Weather Terms and Definitions
♦ Advisories ♦ Apparent Temperature ♦ Barometric Pressure ♦ Cloud Height ♦ Dew Point♦ Fire Weather Index ♦ Heat Index ♦ Humidity ♦ Weather Station ♦ Wind Chill
♦ Wind Direction / Wind Vanes ♦ Connecticut Records ♦ Further Reading ♦ Emergency Preparations
ADVISORIES
Watch
A watch is used when the risk of a hazardous weather or hydrologic event has increased significantly, but its
occurrence, location, and/or timing is still uncertain. It is intended to provide enough lead time so
that those who need to set their plans in motion can do so.
Advisory
Highlights special weather conditions that are less serious than a warning. They are for events that
may cause significant inconvenience, and if caution is not exercised, it could lead to situations that may threaten
life and/or property.
Warning
A warning is issued when a hazardous weather or hydrologic event is occurring, is imminent, or has a very high probability
of occurring. A warning is used for conditions posing a threat to life or property.
APPARENT TEMPERATURE
The apparent temperature is the perceived temperature (in degrees Fahrenheit) derived from using either a combination of temperature and wind (Wind Chill) or temperature and humidity (Heat Index) at a particular point in time. When the temperature falls to 50° F. or less, wind chill is used for the Apparent Temperature. When the temperature rises above 80° F., the heat index is used for Apparent Temperature. Between 51° and 80° F., the Apparent Temperature is the ambient air temperature.
BAROMETRIC PRESSURE
Barometric pressure, also referred to as atmospheric pressure, measures the weight of the atmosphere above us. A rising pressure indicates cooling temperatures and cloudless skies while a falling pressure indicates deteriorating weather conditions. Since barometric pressure varies according to altitude, weather forecasters adjust their barometric pressure readings for sea level so that interpretations of the data are standard.
CLOUD HEIGHT
Actual cloud height measurements are made using a device called a ceilometer which fires a laser signal into the sky
and measures the time it takes for some of the light to be reflected back to earth. Since ceilometers
tend to be beyond the financial reach of weather hobbyists, a close approximation of cloud height can be determined
by a formula using temperature and dew point readings:
Cloud Base Altitude = ((((temperature - dew point) / 4.5) * 1000) + weather station's
altitude),
where temperature and dew point are in Fahrenheit degrees and the weather station's altitude is
in feet above sea level. The resulting Cloud Height is in feet.
Example: Temperature=53.1°; Dew Point=46.0°; Altitude=85.3feet
Cloud Height = ((((53.1 - 46.0) / 4.5) * 1000) + 85.3)
= ((((7.1) / 4.5) * 1000) + 85.3)
= (((1.5777) * 1000) + 85.3)
= (1577.7 + 85.3)
= 1663.0 ft
DEW POINT
Dew point is the temperature at which dew will form. The higher the dew point value, the higher the
moisture content of the air at a given temperature. Dew point temperature is defined as the temperature
to which the air would have to cool (at constant pressure and constant water vapor content) in order to reach
saturation. A state of saturation exists when the air is holding the maximum amount of water vapor
possible at the existing temperature and pressure.
When the dew point temperature and air temperature are equal, the air is said to be saturated. Dew point
temperature is NEVER GREATER than the air temperature. Therefore, if the air cools, moisture must be
removed from the air and this is accomplished through condensation. This process results in the
formation of tiny water droplets that can lead to the development of fog, frost, clouds, or even precipitation.
For more information, go to
http://simple.wikipedia.org/wiki/Dew_point
FIRE WEATHER INDEX
The Fire Weather Index is a measure of six standard weather components that provide numerical ratings of
relative wildland fire potential.
The first three components are fuel moisture codes that follow daily changes in the
moisture contents of three classes of forest fuel:
Fine Fuel Moisture Code (FFMC) - A numerical rating of the moisture content of litter and other cured
fine combustible materials. This code is an indicator of the relative ease of ignition and flammability of fine fuel.
Duff Moisture Code (DMC) - A numerical rating of the average moisture content of loosely compacted
organic layers of moderate depth. This code gives an indication of fuel consumption in moderate duff layers and
medium-size woody material.
Drought Code (DC) - A numerical rating of the average moisture content of deep, compact, organic
layers, such as decaying leaves and branches covering a forest floor. This code is a useful indicator of seasonal
drought effects on forest fuels, and amount of smouldering in deep duff layers and large logs.
