The force exerted on a surface by the weight of the air above it.
A measure of human discomfort due to combined heat and humidity. It measures
the increased physiological heat stress and discomfort associated with higher
than comfortable humidities. The apparent temperature is less than the actual
air temperature when the humidity is relatively low and that the apparent
temperature indicates the reduced stress and increased comfort associated with
the higher rate of evaporative cooling of the skin.
Apparent temperatures greater than 80°F (27°C) are likely to produce some
discomfort. Values in excess of 105°F (41°C) may be dangerous and even
life-threatening, with severe heat exhaustion or heat stroke possible if the
exposure is prolonged or physical activity is high. The degree of stress may
vary with age, health, and body characteristics.
The apparent temperature does not consider the effects of air movement (wind
speed) or exposure to sunshine on the degree of discomfort or stress.
An instrument used for measuring air pressure. The two most common types are
the mercury barometer and the aneroid barometer.
The actual pressure value indicated by a pressure sensor.
The amount and direction of change in barometer readings over a three-hour
It gauges the amount of heating or cooling needed for a building using 65°F
as a baseline. To compute heating/cooling degree-days, take the average
temperature for a day and subtract the reference temperature of 65°F. If the
difference is positive, it is called a Cooling Degree Days. If the
difference is negative, it is called a Heating Degree Days. The
magnitude of the difference is the number of days.
For example, if your average temperature is 50°F for a day in September, the
difference of the average temperature for that day and the reference temperature
of 65°F would yield a minus 15°. Therefore, you know that you are going to have
Heating Degree Days that day. Since the magnitude of the difference is 15°, you
know that you are going to have 15 Heating Degree Days. Electrical, natural gas,
power, and heating, and air conditioning industries utilize heating and cooling
degree information to calculate their needs.
Dew Point (Dew-Point Temperature)
A measure of atmospheric moisture. The temperature to which air must be
cooled, at constant pressure and moisture content, in order for saturation to
occur. The higher the dew point, the greater amount of water vapor in the air
The energy produced by an oscillating electrical
(and magnetic) field, transmitted by photons.
A process by which liquid changes into a gas or vapor.
Combination of evaporation from free water surfaces and transpiration of
water from plant surfaces to the atmosphere.
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. To find the Heat Index (HI), look at the Heat Index (HI) Chart
below. As an example, if the air temperature is 90°F (found at the left side of
the table) and the Relative Humidity (RH) is 70% (found at the top of the
table), the Heat Index (HI)--or how hot it actually feels--is 106°F. This is at
the intersection of the row 90°F and the 70% column.
This index was devised for shady, light wind conditions. Exposure to full
sunshine can increase Heat Index (HI) values by up to 15°F. Also strong winds,
particularly with very hot, dry air, can be extremely dangerous. Any value Heat
Index (HI) greater than 105°F is in the Danger Category. When the Heat
Index is between 105-115°F for 3 hours or more, a Heat Advisory will be
issued by the local National Weather Service Forecast Office.
||Heat Index/Apparent Temperature (°F)
||General Affect on People in High Risk Groups
||130°F or Higher
||Heat/Sunstroke HIGHLY LIKELY with
||105°F - 130°F
||Sunstroke, heat cramps, or heat exhaustion LIKELY,
and heatstroke POSSIBLE with prolonged exposure and/or
||90°F - 105°F
||Sunstroke, heat cramps, or heat exhaustion
POSSIBLE with prolonged exposure and/or physical activity
||80°F - 90°F
||Fatigue POSSIBLE with prolonged exposure
and/or physical activity
See Apparent Temperature.
Generally, a measure of the water vapor content of the air. Popularly, it is
used synonymously with relative humidity.
Energy emitted in the form of electromagnetic waves. Radiation has differing
characteristics depending upon the wavelength. Radiation from the Sun has a
short wavelength (ultra-violet) while energy re-radiated from the Earth's
surface and the atmosphere has a long wavelength (infra-red).
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.
The electromagnetic radiation emitted by the sun.
Water discharged into the atmosphere from plant surfaces.
The energy range just beyond the violet end of the visible spectrum.
Although ultraviolet radiation constitutes only about 5 percent of the total
energy emitted from the sun, it is the major energy source for the stratosphere
and mesosphere, playing a dominant role in both energy balance and chemical
UV (Ultraviolet) Index
This index provides important information to help you plan your outdoor
activities in ways that prevent overexposure to the sun's rays. It was designed
by the National Weather Service and the Environmental Protection Agency (EPA).
Unlike some countries' indices, the United States UV Index is not based upon
surface observations. Rather, it is computed using forecasted ozone levels, a
computer model that relates ozone levels to UV incidence on the ground,
forecasted cloud amounts, and the elevation of the forecast cities.
Currently, the computation of the UV Index does not include the effects of
variable surface reflection (e.g., sand, water, or snow), atmospheric pollutants
or haze. By following the few simple precautions in the table below, you can
greatly reduce your risk of sun related injuries (blistering sunburns, as well
as longer-term problems like skin cancer and cataracts).
|UV Index Value
||Time to Burn
||Actions to take at noon
||Apply SPF sunscreen
||Apply SPF sunscreen, wear a hat.
||Apply SPF 15, wear a hat
||Apply SPF 15 to 30 sunscreen, wear a hat and sunglasses. Limit midday
||Apply SPF 30 sunscreen. Wear a hat, sunglasses, and protective
|Time to burn and actions apply to people with a Type II,
fair skin that sometimes tans and usually burns. People with lighter skin
need be more cautious. People with darker skin may be able to tolerate
more exposure. But even dark skin can burn.
When the Index is High or Very High, try to minimize your outdoor activities
between the peak hours of 10 AM and 4 PM when the sun is most intense. When the
Index is 10 or higher, stay indoors if possible, otherwise be sure to take all
the other necessary precautions.
The wind chill is the effect of the wind on people and animals. The wind
chill temperature is based on the rate of heat loss from exposed skin caused by
wind and cold and is to give you an approximation of how cold the air feels on
As the wind increases, it removes heat from the body, driving down skin
temperature and eventually the internal body temperature. Therefore, the wind
makes it FEEL much colder. If the temperature is 0°F and the wind is
blowing at 15 mph, the wind chill temperature is -19°F. At this level, exposed
skin can freeze in just a few minutes.
The only effect wind chill has on inanimate objects, such as car radiators and
water pipes, is to shorten the amount of time for the object to cool. The
inanimate object will not cool below the actual air temperature. For example, if
the temperature outside is -5°F and the wind chill temperature is -31°F, then
your car's radiator temperature will be no lower than the air temperature of
See many more weather terms in the
National Weather Service's JetStream Online Weather School