Beginning a new series today on how to read those confusing weather maps I put up every day – hope this series proves to be beneficial to you.


Temperature Contours

This surface meteorological chart shows the temperature pattern (in degrees Fahrenheit) over the continental United Sates and is updated every hour.

Surface temperature reported at each station are contoured every five degrees Fahrenheit. Areas of warm and hot temperatures are depicted by orange and red colors and cold temperatures (below freezing) are shaded blue and purple. Areas of sharp temperature gradients (several contours close to each other) tend to be associated with the position of surface fronts. Fronts separate air-masses of different temperature and moisture (and therefore density) characteristics.

Clicking on this map will show the current contours.


Interpreting Surface Observation Symbols
a quick overview

Temperature:The value highlighted in yellow located in the upper left corner is the temperature in degrees Fahrenheit. In this example, the reported temperature is 64 degrees.
Weather Symbol:The weather symbol highlighted in yellow indicates the type of weather occurring at the time the observation is taken. In this case, fog was reported. If there were thunderstorms occurring when the observation was taken, then the symbol for thunderstorms would have appeared instead.
Dew Point Temperature:The value highlighted in yellow located in the lower left corner is the dew point temperature in degrees Fahrenheit. In this example, the reported dew point temperature is 58 degrees.
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.
Sea Level Pressure:The value highlighted in yellow located in the upper right corner represents the last three digits of the sea level pressure reading in millibars (mb).
Wind Barb:The symbol highlighted in yellow is known as a wind barb. The wind barb indicates wind direction and wind speed.

Wind barbs always point in the direction the wind is blowing “from”. As is the case of the diagram below, the orientation of the wind barb indicates winds from the Northeast.

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 (mi/hr)

1 Knot = 1.9 Kilometers per Hour (km/hr)

Using the table below, each short barb represents 5 knots, while each long barb represents 10 knots. A single long barb and a short barb is 15 knots, simply by adding the value of each barb together (10 knots + 5 knots = 15 knots).

Pennants are 50 knots. Therefore, the last wind barb in the chart below has a wind speed of 65 knots. (50 knots + 10 knots + 5 knots). If only a station circle is plotted, the winds are calm.

 


Interpreting Upper Level Observation Symbols
a quick overview

Temperature:The value highlighted in yellow located in the upper left corner is the temperature in degrees Celsius. In this example, the reported temperature is -14 degrees.
Dew Point Temperature:The value highlighted in yellow located in the lower left corner is the dew point temperature in degrees Celsius. In this example, the reported dew point temperature is -34 degrees.
Geopotential Height:The value highlighted in yellow located in the upper right corner represents the geopotential height in meters (m).
Wind Barb:The symbol highlighted in yellow is known as a wind barb. The wind barb indicates wind direction and wind speed.

Geopotential Height

Geopotential height approximates the actual height of a pressure surface above mean sea-level. Therefore, a geopotential height observation represents the height of the pressure surface on which the observation was taken.

Since cold air is more dense than warm air, it causes pressure surfaces to be lower in colder air masses, while less dense, warmer air allows the pressure surfaces to be higher. Thus, heights are lower in cold air masses, and higher in warm air masses.

A line drawn on a weather map connecting points of equal height (in meters) is called a height contour. That means, at every point along a given contour, the values of geopotential height are the same. An image depicting the geopotential height field is given below.

Source: http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cyc/upa/trgh.rxml

500mb Geopotential Height
Geopotential Heights
Figure 1
Geopotential Height Anomalies

Geopotential height anomalies consist of deviations in the geopotential height field from average values. In Figure 2 below, (which displays the geopotential height anomalies from Figure 1), it is evident that areas with lower geopotential heights correlate with negative geopotential height anomalies. The height anomalies map below indicates that the geopotential heights are much below average for this time of year over the eastern United States, and implies colder than average temperatures across this region.

 

500mb Geopotential Height Anomalies
Geopotential Heights
Figure 2


Units of Temperature
from fahrenheit to celsius to kelvin and back
Degrees Fahrenheit, (developed in the early 1700’s by G. Daniel Fahrenheit), are used to record surface temperature measurements by meteorologists in the United States. However, since most of the rest of the world uses degrees Celsius (developed in the 18th Century), it is important to be able to convert from units of degrees Fahrenheit to degrees Celsius:

Kelvin is another unit of temperature that is very handy for many scientific calculations, since it begins at absolute zero, meaning it has no negative numbers. (Note…the word “degrees” is NOT used with Kelvin.) The way to convert from degrees Celsius to Kelvin is:

The three different temperature scales have been placed side-by-side in the chart below for comparison.


