Imagine a record turntable. (Remember those things before CDs?
) Now looking at the turntable from above, an abject placed in the centre will have a speed of zero - it is just spinning around, not going anywhere.
An object placed on the edge will have the greatest speed, as it zooms around the outside of the circle.
Now stop the record player, and chalk a straight line from the centre to the edge. With the record player stopped, there would be no problem walking along this straight line (if you where small enough.)
Now start the record player up again. shrink yourself and stand in the centre - your speed is zero (you are just spinning around) and in front of you is the straight chalked line. After things have settled down, and the fluid on your inner ear has matched the speed of rotation of the record player, you do not notice that it is spinning - it looks stationary to you.
Now for the sobriety test - you start walking along the straight line you have chalked. With each step towards the edge of the record player you actually have to "speed up" to match the speed of the bigger circle you have stepped into. When the record player was still, walking the line was no problem. but now that it is turning, with each step you feel forced to the left (for a clockwise spinning player). This "force" you feel is the reaction to the acceleration necessary for you to speed up to match the speed of the greater circle you have stepped into, and is termed the "coriolis force".
It is an imaginary force we use to explain an apparant "force" in an accelerating (turning) frame of reference, when we consider that frame to be stationary.
Why does it get less towards the equator? On our record above, as you approach the edge, it wouldn't get less.
Look at the Earth from the North pole, looking down. You see a "circle" turning anti-clockwise. because you are looking at a globe, one step from the North pole towards the equator is the same as one step on the circle, but the Earth's surface is curving away from you - so each step on the Earth's surface is a smaller step towards the "edge of the circle" until you reach the equator where each step is simply a step away from the north pole, and doesn't move you towards the edge of the circle at all.