- Instrument for measuring altitude.
- Vertical elevation above mean sea level or any particular
gaseous or air portion
of the physical environment
that encircles a planet. In the case of the earth, it is held more
or less near the surface by the earth's gravitational attraction.
The divisions of the atmosphere include the troposphere,
and the exosphere.
- The pressure
exerted by the
atmosphere at a given point. Its
measurement can be expressed in several ways. One is in millibars.
Another is in inches or millimeters of mercury (Hg).
- Instrument for measuring
Pressure - The pressure
exerted by the atmosphere
at a given point. Its measurement can be expressed in several
ways. One is in millibars.
Another is in inches or millimeters of mercury (Hg).
- A temperature scale based on the freezing point of water as 0°
and boiling point as 100°. Named for
Cirrus - One of the
three basic cloud
forms (the others are cumulus and stratus).
It is also one of the three high cloud
types. Cirrus are thin, wispy clouds composed of ice crystals
and often appear as veil patches or strands. In the mid-latitudes,
cloud bases are usually found between 20,000 to 30,000 feet, and
it is the highest cloud that forms in the sky, except for the
tops, or anvils, of cumulonimbus,
which occasionally build to excessive heights.
time between the moment of sunset, when the sun's apparent upper
edge is just at the horizon, until the center of the sun is 6°
directly below the horizon.
One of the three basic cloud forms
(the others are cirrus
It is also one of the two low cloud
types. A cloud that develops in a vertical direction from the base
(bottom) up. They have flat bases and dome- or cauliflower-shaped
upper surfaces. The base of the cloud is often no more than 3,000
feet above the ground, but the top often varies in height. Small,
separate cumulus are associated with fair weather (cumulus
humilis). With additional heating
from the earth's surface, they can grow vertically throughout the
day. The top of such a cloud can easily reach 20,000 or more into
Under certain atmospheric
conditions, these clouds can develop into larger clouds, known as
towering cumulus (cumulus
congestus), and may produce a rain shower.
Further development may create a cumulonimbus.
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
Doppler Radar -
Special radar which detects changes in frequency due to movement
or air and airborne moisture, thereby able to measure direction
and speed of winds and intensity of storms.
- the conversion of water from
a liquid into a gas.
- The amount of water
transferred from the soil to the atmosphere. Sum of evaporation
and transpiration. Reference Evapotranspiration (ETo) is a term
used to estimate the evapotranspiration rate of a reference crop
expressed in either inches or millimeters. ETo varies by
location, time, and weather conditions. The main factors that
influence ETo include incoming radiation (energy from the sun),
outgoing radiation (sensible energy leaving the earth), the amount
of moisture in the air, air temperature, and wind speed. ETo can
be estimated quite accurately through the use of a "model" (a
series of complex mathematical equations). The "model" that is
used commonly is a version of Penman's equation modified by
Pruitt/Doorenbos (Proceedings of the International Round Table
Conference on "Evapotranspiration", Budapest, Hungary. 1977).
Temperature Scale - A temperature
scale where water at sea level
has a freezing point
of +32Â°F and a boiling point
of +212Â°F. More commonly used in areas that observe the English
system of measurement. Created in 1714 by Gabriel Daniel
Fahrenheit (1696-1736), a German physicist, who also invented the
alcohol and mercury thermometers.
aggregate of minute water droplets
suspended in the atmosphere
at or near the surface of the earth, reducing horizontal visibility
to less than 5/8 statute miles. It is created when the temperature
and the dew point
of the air have become the same, or nearly the same, and
sufficient condensation nuclei
are present. It is reported as "FG" in an observation
and on the METAR.
- The Moon's illuminated side is
facing the Earth. The Moon appears to be completely illuminated by
- On the Beaufort Wind Scale,
a wind with
speeds from 28 to 55 knots (32
to 63 miles per hour). For marine interests, it can be categorized
as a moderate gale (28 to 33 knots), a fresh gale (34 to 40
knots), a strong gale (41 to 47 knots), or a whole gale (48 to 55
knots). In 1964, the World Meteorological Organization
defined the categories as near gale (28 to 33 knots), gale (34 to
40 knots), strong gale (41 to 47 knots), and storm (48 to 55
- A sudden
significant increase in or rapid fluctuations of wind speed.
Peak wind must reach at least 16 knots (18 miles per hour) and the
variation between peaks and lulls is at least 10 knots (11.5 miles
per hour). The duration is usually less twenty seconds.
