Weather & Climate

Weather vs Climate

Weather describes the short-term atmospheric conditions at a specific place and time — temperature, rainfall, wind speed and direction, cloud cover, and humidity. Weather changes from hour to hour and day to day.

Climate is the long-term average of weather conditions for a region, typically measured over a period of 30 years or more. It describes the expected patterns, not the day-to-day variability.

"Climate is what you expect; weather is what you get."

Meteorology is the study of weather; climatology is the study of climate.

Weather is measured using instruments:

• Thermometer — temperature
• Rain gauge — rainfall
• Anemometer — wind speed
• Wind vane — wind direction
• Barometer — atmospheric pressure
• Hygrometer — humidity

Factors Affecting Climate

Six main factors control the climate of any location:

1. Latitude

Latitude is the angle north or south of the equator. It is the single most important factor affecting temperature:

• At the equator (0°): the Sun's rays strike the surface directly (small angle of incidence); energy is concentrated in a small area → hot
• At the poles (90°N/S): the Sun's rays arrive at a low angle; energy is spread over a much larger area and passes through more atmosphere → cold

Temperature decreases as latitude increases (moving away from the equator).

2. Altitude

Temperature decreases by approximately 6.5°C per 1000 m of altitude gained. This environmental lapse rate exists because:

• Air pressure decreases with altitude
• Air expands and cools as it rises
• Less atmosphere above to trap heat

This is why mountain peaks are cold even in tropical regions (e.g., Mount Kilimanjaro in Tanzania is near the equator but has permanent snow on its summit).

3. Distance from the Sea (Continentality)

Water has a high specific heat capacity — it heats and cools much more slowly than land. This gives coastal areas a maritime climate: mild, stable temperatures year-round with relatively high rainfall.

Inland areas have a continental climate: hotter summers and colder winters (greater temperature range), and drier conditions because moisture from the sea does not reach far inland.

Example: London (coastal) has milder winters than Moscow (inland at similar latitude).

4. Prevailing Winds

The dominant wind direction brings the characteristics of whatever surface it has blown over:

• Onshore winds (from the sea): moist, mild, brings rainfall
• Offshore winds (from land): dry, more temperature extremes
• Trade winds blow from subtropical high pressure zones toward the equator

The UK's prevailing winds blow from the southwest, bringing mild, wet conditions from the Atlantic Ocean.

5. Ocean Currents

Warm ocean currents transfer heat from tropical to polar regions, warming the coastlines they flow past. Cold currents cool coastal areas.

• The Gulf Stream / North Atlantic Drift flows from the Gulf of Mexico northeastward past Western Europe, warming the UK and Norway far beyond what their latitude would suggest
• The Canary Current (cold) flows south along the Iberian Peninsula, cooling the coasts of Portugal and Morocco

Thermohaline circulation (the "ocean conveyor belt") is a global system of ocean currents driven by differences in water density (temperature and salinity). It transports heat around the entire planet.

6. Aspect and Relief

• Aspect: slopes facing the Sun (south-facing in Northern Hemisphere) receive more solar radiation and are warmer and drier
• Relief: mountains force air to rise, cool, and drop rain on the windward side (orographic rainfall); the leeward side receives less rain (rain shadow effect)

Climate Zones

Earth's surface is divided into broad climate zones based on temperature and precipitation patterns:

Climate zone	Typical location	Temperature	Rainfall	Example regions
Tropical rainforest	0–10° of equator	Hot year-round (25–30°C)	Very high (2000+ mm/year)	Amazon, Congo basin
Savanna	10–20° latitude	Hot; distinct wet/dry seasons	Seasonal (500–1500 mm)	East Africa, northern Australia
Desert (arid)	~25–35° latitude	Extreme heat in day; cold at night	Very low (<250 mm/year)	Sahara, Atacama
Mediterranean	~30–45° latitude	Hot dry summers; mild wet winters	Moderate (400–900 mm)	Mediterranean coast, California
Temperate	40–60° latitude	Mild; four seasons	Moderate (500–1000 mm)	Western Europe, New Zealand
Continental	40–65° latitude (inland)	Large seasonal temperature range	Low to moderate	Central Russia, central USA
Polar/Tundra	65°+ latitude	Extremely cold	Very low (precipitation as snow)	Arctic, Antarctica

The Greenhouse Effect and Climate Change

The greenhouse effect is a natural process that keeps Earth warm enough to support life:

1. Solar radiation passes through the atmosphere and warms Earth's surface
2. The surface emits infrared (heat) radiation
3. Greenhouse gases (CO2, methane, water vapour, nitrous oxide) in the atmosphere absorb some of this outgoing radiation and re-emit it in all directions, trapping heat

Without the greenhouse effect, Earth's average temperature would be about -18°C instead of +15°C.

Enhanced greenhouse effect (climate change): human activities (burning fossil fuels, deforestation, agriculture) have increased greenhouse gas concentrations, trapping more heat and raising global average temperatures. Consequences include:

• Rising sea levels (thermal expansion and melting ice)
• More frequent extreme weather events
• Shifting climate zones and biomes
• Ocean acidification (CO2 dissolves in seawater forming carbonic acid)

Worked Example

Q: Why is London (UK) milder and wetter than Moscow (Russia), even though both are at similar latitudes?

Both cities are at roughly 51–56°N, yet their climates differ greatly:

• Distance from the sea: London is close to the Atlantic Ocean. Water heats and cools slowly, moderating temperatures — mild winters and warm summers. Moscow is deep inland with little moderating sea influence — very cold winters and hot summers.
• Prevailing winds: The UK receives westerly winds from the Atlantic, bringing moisture and mild air. Moscow is exposed to cold continental air masses from Central Asia in winter.
• Ocean currents: The North Atlantic Drift carries warm water toward northwest Europe, further warming the UK's climate.

This illustrates how latitude alone does not determine climate — proximity to the sea, prevailing winds, and ocean currents all play crucial roles.

Key Terms

• Weather — short-term atmospheric conditions at a place and time
• Climate — long-term average weather conditions for a region (30+ years)
• Latitude — angular distance north or south of the equator; key control of temperature
• Altitude — height above sea level; temperature decreases with altitude
• Continentality — the effect of being far from the sea; produces greater temperature extremes
• Maritime climate — climate influenced by proximity to the sea; mild and moist
• Continental climate — inland climate; greater temperature range; drier
• Greenhouse effect — warming of Earth by atmospheric gases trapping outgoing infrared radiation
• Orographic rainfall — rain produced when moist air is forced to rise over mountains

Common Mistakes

• Confusing weather (short-term, variable) with climate (long-term, average) — a single hot day is weather; a region consistently having hot summers is climate
• Thinking altitude and latitude are the same — latitude is distance from equator (north-south); altitude is height above sea level
• Saying the greenhouse effect is entirely bad — the natural greenhouse effect is essential for life; the enhanced greenhouse effect from human emissions is the concern
• Forgetting that polar regions receive very little precipitation — they are technically cold deserts

Tips and Tricks

• Remember the six climate factors with LOWCAP: Latitude, Ocean currents, Wind, Continentality (distance from sea), Altitude, Prevailing aspect
• Latitude rule: closer to equator = hotter; every 1 degree of latitude = approximately 110 km of distance
• Altitude rule: 1000 m up = 6.5°C cooler — mountains are cold even near the equator
• Continental vs maritime: inland = extreme temperatures; coastal = mild and moist year-round