Which Hawaiian Coastline Receives The Most Annual Rainfall

With its lush green landscapes and abundant rainbows, Hawaii is known across the world for its heavy rainfall. But when it comes to which coastline sees the most precipitation each year, the answer may surprise you.

If you’re short on time, here’s a quick answer to your question: the windward northeastern sides of the Hawaiian islands receive significantly more rainfall than the leeward southwestern sides. This is due to the direction of the northeasterly trade winds and the orographic effect.

In this article, we’ll take an in-depth look at the climate patterns that lead to this discrepancy in rainfall between Hawaiian coasts. We’ll analyze rainfall data from various locations, examine the mechanisms that produce rainfall, and determine exactly how much wetter the windward sides of the islands are.

Rainfall Overview of Hawaii

Total annual rainfall

The Hawaiian islands experience a wide variation in average annual precipitation due to the islands’ location in the trade wind belt and mountainous terrain (Giambelluca et al. 2013). Windward sides of the islands receive much more rain than the leeward, sheltered sides.

For example, the wettest place in the state is on the windward slope of Mount Waiʻaleʻale on Kauaʻi, with an astounding average of 460 inches (about 38 feet) of rain per year. Meanwhile, the leeward Kona coast of the Big Island is quite dry in comparison, with many places averaging less than 25 inches annually.

Seasonal variation

Across most of Hawaii, there is a distinct wet season lasting approximately from October through April. Nearly three quarters of the state’s annual rainfall occurs during these months (Giambelluca et al. 2014).

The extra winter precipitation is tied to an increase in storms and Kona lows that deliver heavy rainfall events.

Conversely, the spring and summer months tend to be much drier. For example, Honolulu sees only about 2 inches of rain during its driest month of June. During the wettest month, March, average rainfall increases nearly 10 times to about 18 inches.

Most leeward areas have a very pronounced dry season during May through September.

El Niño Southern Oscillation effects

The El Niño Southern Oscillation (ENSO) climate pattern can dramatically impact Hawaiian rainfall on an interannual timescale. During a warm El Niño phase, lower sea level pressure west of Hawaii reduces the strength of the trade winds.

This leads to fewer storms and suppression of rainfall, especially during the normally wet winter months. The largest El Niño events can result in severe drought across the islands (Chu and Chen 2005).

Conversely, the opposite La Niña phase produces stronger trade winds and a surge of storms. La Niña winters tend to be exceptionally wet, with flooding a threat during significant events. Overall, ENSO effects contribute to marked variability in Hawaii’s rainfall from year to year.

Mechanisms Causing Hawaiian Rainfall

Trade winds

The northeast trade winds are a consistent and dominant weather pattern affecting the Hawaiian islands. These winds blow from the northeast and drive moisture-laden air up the windward sides of the islands, leading to more rainfall on eastern exposures.

The winds force air to rise rapidly along mountains and volcanoes through a process called orographic lifting. This cooling of the air results in condensation and precipitation. The islands of Hawaii and Maui see the highest rainfall totals from trade winds.

For example, the town of Hilo on the east side of Hawaii receives over 120 inches of rain per year on average.

Orographic lifting

As trade winds blow moist air across the Hawaiian islands, the tall volcanic peaks and mountains force the air to rise in elevation. This rapid uplift, known as orographic lifting, cools the air quickly. Cooler air cannot hold nearly as much moisture, so condensation occurs.

This leads to cloud formation and heavy rainfall on windward mountain slopes. The steep mountains of east Maui and Hawaii see the most pronounced orographic rainfall, with as much as 300-400 inches annually in the wettest locations.

Convergence zones

Convergence zones are areas where winds and weather patterns collide and are forced upwards. This rising air cools and moisture condenses into clouds and rain. An intermittent area of low pressure north of the islands draws winds from differing directions together.

This convergence zone shifts location day-to-day but focuses rainfall when it occurs. Thunderstorms also occasionally develop in convergence zones around the islands. While less persistent than trade wind rainfall, convergence zones can deliver short intense downpours.

One study showed a 16 inch, 24-hour rainfall event over Oahu from a convergent thunderstorm.

Rainfall Analysis by Island and Coastline

Kauai

Kauai, known as the “Garden Isle” for its lush greenery, experiences some of the highest rainfall totals in Hawaii. The northeast side of the island, which faces into the prevailing northeasterly trade winds, receives over 400 inches of rain per year on average.