Heatstroke - Heatstroke is
life-threatening. The victim's temperature control system, which
produces sweating to cool the body, stops working. The body
temperature can rise so high that brain damage and death may
result if the body is not cooled quickly.
Sunstroke - Another term for heatstroke. For additional information
please visit the
Cross web site.
Cramps - Heat cramps are muscular
pains and spasms due to heavy exertion. They usually involve the
abdominal muscles or legs. It is generally thought that the loss
of water from heavy sweating causes the cramps.
Heat Exhaustion - Heat exhaustion
typically occurs when people exercise heavily or work in a warm
humid place where body fluids are lost through heavy sweating.
Blood flow to the skin increases, causing blood flow to decrease
to the vital organs. This results in a form of mild shock. If not
treated, the victim's condition will worsen. Body temperature will
keep rising and the victim may suffer heatstroke.
Heat Index (Real Feel Temperature) -
A number in degrees Fahrenheit that tells how hot it really feels
when relative humidity is added to the actual air temperature.
Exposure to full sunshine can increase the heat index by 15
|Formula to calculate
If you know the relative
humidity and the dry air temperature, then you can use
the following equation to calculate the heat index.
Heat index (HI), or
apparent temperature (AI) = -42.379 + 2.04901523(Tf) +
10.14333127(RH) - 0.22475541(Tf)(RH) - ((6.83783 x 10-3)(Tf2)
- ((5.481717 x 10-2)(RH2) +
((1.22874 x 10-3)(Tf2)(RH)) +
((8.5282 x 10-4)(Tf)(RH2)) -
((1.99 x 10-6)(Tf2)(RH2))
Note: In order for
the Heat Index formula to work correctly, you must use
the relative humidity in percent form. In other words,
if the relative humidity is 65%, use 65 for RH in the
formula, not .65.
Wave - Prolonged period of excessive
heat and humidity. The National Weather Service steps up its
procedures to alert the public during these periods of excessive
heat and humidity.
Humidity - Generally, a measure of the water
vapor content of the air. Popularly, it is used synonymously with
relative humidity. Also see Relative Humidity.
that is composed of minute ice
particles. It occurs in very low temperatures
under clear, calm
conditions in the polar latitudes
and may produce a halo
around the sun or moon.
- An area
of strong winds that
are concentrated in a relatively narrow band in the upper troposphere
of the middle latitudes
regions of the Northern and Southern Hemispheres. Flowing in a
semi-continuous band around the globe from west to east, it is
caused by the changes in air temperature
where the cold polar air
moving towards the equator
meets the warmer equatorial air moving polar-ward. It is marked by
a concentration of isotherms
and strong vertical shear.
nautical unit of speed equal to the velocity at which one nautical mile
is traveled in one hour. Used primarily by marine interests and in weather observations.
A knot is equivalent to 1.151 statute miles per hour or 1.852
kilometers per hour.
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 90Â° North and South latitude.
- Here are the lightning abbreviations you will find on our
tracking system. Also, click
here for in-depth explanation.
Ranging and ACquisition sub process.
process to determine strike locations.
||Positive Cloud to
||Negative Cloud to
||Positive In Cloud or
||Negative In Cloud or
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.
An eclipse of
the moon occurs when the earth is in a direct line between the sun
and the moon. The moon does not have any light of its own,
instead, it reflects
the sun's light. During a lunar eclipse, the moon is in the
earth's shadow. It will often look dim and sometimes copper or
orange in color.
Mean Sea Level - The average height of the sea
surface water level. For the United States, it is computed by
averaging the levels of all tide stages
over a nineteen year period, determined from hourly height
readings measured from a fix, predetermined reference level. It is
used as a basis for determining elevations,
as the reference for all altitudes
in upper air measurements, and as the level above which altitude
is measured by a pressure altimeter
for aviation. Often referred to as MSL.
Mean Temperature - The average of temperature
readings taken over a specified amount of time. Often the average
of the maximum and minimum
- Acronym for
METeorological Aerodrome Report. It is the primary observation
code used in the United States to satisfy requirements for
reporting surface meteorological data. Minimum reporting
requirements includes wind, visibility, runway visual range,
present weather, sky condition, temperature, dew point, and
- A small, concentrated downburst affecting an area less than 4
kilometers (about 2.5 miles) across. Most micro bursts are rather
short-lived (5 minutes or so), but on rare occasions they have
been known to last up to 6 times that long. Case Mid-level
cooling. Local cooling of the air in middle levels of the
atmosphere (roughly 8 to 25 thousand feet), which can lead to
destabilization of the entire atmosphere if all other factors are
equal. Mid-level cooling can occur, for example, with the approach
of a mid-level cold pool.
