NWS Weather Conditions
Station Measurement Definitions & Calculations
NWS WEATHER CONDITIONS
Blizzard (BLZD): Sustained wind or frequent gusts of 35 mph or greater; and considerable falling and/or blowing snow (i.e.,
reducing visibility frequently to less than 1/4 mile. These conditions must persist for 3 hours or longer in order for the storm to be
classified as a blizzard.
Blowing Snow Advisory: Issued when wind driven snow reduces surface visibility, possibly hampering travel. Blowing snow may
be falling snow, or snow that has already accumulated but is picked up and blown by strong winds. This advisory was discontinued
beginning with the 2008-2009 winter storm season, replaced by the "Winter Weather Advisory for Snow and Blowing Snow."
Heavy Snow: Snowfall accumulating to 4" or more in depth in 12 hours or less; or, snowfall accumulating 6 inches or more in
depth in 24 hours or less.
Heavy Snow Warning: May be issued instead of a Winter Storm Warning when heavy snow is the only significant winter weather
expected. Issued by the National Weather Service when snowfall of 6" or more in 12 hours or 8" or more in 24 hours is either
imminent or occurring. These criteria are specific for the Midwest and may vary regionally.
High Wind Advisory: Issued by the National Weather Service when high wind speeds may pose a hazard. The criteria varies from
state to state. In Michigan, for example, the criteria is sustained non-convective (not related to thunderstorms) winds greater than
or equal to 30 mph lasting for one hour or longer, or winds greater than or equal to 45 mph for any duration.
High Wind Warning: Issued by the National Weather Service when high wind speeds may pose a hazard or is life threatening.
Criteria varies from state to state. In Michigan, for example, the criteria is sustained non-convective (not related to thunderstorms)
winds greater than or equal to 40 mph lasting for one hour or longer, or winds greater than or equal to 58 mph for any duration.
Ice Storm Warning: May be issued instead of a Winter Storm Warning when significant ice accumulation is the only significant
winter weather expected. Issued by the National Weather Service when freezing rain produces a significant and possibly damaging
accumulation of ice. The criteria for this warning varies from state to state, but typically will be issued any time more than 1/4" of
ice is expected to accumulate in an area.
Wind Chill Advisory: The National Weather Service issues this product when the wind chill could be life threatening if action is not
taken. Criteria varies from state to state. Had been known to be issued when wind chills of -15 to -24 degrees are expected.
Wind Chill Warning: Issued when the wind chill is life threatening. Varies from state to state. For example, had been Issued when
wind chills of -25 degrees or below are expected.
Winter Storm Warning: Issued by the National Weather Service when a winter storm is producing or is forecast to produce heavy
snow or significant ice accumulations. The criteria for this warning can vary from place to place, and can include expected
conditions such as a combination of heavy snow, freezing rain, sleet, blowing & drifting snow or excessive wind chill.
Winter Storm Watch: Issued to inform the public of the possibility that one or more of the following events may occur. Product is
issued b the National Weather Service when there is a potential for heavy snow or significant accumulations, usually at least 24-36
hours in advance. The criteria for this watch can vary from place to place, but generally includes the following possibilities:
Excessive wind chill
Significant accumulations of ice or sleet.
A Winter Storm Watch is usually issued 24-36 hours in advance of the possible event.
Winter Weather Advisory: A Winter Weather Advisory is issued by the National Weather Service of the United States when a low
pressure system produces a combination of winter weather that presents a hazard, but does not meet warning criteria. A Winter
Weather Advisory is similar to significant weather advisory, but a winter weather advisory is an official product. A similar warning is
issued by Environment Canada's Meteorological Service of Canada offices.
Winter Weather Advisory for Snow and Blowing Snow: Issued when wind driven snow reduces surface visibility, possibly
hampering travel. Blowing snow may be falling snow, or snow that has already accumulated but is picked up and blown by strong
winds. This advisory was implemented beginning with the 2008-2009 winter storm season, replacing the "Blowing Snow Advisory."
