The completed isobars usually reveal a few standard patterns. The stronger the winds, the closer the isobars. This is reversed in the southern hemisphere. Thus, in the northern hemisphere, the lower pressure is to the left of the wind, the higher pressure is to the right. Recalling lesson 3, it is the rotation of the Earth which deflects moving air on course from high to low pressure. The meteorologist in drawing isobars has an additional aid in the plotted wind directions of each station.
By definition, isobars can never cross each other (see Figure 8.2). Usually, they are drawn at intervals of 2 or 4 millibars. They are of the same nature as height contours on a geographical map. The first step is to draw on the isobars - lines along which the pressure is the same. When plotting of the meteorological observations is completed, the forecaster then proceeds with the synopsis and analysis of the chart. In Figure 8.1 the pressure is 1010.5 millibars. For example, a pressure of 987.8 millibars would be written as 878 1014.3 millibars as 143. The hundreds figure for the pressure is omitted as being understood since the pressure is almost always between 9 millibars. Pressure at a station is standardised to sea-level pressure measured in millibars. Temperature is measured in degrees Celsius to the nearest whole degree. The wind speed is denoted by "feathers" on the wind arrow, a short feather indicating 5 knots, a larger one 10 knots, a long and short one 15 knots and so on. In Figure 8.1, the wind is from the northeast. Wind direction is represented by an arrow pointing in the direction from where the wind is coming. The greater the fill, the more the cloud cover, measured in eighths or oktas. For example cloud cover at a particular time is indicated by filling in certain portions of the circle. Others, like the occurrence of rain, snow, cloud and fog are plotted as internationally agreed symbols (see Figure 8.1). Elements like temperature and pressure are entered as plain figures. The weather report for each station is then plotted in and around the circle. The first stage in preparing a synoptic chart is to chart the position of each meteorological station.
Meteorologists plot isobaric patterns on synoptic charts. But to be able to predict and forecast weather it is necessary to understand the isobaric patterns associated with fronts and depressions, anticyclones and high pressure ridges. With an understanding of how the air moves and how clouds and rain form, much prediction can be made by simply observing the sky overhead, observing wind direction and noting the temperature and humidity of the air.
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