An inversion is an increase of temperature with height. There are several ways they can be created which include: (1) high pressure subsidence (2) WAA in the middle levels of the troposphere (3) radiational cooling of the earth's surface (4) warm air flowing over a large cold water body (5) the frontal inversion and (6) the tropopause inversion (warming by absorption of shortwave radiation by ozone) We will take a closer look at each of these processes. (1) High pressure promotes sinking air. As air sinks it warms adiabatically. Sinking air will warm at the dry adiabatic lapse rate due to the air being unsaturated. When high pressure promotes sinking air, the air will not sink at a uniform rate at all levels of the troposphere. A deep high pressure will have the greatest sinking motion in the middle levels of the troposphere (near level of non-divergence). The inversion will develop where the strongest sinking motion is taking place. With deep high pressure this will generally be located between 850 and 600 millibars. Even if a true inversion is not produced by the high pressure, the lapse rate will likely be stable in the middle levels of the troposphere (less than WALR). (2) WAA into the middle levels of the troposphere can occur by way of differential advection or WAA increasing with height from the surface to the middle levels of the troposphere. WAA propagating into the middle levels of the troposphere can occur as dry hot air in higher elevation regions advects from its source region. Upon leaving the source region, this dry and warm air advects into the middle levels of the troposphere upon moving into a region with a lower elevation. The inversion this creates is commonly called a cap or lid. WAA can increase with height due to surface friction or a shallow layer of cold dense air in the PBL. If the WAA is greater at 700 millibars than 850 millibars, this will act to create an inversion over time. (3) The most common inversion is the radiational inversion. The earth is cooled at night by longwave radiation emission to space. This is maximized on clear nights with light wind and dry air. Air in the lower PBL will cool much more rapidly than air at the top of the PBL at night. This will cause an inversion that at times can be quite impressive. These inversions generally erode rapidly once daytime heating warms the lower PBL. (4) A cold body of water will chill the air above it. The air chills due to the conduction of heat away from the air toward the cold body of water. This is common over the Great Lakes in the early summer. Warm air will advect over the Great Lakes that are recovering from cold winter waters. The chilling of air near the surface results in a temperature increase with height. (5) A shallow cold front can create an inversion. This is the second most common type of inversion. The inversion created from a cold front is especially evident when a shallow layer of polar air moves into lower latitudes. The air associated with the shallow air mass is colder than the air aloft, thus creating an inversion. Inversions promote stability within the vertical layer of the troposphere they exist. Keep in mind that precipitation can still occur when there is an inversion in the troposphere (especially frontal and WAA inversions) because rising air can begin ABOVE the inversion boundary. (6) An inversion in the tropopause is created by the absorption of shortwave radiation by ozone. This inversion occurs nears the 150 millibar level, but can be a little higher or lower depending on season and weather. The tropopause inversion and the extreme stability associated with it inhibit UVV's into the stratosphere. |