Weather refers to the condition of the Earth’s atmosphere at a specific location and moment in time. This dynamic state results from the atmosphere constantly attempting to achieve balance through the movement and exchange of energy and matter. All weather phenomena are driven by a limited number of measurable atmospheric properties. Meteorologists constantly record these properties to understand and forecast the short-term fluctuations of our atmosphere. This article defines the six primary elements that combine to create the weather experienced on Earth.
Air Temperature
Air temperature measures the amount of heat energy present in the atmosphere, representing the kinetic energy of the air molecules. When air is heated, it expands, making warmer air less dense than cooler air.
Differences in air density are foundational to all weather processes because they drive vertical atmospheric motion. Warmer, less dense air rises due to buoyancy, while cooler, denser air sinks under gravity. This rising and sinking motion, known as convection, is the primary mechanism for transferring heat and moisture. The resulting vertical movement dictates atmospheric stability and initiates the formation of clouds and precipitation.
Temperature also governs how much water vapor the atmosphere can hold. Measuring air temperature is routinely done using a thermometer, providing a basic metric for forecasting atmospheric conditions.
Atmospheric Pressure and Wind
Atmospheric pressure is the force exerted by the column of air above a given point. It is commonly measured in millibars (mb) using a barometer. Gravity causes air to be densely packed near the surface, which is why pressure decreases rapidly with altitude.
Variations in pressure across the surface are created largely by the uneven heating of the atmosphere. When air is heated and rises, it creates lower surface pressure; conversely, sinking cold air creates higher pressure. These pressure differences establish a pressure gradient force, which directly drives horizontal air movement.
Wind is the air moving horizontally from areas of high pressure to areas of low pressure. Wind speed is determined by the steepness of the pressure gradient; air moves faster when the pressure difference over a short distance is greater. Anemometers measure the velocity of this moving air.
The movement of wind redistributes heat and moisture across the planet, playing a significant role in weather system development. Wind direction is also influenced by the Earth’s rotation, known as the Coriolis effect, which causes moving air to deflect and form large-scale circulation patterns.
Atmospheric Moisture
Atmospheric moisture refers to the amount of water vapor present in the air. Relative humidity is the measure most commonly used in weather reports, expressed as a percentage.
Relative humidity indicates the ratio of the current water vapor in the air compared to the maximum amount the air can hold at that temperature. Because warmer air holds more water vapor than cold air, relative humidity changes if the temperature changes. A reading of 100% relative humidity means the air is completely saturated.
The dew point is a temperature-based measurement that indicates the temperature to which the air must be cooled to become saturated. When the air temperature drops to the dew point, water vapor condenses into liquid form, resulting in dew, fog, or the formation of cloud droplets.
Water Release
Precipitation occurs when water falls from the atmosphere to the Earth’s surface. This process begins after water vapor condenses around tiny particles, known as condensation nuclei, forming microscopic cloud droplets. These droplets are initially too small to fall against the upward air currents sustaining the cloud.
For precipitation to occur, the droplets must grow large enough to overcome air resistance and gravity. In warmer clouds, this growth happens through the collision-coalescence process. Larger droplets collide with and merge with smaller droplets, growing heavier until they fall as rain.
In colder clouds, the Bergeron process is responsible for growth. This involves the co-existence of supercooled water droplets and ice crystals, where water vapor deposits directly onto the ice crystals, causing them to rapidly grow into snowflakes. The resulting precipitation can take various forms, including rain, snow, sleet, or hail, depending on the temperature profile below the cloud.
Energy Input
Solar radiation drives nearly all weather phenomena on Earth. This incoming energy is absorbed by the Earth’s surface and atmosphere. The amount of heat absorbed directly dictates the air temperature, which is the starting point for atmospheric circulation.
Solar radiation is not distributed evenly across the globe. Equatorial regions receive more concentrated, direct energy, while the poles receive energy spread over a larger area. This imbalance, known as differential heating, establishes a fundamental temperature gradient between the equator and the poles.
This global temperature difference creates the large-scale atmospheric pressure variations that generate wind and ocean currents. Solar energy also powers the entire water cycle by driving evaporation, which introduces moisture into the atmosphere.