Nautical Mile - A unit of length used in marine
navigation that is equal to a minute of arc of a great circle on a
sphere. One international nautical mile is equivalent to 1,852
meters or 1.151 statue miles.
- National Oceanic and
NWS - National
- The amount of sky cover
for a cloud layer that is 8/8ths, based on the summation layer amount
for that layer.
A nearly colorless gas and a
form of oxygen (O2). It is composed of an oxygen
molecule made up of three oxygen atoms instead of two.
Ozone Layer -
layer that contains a high proportion of oxygen that
exists as ozone. It
acts as a filtering mechanism against incoming ultraviolet radiation.
It is located between the troposphere
and the stratosphere,
around 9.5 to 12.5 miles (15 to 20 kilometers) above the earth's
- Any and all forms of water,
liquid or solid, that falls from clouds and
reaches the ground. This includes drizzle, freezing drizzle, freezing rain, hail, ice crystals, ice pellets, rain, snow, snow pellets,
and snow grains.
The amount of fall is usually expressed in inches of liquid water
depth of the substance that has fallen at a given point over a
specified time period.
Probability of Precipitation (POP)
- is the likelihood of occurrence
(expressed as a percent) of a precipitation event at any given
point in the forecast area. The NWS uses two different methods
to indicate the chance of precipitation for a specific area:
numerical or in non- numerical terms. The "Expression of
Uncertainty" category is used for widespread precipitation and
the "Equivalent Areal Coverage" for convective (i.e., showery)
events. Below is a table of these two methods with the
Expression of Uncertainty
Equivalent Areal Coverage
are other qualifying terms which are
used with the above non-numerical
expressions. For example:
For duration - brief,
occasional, intermittent, frequent.
< .01 inch per hour
.01 to .10 inch per hour
.10 to .30 inch per hour
greater than .30 inch per hour
Quantitive Precipitation Forecast
- A forecast
of rainfall, snowfall
or liquid equivalent of snowfall.
Quasi-Stationary Front - A front
which is nearly stationary or moves very little since the last synoptic
position. Also known as a stationary front.
- 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.
Real Feel Temperature = See Heat
Sky Cover Terminology -
Predominate Sky Condition
Day or Night Terminology
- 2/8 Opaque Clouds
- 5/8 Opaque Clouds
- 7/8 Opaque Clouds
Snowboard - Platform device for measuring snow
Making a Snowboard:
To measure snow that will avoid the melting issues associated
with pavement and the potential overestimation problems with
tall grass is to make your own snowboard. This is simply a piece
of plywood approximately 2 feet by 2 feet that should be painted
white in order to reflect incoming solar radiation. Before an
expected snowfall, place the snowboard in a flat area of your
yard away from trees and other obstructions. Be sure to mark the
location of your board with a stake or a reflector so you'll
know its location once it gets covered by snow. You can then
take your measurements directly from the board. Of course, if
strong winds have swept it clean, you will need to take your
Guidelines for measurement have been set by
Radiation - The
electromagnetic radiation emitted by the Sun. The entire energy
range of electromagnetic radiation is specified by frequency,
wavelength, or photon energy. The Solar Spectrum is the
distribution of the radiation by wavelength. The low end of the
spectrum is infrared radiation (heat), and passes through the
colors of visual light from red through violet, through
ultraviolet radiation, x-rays, and gamma rays.
Solar Irradiance is the amount of solar energy that arrives at a
specific area of a surface during a specific time interval
(radiant flux density). A typical unit is W/m².
The Solar Constant is the amount of solar power flux that passes
through the mean Earth orbit. Although not strictly constant due
to variations in the suns output, the currently accepted value is
1367 W/m2. Note that Earth-based instruments record lower values
of solar power flux because of atmospheric attenuation.
Solar Energy is the ability of the Sun's energy to do work, such
as raise the temperature of water or excite electrons in a
photovoltaic cell. Typical units of measure are kilowatt-hours
per square meter (kWh/m²), mega joules per square meter (MJ/m²),
langleys (L) (equal to cal/cm²), or British thermal units per
square foot (Btu/ft²).