300 mb Winds and Heights

eta model forecast
 


Latest ETA Model 12 HR Forecast Panel
300 mb forecasted fields for geopotential height, wind speed and wind vectors. 300 mb charts depict conditions in the upper troposphere (roughly 9000 meters) where most of the weather producing phenomena occur, otherwise known as the jet stream level.

Geopotential height approximates the actual height of a pressure surface above mean sea-level and is represented by the solid white contours. The geopotential height field is given in meters with an interval of 120 meters between height lines. The 300 mb height field encircling the globe consists of a series of troughs and ridges, which are the upper air counterparts of surface cyclones and anticyclones. The distance from trough to trough (or ridge to ridge) is known as a longwave. Embedded within the longwaves are shortwaves, which are smaller disturbances often responsible for triggering surface cyclone development.

Wind vectors provide information about wind direction and wind speed and are drawn here as tiny red arrows. Wind vectors point towards the direction in which the wind is blowing and the longer the wind vector, the stronger the wind. The unit of magnitude for wind speed as depicted by the wind vectors is meters per second.

Wind speed is represented by the color filled regions and the intensity is indicated by the color code located in the lower left corner of the forecast panel. Wind speeds are given in knots with an interval of 20 knots between wind speed contours, also called isotachs. Wind speeds of less than 60 knots are represented by shades of blue while winds exceeding 120 knots are depicted in shades of red.

This information is useful in locating the jet stream, a narrow band of relatively strong winds encircling the earth in the upper troposphere. Wind speed maxima embedded within the jet stream, called jet streaks, are localized regions of high atmospheric energy that play a vital role in the development of surface low pressure centers. The closer together the height contours, the stronger the wind speed, which is why jet streaks are found where height contours are packed closely together.

Air Masses

uniform bodies of air
An air mass is a large body of air that has similar temperature and moisture properties throughout. The best source regions for air masses are large flat areas where air can be stagnant long enough to take on the characteristics of the surface below. Maritime tropical air masses (mT), for example, develop over the subtropical oceans and transport heat and moisture northward into the U.S.. In contrast, continental polar air masses (cP), which originate over the northern plains of Canada, transport colder and drier air southward.

Once an air mass moves out of its source region, it is modified as it encounters surface conditions different than those found in the source region. For example, as a polar air mass moves southward, it encounters warmer land masses and consequently, is heated by the ground below. Air masses typically clash in the middle latitudes, producing some very interesting weather.


Cold Advection

cold air moves into a warmer region
Cold advection is the process in which the wind blows from a region of cold air to a region of warmer air. The following animation depicts a very simple example of cold advection. The horizontal lines are isotherms in degrees Fahrenheit and the arrows represent wind vectors. Winds are blowing from a region of cold air to a region of warmer air, which results in cooling of the warmer region. As the cold advection persists, temperatures in the warmer region will begin to decrease as the colder air moves into the region of warmer air.


Animation by: Van Dorn
The net result of cold advection is to make a region cooler. The animation below shows (in a very general sense) how cold advection can lead to sinking motion. Cold advection is occurring in Figure A while Figure B shows a vertical cross section through the region of cold advection. In Figure B, the horizontal lines are isobars and the arrows represent wind vectors. It is important to note that Figure A is along the ground and that Figure B is from the ground up to a higher level in the atmosphere, directly above the region of cold advection.


Animation by: Van Dorn
With the onset of cold advection (Figure A), the isobar in Figure B starts to bend downward since colder air is more dense and occupies less room than warmer air. The bending of the isobar due to cold advection creates a localized area of low pressure (“L” in Figure B”), thus altering the pressure gradient force. As air moves from the regions of high pressure (“H” in Figure B) to the local region of lower pressure, air is pushed downward from above, which is the sinking motion that is caused by cold advection.


