Once the boundary layer of the air adheres to the wing by  friction, further resistance to the airflow is caused by the  viscosity, the tendency of the air to stick to itself. When  these two forces act together to resist airflow over a wing,  it is called drag.  Pressure is the force applied in a perpendicular direction to the  surface of an object. Often, pressure is measured in pounds of  force exerted per square inch of an object, or PSI. An object  completely immersed in a fluid will feel pressure uniformly  around the entire surface of the object. If the pressure on one  surface of the object becomes less than the pressure exerted  on the other surfaces, the object will move in the direction  of the lower pressure.  Although there are various kinds of pressure, pilots are  mainly concerned with atmospheric pressure. It is one of  the basic factors in weather changes, helps to lift an aircraft,  and actuates some of the important flight instruments. These  instruments are the altimeter, airspeed indicator, vertical  speed indicator, and manifold pressure gauge.  Air is very light, but it has mass and is affected by the  attraction of gravity. Therefore, like any other substance,  it has weight, and because of its weight, it has force. Since  air is a fluid substance, this force is exerted equally in all  directions. Its effect on bodies within the air is called pressure.  Under standard conditions at sea level, the average pressure  exerted by the weight of the atmosphere is approximately  14.70 pounds per square inch (psi) of surface, or 1,013.2  millibars (mb). The thickness of the atmosphere is limited;  therefore, the higher the altitude, the less air there is above.  For this reason, the weight of the atmosphere at 18,000 feet  is one-half what it is at sea level.  The pressure of the atmosphere varies with time and location.  Due to the changing atmospheric pressure, a standard  reference was developed. The standard atmosphere at sea  level is a surface temperature of 59 °F or 15 °C and a surface  pressure of 29.92 inches of mercury ("Hg) or 1,013.2 mb.  [Figure 4-2]  A standard temperature lapse rate is when the temperature  decreases at the rate of approximately 3.5 °F or 2 °C per  thousand feet up to 36,000 feet, which is approximately –65  °F or –55 °C. Above this point, the temperature is considered  constant up to 80,000 feet. A standard pressure lapse rate is  when pressure decreases at a rate of approximately 1 "Hg  per 1,000 feet of altitude gain to 10,000 feet.  [Figure 4-3]  The International Civil Aviation Organization (ICAO) has  established this as a worldwide standard, and it is often  referred to as International Standard Atmosphere (ISA) or  ICAO Standard Atmosphere. Any temperature or pressure  that differs from the standard lapse rates is considered  nonstandard temperature and pressure.  Figure 4-3.  Properties of standard atmosphere.  Figure 4-2.  Standard sea level pressure.  Atmospheric Pressure  Pressure  30  25  20  15  10  5  0  Inches of  Mercury  Millibars  1016  847  677  508  339  170  0  29.92  Standard  Sea Level  Pressure  Hg  1013  Standard  Sea Level  Pressure  mb  Atmospheric Pressure Altitude (ft)  Pressure (Hg)  Temperature  (°C)  (°F)  Standard Atmosphere  0  1,000  2,000  3,000  4,000  5,000  6,000  7,000  8,000  9,000  10,000  11,000  12,000  13,000  14,000  15,000  16,000  17,000  18,000  19,000  20,000  29.92  28.86  27.82  26.82  25.84  24.89  23.98  23.09  22.22  21.38  20.57  19.79  19.02  18.29  17.57  16.88  16.21  15.56  14.94  14.33  13.74  15.0  13.0  11.0  9.1  7.1  5.1  3.1  1.1  -0.9  -2.8  -4.8  -6.8  -8.8  -10.8  -12.7  -14.7  -16.7  -18.7  -20.7  -22.6  -24.6  59.0  55.4  51.9  48.3  44.7  41.2  37.6  34.0  30.5  26.9  23.3  19.8  16.2  12.6  9.1  5.5  1.9  -1.6  -5.2  -8.8  -12.3  4-3 