Reading climate from maps and data
Climate is the usual weather of a place, measured over about 30 years. Two numbers tell most of the story: the mean temperature (average of the year) and the precipitation (all rain, snow and hail added up, in millimetres).
A climate diagram shows 12 months. The line is temperature; the bars are rainfall. A map with isotherms (lines joining places of equal temperature) or isohyets (equal rainfall) shows the pattern over a whole region.
How to read it: (1) find the warmest and coldest month; (2) find the wettest and driest month; (3) subtract coldest from warmest to get the annual range; (4) add the monthly rain to get the yearly total.
What shapes regional climate
Inside a region four things matter most.
- Distance from the sea. Water warms and cools slowly. Near the sea winters are milder and summers cooler. Far inland the range between summer and winter is larger.
- Height. Air gets cooler by about 6.5 °C for every 1000 m you climb.
- Relief and wind. Moist air pushed up a mountain cools and drops rain or snow. So mountains get more precipitation than the plains.
- Air masses. Wet air from the ocean, cold air from the north, dry air from the interior meet over a region and change weather from year to year.
Regional climate and the growing season
The growing season is the number of days in a year when the daily temperature stays above about 5 °C, so plants can grow. It is long on warm plains, short at height or far north.
It decides what a farmer can grow and when to sow. On plains with about 220 growing days cereals, sugar beet and potatoes do well. In the mountains with only about 120 days, grass for animals and forest are common. A late frost or a dry spring can shorten the useful season.
River network
A river network is the main river and the streams (tributaries) that join it. The land that drains into one river is its drainage basin. The line of high ground between two basins is the watershed.
Rivers start high, in places with a lot of rain or melting snow, and flow downhill to the sea or a lake. Near the source they are fast and narrow; near the mouth they are slow and wide. Flow also changes through the year: high after snow melts or heavy rain (flood), low in dry summer (low water). This yearly pattern is the river's regime.
Example: the Vistula and Oder in Poland start in the southern mountains and flow north into the Baltic Sea.
Lakes and reservoirs
Natural lakes fill hollows. In northern Europe thousands of lakes sit in hollows left by ice sheets that melted about 10,000 years ago (Poland's lake districts are an example).
Reservoirs are artificial lakes behind a dam built across a river. They store water for drinking, farms and factories, help to control floods, and can make electricity. Their cost: land is flooded, fish paths are blocked and mud collects.
Water shortage
A water shortage happens when the water people use is more than the water that is available. Think of a simple balance: water in (rain and snow-melt) minus water used = water left.
Causes: little rain or a dry year, too much use in farms, factories and cities, wasted water from leaks, and polluted rivers that cannot be used. A common measure: a country with less than about 1,700 cubic metres of renewable water per person per year is under water stress. Ways to help: save water, fix leaks, reuse water, store rain, plant trees and wetlands, and use drip irrigation.
Key formulas and definitions
- Annual range of temperature = warmest month mean − coldest month mean
- Cooling with height ≈ 6.5 °C per 1000 m
- Water left = water in (rain + melt) − water used
- Growing season = number of days with mean temperature above about 5 °C
- Key terms: isotherm, isohyet, drainage basin, watershed, tributary, regime, reservoir, water stress
Worked examples
1. A town has a warmest month of 18 °C and a coldest month of −2 °C. Find the annual range.
Range = 18 − (−2) = 20 °C.
2. A coastal town is at sea level and averages 9 °C. A village on a mountain 1500 m higher is in the same region. About what mean temperature would you expect there?
Cooling = 6.5 × 1.5 = 9.75 °C. So the village is about 9 − 9.75 = about −1 °C on average.
3. Monthly rain in mm for a year is 40, 30, 35, 40, 55, 70, 85, 75, 55, 45, 45, 45. Find the yearly total and the wettest month.
Add: 40+30+35+40 = 145; 55+70+85+75 = 285; 55+45+45+45 = 190. Total = 145+285+190 = 620 mm. Wettest month is the 7th (July), 85 mm.
4. A city's reservoir gets 50 million litres a day from the river but the city uses 65 million litres a day. What is the daily balance and what happens in 10 days?
Balance = 50 − 65 = −15 million litres per day. In 10 days the reservoir loses 150 million litres. It will keep falling, so a shortage is coming unless use falls or inflow rises.
Common mistakes
- Saying mountains are always drier. Their windward slopes usually get more rain and snow than the plains.
- Mixing up weather and climate. Weather is today; climate is the 30-year pattern.
- Thinking a reservoir makes new water. It only stores water that the river brought.
- Calling a tributary the main river, or the watershed a river. The watershed is a line of high ground.