Warm Advection

warm air moves into a cooler region
Warm advection is the process in which the wind blows from a region of warm air to a region of cooler air. The following animation depicts a very simple example of warm advection. The horizontal lines are isotherms in degrees Fahrenheit and the arrows represent wind vectors. Winds are blowing from a region of warm air to a region of colder air, which results in a warming of the colder region. As the warm advection persists, temperatures in the colder region will begin to increase as the warmer air moves into the region of colder air.


Animation by: Van Dorn
The net result of warm advection is to make a region warmer. The animation below shows (in a very general sense) how warm advection can produce upward motion. Warm advection is occurring in Figure A while Figure B shows a vertical cross section through the region of warm advection. It is important to realize that Figure A is along the ground and that Figure B is from the ground up to a higher level in the atmosphere, directly over the region of warm advection.


Animation by: Van Dorn
With the onset of warm advection (Figure A), the isobar in Figure B starts to bend upward since warmer air is less dense and occupies more space than colder air. The bending of the isobar due to warm advection creates a localized area of high pressure (“H” in Figure B), thus altering the pressure gradient force. As air moves from the local region of high pressure to the regions of lower pressure (“L” in Figure B), air is drawn upward from below, which is the rising motion produced by warm advection.


850 mb Temperature Advection

an indicator of surface changes to come
Warm advection at 850 mb is often indicative of rising temperatures at the surface, while cold advection at this level often precedes falling temperatures. Regions of strongest temperature advection are found where geopotential height contours (blue) and isotherms (red) are nearly perpendicular to each other. For example, on the following 850 mb surface, the strongest cold advection is occurring from Montana to New Mexico, while the strongest warm advection is occurring from eastern Texas into Illinois.

The influence of thermal advection at 850mb is typically felt at the surface about a day later (map below). In the area under cold advection, temperatures ranged from the 20’s to 40’s, with winds generally out of the northwest bringing colder air southward from Canada.

Regions in the area of warm advection, however, were experiencing temperatures in the 60’s, providing these areas with a Spring day in early January.


Moisture Advection

along the 850 mb surface
Moisture advection is horizontal transport of moisture, which plays a very important role in the development of precipitation. If little moisture is available, it is unlikely that precipitation will form. However, if a cyclone is supplied with an abundance of moisture, there is an increased likelihood that heavy precipitation will develop. Regions of moisture advection are often co-located with regions of warm advection. For the regions of greatest moisture advection, look for areas where the geopotential height contours (blue) and isodrosotherms (dashed red) are nearly perpendicular (map below).

The greatest moisture advection was occurring from Texas into Illinois, as moist air from the Gulf of Mexico was being advected northward by southerly winds ahead of an intensifying low pressure system. This rich moisture supply was enough for showers and thunderstorms to develop as indicated by the radar echoes stretching from Ohio southward to Louisiana (below).

Notice how the precipitation was located in the region where the strongest moisture advection was occurring. Also note that the areas experiencing dry advection (the western states, which were under advection of drier air from the north) had no rainfall.


Vorticity Advection

leads to rising/falling pressures at the surface
Vorticity is the localized rotation of the air. Air that rotates counterclockwise, such as in cyclones and troughs, is said to have positive vorticity. Clockwise rotating air, such as in high pressure systems and ridges, has negative vorticity. The advection of vorticity at high levels will result in a response at the surface which will attempt to offset the effects of the advection. More specifically, vorticity advection is indicative of rising motion/falling pressures at the surface. For example, look at this 500 mb map for 12Z, October 29, 1995.

Now look at these two maps of surface pressure (solid lines) from 12Z October 29,1995 and 0Z October 30,1995.

Notice how the surface low has deepened in the area of strong vorticity advection.


Colder air masses are termed polar or arctic, while warmer air masses are deemed tropical. Continental and superior air masses are dry while maritime and monsoon air masses are moist. Weather fronts separate air masses with different density (temperature and/or moisture) characteristics.

In meteorology, an air mass is a volume of air defined by its temperature and water vapor content. Air masses cover many hundreds or thousands of square miles, and adapt to the characteristics of the surface below them. They are classified according to latitude and their continental or maritime source regions. Colder air masses are termed polar or arctic, while warmer air masses are deemed tropical. Continental and superior air masses are dry while maritime and monsoon air masses are moist. Weather fronts separate air masses with different density (temperature and/or moisture) characteristics. Once an air mass moves away from its source region, underlying vegetation and water bodies can quickly modify its character. Classification schemes tackle an air mass’ characteristics, and well as modification.