Atmospheric Pressure - The weight of the air making up our atmosphere exerts a pressure on the surface of the earth. This
pressure is known as "Atmospheric" pressure. Generally, the more air above an area, the higher the atmospheric pressure. The
amount of atmospheric pressure is therefore different at different altitudes. Atmospheric pressure is less on a mountain top than
it is at sea level. The atmospheric pressure as measured at sea level is assigned the standard value of one (1) atmosphere, and is
equal to 14.6959488 pounds per square inch. One standard used to ascertain the atmospheric pressure is a device known as a
mercury barometer. A mercury barometer has a glass column, or tube, that's on average 30 inches in height. It is closed or sealed
at the top, but open at the bottom where it is part of a mercury filled reservoir. The mercury in the tube adjusts its level until the
weight of the mercury is equal to the atmospheric force applied to the mercury in the reservoir. Atmospheric pressure is not a
constant at any fixed location, but will vary with changing weather conditions. As the weight of the atmosphere changes due to
weather, the level of the mercury in the tube would then also vary accordingly. High pressure conditions force more mercury into
the tube, while lower pressures result in less mercury in the tube. The height of the mercury in the tube is measured in inches. A
standard atmosphere of 1, or 14.69 lbs per square inch, will raise the level of the mercury to a height of 29.92 inches at sea level.
Thus, we have the standard measurement on what we would call a standard (dry) day of 29.92" of Mercury, or 29.92" Hg, of
atmospheric pressure, which equates to 14.6959488 lbs/in
The United States and Britain still use these older measurement units. In 1960 the International System of Units, SI, was
developed. This system of units is based upon the metric meter/kilogram/second (mks) system, which Britain and the USA are
both slow to embrace.
Pressure as measured in the SI "mks" system is defined in terms of the Pascal, and equals a force of one Newton per square meter
(in turn, a Newton is the force required to give a 1 kilogram mass an acceleration of 1 meter per second per second.) The Pascal is
a very small amount of pressure, so we often use KiloPascals (kPa), equal to one thousand Pascals. Using the SI system of units,
one atmosphere is equal to 101.325 kPa, or 101,325 Pa.
The sciences (meteorology) involving weather have adopted the "bar" as the unit of pressure measurement in addition to the old,
standard "English" units. A "bar" is equal to 1x10^5 Pascal. This pressure is most often expressed in terms of millibars of pressure
to avoid using a lot of decimal points. A pressure of 1 atmosphere is equal to 1000 millibars or 1 bar. If you run the math, you'll see
then that a standard atmospheric pressure, as defined using English units of measurement of 29.92" of Hg, or 14.69 pounds per
square inch, is equal to 1 bar or 1,000 millibars under the SI system.
Barometric Pressure - Atmospheric pressure varies with both altitude and weather changes. At sea level, given a standard day
(implies dry air), we will measure an atmospheric pressure of 29.92" of Mercury (29.92" Hg), or 1 bar (1000mb). At that very same
moment in time, if you measure the atmospheric pressure at an altitude of 5,000 feet (Denver, etc.), you will measure an
atmospheric pressure of only 24.89" Hg (or only 12.23 pounds square inch). The pressure in SI units at 5,500 feet is 827 millibars.
To work from a common reference point, Meteorologists are interested in measuring the changes in atmospheric pressure due
only to the effects of weather phenomenon and must therefore somehow discount pressure values effected by differences in
altitude. To normalize the effects of altitude on atmospheric pressure, i.e., to compensate for differences in pressure readings
associated with different altitudes at different locations, atmospheric pressure is converted to an equivalent pressure referenced
to sea level. This referenced measurement has been assigned the name "Barometric" pressure. Our station actually measures
atmospheric pressure then converts this value to barometric pressure based upon our altitude (5445 feet). Atmospheric pressure,
and subsequently Barometric pressure as well, is in America generally measured in units of Mercury. We use the standard of 1
atmosphere being able to push a column of mercury up a tube to a level of 29.9246899 inches of mercury. Therefore, when the
barometric pressure of one atmosphere at sea level is measured to be 29.92" of Hg, then our station at our altitude will report an
equivalent of that value (one atmosphere), i.e., 29.92" of Hg in spite of our altitude, given all other parameters remain constant.
