Barometric formula - Wikipedia
We will examine how air temperature, number density, and pressure change in Located from the surface of the Earth to approximately 11 kilometers in altitude. The density altitude is the altitude relative to standard atmospheric conditions at which the air The density altitude can also be considered to be the pressure altitude Both an increase in the temperature and a decrease in the atmospheric pressure, The lift generated by the aircraft's airfoils, and the relation between its . Using a barometer to measure altitude is a well established technique. derived here will probably fail to accurately predict the pressurealtitude relationship. tute the real values of base pressure and temperature for those.Why does temperature decrease with higher altitude? In Hindi
One might then expect different components to have different scale heights determined by their molecular weight. In particular, considering the difference in molecular weight between N2 and O2, one might expect the O2 mixing ratio to decrease with altitude. However, gravitational separation of the air mixture takes place by molecular diffusion, which is considerably slower than turbulent vertical mixing of air for altitudes below km problem 4.
Turbulent mixing thus maintains a homogeneous lower atmosphere. Only above km does significant gravitational separation of gases begin to take place, with lighter gases being enriched at higher altitudes. During the debate over the harmful effects of chlorofluorocarbons CFCs on stratospheric ozone, some not-so-reputable scientists claimed that CFCs could not possibly reach the stratosphere because of their high molecular weights and hence low scale heights.
In reality, turbulent mixing of air ensures that CFC mixing ratios in air entering the stratosphere are essentially the same as those in surface air. Exercise The cruising altitudes of subsonic and supersonic aircraft are 12 km and 20 km respectively. What is the relative difference in air density between these two altitudes?
Pressure with Height: pressure decreases with increasing altitude
The air density at 20 km is only a third of that at 12 km. The high speed of supersonic aircraft is made possible by the reduced air resistance at 20 km.
These layers are called temperature inversions and can be very important in weather forecasting. One situation where temperature inversions frequently form is just above the ground surface during long winter nights caused by the air contacting the cold ground surface.
The Atmosphere and the Weather
This type of inversion will persist into the early morning until the sun is able to heat the ground surface See Figure X. Isothermal constant temperature layer that exists above the troposphere. On average, it is found between about 11 and 20 kilometers. At these levels the air temperature is much colder than that found near the ground surface.
Separates the troposphere from the stratosphere. Acts like a "lid" on rising air motion.
Change in the Atmosphere with Altitude
Because it is difficult for tropospheric air to rise into the tropopause region, clouds are typically confined below the tropopause region See Figure Y. Layer in the atmosphere where the jet stream exists. Extends from 20 to 48 kilometers above the surface average location. Temperature increases with altitude because ozone gas molecules, present in this layer, absorb ultraviolet sunlight creating heat energy.
The layer of higher ozone concentrations, which reaches a maximum between 20 and 30 km above sea level, is also called the ozone layer. Ozone in the stratosphere protects life from harmful exposure to the sun's ultraviolet radiation. Even though we refer to an "ozone layer", keep in mind that ozone molecules account for a very small percentage of all air molecules in the stratosphere. Even within the ozone layer, ozone is still a trace constituent.
Density Air density can be defined as the number of air molecules per unit volume number density. Near sea level there are about 2.
Air molecules are held near the earth by gravity.
In other words, air has weight. Weigh an empty bag, then fill it with air, it now weighs more. In addition gases, like air, are easily compressed, i.
In other words, we say gases are compressible because they can easily be squeezed into a smaller volume. Solids and liquids on the other hand are not easily compressed. The weight of all of the air above a given point in the atmosphere squeezes air molecules closer together, which causes their numbers in a given volume to increase increase in number density.
The more air above a level and hence the more weight of air above a levelthe greater the squeezing effect or compression. Since air density is the number of air molecules in a given space volumeair density is typically greatest at the surface or sea level where it is squeezed by the weight of the entire atmosphere above and decreases as we move up in the atmosphere because the weight of air above becomes less and hence there is less of a squeezing effect See Figure Z.
Pressure Atmospheric air pressure results from the Earth's gravitational pull on the overlying air.
- Barometric formula
Without gravity holding the atmosphere just above the ground surface, air molecules would spread out, and the gas pressure would be close to zero. The weight of the atmosphere acts as a force upon the underlying surface of the Earth. The amount of force excerted over an area of surface is called atmospheric pressure or air pressure.
Near sea level, the average air pressure is about In this class we will use the unit millibars mb to specify air pressure.