Summary — What This Chapter Covers

1. Collecting Weather Data

The Stevenson Screen

Think of the Stevenson screen as a "fair test box" for measuring air temperature and humidity. If you just left a thermometer sitting on the ground in direct sun, you wouldn't be measuring air temperature at all — you'd be measuring how hot that one patch of ground got, plus the effect of direct sunlight cooking the thermometer itself. The Stevenson screen exists to strip all of that away so every weather station on Earth is measuring the same thing, the same way.

Every design feature solves one specific measurement problem:

Wind Direction & Wind Speed

Wind instruments answer two separate questions: where is the wind coming from? (direction) and how fast is it moving? (speed). Both matter because wind direction tells you what kind of air mass — and therefore what temperature and moisture — is being carried towards you.

Wind vane (direction)

A wind vane has an arrow on a shaft that swings freely. The wind pushes the broad "tail" end of the arrow, which forces the narrow pointed end to swing round and point into the wind — that is, it points towards the direction the wind is coming from, not the direction it's blowing towards. This is a classic point of confusion, so lock it in: a "northerly wind" is blowing from the north towards the south.

Anemometer (speed)

An anemometer has 3–4 cups mounted on arms at the top of a tall pole (usually 10 m — high enough that friction with the ground doesn't slow the wind down and give a false low reading). Stronger wind spins the cups faster, and a counter converts rotations into a speed reading, usually in metres per second (m/s) or kilometres per hour (km/hr). Digital handheld versions display the reading directly and can even send data straight to an app.

Both instruments share a placement rule: they must be sited well away from buildings and trees, because solid obstacles create wind tunnels or block/slow airflow, corrupting the reading. The rule of thumb is at least three times the height of the nearest obstacle.

Rain Gauge (Precipitation)

Precipitation means any water falling to Earth — not just rain, but hail, mist, sleet and snow too. It's measured in millimetres (mm). A rain gauge is a metal cylinder with a funnel on top that channels water into a collecting jar, which is partly buried in the ground so it stays stable and roughly the same temperature as its surroundings.

Each day at the same time, whatever has collected is poured into a special tapered measuring cylinder — tapered so that small amounts of rain still produce a noticeable, easily-readable rise in level (accuracy at low volumes). You read the water level at eye level, at the bottom of the meniscus (the curved surface of the water). If the amount is too small to register properly, it's recorded as "trace."

Temperature — Six's Thermometer & Thermograph

A Six's thermometer (maximum-minimum thermometer) is a clever U-shaped tube that records the highest and lowest temperatures reached since it was last reset — useful because nobody can stand watching a thermometer all day. Small metal markers ("indices") get pushed along by mercury as the temperature rises and falls, and stay at the most extreme point reached, even after the temperature changes again.

A thermograph gives a continuous record instead of two snapshot values. Inside it, an exposed bimetallic strip physically deforms (bends) as temperature changes. That tiny bend is mechanically amplified through levers connected to a pen, which traces a continuous curve onto graph paper wrapped around a slowly rotating drum. As a rule of thumb, a 1 mm vertical movement on the trace represents about 1°C.

Humidity — The Hygrometer

Humidity measures how much water vapour is in the air. Warm air can hold more water vapour than cold air — this is the single fact that explains almost everything about how a hygrometer works.

A hygrometer uses two thermometers side by side: a normal "dry bulb" thermometer measuring plain air temperature, and a "wet bulb" thermometer whose bulb is wrapped in wet muslin cloth, fed by a wick dipped in water. If the surrounding air isn't already saturated, water evaporates off the wet muslin — and evaporation always requires energy, which it pulls from the bulb itself as heat, cooling it down. So the wet bulb reads lower than the dry bulb. The bigger that gap (the "depression"), the drier the air. If the air is already saturated (holding all the moisture it possibly can), no evaporation can happen, so both thermometers show the same reading.

Pressure — Barometers & the Barograph

Air has weight, and that weight presses down on the Earth's surface as atmospheric pressure, measured in millibars (mb). Average sea-level pressure is 1013 mb. There are three ways of measuring it:

Sunshine Hours — Campbell-Stokes Recorder

A Campbell-Stokes sphere is a glass ball mounted in a metal frame, positioned so it focuses sunlight — like a magnifying glass — onto a specially marked card sitting below the focal point. When the sun is strong enough, the focused rays scorch a burn line into the card as the sun moves across the sky through the day. At the end of the day, you measure the total length of the burn line and convert it into hours and minutes of actual bright sunshine (cloud cover breaks the burn line, which is exactly the point).

Placement matters: it must be south-facing in the Northern Hemisphere (or north-facing in the Southern Hemisphere), somewhere fully exposed with no shade at any point in the day — commonly on a roof or raised stand.

