Kauai's nickname is The Garden Island.  This is because of its lush vegetation due to its geographical location and weather patterns - not because it’s an island composed of passionate gardeners!! :)

                                 (The major regions of Kauai are defined by their
                                   location on the island relative to the prevailing trade winds)

Although Kauai’s climate is tropical, it consists of many microclimates, including arid areas, rainforests, temperate and tundra climates.  Due to it's differing elevations, pressure variations, rainfall, wind and humidity; temperatures are not as extreme as you would find in a true tropical environment, so many people regard Kauai as a subtropical region.

  

Trade winds blow across Kauai from the northeast. This means that the north and east sides of Kauai are on the windward side of the island.  These sides receive the most rain.  The south and west sides of the island are on the leeward side of the island, which receive less rain - rain has already fallen on the windward side.

(A satellite image of the islands. Notice the pattern of the clouds, as well as the vegetation primarily on the northeast sides of the islands because they face the tradewinds, which bring humidity) 

 

(an example of precipitation on the leeward side of a mountain)

Trade winds in combination with Kauai’s geographic location create Kauai’s temperature.  Kauai has two seasons, summer- from may to October and winter - from September to April.  Temperatures range between 85 and 69 degrees depending where you are on the island.  Coastline temperatures average around 72 degrees in the winter months.  During spring and summer the average is around 82 degrees. 

(Annual precipitation rates for Kauai)

These two graphs show Kauai's difference in precipitation rates within the regions of the island:

(Poipu, Kauai receives much less rain throughout the year than Princeville, which is on the leeward side) 

As you can see, some areas receive much less rain than others.  Precipitation is common throughout the island, however, the vast majority of rain showers are brief.  Even heavy showers in the west and north are unique because thunder and lightening are rare.  

 ("Anuenue" is Hawaiian for rainbow.  Note: The outer rainbow is the reverse of the more vividly colored inner one.)

Rainbows occur frequently all over Kauai.  Droplets of precipitation act like prisms.  In white light the colors bend and spread, causing these beautiful sights.  

*(technically there's a region in India which averages a few
more inches of rain per, however, Mount Waialeale
wins because it's precipitation rates are more consistent) 

Mount Waiʻaleʻale (Hawaiian for “over flowing water”) is known as the wettest spot on earth, averaging 450 inches of rain yearly.  It is Kauai’s second highest peak with an elevation of 5,148 feet.  It is located in the center of Kauai.  It has had a huge impact on many formations throughout the island.

Why does Mount Waiʻaleʻale receive sooo much rain? 

Kauai is positioned at the top of the main island chain.  It is conical in shape, which exposes all sides of  Mount Waiʻaleʻale to trade winds and their moisture.  When moist air rises and flows over a mountain, on the windward side it is called orographic rainfall.  

(A diagram of the orographic rainfall process)

(Adiabatic cooling)
 Note: on the Wind ward side, cumulus clouds usually form above because of the tradewinds.  

As air is heated it expands and becomes less dense.  Because it is lighter, it rises above the cooler air.  As it raises it continues to expand because there is less pressure higher in the atmosphere. This process is called adiabatic cooling and is common among elevated landforms.  The result of adiabatic cooling is precipitation.  In addition, Mount Waiʻaleʻale is below the trade wind inversion layer of 6,000 feet.  This means that clouds produced by trade winds can’t rise, there for they get stuck in one spot, hence the large average amount of rain in this particular area. 

Below Mount Waiʻaleʻale is an extinct caldera.  It is now a plateau, called the Alakai Swamp.  It is the world's highest rainforest and swampland (4,000 to 4,500 feet above the Pacific) and was formed by excess water drainage from Mount Waiʻaleʻale.  Alakai Swamp is a Montane Forest, meaning it has persistent precipitation and low-cloud cover, which retains moisture.  Because of its elevation it is considered to be one of the most unique 'swamps' in the world.    

External links:

Data Base for Hawaii

Hydrology.php

Kauai

http://www.soest.hawaii.edu/coasts/publications/hawaiiCoastline/kauai.html