A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorological phenomena. In surface weather analyses, fronts are depicted using various colored lines and symbols, depending on the type of front. The air masses separated by a front usually differ in temperature and humidity. Cold fronts may feature narrow bands of thunderstorms and severe weather, and may on occasion be preceded by squall lines or dry lines. Warm fronts are usually preceded by stratiform precipitation and fog. The weather usually clears quickly after a front’s passage. Some fronts produce no precipitation and little cloudiness, although there is invariably a wind shift.

Cold fronts and occluded fronts generally move from west to east, while warm fronts move poleward. Because of the greater density of air in their wake, cold fronts and cold occlusions move faster than warm fronts and warm occlusions. Mountains and warm bodies of water can slow the movement of fronts. When a front becomes stationary, and the density contrast across the frontal boundary vanishes, the front can degenerate into a line which separates regions of differing wind velocity, known as a shearline. This is most common over the open ocean.

Continental Polar Air Masses

cold temperatures and little moisture
Those who live in northern portions of the United States expect cold weather during the winter months. These conditions usually result from the invasion of cold arctic air masses that originate from the snow covered regions of northern Canada. Because of the long winter nights and strong radiational cooling found in these regions, the overlying air becomes very cold and very stable. The longer this process continues, the colder the developing air mass becomes, until changing weather patterns transport the arctic air mass southward.

 

Arctic air masses move about as a shallow area of high pressure, commonly known as an “Arctic High”. Northerly winds associated with a cyclone and trailing anticyclone, (the center of the arctic air mass), transport the colder air southward. Since the terrain is generally flat and free of any significant topographical features, arctic air masses entering the United States and can easily slide all the way to Texas and Florida.

 

Below is a map of surface observations and the leading edge of a large arctic air mass blanketing much of the United States has been highlighted by the blue line. The center of this air mass is a high pressure center located in northern Montana (indicated by the blue “H”).

 

From these reports, we see that most stations in the arctic air mass generally exhibit relatively colder temperatures, with lower dew point temperatures, and winds generally out of the north. Notice that on the other side of the blue boundary, outside of this air mass, surface conditions are much different, which indicates the presence of an entirely different air mass.


Maritime Tropical Air Masses

warm temperatures and rich in moisture
Maritime tropical air masses originate over the warm waters of the tropics and Gulf of Mexico, where heat and moisture are transferred to the overlying air from the waters below. The northward movement of tropical air masses transports warm moist air into the United States, increasing the potential for precipitation.

Tropical air masses are generally restricted to the southern states during much of the winter. However, southerly winds ahead of migrating cyclones occasionally transport a tropical air mass northward during the winter season.

Below is a map of surface observations and the leading edge of a tropical air mass surging northward into the Ohio Valley has been highlighted in red. Southerly winds behind the boundary signify the continued northward transport of warm moist air.

 

From these reports, we see that most stations in the tropical air mass generally exhibit relatively warmer temperatures, with higher dew point temperatures, and winds generally out of the south. Notice that on the other side of the red boundary, outside of this air mass, surface conditions are much different, which indicates the presence of an entirely different air mass.

Fronts

the boundaries between air masses
A front is defined as the transition zone between two air masses of different density. Fronts extend not only in the horizontal direction, but in the vertical as well. Therefore, when referring to the frontal surface (or frontal zone), we referring to both the horizontal and vertical components of the front.

Stationary Front

a front that is not moving
When a warm or cold front stops moving, it becomes a stationary front. Once this boundary resumes its forward motion, it once again becomes a warm front or cold front. A stationary front is represented by alternating blue and red lines with blue triangles pointing towards the warmer air and red semicircles pointing towards the colder air.

A noticeable temperature change and/or shift in wind direction is commonly observed when crossing from one side of a stationary front to the other.


Image by: WXP Purdue
In the map above, temperatures south of the stationary front were in the 50’s and 60’s with winds generally from the southeast. However, north of the stationary front, temperatures were in the 40’s while the winds had shifted around to the northeast. Cyclones migrating along a stationary front can dump heavy amounts of precipitation, resulting in significant flooding along the front.

