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. 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. 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.