One engine, uneven fuel
Weather is what the atmosphere is doing at a given place and time. Almost all of it traces back to one fact: the sun heats Earth unevenly. The tropics receive more direct sunlight than the poles, land warms and cools faster than water, and day gives way to night. Air tries to even out those differences, and the movement involved is wind, cloud and storm.
This article is about those everyday mechanisms. How forecasters predict them is a separate topic.
Pressure and wind
Air has weight, and the pressure it exerts varies from place to place. When air is warmed it expands and rises, leaving lower pressure near the surface. Where air cools and sinks, pressure near the surface is higher. Air flows from higher toward lower pressure, and that flow is wind. The bigger the pressure difference across a distance, the stronger the wind tends to be.
On a rotating planet, moving air does not travel in a straight line from high to low. Earth’s rotation deflects it, a result known as the Coriolis effect. In the Northern Hemisphere air tends to circle clockwise around high-pressure centers and counterclockwise around low-pressure centers, with the pattern reversed in the Southern Hemisphere.
Highs and lows
In general, low-pressure systems are associated with rising air, clouds and unsettled weather. As the air rises it cools, and moisture in it can condense. High-pressure systems are associated with sinking air, which tends to suppress cloud, so conditions are often calmer and clearer. These are tendencies rather than guarantees, and local conditions such as fog or a temperature inversion can complicate matters.
Moisture and clouds
Air always holds some water vapor, and warm air can hold more than cold air. When air rises, it expands and cools. If it cools enough, the vapor condenses onto tiny particles such as dust or salt, forming the droplets or ice crystals that make up a cloud. This is the same process that makes dew on a cold glass.
Rain or snow falls when droplets or crystals grow too heavy to remain suspended. The wider cycle that moves water from oceans to sky to land and back is covered in the water cycle, and the heat released when vapor condenses is an important energy source for storms.
Humidity is often reported as a percentage. Relative humidity compares the water vapor in the air to the most it can hold at that temperature, which is why it often rises at night as the air cools, even if no moisture was added.
Air masses and fronts
Large bodies of air can take on the temperature and moisture of the region where they form. When two different air masses meet, the boundary between them is called a front.
- At a cold front, colder, denser air pushes under warmer air and forces it upward. This can bring a narrow band of heavy showers or thunderstorms, followed by cooler, often clearer conditions.
- At a warm front, warmer air slides up over colder air more gradually. This often brings broader, steadier cloud and rain ahead of the front, with warmer air behind it.
- Where the boundary barely moves, it is called a stationary front, and cloudy, wet conditions can linger.
Fronts are often found in low-pressure systems in the middle latitudes, which is why a change in wind direction, temperature and pressure often arrives together.
Thunderstorms and other intense weather
On a warm, humid day, the ground heats the air above it, and pockets of air can rise rapidly. If the rising air stays warmer than its surroundings, it keeps going, building a tall cloud. Within a thunderstorm cloud, ice, water droplets and moving air interact to separate electric charge, and lightning is the sudden discharge. Thunder is the sound produced as the air heated by the flash expands quickly.
Larger and more organized storms, such as tropical cyclones, draw their energy from warm ocean water and are shaped by the planet’s rotation. Their details lie beyond everyday weather, but the same ingredients of heat, moisture and rising air are at work.
Weather versus climate
Weather describes conditions over hours and days. Climate describes the pattern averaged over many years. A cold week does not on its own say much about climate, and a warm decade is not explained by one hot afternoon. The drivers of long-term change, such as the greenhouse effect, are studied separately from day-to-day variation.
A simple way to read the sky
A few observations offer clues, though none is foolproof. Falling pressure often accompanies approaching unsettled weather, and rising pressure often accompanies clearing. A shift in wind direction can signal a front passing. High, thin cloud sometimes arrives ahead of a warm front, while tall, dark clouds with towering tops point toward showers or storms. For anything affecting safety, such as severe storms, rely on official local warnings rather than guesses from the sky.