Now, we can follow barometric pressure changes with local weather conditions, making this measurement an important weather
forecasting tool. High barometric pressures generally are associated with fair weather, while low pressures are associated with
poor weather. Thus, rising barometric pressures indicate improving weather conditions, while falling pressures indicate
deteriorating weather conditions.
Dew Point - Dew point is the temperature to which air must be cooled in order for it to reach saturation (defined as 100% relative
humidity), providing there is no change in water vapor content. Dew point is an important measurement used to predict the
formation of dew, frost, and fog. If dew point and temperature are close together in the late afternoon when the air begins to turn
colder, fog is likely during the night. Dew point is also a good indicator of the air's actual water vapor content, unlike relative
humidity, which takes the air's temperature into account. High dew point indicates high water vapor content; low dew point
indicates low water vapor content. Additionally, a high dew point indicates a better chance of rain, severe thunderstorms, and/or
tornados. You can also use dew point to predict the minimum overnight temperature. Provided no new fronts are expected
overnight and the afternoon relative humidity is greater than or equal to 50%, the afternoon's dew point gives you an idea of what
minimum temperature to expect overnight, since the air can never get colder than the dew point. Dew Point also helps predict low
cloud levels. High dew point signifies moist air. An approximate cloud base calculation allows 400ft for every 1 degree difference
between temperature and dew point.
ET - Evapotranspiration - Evapotranspiration (ET) is the amount of water that moves from the ground (and plants on the ground)
to the atmosphere through both evaporation and transpiration. It is primarily important to people who are monitoring plant
growth and associated water usage. Measuring actual ET for a given location requires the measurement of weather variables at
different heights at the same location and is beyond the capabilities of the current Davis weather stations. Instead, a single set of
weather data measurements are used to calculate a Reference ET (ETo). ETo is the amount of ET that is expected at a location with
specified reference conditions under the actual weather conditions. The two most common reference conditions used for
agricultural purposes are the grass reference (ETo) which consists of an extensive surface of well-watered grass that completely
shades the ground and is uniformly clipped to a few inches in height, or the alfalfa reference (ETr) similar to the grass reference
but using alfalfa instead of grass, and at different height. The Davis ET calculations all calculate ETo from a grass reference.
The Texas A&M Irrigation Technology Program describes Evapotranspiration (ET) as follows: "Evapotranspiration (ET) is a
measurement of the total amount of water needed to grow plants and crops. This term comes from the words evaporation (i.e.,
evaporation of water from the soil) and transpiration (i.e., transpiration of water by plants). Different plants have different water
requirements, so they have different ET rates.
Since there are thousands of cultivated plants, we have tried to simplify matters by establishing a standard ET rate for general
reference and use. The standard is referred to as the potential evapotranspiration ETo (pet). This is the potential ET since we are
assuming the crop is in a deep soil and under well watered conditions. The standard crop we are using is a cool season grass
which is 4-inches tall. The technical term for this is the "Potential Evapotranspiration of a Grass Reference Crop" or "ETo" for short.
ETo depends on the climate and varies from location to location. Special weather stations are used to collect the climatic data for
calculating ETo,including temperature, dew point temperature (relative humidity), wind speed, and solar radiation.
The water requirements of specific crops and turf grasses can be calculated as a fraction of the ETo. This "fraction" is the called the
crop coefficient (Kc) or turf coefficient (Tc). Crop coefficients vary depending on the type of plant and its stage of growth.
