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How to read a 500 mb map
A 500 mb map shows the shape of the atmosphere about halfway up, at roughly 18,000 feet. Contour lines mark the height where the air pressure is 500 millibars, in decameters, so 564 means 5,640 meters. Low heights form troughs, which usually go with stormy weather, and high heights form ridges, which usually go with calmer weather. Storms at the surface tend to follow the pattern aloft.
By Weather Decision Solutions. Published . Updated .
What 500 mb is
Air pressure falls as you climb. A weather map at 500 mb does not show the air at a fixed height. It shows how high you have to go to find the place where the pressure has dropped to 500 millibars, about half of what it is at sea level. NOAA’s JetStream describes the 500 mb level as about the middle of the atmosphere, between about 16,000 and 20,000 feet. It is high enough to be above most of the friction and clutter near the ground, so the broad pattern steering storms shows up clearly.
Reading the heights
The contour lines are heights, labeled in decameters (dam), which is tens of meters, with the last zero dropped. The label 564 means 5,640 meters, or about 18,500 feet. Heights are highest in warm air and lowest in cold air, because warm air is thicker and pushes the pressure surface higher. That is why a 500 mb map doubles as a map of where the cold and warm air masses are.
Ridges, troughs and the flow between them
- A ridge is a bulge of higher heights, bending the contours toward the pole. Under a ridge, air tends to sink, skies are often clearer, and temperatures run warmer.
- A trough is a dip of lower heights, bending the contours toward the equator. Troughs go with cooler air and more active weather.
- The wind blows roughly parallel to the contours, with lower heights to its left. Where contours crowd together, the wind is stronger.
- The big waves, a handful of them around the hemisphere, are called longwaves. They set up the broad temperature pattern, such as a cold East and a mild West.
Shortwaves and vorticity
Smaller ripples riding along the larger waves are called shortwaves. They are often the trigger for a particular storm. Forecasters find them with vorticity, a measure of how much the air is spinning. NOAA’s JetStream notes that vorticity maxima help locate shortwave troughs embedded in the longwaves, and that increasing vorticity goes with rising air and precipitation. Absolute vorticity is highest in troughs and lowest in ridges.
On a map that shows vorticity as colors with height contours drawn over it, look for a bullseye of high vorticity moving along the flow. The surface storm usually develops near it and ahead of it.
How the upper level relates to storms at the surface
A surface low is not the whole story. It lives under a column of air, and the pattern aloft helps decide whether it strengthens, stalls or races away. A trough approaching from the west with a strong shortwave helps lift air ahead of it, which supports surface lows and precipitation. A trough that is slowing down or digging south can drag a storm along the coast. A ridge that builds ahead of a trough can steer a storm inland or block it.
For a nor’easter, the position of the trough, how much it tilts, and the strength of the ridge to the east all feed into the track. This is why forecasters check the 500 mb pattern when they compare storm tracks with the 40/70 benchmark.
Using height anomalies
A height anomaly is the height at a point minus the long-term average height for that date. The raw map shows the pattern, and the anomaly shows how unusual that pattern is for the date. Positive anomalies mean higher heights than normal, which usually means ridging and warmer air. Negative anomalies mean lower heights than normal, which usually means a trough and cooler air. A large anomaly over a region often lines up with a persistent temperature pattern there.
Anomaly maps are especially useful for longer forecasts, where the exact position of every wave is uncertain but the large-scale lean still carries information. They show where the pattern leans over a week or a month, which is a different job from tracking one storm.
A step-by-step read
- Check the model, run and valid time at the top of the map.
- Find the large troughs and ridges. Where is the cold-air trough, and where is the warm ridge?
- Follow the flow. Notice where the contours are packed tightly and the wind is strong.
- Find shortwaves. On a vorticity map, look for bullseyes moving along the flow.
- Match the pattern to the surface. Where is the surface low relative to the trough?
- Compare with the previous run, another model and the ensemble at the same valid time. A trough that is stronger or slower changes the surface forecast.
500 mb in 4070
In Model Explorer, GFS, ECMWF, AIFS and AI-GFS offer 500 mb heights and wind, and GFS also offers 500 mb vorticity with heights. CFSv2 Monthly shows the 500 mb height as a monthly mean and as an anomaly. 500 mb is part of 4070 Pro, with a seven-day free trial. See also GFS, ECMWF IFS and CFSv2.
The pattern aloft sets the stage, not the outcome at your address. Official National Weather Service forecasts, watches and warnings remain the authority for what is expected where you live.
Common questions
What is a 500 mb map?
It is a map of the height at which air pressure is 500 millibars, about halfway up the atmosphere or roughly 18,000 feet. The height contours show ridges and troughs that steer weather systems at the surface.
What does 564 mean on a 500 mb map?
Heights are labeled in decameters with the last zero dropped, so 564 means 5,640 meters, or about 18,500 feet. Lower numbers mean colder air and a trough, and higher numbers mean warmer air and a ridge.
What is a trough on a 500 mb chart?
A trough is a dip of lower heights, with the contours bending toward the equator. It goes with cooler air and more active weather, and surface storms often form near and ahead of one.
What is a shortwave?
A shortwave is a smaller ripple moving along the larger flow aloft. It often triggers a particular storm, and forecasters find it by looking for maxima of vorticity.
What is a 500 mb height anomaly?
It is the height at a point minus the long-term average for that date. Positive anomalies usually mean ridging and warmer air, negative anomalies usually mean troughing and cooler air.
Why do forecasters look at 500 mb?
It is about the middle of the atmosphere, above most surface clutter, so it shows the broad pattern steering storms. Forecasters use it to judge where storms will go, how strong they may get and where cold or warm air will settle.
How does the 500 mb pattern relate to a nor’easter?
The position, tilt and speed of the trough and the strength of the ridge to its east all help set the track of the surface low. A small change in the pattern aloft can move the track and with it the rain-snow line.
Sources
- NOAA JetStream, 500 mb constant pressure chart. Heights are in decameters, so 564 means 5,640 meters; 500 mb is about the middle of the atmosphere; vorticity maxima help locate shortwave troughs.
- NOAA JetStream, Absolute vorticity. Absolute vorticity is highest in troughs and lowest in ridges.
- NOAA WPC, Ensemble training. An ensemble is two or more forecasts verifying at the same time; the high-resolution control is the best member only about 5 to 7 percent of the time; the GFS ensemble mean began to beat the operational GFS for 500 mb heights at about day 3.5; a multi-model ensemble may verify better than one from a single model.
- NWS State College, Snow storm types. In a nor’easter the heaviest snow falls north and west of the low’s track, a fairly consistent line between rain, mixed precipitation and snow moves along with the storm, and an uncertain track makes the timing of changes very difficult.
Look at the pattern aloft.
500 mb heights and wind are available for GFS, ECMWF, AIFS and AI-GFS in Model Explorer, part of 4070 Pro. Start with seven days free.