The town of Mount Waialeale is one of the rainiest spots on Earth, with over 460 inches of rain annually. Meanwhile, the southwest side of Kauai lies in the rain shadow of the central mountains and receives less than 30 inches of precipitation per year.

Oahu

Oahu shows a dramatic difference in rainfall between its windward (northeast) and leeward (southwest) sides. The windward side can receive over 150 inches of rain per year, especially in the steep Koolau Mountain Range. For example, the trail to Manoa Falls averages about 200 inches of rain annually.

Just a few miles away on the leeward side, the Waianae Mountain Range blocks rain clouds, creating much drier conditions. The leeward sides of Oahu receive less than 25 inches of precipitation per year.

Maui

The unique geography of Maui, with the West Maui Mountains blocking rain from the northeast and the Haleakala volcano creating a rain shadow effect on the south coast, produces striking differences in annual rainfall.

The northeast side of the West Maui Mountains records Hawai’i’s second highest average rainfall, with about 360 inches per year in Waihee. In contrast, the southwestern slopes of Haleakala Volcano only receive around 15 inches of rain annually due to the rain shadow effect.

Hawaii Island (Big Island)

On Hawaii Island, also known as the Big Island, the northeast side faces into the prevailing trade winds and records the island’s highest rainfall totals, averaging about 200 inches per year. For example, Hilo on the windward coast measures over 130 inches of rain annually.

The Kohala and Kona districts on the leeward west coast only receive between 10-15 inches of rain per year since tall volcanoes block rain clouds. Overall, you can easily drive from a wet tropical rainforest to an arid desert in less than an hour on Hawaii Island!

Rain Shadow Effect

The rain shadow effect describes how tall mountains block rain clouds on the leeward side, creating a “shadow” of dryness. As moisture-laden trade winds blow onshore and rise up mountain slopes, the air cools and condenses to produce rain.

By the time the air descends the leeward mountain slopes, much of the moisture has already precipitated out. The remaining drier air and sunny skies produce desert-like landscapes downwind of the mountains.

Hawaii’s dramatic rainfall differences between windward and leeward areas result from this rain shadow phenomenon.

Which Coastline Receives the Most Rain?

When it comes to rainfall in Hawaii, there is a clear difference between the windward (east facing) and leeward (west facing) sides of the islands. The windward sides tend to be much wetter as moist air is pushed up the mountains by the prevailing northeasterly trade winds, forcing precipitation.

The leeward sides are in the rain shadow of the mountains and typically receive significantly less rainfall.

More specifically, the windward side of the Big Island Hawaii receives the most rainfall of any coastline in the Hawaiian islands. Parts of the Hilo and Puna districts on the east side of the Big Island record over 120 inches (3,000 mm) of rain per year on average.

By contrast, the Kona coast on the leeward side receives less than 25 inches (650 mm).

There are several reasons why the northeast coast of the Big Island experiences such an enormous amount of rainfall:

• It faces directly into the prevailing northeasterly trade winds that bring moist air from over the ocean.
• It has two 13,000+ feet high volcanoes, Mauna Loa and Mauna Kea, which force rising moist air to produce rain through orographic lifting.
• Its sheer size creates a massive barrier for moist winds flowing from east to west across the island.

Other windward coasts do receive ample rain as well, just not to the extent of the Big Island’s massive mountainous terrain. For example, over 250 inches (6,400 mm) of rain falls per year on the summit of Mt. Waialeale on Kauai.

But near sea level on the same coastline, the town Hanalei still receives almost 100 inches annually.

Meanwhile, the leeward Kona coast of the Big Island is in a severe rain shadow and hot, dry air descending the mountains produces arid, desert-like conditions. Kailua-Kona averages just about 25 inches (650 mm) of rain per year with many sunny days.

It is hard to understate the contrast in climate and scenery on opposite sides of the same island!

Conclusion

As the data shows, the northeastern windward sides of Hawaii’s islands receive substantially higher rainfall than the southwestern leeward coasts each year. This discrepancy is driven by the direction of the northeasterly trade winds blowing moist air upslope, resulting in orographic precipitation.

Kauai’s Mt. Waialeale, Hawai’i Island’s Hilo area, and Maui’s Haleakala Crater stand out for their extreme rainfall. Meanwhile, leeward locations like Kona and Lihue see as little as 20-25% of their windward counterparts.

So next time you visit Hawaii and want to maximize your chances of seeing rainbows, be sure to spend time on the lush green windward coasts.

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