Weather Theory

At sea level pressure, air can hold 9 g H O/cubic meter of air at 10 °C 2 17 g H O/cubic meter of air at 20 °C 2 30 g H O/cubic meter of air at 30 °C 2 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%. 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.) 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. Figure 12-20. Relationship between relative humidity, temperature, and dewpoint. As moist, unstable air rises, clouds often form at the altitude Explanation: where temperature and dew point reach the same value. When With an outside air temperature (OAT) of 85 °F at the surface lifted, unsaturated air cools at a rate of 5.4 °F per 1,000 feet and dew point at the surface of 71 °F, the spread is 14°. Divide and the dew point temperature decreases at a rate of 1 °F per the temperature dew point spread by the convergence rate of 1,000 feet. This results in a convergence of temperature and 4.4 °F, and multiply by 1,000 to determine the approximate dew point at a rate of 4.4 °F. Apply the convergence rate height of the cloud base. to the reported temperature and dew point to determine the height of the cloud base. Methods by Which Air Reaches the Saturation Point If air reaches the saturation point while temperature and Given: dew point are close together, it is highly likely that fog, low Temperature (T) = 85 °F clouds, and precipitation will form. There are four methods by which air can reach the saturation point. First, when warm Dew point (DP) = 71 °F air moves over a cold surface, the air temperature drops and Convergence Rate (CR) = 4.4° reaches the saturation point. Second, the saturation point may T – DP = Temperature Dew Point Spread (TDS) be reached when cold air and warm air mix. Third, when air cools at night through contact with the cooler ground, air TDS ÷ CR = X reaches its saturation point. The fourth method occurs when X × 1,000 feet = height of cloud base AGL air is lifted or is forced upward in the atmosphere. Example: As air rises, it uses heat energy to expand. As a result, the rising 85 °F – 71 °F = 14 °F air loses heat rapidly. Unsaturated air loses heat at a rate of 14 °F ÷ 4.4 °F = 3.18 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 3.18 × 1,000 = 3,180 feet AGL brings clouds, rain, and other critical weather situations. The height of the cloud base is 3,180 feet AGL. 12-14