Cold Front

transition zone from warm air to cold air
A cold front is defined as the transition zone where a cold air mass is replacing a warmer air mass. Cold fronts generally move from northwest to southeast. The air behind a cold front is noticeably colder and drier than the air ahead of it. When a cold front passes through, temperatures can drop more than 15 degrees within the first hour.

Symbolically, a cold front is represented by a solid line with triangles along the front pointing towards the warmer air and in the direction of movement. On colored weather maps, a cold front is drawn with a solid blue line.

There is typically a noticeable temperature change from one side of a cold front to the other. In the map of surface temperatures below, the station east of the front reported a temperature of 55 degrees Fahrenheit while a short distance behind the front, the temperature decreased to 38 degrees. An abrupt temperature change over a short distance is a good indicator that a front is located somewhere in between.

If colder air is replacing warmer air, then the front should be analyzed as a cold front. On the other hand, if warmer air is replacing cold air, then the front should be analyzed as a warm front. Common characteristics associated with cold fronts have been listed in the table below.

Before Passing While Passing After Passing
Winds south-southwest gusty; shifting west-northwest
Temperature warm sudden drop steadily dropping
Pressure falling steadily minimum, then sharp rise rising steadily
Clouds increasing: Ci, Cs and Cb Cb Cu
Precipitation short period of showers heavy rains, sometimes with hail, thunder and lightning showers then clearing
Visibility fair to poor in haze poor, followed by improving good, except in showers
Dew Point high; remains steady sharp drop lowering

Table adapted from: Ahrens, (1994)

Finding Cold Fronts Using Wind Direction

shift from south-southwest to west-northwest
Cold fronts are not always identifiable by simply examining the temperature field alone. Other fields must also be taken into consideration. For example, below is a surface weather map with an analyzed low pressure center (red “L”) and associated cold front (blue line) and warm front (red line). The numbers are surface temperature reports and the symbols are wind barbs, indicating wind direction and wind speed.

At the time this map was generated, temperatures ahead of the cold front were in the 50’s, while behind the front, temperatures were only slightly colder (in the 40’s and 50’s). However, notice the change in wind direction (as indicated by the wind barbs) from one side of the cold front to the other. Winds ahead of the cold front were generally from south-southwest, while behind the front, winds had shifted around and were blowing out of the west. This sudden shift in wind direction was the key indicator that a cold front was present.

A sudden change in wind direction is commonly observed with the passage of a cold front. Before the front arrives, winds ahead of the front (in the warmer air mass) are typically out of the south-southwest, but once the front passes through, winds usually shift around to the west-northwest (in the colder air mass).

Finding Cold Fronts Using Dew Points

lower dew point temperatures behind the cold front
Another indication of a possible frontal passage is a change in the air’s relative humidity. The air mass ahead of a cold front is typically more moist than the air mass behind it. The surface map below contains reports of temperature, dew point temperature, and wind barbs. Higher dew points indicate a higher moisture content of the air. Ahead of the cold front analyzed below, dew point temperatures were generally in the 50’s, while behind the front, dew point values dropped off into the 30’s and 40’s.

This decrease in dew point temperature indicated the presence of drier air behind the cold front. A noticeable change in the air’s relative humidity is commonly observed with the passage of a cold front. Before the front arrives, the air typically feels more humid (in the warmer air mass), but once the front passes through, the humidity decreases and the air feels drier.


Cyclones and Associated Cold Front

leading edge of colder air mass
Below is a simple model of a cyclone with a cold front extending to the south from the center of low pressure and a warm front extending to the east ahead of the storm.

At low levels, several air masses of distinctly different origin may be found in varying parts of the cyclone. The cold front marks the leading edge of a colder and drier air mass being wrapped southeastward by north-northwesterly winds behind the low.

Clouds and precipitation usually develop along and ahead of the cold front as the colder air mass lifts the warm moist air ahead of it.

Precipitation Along a Cold Front

lifting the warm moist air ahead of it
The animation below is a sequence of vertical cross sections that depict the development of precipitation ahead of and along a cold front. The surging blue mass represents colder air behind the cold front (solid blue line) while the yellow shading indicates the warm moist air mass ahead of the front.


