There is a big gulf of difference in temperature between the thickly foliaged forested regions and the open grounds. Forests are like sponges which retain moisture in the soil. They also prevent the air from being heated rapidly during the day and from being cooled quickly during the night. The dense vegetation of the Amazon jungle cuts off much of the in-coming insolation and in many places sunlight never ever reaches the ground.
It is, in fact, cool in the jungle and its shade temperature is a few degrees lower than that of the open spaces within the similar latitudes elsewhere. During the day trees lose water by evaporation from the soil and other surfaces and by transpiration from the plants. Thus, the air above is cooled. Relative humidity, however, increases and mist and fog may also form.
The more water a soil can retain, the less rapidly it heats or cools. Dark coloured soils and surfaces absorb more of the Sun’s heat than the light coloured ones. On the other hand, light coloured soils reflect more heat than darker soils. In fact, dry soils like sands are very sensitive to temperature changes, whereas wet soils, like clay, retain much moisture and warm up or cool down more slowly.
Rocky and sandy soil is heated more rapidly and also cooled more rapidly. Alluvial soil can retain water, while on rocky soil the rainfall run quickly of the surface and is lost. The great range of temperature in the deserts during the day, is partly due to the nature of its soil.
Thus, the difference in the nature of the soil is also responsible for the variation of the temperature.