Lightning Distance Calculator

This simple lightning distance calculator can estimate how far away a storm is based on the time difference between a flash of lightning and the thunder ⚡⚡⚡

Lightning distance1.716 km1.066 miles — light arrives essentially instantly, so the delay is entirely the sound's travel time

Air density1.20411 kg/m³98.29% of the sea-level standard 1.225 kg/m³

Dynamic viscosity18.1341 µPa·sSutherland's law — and note air gets MORE viscous as it warms, the opposite of a liquid

Kinematic viscosity1.506e-5 m²/s15.0602 centistokes — this rises steeply with temperature because density falls as viscosity rises

Speed of sound343.234 m/s√(γR_dT) — depends only on temperature, not on pressure, because density and pressure change together

Specific volume0.83049 m³/kg

Molar volume here24.055 litrescompared with 22.414 at 0 °C and one atmosphere

Standard flow100 SCFMreferenced to 101.325 kPa and 20 °C — SCFM is a MASS flow in disguise, which is why it is the only fair way to compare compressors

Ratio to actual flow1×the air is thinner than standard

Mass flow0.05683 kg/s204.58 kg an hour

The 3-second rule1.67 kmabout 3 seconds per kilometre, or 5 per mile — close enough for judging a storm

Within the 30-30 danger rule?YES — seek sheltera delay under 30 seconds puts the strike within about 10 km, which is inside the range of the next one

The formula

ρ = p ÷ R_dT_v; SCFM = ACFM × (p/p_s)(T_s/T)

Air gets more viscous as it warms

Liquids thin out when heated; gases do the opposite. In a liquid, viscosity comes from intermolecular attraction that warming overcomes. In a gas it comes from molecules carrying momentum between layers, and warming makes them move faster and transfer more. Sutherland's law captures that, and it means air's kinematic viscosity rises steeply with temperature — density falling while viscosity rises.

The speed of sound in air depends only on temperature, not on pressure. Pressure and density change together in the ratio that appears in the formula, so they cancel. This is why the speed of sound falls with altitude in the troposphere (it is getting colder) and then stays constant through the isothermal stratosphere despite the pressure continuing to drop.

SCFM is a mass flow wearing volumetric clothing. Comparing compressors on actual CFM is meaningless because a cubic foot of dense air contains more air than a cubic foot of thin air; correcting to standard conditions makes the comparison fair. Beware that "standard" has several definitions — 20 °C is the compressed-air industry convention, while 0 °C and 15 °C are used elsewhere.