5.3 Weather and Climate

Weather and climate shape daily lives and long-term environmental conditions. Weather is the day-to-day state of the atmosphere, while climate represents long-term patterns over decades or more.

Understanding these concepts matters for predicting short-term events like storms and for tracking long-term trends like shifting growing seasons. Factors like temperature, humidity, and air pressure drive weather, while global patterns and geographical features define climate zones worldwide.

Weather vs Climate

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Defining Weather and Climate

Weather refers to the day-to-day atmospheric conditions in a specific location.

• Includes temperature, humidity, precipitation, and wind
• Can change rapidly over short periods (hours or days)
• Examples: a sunny afternoon, a thunderstorm, a cold front passing through

Climate describes the long-term average weather patterns in a region, typically measured over a period of 30 years or more. It represents the expected conditions for a given location and time of year.

• Hot and humid year-round in tropical rainforests
• Dry and arid in deserts like the Sahara
• Cold and snowy in polar regions like Antarctica

A helpful way to remember the difference: climate is what you expect, weather is what you get.

Comparing and Contrasting Weather and Climate

• Weather is short-term and highly variable; climate is long-term and more stable
• Weather can be influenced by local factors (topography, urban heat islands), while climate is shaped by global factors (latitude, ocean currents, atmospheric circulation patterns)
• Weather forecasts predict conditions for the near future (days to weeks), while climate projections estimate trends over decades to centuries
• Both matter for practical decisions in agriculture, transportation, energy production, and disaster preparedness

Factors Influencing Weather

Temperature, Humidity, and Air Pressure

Temperature measures the average kinetic energy of molecules in the atmosphere.

• Influenced by latitude (how directly the sun's rays hit), altitude (air is thinner and cooler higher up), and proximity to large bodies of water (water heats and cools more slowly than land)
• Affects air density, pressure, and the formation of convection currents

Humidity is the amount of water vapor present in the atmosphere.

• Influences cloud formation, precipitation, and how hot it feels (the heat index)
• Can be measured as absolute humidity ($\text{g/m}^3$), relative humidity (%), or dew point temperature
• Relative humidity is the most commonly reported: it tells you how close the air is to being fully saturated. At 100% relative humidity, the air can't hold any more water vapor, and condensation begins.

Air pressure is the force exerted by the weight of the atmosphere on a given surface area.

• Warm air is less dense and rises, creating low pressure areas. Cool air is denser and sinks, creating high pressure areas.
• Pressure decreases with altitude because there's less atmosphere above you
• Air moves from high to low pressure, which is what creates wind

Wind and Atmospheric Circulation

Wind is the horizontal movement of air from areas of high pressure to areas of low pressure. Three main factors shape wind patterns:

1. Temperature differences create pressure gradients that get air moving in the first place
2. The Coriolis effect deflects moving air due to Earth's rotation (to the right in the Northern Hemisphere, to the left in the Southern Hemisphere)
3. Friction from the Earth's surface slows wind near the ground

Global wind patterns like the trade winds and westerlies are driven by unequal heating of Earth's surface. This unequal heating sets up three large atmospheric circulation cells in each hemisphere:

• Hadley cells (0°–30°): Warm air rises at the equator, flows poleward, cools, and sinks around 30° latitude
• Ferrel cells (30°–60°): Air circulates between the Hadley and Polar cells, driven partly by the other two
• Polar cells (60°–90°): Cold air sinks at the poles and flows toward 60° latitude

Local wind patterns result from smaller-scale temperature differences. Sea breezes blow from water to land during the day (land heats faster), and the pattern reverses at night. Mountain-valley breezes work similarly, with air flowing upslope during the day and downslope at night.

Air Mass Formation and Characteristics

Types and Source Regions of Air Masses

An air mass is a large body of air with relatively uniform temperature and humidity throughout. Air masses form over source regions where air sits long enough to take on the temperature and moisture properties of the surface below.

The naming system uses two parts: the surface type (continental or maritime) and the latitude (polar or tropical).