We are using the standardized Penman-Monteith method to calculate ETo from the weather station data. This is one of a number
of methods that can be used to determine ETo and ET. Several organizations, such as the International Committee on Irrigation
and Drainage and the Water Requirements Committee of the American Society of Civil Engineers, have proposed establishing the
Penman-Monteith method as a world-wide standard. Such a standard would help facilitate the sharing of ETo data and
development of crop coefficients."
Heat Index [NOAA.GOV] - About 237 Americans succumb to the taxing demands of heat every year. Our bodies dissipate heat by
varying the rate and depth of blood circulation, by losing water through the skin and sweat glands, and as a last resort, by panting,
when blood is heated above 98.6 ºF. Sweating cools the body through evaporation. However, high relative humidity retards
evaporation, robbing the body of its ability to cool itself.
When heat gain exceeds the level the body can remove, body temperature begins to rise, and heat related illnesses and disorders
The Heat Index (HI) is the temperature the body feels when heat and humidity are combined. The chart below shows the HI that
corresponds to the actual air temperature and relative humidity. (This chart is based upon shady, light wind conditions. Exposure
to direct sunlight can increase the HI by up to 15 ºF.
(Due to the nature of the heat index calculation, the values in the tables below have an error +/- 1.3 ºF.)
Heating/Cooling Degree Days - A "degree day" is a unit of measure for recording how hot or how cold it has been over a 24-hour
period. The number of degree days applied to any particular day of the week is determined by calculating the mean temperature
for the day and then comparing the mean temperature to a base value of 65 ºF. (The "mean" temperature is calculated by adding
together the high for the day and the low for the day, and then dividing the result by 2.)
If the mean temperature for the day is, say, 5 degrees higher than 65, then there have been 5 cooling degree days. On the other
hand, if the weather has been cool, and the mean temperature is, say, 55 degrees, then there have 10 heating degree days (65
minus 55 equals 10).
Why do we want or need to know the number of "degree days?" It is a good way to generally keep track of how much demand
there has been for energy needed for either heating or cooling buildings. The cooler the weather, the larger the number of
"heating degree days"... and the larger the number of heating degree days, the heavier the demand for energy needed to heat
buildings. Likewise, The warmer the weather, the larger the number of "cooling degree days"... and the larger the number of
cooling degree days, the heavier the demand for energy needed to cool buildings.
Where Can I Find the Actual Number of Degree Days Accumulated in Recent Months?"
Degree day calculations are made at the end of each day and sent out the following morning in a National Weather Service (NWS)
product called "Climate Report." Addressing our case here in Lakewood, CO, one would visit the NWS's web site at
"http://www.weather.gov/climate/index.php?wfo=bou" and select the "Monthly Weather Summary for Denver, CO...Most Recent.
You would then drop down on the presented "Climate Report" page to find the "DEGREE_DAYS" presentation. You would see for
the month of April, 2007 for example, we had a total of 544 Heating Degree Days, and a total of 5 Cooling Degree Days.
Humidity - The term "humidity" itself refers to the amount of water vapor in the air. However, the total amount of water vapor
that the air can hold varies with the air's temperature and pressure. Relative Humidity takes into account these factors and offers
a humidity reading which reflects the amount of water vapor in the air as a percentage of the amount the air is capable of holding.
Relative humidity is therefore not an actual direct measurement of the amount of water vapor in the air, but a calculated ratio of
the air's water vapor content to its capacity.
Knot - 1 knot = 1.1508 mph.
Solar Radiation - What we call "current solar radiation" is technically known as "Global Solar Radiation." This is a measure of the
intensity, the energy, of the sun's radiation reaching a horizontal surface. The irradiance includes both the direct component from
the sun and the reflected component from the rest of the sky. The solar radiation reading gives a measure of the amount of solar
radiation hitting the solar radiation sensor at any given time, expressed in Watts/Square Meter (W/M^2). If you observe the sunrise
and sunset times, you'll see that on a day with minimal clouds you will have solar radiation readings that begin with sunrise and
end at sunset.
