Measuring weather — instruments and synoptic charts

Weather is the state of the atmosphere at a particular place and time. CCEA examiners expect you to know the instruments used to measure weather elements, how to read synoptic (weather) charts, and to interpret weather maps.

Weather elements and their instruments

Weather element	Unit	Instrument
Temperature	°C	Thermometer (max/min thermometer records daily range)
Rainfall (precipitation)	mm	Rain gauge (funnel collecting rain into a measuring cylinder)
Wind speed	km/h or knots / Beaufort scale	Anemometer (spinning cups)
Wind direction	Points of compass (N, NE, E…)	Wind vane (arrow points INTO the wind)
Air pressure	millibars (mb)	Barometer (aneroid or mercury)
Cloud cover	Oktas (eighths of sky covered)	Observer's estimate
Humidity	%	Hygrometer or wet-and-dry bulb thermometer (psychrometer)
Sunshine hours	Hours	Campbell-Stokes sunshine recorder (glass sphere burns a trace)

Key principles:

• Weather instruments must be placed in a Stevenson Screen — a white, louvred wooden box that shields thermometers from direct sunlight while allowing free air circulation.
• The screen is mounted 1.25 m above the ground, away from buildings and trees.
• Rain gauges are placed in the open, away from buildings, with the rim 30 cm above the ground to prevent splash-in.

Synoptic charts

A synoptic chart is a weather map that uses standard symbols to show weather conditions across a wide area at one moment in time. Key features:

Isobars: lines connecting places of equal air pressure. Closely spaced isobars indicate strong pressure gradients → strong winds. Widely spaced isobars indicate light winds.

Pressure systems:

• Low pressure (depression/cyclone): isobars form roughly circular patterns with the lowest pressure at the centre. Typically brings clouds, rain, and strong winds.
• High pressure (anticyclone): isobars form roughly circular patterns with highest pressure at centre. Typically brings dry, settled weather; clear skies in summer (hot); clear skies in winter (cold/frosty).

Fronts: boundaries between air masses of different temperatures:

• Warm front: shown as a line with red semicircles. Warm air slowly rises over cold air → prolonged, gentle rain.
• Cold front: shown as a line with blue triangles. Cold air aggressively undercuts warm air → heavy, short-lived rain and thunderstorms.
• Occluded front: forms when a cold front catches up with a warm front → more complex, mixed precipitation.

Wind symbols: arrows show wind direction (arrow points in direction wind is blowing FROM); barbs on the arrow indicate wind speed (one full barb = 10 knots).

Reading and interpreting a synoptic chart

When interpreting a synoptic chart:

1. Identify high and low pressure systems and their centres.
2. Read isobar spacing to determine wind strength (close = strong).
3. Identify fronts and predict associated weather.
4. Note wind direction (winds circulate ANTICLOCKWISE around lows in the Northern Hemisphere; CLOCKWISE around highs — due to the Coriolis effect).

NI context: Northern Ireland's weather is dominated by Atlantic depressions tracking northeast across the British Isles. NI receives approximately 1,050 mm of precipitation per year (higher in uplands — the Mourne Mountains can receive 2,000+ mm).