Time the Thunder
Tap the moment you see the flash
Thunder timing gives distance only — the storm's compass direction is unknown.
*If the storm is heading straight toward you at a typical 25 mph (40 km/h). Storms vary from nearly stationary to 60+ mph.
⚡ When Thunder Roars, Go Indoors
That's the National Weather Service rule: if you can hear thunder at all, lightning is close enough to strike you — bolts can hit more than 10 miles from the rain. Stay sheltered until 30 minutes after the last thunder.
Storm Trend Tracker
Log the flash-to-bang time of several strikes a few minutes apart. The trend reveals whether the storm is approaching or moving away — and how fast.
Tip: you can also time a strike on the Calculator tab and tap "Add to tracker". Strikes are saved on this device.
| # | Time of day | Flash-to-bang | Distance |
|---|
📈 Reading the Trend
The "closing speed" is how fast the distance between you and the lightning is shrinking or growing — the storm's component of motion toward you. A storm passing to one side will appear to approach, stall, then recede.
Distances assume the air temperature set on the Calculator tab.
Lightning Safety Zones
Based on your flash-to-bang distance. One honest caveat first: there is no fully safe distance outdoors — strikes have hit over 10 miles from the parent storm.
🚨 Extreme Danger
0 – 1 mile
Under 5 seconds · 0–1.6 km
- The storm is essentially overhead
- Be inside a substantial building or hard-topped vehicle NOW
- Stay off corded electronics and plumbing
- Keep away from windows and doors
⚠️ High Risk
1 – 3 miles
5–15 seconds · 1.6–4.8 km
- Lightning routinely strikes this far from the cell
- Move to real shelter immediately — don't wait it out
- Get out of and away from water
- Avoid open fields, hilltops, and lone trees
⚡ Take Action
3 – 6 miles
15–30 seconds · 4.8–9.7 km
- You are within documented strike range
- Head to shelter now, not "in a few minutes"
- Wrap up outdoor activities
- Use the Tracker tab to watch the trend
👁️ Still in Range
6 – 10+ miles
30–50+ seconds · 9.7–16+ km
- "Bolts from the blue" strike from this distance
- If you can hear thunder, you can be hit
- Plan your route to shelter and start moving
- Thunder is rarely audible beyond ~10 miles
🏠 Safe vs. Not Safe
SAFE:
- ✅ Substantial buildings with wiring and plumbing (they conduct the strike to ground)
- ✅ Hard-topped metal vehicles with the windows closed
NOT SAFE:
- ❌ Open structures — gazebos, pavilions, dugouts, porches
- ❌ Small sheds, tents, and lean-tos
- ❌ Convertibles, golf carts, bikes, motorcycles
- ❌ Under trees — a leading cause of lightning casualties
🧍 If You're Caught Outside
The honest answer from the National Weather Service: nothing you do outside makes you safe — including the old "lightning crouch," which the NWS no longer recommends because it provides little real protection.
- Keep moving briskly toward the nearest substantial building or vehicle
- Get off ridges, peaks, and away from isolated tall objects
- Spread out if you're in a group (reduces multiple casualties)
- Never shelter under a tree or lie flat on the ground
⏱️ The 30-Minute Rule
Wait 30 minutes after the last thunder before resuming outdoor activities. Many lightning casualties happen before the rain arrives or after people think the storm has passed. (You may also see the older "30-30 rule" — the 30-second half has been replaced by "any thunder = go indoors," but the 30-minute wait still stands.)
Know Your Thunderstorm
Four basic flavors of thunderstorm, from a passing rumble to a tornado factory. Knowing which one you're under tells you how long it will last and how seriously to take it.
Single-Cell ("Popcorn")
One updraft that builds, rains itself out, and dies. The classic brief summer-afternoon storm.
Multicell Cluster
A family of cells at different life stages; new cells keep forming on the flank, so the storm outlives any single cell.
