Reading Climate Graphs and Understanding Climate Factors
A climate graph (climograph) shows the average monthly temperature as a line and the average monthly rainfall as bars for a place, and the main factors that affect climate are latitude, altitude, distance from the sea, ocean currents, prevailing winds and relief. In brief: read the temperature line for warmth and seasonality (maximum, minimum and annual range) and the bars for the amount and timing of rainfall (total and wet or dry seasons). Then explain the pattern using the factors: low latitudes are warmer; temperature falls about 6.5 °C per 1,000 m of height; coasts are milder than interiors; warm currents warm and cold currents cool the coast; winds from the sea bring rain; mountains cause rain on the windward side and a rain shadow on the leeward side.
Climate graphs are the standard way of summarizing the climate of a place. This guide explains how to read and describe a climate graph, describes each factor that affects climate, compares the factors in a table, works through a complete reading of a graph with computed values, outlines the history of climate classification and corrects common mistakes.
How do you read a climate graph?
A climate graph has the months J to D along the bottom, a temperature scale on one side (°C) for a red line graph, and a rainfall scale on the other side (mm) for blue bars. To describe it, follow these steps:
- Temperature: state the highest and lowest monthly means, with the months, and calculate the annual temperature range (highest − lowest). Is it hot, warm, mild or cold? Is the range large (continental) or small (maritime or tropical)?
- Precipitation: add the monthly values to get the total, state the wettest and driest months and describe the pattern: evenly spread, a distinct wet and dry season, or winter or summer maximum.
- Seasons: note when the high temperatures and rain occur, and whether they coincide.
- Interpret: use numbers from the graph, name the likely climate type and hemisphere (the warmest months in December to February indicate the Southern Hemisphere), and explain the pattern with the climate factors.
What factors affect climate?
- Latitude: the angle of the Sun’s rays and the length of the day vary with latitude. Near the equator, the Sun is nearly overhead and the energy is concentrated, so it is hot all year; near the poles the rays are oblique and spread out, and the polar night lasts for months. The seasonal range increases with latitude.
- Altitude: the temperature decreases by about 6.5 °C for each 1,000 m of height (the environmental lapse rate), because the air is thinner and absorbs less of the heat radiated from the ground.
- Distance from the sea (continentality): water heats and cools more slowly than land, so coasts have cooler summers and milder winters, and interiors have a larger temperature range and less rain.
- Ocean currents: warm currents (the North Atlantic Drift, bringing mild winters to northwest Europe) raise the temperature of the coast; cold currents (the Humboldt or Peru Current, associated with the Atacama Desert) cool it and reduce rain.
- Prevailing winds: winds from the ocean carry moisture and bring rain; winds from the interior of a continent are dry. The direction of the prevailing wind therefore determines which coasts are wet.
- Relief: mountains force the air to rise, so the windward side receives heavy rain, while the leeward side is a dry rain shadow. Slope direction (aspect) also affects temperature: in the Northern Hemisphere south-facing slopes are warmer.
Climate factors compared
| Factor | Effect on temperature | Effect on rainfall | Example |
|---|---|---|---|
| Latitude | Cooler toward the poles | High near the equator, low near 30° | Rainforest vs. desert |
| Altitude | About 6.5 °C cooler per 1,000 m | Often more rain on mountains | Andean highlands |
| Distance from sea | Larger range inland | Less rain inland | Siberia vs. coast of Ireland |
| Ocean currents | Warm or cool the coast | Warm: more; cold: less | North Atlantic Drift |
| Prevailing winds | Depends on source | Onshore wind brings rain | Westerlies in Western Europe |
| Relief | Aspect effects | Windward wet, leeward dry | Rain shadow of the Sierra Nevada |
Worked example: reading a climate graph
A climate graph (hypothetical place) shows monthly mean temperatures (°C) from January to December of 9, 10, 12, 15, 19, 23, 26, 26, 23, 18, 13 and 10, and monthly rainfall (mm) of 70, 60, 50, 40, 30, 15, 5, 8, 30, 60, 80 and 85.
- Temperature extremes: the warmest months are July and August (26 °C) and the coolest is January (9 °C). The annual range is 26 − 9 = 17 °C.
- Mean annual temperature: the sum is 204, so the mean is 204 ÷ 12 = 17.0 °C.
- Total rainfall: the sum of the monthly values is 533 mm.
- Seasonal pattern: the driest month is July (5 mm) and the wettest is December (85 mm). June to August have 15 + 5 + 8 = 28 mm, which is 28 ÷ 533 = 5.3 percent of the year’s total, so the summers are hot and dry, and the winters mild and wet.
- Interpretation: the warmest months are July and August, so the place is in the Northern Hemisphere. Hot dry summers, mild wet winters and a moderate range are typical of a Mediterranean climate, found on the western sides of continents at latitudes of about 30°–40°, where the subtropical high-pressure belt moves over in summer and westerly winds bring rain in winter.
- Altitude effect: a town at the same latitude but 1,500 m higher would have a mean annual temperature of about 17.0 − 1.5 × 6.5 = 7.25 °C, so about 10 °C cooler.
How were climates classified?
The Greeks divided the world into climatic zones (hot, temperate and cold) by latitude. The modern systematic classification was developed by the German climatologist Wladimir Köppen from 1884, with a later version published in 1918 and revised by Rudolf Geiger. The Köppen–Geiger system uses temperature and precipitation to define five main groups: tropical (A), dry (B), temperate (C), continental (D) and polar (E). Climate graphs became a standard tool in geography teaching in the 20th century, and today digital data allow the climate of every grid cell on Earth to be mapped.
Which mistakes are common?
Mistake 1: Reading the wrong scale
Temperature is a line on one axis (°C) and rainfall is bars on the other (mm). Check which is which.
Mistake 2: Describing the graph without figures
Quote values from the graph, such as the temperature range and the total rainfall.
Mistake 3: Forgetting to find the hemisphere
The warmest months show whether the place is north (June to August) or south (December to February) of the equator.
Mistake 4: Giving only one factor as the explanation
Climate is usually the result of several factors together, such as latitude, altitude and the distance from the sea.