These activities build on the content that follows. 

Hahai no ka ua i ka ululāʻau.

Rains always follow the forest.

The rains are attracted to forest trees.

Knowing this, Hawaiians hewed only the trees that were needed.

—ʻŌlelo Noʻeau #405, Mary Kawena Pukui

Rain Around the Islands

Rain in Hawaiʻi is location specific. Some areas are very dry, and others are almost always rainy. The Western (leeward) sides of the islands tend to be drier than the Eastern (windward) sides (Fig. 1). The dryest location in Hawaiʻi is the summit of Mauna Kea, Hawaiʻi Island, which gets less than 10 inches of rain per year. On the other hand, the windward slope of Haleakalā, Maui, gets more than 400 inches of rain each year. 

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Fig. 1. Rainfall varies between the windward and leeward sides of the Hawaiian Islands. 

Image copyright and source

Image Courtesy of the Rainfall Atlas of Hawaii

Giambelluca, T.W., Q. Chen, A.G. Frazier, J.P. Price, Y.-L. Chen, P.-S. Chu, J.K. Eischeid, and D.M. Delparte, 2013: Online Rainfall Atlas of Hawai‘i. Bull. Amer. Meteor. Soc. 94, 313-316, doi: 10.1175/BAMS-D-11-00228.1.

Most of the rain in Hawaiʻi is caused by winds interacting with the mountains (Fig. 2). The normal trade wind direction is East-Northeast. Therefore, the average rainfall for the East facing sides of the islands is higher than for the West facing sides. General rainfall patterns occur in the following manner:

1. As the trade winds contact the windward mountains, air is forced upward.
2. Rising air cools because of colder temperatures in the upper atmosphere.
3. As the air cools, water vapor in the air condenses on small particles called condensation nuclei to form clouds, water droplets, and rain. 
4. Plants capture rain and help it to slowly move through the ground.
5. Once the water is released from the air, there is less water to fall on the other side of the mountain, or leeward side.

The mountains bring the rain by physically pushing air up and cooling it. But, the forest is needed to capture the rain. The complex, multi-layered nature of the native, Hawaiian forests are very good at capturing water from clouds and from rain. Humans have cleared land for grazing, harvested forests for trees, and allowed invasive animals and plants to destroy the native forests. Efforts are now underway to restore native forests in order to help cature fresh water and help to recharge underground aquifers.

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Fig. 2. The patterns of rainfall around the Hawaiian Islands is due in large part to the interaction of the winds and landscape. 

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Image Courtesy of Flickr

Weather

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Fig 3. A rainbow forms over the big island of Hawai'i after a rain storm passes through. 

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Image courtesy of Wikimedia Commons

Weather is all around us (Fig. 3). It has a large effect on our daily lives. The weather affects how and where we live, what we do each day, what we wear, and what we eat. Weather is made up of many different atmospheric factors, like:

• wind
• precipitation
• humidity
• temperature
• pressure
• sunshine
• clouds
• visibility

Meteorology is the study of weather. A person who studies weather is called a meteorologist. Meteorologists make weather predictions based on the interactions of atmospheric factors. 

Climate

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Fig. 4. Climate represents weather conditions over long periods of time.

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Image courtesy of NASA

Climate is a term used to describe average weather conditions (Fig. 4), in a particular place, over a long period of time. The weather changes from day to day, but climate changes take place over many years (for example, more than 30 years). This means that, even though it may rain sometimes in the desert, on average the climate is hot and dry. The Antarctic has a very cold and dry climate. Tropical places, like Hawai‘i, have warm, humid climates. 

The Water Cycle

The water cycle is the recylcying of water through evaporation, transpiration, and precipitation (Fig. 5). During rainy periods, plants, soils, rivers, and lakes take up water.

Periods of hot, dry weather can lead to water evaporation and transpiration. Plants and soil dry out. Streams and rivers shrink to lower water levels.

But, over the course of several years, a balance occurs in the water cycle so the amount of water entering the system is equal to the amount exiting. This balance, or equilibrium, in the hydrologic cycle helps stabilize the climate.

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Fig. 5. The water cycle has a big influence on weather and climate. 

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Image courtesy of NOAA

Climate is affected by the natural fluctuation of the balance of water entering and exiting the system on a global scale. The availability of water depends on the ocean and the atmosphere—as well as on large freshwater lakes and glaciers. Natural hazards like floods, droughts, sea level rise, and elevated sea surface temperature affect daily weather events, but they can also have long-term impacts on the climate.

Climate Changes

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Fig. 6. A flooded intersection (near the corner of Keeaumoku & Makaloa Streets in Honolulu) reveals the potential dangers of excess rain. This photo was taken in 2006.

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Image courtesy of Wikimedia Commons

Scientists are studying how different factors impact the climate. Scientists also observe local weather for patterns that can be predicted on reliable timescales. The majority of climate scientists agree that one of the biggest threats to our Earth is the rapid, global climate change taking place as a result of human influence. The energy and resources used by humans are converted into gases that change the chemistry of the atmosphere. Climate change can alter the water cycle by increasing (1) the likelihood of floods in usually dry regions and (2) droughts in historically wet regions (Fig. 6).

As a result of Earth's warming climate, many of the world’s glaciers are melting. These melting glaciers can significantly raise the water level in the ocean (see the grade 5 activity on rising sea levels!). The warming climate also changes weather patterns in some areas, making weather harder to predict and ultimately producing more severe weather as a result. In tropical regions such as Hawai‘i, global climate change can contribute to a rise in sea surface temperatures. This rise in temperature is important because even small increases can have big effects on coral reefs, causing bleaching that eventually can lead to the death of both corals and the associated ecosystems.