HEAT INDEX
The combination of air temperature and humidity that gives a description of how the temperature feels. This is not the actual air temperature.  SEE CHARTS BELOW.

HEATING DEGREE DAY
One heating degree day is given for each degree that the daily mean temperature is below 65 degrees a given temperature. It is used as an indication of fuel consumption. Refer to cooling degree day.

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.

LATITUDE
The location north or south in reference to the equator, which is designated at zero (0) degrees. Parallel lines that circle the globe both north and south of the equator. The poles are at 90 degrees North and South latitude.

LONGITUDE
The location east or west in reference to the Prime Meridian, which is designated as zero (0) degrees longitude. The distance between lines of longitude are greater at the equator and smaller at the higher latitudes, intersecting at the earth's North and South Poles. Time zones are correlated to longitude. See Greenwich Mean Time.

MOON PHASE
The moon phase is caused by sun rays reflecting off the moon's surface while it moves around the earth. The sun illuminates half of the moon at any time while the moon orbits around the earth. The variation in the angle made by the earth-moon line with respect to the earth-sun line causes changing phase of the moon.

The moon completes one revolution around the earth in 27.322 days with respect to the background stars. This is called the SIDERIAL period of the moon. During this same time the earth moves about 27 degrees along its orbit around the sun. As a result, the moon takes about two extra days to complete the cycle with respect to the sun-earth line. This longer cycle of the moon that takes about 29.57 days is called SYNDONIC period of the moon. The longer cycle is considered as Lunar month.

PRESSURE ALTITUDE
Atmospheric or barometric pressure expressed in terms of altitude which corresponds to that pressure in the standard atmosphere.

RATE OF CHANGE
The derivative or change in a parameters value with respect to time. Virtual Weather Station calculates the rate of change by calculating the derivative of a parameter, and then filtering it over one hour. Thus, the rate of change equation factors all of the measurements taken in the last hour, and may not exactly match the change in one hour.

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.

SEA LEVEL PRESSURE
The atmospheric pressure at mean sea level either directly measured by stations at sea level or empirically determined from the station pressure and temperature by stations not at sea level. Used as a common reference for analyses of surface pressure patterns.

SUNRISE
The daily appearance of the sun on the eastern horizon as a result of the earth's rotation. In the United States, it is considered as that instant when the upper edge of the sun appears on the sea level horizon. In Great Britain, the center of the sun's disk is used instead. Time of sunrise is calculated for mean sea level.
See sunset for comparison.

SUNSET
The daily disappearance of the sun below the western horizon as a result of the earth's rotation. In the United States, it is considered as that instant when the upper edge of the sun just disappears below the sea level horizon. In Great Britain, the center of the sun's disk is used instead. Time of sunset is calculated for mean sea level. See sunrise for comparison.

VAPOR PRESSURE
The pressure exerted by water vapor molecules in a given volume of air.

VIRTUAL TEMPERATURE
Virtual temperature is a fictitious temperature that takes into account moisture in the air. The formal definition of virtual temperature is the temperature that dry air would have if its pressure and specific volume were equal to those of a given sample of moist air. Virtual temperature allows meteorologists to use the equation of state for dry air even though moisture is present.

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. Refer to Comfort Index chart below.

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 RUN
The distance or length of flow of the air past a point during a given interval of time.

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.

The Heat Stress Index calculates the body's reaction to heat and humidity, or the Heat Index. High temperatures and humidity stress the body's ability to cool itself, and heat illness becomes a special concern during hot weather.

Current

Do not base important decisions on this heat stress index! Everybody's reaction to heat is different.

The comfort index calculates the body's reaction to heat, cold, humidity and wind chill. High temperatures and humidity stress the body's ability to cool itself, and low temperature can stress the body's ability to heat itself.

Current

Do not base important decisions on this comfort index! Everybody's reaction to heat and cold is different.

The Beaufort scale is a system of recording wind velocity (speed) devised in 1806 by Francis Beaufort (1774–1857). It is a numerical scale ranging from 0 for calm to 12 for a hurricane.

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