Temperature - [Wikipedia] Temperature is a physical property of a system that underlies the common notions of hot and cold;
something that is hotter has the greater temperature. Temperature is one of the principal parameters of thermodynamics. The
temperature of a system is related to the average energy of microscopic motions in the system. For a solid, these microscopic
motions are principally the vibrations of the constituent atoms about their sites in the solid. For an ideal mon-atomic gas, the
microscopic motions are the translational motions of the constituent gas particles.
Temperature is measured with thermometers that may be calibrated to a variety of temperature scales. Throughout the world
(except for in the U.S.), the Celsius scale is used for most temperature measuring purposes. The entire scientific world (the U.S.
included) measures temperature using the Celsius scale, and thermodynamic temperature using the Kelvin scale. Many
engineering fields in the U.S., especially high-tech ones, also use the Kelvin and Celsius scales. The bulk of the U.S. however, (its lay
people, industry, meteorology, and government) relies upon the Fahrenheit scale. Other engineering fields in the U.S. also rely
upon the Rankine scale when working in thermodynamic-related disciplines such as combustion.
Intuitively, temperature is a measure of how hot or cold something is. Microscopically, temperature is the result of the motion of
particles which make up a substance. Temperature increases as the energy of this motion increases. The motion may be the
translational motion of the particle, or the internal energy of the particle due to molecular vibration or the excitation of an electron
energy level. Although very specialized laboratory equipment is required to directly detect the translational thermal motions,
thermal collisions by atoms or molecules with small particles suspended in a fluid produces Brownian motion that can be seen
with an ordinary microscope. The thermal motions of atoms are very fast and temperatures close to absolute zero are required to
directly observe them. For instance, when scientists at the NIST achieved a record-setting cold temperature of 700 nK (1 nK = 10−9
K) in 1994, they used optical lattice laser equipment to adiabatically cool cesium atoms. They then turned off the entrapment
lasers and directly measured atom velocities of 7 mm per second in order to calculate their temperature.
Molecules, such as O2, have more degrees of freedom than single atoms: they can have rotational and vibrational motions as well
as translational motion. An increase in temperature will cause the average translational energy to increase. It will also cause the
energy associated with vibrational and rotational modes to increase. Thus a diatomic gas, with extra degrees of freedom like
rotation and vibration, will require a higher energy input to change the temperature by a certain amount, i.e. it will have a higher
heat capacity than a mon-atomic gas.
The process of cooling involves removing energy from a system. When there is no more energy able to be removed, the system is
said to be at absolute zero, which is the point on the thermodynamic (absolute) temperature scale where all kinetic motion in the
particles comprising matter ceases and they are at complete rest in the ?classic? (non-quantum mechanical) sense. By definition,
absolute zero is a temperature of precisely 0 kelvin (−273.15 ºC or −459.67 ºF).
Comparison of Temperature Scales
(1) The temperature scale is in disuse, and of mere historical interest.
(2) Normal human body temperature is 36.8 ºC +/- 0.7 ºC, or 98.2 ºF +/- 1.3 ºF. The commonly given value 98.6 ºF is simply the exact
conversion of the nineteenth-century German standard of 37 ºC. Since it does not list an acceptable range, it could therefore be said to
have excess (invalid) precision. Some numbers in this table have been rounded off.
THW Index: The THW index combines air temperature, wind chill index, and heat index to produce a more accurate apparent
temperature. This is how the temperature will feel when you are out of the sun.
THSW Index: Same concept as for theTHW index, but THSW index includes the effects of the sun's solar energy and is the most
useful measure of how it would feel if you were standing directly in sunlight. Parameters Used: Temperature, Humidity, Solar
Radiation, Wind Speed, Latitude & Longitude, Time and Date. Like Heat Index, the THSW Index uses humidity and temperature to
calculate an apparent temperature. In addition, THSW incorporates the heating effects of solar radiation and the cooling effects of
wind (like wind chill) on our perception of temperature. The formula used to calculate THSW by our Vantage Pro 2 Plus system
was developed by Steadman (1979). The following describes the series of formulas used to determine the THSW or Temperature-
Humidity-Sun-Wind Index. Thus, this index indicates the level of thermal comfort including the effects of all these values. This
Index is calculated by adding a series of successive terms. Each term represents one of the three parameters: (Humidity, Sun &
Wind). The humidity term serves as the base from which increments for sun and wind effects are added.
