14 Weather Maps and Satellite Imagery Warm front Fig. 14.5(b) Synoptic chart symbols.
Pressure values, shown by isobars, are measured in hectopascals. They are usually shown at four-hectopascal intervals (Fig. 14.6).
Winds generally blow parallel to isobars. In a depression the overall wind direction will be anticlockwise. A pattern of close isobars indicates strong winds. The word LOW or the letter L marks the centre of a depression.
The wind pattern associated with an anticyclone is clockwise. The pattern of isobars is well-spaced, indicating little or no winds, which can result in fog. The word HIGH or the letter H identifies the centre of an anticyclone.
998 1011
Hectopascals are the agreed units of measurement for atmospheric pressure.
1 millibar (mb) = 1 hectopascal (hPa)
Cold front Occluded front
1011 1033
Fig. 14.6(a) The chart shows a depression of 998 hectopascals, with an occluded front and associated warm and cold fronts to the west/north-west.
Fig. 14.6(b) Note the bands of cloud associated with the fronts to the west.
Fig. 14.6(c) Rain associated with the warm front is shown in lighter blue, while a band of heavy rain (dark blue) associated with the cold front lies approaches from the west.
CHECK YOUR LEARNING
1. Name three occupations or activities which rely on weather forecasts. 2. Name three different types of fronts and draw the symbols used to represent each type.
3. What are isobars? What units are used to measure them? 4. Name two characteristics of an anticyclone? 5. Copy the chart below into your copybook and then match the correct number to the correct description
a. Stable weather conditions b. Warm air rising quickly c. Heavy rain showers d. Light winds
e. Measurement of pressure i. Warm front ii. Hectopascals
iii. Well-spaced isobars iv Cold front v. Anticyclone
A-Z
Synoptic chart: A weather chart showing information on atmospheric pressure patterns, wind, temperature and precipitation.
Isotherm: A line on a map joining places of equal temperature.
Isohyet: A line on a map joining places of equal precipitation.
Isotach: A line on a map joining places of equal wind speed.