Squall Line
A solid line of storms, often hundreds of miles long, racing ahead of a cold front with a wall of wind on its leading edge.
Supercell
A single rotating updraft (mesocyclone) — the rarest type but responsible for most violent tornadoes and giant hail.
Typical lightning flash rates by storm type
Approximate ranges of total flash rate (in-cloud + cloud-to-ground) from storm research. Individual storms vary widely — severe supercells have exceeded 200 flashes per minute.
🌩️ A Useful Field Sign
A sudden, sharp jump in lightning frequency — strikes coming noticeably faster — often precedes severe weather at the ground by 10–30 minutes. Researchers call it a "lightning jump," and it's one more reason to head indoors when the thunder starts stacking up.
The Science of Thunder & Lightning
🌩️ Why Thunder Lags Behind Lightning
Light covers the distance to your eyes in a few millionths of a second. Sound plods along at roughly 1,126 feet per second (343 m/s) in 68°F air — about a mile every 4.7 seconds. The lag between flash and bang is therefore a direct measurement of distance:
Distance (miles) ≈ seconds ÷ 5 · Distance (km) ≈ seconds ÷ 3This tool sharpens the rule of thumb by using the actual speed of sound for your air temperature.
🌡️ Temperature's Effect on Sound
Sound moves faster in warm air because the molecules carrying the wave move faster. The physics:
v = 331.3 × √(1 + T/273.15) m/s (T in °C)≈ 1087 + 1.1 × (°F − 32) ft/s
A common published shortcut, "1087 + 1.1 × °F," forgets to subtract 32 and overstates the speed by about 3% — one of the bugs this version fixes. Humidity adds a further small boost (about +0.4% in warm, saturated air) because water vapor is lighter than the air it displaces.
Speed of sound vs. air temperature
The curve below is the real physics; the marker shows the temperature currently set on the Calculator tab.
⚡ How Lightning Forms
Inside a thunderstorm, ice crystals and soft hail (graupel) collide millions of times, transferring charge: positive to the small crystals carried up, negative to the heavier graupel below. When the electric field grows strong enough, the air breaks down and a channel of plasma — lightning — equalizes the charge.
- Temperature: the channel reaches ~50,000°F — about five times hotter than the Sun's surface
- That heat is the thunder: air around the bolt expands explosively, creating the shock wave you hear
- Speed: the visible return stroke races upward at tens of millions of mph
- Frequency: Earth sees ~100 lightning flashes every second
- Why it rumbles: the bolt is miles long, so sound from different parts of the channel arrives at different times
⚡ Types of Lightning
- Intra-cloud (~75%): flickers within one cloud — the classic "sheet lightning"
- Cloud-to-ground (~25%): the dangerous kind this tool measures
- Positive strikes (~5% of CG): from the storm's high anvil — up to 10× the current, and they can hit many miles from the rain ("bolt from the blue")
- Ball lightning, sprites & jets: rare and exotic — sprites flash high above storms, into the upper atmosphere
🎯 Lightning by the Numbers (US)
- Lifetime odds of being struck: about 1 in 15,000
- ~25 million cloud-to-ground strikes per year
- ~20 deaths and ~240 injuries per year
- About 90% of strike victims survive — often with lasting injuries
- Peak season: June–August, mid-to-late afternoon
🌈 Rare Phenomena Worth Knowing
"Heat lightning" isn't a separate thing — it's an ordinary storm so far away (40+ miles) that the thunder can't reach you. Thunder is rarely audible past ~10 miles.
Volcanic lightning: ash particles colliding in an eruption plume generate their own storms.
Catatumbo lightning (Venezuela): one lake sees storms ~260 nights a year — the lightning capital of Earth.
Fulgurites: when lightning hits sand, it fuses a glass tube along the strike path — a fossil of the bolt.
Thundersnow: lightning during snowstorms; the snow muffles the thunder to a ~2–3 mile radius.