Weather is the state of the atmosphere at a specific time and location. To describe and forecast this state, meteorologists rely on measurable variables, often called the elements of weather. By precisely measuring these variables, scientists characterize the atmosphere’s immediate state and project its future changes.
Measuring Heat and Pressure
Temperature is a fundamental weather element, representing the degree of hotness or coldness of the air. This measurement is proportional to the average kinetic energy of the air molecules. It is measured using a thermometer, which historically relied on the expansion and contraction of liquids like mercury, but modern systems often use electronic resistance temperature detectors (RTD).
For an accurate reading of air temperature, the sensor must be protected from direct sunlight, which would otherwise artificially raise the measurement. Weather stations achieve this by placing the thermometer inside a white, louvered enclosure, such as a Stevenson screen, two meters above the ground. Differences in air temperature are primary drivers of air movement.
Barometric pressure is another foundational element, defined as the force exerted on a surface by the weight of the air column above it. This pressure is measured with a barometer, which can be a traditional mercury column, a sealed metal aneroid cell, or a modern capacitive sensor. Standard sea-level pressure averages around 1013.25 hectopascals (hPa), also known as millibars (mb).
Changes in barometric pressure are closely linked to shifts in weather patterns. Generally, rising pressure indicates fair, stable weather, while a sudden drop often signals the arrival of a storm. The movement of air from areas of high pressure to areas of low pressure is the driving mechanism for wind, directly connecting pressure to atmospheric motion.
The Role of Atmospheric Moisture
Atmospheric moisture determines the potential for clouds and precipitation. Relative humidity (RH) quantifies this moisture, expressing the amount of water vapor in the air as a percentage of the maximum amount the air can hold at that specific temperature. Because warmer air holds more moisture than cold air, relative humidity changes with both temperature and actual moisture content.
A more absolute measurement of moisture is the dew point, which is the temperature to which the air must be cooled for saturation to occur, resulting in a relative humidity of 100%. When the air temperature drops to the dew point, water vapor begins to condense into liquid water. This value is a better indicator of the air’s actual moisture content and how muggy it will feel outside.
When air cools to its dew point while suspended in the atmosphere, water vapor condenses around microscopic particles like dust or pollen, forming clouds. Clouds are visible masses of tiny liquid water droplets or ice crystals. Cloud cover determines the amount of solar radiation reaching the surface and influences the Earth’s energy balance.
Movement and Water Delivery
Wind is the horizontal movement of air, driven by pressure differences established by temperature variations. Air moves from a high-pressure zone to a low-pressure zone, and this movement is quantified by both speed and direction. Wind speed is measured using an anemometer, which often uses rotating cups or a propeller to catch the airflow.
The direction of the wind is equally important, measured by a wind vane that aligns itself with the flow, indicating the direction from which the air is blowing. For instance, a west wind is air moving from the west toward the east. Both wind speed and direction are recorded to understand the transport of heat and moisture across the globe.
The process of water delivery from the atmosphere to the Earth’s surface is known as precipitation, which occurs when condensed water in clouds becomes too heavy to remain suspended. Rain is the most common form, consisting of liquid water droplets that fall when the air temperature below the cloud is above freezing. The distinct types of precipitation are determined by the temperature profile in the lower atmosphere. These forms include:
- Snow: Forms when water vapor freezes directly into hexagonal ice crystals within the cloud and remains frozen during descent.
- Sleet: Occurs when snow melts into rain after passing through a warm layer, then refreezes into small ice spheres in a deeper layer of sub-freezing air near the surface.
- Freezing rain: Falls as liquid water that freezes only upon contact with a surface that is below the freezing point.
- Hail: Composed of layered lumps of ice that form within strong thunderstorms with powerful updrafts that repeatedly lift water droplets into very cold regions of the cloud.