Why are all parts of the earth not heated evenly?

Answer 1

Because you have temperature differences that come from the sea or air moving above our heads not wanting to mix. Seas of hot air not wanting to mix with those of cold, those are moved around by corleous forces in large predictable hoops about the face of the earth. In my opinion these can cause some pretty stark temperature differences... its like dipping your leg into the water at the beach its nice and warm and then suddenly (in the space of centimeters) it gets really cold.

The sun is like clockwork, your seeing the sun through its passage of seasons, (its due to the sun hitting a progressively steeper face of the earth with respect to the beam's axis as the latitude climbs).

Similarly, a beam from the sun hitting a surface's head on can be said to make a circle, and when you increase the angle from the vertical, the more the circle looks like an ellipse, and the more elliptical it gets the more area the same beam covers! The heating effectiveness of the rays comes from the ratio (power of the beam)/(area illuminated), so the larger the area illuminated the less heating takes place. All in all, the larger the angle is from the vertical, the colder that surface will be from the light. Likewise, the higher or lower the latitude angle (and even longitude) on earth, the less heat will be offered, the colder the air will be.

In summation, even if there were not enough atmosphere to make an arguable difference, the sun's striking of the earth's surface would still cause temperature inequalities.