Weather and climate are not the same thing
Weather is the state of the atmosphere at a particular place and time: its temperature, humidity, wind, cloudiness and precipitation. Climate is the long-term pattern of that weather, usually described over decades. One unusually cold week says little about the climate of a region, and confusing the two is behind many popular climate claims. This article is about the first of these, the day-to-day behavior of the air.
The Sun is the engine
Almost all weather is powered by sunlight. Earth is heated unevenly. The tropics receive sunlight at a steep angle and warm up the most, while the polar regions receive it at a shallow angle and stay colder. Land warms and cools faster than water, so coasts and open oceans behave differently. Earth’s axis is tilted by roughly 23.5 degrees, which shifts which hemisphere gets the most direct sunlight through the year and produces the seasons.
The atmosphere acts as a huge system for moving heat from warm regions toward cold ones. Nearly all weather happens in its lowest layer, the troposphere, which is only roughly 8 to 18 kilometers deep depending on latitude and season. Gases in the air, including water vapor and carbon dioxide, also absorb and re-emit heat; this natural greenhouse effect is part of why the surface is warm enough for liquid water.
Air pressure and wind
Air has weight, and air pressure is the force of the air above pressing down on a surface. Average pressure at sea level is about 1,013 hectopascals (also called millibars), though it varies from place to place and hour to hour.
Warm air expands and becomes less dense, so it tends to rise. Where air rises, surface pressure is usually lower. Cool air is denser and tends to sink, which usually means higher surface pressure. Wind is air moving from higher toward lower pressure, a bit like water running downhill.
Because Earth rotates, moving air does not travel in a straight line. This deflection, called the Coriolis effect, bends winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The result is that air circles around pressure centers. In the Northern Hemisphere, wind spirals counterclockwise into a low-pressure area and clockwise out of a high.
Low-pressure areas generally have rising air, which is linked to clouds and precipitation. High-pressure areas have sinking air, which usually brings clearer, calmer conditions. These are tendencies, not guarantees.
Humidity, clouds and precipitation
The air always holds some water in the form of invisible vapor, supplied by evaporation from oceans, lakes, soil and plants as part of the water cycle. Warm air can hold more vapor than cold air. Relative humidity compares how much vapor is present with how much the air could hold at that temperature. The dew point is the temperature to which air must cool for vapor to begin condensing.
When air rises, it moves into lower pressure, expands and cools. If it cools to its dew point, vapor condenses onto tiny particles such as dust or sea salt, forming cloud droplets or ice crystals. This change is one of the everyday states of matter in action: gas turning into liquid, which also releases heat into the surrounding air.
Cloud droplets are very small, and they fall as rain, snow, sleet or hail only after growing large or heavy enough through collisions and freezing. Air can be lifted in several ways: by warming from the ground, by being pushed up over hills and mountains, by converging with other air, or by riding up over a front.
Air masses and fronts
An air mass is a large body of air with fairly uniform temperature and moisture, taking on the character of the region where it formed. A front is the boundary where two different air masses meet. Because the air masses do not mix easily, the boundary is where much of the change in weather happens.
- Cold front: Dense cold air pushes under warmer air and lifts it quickly. This often produces a narrow band of showers or thunderstorms, followed by cooler, drier air and clearing skies.
- Warm front: Warmer air slides gradually up and over cooler air. Clouds often thicken over a wide area, with steadier, longer-lasting rain or snow, and temperatures rise afterward.
- Stationary front: Neither air mass advances much, so cloudy or wet weather can linger over the same area.
- Occluded front: A faster cold front overtakes a warm front, lifting the warm air off the ground. Mature storms in middle latitudes often reach this stage.
Many large storms in the middle latitudes form along fronts, as a low-pressure center develops and the air begins to rotate around it.
The jet stream and large-scale patterns
High above the surface, narrow bands of very fast wind called jet streams flow roughly from west to east. They form where there are strong temperature contrasts and tend to wander in waves. These waves help steer lows and highs near the surface, which is one reason weather in the middle latitudes usually moves from west to east. In the tropics, steadier trade winds blow generally from east to west.
Local effects
Geography adds detail. A sea breeze can cool a coast on a sunny afternoon, because land heats faster than the nearby water and air flows inland to replace the rising warm air. Mountains force air upward on their windward side, often producing clouds and rain there and leaving a drier “rain shadow” on the far side. Large lakes can add moisture and heat to cold air passing over them, sometimes enhancing snowfall downwind.
Where the dramatic weather fits in
Thunderstorms occur when warm, moist air rises strongly; the electrical side of this is covered in the discussion of lightning strikes. Tropical cyclones draw their energy from warm ocean water, a process described under hurricane formation. And meteorologists combine measurements with physics-based computer models to predict all of this, a field known as weather forecasting. Each of these builds on the same basic ingredients: uneven heating, moving air, pressure differences and water changing form.