Animation by: Hall
As the front advances, the colder air lifts the warmer air ahead of it (red arrows). The air cools as it rises and the moisture condenses to produce clouds and precipitation ahead of and along the cold front. In contrast to lifting along a warm front, upward motions along a cold front are typically more vigorous, producing deeper clouds and more intense bands of showers and thunderstorms. However, these bands are typically quite narrow and move rapidly just ahead of the cold front.

A Closer Examination of the Animation:
Initially, the cold air mass wedges into the warmer air mass ahead of it, (separated from each other by the cold front). The lighter warm air is lifted upwards by the denser cold air and if enough water vapor condenses, clouds develop.

If condensation of water vapor persists, precipitation may develop, typically in a narrow band just ahead of the cold front.

Due to the steep slope of a cold front, vigorous rising motion is often produced, leading to the development of showers and occasionally severe thunderstorms.

Warm Front

transition zone from cold air to warm air
A warm front is defined as the transition zone where a warm air mass is replacing a cold air mass. Warm fronts generally move from southwest to northeast and the air behind a warm front is warmer and more moist than the air ahead of it. When a warm front passes through, the air becomes noticeably warmer and more humid than it was before.

Symbolically, a warm front is represented by a solid line with semicircles pointing towards the colder air and in the direction of movement. On colored weather maps, a warm front is drawn with a solid red line.

There is typically a noticeable temperature change from one side of the warm front to the other. In the map of surface temperatures below, the station north of the front reported a temperature of 53 degrees Fahrenheit while a short distance behind the front, the temperature increased to 71 degrees. An abrupt temperature change over a short distance is a good indication that a front is located somewhere in between.

If warmer air is replacing colder air, then the front should be analyzed as a warm front. If colder air is replacing warmer air, then the front should be analyzed as a cold front. Common characteristics associated with warm fronts have been listed in the table below.

Before Passing While Passing After Passing
Winds south-southeast variable south-southwest
Temperature cool-cold, slow warming steady rise warmer, then steady
Pressure usually falling leveling off slight rise, followed by fall
Clouds in this order: Ci, Cs, As, Ns, St, and fog; occasionally Cb in summer stratus-type clearing with scattered Sc; occasionally Cb in summer
Precipitation light-to-moderate rain, snow, sleet, or drizzle drizzle or none usually none, sometimes light rain or showers
Visibility poor poor, but improving fair in haze
Dew Point steady rise steady rise, then steady

Table adapted from: Ahrens, (1994)

Finding Warm Fronts Using Wind Direction

shift from east-southeast to south-southwest
Warm fronts are not always identifiable by simply examining the temperature field alone. Other fields must also be taken into consideration. For example, below is a surface weather map with an analyzed low pressure center (red “L”) and associated cold front (blue line) and warm front (red line). The numbers are surface temperature reports and the symbols are wind barbs, indicating wind direction and wind speed.

At the time this map was generated, temperatures ahead of the warm front were in the 40’s, while behind the front, temperatures were only slightly warmer (in the 50’s). However, notice the change in wind direction (as indicated by the wind barbs) from one side of the warm front to the other. Winds ahead of the warm front were generally from the east, while behind the front, winds had shifted around and were blowing out of the south. This sudden shift in wind direction was the key indicator that a warm front was present.

A sudden change in wind direction is commonly observed with the passage of a warm front. Before the front arrives, winds ahead of the front (in the cooler air mass) are typically from the east, but once the front passes through, winds usually shift around to the south-southwest (in the warmer air mass).

Finding Warm Fronts Using Dew Points

higher dew point temperatures behind the warm front
Another indication of a possible frontal passage is a change in the air’s relative humidity. The air mass behind a warm front is typically more moist than the air mass ahead of the front. The surface map below contains reports of temperature, dew point temperature, and wind barbs. Higher dew points indicate a higher moisture content of the air. Ahead of the warm front analyzed below, dew point temperatures were generally in the 40’s, while behind the front, dew point values climbed into the 50’s.

This increase in dew point temperature indicated that the air behind the warm front contained more moisture. A noticeable change in the air’s relative humidity is commonly observed with the passage of a warm front. Before the front arrives, the air typically feels less humid than after the warm front passes through.