Meteorology 2

Pressure Measurement There are two main types of pressure and both are measured using a barometer, the two types are: Station Pressure - the actual atmospheric pressure at the elevation of the observing station Mean Sea Level Pressure - the atmospheric pressure at mean sea level and is used as a standard to allow comparisons of pressure at different altitudes Atmospheric Pressure and Altitude Clearly as we move up into the atmosphere, there is less mass of air above us, and therefore less force so in general, but not always: Air pressure decreases as we ascend Air pressure increases as we descend This being the basis for how an altimeter works, it measures air pressure and differences in air pressure over different altitudes The effects of altitude on flight At higher altitudes there is "less" air available and since lift depends on moving air over an aerofoil with less air, there is less lift at a certain speed. Therefor at higher altitudes, a wing (or propeller) needs to push through the air at higher speeds to generate the same lift as at a lower altitude and at some altitude an aircraft simply won't be able to generate enough lift to support itself. Pressure Altitude Pressure altitude is the height above sea level corresponding to a given barometric pressure under standard air conditions. It is the altitude indicated on an altimeter when it is set to 29.92 inHg (the standard pressure setting) It is also the altitude setting that aircraft use when flying above 18,000 feet in North America in order to standardize the indicated altitudes of all aircraft. Density Altitude Density altitude is not a measure of altitude above ground or sea level. It is a measure of air density in a given location at the present temperature It is pressure altitude corrected for non-standard temperature (colder temperatures lead to more dense air) It is also important in determining the performance of the aircraft since, as we mentioned earlier, the "amount" or density of the air affects lift and engine performance. Temperature and Altitude In the lowest level of the atmosphere (the troposphere) up to about 50,000 feet, temperature generally decreases as we increase in altitude. Under certain conditions however the temperature may be warmer at a higher altitude than below we call the phenomenon an inversion.