Atmospheric pressure is the force exerted on the Earth’s surface by the weight of the air column above it. Meteorologists use barometers to measure this force, typically expressing readings in units such as millibars (mb) or hectopascals (hPa). Weather maps rely on standardized visual symbols to quickly communicate these measurements across a wide area. These symbols allow forecasters and the public to instantly identify areas of pressure variation, which correlates with different weather patterns.
Identifying High Pressure
The universal symbol used to denote a center of high pressure on a weather map is the capital letter ‘H’. This designation marks an area where the atmospheric pressure is greater than the surrounding regions, signifying the core of an anticyclone. The ‘H’ is utilized in surface weather analysis as the primary identifier for these large-scale meteorological features.
On professional weather charts, the ‘H’ is often accompanied by a numerical value. This number indicates the actual pressure reading at the center of the system, usually measured in millibars or hectopascals. This combination offers both immediate identification and precise meteorological information, such as confirming a relatively strong high-pressure system with a reading of 1025 mb or hPa.
Contrasting Symbols in Weather Maps
The ‘H’ for high pressure is always presented in contrast with the symbol for low pressure, the capital letter ‘L’. This pairing is fundamental to reading a weather map, as atmospheric systems are defined by the relationship between these two pressure centers. A common convention is to color the high-pressure ‘H’ blue, while the low-pressure ‘L’ is colored red.
Lines encircling both the ‘H’ and ‘L’ are called isobars, which connect points on the map that share the same atmospheric pressure. The spacing of these isobars provides insight into wind speed; closely packed lines indicate a steep pressure gradient and stronger winds. Air naturally moves from higher pressure toward lower pressure, and this movement generates wind.
What High Pressure Implies for Weather
A high-pressure system, or anticyclone, is characterized by air descending from the upper atmosphere toward the surface. This downward motion, known as subsidence, causes the air mass to warm and become drier. The warming process suppresses the formation of clouds because the air cannot rise and cool enough for water vapor to condense.
The result of this sinking, stable air is fair weather, marked by clear skies and an absence of precipitation. Winds near the center of a high-pressure system are often light or calm because the air diverges, or moves outward, from the center. The Earth’s rotation causes the air to circulate, leading to a clockwise wind pattern around the ‘H’ in the Northern Hemisphere and a counterclockwise pattern in the Southern Hemisphere.