The final three components are fire behaviour indexes, representing rate of spread, amount of available fuel, and fire intensity;
their values increase as fire weather severity worsens:
Initial Spread Index (ISI) - A numerical rating of the expected rate of fire spread. It combines
the effects of wind and the FFMC on the rate of spread without the influence of variable quantities of fuel.
Buildup Index (BI) - A numerical rating of the total amount of fuel available for combustion that combines
the DMC and the DC.
Fire Weather Index (FWI) - Information from the ISI and BUI is combined to provide a numerical rating of
fire intensity – the Fire Weather Index. The FWI indicates the likely intensity of a fire.
HEAT INDEX
The Heat Index is a measure of how hot it really feels when relative humidity is factored in with the actual
air temperature.
The chart below depicts the apparent temperature when the relative humidity and actual temperature are
known, i.e., if the temperature is 96° and the relative humidity is 60%, the temperature will feel like
a dangerous 116°.
HUMIDITY
Humidity, often referred to as relative humidity, refers to the amount of water vapor in the air relative to
the temperature. Since humidity affects how we feel in our daily lives, humidity becomes an important factor
in daily weather forecasts. On a hot, humid day, if the humidity is close to 100%, the sweat on our bodies
will not evaporate, which makes feel even hotter. On a cool, dry day, with the humidity low, the moisture
from our body tends to evaporate more easily. Relative humidity, temperature and dew point all work very
closely together when forecasting weather.
For more information, go to
http://simple.wikipedia.org/wiki/Humidity
WEATHER STATION
The weather station here at AF1HS has thermometers for measuring outside and inside temperatures, a barometer for
measuring any changes in barometric pressure, a hygrometer for measuring humidity levels, an anemometer for measuring
both wind speed and wind direction, and last but not least, a rain guage for measuirng daily rainfall. All of these
measurements are compiled at the computer running Weather Display software and forwarded to the web server
using WeatherDisplay Live software. This allows viewers to view live weather data from the
weatherstation over the web in nearly real time. Data from the daily readings are stored in log files
allowing viewers to "look back" in time to weather history at the station.
For more information, go to
http://www.weather-display.com/index.php
http://en.wikipedia.org/wiki/Temperature
http://en.wikipedia.org/wiki/Barometer
http://en.wikipedia.org/wiki/Hygrometer
http://en.wikipedia.org/wiki/Anenometer
http://en.wikipedia.org/wiki/Rain_gauge
WIND CHILL
Wind chill describes how cold people feel when cold air is pushed by wind currents. It is related to the
rate of heat loss from skin exposed to the combined wind and cold temperatures. As wind velocities increase,
heat is drawn from the body which lowers the internal body temperature. This can result in varying levels
of frostbite and hypothermia.
The chart below indicates the effect of wind speed and temperature and the resulting wind chill that effects the body.
As an example, if the temperature is -15° and the wind is blowing at 25 mph, the wind chill temperature feels like
-44° and you can expect frostbite to set in within 10 minutes.
For more information, go to
http://en.wikipedia.org/wiki/Wind_chill
WIND DIRECTION / WIND VANES
Knowing the direction of the wind is an important part of predicting weather because wind brings us our weather. A wind vane, also called a weather vane, is a tool for measuring wind direction and was probably one of the first weather instruments ever used. To determine wind direction, a wind vane spins and points in the direction from which the wind is coming and generally has two parts, or ends: one that is usually shaped like an arrow and turns into the wind and one end that is wider so that it catches the breeze. The arrow will point to the direction the wind is blowing from so if it is pointing to the east, it means the wind is coming from the east. Therefore a west wind is blowing from the west.
CONNECTICUT RECORDS
High: 106° on July 15, 1995 in Danbury
Low: -32° on February 16, 1943 in Falls Village
All-time highest temperature on record in Hartford: 103° on July 22, 2011
24-hour Rainfall: 12.77" on August 19, 1955 in Burlington
Maximum Annual Rainfall: 78.53" in 1955 in Burlington
Minimum Annual Rainfall: 23.60" in 1965 in Baltic
All-Time 24-hour and Greatest Single Snowstorm Record in Hartford: 24.0" on January 11-12, 2011
All-Time 24-hour Snowfall Record in New Haven: 28.5" on January 11-12, 2011
Snowiest month on record in Hartford: 57.0" in January, 2011
Snowiest month on record in New Haven: 56.6" in January, 2011
For Fun Facts about CT weather, go to
http://www.ct.gov/dep/cwp/view.asp?a=2688&q=322366
FURTHER READING
National Weather Service Glossary
NWS Weather Website for Kids
Federal Meterological Handbook No. 3
USA TODAY Weather Graphic Index
Weather and Climate Basics
