## Pilot's Handbook of Aeronautical Knowledge Weather Theory Atmospheric Stability

Pilot's Handbook of Aeronautical Knowledge

Preface

Acknowledgements

Appendix

Glossary

Index

 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. Given: 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 Example: 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. Explanation: 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. Methods by Which Air Reaches the Saturation Point 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 complete 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.
 Figure 11-20. Relationship between relative humidity, temperature, and dewpoint.

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