Atmospheric pressure is greatest at ground level and decreases rapidly with altitude. This change in pressure is important in many situations, from weather forecasting to mountain climbing and aviation.
Basic Knowledge
Atmospheric pressure is produced by the weight (gravity) of the air above a given altitude. Standard atmospheric pressure at ground level is 1013.25 hPa (1 atmosphere = 101325 Pa).
As altitude increases, the amount of air decreases, so atmospheric pressure falls. As a rule of thumb, atmospheric pressure is approximately halved for every 5.5 km increase in altitude. (At an altitude of 10 km ≈ 264 hPa, and at 20 km ≈ 55 hPa)
Equation for changes in pressure: P = P₀ × exp(−h/H). H is the scale height (approximately 8.5 km).
Differences in atmospheric pressure create high- and low-pressure systems, causing atmospheric flow (wind).
・High-pressure system: Pressure is higher than in the surrounding area. Descending air. Fair weather. In the Northern Hemisphere, winds flow outward clockwise.
・Low-pressure system: Pressure is lower than in the surrounding area. Rising air. Cloudy or rainy weather. In the Northern Hemisphere, winds flow inward counterclockwise.
At high elevations, pressure and the partial pressure of oxygen decrease, increasing the risk of altitude sickness. The pressure at the summit of Mount Fuji (3776 m) is approximately 630 hPa (approximately 62% of the pressure at ground level).
Standard atmospheric pressure（1013.25 hPa = 1 atm = 760 mmHg）
Scale height（The altitude difference at which atmospheric pressure becomes 1/e≈37%. Approximately 8.5 km）
High-pressure system（Descending air・fair weather. Northern Hemisphere=wind flows outward clockwise）
Low-pressure system（Rising air・bad weather. Northern Hemisphere=wind flows inward counterclockwise）
Isobar（A line connecting points with the same atmospheric pressure. The closer the spacing, the stronger the wind）
In Depth
The atmosphere is a compressible fluid, so density also decreases with altitude. In the troposphere, the adiabatic lapse rate is dominant, and air expands and cools as it rises. This cooling causes water vapor to condense and clouds to form.
In weather observations, a radiosonde(a weather instrument suspended from a balloon) is used to measure temperature, humidity, and wind speed in the upper atmosphere. In Japan, radiosondes are launched twice a day (9 a.m. and 9 p.m.) at locations throughout the country.
The international unit of atmospheric pressure is Pa (pascal), but hPa (hectopascal) is used in meteorology. The formerly used millibar (mb) has the same numerical value because 1 hPa = 1 mb.
- Standard atmospheric pressure: 1013.25 hPa (at ground level)
- Pressure is approximately halved for every 5.5 km increase in altitude
- High-pressure system=descending air・fair weather
- Low-pressure system=rising air・clouds・rain
- Northern Hemisphere: high-pressure system=clockwise, low-pressure system=counterclockwise
- Pressure at the summit of Mount Fuji is approximately 62% of that at ground level
Points to Note
① High- and low-pressure systems are determined not by “absolute pressure values” but by whether pressure is relatively high or low compared with the surroundings. ② Wind directions around high- and low-pressure systems are reversed in the Northern and Southern Hemispheres (the direction of the Coriolis force is reversed). ③ Narrowly spaced isobars=large pressure-gradient force=strong winds.
Practice
- What is standard atmospheric pressure in hPa?
- In the Northern Hemisphere, state the direction of winds flowing into a low-pressure system (clockwise/counterclockwise).
- Using “pressure” and “oxygen,” explain why altitude sickness is likely to occur at high elevations.