CORRELATION OF WIND AND WEATHER

Inasmuch as the vast majority of clouds are formed by upward motion of air, and vertical convergence of air can be deduced from the horizontal components of direction and speed of the wind field taken over large areas, many weather patterns in general, and specifically tropical weather patterns, have a direct relation-ship to streamline patterns at all levels.

Complete correlation of wind and weather requires analysis of the wind field at several levels; however, data and time limitations usually permit correlation of only two levels: the low-level streamlines between 2,000 and 5,000 feet and the streamlines of the 200-mb level. The correlation rules are as follows:

1. In regions of moderate to strong divergence in the low-level streamlines, the predominant cloud is usually cumulus humilis with less than one-half coverage. When great vertical shear accompanies the moderate divergence, the cumulus is drawn out and sheared off and may be accompanied by dependent stratocumulus.

2. North of the trade wind maximum, in the Northern Hemisphere, the vertical shear in the lower layers is usually great and the trade wind inversion low. The predominant cloud here is stratocumulus. The clouds exist in broken patches if the low-level-streamline pattern is divergent. Even with weak to moderate convergence at this level, very little cumulus forms; this usually results in an increase in the amount of stratocumulus west and south of the subtropical high-pressure centers.

3. When there is little vertical shear in the lower layers, and the low-level streamlines show weak to moderate convergence south of the trade wind maximum, expect deeper cumulus. The amount of cumulus will also increase over that expected in divergent flow, but not to a marked extent. Even with very strong convergence, the cumuliform clouds show many breaks, the general effect of increased convergence being to increase the height of the convective clouds.

4. An asymptote of convergence in the low-level streamlines is accompanied by a line of cumulus congestus or cumulonimbus if the asymptote coincides with an isotach minimum in the wind field.

5. Polar fronts that penetrate into the tropics rarely reach 15N or S latitude over oceanic regions. Such fronts are often detectable as a line of towering cumulus. The highly modified front may, in such cases, appear on the low-level streamline chart as an asymptote of convergence running east to west or northeast to southwest from the neutral point between two middle-latitude anticyclones (high-pressure centers). In most cases, temperature contrast across such fronts is negligible, even though the cloud line may persist for some time after air mass contrasts have disappeared.

6. An asymptote of convergence in the lower streamline field is not necessarily accompanied by a cloud line. If strong speed divergence occurs along part of the line, there may be very little heavy clouds or precipitation accompanying it; furthermore, bad weather may be found some distance from the line where speed convergence predominates.