A barometer does not measure humidity; it is an instrument specifically designed to measure atmospheric pressure, which is the weight of the air pressing down on the Earth’s surface. Humidity, the amount of water vapor in the air, is measured by a different device called a hygrometer. Although measured separately, these two atmospheric properties are intrinsically linked in the complex patterns that form our weather.
The Role of the Barometer
A barometer’s sole function is to quantify atmospheric pressure, which is the force exerted by the column of air stretching from the instrument up to the edge of space. This measurement is profoundly useful for meteorologists because changes in pressure correspond directly to shifts in weather systems. A rising pressure typically indicates the approach of fair, stable weather, while a rapidly falling pressure often signals an incoming storm or precipitation.
The two primary types of barometers are the mercury and the aneroid models. A traditional mercury barometer measures pressure by balancing the weight of the air against a column of mercury in a glass tube. The height of the column indicates the pressure.
The aneroid barometer, which is more common today, uses a small, flexible metal capsule called an aneroid cell. This cell expands or contracts in response to pressure changes, with mechanical linkages moving a pointer on a dial. Readings are commonly expressed in units like inches of mercury (inHg) or millibars (mbar).
Measuring Atmospheric Moisture
Humidity is a measure of the water vapor content in the air, typically expressed as Relative Humidity (RH). RH represents the amount of moisture present as a percentage of the maximum amount the air can hold at a specific temperature. Warmer air can hold significantly more water vapor than colder air, making temperature an important factor in this measurement.
The instrument used to measure atmospheric moisture is the hygrometer. Modern hygrometers often employ electronic sensors that rely on changes in electrical properties to determine the humidity level. A common type is the capacitive hygrometer, which measures the change in electrical capacitance across a thin polymer film as it absorbs or releases water vapor.
Another established method uses a psychrometer, which consists of two thermometers: a dry-bulb and a wet-bulb. The wet-bulb is cooled by evaporation. The difference in temperature between the two bulbs is then used with a psychrometric chart to calculate the relative humidity.
Why Pressure and Humidity Are Linked in Weather
The connection between atmospheric pressure and humidity is an indirect but important one, explaining why people often associate the two. Weather systems are fundamentally driven by the movement of air, and pressure systems govern this movement. High-pressure systems are characterized by sinking air, which warms up as it descends, causing the air mass to be stable and typically leading to clear skies and lower relative humidity.
Conversely, low-pressure systems are areas where air is rising. As this air ascends, it cools, which reduces its capacity to hold water vapor. This cooling causes water vapor to condense into clouds and often results in precipitation and higher relative humidity levels near the surface. This correlation allows meteorologists to use pressure readings as an effective tool for forecasting conditions that are often humid or dry.