Station circles, recognising systems, and satellite images

Overview

Reading wind, cloud and weather off a station circle, telling a depression from an anticyclone on a chart, and what visible and infrared satellite images each show.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   How is wind shown at an individual station on a weather chart?

   Model answer

   A straight shaft is drawn into the station circle, pointing back along the direction the wind is coming from, so a shaft running out to the south-west of the circle means a south-westerly wind. Feathers or barbs on the end of the shaft show speed: each full feather represents 10 knots, a half feather 5 knots and a solid triangular pennant 50 knots, so a shaft with one full and one half feather is a 15 knot wind. A circle with no shaft means calm.

2. 2

   How are cloud cover and weather shown at an individual station?

   Model answer

   Cloud cover is shown by shading of the station circle in eighths, called oktas: an empty circle means clear sky, a circle with one quarter shaded means two oktas, a half-shaded circle four oktas, and a completely filled circle means overcast at eight oktas. Present weather is shown by a small standard symbol placed to the left of the circle, such as dots for rain, a comma for drizzle, asterisks for snow, a triangle for a shower and a zigzag arrow for a thunderstorm, and figures for temperature and dew point are printed alongside.

3. 3

   How would you recognise a depression on a synoptic chart?

   Model answer

   Look for a set of closed, roughly circular isobars with the lowest value at the centre, marked L, usually somewhere between 950 and 1000 millibars. The isobars will be closely spaced, showing strong winds, and the wind will be circulating anticlockwise and slightly inwards. Fronts will be drawn radiating from the centre, typically a warm front curving ahead to the east and a cold front trailing back to the south-west, with an occlusion between them near the centre if the system is mature.

4. 4

   How would you recognise an anticyclone on a synoptic chart?

   Model answer

   Look for a set of closed, roughly circular isobars with the highest value at the centre, marked H, usually between 1020 and 1040 millibars. The isobars will be widely spaced, showing light winds, the circulation will be clockwise and slightly outwards, and there will be no fronts within the system because it contains a single air mass. Station circles beneath it will show little or no cloud.

5. 5

   A station lies about 200 kilometres ahead of an approaching warm front, with pressure falling and a freshening south-easterly wind. What weather should be forecast for it over the next twelve hours?

   Model answer

   Cloud will thicken and lower from cirrus through cirrostratus and altostratus to nimbostratus, pressure will continue to fall steadily, and steady moderate rain will set in for several hours. As the warm front arrives the rain will ease to drizzle, temperature will rise noticeably, the wind will veer to south-westerly and the fall in pressure will level off. The station will then be in the warm sector, with mild muggy air, low cloud, drizzle and poor visibility until the cold front follows.

6. 6

   What does a visible satellite image show, and what is its main limitation?

   Model answer

   A visible image records sunlight reflected back from the Earth, so it shows what an observer in space would see. Thick cloud is bright white because it has a high albedo, thin cloud is grey, and land and sea are dark. It gives an excellent picture of cloud extent and structure, but because it depends on sunlight it is useless at night, and it cannot distinguish a low thick sheet of stratus from a high thick storm cloud, since both look equally white.

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