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As you begin to analyze the weather occurring over the tropics, it becomes very apparent that you are working with limited data. This forces you to make the most of every weather observation. You will have to learn which stations transmit reliable and representative data. Reports judged reliable should be plotted and adhered to religiously. This applies to both surface and upper-air data.

Surface Data

Weather observations taken in the tropics do not differ from those taken anywhere else in the world. It is the level of importance that is placed on the elements that must be understood.

STATE OF THE SKY.— Unfortunately, cloud reports are often one of the least satisfactory items in the surface report. Usually clouds cannot be measured, but must be estimated. The present codes do not permit an observer to describe properly the various states of the sky found in low latitudes. The observer’s entry nearly always entails some arbitrary decision. Some observers regularly transmit the same combination of low, middle, and high clouds.

In spite of these drawbacks, much can be extracted from the cloud data. Emphasis must be placed on the presence or absence of low, middle, or high clouds as groups. Division into one of nine types within each group should generally be disregarded, especially with reference to middle and high clouds.

Low Clouds.— Cumulus is the predominant type of cloud found in the tropics. Their development, vertical and horizontal extent, and persistence are controlled by several factors: (1) horizontal convergence in the wind field, (2) depth of the moist layer, (3) orography, and (4) vertical stability of the air mass. There are many signifi-cant types of cumulus, and they usually range in some intermediate form between cumulus humilis and cumulus congestus. Stratus and strato-cumulus are also found in certain regions of the tropics.

A popular conception of the cloud distribution in the tropics pictures the equatorial region as a tremendous factory continuously producing cumulonimbus (Cb) clouds, which rise to spec-tacular heights. THIS IS ONLY TRUE OF CERTAIN REGIONS. Although records indicate that Cb clouds are more common in tropical than in polar regions, they also show that except for such places as central Africa, southeast Asia, Indonesia, the Amazon valley, and the southern United States of America, cumulonimbus is the exception rather than the most common cloud type. In fact, certain equatorial regions are known to report few, if any, Cb clouds throughout the year.

Middle Clouds.— These clouds are found everywhere in the tropics and in any season of the year; no appreciable seasonal variation occurs.

High Clouds.— Cirriform clouds in varying amounts are also found everywhere in the tropics. Isolated maximums of high clouds are found near the equator, some of which maybe attributed to the anvil tops of the cumulonimbus. An overall seasonal variation in high cloudiness is not evident in the tropics, although certain areas do experience seasonal changes.

SURFACE WIND.— In the tropics, topog-raphy and coastal features may render the surface wind completely unrepresentative, because they produce localized diurnal wind regimes. A representative wind can sometimes be obtained by subtracting the local effects. Ship winds are reliable and can usually be accepted at face value. Ship winds are least representative in or near heavy shower activity. The most representative wind found in the tropics is that which occurs during the day over flat stretches of land.

SEA LEVEL PRESSURE.— There are certain topographic effects that cause true pressure abnormalities. For example, on the lee side of mountain ranges and in channels between mountainous islands, pressures can read 1 to 3 millibars (mb) lower than normal. These readings impact the isobaric analysis, because they show consistent troughing over these areas. Unless you recognize this feature and take it into account in your analysis, you will interpret the pressure pattern incorrectly.

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