Hawaiʻi is called the Rainbow State for a reason. The license plates have carried a rainbow graphic since 1991. Local weather forecasters talk about rainbows the way mainland ones talk about clouds. Visitors spend a week here and go home with a phone full of them without really thinking about why.

The nickname is earned. Rainbows are a daily product of how Hawaiian weather actually works, and summer is one of the best times of year to catch one. Come between June and September and you will almost certainly see a handful during your trip. A little meteorology up front is the difference between walking past one and knowing where to stand.

Here is why rainbows happen so reliably in Hawaiʻi, when summer is prime, and the spots on the main visitor islands where you can tilt the odds heavily in your favor.

A rainbow needs sunlight, airborne water droplets, and an observer with the sun behind them. Hawaiʻi produces those conditions on a loop, nearly year-round.

The engine is the northeasterly trade wind. Trade winds blow across the state roughly 90 percent of summer days, pushing moist Pacific air into the windward side of every island. When that air slams into the mountains (the Koʻolau range on Oʻahu, Haleakalā on Maui, Mauna Kea and Mauna Loa on the Big Island, the Waiʻaleʻale massif on Kauaʻi) it rises, cools, and squeezes out moisture as short, patchy showers. Meteorologists call the mechanism orographic lift, and it is the defining feature of Hawaiian weather.

Those showers are compact. A narrow band of rain passes through and the wind pushes it somewhere else within minutes. Sunshine keeps punching through the gaps. That combination of tight rain cells moving through bright sun is the rainbow factory. On the mainland, rainbows usually require a thunderstorm to clear. In Hawaiʻi they happen in the middle of an otherwise blue-sky afternoon, which is why locals treat them as part of the scenery rather than an event.

Most visitors assume winter is the rainbow season because winter is when the full storms roll through. That logic does work. A January Kona low can leave the islands draped in double rainbows for half a day. But the day-to-day odds are actually better in summer.

Trade winds are most reliable between May and September. Kona weather (winds from the south that shut the trades off) is a winter pattern; summer trades are the steady, predictable kind that keep the windward-side shower factory running. The sun also rides higher in the sky in summer. A rainbow’s center is always directly opposite the sun from the observer, so a high sun produces a low, wide arc. That is the kind that looks huge across a valley rather than the thin vertical slice you get at winter sunset. And summer afternoons produce brief mauka (mountain-side) showers almost daily, so your window of opportunity reopens every afternoon without requiring a storm system.

The best rainbow windows in summer:

• Early morning (around 7 to 9 a.m.) — trade-wind showers that built overnight are still passing over windward slopes. Sun is low in the east, so rainbows appear tall over the western half of the sky.
• Late afternoon (around 3 to 5 p.m.) — mountain-scale convection peaks, producing the afternoon mauka showers that locals schedule around.
• Right after a brief rain cell passes. If you have been driving in a shower for three minutes and punch back into sunshine, look behind you toward the rain. There is almost always a rainbow there.

You cannot predict exactly where a rainbow will form, but you can stack the odds. The trick is to pick a spot where you are looking at the windward mountains from a position that puts the sun behind you, ideally in the morning or late afternoon. Here are spots that reliably deliver on each of the main visitor islands.

Oʻahu — Nuʻuanu Pali Lookout

The Pali funnels wind and moisture straight up the cliff face. The lookout faces northeast toward Kāneʻohe Bay, so the geometry you want is late afternoon, when the sun has swung west and sits behind you. Watch for trade-wind showers drifting across the windward coast below and a rainbow will pop against the green wall of the Koʻolau. Note: the state parking fee applies to non-residents.

Oʻahu runner-up — Mānoa Valley

The Mānoa Falls trail runs up one of the wettest valleys on the island. The upper reaches of Mānoa receive around 150 inches of rain a year, and the valley opens toward town with the sun behind you in the afternoon. Full-arc rainbows over the university side of the valley are a regular evening occurrence.

Maui — Nāpili and Kāʻanapali, looking mauka

Western Maui’s resort coast faces the afternoon sun while the West Maui Mountains behind it catch constant trade-wind showers. From Nāpili Bay or any of the Kāʻanapali resorts, you can sit with a drink around 4 p.m. and watch rainbows form against the green mountains behind you. This is the classic postcard framing.

Big Island — Rainbow Falls, Hilo

The waterfall is named for the rainbows that form in its mist when the low eastern sun hits the spray. Rainbow Falls State Park is about five to ten minutes from downtown Hilo, free to enter, and the viewing platform puts you exactly where you need to stand. Mornings are the window, typically between roughly 8 and 10 a.m. By noon the sun angle has moved on.

Big Island runner-up — ʻAkaka Falls

ʻAkaka Falls north of Hilo is a 442-foot drop into a fern gulch that traps mist in the canyon. Morning light slants in from the east and produces ribbon rainbows below the falls. The paved loop is short and easy.

Kauaʻi — Hanalei Valley Overlook

From the Hanalei Valley overlook off Kūhiō Highway, you look southwest into a wide taro-patch valley with the Namolokama ridge wall at the back. That view works best in morning, when the sun is rising in the east behind you and a mauka shower is still hanging over the ridge. This is probably the single most photogenic rainbow vantage in the state on a good summer morning.

Kauaʻi runner-up — Wailua Falls, east side

Early morning (before 9 a.m.) a rainbow frequently appears in the mist at the base of the 80-foot double falls. Pull-in parking right at the overlook, no hike.

Phones underperform on rainbows. The dynamic range between a bright sky and the thin prism of color is brutal, and auto-exposure tends to blow the background out and leave the rainbow looking washed.

A few adjustments help on any modern phone:

• Tap the bright sky to lock focus, then drag the exposure slider down one or two stops. A slightly darker frame will show the rainbow colors dramatically more.
• Turn HDR off. It averages tones across the frame, which is the opposite of what you want.
• Shoot with a polarizing filter if you have one. Polarizers can either strengthen or erase a rainbow depending on rotation. Turn the filter until the rainbow peaks.

A waterproof phone case is worth having on any Hawaiʻi trip. Rainbow conditions are, by definition, conditions where you are standing in intermittent rain. Most of the best shots in this article come from moments where someone had a phone out in a brief shower.

For composition, the cliché is to put the rainbow at the center. The more interesting shot is to put something Hawaiian in the foreground and let the rainbow arc over it: a palm tree, or a taro patch with the arc coming down behind it.

The rainbow on the state license plate is not decorative. In Hawaiian tradition, ʻānuenue (the word for rainbow) carries meaning. It appears in oli (chant) and moʻolelo (story) as a marker of presence: of aliʻi traveling, or of an important arrival at a significant place. The University of Hawaiʻi athletic teams are the Rainbow Warriors and Rainbow Wahine for the same reason.

None of which is required reading to enjoy seeing one. But the next time you are standing at the Pali lookout and a rainbow forms over Kāneʻohe Bay, or you are on the Mānoa trail and one appears across the valley, it is worth knowing that people have been noticing them and naming them here for a very long time. You are doing the same thing.

You do not need a plan. Rainbows will find you in Hawaiʻi whether you want them to or not. But if you are the kind of traveler who wants to stack the deck, build one morning or one afternoon around a windward-side viewpoint (any from the list above), check the forecast the night before for trade-wind speed and cloud cover, and be there at the right solar angle.

The sunrise/sunset tables will tell you when the sun will be low enough to produce a tall arc. For most of summer on Oʻahu that is before 9 a.m. and after 3:30 p.m.

Then be patient. A rainbow in Hawaiʻi is usually about ten minutes away.