Climate is the long-term pattern of weather in a given place, averaged over decades rather than days. Scientists use a standard 30-year window to calculate what’s “normal” for a location. So while weather tells you whether to grab an umbrella today, climate tells you whether a city is rainy, dry, hot, or cold in general. The current official reference period runs from 1991 to 2020, and the World Meteorological Organization requires every member nation to recalculate these averages at least every 30 years.
Five Major Climate Types
The most widely used system for describing climates was developed by climatologists Wladimir Köppen and Rudolf Geiger. It divides the world into five broad groups based on temperature and precipitation patterns, each producing very different day-to-day conditions depending on where you live.
Tropical (A): Found near the equator, these climates stay warm year-round, with every month averaging above 64°F (18°C). Tropical rainforests sit in this group, with annual temperatures hovering between 68°F and 86°F and more than 80 inches of rain per year. The temperature barely shifts between seasons. In fact, the difference between daytime highs and nighttime lows on a single day (10 to 12°F) is often larger than the difference between the warmest and coolest months (just 3 to 4°F). Tropical monsoon and savanna climates also fall here, distinguished by whether they have a pronounced dry season.
Arid (B): Deserts and dry steppes, where evaporation outpaces rainfall. In the driest zones, classified as “hyperarid,” annual rainfall is less than 3% of what would evaporate under the same conditions. Semi-arid regions get more moisture but still lose more water to evaporation than they receive. These climates range from scorching subtropical deserts, where frost is rare, to cold mid-latitude deserts with freezing winters.
Temperate (C): Mild climates with distinct warm and cool seasons. The humid subtropical climate common in the southeastern United States fits here: hot summers above 72°F, cool winters below 64°F, and rain spread throughout the year. Mediterranean climates, with dry summers and wet winters, also belong to this group.
Continental (D): Found in interior regions of large landmasses, these climates swing between hot summers and severe winters. Subarctic areas at the extreme end can see brutally cold winters with cool, short summers.
Polar (E): The coldest climates on Earth, where even the warmest month stays below 50°F (10°C). Tundra regions have no true summer, and ice cap zones are covered in permanent ice and snow.
What Determines Your Local Climate
Two places at the same latitude can have vastly different climates. Several geographic factors interact to shape what you experience.
Latitude is the most fundamental driver. As you move away from the equator, average yearly temperatures drop. Latitude also creates global pressure belts that influence precipitation: areas near the equator and around 60° north and south tend to be wet, while zones around 30° north and south are often dry, which is why many of the world’s deserts sit at those latitudes.
Elevation works like latitude in miniature. Temperatures decrease as you climb, which is why a mountain city near the equator, like Quito, Ecuador, can be cool year-round despite its tropical latitude.
Proximity to water has a powerful moderating effect. Coastal areas experience smaller swings between daytime and nighttime temperatures and between summer and winter. Inland locations near the center of a continent tend to have wider temperature extremes and drier conditions, since moisture-carrying air loses water content as it travels over land.
Mountain ranges create sharp climate boundaries. The side facing incoming wind (the windward side) gets cooled and receives heavy precipitation. The opposite side (the leeward side) is warmer and drier, a phenomenon called a rain shadow. The Cascade Range in the Pacific Northwest is a textbook example: lush forests on the western slopes, dry grassland to the east.
Ocean currents redistribute heat across the globe. Warm currents raise temperatures along eastern coastlines, while cold currents cool western coastlines. This is why Portugal feels milder than Newfoundland despite sitting at nearly the same latitude.
Microclimates in Cities
Climate can vary meaningfully over short distances, especially in urban areas. Buildings, roads, and pavement absorb and re-emit solar heat far more efficiently than forests or open land. In the United States, this “heat island” effect raises daytime temperatures in cities by 1 to 7°F compared to surrounding rural areas, and nighttime temperatures by 2 to 5°F. Tall buildings and narrow streets can also block natural wind flow, trapping warm air. If you live in a dense urban neighborhood, your local climate may be noticeably warmer than a suburb just a few miles away.
How Climate Is Shifting
The 30-year averages that define climate are not static. Global average surface temperature in 2024 was 1.60°C warmer than pre-industrial levels (roughly 1850 to 1900), making it the hottest year on record and the first calendar year to clearly exceed the 1.5°C threshold set by the Paris Agreement. Atmospheric carbon dioxide is projected to reach 425.7 parts per million in 2025, about 52% above pre-industrial concentrations.
These shifts mean the climate “normals” for many locations are already outdated by the time they’re published. A city’s 1991 to 2020 average may understate how warm or wet conditions have become in recent years. Practically, this shows up as hotter summers, shifting growing seasons, altered rainfall patterns, and more frequent extreme weather events in many regions around the world.