Cold Front 9 August 2005 12 UTC

The isentropes show a typical configuration preceeding the passing of an active cold front.

At location 1 (over southern Finland and on the Gulf of Finland) the atmosphere is highly potentially unstable; equivalent potential temperature decreases by over 10 degrees from near surface layers up to 500 hPa. The potentially unstable layer is also quite deep, up to the mid-troposphere.

At location 2 (east of Estonia and Latvia) the typical tightening isentropes denote the location of the upper cold front, isentropes inclining backwards with height.

At location 3 the warm air at lowest layers of the atmosphere disrupts the cold front structure and causes the isentropes to tilt forward by height. This further strengthens the potential instability of the air as values of lower Theta-e between 700-850 hPa overrun the higher values near the surface. The leading edge of the surface cold front is roughly at the location marked with number 3.

Strong low-level convergence can be found both within the surface cold front (east of Estonia and Latvia) and ahead of it (near the Finnish Gulf). This pre-frontal convergence is rather intensive and extends up to 700 hPa.

Associated with the low-level convergence also the vertical motion has double structure: the ascending motion at the cold front is the most intensive and at maximum at the middle troposphere (450-650 hPa). A slightly smaller vertical motion maximum associated with the pre-frontal convergence is also apparent.

Relative humidity shows that the air is very moist between from the surface up to 650 hPa. The cold front cloudiness is seen as high relative humidity values extending up to the tropopause. Sinking dry air at the rear of the cold front is seen in the rightmost part of the image.

Cold advection structure suggests that the Cold front is mostly located within a cold advection overrunning the front at the middle layers of the atmosphere. This in turn contributes to the unstable conditions to become even more unstable ahead of the front.