Accuracy

The extent to which results of a calculation or the readings of an instrument approach the true values of the calculated or measured quantities. ²

Apparent Temperature

A measure of human discomfort due to combined heat and humidity (e.g., heat index). ¹

Atmospheric Pressure

The pressure exerted by the earth's atmosphere at any given point, determined by taking the product of the gravitational acceleration at the point and the mass of the unit area column of air above the point. ¹

Barometric Pressure

see Atmospheric Pressure

Dew Point

A measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation (assuming air pressure and moisture content are constant). A higher dew point indicates more moisture present in the air. It is sometimes referred to as Dew Point Temperature, and sometimes written as one word (Dewpoint). ¹

Heat Index

The Heat Index (HI) or the "Apparent Temperature" is an accurate measure of how hot it really feels when the Relative Humidity (RH) is added to the actual air temperature. ¹

Humidity

Generally, a measure of the water vapor content of the air. Popularly, it is used synonymously with relative humidity. ¹

Precision

The quality of being exactly defined. ²

Rain

Precipitation that falls to earth in drops more than 0.5 mm in diameter. ¹

Rainfall

The amount of precipitation of any type, primarily liquid. It is usually the amount that is measured by a rain gauge. Refer to rain for rates of intensity and the quantitative precipitation for forecasting. ¹

Range

The difference between the maximum and minimum of a given set of numbers. ²

Relative Humidity

A dimensionless ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated. Since the latter amount is dependent on temperature, relative humidity is a function of both moisture content and temperature. As such, relative humidity by itself does not directly indicate the actual amount of atmospheric moisture present. See dew point. ¹

Resolution

1. The degree to which nearly equal values of a quantity can be discriminated. 2. The smallest measurable change in a quantity. 3. The least value of a measured quantity that can be distinguished. ²

Temperature

The temperature is a measure of the internal energy that a substance contains. This is the most measured quantity in the atmosphere. ¹

Temperature-Humidity-Wind (THW) Index

The THW Index uses humidity, temperature and wind to calculate an apparent temperature.

Variable

Something that can assume different values or states. ²

Wind Chill

Reference to the Wind Chill Factor; increased wind speeds accelerate heat loss from exposed skin, and the wind chill is a measure of this effect. No specific rules exist for determining when wind chill becomes dangerous. As a general rule, the threshold for potentially dangerous wind chill conditions is about -20°F. ¹

Wind Chill Factor

Increased wind speeds accelerate heat loss from exposed skin. No specific rules exist for determining when wind chill becomes dangerous. As a general rule, the threshold for potentially dangerous wind chill conditions is about -20°F. ¹

Wind Direction

The true direction from which the wind is blowing at a given location (i.e., wind blowing from the north to the south is a north wind). It is normally measured in tens of degrees from 0 to 360. ¹

Wind Speed

The rate at which air is moving horizontally past a given point. It may be a 2-minute average speed (reported as wind speed) or an instantaneous speed (reported as a peak wind speed, wind gust, or squall). ¹

1. Definition from the National Weather Service Glossary
2. Definition from the American Meteorological Society Glossary