On average, temperature decreases with height in the troposphere. The troposphere is bounded by the earth's surface and the tropopause (located at about 150 millibars in the tropics and 300 mb at high latitudes). In the troposphere on any given day, there are various layers between the surface and the tropopause where the lapse rates are far from average (the average environmental lapse rate is near 6.5 degrees C/km). Rawinsonde weather balloons measure temperature as they rise (this is the plot of the environmental lapse rate). Under certain conditions, the lapse rate in the atmosphere (defined as the change in temperature with height) is much above or much below normal. An example of a lapse rate much below normal is an inversion. In an inversion the temperature actually increases with height. An inversion promotes atmospheric stability (stability is common when warm air is vertically higher than cooler air). The location of the inversion when it does exist is within or near the boundary layer (but can vary depending on elevation and the current weather pattern). An inversion can exist at the surface also on a clear night. The surface cools through longwave radiation emission. This results in an inversion since the air near the surface is cooler than the air higher up (i.e. 850 millibars). The lapse rate can be well above normal also. An example is in the case of solar heating of the earth's surface. The sun warms the earth's surface resulting in daytime temperatures that are much warmer near the surface and cool significantly with height. Cold air advection into the mid-levels of the atmosphere and warm air advection into the low levels can also lead to a steep (much greater than normal) lapse rate. A steep lapse rate is indicative of an unstable troposphere. Warm air under cold air will support convective mixing. Convection is not only the development of thunderstorms. Any positively buoyant rising air can be labeled convection. Even though you can not see it with your eyes, convective rising is occurring everywhere in the low levels of the troposphere (especially during the day). If you could see the convection, it would look like a slowly boiling pot of water. This convection mixes air from the surface to higher in the troposphere. In summary, although the average environmental lapse rate is about 6.5 C/km, the lapse rates in the actual troposphere can vary dramatically day to day and day versus night and weather situation to weather situation. |