HUMIDITY FACTOR: The first term is humidity. This term is determined in the same manner as the Heat Index. This term
serves as a base number to which increments of wind and sun are added to come up with the final THSW Index
SUN FACTOR: The second term is sun. This term, Qg, is actually a combination of four terms (direct incoming solar, indirect
incoming solar, terrestrial, and sky radiation). The term depends upon wind speed to determine how strong an effect it is.
The value is limited to between −20 and +130 W/m2 in the Vantage Pro2 console firmware and WeatherLink software
versions 5.6 or later.
WIND FACTOR: The third term is wind. Depending upon the version of firmware or software, this term is determined in part
by a lookup table (for temperatures above 50 ºF)and in part by the wind chill calculation, or uses an integrated table that is
used both for calculation of this term and for wind chill. With this in mind, the following criterion apply with later versions
referring to Vantage Pro2 console firmware revision May 2005 or later or WeatherLink version5.6 or later:
- At 0 mph, this term is equal to zero.
- For temperatures at or above 68 ºF and wind speeds above 40 mph,the wind speed is set to40 mph. For later versions,
there is no upper limit on wind speed.
-For temperatures at or above 130 ºF, this term is set equal to zero. For later versions of this algorithm: WeatherLink uses
144 ºF as the threshold; Vantage Pro2console firmware143 ºF. This is based on a best-fit regression of the Steadman 1979
wind table. The differences are reflective of the higher resolution used in the WeatherLink software.
- For temperatures below 50 ºF (later versions use the new wind chill formula result here (calculate the wind chill increment
using the difference between the air temperature and wind chill)):
-- For the earlier display console versions and WeatherLink version5.0 or 5.1: use the wind chill calculation as the base
--For the WeatherLink software (versions 5.2 through 5.5.1): use the new heat index formula (as described in the heat index
section) as the base temperature and calculate the wind chill increment using the difference between the air temperature
and wind chill (which is always a negative number). The resulting value is the wind term, which will be added to the humidity
term and subsequently the sun term as indicated below. Note: The WeatherLink software(version 5.2 through 5.5.1) offers a
variable does not include the sun term in its calculation. It shows the result as the "THW Index"or "Temperature-Humidity-
Wind Index." This value indicates the"apparent" temperature in the shade due to these factors.
Steadman, R. G., 1979: The Assessment of Sultriness, Part II: Effects of Wind, Extra Radiation
and Barometric Pressure on Apparent Temperature. Journal of Applied Meteorology,
"Media Guide to NWS Products and Services", National Weather Service Forecast Office,
Monterey, CA, 1995.
Quayle, R. G. and Steadman, R. G., 1998: The Steadman Wind Chill: An Improvement over
Present Scales. Weather and Forecasting, December 1998.
UV (Ultra-violet) Index - The UV Index is a measure of the amount of skin damaging UV radiation reaching
the earth's surface. The amount of UV radiation reaching the surface at any given time is primarily related
to the elevation of the sun in the sky, the amount of ozone in the stratosphere, and the amount of clouds
present. The UV Index can range from 0 (when it is nighttime) to 15 or 16 (in the tropics a thigh elevations
under clear skies). UV radiation is greatest when the sun is highest in the sky and rapidly decreases as the
sun approaches the horizon. The higher the UV Index, the greater the dose rate of skin damaging (and eye
damaging) UV radiation. Consequently, the higher the UV Index, the shorter the time before skin damage
There are several effects experienced as a result of overexposure to UV radiation: 1) a severe sunburn
following an intense short term overexposure, and 2) the more serious skin cancers developing after long term overexposure.
