The preceding paragraphs refer to air that is perfectly dry. In  reality, it is never completely dry. The small amount of water  vapor suspended in the atmosphere may be almost negligible  under certain conditions, but in other conditions humidity  may become an important factor in the performance of an  aircraft. Water vapor is lighter than air; consequently, moist  air is lighter than dry air. Therefore, as the water content  of the air increases, the air becomes less dense, increasing  density altitude and decreasing performance. It is lightest or  least dense when, in a given set of conditions, it contains the  maximum amount of water vapor.  Humidity, also called relative humidity, refers to the amount  of water vapor contained in the atmosphere and is expressed  as a percentage of the maximum amount of water vapor the  air can hold. This amount varies with temperature. Warm air  holds more water vapor, while cold air holds less. Perfectly  dry air that contains no water vapor has a relative humidity  of zero percent, while saturated air, which cannot hold any  more water vapor, has a relative humidity of 100 percent.  Humidity alone is usually not considered an important factor  in calculating density altitude and aircraft performance, but  it is a contributing factor.  As temperature increases, the air can hold greater amounts  of water vapor. When comparing two separate air masses,  the first warm and moist (both qualities tending to lighten  the air) and the second cold and dry (both qualities making  it heavier), the first must be less dense than the second.  Pressure, temperature, and humidity have a great influence  on aircraft performance because of their effect upon density.  There are no rules of thumb that can be easily applied, but  the affect of humidity can be determined using several online  formulas. In the first example, the pressure is needed at the  altitude for which density altitude is being sought. Using  Figure 4-2,  select the barometric pressure closest to the  associated altitude. As an example, the pressure at 8,000 feet  is 22.22 "Hg. Using the National Oceanic and Atmospheric  Administration (NOAA) website (www.srh.noaa.gov/  epz/?n=wxcalc_densityaltitude) for density altitude, enter  the 22.22 for 8,000 feet in the station pressure window. Enter  a temperature of 80° and a dew point of 75°. The result is a  density altitude of 11,564 feet. With no humidity, the density  altitude would be almost 500 feet lower.  Another website (www.wahiduddin.net/calc/density_  altitude.htm) provides a more straight forward method of  determining the effects of humidity on density altitude  without using additional interpretive charts. In any case, the  effects of humidity on density altitude include a decrease in  overall performance in high humidity conditions.  Newton’s Second Law: “Force is equal to the change in  momentum per change in time. For a constant mass, force  equals mass times acceleration.”  This means that nothing starts or stops moving until some  outside force causes it to do so. An aircraft at rest on the ramp  remains at rest unless a force strong enough to overcome  its inertia is applied. Once it is moving, its inertia keeps  it moving, subject to the various other forces acting on it.  These forces may add to its motion, slow it down, or change  its direction.  Newton’s First Law: “Every object persists in its state of rest  or uniform motion in a straight line unless it is compelled to  change that state by forces impressed on it.”  The fundamental physical laws governing the forces acting  upon an aircraft in flight were adopted from postulated  theories developed before any human successfully flew  an aircraft. The use of these physical laws grew out of the  Scientific Revolution, which began in Europe in the 1600s.  Driven by the belief the universe operated in a predictable  manner open to human understanding, many philosophers,  mathematicians, natural scientists, and inventors spent their  lives unlocking the secrets of the universe. One of the most  well-known was Sir Isaac Newton, who not only formulated  the law of universal gravitation, but also described the three  basic laws of motion.  The formulation of lift has historically been an adaptation  over the past few centuries of basic physical laws. These  laws, although seemingly applicable to all aspects of lift,  do not explain how lift is formulated. In fact, one must  consider the many airfoils that are symmetrical, yet produce  significant lift.  In order to achieve flight in a machine that is heavier than air,  there are several obstacles we must overcome. One of those  obstacles, discussed previously, is the resistance to movement  called drag. The most challenging obstacle to overcome in  aviation, however, is the force of gravity. A wing moving  through air generates the force called lift, also previously  discussed. Lift from the wing that is greater than the force of  gravity, directed opposite to the direction of gravity, enables  an aircraft to fly. Generating this force called lift is based on  some important principles, Newton's basic laws of motion,  and Bernoulli's principle of differential pressure.  Effect of Humidity (Moisture) on Density  Theories in the Production of Lift  Newton’s Basic Laws of Motion  4-5 