong gale (41 to 47 knots), or a whole gale (48 to 55 knots). In 1964, the World Meteorological Organization defined the categories as near gale (28 to 33 knots), gale (34 to 40 knots), strong gale (41 to 47 knots), and storm (48 to 55 knots).

HUMIDITY
The amount of water vapor in the air. It is often confused with relative humidity or dew point. Types of humidity include absolute humidity, relative humidity, and specific humidity.

HYGROMETER
An instrument that measures the water vapor content of the atmosphere. See the psychrometer as an example.

KNOT
A nautical unit of speed equal to the velocity at which one nautical mile is traveled in one hour. Used primarily by marine interests and in weather observations. A knot is equivalent to 1.151 statute miles per hour or 1.852 kilometers per hour.

MOISTURE
Refers to the water vapor content in the atmosphere, or the total water, liquid, solid or vapor, in a given volume of air.

PREVAILING WIND
A wind that blows from one direction more frequently than any other during a given period, such as a day, month, season, or year.

PSYCHROMETER
An instrument used to measure water vapor content of the atmosphere. It consists of two thermometers, a wet bulb and dry bulb. May also be referred to as a sling psychrometer.

RELATIVE HUMIDITY
A type of humidity that considers the ratio of the actual vapor pressure of the air to the saturation vapor pressure. It is usually expressed in percentage.

SATURATE
To treat or charge something to the point where no more can be absorbed, dissolved, or retained. In meteorology, it is used when discussing the amount of water vapor in a volume of air.

SATURATION POINT
The point when the water vapor in the atmosphere is at its maximum level for the existing temperature.

SEA BREEZE
A diurnal coastal breeze that blows onshore, from the sea to the land. It is caused by the temperature difference when the surface of the land is warmer than the adjacent body of water. Predominate during the day; it reaches its maximum early to mid afternoon. It blows in the opposite direction of a land breeze.

WET BULB DEPRESSION
Dependent on the temperature and the humidity of the air, it is the difference between the dry bulb and the wet bulb readings.

WET BULB THERMOMETER
A thermometer used to measure the lowest temperature in the ambient atmosphere in its natural state by evaporating water from a wet muslin-covered bulb of a thermometer. The wet bulb temperature is used to compute dew point and relative humidity. One of the two thermometers that make up a psychrometer.

WIND
Air that flows in relation to the earth's surface, generally horizontally. There are four areas of wind that are measured: direction, speed, character (gusts and squalls), and shifts. Surface winds are measured by wind vanes and anemometers.

WIND CHILL INDEX
The calculation of temperature that takes into consideration the effects of wind and temperature on the human body. Describes the average loss of body heat and how the temperature feels. This is not the actual air temperature. For an example, check out the wind chill chart.

WIND DIRECTION
The direction from which the wind is blowing. For example, an easterly wind is blowing from the East, not toward the East. It is reported with reference to true north, or 360 degrees on the compass, and expressed to the nearest 10 degrees, or to one of the 16 points of the compass (N, NE, etc.).

WIND SPEED
The rate of the motion of the air on a unit of time. It can be measured in a number of ways. In observing, it is measured in knots, or nautical miles per hour. The unit most often used in the United States is miles per hour.

ABSOLUTE HUMIDITY
A type of humidity that considers the mass of water vapor present per unit volume of space. Also considered as the density of the water vapor. It is usually expressed in grams per cubic meter.

ANEMOMETER
An instrument which measures wind speed.

BEAUFORT WIND SCALE
A system of estimating and reporting wind speeds. It is based on the Beaufort Force or Number, which is composed of the wind speed, a descriptive term, and the visible effects upon land objects and/or sea surfaces. The scale was devised by Sir Francis Beaufort (1777-1857), hydrographer to the British Royal Navy.

CELSIUS TEMPERATURE SCALE
A temperature scale where water at sea level has a freezing point of 0 degrees C (Celsius) and a boiling point of +100 degrees C. More commonly used in areas that observe the metric system of measurement. Created by Anders Celsius in 1742. Same as Centigrade. In 1948, the Ninth General Conference on Weights and Measures replaced "degree centigrade" with "degree Celsius."

CONDENSATION
The process by which water vapor undergoes a change in state from a gas to a liquid. It is the opposite physical process of evaporation.

DEW POINT
The dew point is the temperature at which dew would form assuming all other conditions remained the same. The dew point is a function of the air temperature and humidity. The dew point temperature can never be higher than the air temperature. If the dew point temp and air temp are the same, than the humidity would be 100%.

The dew point is a very good measure of comfort. If the dew point is high, the temperature and humidity must also be high, and you are probably sweating profusely even while standing still. If the dew point is low, then either the temperature or humidity or both are very low, and you are feeling quite comfortable. This is a better gauge for comfort than temperature or humidity alone. It could be quite warm but very dry (low dew point) and you would feel comfortable. It could also be very humid but cool or cold (low dew point) and you would feel comfortable. The temperature to which air must be cooled at a constant pressure to become saturated.

DRY BULB THERMOMETER
A thermometer used to measure the ambient temperature. The temperature recorded is considered identical to air temperature. One of the two thermometers that make up a psychrometer.

EVAPORATION
The physical process by which a liquid, such as water, is transformed into a gaseous state, such as water vapor. It is the opposite physical process of condensation.