WeatherHut.com’s Weather Glossary
Contents
Our weather glossary will help you understand many of the terms used in
weather monitoring and forecasting.
Humidity
Humidity measures the amount of water vapor (water in gas form) in the atmosphere.
It is often written as a percentage (see relative humidity).
Most of the water vapor in the air comes from evaporation of water from
the sea and land. Evaporation is the name of the process for liquid water
becoming water vapor. The opposite process, water vapor coming liquid water,
is called condensation.
Water vapor in the atmosphere has many important effects on weather. It is
the source of all condensation, producing clouds and fogs, and rain. Water
vapor helps warm the air by absorbing energy from the sun and is source
of energy for storms.
Relative Humidity
Relative humidity measures the water vapor (water in gas form) content of
the atmosphere relative to the content at saturation. Saturated air has
a relative humidity of 100%. Near the Earth’s surface, relative humidity
is usually above 30%.
The water vapor saturation point is very sensitive to temperature. At 86°F (30°C),
a volume of air can contain up to 3% water vapor before the relative humidity reaches
100%. At 68°F (20°C), the same volume of air can contain less than 2% water vapor
before the relative humidity reaches 100%. Relative humidity can reach 100% in three
different ways in the atmosphere. The most common way is when air rising cools by rising
up a mountain side or mixing with a cooler mass of air (see air mass). Two air masses
at different temperatures, each with a relative humidity less than 100%, mixed together
can have a relative humidity of 100%. Relative humidity can also reach 100% by
evaporation of more water into the air.
Water vapor content in the atmosphere is driven by evaporation and condensation.
When the relative humidity is below 100%, evaporation occurs more often than
condensation. When relative humidity exceeds 100%, when a rising mass of air cools,
for example, condensation occurs more easily than evaporation. In the atmosphere,
condensation occurs on tiny particles in the air such as smog, dust or ice crystals.
These are called nucleation sites.
Related Websites:
Water Vapor Myths: A brief Tutorial
http://fermi.jhuapl.edu/people/babin/vapor/index.html
Relative humidity calculator
http://members.nuvox.net/~on.jwclymer/rh.html
Water Vapor
http://en.wikipedia.org/wiki/Water_vapor
Evaporation
Evaporation is the name of the process for liquid water becoming water vapor.
The opposite process, water vapor coming liquid water, is called condensation.
Condensation
Condensation is the name of the process for water vapor, or water in gas form,
becoming liquid water. The opposite process, liquid water becoming water vapor
is called evaporation.
Dew Point Temperature
Photo Credit: U.S. Fish and Wildlife Service
The dew point is the temperature where objects, such as a blade of grass or
window pane, will become wet with dew from the air. When the dew point falls below
the freezing point of water (32 °F, 0 °C), the dew point is known as the frost point.
Dew is the water droplets that formed on cold surfaces exposed to the air. This
process is driven by the humidity of the air, the temperature of the air and the
temperature of the surface. When a surface is at, or below, the dew point temperature,
droplets of dew can appear.
Dew often occurs on clear still nights and can form on most surfaces such as plants,
decks, grass and windows. Dew can harm plants because it may stimulate the growth of
harmful fungi. It can also be a nuisance to astronomers where dew can hinder viewing or
damage expensive equipment. Dew may be a hazard in the morning making pathways or decks slippery.
On clear nights outdoor surfaces will radiate energy into space and can become cooler
than the air. Microscopic droplets of water vapor from the air are continually condensing
onto surfaces then evaporating away. It becomes harder for the water condensed on the
surface to evaporate when the surface temperature cools, because the cold surface cools
the nearby air. As the air temperature drops, the relative humidity increases. As long
as the humidity near the surface stays below 100%, any condensing water will very quickly
evaporate again. If the surface is cold enough, the dew point temperature, the local
relative humidity can reach 100% or more. Then, water will condense on the surface much
more quickly than it evaporates and water droplets will build up on the surface.
Related Websites:
What Exactly is the Dew Point?
http://weathersavvy.com/Q-dew_point1.html
Dew Point in Wikipedia
http://en.wikipedia.org/wiki/Dew_point
Preventing dew forming on telescopes
http://www.telescopes.com/extra-info/fightingdew.html
Wind Chill Temperature
The wind chill temperature measures the cooling power of wind on bare skin. It is
an indication of how cold animals and people feel outside. The wind chill temperature
is always equal to (in still air) or below the air temperature. Wind chill temperature
has no effect on inanimate objects such as radiators and water pipes. A water pipe will
be at air temperature (unless it has hot or cold water flowing through it).
Metabolism maintains our internal temperature at around 98.6 °F. We are most comfortable
when the air temperature is between 60 and 80 °F. In this environment our skin temperature
is about 90 °F because heat from our body escapes into the environment cooling our skin.
If wind is blowing past our skin, more heat escapes into the environment reducing the skin
temperature further and we feel colder. The wind chill temperature is a measure of how much
colder we feel when the wind is blowing. It is the temperature, at some standard wind speed
(typically walking speed), that would produce the same amount heat loss through the skin as
a given air temperature and wind speed. For example, if the air temperature is 40 °F and
there is a 5 mph wind blowing into your face, the air temperature would feel more like 36 °F
on a windless day. If the wind increased to 25 mph, the air temperature would feel more
similar to 29 °F on a windless day.
Wind chill temperature ignores the warming effect of sunshine, which helps to reduce
the cooling effect of the wind. Sunshine can add 10 to 18 °F to the wind chill temperature
on a sunny day.
