Although the high temperature on a barotropic day occurs in the late afternoon, the greatest temperature rises tend to occur in the morning hours. On a barotropic day, the skies tend to be clear at night and during the morning hours. Night cooling results in a low-level inversion. The coolest air is adjacent to the earth's surface as a result of cooling due to longwave emission. On a barotropic night the winds tend to be light due to the development of the shallow inversion. Light winds allow for a shallow layer of relatively cool air to build at the surface. As the sun rises and skies are clear, solar energy is added to the earth's surface. At the point solar radiation becomes greater than the longwave emission from the earth's surface, the temperature will begin to rise. As the morning progresses, the warming surface will warm the air adjacent to the surface. Since the low-level inversion is relatively shallow, the earth's surface can quickly warm the air adjacent to the earth's surface under the inversion. During the morning the low-level inversion erodes due to solar heating of the earth's surface. This process is termed "A mixing out of the low-level inversion". During the course of the morning hours the low-level lapse rate switches from an inversion to nearly dry adiabatic. This transformation in lapse rate produces a rapid rise in temperature. Once the low-level lapse rate becomes dry adiabatic, warming slows beyond this point because the air warmed by the surface must now mix with a much deeper layer of lower tropospheric air. This deep mixing allows wind to be much stronger during the day as compared to night and also moderates further warming of the air temperature. Temperatures tend to rise most rapidly between 2 and 7 hours after the sun rises. Before two hours, the sun angle is too low to significantly warm the surface (the earth is still radiating more energy away than the sun is supplying). After 7 hours, the low-level inversion has mixed out to a dry adiabatic lapse rate. Below is a typical progression of temperatures on a hot dry summer barotropic day: 5:30 am (sunrise): 69 F 6 am: 68 F 7 am: 70 F 8 am: 75 F 9 am: 82 F 10 am: 89 F 11 am: 95 F Noon: 99 F 1 pm: 101 F 2 pm: 102 F 3 pm: 103 F 4 pm: 104 F 5 pm: 102 F 6 pm: 100 F 7 pm: 97 F 8 pm: 94 F 8:30 pm (sunset): 92 F |