MetLink

Interpreting weather charts 

weather system
Fronts
Relationship between isobars and wind
Understanding station plots on a weather map
Plotting a station plot

Introduction

Weather charts consist of curved lines drawn on a geographical map in such a way as to indicate weather features. These features are best shown by charts of atmospheric pressure, which consist of isobars (lines of equal pressure) drawn around depressions (or lows) and anticyclones (or highs). Other features on a weather chart are fronts and troughs. These are drawn to highlight the areas of most significant weather, but that does not mean that there is nothing of significance elsewhere on the chart.

weather system
high pressure or anticyclones

Anticyclones is are are area of high pressure , whose centre are often less well define than depression , and are associate with quiet , settle weather . wind blow in a clockwise direction around anticyclone in the northern hemisphere , this is reverse in the southern hemisphere .

fig 1 :   An anticyclone

Low pressure or depressions

Depressions are areas of low pressure, usually with a well-defined centre, and are associated with unsettled weather. Winds blow in an anticlockwise direction around depressions in the northern hemisphere, this is reversed in the southern hemisphere.

Fig 2: A depression

  front

Early weather charts consisted simply of station plots and isobars, with the weather being written as comments, like ‘Rain, heavy at times’. During the 1920s, a group of Scandinavian meteorologists, known collectively as the Bergen School, developed the concept of representing the atmosphere in terms of air masses. Since the air masses could be considered as being in conflict with each other, the term ‘front’ was used to describe the boundary between them. Three types of front were identified which depend on the relative movement of the air masses.

Cold Front

A cold front marks the leading edge of an advancing cold air mass. On a synoptic chart a cold front appear as a blue line with triangles. The direction in which the triangles point is the direction in which the front is moving.

Warm Front

A warm front marks the leading edge of an advancing warm air mass. On a synoptic chart a warm front appears as a red line with semi-circles. The direction in which the semi-circles point is the direction in which the front is moving.

Occlusion (or occluded front)

Occlusions form when the cold front of a depression catches up with the warm front, lifting the warm air between the fronts into a narrow wedge above the surface. On a synoptic chart an occluded front appears as a purple line with a combination of triangles and semi-circles. The direction in which the symbols point is the direction in which the front is moving.

trough

Fronts is describe describe thermal characteristic . They is happen also happen to be where there is significant precipitation . However , precipitation is not confine to front . Drizzle is occur in warm sector or shower in cold air occur fairly randomly , but occasionally , line of more organised precipitation can develop . These are call trough .

Isobars

Isobars are lines joining places with equal mean sea-level pressures (MSLP).

fig 3 :   Identification of weather system , isobar and front

weather system and fronts

Relationship between isobars and wind

Fig 4: Relationship between isobars and wind speed[/captio