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Tuesday, October 27, 2020

Explanation on Pressure

Explanation on Pressure

Pressure and its variation have important applications to the prediction of winds and weather. Pressure is the force exerted by the moving molecules of a gas on given area. Variations in pressure between one location and another cause air to move both horizontally and vertically and are very important in weather forecasting.

Atmospheric pressure is defined as the weight of column of the atmosphere with a given cross sectional area, a square inch or square centimeter for example.

In aviation, there is also a commonly used pressure known as standard pressure. Standard pressure refers to an established or standard value that has been created for atmospheric pressure. This standard pressure value is 29.92 inches of mercury (“Hg), 1,013.2 hectopascal (hPa), or 14.7 psi. It is part of a standard day that has been established that includes a standard temperature of 15 °C at sea level. Specific standard day values have also been established for air density, volume, and viscosity.

**Some important pressure terms that should know:

Isobars- Isobars are lines of constant pressure

Pressure Gradient - A pressure gradient is a change in pressure over a horizontal distance. A pressure gradient of 4mb per 100nm is average in mid latitudes. The pressure gradient is revealed by the pattern of isobars. The closer the isobars are to each other, the stronger the pressure gradient.

High- A high is an area of high pressure surrounded on all sides by lower pressure. This is not necessarily low pressure as in a low pressure system. It is relative. Another name for a High is an anti-cyclone.

Low- A low is an area of low pressure surrounded on all sides by higher pressure. "Relatively' applies to the definition of a low pressure area as well, the surrounding pressure is relatively higher. Another name for a low is cyclone.

Ridge- A ridge is a elongated region of relatively high pressure.

Trough- A trough is an elongated region of relatively low pressure.

In practice, as an aircraft climbs, for the comfort of the passengers, the pressurization system will gradually increase the cabin altitude and the differential pressure at the same time. If the aircraft continues to climb once the maximum differential pressure is reached, the differential pressure will be maintained while the cabin altitude climbs. The maximum cruise altitude will be limited by the need to keep the cabin altitude at or below 8,000 ft.

When an aircraft is flown at high altitude, it burns less fuel for a given airspeed than it does for the same speed at a lower altitude. This is due to decreased drag that results from the reduction in air density. Bad weather and turbulence can also be avoided by flying in the relatively smooth air above storms and convective activity that occur in the lower troposphere. To take advantage of these efficiencies, aircraft are equipped with environmental systems to overcome extreme temperature and pressure levels. While supplemental oxygen and a means of staying warm suffice, aircraft pressurization and air conditioning systems have been developed to make high altitude flight more comfortable.


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