How to read a weather map: isobars, fronts and symbols

To read a weather map, look at three things: the isobars, which join places with the same air pressure; the H and L letters, which mark highs and lows; and the colored lines with triangles or semicircles, which are the fronts. With those three elements you can tell where the wind will come from, whether it will be strong and whether rain is on its way.

The map you see on TV is almost always a surface map, with pressure corrected to sea level. Meteorologists also use upper-air maps, such as the 500 hPa chart, which show what the surface map cannot.

Key takeaways

• Isobars are lines joining points of equal air pressure; on many national charts, such as AEMET's, they are drawn every 4 hPa (4 millibars).
• The closer together the isobars, the stronger the wind.
• H marks a high (settled weather) and L a low (clouds, wind and rain); in the Northern Hemisphere wind turns clockwise around highs and counterclockwise around lows, and the reverse in the Southern Hemisphere.
• Cold fronts are blue with triangles, warm fronts red with semicircles, occluded fronts purple with both, and stationary fronts alternate red and blue.
• On a station plot, a half barb on the wind arrow means 5 knots, a full barb 10 knots and a triangular pennant 50 knots.

What are isobars on a weather map?

Isobars are the lines on a weather map that join the places where air pressure, corrected to sea level, is the same. Each line carries a value in hectopascals (hPa) or millibars, which are the same unit, such as 1008, 1012 or 1016. On many charts there is one isobar every 4 hPa.

Isobars close around pressure centers, just as the contour lines on a topographic map ring a hill or a valley. Average sea level pressure is about 1013 hPa (29.92 inches of mercury); strong highs exceed 1030 hPa and deep lows fall below 970 hPa. US TV forecasts often quote pressure in inches of mercury, but the maps themselves are drawn in millibars.

What does the spacing of isobars tell you?

The spacing of isobars tells you the strength of the wind: the closer together they are, the faster pressure changes over a distance and the harder the wind blows. Widely spaced isobars mean light winds or calm.

It is the same logic as a hiking map, where tight contour lines mean a steep climb: tightly packed isobars mean strong wind on land and gales at sea.

How to read wind direction on a weather map

Wind blows almost parallel to the isobars, but near the ground friction makes it cross them at an angle toward lower pressure. In the Northern Hemisphere it turns counterclockwise around a low and clockwise around a high; in Australia, New Zealand and the rest of the Southern Hemisphere, it is the opposite.

A practical rule is Buys Ballot's law: in the Northern Hemisphere, if you stand with your back to the wind, low pressure is on your left. In the Southern Hemisphere, it is on your right.

What do H and L mean on a weather map?

H marks the center of a high pressure system, or anticyclone, and L the center of a low pressure system, or depression. US maps from the National Weather Service usually show highs as a blue H and lows as a red L, with the central pressure written next to the letter. Spanish maps use A (alta) and B (baja).

Near an H the weather is usually stable and dry, and near an L there are clouds, wind and precipitation, although in winter a high can also bring fog and frost. We explain the differences in low pressure vs high pressure weather.

Weather map symbols and what they mean

Fronts are the colored lines that separate air masses of different temperature, and each type has its own symbol: the triangles or semicircles point in the direction the front is moving.

Front	Color	Symbol	Weather it brings
Cold	Blue	Triangles	Showers, thunderstorms, a drop in temperature and a wind shift
Warm	Red	Semicircles	Lowering clouds and light, steady rain
Occluded	Purple	Triangles and semicircles on the same side	Cloud and rain; appears as a low matures
Stationary	Alternating red and blue	Semicircles and triangles on opposite sides	Persistent cloud and rain over the same area

Fronts trail out of lows and curve around them. A cold front moves faster than a warm front, and when it catches up, an occluded front forms. To find out what happens when one reaches your city, read what a cold front is and the weather it brings. Many maps also show troughs, elongated areas of low pressure without a front, often as dashed lines.

Station plots and wind barbs

Detailed maps, like the surface analyses from the NWS, add station plots: a small circle for each weather station with numbers and a wind arrow around it. The circle is filled in according to cloud cover, empty for clear skies and solid for overcast. On US plots, temperature appears at the upper left and dew point at the lower left, in °F.