Related Websites:
National Weather Service Wind Chill Temperature Index
http://www.weather.gov/om/windchill/
Wind Chill in Wikipedia
http://en.wikipedia.org/wiki/Wind_chill
Wind Chill and Humidex: The ridiculous got a temperature
http://ptaff.ca/humidex/?lang=en_CA
Barometric Pressure or Atmospheric Pressure
Barometric, or atmospheric, pressure measures the weight of air in the atmosphere above as a pressure.
The term barometric pressure comes from the instrument once commonly used for measuring
atmospheric pressure: a barometer.
Atmospheric pressure is greatest at sea level, when the column of air above is largest, and
decreases with rising altitude. Pressure is commonly measured in millimeters
of mercury (mm-Hg), hecto-Pascals (hPa) or inches of mercury. At sea level, the average
pressure is about 760 mm-Hg, 1013 hPa or 30 in-Hg.
Changes in atmospheric pressure are one of the most commonly used ways to forecast changes
in the weather because weather patterns are carried around in regions of high and low pressure
(see Air Mass). A slowly rising atmospheric pressure,
over a week or two, typically indicates settled weather that will last a long time. A sudden
drop in atmospheric pressure over a few hours often forecasts an approaching storm, which will
not last long, with heavy rain and strong winds. Pioneering meteorologist Vice-Admiral Robert Fitzroy once
noted: “long foretold, long last; short notice, soon past”.
By carefully watching the pressure on a barometer, you can forecast local weather using
these simple guidelines:
- Decreasing barometric pressure indicates storms, rain and windy weather.
- Rising barometric pressure indicates fair, dry, and colder weather.
- Slow, regular and moderate falls in pressure suggest a low pressure area is passing some distance away.
Marked changes in the weather are unlikely.
- Small rapid decreases in pressure indicate a nearby change in weather. They are usually followed by
short lasting wind and showers.
- A quick drop in pressure over a short time indicates a storm is likely in 5 to 6 hours.
- Large, slow and sustained decreasing pressure forecasts a long period of poor weather. The
weather will be more pronounced if the pressure started rising before it began to drop.
- A rapid rise in pressure, during fair weather and average, or above average pressure,
indicates a low pressure cell is approaching. The pressure will soon decrease forecasting poorer weather.
- Quickly rising pressure, when the pressure is low, indicates a short period of fair weather is likely.
- A large, slow and sustained rise in pressure forecasts a long period of good weather is on its way.
For pressure tendency, the definitions used by these guidelines are:
| Term |
Pressure change
over 3 hours |
hPa |
in-Hg |
mm-Hg |
| Steady |
Less than |
0.1 |
0.003 |
0.08 |
| Slowly rising or falling |
|
0.15 to 1.5 |
0.003 to 0.04 |
0.08 to 1.1 |
| Rising or falling |
|
1.6 to 3.5 |
0.05 to 0.1 |
1.2 to 2.6 |
| Quickly rising or falling |
|
3.6 to 6.0 |
0.1 to 0.18 |
2.7 to 4.5 |
| Rapidly rising or falling |
More than |
6.0 |
0.18 |
4.5 |
Guidelines and table adapted from:
Weather Doctor’s Weather Eyes.
Related Websites:
Wikipedia article on Atmospheric Pressure
http://en.wikipedia.org/wiki/Atmospheric_pressure
Extreme High Altitude Conditions Calculator
http://bpesoft.com/s/wleizero/xhac/?M=p
Weather Doctor’s Weather Eyes
http://www.islandnet.com/~see/weather/eyes/barometer3.htm
Air Mass
An air mass is a large volume of air, which may cover many hundreds
or thousands of square miles. They are shown on weather maps as zones
of high or low pressure (see high pressure air mass and
low pressure air mass). The temperature, atmospheric or barometric
pressure and water vapor content are fairly uniform across an air-mass,
though all three change with altitude.
There are about 50 air-masses scattered across the surface of the planet
at any one time dominating our weather. Air masses are separated by narrow
transition zones called fronts. Sometimes these transition zones are subtle,
revealed only by a shift in wind direction. Other times the transitions
are tumultuous, with conflicts between hot and cold air masses producing
heavy rain, thunderstorms or snow.
High Pressure Air Mass
Zones of high pressure, or anticyclones, are formed by a sinking column of air
at the centre of an air mass. They typically provide fine weather, with no rain
and clear skies and can remain in place for many weeks. In the summer-time, high
pressure zones provide hot, sunny days. In the winter, high pressure zones
produce cooler weather, fog and frosts.
The high pressure air forms when descending air reaches the surface. If the air
is descending more quickly than it can flow away, the falling mass of air
accumulates. The larger mass of air increases the local atmospheric pressure
(see barometric pressure).
Clouds usually do not form in regions of high pressure because the descending
mass of air gets warmer as it sinks. This is because warming the air decreases
its relative humidity (see relative humidity)
making it less likely relative humidity will reach 100%. Clouds will not
form until relative humidity reaches 100%.
Low Pressure Air Mass
Zones of low pressure are formed by a rising column of air at the centre
of an air mass. They typically produce cloudy, wet weather.
The low pressure air forms when air is ascending more quickly than
it can be replaced by air flowing in from surrounding areas. This flow
reduces the mass of air in an area reducing the local barometric pressure
(see barometric pressure).
Low pressure zones are usually accompanied by cloudy wet weather. Air in
a low pressure zone cools as it rises because the gas expands as altitude
increases. Warm air has a lower relative humidity
(the ratio of water content to saturation point) than cooler air. So,
as the air rises and cools, humidity will
increase. If the air temperature reaches the dew point temperature,
the point when humidity is 100%, droplets of water will begin to form on
tiny particles in the air such as smog, dust or ice crystals.
These are called nucleation sites. When droplets of water condense in the
air over a large enough area they can be seen from the ground as clouds.
If there is a large supply of water vapor, some of it comes back to
earth as rain.
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