APES 4.5-4.6: Earth's Geography and Climate

Climate

Climate is the long-term statistical pattern of weather in a region, including both average conditions and variability in variables such as temperature, precipitation, humidity, pressure, and wind.

How does climate differ from weather?

Weather describes short-term atmospheric conditions, whereas climate describes the long-term average and variability of those conditions. A useful summary is that climate is what is expected, while weather is what occurs at a particular time.

Why is a 30-year period commonly used to describe climate?

Thirty years is long enough to reduce the influence of year-to-year anomalies, such as El Niño, while remaining short enough to reveal longer-term trends. A 30-year arithmetic average of a climate variable is called a climate normal.

Climate normal

A climate normal is a reference average of a climate variable, usually calculated over 30 years, used to compare current conditions with what is considered typical.

Which variables are commonly used to characterize climate?

Common climate variables include air temperature, precipitation, atmospheric pressure, wind, humidity, cloud amount, visibility, solar radiation, and evaporation rate.

Which geographic factors influence the climate of a location?

Latitude, longitude, altitude, terrain, land use, nearby bodies of water, ocean currents, and proximity to mountains or oceans can all affect local climate.

Why does latitude strongly influence climate?

Latitude affects the angle at which solar radiation reaches Earth's surface and therefore the amount of energy received per unit area. It also helps determine the length of daylight and seasonal changes.

How can altitude affect climate?

Temperature generally decreases with increasing altitude, so high-elevation regions are often cooler than nearby lowlands. Changes in elevation can shift climate zones and the organisms adapted to them.

How do oceans and ocean currents influence regional climate?

Water has a high heat capacity, so oceans absorb and release large amounts of energy while moderating nearby temperatures. Ocean currents redistribute heat between the tropics and higher latitudes and can make some coastal regions warmer or cooler than expected.

Thermohaline circulation

Thermohaline circulation is large-scale ocean circulation driven by differences in water density caused mainly by temperature and salinity. It transports heat and can produce substantial regional climate differences.

What is a rain shadow?

A rain shadow is a relatively dry region on the leeward side of a mountain range. Moist air rises on the windward side, cools and loses moisture as precipitation, then descends on the leeward side and becomes warmer and drier.

How does topography create wet and dry sides of a mountain range?

As air is forced upward over the windward side, it expands, cools, and condenses, producing precipitation. After crossing the summit, the air descends, warms, and becomes drier, creating a rain-shadow region.

Köppen climate classification

The Köppen system classifies climates primarily using characteristic temperature and precipitation patterns and relates climate zones to typical biomes. Its major groups include tropical, dry, temperate, continental, and polar climates.

What are the major climate zones?

The major climate zones are tropical, dry, temperate, continental, and polar. They are distinguished primarily by long-term patterns of temperature and precipitation.

What is the normal state of the tropical Pacific during the ENSO cycle?

Normally, easterly trade winds push warm surface water toward the western tropical Pacific. This allows cold, nutrient-rich water to upwell near the eastern Pacific coast of South America.

What happens during an El Niño event?

During El Niño, tropical Pacific trade winds weaken or may reverse, allowing warm surface water to move eastward. Upwelling of cold, nutrient-rich water decreases near the eastern Pacific, altering precipitation, temperature, and marine productivity patterns.

What happens during a La Niña event?

During La Niña, tropical Pacific trade winds become stronger than normal. Warm water is pushed farther west, while cold-water upwelling in the eastern and central tropical Pacific intensifies.

How do El Niño and La Niña generally differ?

El Niño is characterized by unusually warm surface waters in the central and eastern tropical Pacific, weakened trade winds, and reduced eastern-Pacific upwelling. La Niña is characterized by unusually cool surface waters there, stronger trade winds, and enhanced upwelling.

What is ENSO?

The El Niño–Southern Oscillation, or ENSO, is a recurring climate pattern involving changes in tropical Pacific Ocean temperatures, trade winds, air pressure, ocean circulation, and global weather patterns. Its warm phase is El Niño and its cool phase is La Niña.

How can ENSO affect ecosystems and human activities?

ENSO changes the distribution of heat and moisture, influencing regional precipitation, droughts, floods, storms, and temperatures. El Niño's reduced upwelling can lower marine nutrient availability and disrupt fisheries, while La Niña often produces contrasting oceanic and atmospheric conditions.