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Introduction

Synoptic charts are featured in weather reports on television and in newspapers everyday. Not everyone, however, knows how to read a synoptic chart. This chapter will discuss the basic skills needed to interpret a synoptic chart.

What is a synoptic chart?

A synoptic chart is the scientific term for a weather map. Synoptic charts provide information on the distribution, movement and patterns of air pressure, rainfall, wind and temperature. This information is conveyed using symbols, which are explained in a legend. Synoptic charts are used to report on the current weather and to predict future weather patterns.

Air pressure

The most important feature of a synoptic chart is the fine black lines called isobars. While isobars are similar to contour lines, which are used on topographic maps, they provide different information. Contour lines connect points of equal altitude, while isobars connect points which share the same atmospheric (air) pressure. The closer contour lines are, the steeper the slope is, and the closer isobars are, the stronger the winds are.

Air pressure is essentially the weight of the air. Since humans cannot feel the weight of the air unaided, a barometer is used to measure air pressure in hectopascals (hPa). These measurements can be seen on synoptic charts.

Air pressure systems usually move from west to east, but change shape and position as they move. Since the average air pressure at sea level is 1013 hPa, any measurement above this number is called a high pressure system and is considered to be an area of sinking air. A high pressure system generally means that the weather is fine and settled. On a map, high pressure systems are often marked by an 'H'. These systems are often known as anticyclones. In a high pressure system, winds move in an anti-clockwise direction in the Southern Hemisphere and clockwise in the Northern Hemisphere.

Conversely, any measurement below 1013 hPa is called a low pressure system and is thought to be an area of rising air. A low pressure system usually means unsettled weather, which could turn into rain. On a map, it is marked by an 'L'. These systems are often referred to as cyclones. In a low pressure system, winds move in a clockwise direction in the Southern Hemisphere and anti-clockwise in the Northern Hemisphere.

When two masses of air with differing characteristics (warmer or colder) collide with one another, it is called a front. A warm front usually results in an increase in temperature or even light showers. A cold front can lead to cooler temperatures and rain. Synoptic charts depict warm fronts using thicker lines with attached semi circles, while cold fronts are represented by attached triangles. The air in fronts always travels in the direction that the semi-circles or triangles are pointing in on the map.

See image 1

Precipitation: rainfall

Precipitation includes snow, hail and dew. The most common form of precipitation, however, is rainfall. Rainfall is a regular feature on synoptic charts, since it has a significant impact on the daily activities of many people. On a synoptic chart, rainfall is usually shown using shading. This shading is often explained in the legend as representing the areas which received rainfall in the last 24 hours. Not all synoptic charts show rainfall using shading. Rainfall can still be determined, however, by referring to the isobars on the synoptic chart. Low pressure systems and cold and warm fronts are usually a good indication of rain.

See image 2

Wind direction and speed

Wind generally refers to the horizontal movement of air. The Earth produces local winds, which include land breezes and sea breezes, as well as permanent global winds, such as Trade Winds and the Polar Easterlies. While synoptic charts do not distinguish between the different types of winds, some do show wind direction and wind speed.

Wind direction can be measured using a weather vane, which is a device that turns on an axis to point in the direction of the wind. Wind speed can be measured using an anemometer, which is a device that uses rotating cups of pressure differences to determine speed.

On a synoptic chart, wind speed and direction are shown using a wind barb (a dot with a straight line attached). The direction that the stem of the barb is pointing in indicates the direction that the wind is coming from. To represent an increase in speed, lines are added to the barb to make it appear like an arrow. The greater the wind speed, the more 'vanes' the barb has. The exact meanings of the various forms of the barb can be found in the legend. It is also possible to refer to the closeness of the isobars to determine how windy it is.

See image 3

Temperature

Synoptic charts do not usually feature any symbolic or numerical measurements of temperature (degrees of hotness or coldness). Temperature recordings and forecasts are, instead, displayed in an accompanying table where they can be easily identified and interpreted by the general public. There are ways, however, in which synoptic charts can be used to give a basic indication of temperature.

A cold front, for example, brings cool temperatures since it involves cold air forcing warm air upwards. A wind coming from the ocean onto the land will also bring cooler temperatures. It has been proven that a wind coming from the tropical area of northern Australia is likely to bring warmer temperatures.


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Question 1/5

1. What does an anemometer measure?

Wind speed

Wind direction

Air pressure

Humidity

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