Atmospheric
Atmospheric science terms — from barometric pressure to evaporative cooling. How the air itself creates the weather you experience.
Atmospheric Pressure
The weight of the air pressing down on the Earth's surface. Drops in atmospheric pressure precede most major storms and are responsible for the joint and ear discomfort many people feel before bad weather.
Barometric Pressure
Another name for atmospheric pressure, measured by a barometer. Falling barometric pressure typically signals incoming rain or snow, while rising pressure usually means clear, stable weather is on the way.
Convective Heat Loss
The process by which moving air strips heat from a warm surface, including human skin. Convective heat loss is why a windy 40°F day feels far colder than a calm 40°F day.
Dew Point
The temperature at which the air becomes saturated and water vapor condenses into liquid. A high dew point on a hot day produces oppressive humidity; a low dew point makes the air feel dry.
Evaporative Cooling
The process by which sweat evaporates off the skin, removing heat and cooling the body. Synthetic running fabrics enable this process; cotton blocks it.
Heat Index
A measure of how hot the air actually feels when humidity is factored in with the air temperature. A 90°F day with 70% humidity has a heat index near 105°F.
High-Pressure System
An area where atmospheric pressure is higher than its surroundings, characterized by sinking air, clear skies, and stable weather. High-pressure systems usually mean dry, calm conditions.
Humidity
The amount of water vapor in the air, usually expressed as relative humidity. High humidity makes hot weather feel hotter and cold weather feel sharper.
Low-Pressure System
An area where atmospheric pressure is lower than its surroundings, characterized by rising air, cloud formation, and unstable weather. Low-pressure systems are responsible for most rain and snow events.
Precipitation
Any form of water — liquid or solid — that falls from clouds to the ground, including rain, snow, sleet, hail, and freezing rain.