Weather and Climate
Atmosphere science rests on a distinction that trips up many students: weather and climate describe the same variables but across completely different timescales.
Weather refers to the specific atmospheric conditions at a particular place and time, including temperature, humidity, air pressure, precipitation, wind speed, and cloud cover.
Climate is the long-term average of atmospheric conditions in a region, calculated over approximately 30 years.
Weather is what you experience walking outside on a given morning. It shifts over hours or days as air masses, pressure systems, and local energy exchanges interact. Climate is the statistical pattern built from decades of those daily records: it tells you what to expect in a region across seasons and years.
The World Meteorological Organization uses 30-year intervals as the standard climatological baseline. The most recent official period ran from 1981 to 2010; many agencies are now shifting to 1991–2020 to reflect ongoing warming trends. The 30-year window is long enough to smooth out short-term fluctuations but still captures recent shifts in the climate system.
| Feature | Weather | Climate |
|---|---|---|
| Timescale | Hours to days | ~30 years |
| Scope | Localised and variable | Regional to global |
| Example | A thunderstorm tonight | The monsoon arriving every summer |
| Determined by | Passing air masses, pressure systems | Solar radiation, ocean currents, atmospheric circulation |
The 30-year baseline is a human statistical construct, not a natural boundary.
Its definition shapes what counts as "normal" climate, which in turn influences how we identify and measure climate change. As baselines shift, recorded anomalies shift with them.
What Determines Regional Climate
Several interacting factors set the long-term climate of any region. Understanding them also explains why the same latitude can host very different ecosystems.
Latitude governs the angle at which solar radiation strikes the surface. At the equator, sunlight arrives nearly vertically and is concentrated over a small area. At the poles, the same energy spreads across a much larger surface and must travel through a greater thickness of atmosphere, producing far colder conditions.
Altitude reduces temperature with height. The environmental lapse rate (the actual average fall in temperature through the real troposphere) is approximately 6.5°C per 1,000 m. High-altitude ecosystems, even in the tropics, can therefore resemble polar biomes.
Ocean currents redistribute heat along coastlines. Warm currents such as the Gulf Stream raise coastal temperatures significantly above the global average for that latitude; cold currents such as the Humboldt Current cool adjacent coasts and suppress rainfall, sometimes generating coastal deserts.
Distance from the sea controls how much the ocean buffers temperature swings. Coastal (maritime) climates have mild, wet conditions year-round because water absorbs and releases heat slowly. Continental interiors, cut off from that buffer, experience large seasonal extremes.
Prevailing winds carry moist or dry air masses across regions. The westerlies that dominate mid-latitude Europe and North America, and the trade winds in the tropics, are both products of large-scale atmospheric circulation.
Topography shapes local precipitation through orographic lift: moist air forced over mountains deposits rain on the windward side and descends dry on the leeward, creating rain shadows.
Human activities now modify climate at both local and global scales. Urbanisation creates heat islands. Deforestation alters albedo and evapotranspiration. Fossil fuel combustion raises atmospheric greenhouse gas concentrations, driving global warming.