Tips and Tricks to Understand Isobars on Weather Maps



  1. Read a Weather Map Step 1.jpg

    Understand general concepts of the weather. What most people are concerned with is precipitation, which, in meteorology (the study of weather), is any form of water that falls onto the Earth’s surface. Forms of precipitation include rain, hail, snow, and sleet. High pressure implies fair weather and low pressure is usually associated with precipitation.

  2. Read a Weather Map Step 2.jpg

    Find a weather map. Watch out for one on the TV news, online, or in your local newspaper. (Other sources may include magazines and books, but they may not be current.) Newspapers are a convenient method to find a weather map, as it is cheap, reliable, and can be cut apart so you can carry it with you while learning to interpret the symbols.

    • If possible, find a map covering a smaller area – these can be easier to interpret. Focusing on a larger scale may be difficult for the beginner. On the map, notice the location, lines, arrows, patterns, colors, and numbers. Every sign counts and all are different.
  3. Read a Weather Map Step 3.jpg

    Read the air pressure. This is the weight or pressure the air exerts on the ground and is measured in millibars. It is important to be able to read air pressure because pressure systems are associated with certain weather patterns. To read air pressure, check for isobars (iso = equal, bar = pressure) – plain, curved lines that indicate areas of equal air pressure. Isobars play a major role in determining the speed and direction of wind. When the isobars form concentric closed (but not always round) circles, the smallest circle in the center indicates a pressure center. This can be either a high-pressure system (depicted by an “H” in English, “A” in Spanish) or a low pressure system (depicted by an “L” in English, “B” in Spanish). Air does not flow “down” pressure gradients; it flows “around” them due to the Coriolis effect (Earth spinning). Hence, wind direction is indicated by the isobars, counterclockwise around lows (cyclonic flow) and clockwise around highs (anticyclonic) in the northern hemisphere, thus creating wind. The closer the isobars are to one another, the stronger the winds.:

    • Low Pressure System (Cyclone): Increased cloudiness, winds, temperatures, and chance of precipitation. Represented on a weather map by isobars that are close together, arrows traveling clockwise (Southern Hemisphere) or counter-clockwise (Northern Hemisphere), usually with a “T” in the middle isobar, which forms a round circle (the letter can vary, however, depending on the language the weather report is presented in). Radar imagery can show low-pressure systems. Tropical cyclones (South Pacific) are also named hurricanes aroundAmerica or typhoons in coastal Asia.
      Read a Weather Map Step 3Bullet1.jpg
    • High Pressure System: Indicates clear, calm conditions with reduced chance of precipitation. Drier air usually results in a greater range of high and low temperatures. Represented on a weather map as isobars with an “H” in the middle isobar and arrows showing which direction the wind is flowing (clockwise in Northern Hemisphere, counter-clockwise in the Southern Hemisphere). Like cyclones, they can also be shown with radar imagery.
  4. Read a Weather Map Step 4.jpg

    Observe the types and movement of fronts. These mark the boundary between warmer air on one side and colder air on the other. If you are close to a front and you know the front is moving towards you, you can expect a change in weather (e.g. cloud formation, precipitation, thunderstorms, and wind) when the front boundary passes over you. Mountains and large bodies of water can distort its path. On a weather map, you will notice some lines that have semi-circles or triangles on either side, or both (shown here). These indicate the boundaries for various types of fronts:

    • Cold frontRainfall can be torrential and wind speeds can be high. Blue lines with triangles on one side represents cold fronts on weather maps. The direction the triangles point is the direction in which the cold front is moving.
    • Warm front: Often brings a gradual increase in rainfall as the front approaches, followed by prompt clearing and warming after the front passes. If the warm air mass is unstable, the weather might be characterized by prolonged thunderstorms. A red line with semi-circles on one side represents warm fronts. The side the semi-circles are on represent the direction in which the warm front is heading.
    • Occluded front: Formed when a cold front overtakes a warm front. Associated with various weather events (possibly thunderstorms) depending on whether it is a warm or cold occlusion. The passing of an occluded front usually brings drier air (lowered dew point). A purple line with semi-circles and triangles both on the same side represents occluded fronts. Whichever side they’re on, is the direction the occluded front is going.
    • Stationary front: Indicates a non-moving boundary between two different air masses. These fronts have long continuous rainy periods that linger for extended periods in one area and move in waves. A semi-circle bordering one side and triangles along the opposite side represents that the front is not moving in any direction.
  5. Read a Weather Map Step 5.jpg

    Read the station models at each point of observation. If your weather map has station models, each one will plot the temperature, dew-point, wind, sea level pressure, pressure tendency, and ongoing weather with a series of symbols.

    • Temperature is generally recorded in Celsius degrees and rainfall, in millimeters. In the US, temperatures are in Fahrenheit and rainfall is measured in inches.
    • Cloud cover is indicated by the circle in the middle; the extent to which it is filled indicates the degree to which the sky is overcast.
    • Wind barbs point in the direction of the wind. Lines or triangles coming off the main line at an angle indicate wind strength: 50 knots for every triangle, 10 knots for every full line, 5 knots for every half line.



Isobarslines of constant pressureA line drawn on a weather map connecting points of equal pressure is called an “isobar”. Isobars are generated from mean sea-level pressure reports and are given in millibars.

The diagram below depicts a pair of sample isobars. At every point along the top isobar, the pressure is 996 mb while at every point along the bottom isobar, the pressure is 1000 mb. Points above the 1000 mb isobar have a lower pressure and points below that isobar have a higher pressure.

Any point lying in between these two isobars must 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 isobar maps are likewise available every hour. The solid blue contours (in the map below) represent isobars and the numbers along selected contours indicate the pressure value of that particular isobar.

Such maps 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 stronger pressure gradients.


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