What factors determine the average monthly temperatures?
Seasons and degrees latitude effect the climate of a region and, thus, the average monthly temperature.
Seasons are due to the tilt of Earth on its axis and Earth's orbit around the sun throughout the year. The Earth's axis is tilted at approximately 23.5 degrees, so the Earth is never exactly straight up and down. The hemisphere of Earth that is leaning into the sun experiences summer or spring. Likewise, the hemisphere of Earth that is learning away from the sun is experiencing fall or winter.
There are variations of temperature within the same hemisphere during any season. These variations are due to differences in latitude. Areas that are at or near the equator receive more direct sunlight than areas further north or south of the equator. Tis means that the sun hits the Earth with a greater angle at the poles. Thus, these equatorial regions will experience higher average monthly temperatures than the polar regions.