Melanoma, the more deadly of the two types of skin cancer, occurs when the person has been subjected to several intense short
term overexposures. Non-melanoma skin cancers, which are almost 100% curable, occur in people who are overexposed for very
long periods of time, such as construction workers, farmers, or fishermen. Long term overexposure to UV radiation has been
linked to the formation of cataracts in the eyes as well.
The UV Index forecast indicates the probable intensity of skin damaging ultraviolet radiation reaching the surface during the solar
noon hour (11:30-12:30 local standard time or 12:30-13:30 local daylight time). The greater the UV Index is the greater the amount
of skin damaging UV radiation. How much UV radiation is needed to actually damage one's skin is dependent on several factors.
But in general the darker one's skin is, that is the more melanin one has in his/her skin, the longer (or the more UV radiation) it
takes to cause erythema (skin reddening).
Wind Speed - Wind Speed is the current sustained wind, while Wind Gust is the current intermittent burst of wind speed. Wind
speed is measured by the weather station's anemometer. The station calculates a 10-minute average wind speed and a dominant
10-minute wind direction as "wind speed."
Wind Gusts - Intermittent bursts of wind are generally considered to be "gusts" when the wind speed reaches 16 knots or
18.4mph (1 knot = 1.1508 mph) and the variability of the wind from highest point to lowest is more than 9 knots or 10.4 mph. A
gust will usually be defined as less than 20 seconds in duration and is the maximum speed reached by the wind. With personal
weather stations, the definition of Gust varies by manufacturer and software used. Some software packages do not send
information on wind gusts to online weather specialty sites such as Wunderground.com, so the "gust" reading you will see may be
the highest measured wind reading.
Wind run: Wind run is measurement of the "amount" of wind passing the station during a given period of time, expressed in
either "miles of wind" or "kilometers of wind". Wind run is calculated by multiplying the average wind speed for each archive
record by the archive interval.
Average Wind Speed = 5 mph
Archive Interval = 30 minutes (0.5 hours)
Wind Run = 5 mph x 0.5 hours = 2.5 miles of wind
Wind Chill - Parameters Used: Outside Air Temperature and Wind Speed. Wind chill takes into account how the speed of the wind
affects our perception of the air temperature. Our bodies warm the surrounding air molecules by transferring heat from the skin.
If there's no air movement, this insulating layer of warm air molecules stays next to the body and offers some protection from
cooler air molecules. However, wind sweeps that comfy warm air surrounding the body away. The faster the wind blows, the faster
heat is carried away and the colder the environment feels. The new formula was adopted by both Environment Canada and the
U.S. National Weather Service to ensure a uniform wind chill standard in North America. The formula is supposed to more closely
emulate the response of the human body when exposed to conditions of wind and cold than the old formula did.
