|Description||The physical processes which result in the formation of clouds and the production of precipitation
have been described by numerous meteorologists. The genetical classification has been
summarized by Petterssen as follows:
(1) Clouds and precipitation types which form in unstable air masses,
(2) Clouds and precipitation types which form in stable air masses,
(3) Clouds and precipitation types which form in connection with quasi-horizontal inversions
in the free atmosphere, and
(4) Frontal clouds and precipitation forms.
In general, the procedure adopted for the forecasting of these meteorological elements can also be
subdivided into the same four categories.
Since the distribution of the elements in the atmosphere is never constant with time, the
problem of forecasting cloudiness and precipitation is sometimes diffcult. A detailed discussion
of qualitative and quantitative prognostic methods can be found in various standard meteorological
texts. However, present methods are not entirely adequate and it is the aim of this investigation
to contribute to the solution of the forecast problem.
This study has been divided into two main sections, viz.,
(1) Clouds and precipitation types which occur at frontal surfaces as the result of frontal lifting,
(2) Clouds and precipitation types which occur within air masses as the result of horizontal
convergence within the wind field.
(1) Frontal Surfaces. In a recent study of fronts and frontogenesis, Petterssen and Austin have investigated the processes that tend to create wind shear, or vorticity, along fronts. It was
found that, in general, an increase or decrease in shear is accompanied by a change in the vertical
velocity field at a front. Since cloudiness and precipitation arise from the ascending velocities at
frontal surfaces, the possibility of forecasting the change in horizontal shear at a front will be
Furthermore, it has been shown that the vertical velocity of either air mass at a frontal
surface can be determined from the velocity of the front and the horizontal wind velocity. The
wind velocities in the free atmosphere can be obtained with a reasonable degree of accuracy, but
the front velocity cannot always be evaluated with the same accuracy. Because it is important to
determine the magnitude or at least the sign of the vertical velocities, a kinematical study will be
made of the displacement of frontal surfaces.
(2) Horizontal Convergence. In 1931 Giao published an explanation of the origin of clouds
and precipitation in the vicinity of moving cyclones, without introducing the concept of frontal
surfaces. Giao claimed that most condensation phenomena could be attributed to the cooling
produced by local pressure changes and the horizontal convergence which arises from the meridional
component of the wind velocity. The conclusions are conveniently summarized by Haurwitz, together with a short discussion of Giao's theory.
Since some systems of clouds and precipitation types may result solely from convergence in
the horizontal flow, an attempt wil be made to evaluate this effect quantitatively.||en