Heather...you've picked one tough topic to put in really simple terms, but here goes.
Because we live on a globe that spins, a force arises called the Coriolis Force. The Coriolis force arises (I am seriously simplifying here, so anyone who knows the math, cut me slack, please) because as well as I can put it, different parts of the planet spin at different speeds. The coriolis insures that any large scale, long lasting motion (like a tropical cyclone, or, for that matter a "plain old" high or low pressure system) "spins".
Let's start here...imagine you are on the equator. The earth is about 24,900 miles in circumference there. In one day--24 hours--you'd travel 24,900 miles if you were standing on the equator. Importantly, you'd travel toward the EAST really fast, because that's the direction in which the earth spins.
Now, imagine that you are just steps away form the north pole. The Earth has a very small circumference there--it's not as far "around" as it is at the equator. Now, at the pole, you only have to cover a few feet in one day--you wouldn't travel very far in 24 hours at all! your eastward speed is MUCH slower there.
The further north from the equator you go, the slower the speed at which you go east...follow?
OK, so...imagine an air parcel at the equator that starts moving north toward the north pole (north is important!). As it starts out northward, it is still going to be moving at that same speed...really fast to the east. However, as it gets farther north, the ground you are passing over will have an eastward component that is slower and slower (the closer you get to the pole, the slower). So the parcel that left the equator will be shooting ahead to the east as it goes north becasue it has a faster eastward component than the surface it's over.
Viewed from above, the parcel would curve to the right (eastward). If you think through the same example (sticking with the northern hemisphere) of an air parcel released from the north pole toward the equator, you'll see that it moves to the right too.
Here is a simple image...focus on the northern hemisphere (all is different in the southern hemisphere).
http://www.prim.net/dessins/coriolisico.jpg
See all the arrows go to the right in the northern hemisphere?
OK, so let's generalize...for any long lasting, large scale event...like the movement of air parcels, coriolis has an impact.
in the northern hemisphere, that impact is to cause the air parcel to deflect to the right of its forward motion.
Now imagine a hurricane. It is a very strong center of low pressure. Air moves in toward low pressure like a hurricane. But, instead of moving directly in (a straight line, no "spin"), it breaks to the right. That creates the counterclockwise motion we're used to seeing in low pressure systems...like hurricanes...in the northern hemisphere.
Here's an image that shows airflow getting bent to the right and creating the counterclockwise spin around lows (and clockwise around highs).
http://www.earth.rochester.edu/fehnlab/ ... ig17_5.jpg
Hope it helps!
For fun, here is a shockwave game that simulates Coriolis and has a little thing that explains it too!
http://www.eoascientific.com/campus/ear ... nteractive