35.74 + 0.6215T - 35.75 * (V 0.16 ) + 0.4275T * (V 0.16 )
Any place where the result yields a wind chill temperature greater than the air temperature, the wind chill is set equal to the air
temperature. This always occurs at wind speeds of 0 mph or temperatures above 76 ºF. This also occurs at lower wind speeds with
temperatures between 0 ºF and 76 ºF. The new formula takes into account the fact that wind speeds are measured "officially" at 10
meters (33 feet) above the ground, but the human is typically only 5 to 6 feet (2 meters) above the ground. So, anemometers still
need to be mounted as high as possible (e.g., rooftop mast) to register comparable wind speed readings and wind chill values. Our
newer version of the formula addresses the fact that the latest National Weather Service (NWS) formula was not designed for use
above 40 ºF. The result of the straight NWS implementation was little or no chilling effect at mild temperatures. This updated
version provides for reasonable chilling effect at mild temperatures based on the effects determined by Steadman (1979) (see
THSW Index section), but as with the new NWS formula, no upper limit where chilling has no additional effect. This later version for
the console table only differs in that whole degrees and less resolution in the table are used for code and memory space
conservation. As with previous versions of the wind chill formula, any place where the result yields a wind chill temperature
greater than the air temperature, the wind chill is set equal to the air temperature. This always occurs at wind speeds of 0 mph or
temperatures at or above 93.2 ºF (34 ºC). This also occurs at lower wind speeds with temperatures between 0 ºF (-18 ºC) and 93.2
ºF (34 ºC). As per Steadman (1979), 93.2 ºF (34 ºC) is the average temperature of skin at mild temperatures, thus temperatures
above this value will actually create an apparent warming effect (see THSW Index section). The Vantage Pro and Vantage Pro2
console uses the "10-minute average wind speed" to determine wind chill, which is updated once per minute. When 10-minute of
wind speed data is unavailable, it uses a running average until 10-minutes worth of data is collected. The reason an average wind
speed is employed in the Vantage Pro and Vantage Pro2 to calculate wind chill is as follows: The human body has a high heat
capacity, thus high wind speeds have no effect on the body's thermal equilibrium. So, an average wind speed provides a more
accurate representation of the body's response than an instantaneous reading. Also, "official" weather reports (from which wind
chill is calculated) provide average wind speed, so using an average wind speed more closely matches the results that are seen in
"Media Guide to NWS Products and Services", National Weather Service Forecast Office, Monterey, CA, 1995.
"New Wind Chill Temperature Index", Office of Climate, Water and Weather Services, Washington, DC, 2001.
Siple, P. and C. Passel, 1945. “Measurements of Dry Atmospheric Cooling in Subfreezing Temperatures.” Proc. Amer. Philos.
Soc. Steadman, R. G., 1979: T”he Assessment of Sultriness, Part I: A Temperature-Humidity Index Based on Human
Physiology and Clothing Science.” Journal of Applied Meteorology, July 1979
In 2001, NWS implemented an updated Wind Chill Temperature (WCT) index. The change improves upon the former WCT
Index used by the NWS and the Meteorological Services of Canada, which was based on the 1945 Siple and Passel Index.
In the fall of 2000, the Office of the Federal Coordinator for Meteorological Services and Supporting Research (OFCM) formed a
group consisting of several Federal agencies, MSC, the academic community (Indiana University-Purdue University in Indianapolis
(IUPUI), University of Delaware and University of Missouri), and the International Society of Biometeorology to evaluate and
improve the windchill formula. The group, chaired by the NWS, is called the Joint Action Group for temperature Indices (JAG/TI).
JAG/TI's goal is to upgrade and standardize the index for temperature extremes internationally (e.g. Windchill Index).
The current formula uses advances in science, technology, and computer modeling to provide a more accurate, understandable,
and useful formula for calculating the dangers from winter winds and freezing temperatures.
(click on chart for a larger version)
Clinical trials were conducted at the Defense and Civil Institute of Environmental Medicine in Toronto, Canada, and the trial results
were used to improve the accuracy of the new formula and determine frostbite threshold values.
Standardization of the WCT Index among the meteorological community provides an accurate and consistent measure to ensure
public safety. The new wind chill formula is now being used in Canada and the United States.
Specifically, the new WCT index:
Calculates wind speed at an average height of five feet (typical height of an adult human face) based on readings from the
national standard height of 33 feet (typical height of an anemometer)
Is based on a human face model
Incorporates modern heat transfer theory (heat loss from the body to its surroundings, during cold and breezy/windy days)
Lowers the calm wind threshold to 3 mph
Uses a consistent standard for skin tissue resistance
Assumes no impact from the sun (i.e., clear night sky)
Note: Wind Chill Temperature is only defined for temperatures at or below 50 ºF and wind speeds above 3 mph. Bright sunshine
may increase the wind chill temperature by 10 to 18 ºF.
To view the NWS brochure about the new wind chill temperature index, click here.
For more information visit: https://www.weather.gov/safety/cold
Want to see the equation from which HI is derived? Click HERE