Temperature fluctuations and climate pattens arise mainly due to the variations of the physical environment and our star, the sun. It’s what makes our climate in California Mediterranean, with rainy winters and sunny summers and the tropics humid and full of rain.
Most of us are probably aware that the earth’s tilt (approximately 23.5 degrees) is what gives rise to seasonal variation on our planet. It is sometimes believed that the distance of the earth from the sun contributes to our seasons, but this is not true. Also, the earth stays at a constant tilt in the same direction. The area that receives the most sun is due to the earth’s rotating orbit around the sun. Unfortunately, I cannot find a picture that illustrates this concept clearly.
During the northern hemisphere’s winter solstice, the sun’s energy is more spread out and the intensity of the energy is less. A the same time, the southern hemisphere receives more energy from the sun, creating summer-time temperatures.
Likewise, during the summer solstice, the earth will now be tilted toward the sun (during the winter it is tiled away from the sun) and receives more energy than the southern hemisphere. It is also during this time that the North Pole has sunlight for the majority of the day.
So why does the tropics receive such large amounts of rain? This is largely due to what’s called Hadley cells. Near the equator, systems of vertical air movement occurs. As the sun heats up the air near the equator, surface currents meet near the equator (the intertropical convergence) and rises up. It rises because warm air expands and rises. Warming up, air can pick up more moisture. As it rises up to the atmosphere, it cools and starts to condense and thus rain falls.
Once it has rained, the cool air drops back down to about 30 degrees north and south of the equator. Once on the surface, air picks up moisture on the surface, essentially creating arid climates like the Sahara desert, or Mojave. This is the subtropical high pressure belt, where little rain fall occurs. When the air gets to the equator again, the whole process repeats.
So going back to the original question of why the tropical areas receive more rain. Hadley cells occur at 30 and 60 degrees north and south of the equator, but what makes the equator have more rain? It’s because water cycles more rapidly through the tropical atmosphere.
Of course, this is all generally speaking. If you look at a map of annual precipitation for earth, you’ll notice that more rain occurs more below the equator than above it. This is because there’s more water mass in the southern hemisphere, thus more moisture available to be picked up.
Wind patterns also contribute to variations in climate due to physical changes on the environment.  You may notice that for example, if you live in the northern hemisphere, like in California, that mountains create a dividing line between forests and deserts-like environments.  This is because wind flows west, blowing air up to the mountain.  As the air climbs up the mountain, air cools.  Assuming that this air has picked up moisture along the way, precipitation occurs when the air gets cold enough and showers on the slops west of the mountain.  At the peak, all the moisture from the air current is gone and starts picking up moisture east of the mountain, creating arid environments.  These environments are called rain shadows. Sierra Nevada is a good example of this.
Ocean currents play a role as well. ¬† They’re driven by the surface winds and the rotation of the earth. If you’ve ever been in the Atlantic ocean on the eastern coast of United States, you know that the water is very warm compared to the Pacific ocean.¬† This is due to cycling ocean currents.¬† In the Pacific, water from the areas like Alaska move down the coasts of Oregon, Washington and California.¬† So cold water from the north is moving down toward the equator. It’s not until this current meets near the equator that is starts warming up again, giving Japan warm ocean currents.
There’s a few exceptions to this pattern,¬† and they’re called upwellings.¬† Upwellings occur when surface currents diverge, and as they move apart, deep waters tend to rise up vertically.¬† They also occur when strong ocean currents move toward the equator.¬† The cycling of water in upwelling actually creates high areas of biological productivity, producing an area full of rich fisheries.
Ocean currents, wind patterns, intensity of sunlight — they all contribute to our global climate.¬† In some cases, they create seasonal events such as El Nino or El Nina (which follows after El Nino).¬† They’re known as ENSO (El Nino- Southern Oscillation) events.
El Nino occurs when air pressure changes in certain areas.  This in effect causes trade winds to be weakend, allowing warmer water to move east.  This warm water, as well as moisture being carried over decreases development of Atlantic hurricanes.  However, they also create higher rainfall averages to occur in Western United States.
We’re currently in a somewhat El Nina event which is just reverse of El Nino.¬† The winds that were once weak are now strong and warm water is pushed west instead of flowing east.¬† This creates a more drier climate in Eastern United States and tends to increase the development of Atlantic hurricanes.