NOAA FORECASTING ACTIVITY
WEATHER SYMBOLS
A. Why Are Weather Symbols Used?
Some typical surface station plots display cloud
cover, temperature, dewpoint, wind speed and
direction, pressure and weather. The plot at the
lower right indicates light snow as xx.
A meteorologist must convey a lot of information
without using a lot of words. When looking at a weather
map, a meteorologist needs to know where the cold air
is, where the warm air is, where it is raining, what type
of clouds are in the area, and many more things. The reason for this is that forecasts need to be
accurate. But, they also need to be timely.
If too much time is spent making the forecast, it will be late. Not many people want to know
what the weather was doing twenty minutes ago. Most people want to know what the weather is
going to do in the near future. Because of this, weather symbols were invented so that weather
maps could be looked at in a short amount of time.
There are a large number of weather symbols in existence. Some are used for weather like rain,
snow, and lightning. There are also symbols representing the speed of the wind, types of clouds,
air temperature, and air pressure. All of these symbols help meteorologists forecast the weather
in a timely manner.
B. What Do Some Weather Symbols Look Like?
The following image shows many different weather ymbols. Look at the symbols, do they look
like the weather they represent?
Graphic From Purdue University
PROJECT CLOUD COVER
The symbol highlighted in yellow indicates the amount of cloud cover observed at the
time the observation is taken. In this case, broken clouds were reported.
The chart below spans the entire spectrum of cloud cover reports, from clear to
overcast skies.
PROJECT WIND SPEED SYMBOLS
Observed Winds
represented by wind barbs
The symbol highlighted in yellow (in the diagram above) is known as a "Wind
Barb". The wind barb indicates the wind direction and wind speed.
Wind barbs point in the direction "from" which the wind is blowing. In the case of the
diagram below, the orientation of the wind barb indicates winds from the Northeast.
The term easterly means that the winds are from the east. In the example above, the
winds are out of the northeast, or northeasterly. On the other hand, the term
"eastward" means that the winds are blowing towards the east.
Wind speed is given here in the units of "knots" (knt). A "Knot" is a nautical mile per
hour.
1 Knot = 1.15 Miles Per Hour (MPH)
1 Knot = 1.9 Kilometers Per Hour (KM/HR)
Each short barb represents 5 knots, each long barb 10 knots. A long barb and a short
barb is 15 knots, simply by adding the value of each barb together (10 knots + 5 knots
= 15 knots). If only a station circle is plotted, the winds are calm.
Pennants are 50 knots. Therefore, the last wind example in the chart below has a wind
speed of 65 knots. (50 knots + 10 knots + 5 knots).
PROJECT ISOBARS
Isobars
lines of constant pressure
A line drawn on a weather map connecting points of equal pressure is called an isobar.
The isobars are generated from mean sea level pressure reports and the pressure
values are given in millibars.
The diagram below depicts a pair of isobars. At every point along the top isobar, the
pressure is 996 mb and at every point along the bottom isobar the pressure is 1000
mb.
Any point in between these two isobars will have a pressure somewhere between 996
mb and 1000 mb. Point A, for example, has a pressure of 998 mb and is therefore
located between the 996 mb isobar and the 1000 mb isobar.
Sea level pressure reports are available every hour, which means that maps of isobars
are likewise available every hour. The solid blue contours are isobars and the numbers
along particular contours indicate the pressure value of the isobar.
Surface maps of isobars are useful for locating areas of high and low pressure, which
correspond to the positions of surface cyclones and anticyclones. A map of isobars is
also useful for locating strong pressure gradients which are identifiable by a tight
packing of the isobars. Stronger winds are associated with larger gradients in pressure.
WEATHER MAPS
A. The History Behind Weather Maps
Weather maps can display current observations and numerical model output. Meteorologists
frequently access this data on their computer workstations.
The weather map was not developed until the telegraph was working. Back in the 1870's, the
first weather map came to be. At that time, the government meteorologists worked for the US
Army Signal Service. Later, this department became the Weather Bureau and now it is known as
the National Weather Service.
In order to make a weather map, stations from around the country would take observations.
Meteorologists recorded temperature, wind speed and direction, and pressure. This data was
then sent to one location, Washington, DC, where it was analyzed by hand.
Because a large part of the US was not well populated in the late 1800s and early 1900s, the
weather maps of the day were missing observations from most of the central Plains states. Also,
meteorologists did not always understand what the maps they were analyzing meant. Many of
the theories used today in forecasting had not yet been developed.
B. How Do Weather Maps Help Meteorologists?
The weather map is the most valuable tool that the meteorologist uses to forecast the weather.
Without this tool, it would be very hard to predict what the weather was going to do. Weather
maps summarize what is happening in the atmosphere at a certain time, and it would be very
difficult to predict changes in the weather without these maps.
By looking at weather maps from different heights in the atmosphere, a meteorologist can make
a three-dimensional picture, in their heads, of what is happening in the atmosphere. They can
tell whether a particular area has high or low pressure, whether it may rain, and many other
things just by looking at a weather map.
Today, weather observations are taken from thousands of locations. This data is then sent to
Washington, DC where it is analyzed. Computers are the main tool creating weather maps since
they can handle large amounts of data quickly. Forecasts have become much more accurate
since meteorologists have better weather maps than in the past.
WEATHER SYSTEMS
A. Why Do They Exist?
Infrared satellite image of North America
Our spinning planet is covered with moving air. Some of this air is over the ocean and some is
over the land. The ground gets warm faster than water, but also gets cold faster than water.
That is why it is normally colder over land in the winter, and warmer over land in the summer.
When the ocean is warmed by the sun, the water absorbs heat and warms the air above it.
At the same time, the land can become cold and cool the air above it. Because of heating and
cooling, air can move up and down. When a pocket of air gets too warm, it rises until it is the
same temperature as the surrounding air. Just the opposite happens when a pocket of air gets
cooled, it sinks. The combination of warm and cold areas, and rising and sinking air results in
what we call weather systems.
B. How Do They Move?
Areas of high pressure, called Highs, have air that sinks. This air tends to be cold and produces
clear skies and fair weather. Areas of low pressure, called Lows, have warm air that rises. They
can cause clouds to form, rain to fall, and storms to occur. Weather systems in the United States
and within the mid-latitudes, between 30° N and 60° N and between 30° S and 60° S, usually
move from West to East. Weather systems in other latitudes move from East to West.
WEBSITE LINKS
Weather Graphics: http://ww2010.atmos.uiuc.edu/(Gh)/wx/surface.rxml#obs
Daily Weather Map: http://www.hpc.ncep.noaa.gov/dwm/dwm.shtml
Satellite Images: http://www.goes.noaa.gov/index.html
Wind Chill: http://www.nws.noaa.gov/om/windchill/index.shtml