Solar radiation data provide information on how much of the sun's
energy strikes a surface at a location on earth during a
particular time period. The data give values of energy per unit of
area. By showing naturally occurring changes in the amount of
solar radiation over the course of days, months, and years, these
data determine the amount of solar radiation for a location.
here for in-depth explanation.
- One of
the three basic cloud forms
(the others are cirrus and cumulus. It
is also one of the two low cloud
types. It is a sheetlike cloud that does not exhibit individual
elements, and is, perhaps, the most common of all low clouds.
Thick and gray, it is seen in low, uniform layers and rarely
extends higher than 5,000 feet above the earth's surface. A veil
of stratus may give the sky a hazy
appearance. Fog may
form from a stratus cloud that touches the ground. Although it can
produce drizzle or snow, it
rarely produces heavy precipitation.
Clouds producing heavy precipitation may exist above a layer of
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.
- 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.
- Measurement of heat and/or
Numerical temperature values are
represented in NWS forecasts in four
"around," or "about" a specific value
rounded to the nearest five zero. Above
100°F or below 10°F, any number will be
used. For example:
40, AROUND 15, ABOUT 85, or NEAR 106.
2. A general
range where the terms are defined by the
50's (50 - 54)
specific range rounded to the nearest
five or zero (except ranges below 10°F
or above 100°F, any number may be used).
For example, 70 to 75 or 102 to 108.
MID 50's (53 - 57)
UPPER 50's (56 - 59)
50's (50 - 59)
for in-depth explanation.
uses temperature, humidity and wind to calculate an apparent
temperature that incorporates the
cooling effects of wind on our perception of temperature.
Temperature-Humidity-Wind-Sun (THWS) Index uses humidity,
temperature, the cooling effects of wind and the heating effects
of direct solar radiation to calculate an apparent temperature.
- A violently rotating column of air in contact with the ground
and extending from the base of a thunderstorm. A condensation
funnel does not need to reach to the ground for a tornado to be
present; a debris cloud beneath a thunderstorm is all that is
needed to confirm the presence of a tornado, even in the total
absence of a condensation funnel.
(UV) Index - The UV
Index describes the levels of exposure to UV rays. The Index
predicts UV levels on a 0-10+ scale:
sunscreen (spf 15+)
sunscreen (spf 15+)
keep to shady areas
sunscreen (spf 15+)
keep to shady areas or stay indoors
as much as possible
take precautions when outdoors
The US 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.
The calculation starts with measurements of current total ozone
amounts for the entire globe, obtained via two satellites operated
by the National Oceanic and Atmospheric Administration (NOAA).
These data are then used to produce a forecast of ozone levels for
the next day at various points around the country. A radiative
transfer model is then used to determine the amount of UV
radiation reaching the ground from 290 to 400 nm in wavelength,
using the time of day (solar noon), day of year, and latitude.
As an example, assume the following UV levels for each wavelength
are predicted for a given location (these are totally made up
numbers, and not even the ratios represent reality):
This information is then
weighted according to how human skin responds to each wavelength;
it is more important to protect people from wavelengths that harm
skin than from wavelengths that do not damage people's skin. The
weighting function is called the McKinlay-Diffey Erythema action
spectrum. For illustration purposes only (these numbers are not
correct), assume 290nm radiation causes three times as much damage
as 350nm radiation and five times as much damage as 400 nm
radiation. Then, if in some unit 290nm UV radiation did 15 units
of damage, 350nm radiation would do 5 units and 400 nm radiation
would do 3 units. At each wavelength, multiply the actual incoming
radiation level by the weighting:
These weighted irradiances are
summed up, or integrated, over the 290 to 400 nm range resulting
in a value representing the total effect a given day's UV
radiation will have on skin. For our example, the total is 400.
These estimates are then adjusted for the effects of elevation and
clouds. UV at the surface increases about 6% per kilometer above
sea level. Clear skies allow 100% of the incoming UV radiation
from the sun to reach the surface, whereas scattered clouds
transmit 89%, broken clouds transmit 73%, and overcast conditions
transmit 31%. If we assume that the example location is at 1
kilometer in elevation, and that there will be broken clouds, then
the calculation is: 400 x 1.06 x 0.73 = 309.5
Once adjusted for elevation and clouds, this value is then scaled
(divided) by a conversion factor of 25 and rounded to the nearest
whole number. This results in a number that usually ranges from 0
(where there is no sun light) to the mid teens. This value is the
UV Index. Thus, the UV Index for the example city would be: 309.5
/ 25 = 12.4, rounded to 12
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. Source: USEPA
- The pressure
exerted by the molecules of a given vapor. In meteorology,
it is considered as the part of total atmospheric pressure
due to the water vapor
content. It is independent of other gases or vapors. Vapor Trail - A cloudlike streamer or trail often seen
behind aircraft flying in clear, cold, humid air.
