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. This compares favorably with the average annual diurnal temperature range over continents, which is 5 to 10 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. 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. Over most of the tropics, the reports of current weather nearly always involve some form of precipitation.