and Tm air from the Gulf also results in extensive stratus formation

over adjacent land areas.

Overrunning by Tm air is not as frequent as

with the Pc air mass. The small migratory highs of either Pm or Pc air that move down from the continental land mass result in clear skies, no fog, and little or no turbulence.

a. Winter

The northwestern Gulf of Mexico is a winter spawning

ground for cyclones, which form in association with quasi-stationary cold fronts, especially Pc types. Low pressure systems usually develop on the front near the Texas or Mexican coast in association with a trough moving eastward from southern Arizona. These waves usually travel eastward rapidly, once movement begins, but in the development stage, low ceilings and continuous rain persist for 24 hours or longer. The principal track of the associated low centers parallels the Gulf Coast before passing across northern Florida or southern Georgia into the Atlantic. Major development of the low center into a dangerous extratropical storm ordinarily does not occur until the system reaches the

Atlantic.

Fair weather in the northwestern Gulf is confined largely to the closed centers of rapidly moving (eastward or southeastward) high pressure cells that pass directly over the area. This limits the duration of fair weather to about 48 hours during the winter season.

During March, Pc fronts usually become quasi-stationary

farther north than in February, so that the principal track of low centers lies just inland along the northern Gulf Coast. However, quasistationary fronts continue to frequent the northern Gulf region and

cause persistent periods of low ceilings and rain lasting 24 hours or longer. Fair weather is still largely associated with the closed centers of migratory anti-cyclones.

Cold fronts enter the Gulf much less frequently in April and almost never in May, so that frontal weather becomes less of a problem than in winter and early spring months.

The semi-permanent subtropical anti-cyclone is well established over the South Atlantic in May. Westerly systems are too weak to penetrate the strong ridge of high pressure extending westward across the Gulf of Mexico, and the season is too early for easterly systems to exert an important influence on the weather; consequently, May can be considered as having the fairest weather of the year.

Easterly circulation is predominant in June so that

easterly disturbances and occasionally more intense tropical disturbances begin to appear in the Gulf of Mexico during this month.

There are no significant differences in the number of tropical disturbances, amount of cloud cover, or visibility between June and

July. Because of warmer sea surface temperatures, air mass thundershower activity increases during the period from early morning to

early evening.

The Azores-Bermuda high is very strong during July and August and dominates the weather pattern over the Gulf as discussed earlier.

Easterly disturbances and tropical storms in the Gulf of Mexico increase significantly in August and reach a peak in September. Tropical storms follow two principal paths into the Gulf during August, one through the Straits of Florida, the other through the Yucatan Channel.

Thunder

shower activity is slightly more frequent than in July, and occurs mainly from early morning to mid-afternoon.

d. Fall

The July-August weather pattern just discussed continues

on through September. Easterly disturbances and tropical storms reach their peak frequencies during this month. Thunder shower activity decreases from the August frequency and occurs earlier in the day.

October is the transitional month, when easterly systems weaken, and westerly systems are not yet strong enough to reach the Gulf with any regularity. Continental air masses, usually of the Pm type, may occasionally move over the Gulf and produce several days of nearly cloudless skies. Tropical disturbances may still enter the Gulf during October, but their frequency in the northwestern Gulf is about one-half of September's.

Very few thunderstorms occur in October.

Tropical storms seldom enter the Gulf of Mexico during November.

tropical anti-cyclones over the South Atlantic are migratory and westerly systems control the weather in the Gulf and adjacent land

Sub

two-tenths or less approximately 30 percent of the time, and eighttenths or more about 40 percent of the time.

Visibilities less than

five nautical miles (M) are slightly more frequent in January and February than in December because of more frequent fogs induced by colder water temperatures, and are also more frequent in the coastal waters than in the central Gulf area for the same reason. In coastal waters, visibilities of less than five M are reported in about 15 percent of ship observations during January and February, compared to 10 percent in waters further offshore. During December, a visibility

of less than five M is reported about 10 percent of the time with less geographic variation.

b. Spring

Considerable cloudiness persists through April, but

breaks off sharply in May. Sky cover of two-tenths or less occurs approximately 30 percent of the time. In May, two-tenths or less

sky cover is reported about 30 percent of the time, but the percentage frequency of cloud cover eight-tenths or more decreases to only 20

percent.

Visibilities improve sharply in April, compared to March,

as the sea surface temperatures rise.

Frequency of visibilities less

than five M is 10 percent in March and 5 percent in April in the

northwestern Gulf. During May, restrictions to visibilities rarely

occur except possibly under infrequent thunder showers.

c.

Summer

The frequency of sky cover two-tenths or less over the

northwestern Gulf Coast is about 30 percent, and the frequency of sky cover eight-tenths or more is about 20 percent. No restriction to visibility occurs except in heavy rainshowers.

a.

Fall

September visibilities and sky cover are similar to

those present in summer.

During October and most of November, cloud

cover two-tenths or less is reported by approximately 50 percent of the ship observations off the middle and upper Texas Coast, and the Farther offshore, the frequency of cloud cover

Louisiana Coast.

eight-tenths or more is about 30 percent.

Advection and frontal fogs

begin to appear in November, reducing the visibility to less than 5 M

about 10 percent of the time near the coast and about 5 percent of the

The Azores-Bermuda atmospheric high pressure cell dominates the circulation over the Gulf, particularly during the spring and