A vapor trail is created when the water vapor
from the engine exhaust gases are added to the atmosphere.
Also called a contrail,
for condensation trail. Visibility - A measure of the opacity of
and therefore, the greatest distance one can see prominent objects
with normal eyesight. The National Weather Service
has various terms for visibility. Surface visibility is the
prevailing visibility determined from the usual point of observation. Prevailing visibility
is considered representative of visibility conditions at the
station. Sector visibility is the visibility in a specified
direction that represents at least a 45 degree arc of the horizon
circle. Tower visibility is the prevailing visibility determined
from the airport traffic control tower (ATCT) at stations that
also report surface visibility.
Wet Bulb Temperature -
The air temperature drops after it
rains. In the summertime a thunderstorm can break the oppressive
heat of the day. As it precipitates, evaporation occurs and the
air temperature drops. The wet bulb temperature is the
temperature at which no more evaporation will occur, and thus no
further decrease in the temperature. The air will continue to
cool until the air can evaporate no more moisture. The
temperature, when the cooling continues until the evaporation
stops and the air becomes saturated, is the wet bulb temperature.
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.
Wind Chill is the term
used to describe the rate of heat loss on the human body resulting
from the combined effect of low temperature and wind. When wind
blows across the skin, it removes the insulating layer of warm air
adjacent to the skin. When all factors are the same, the faster
the wind blows, the greater the heat loss, which results in a
colder feeling. As winds increase, heat is carried away from the
body at a faster rate, driving down both the skin temperature and
eventually the internal body temperature.
While exposure to low wind chills can be life threatening to both
humans and animals alike, the only effect that wind chill has on
inanimate objects, such as vehicles, is that it shortens the time
that it takes the object to cool to the actual air temperature (it
cannot cool the object down below that temperature).
Wind Direction and Speed
- A forecast wind (direction and speed)
is included in the first three periods
of the forecast. The wind is included in
the fourth period if considered
Wind Chill Temperature is only defined
for temperatures at or below 50 degrees
F and wind speeds above 3 mph. Bright
sunshine may increase the wind chill
temperature by 10 to 18 degrees F.
1. Wind direction is the
direction where the wind is coming FROM
and is based on an 8-point compass (NE,
E, SE, etc.). Light wind (usually 5 mph
or less) will be handled in the
SOUTH WINDS (if direction is known),
LIGHT AND VARIABLE WINDS , or
LIGHT WINDS (where "light" implies a
variable wind direction).
speed will be given in miles per hour.
Following is a list of terms sometimes
used to describe the wind speed.
light or light and variable
used or breezy for mild weather
brisk for cold weather
breezy (mild weather), brisk
mph or greater
strong, damaging, dangerous,
A forecast can contain a peak wind speed
in gusty situations. For example,
"NORTHWEST WIND 20 TO 30 MPH WITH
OCCASIONAL GUSTS TO 40 MPH.")
- A sudden higher change of
wind speed outside the normal speed at the time. (Example: Wind
currently at 5 miles per hour suddenly gusts up to 12 miles per
hour and either settles down to zero or a lower average speed).
Wind run is the
total distance that the wind has traveled during a period of
time in a particular direction. Wind run has a direction and a
- Sensor for wind direction measurement.
- The portion of the
electromagnetic spectrum that has a very short
length. It has a wave length longer than gamma rays, yet
light. X-rays can penetrate various thicknesses of all
solids, and when absorbed by a gas, can result in ionization.
that is given golden, or yellow, appearance by the presence of
pine or cypress pollen in it.
The position of the sun during the course of the year as it
appears to move though successive constellations. Also, the band
where the ecliptic
runs centrally through the celestial sphere and contains the sun, the moon, and all the
planets except Venus and Pluto.
- One of several names for the twenty-four hour time which is
used throughout the scientific and military communities.
Related terms: Universal
Time Coordinate (UTC) and Greenwich
Mean Time (GMT).
was compiled by WXWP-NJ
- Top -