Understanding Weather Images

How to read and interpret weather images

A weather map depicts meteorological features such as wind, pressure and temperature in a particular place at a particular time. They can be used to show past and present data as well as predictions for future weather patterns. Some weather maps show a combination of various meteorological features to present a complete story.

Similar to map reading, weather maps convey detailed information through symbols and lines, as well as real-life satellite imagery. For Australia, the Bureau of Meteorology is a reliable starting point for finding weather maps.

While there are many ways to convey weather information on a weather map, we’ll run through a few of the most common below.

  1. Station model maps
    Across Australia, we have thousands of weather stations that collect detailed local data including everything from cloud type and cover right through to rainfall, pressure and wind also. When combined, local data from across the country can provide powerful insights into likely weather patterns in the future. To present the large amount of data in a meaningful way, meteorologists have developed a key to denote the weather at a particular location in shorthand.

    Plotting multiple weather stations over a larger scale can be a helpful way to observe current weather patterns and predict future ones:

    On generalised maps, barometric pressures recorded at individual weather stations are summarized into a contour map, where points of equal pressure are joined up with a line. These lines are known as isobars, and create a map with labelled contours so readers can view trends (see next map).

  2. Pressure maps

    Internet Archive Book Images via Foter.com

    Pressure maps depict areas of high and low air pressure, which in turn, indicate likely weather conditions. They are generated from local weather station data reporting pressure readings.

    In general, high-pressure systems mean stable air and good weather conditions. Conversely, low-pressure systems mean less stable air: clouds can form with subsequent rainy and stormy conditions.

    Low or high air pressure systems are caused by the cooling and heating of air, and are dynamic, with continual 3-dimensional flow of air between the systems. That’s why air pressure and weather patterns can vary dramatically from day to day and between different areas.

  3. Temperature maps
    These are the sort of weather maps you are probably most familiar with as they are most commonly used in media to give readers a quick overview of weather conditions. Often combined with rainfall and cloud cover data, these temperature maps are great at conveying a basic forecast information to the general public.

    Icons source: http://www.jschreiber.com/archives/2004/08/free_weather_ic.html

    Temperature maps may also use isotherms. These are lines that connect points of the same temperature, a bit like a topographic map {link to map reading post} where connected lines indicate points of the same elevation. Colour is also often used to show temperature – orange/red being hotter, purple/blue being cooler. These maps are helpful to view past trends and predict future weather patterns at large scale.

    For instance, the image below is the maximum temperature across Australia on 7th January 2013 (pretty hot!).

  4. Streamline maps
    Streamline maps indicate wind patterns and are particularly helpful in tropical areas where pressure gradients are weak and are bad predictors of wind condition. They are based on pressure readings but transform the data into more useful images that show actual wind patterns. Streamlines are shown in brown below.
  5. Front maps
    When we talk about ‘fronts’ in the context of weather, we mean a boundary separating two masses of air of different densities . These air masses are dynamic, moving, and are depicted on weather maps with the following symbols.

    Weather map symbols: 1. Cold front (triangles point in the direction of travel); 2. Warm front (semi-circles point in the direction of travel); 3. Occluded front; 4. Stationary front.

    1. Cold front
      A cold front is at the leading edge of a temperature drop off, that is, a change from warmer weather to cooler weather. These cold fronts often bring rain, hail and heavy thunderstorms. They tend to produce much swifter changes in weather because they move up to twice as quickly as warm fronts. Cold fronts are associated with low-pressure areas. Once a cold front has passed, the air is cooler and drier than before.
    2. Warm front
      A warm front is at the leading edge of a warm air mass, and moves slower than a cold front. Fog often precedes a warm front, followed by clearing and warming afterwards. The air is warming and more humid in the wake of a warm front.
    3. Occluded front
      An occluded front occurs when a cold front overtakes a warm front. The two fronts curve around, resulting in a variety of weather patterns, thunderstorms possible, but usually just associated with drying of air.
    4. Stationary front
      A stationary front is a boundary between two air masses that is non-moving. In this case, neither air masses are strong enough to replace the other. A wide variety of weather patterns can occur, but most commonly, long periods of cloud and rain but restricted to a narrow band.

Radar Finding, reading and interpreting radar images

Weather Radars (also called weather surveillance radar and Doppler weather radar) can detect precipitation (i.e. rainfall) in real time.

RADAR stands for RAdio Detecting And Ranging. It uses radio waves to detect location and quantity of precipitation in the atmosphere. The basic principle is similar to an echo that you experience if you shout out aloud in a cave and hear your voice bounce back off the wall and call again. Likewise, a RADAR unit sends out radio waves that are reflected and scattered back and based on how much of the echoed waves are received, the RADAR unit can detect where and how much precipitation exists.

This reflectivity of radio waves was discovered by German Heinrich Hertz in 1887, and has been used extensively ever since to understand more about the atmospheric makeup of the earth.

Forty years earlier, in 1842, the Austrian physicist Christian Doppler discovered what we now know as the Doppler Effect. The Doppler effect is the phenomenon that objects moving towards us have a higher frequency than objects moving away from us. Think of when an ambulance with its siren going drives past: it sounds high when it’s approaching (it has a high frequency) and drops to a lower pitch (a lower frequency) when it’s driving away. The same principle can be translated to Radio waves, and in this way, Weather Radars can detect position and intensity as well as the speed of precipitation. Moreover, since rainfall generally moves with the wind, Radar technology can also detect wind velocity.

The Bureau of Meteorology collects and publishes Radar information every 10 minutes, and also uses Radar information to generate localised weather warnings (e.g. severe thunderstorms in Sydney CBD). The image below is a typical RADAR image from the Bureau of Meteorology, where precipitation is superimposed over a geographical map. The colour code of 15 colours indicates how intense the rainfall is, from very light in white, right through to extremely heavy or hailstones in red and black.

Supplied by BOM

Satellite images Finding, reading and interpreting satellite images

Satellite images show real-time images of weather formations, including clouds and lightning. Images of cloud formations are another tool that meteorologists can use to predict rainfall patterns, storms and major weather formations like cyclones.

The Bureau of Meteorology now has High definition Satellite images from the Himawari-8 satellite, a geostationary weather satellite operated by the Japan Meteorological Agency.