Barometric Pressure
Barometric Pressure[edit]
Barometric pressure is the weight of the air pressing down at the Earth’s surface. It is sometimes called atmospheric pressure because it represents the combined mass of the atmosphere above a given point. This pressure constantly changes with weather systems and elevation, and those changes provide valuable clues about future conditions.
For canyoneers, barometric pressure is not just an abstract meteorological concept. A falling barometer often signals unsettled weather or approaching storms, while rising pressure suggests clearing skies. Because flash floods can be triggered by even distant rainfall, being able to read and interpret pressure trends is an important forecasting tool before committing to a canyon.
Units and Measurement[edit]
Barometric pressure can be expressed in several different units. In the United States, the most common unit is inches of mercury (inHg), named after the traditional mercury barometer. Other units include millibars (mb) or hectopascals (hPa), which are standard in scientific and international forecasts. All of these are equivalent ways of describing the same value:
- 1 atmosphere (atm) = 29.92 inHg
- 1 atm = 760 millimeters of mercury (mmHg)
- 1 atm = 1013.25 millibars (mb) or hectopascals (hPa)
- 1 atm = 101,325 pascals (Pa)
Most modern forecasts will use inHg in U.S. reports and mb/hPa internationally.
Station Pressure and Sea-Level Pressure[edit]
Pressure decreases as elevation increases, so a barometer at high altitude will always read lower than one at sea level. The raw measurement at a specific location and altitude is called station pressure. While useful for aviation or scientific purposes, station pressure cannot be directly compared between places at different elevations.
To make values comparable, meteorologists convert station pressure to an equivalent value at sea level. This is what is reported on weather maps and forecasts. When you hear a forecaster say “a high-pressure system is moving in,” they are referencing the sea-level adjusted pressure, not the raw station reading.
Interpreting Pressure Changes[edit]
More than the absolute number, the trend in barometric pressure reveals the most about upcoming weather:
- Rising pressure usually means improving, clearing, and more stable skies.
- Falling pressure signals increasing clouds and a higher chance of precipitation.
- Rapid drops in pressure are strong indicators of storm systems approaching.
- Steady pressure suggests little immediate change.
Because canyoneers often operate in terrain where visual signs of weather may be hidden until too late, a quick glance at pressure changes can provide advance warning that conditions are shifting.
Pressure Ranges[edit]
At sea level, pressures are usually grouped into three categories:
- **High pressure (over 30.20 inHg):** Associated with clear skies, calm winds, and stable conditions.
- **Normal pressure (29.80–30.20 inHg):** Indicates relatively steady weather with little immediate change expected.
- **Low pressure (below 29.80 inHg):** Linked to warm, moist air and unsettled weather, often bringing rain or storms.
The speed of change also matters. A slow fall might only bring light rain. A rapid fall, especially below 29.80 inHg, often points to a significant storm front.
Application to Canyoneering[edit]
Barometric pressure is particularly relevant in canyoneering because slot canyons magnify the consequences of unexpected rainfall. Unlike hikers on open trails, canyoneers cannot always see developing storms on the horizon, nor can they easily retreat once committed to the descent. Monitoring barometric trends can give a crucial early signal before clouds build or radar shows a storm cell moving into the watershed. A rapid pressure drop before a canyon trip should be treated as a red flag to recheck forecasts or reconsider entering.
Tools and Resources[edit]
Modern apps and websites make pressure tracking accessible. One example is BarometricPressure.app, which provides live pressure maps, historical charts, and forecasts across the United States. Combining these tools with standard forecasts and radar imagery gives canyoneers a more complete picture of storm potential.