Quick Answer: Heavy rain degrades GPS accuracy in two ways: water in the atmosphere and on the watch itself attenuates and scatters the satellite signal, and rain usually comes with dense cloud that reduces signal quality further. Add a wet sleeve over the watch and the reflective effects of wet buildings and trees, and instant pace becomes unreliable. Average pace over the full run stays reasonably accurate; the per-second number is what falls apart.

The fix is to stop watching instant pace on wet runs and use lap pace or effort instead.

What is actually interfering with the signal?

Several things at once, and instant pace is the most fragile output.

Water attenuates the GPS signal. GPS operates at frequencies that are absorbed to some degree by water, and heavy precipitation puts a great deal of water between the satellite and your wrist. The effect from rain alone is modest, and it is real, and it stacks with everything else.

A wet watch face and a wet sleeve matter more than the rain in the air. A film of water directly on the antenna, and especially a soaked jacket sleeve pulled over the watch, degrades reception noticeably. This is the single most controllable factor and the one most people get wrong on wet runs, because pulling the sleeve down is the natural response to cold hands.

Cloud cover comes with the rain. Dense, moisture-laden cloud is not neutral, and heavy overcast conditions correlate with poorer signal quality.

Wet surfaces reflect. GPS accuracy suffers from multipath error — the signal bouncing off surfaces and arriving at the receiver by more than one path, at slightly different times. Wet building faces, wet roads, and wet foliage are more reflective than dry ones, which increases multipath in exactly the environments where it was already a problem.

The reason this shows up as wild pace numbers rather than a slightly wrong distance is arithmetic. Instant pace is calculated from a very short distance over a very short time, so a small positional error becomes a large pace error. A position that is off by 10 feet in one second produces a dramatic apparent speed change. The same error averaged over a mile is negligible.

Why does average pace still look fine?

Because the errors are largely random and cancel out over distance.

GPS positional error under normal conditions is roughly in the range of a few meters, and in poor conditions somewhat more. Critically, that error is not consistently in one direction — the position wanders around the true position rather than drifting steadily away from it.

Over one second, the wander dominates the actual movement, and instant pace is nonsense. Over a mile, your actual movement dominates the wander, and the number is close to right.

This is why the honest summary is that your watch is more accurate than it appears. Total distance on a wet run is usually within a percent or two of correct, even when the instant pace was jumping around by a minute per mile.

There is one systematic error worth knowing about, though. GPS wander tends to add distance rather than subtract it, because a wandering path is longer than a straight one. On a run with a lot of turns, in a city, in the rain, watches commonly over-report distance slightly. This is also why GPS distances on a track are famously unreliable.

The practical implication is that the number to trust on a wet run is the lap pace or the overall pace, not the current pace. Most watches let you configure which fields are displayed, and swapping instant pace for lap pace on your rain screen is a five-minute setup change that removes the entire frustration.

What Nobody Tells You About Watches in Rain

The sleeve is the biggest fixable factor. A soaked jacket sleeve over the watch degrades reception more than the rain in the air does. Wear the watch outside the sleeve, even if it means a colder wrist.

Touchscreens stop working in rain, and that is not a fault. Water on a capacitive screen registers as touches. Most watches have a water lock or button-only mode for exactly this, and knowing where it is before you need it saves a lot of mid-run frustration.

Heart rate from the wrist also degrades when wet. Water between the sensor and skin disrupts the optical reading, and a wet, cold wrist has reduced peripheral blood flow, which makes the reading worse. Wrist heart rate is often less reliable than GPS pace on a cold wet run, and a chest strap resolves it completely.

Auto-pause behaves badly in rain. Pace fluctuation can trigger auto-pause when you have not stopped, or fail to trigger when you have. Turning it off for wet runs makes the data cleaner.

Starting your run before the watch has a solid fix compounds everything. In poor conditions, acquisition takes longer. Waiting an extra 30 seconds under open sky before starting produces a noticeably better track.

Some watches let you enable additional satellite systems — multi-band or all-systems modes — which improves accuracy in poor conditions at the cost of battery. For a wet, urban run, that trade is usually worth it.

How should you actually run when the data is unreliable?

By effort, with the watch as a record rather than a guide.

Set up a rain screen. Lap pace, average pace, distance, and time. No instant pace. This alone converts the run from frustrating to normal.

Run by effort and breathing. On a wet run, effort at a given pace is genuinely different anyway — wet shoes are heavier, footing is less certain, and wind and cooling change the cost. Chasing a pace number in the rain means chasing a number that no longer describes the same effort.

Use manual laps at known points. If you run a familiar route, pressing lap at consistent landmarks gives you real, comparable pace data across sessions regardless of GPS quality.

Treat wet-run data as its own category. Comparing a rainy 5-mile run to a dry one is comparing different conditions, not different fitness. Noting the conditions in the run title takes three seconds and makes your training log actually interpretable months later.

Expect the first autumn rain runs to feel worse than mid-winter ones. You are not yet adapted, your gear choices are still summer-calibrated, and the contrast with dry summer running is at its sharpest. This is a well-known pattern and it resolves within a few weeks of consistent wet-weather running.

One safety note that outranks all of the data considerations. Visibility drops faster in rain than most runners expect, both yours and drivers’. Reflective gear and a light matter more on a wet afternoon than on a clear evening at the same hour, and as the light fades earlier through September this compounds week by week.

This is general training information, not medical advice.

FAQ: Wet Weather GPS Questions, Answered

Will a more expensive watch fix this?

Partly. Multi-band GPS receivers handle poor conditions and urban multipath substantially better. They do not eliminate the sleeve problem or the fundamental fragility of instant pace.

Is my distance actually wrong on rainy runs?

Usually within a percent or two, and slightly over-reported if the route has many turns. Distance holds up far better than instant pace.

Should I use a foot pod instead?

A foot pod or accelerometer-based pace is genuinely more stable in poor GPS conditions, and many watches will blend the two. If you run a lot in rain or in dense urban areas, it is the most effective single upgrade.

TL;DR:

• Heavy rain degrades GPS through atmospheric water, a wet watch and sleeve, dense cloud, and increased reflection off wet surfaces.
• Instant pace is calculated over a tiny distance, so small position errors become huge pace errors; average and lap pace stay accurate.
• Wear the watch outside your sleeve, set up a rain screen without instant pace, and turn off auto-pause.
• Wrist heart rate degrades in the wet too — a chest strap is the reliable option for rainy sessions.

Spend five minutes setting up a rain data screen before the next wet run. It removes the single most annoying part of running in bad weather.