NWS WEATHER CONDITIONS
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.
Blizzard (BLZD): Sustained wind or frequent gusts of 35 mph
or greater; and considerable falling and/or blowing snow (i.e.,
reducing visibility frequenctly 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.
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
- Blizzard conditions
- Heavy snow
- 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 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.
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
occuring. These criteria are specific for the Midwest and may vary
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
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."
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.
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.
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.
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 (metrology) 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
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
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 cloudbase
calculation allows 400ft for every 1 degree difference between
temperature and dewpoint.
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 (described in detail below)
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 ?
well watered grass that completely shades the ground, is uniformly
clipped to a few inches in height ? and the alfalfa reference ?
similar to the grass reference with alfalfa instead of grass, and a
different height. The Davis ET calculations all calculate ETo for a
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 may develop.
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.)
Want to see the equation from which HI is derived? Click
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
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
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 monatomic gas, the
microscopic motions are the translational motions of the constituent
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 monatomic 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
Comparison of temperature scales
ºR (ºRè, ºRe)
|Lowest recorded surface temperature on
(Vostok, Antarctica - July 21, 1983)
|Fahrenheit's ice/salt mixture
|Ice melts (at standard pressure)
|Average surface temperature on Earth
|Average human body temperature (2)
|Highest recorded surface temperature on
(Al 'Aziziyah, Libya - September 13, 1922)
|Water boils (at standard
|The surface of the Sun
(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
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 the
THW 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
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.
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 temperature.
The second 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 version
5.6 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 to
40 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 Pro2
143 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
- 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
-- For the earlier display console versions and WeatherLink version
5.0 or 5.1: use the
wind chill calculation as the base temperature.
--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.
The third 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.
Steadman, R.G., 1979: The Assessment of Sultriness, Part II: Effects
of Wind, Extra Radiation
and Barometric Pressure on Apparent Temperature. Journal of Applied
"Media Guide to NWS Products and Services", National Weather Service
Monterey, CA, 1995.
Quayle, R.G. and Steadman, R.G., 1998: The Steadman Wind Chill: An
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 at
high 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 occurs.
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
Windchill - 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
35.74 + 0.6215T - 35.75 * (V 0.16 ) + 0.4275T * (V
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: The 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
Windchill 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
(click on chart for a larger, PDF version)
Clinical trials were conducted at the Defence 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 windchill 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
Lowers the calm wind threshold to 3 mph
Uses a consistent standard for skin tissue
Assumes no impact from the sun (i.e., clear
Note: Windchill Temperature is only defined
for temperatures at or below 50 ºF and wind speeds above 3 mph.
Bright sunshine may increase the windchill temperature by 10 to 18 ºF.
To view the NWS brochure about the new windchill
temperature index, click
For more information visit: