Figure 12-20.  Relationship between relative humidity, temperature, and dewpoint.  As moist, unstable air rises, clouds often form at the altitude  where temperature and dew point reach the same value. When  lifted, unsaturated air cools at a rate of 5.4 °F per 1,000 feet  and the dew point temperature decreases at a rate of 1 °F per  1,000 feet. This results in a convergence of temperature and  dew point at a rate of 4.4 °F. Apply the convergence rate  to the reported temperature and dew point to determine the  height of the cloud base.  With an outside air temperature (OAT) of 85 °F at the surface  and dew point at the surface of 71 °F, the spread is 14°. Divide  the temperature dew point spread by the convergence rate of  4.4 °F, and multiply by 1,000 to determine the approximate  height of the cloud base.  Explanation:  If air reaches the saturation point while temperature and  dew point are close together, it is highly likely that fog, low  clouds, and precipitation will form. There are four methods  by which air can reach the saturation point. First, when warm  air moves over a cold surface, the air temperature drops and  reaches the saturation point. Second, the saturation point may  be reached when cold air and warm air mix. Third, when air  cools at night through contact with the cooler ground, air  reaches its saturation point. The fourth method occurs when  air is lifted or is forced upward in the atmosphere.  As air rises, it uses heat energy to expand. As a result, the rising  air loses heat rapidly. Unsaturated air loses heat at a rate of  3.0 °C (5.4 °F) for every 1,000 feet of altitude gain. No matter  what causes the air to reach its saturation point, saturated air  brings clouds, rain, and other critical weather situations.  Example:  Given:  Methods by Which Air Reaches the Saturation Point  12-14  Temperature (T) = 85 °F  Dew point (DP) = 71 °F  Convergence Rate (CR) = 4.4°  T – DP = Temperature Dew Point Spread (TDS)  TDS ÷ CR = X  X × 1,000 feet = height of cloud base AGL  85 °F – 71 °F = 14 °F  14 °F ÷ 4.4 °F = 3.18  3.18 × 1,000 = 3,180 feet AGL The height of the cloud base is 3,180 feet AGL. A cubic meter of air with 17g of water  vapor at 20 °C is at saturation  or 100% relative humidity. Any further  cooling will cause condensation (fog, clouds, dew) to form. Thus, 20 °C is  the dew point for this situation.  If the same cubic meter of air warms  to 30 °C, the 17 g of water vapor will  produce a relative humidity of 56%.  (17 g is 56% of the 30 g the air could hold at this temperature.)  If the temperature is lowered to 10 °C,  the air can hold only 9 g of water  vapor, and 8 g of water will condense  as water droplets. The relative  humidity will still be at 100%.  At sea level pressure, air can hold  9 g H$_{2}$O/cubic meter of air at 10 °C  17 g H$_{2}$O/cubic meter of air at 20 °C  30 g H$_{2}$O/cubic meter of air at 30 °C 