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PRESSURE TENDENCIES.— Except in the vicinity of a tropical storm, 3-hourly tendencies are nearly useless in the tropics and can never be used as in midlatitudes except as they deviate from normal. There are three reasons for this:

1. The 3-hour synoptic pressure change is so small (on the order of 0.1 mb) as to fall within the unavoidable range of observation errors.

2. The diurnal variation (on the order of 1 mb) completely masks the true synoptic varia-tions.

3. Passage of local cloud systems often causes barometric changes of 0.1 mb or more.

Consequently, the 24-hour pressure change is used in tropical analysis. This eliminates the normal diurnal change from the tendency and also provides for a better relation between the rate of motion of disturbances and the time interval over which pressure changes are measured. The 24-hour change in the tropics is comparable to the 12-hour change in midlatitudes.

In most areas, changes of 3 mb are rare and definitely indicate danger of severe weather developing when a storm is not already in existence. Even values of 1.5 to 2.5 mb warrant careful attention, especially when the change is at or below average pressure values.

TEMPERATURE.— This is one of the least representative elements of the tropical synoptic report. Cloud cover and precipitation cause temperature fluctuations. Clouds shade the hot tropical sun. Rain, evaporating as it falls, cools the air considerably. Temperature readings in showers are therefore quite unrepresentative. Even when a shower passes, the low-level air doesn’t immediately return to its previous state. Care must be taken not to misinterpret the combination of clouds, precipitation, and cooler temperatures as a frontal passage. Temperatures on the lee side of mountainous islands are also unrepresentative. Marked diurnal changes are caused by cool mountain air descending the lee side at night. Diurnal differences of 10°C (20°F) are not uncommon at these locations. This compares favorably with the average annual diurnal temperature range over continents, which is 5 to 10 degrees Celsius. Over the open ocean, the diurnal effect is not nearly as great (2 to 3 degrees Celsius). Diurnal variations are greatest on clear days. The largest temperature variations take place along the fringe areas of the tropics that come under the influence of midlatitude pressure systems. The most representative tropical temperatures are those reported by observation stations located on small, flat islands that are well removed from the effects of large land masses and by ships on the open sea.

DEW POINT.— This element reacts much like the air temperature and is unrepresentative under the same conditions of showers and lee-side cooling. Dew points are not used in tropical analysis as they are in higher latitudes, because evaporation and mixing processes far outweigh the differences found in the modified air masses of the region.

CURRENT AND PAST WEATHER.— Over most of the tropics, the reports of current weather nearly always involve some form of precipitation. However, in some areas (for instance, the Sahara and Northern Australian deserts) dust storms are more important, especially in the dry season. Along the cold water coasts of Africa and America, fog takes first place in importance. Although separation of rainfall into showers and steady precipitation is highly desirable, it is not always done. Making this separation is not always easy. A station may receive fairly steady rain for a long time, yet the rain be derived from cumuliform clouds. Because of the cell character of tropical rain, and again because of local regimes, the current weather report is very often rather meaningless. Many occasions are on record when severely disturbed conditions happened to let up temporarily at the 6-hourly observation period. Here a check of hourly sequences, when available, is most helpful. Again, in some freak situations, stations have been deluged, when all around them conditions were quite normal. Although not all of these phenomena can be spotted, it is advisable to place as much (or more) weight on weather in the past 6 hours as on current weather.

VISIBILITY.— Visibility in the tropics is usually good. Reductions are produced mainly by rain and haze; but in some areas, blowing sand or dust cause reductions, while along cold-water coasts restrictions are fog related. Over the open ocean, haze becomes a concern, especially with regard to carrier operations. Wet haze is produced under very stable conditions. Convection is suppressed and strong surface winds are usually present. As the wind raises salt particles from the sea surface, they absorb the moisture from the air, thereby producing this type of haze. The other type of haze is dry and occurs occasionally when continental air with a high dust content moves over the ocean. The smoke, ash, and haze that accompany volcanic eruptions also reduce risibilities when a large amount of the material is trapped beneath the trade inversion. Such conditions can last for more than a week, with the smoke, ash, and haze spreading over a large area.

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