The wind arrow, or barb, points to the direction the wind is coming from, and its feathers give the speed:

Symbol on the barb	Wind speed
Circle with no barb	Calm
Half barb	5 knots (6 mph, 9 km/h)
Full barb	10 knots (12 mph, 19 km/h)
Pennant (filled triangle)	50 knots (58 mph, 93 km/h)

Add up the symbols to get the total: one full barb and one half barb make 15 knots.

What is the difference between isobars and isotherms?

The difference between isobars and isotherms is what they join: isobars join points of equal pressure, and isotherms join points of equal temperature. Isobars help you see the wind and pressure systems; isotherms help you see masses of cold or warm air.

Line	Joins points of equal	Where it appears	What it shows
Isobar	Pressure	Surface map	Lows, highs and wind
Isotherm	Temperature	Temperature maps, at the surface or aloft	Cold and warm air masses
Height contour (isohypse)	Height of a pressure level	Upper-air maps (500 hPa, 850 hPa)	Troughs, ridges and cut-off lows
Isohyet	Rainfall	Rainfall total maps	Where it has rained most

How do you read a 500 hPa map?

A 500 hPa map shows the height at which pressure reaches 500 hPa, about 5,500 meters (18,000 feet) up, and it reveals the circulation in the middle of the atmosphere. Instead of isobars it has height contours, given in decameters: 552 means 5,520 meters.

How to interpret it:

• Low heights mean cold, dense air; high heights, warm air.
• Troughs: the contours bend into a U shape toward the equator. Ahead of a trough there is often unsettled weather and rain.
• Ridges: the contours form an upside-down U toward the pole. Under a ridge the weather tends to be settled and warm.
• Cut-off lows: closed circles of contours with cold air in the center, cut off from the main flow. In Spain they are called DANAs, which we explain in what is a DANA.

How to read a weather map step by step

To read a surface weather map, go from pressure centers to isobars, then fronts, then change over time.

1. Check the date and time. Is it an analysis (current situation) or a forecast? Charts usually use UTC, often written as Z.
2. Find the nearest H and L to your area and note their pressure.
3. Look at the isobars over your region: close together means wind, far apart means calm.
4. Work out the wind direction from the rotation around each center. A west wind in Ireland, a north wind in Texas or an east wind on the Spanish Mediterranean coast changes the weather completely.
5. Find the fronts and see where their symbols point: if they are heading toward you, clouds and rain are coming.
6. Compare several maps in a row, for example 12 hours apart, to see where the systems are moving.
7. If you can, check the 500 hPa map for troughs or cut-off lows that the surface map does not show.

Using weather maps with Clima de Hoy

The map gives the big picture and the local forecast tells you what will happen in your street. If a map shows a front approaching the UK, open the hourly forecast for London to see when the rain arrives and how the wind and temperature change.

The Clima de Hoy rain forecast map animates the precipitation, wind and clouds forecast by the GFS model, which lets you follow fronts without reading isobars. For the longer trend, such as a high that will settle in for days, the 14-day forecast for New York gives you an idea, with the uncertainty that the last days always carry.

Frequently asked questions

What are isobars on a weather map?

Isobars are lines joining places with the same air pressure at sea level. They are usually drawn every 4 hPa and show where the highs and lows are. The closer together they are, the stronger the wind.

What does it mean when isobars are close together?

It means pressure changes a lot over a short distance, which produces strong wind. When isobars tighten around a low, wind warnings on land and gale warnings at sea are common.

What is the difference between isobars and isotherms?

Isobars join points of equal pressure, and isotherms join points of equal temperature. Isobars show wind and pressure systems, while isotherms help you follow masses of cold or warm air.

What do the triangles and semicircles mean on a weather map?

They mark fronts. Blue triangles show a cold front, red semicircles a warm front, and both together on a purple line an occluded front. The symbols point in the direction the front is moving.

How do you read wind direction on a weather map?

Follow the isobars: wind blows roughly along them, turning counterclockwise around lows and clockwise around highs in the Northern Hemisphere, and the reverse in the Southern Hemisphere. On station plots, the wind barb points to the direction the wind is coming from.