The Blanket Effect
Heavy rain is bad for wind. Imagine millions of water droplets falling from the sky. They create friction against the horizontal wind.
A heavy curtain of rain acts like a wall. It creates drag and turbulence. Often, the wind will die completely inside a heavy rain zone. We call this "rain kill."
The physics behind this is simple but devastating: falling rain droplets have mass and momentum. When horizontal wind encounters a vertical rain curtain, energy is lost to turbulence and drag. Wind speed can drop by 30-70% inside a heavy rain cell.
The Physics
Raindrops create vertical momentum transfer and turbulent drag. Heavy precipitation (rain rate > 10mm/hr) increases air density locally and creates friction against horizontal wind flow, reducing effective wind speed.
Rain rate of 25mm/hr can reduce surface wind by 50% or more
Rain Intensity Spectrum
Not all rain is the same. Meteorologists classify rain intensity by precipitation rate in millimeters per hour:
- Light rain: 0.5-2.5 mm/hr (barely noticeable)
- Moderate rain: 2.5-10 mm/hr (steady rainfall)
- Heavy rain: 10-50 mm/hr (downpour)
- Violent rain: >50 mm/hr (tropical deluge)
For wing foiling, anything above 10mm/hr will significantly degrade wind quality.
Rain Intensity vs Wind Impact
Light Drizzle
<2.5mm/hr
Minimal wind impact, safe to ride
Moderate Rain
2.5-10mm/hr
Wind becomes gusty, visibility reduced
Heavy Downpour
>10mm/hr
Wind killed, dangerous conditions
The Downdraft Exception
In a squall (as discussed in other articles), the rain brings a short burst of wind. But sustained, heavy rain usually means a bad session. The wind becomes gusty and unstable.
A squall line features a strong downdraft that can temporarily increase surface wind to 25-40 knots. This wind surge lasts only 5-15 minutes, followed by heavy rain and wind collapse. Squalls are identifiable by their dark, advancing cloud shelf and sudden temperature drop.
Rain Cell Structure and Wind Zones
A typical rain cell has three distinct zones with different wind characteristics:
- Leading edge (gust front): 15-30 minute window of increased wind (1.5-2× normal) before rain arrives
- Core (heavy rain): 20-40 minutes of wind reduction (30-70% loss) with poor visibility
- Trailing edge (clearance): 30-60 minutes of recovering wind, often stronger than before the cell
Understanding this structure lets you time sessions around rain cells. Ride during the gust front, pack gear during the core, and return for the trailing edge revival.
Quantifying Wind Loss in Rain
The wind reduction from rain is calculable based on precipitation rate:
- 5mm/hr rain: Expect 15-25% wind reduction (20 knots becomes 15-17 knots)
- 15mm/hr rain: Expect 40-60% wind reduction (20 knots becomes 8-12 knots)
- 30mm/hr rain: Expect 60-80% wind reduction (20 knots becomes 4-8 knots)
These values assume moderate to heavy rain falling through horizontal wind. The denser the rain curtain, the greater the momentum transfer and drag effect.
Visibility and Comfort
Rain also smooths the water surface, which looks inviting. But rain drops hitting your eyes at 20 miles per hour hurt. Visibility drops to zero. It becomes difficult to see the shore.
At 15 knots of wind, raindrops strike your face at approximately 24 km/h (15 mph). This is painful and disorienting. Visibility can drop below 100 meters in heavy rain, making navigation back to shore dangerous. Rain also fills your wing with water, adding weight and reducing response.
Practical Rain Tips
Watch radar loops: See if rain is moving toward or away from your spot
Light drizzle is rideable: If you can still see the horizon, you can ride
Wait out heavy cells: Most rain showers pass in 20-40 minutes
Avoid dark cloud bases: Sign of heavy precipitation and possible lightning
Rinse gear thoroughly: Rainwater is dirty; flush your equipment after sessions
Post-Rain Wind Recovery
After a rain cell passes, the wind often returns stronger than before. This is because:
- Evaporative cooling: Rain cools the surface, increasing pressure gradient
- Cleared atmosphere: Rain removes dust and moisture, reducing drag
- Cold pool outflow: Descending air from the storm spreads horizontally
Some of the best wing foiling sessions happen in the 1-2 hour window immediately after a rain system passes.
Rain Intensity Guide:
Light Drizzle
Go for it - minimal impact on wind
Heavy Downpour
Wind quality terrible - wait for it to pass
Lightning and Electrical Hazards
Rain often comes with lightning. If you see a flash or hear thunder, you are already in danger. Lightning can strike water up to 10 miles from the storm cell. Carbon fiber masts and foils conduct electricity.
Follow the 30-30 rule: If the time between lightning flash and thunder is less than 30 seconds, seek shelter immediately. Do not return to the water until 30 minutes after the last thunder.
Summary
If it is just a light drizzle, go for it. If it is a heavy downpour, the wind quality will likely be terrible. Wait for the rain to pass. Use weather radar to track rain cells, and capitalize on the strong post-rain wind that often follows. Never ride in conditions with visible lightning or audible thunder.