Clouds

Clouds form when air rises, cools, and condenses — turning invisible water vapour into visible tiny water droplets or ice crystals that are light enough to float. Clouds only produce precipitation once enough of those droplets/crystals collide and stick together, growing heavy enough to fall through the rising air currents that were holding them up.

Clouds are classified by height and shape, using Latin roots: cirrus (wispy, high), stratus (layered), cumulus (heaped), and nimbus (rain-bearing). You can combine these — cirrostratus, altocumulus, nimbostratus and so on describe a cloud's height and shape at once.

Tropical Storms

A tropical storm is a deep, low-pressure system with spiralling bands of intense wind, forming over warm tropical oceans (sea temperature above 27°C is the trigger). Warm, moist air rises and is pulled towards the centre, spiralling around a surprisingly calm central "eye." The same phenomenon has different names depending where it forms:

Storms can be up to 800 km across, but wind isn't uniform throughout — the strongest and most destructive winds are found in the eyewall, immediately surrounding the calm eye, not at the outer edges. This is why the eye passing over can create a deceptive lull before the storm returns with full force from the opposite direction.

2. Working With Weather Data — Calculations

Rainfall

Rainfall is always measured in mm and always plotted as a bar graph (never a line graph — this is a very common mark-losing mix-up with temperature). Daily totals are added to give weekly, monthly and annual totals.

Temperature

Measured in °C or °F, and always plotted as a line graph. You should be able to calculate:

• Diurnal (daily) range
• Mean daily temperature
• Mean monthly temperature
• Annual range and mean annual range

Wind

Measured in knots, mph, or km/h. Remember direction is always stated as where the wind is coming from. Two special terms:

Pressure

Pressure is measured in millibars (mb), based against the mean sea-level pressure of 1013 mb. Unlike rainfall, it isn't totalled over time — it's recorded on whatever schedule the situation demands (hourly during a developing storm, daily for routine forecasting).

In the Northern Hemisphere: anticyclones (high pressure) have descending air, light winds blowing clockwise, and settled, clear conditions (less cloud formation). Depressions (low pressure) have rising air blowing anticlockwise — that rising air cools, causing condensation, cloud, and often precipitation. This is why depressions bring unsettled weather and anticyclones bring calm, dry spells.

3. Interpreting Weather & Climate Data

Climate Graphs

A climate graph combines mean monthly temperature (line graph) and mean monthly precipitation (bar graph) over a 30-year period, on the same set of axes. To describe one properly in an exam, follow this sequence:

1. Describe the overall shape — is the temperature line steep or gentle? Does it change much across the year?
2. Identify extremes — highest and lowest temperature/rainfall, and which month each occurs in (always quote the unit).
3. Note any anomalies — a value that breaks the general trend.
4. Name the season with most/least rainfall — but remember equatorial climates don't really have seasons, and the Southern Hemisphere's seasons are the reverse of the Northern Hemisphere's (Australian summer runs Nov–March; European summer runs June–Sept).
5. Calculate the temperature range and the annual/average monthly rainfall.

Dispersion Graphs

A dispersion graph plots individual data values on a vertical axis against categories (e.g. different sites) on the horizontal axis. It's useful precisely because it shows whether data clusters tightly together or scatters widely — which a simple average would hide. To read one: check the title, understand both axes, describe the overall pattern, spot any anomalies, and calculate statistics like the mean, median and range if asked.

Wind Rose

A wind rose shows wind direction for a location using rectangles radiating outward like spokes from a central circle, one spoke per compass point. The length of each rectangle shows how many days or times the wind blew from that direction, and colour-coding within the rectangle typically shows speed bands. The centre circle shows how many days/hours had no wind at all (calm conditions).

Wind Barbs

Wind barbs are the little "flag" symbols used on synoptic weather maps to show both wind direction and speed in one compact symbol. The barb points towards the direction the wind is coming from (same "from" rule as the wind vane). The little marks on the tail show speed:

These combine — for example, a double flag plus 3 full barbs plus a half barb = 100 + 30 + 5 = 135 knots.

Isoline & Choropleth Maps

Synoptic Charts

A synoptic chart plots readings from many meteorological stations onto a single map, potentially showing wind speed and direction, pressure patterns (via isobars), weather fronts, cloud cover and temperature all at once. This is the type of map you see on TV weather forecasts, with an "L" marking a depression's centre and an "H" marking an anticyclone's centre, plus front symbols (cold, warm, occluded) tracing storm systems.

What to Memorise

Concepts Checklist

Exam Tips — Common Mistakes & Examiner Traps