Air Masses

uniform bodies of air

Sometimes the reported values (black numbers) are not always located between the correct contours. For example, in the dew point temperature map below, a station in Minnesota (circled in blue) reported a dew point temperature of 52 degrees, but it is located between the 60 and 65 degree contours.

Why is this so? Because contours are plotted to provide a "best-fit" for all reports, which include a very large number of stations. To give you an idea, I've increased the number of reporting stations in the image below.

This is the exact same dew point map, but with many more station reports (black numbers). And this still doesn't include all of them!

In order for every station to be within the correct contours, lines would be zigging and zagging everywhere, making the map unreadable. Fortunately, contour lines are smoothed to make the map readable.

Therefore, it is important to remember that although the contours may not be 100% accurate for every single reporting station, contouring provides READABLE information, as accurately as possible, for a HUGE number of reporting stations.

Back to the top

When the height contours bend strongly to the south, (as in the diagram below), it is called a TROUGH. Strong troughs are typically preceded by stormy weather and colder air at the surface. Below is an example of a trough in an upper-level height field (red contours). The trough axis is denoted by the purple line.

When the height contours bend strongly to the north (as in the diagram below), this is known as a RIDGE. Strong ridges are accompanied by warm and dry weather conditions at the surface. Below is an example of a ridge in an upper-level height field (red contours). The purple line denotes the ridge axis.

The jet stream is a current of fast moving air found in the upper levels of the atmosphere. This rapid current is typically thousands of kilometers long, a few hundred kilometers wide, and only a few kilometers thick. Jet streams are usually found somewhere between 10-15 km (6-9 miles) above the earth's surface. The position of this upper-level jet stream denotes the location of the strongest SURFACE temperature contrast (as in the diagram below).

During the winter months, Arctic and tropical air masses create a stronger surface temperature contrast resulting in a strong jet stream. However, during the summer months, when the surface temperature variation is less dramatic, the winds of the jet are weaker.

Below is an ETA Model forecast panel for 300 mb winds and geopotential heights (white contours). The color filled regions indicate wind speed in knots and is color coded according to the legend at the bottom of the image. The shades of blue indicate winds less than 60 knots, while winds greater than 120 knots are given in shades of red.

The yellow, green and red ribbon on the image above represents the jet stream, and along the East Coast, the region of strongest winds (shaded in red) is a jet streak.

Back to the top

The word "Radar" stands for "Radio Detection and Ranging". Radar images are very helpful in finding precipitation. As an X- ray machine examines the inside of a human body , a radar examines the inside of a cloud .

A radar sends out signals into the atmosphere and if any precipitation is present, the radar signal is reflected back to the radar transmitter.

These returned signals ,called "radar echoes", are used to produce the radar image you see below .

As you can see , these images can be quite colorful.

Back to the top

Cell movement  data depicts in which direction and at what speed individual thunderstorm cells are moving .

 Back to the top

Echo Tops :

DESCRIPTION: The Echo Tops product is approximately the elevation where the top of a given precipitation core is located. Echo top data is indicated by the small white numbers located near some of the radar echoes .These numbers are estimates in feet of the highest cloud tops associated with the radar echoes.

   Back to the top

Precipitation Type  and Tendencies

The type of weather associated with a cluster of radar echoes is identified by letter abbreviations. A table of the common weather symbols has been given below:

These abbreviations appear as small white letters. For example, RW was reported in eastern Texas, which indicates "rain showers".

However, many of these reports are combinations of two or more abbreviations as in South Carolina, where "TRW" was near the yellow and green radar echoes. "TRW" is a combination of "T" (thunderstorms), "RW" (rain showers) and actually means showers and thunderstorms. Further west in northern Georgia, a "TRW++" was reported near the pink and red radar echoes, which indicates the presence of showers and severe thunderstorms in the area. The extra "+" is there to emphasize that these are very intense storms.

Precipitation tendency indicates a change of the intensity of precipitation during the last time period. This information is represented by either a white "NC", "+", or a "-". The "NC" reported in the panhandle of Florida, indicates that the intensity of the precipitation during the past hour has "Not Changed".

In Georgia a "-" reported, which means that the intensity of precipitation has decreased. This makes sense because the line of heavier precipitation (indicated by the yellow, pink and red radar echoes), was moving out of Georgia and into South Carolina. This also explains why there was a "+" indicated in South Carolina, where the intensity of the precipitation is on the increase, due to the heavier showers and thunderstorms approaching from the west.

Back to the top