The need for synoptic charts
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. 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. Meteorologists plot isobaric patterns on synoptic charts.
Plotting weather observations
The first stage in preparing a synoptic chart is to chart the position of each meteorological station. These are marked by a small circle. The weather report for each station is then plotted in and around the circle. Elements like temperature and pressure are entered as plain figures. Others, like the occurrence of rain, snow, cloud and fog are plotted as internationally agreed symbols (see Figure 8.1). For example cloud cover at a particular time is indicated by filling in certain portions of the circle. The greater the fill, the more the cloud cover, measured in eighths or oktas. Wind direction is represented by an arrow pointing in the direction from where the wind is coming. In the figure below, the wind is from the northeast. 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.
Temperature is measured in degrees Celsius to the nearest whole degree. Pressure at a station is standardised to sea-level pressure measured in millibars. The hundreds figure for the pressure is omitted as being understood since the pressure is almost always between 950 and 1050 millibars. For example, a pressure of 987.8 millibars would be written as 878; 1014.3 millibars as 143. Here the pressure is 1010.5 millibars.
When plotting of the meteorological observations is completed, the forecaster then proceeds with the synopsis and analysis of the chart. The first step is to draw on the isobars – lines along which the pressure is the same. They are of the same nature as height contours on a geographical map. Usually, they are drawn at intervals of 2 or 4 millibars. By definition, isobars can never cross each other.
The meteorologist in drawing isobars has an additional aid in the plotted wind directions of each station. It is the rotation of the Earth which deflects moving air on course from high to low pressure. Thus, in the northern hemisphere, the lower pressure is to the left of the wind, the higher pressure is to the right. This is reversed in the southern hemisphere. The stronger the winds, the closer the isobars.
The completed isobars usually reveal a few standard patterns. A set of curved isobars surrounding an area of low pressure reveals a depression, with the wind in the northern hemisphere blowing anticlockwise around its centre. A set of curved isobars surrounding a high pressure reveals an anticyclone, with the winds in the northern hemisphere blowing clockwise around its centre. Open V-shaped isobars with low pressure inside delineates a trough of low pressure; high pressure inside the formation is called a ridge of high pressure. A col is the indefinite isobar configuration between two highs and two lows arranged alternately, and has no particular type of weather associated with it other than light winds. Some of these patterns can be seen in the figure below.
Isobaric structure of frontal depressions
A depression, as its name implies, is a region of low barometric pressure and appears on the synoptic chart as a set of closed curved isobars with winds circulating anticlockwise in the northern hemisphere, clockwise in the southern hemisphere. The warm and cold fronts associated with depressions bring with them characteristically unsettled weather. Depressions vary from between 200 and 2,000 miles in diameter; they may be deep when pressure at their centre is very low and the isobars are tightly packed, or shallow when less well developed.
A depression develops like the propagation of a wave in water. Initially, a uniform boundary or front exists between cold air pushing southwards and warm air pushing northwards. A wave-shaped distortion may appear on the front, and a small low pressure centre develops at the crest of the wave. In the immediately surrounding area the pressure begins to fall. A disturbance of this kind is called a wave depression. As the “wave” develops, a warm sector of air forms, bounded by the warm and cold fronts, which begins to tie over the engulfing cold air. Both the warm and cold fronts originate from the centre of the depression. On the ground, sudden changes in the wind direction may be experienced when fronts pass by.
Wave depressions can grow off the tail ends of primary cold fronts. The depression so formed is then called a secondary depression. New centres may also develop at the point of occlusion within the primary depression. The secondary system can then become the main system, and the primary occluded front becomes caught up in the developing circulation, effectively becoming a third front.