Re: Are there seasons on the equator?

As one might expect, there is little change in temperature at the equator over the course of the year. The temperatures are perpetually at what you in Michigan would call "summer". The figure below (scanned from Peixoto and Oort's "Physics of Climate" (American Institute of Physics, 1992)) shows the difference between January and July temperatures over the whole globe, in degrees C.

Note that when you're not exactly at the equator, the change of temperature with season depends a great deal on whether you're on land or at sea. Temperatures in the central Sahara fluctuate by 20 C annually, while in the Atlantic at the same latitude, they vary by only 5 C. This is because the ocean is able to efficiently store heat during the summer and release it during the winter -- it has a high "heat capacity".

Temperature is fairly constant near the equator, but rainfall varies greatly. Tropical areas do not have "winter" and "summer": they have "dry season" and "wet season". As warm air rises near the equator, it produces heavy rainfall. This zone of rising motion and heavy rain is called the intertropical convergence zone, or ITCZ. The rising motion occurs where the Earth is being heated most strongly, so the ITCZ lies north of the equator in June-August, and south of the equator in December-February. As the ITCZ moves north and south of the equator, the tropics receive more or less rainfall.

This difference can be huge! For example, the Amazon rain forest (just south of the equator) receives on average 1 meter of rainfall during December-February, and just 5 centimeters during June-August. The Congo (just north of the equator) is the other way around: it gets 1 meters of rain during June-August, and 5 centimeters during Dec-Feb. These figures are also taken from Peixoto and Oort (page 166), but I haven't scanned in the picture.

The ITCZ shifts between about 15 degrees north and south, so its effects are felt within that zone. At higher latitudes, seasons are more like what you're used to, progressing from mild to severe as you go farther north.

One special case is the southern part of Asia. Air over the vast landmass of central Asia, especially the high Tibetan Plateau, is strongly heated and then cooled as the seasons change there. Heating and cooling over Asia cause air to rush toward or away from the continent from the ocean to the south. Along with these changing winds come large changes in temperature and rainfall. This is the Asian Monsoon, which dominates the seasonal climate of India, Arabia, and Southeast Asia.

A personal anecdote: I grew up in Hawaii (20 degrees North, surrounded by Pacific Ocean). We were just outside the ITCZ zone. Wintertime daily high temperatures averaged 75 F (24 C), with lows of 60-65 F (17 C). Summertime highs averaged around 85 F (29 C), with lows of 70-75 F (23 C). Rainfall depended a great deal on location, but generally, winters were wet and summers were dry.

NASA's Goddard Space Flight Center has some good information on the Asian monsoon.