• Continental Polar (cP): Cold and dry. Forms over high-latitude land masses like central Canada and Siberia.
• Continental Tropical (cT): Hot and dry. Forms over arid regions like the Sahara and the Australian Outback.
• Maritime Polar (mP): Cold and moist. Forms over high-latitude oceans like the North Atlantic and North Pacific.
• Maritime Tropical (mT): Warm and moist. Forms over subtropical oceans like the Gulf of Mexico and the Caribbean Sea.

Air Mass Modification and Interaction

Air masses change as they move. A continental air mass picks up moisture as it crosses a body of water. A maritime air mass loses moisture as it moves over land. This process is called air mass modification.

When two different air masses meet, the boundary between them is called a front. Fronts are where most interesting weather happens.

• Cold front: A cold air mass pushes under and displaces a warm air mass. The warm air is forced up quickly, often producing thunderstorms and heavy but short-lived precipitation. Temperatures drop after the front passes.
• Warm front: A warm air mass slides up and over a retreating cold air mass. The gradual lifting produces widespread stratus clouds and steady, lighter precipitation. Temperatures rise after the front passes.
• Stationary front: Two air masses meet but neither advances. This can cause prolonged cloudiness and precipitation over the same area for days.
• Occluded front: A faster-moving cold front catches up to a warm front, lifting the warm air completely off the ground. This produces complex weather with mixed precipitation types.

Climate Zones and Weather Patterns

Major Climate Zones

Earth's surface is divided into three main climate zones based on latitude and the angle of incoming solar radiation.

• Tropical zone (23.5°N to 23.5°S, between the Tropics of Cancer and Capricorn)
  • High temperatures, high humidity, and abundant rainfall year-round
  • Examples: Amazon rainforest, Congo Basin, Indonesian archipelago
• Temperate zones (23.5° to 66.5° in both hemispheres)
  • Distinct seasonal changes in temperature and precipitation
  • Examples: Eastern United States, Western Europe, Eastern China
• Polar zones (66.5° to 90° in both hemispheres)
  • Extremely cold temperatures, low humidity, and limited precipitation (mostly snow)
  • Examples: Arctic tundra, Antarctic ice sheet, Greenland ice cap

Köppen Climate Classification System

The Köppen system is the most widely used method for classifying climates. It goes beyond the three basic latitude zones by factoring in temperature, precipitation, and seasonality.

There are five main climate groups:

• A (Tropical): Warm year-round with high rainfall
• B (Arid): Dry climates where evaporation exceeds precipitation
• C (Temperate): Mild winters with moderate precipitation
• D (Continental): Large temperature swings between warm summers and cold winters
• E (Polar): Cold year-round with very little precipitation

Each group is further divided into sub-categories using additional letters. For example:

• Af = Tropical rainforest (hot and wet all year)
• BWh = Hot desert (very dry, high temperatures)
• Cfb = Marine west coast (mild, wet winters; cool summers)
• Dfc = Subarctic (short cool summers, long cold winters)
• ET = Tundra (warmest month below 10°C but above 0°C)

This system gives scientists and geographers a standardized way to describe and compare climates across different regions.

Factors Influencing Climate Patterns

Several large-scale factors work together to determine a region's climate:

• Latitude: Determines how much solar radiation a region receives. Areas near the equator get more direct sunlight and are warmer; areas near the poles get less and are colder.
• Atmospheric circulation: The Hadley, Ferrel, and Polar cells redistribute heat and moisture globally. Deserts tend to form around 30° latitude where dry air sinks in the Hadley cell.
• Ocean currents: Transfer heat between regions and strongly affect coastal climates. The Gulf Stream, for example, carries warm water from the Gulf of Mexico northeast across the Atlantic, keeping Western Europe significantly warmer than other regions at the same latitude.
• Distribution of land and water: Land heats up and cools down faster than water. This creates temperature contrasts that influence precipitation patterns and give coastal areas more moderate climates than continental interiors.
• Topography: Mountains force air upward, causing it to cool and release moisture on the windward side. The leeward side receives much less precipitation, creating a rain shadow. The eastern side of the Andes and the leeward slopes of the Himalayas are classic examples.