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Weather-safety glossary: lightning, hail and heat terms

The words you meet in lightning policies, heat rules and vendor proposals, defined in plain language. Each term has its own link to share, and points to the page that goes deeper.

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Terms to know

NLDN

The National Lightning Detection Network: ground sensors across the continental U.S. that locate strikes by triangulation. The reference record most U.S. safety programmes use.

The full entry

0–9

1×1 km cell: The 1×1 km grid square Flash forecasts. Every site sits in one cell, and every score on the Accuracy Method page is computed per cell.; 1×1 km cells on large sites
30-30 rule: An older rule of thumb: take shelter if the flash-to-bang count is 30 seconds or less, and wait 30 minutes after the last thunder before going back out. Current guidance keeps the 30-minute wait but says to seek shelter as soon as thunder is heard.; The 30-30 rule, explained

A

Advisory · Watch · Warning: Flash's three alert levels for a cell, in rising order of likelihood inside the one-hour window. Your written policy names the level that clears the field.; Lightning prediction
Alert radius: The distance around a site, often 8 or 10 miles in written policies, inside which a detected strike triggers a lightning delay. A larger radius gives more warning and more delays.; Radius vs cell
All-clear: The signal that activity can resume after a lightning delay, usually once a set time (commonly 30 minutes) has passed with no lightning or thunder inside the policy area. Any new strike restarts the clock.; The all-clear rule

B

Bolt from the blue: A strike that leaves the side or top of a storm and comes down miles away, where the sky overhead may still look clear. It is why guidance says no blue sky is proof of safety while thunder can be heard.; How far can lightning strike?

C

CAPE: Convective available potential energy: the energy, in joules per kilogram, available to a rising parcel of air. Higher CAPE allows stronger updrafts, but it is an ingredient, not a forecast that storms will form.
Cloud-to-ground (CG): A lightning flash that reaches the ground. The only kind Flash forecasts and scores; intra-cloud flashes are excluded.; Lightning prediction
Convective initiation: The moment a new thunderstorm first forms. It is the hardest case for a warning that tracks existing storms, because until it starts there is no storm to track.; Lightning prediction vs detection
Critical success index (CSI): Hits divided by hits, misses and false alarms together. A single score that penalises both missed events and false alarms.; False-alarm ratio and lightning metrics

E

Electric field mill: An on-site sensor that measures the strength of the atmospheric electric field at the point where it is mounted. A rising field can precede a nearby strike; the reading describes that one location.; Prediction vs sensors vs detection
Emergency action plan (EAP): The written plan a school, venue or site follows in an emergency. Its lightning section names who monitors the weather, who makes the call, where everyone shelters and how the all-clear is given.; Writing a lightning safety policy

F

False-alarm ratio: Flagged cell-windows with no CG strike, divided by all flagged cell-windows. Tracked alongside the hit rate so a forecast cannot score well by flagging everything.; False-alarm ratio and lightning metrics
First strike: The first CG strike of a storm cell inside a given 1×1 km cell. Detection can only report it; prediction is scored on whether it flagged the cell first.; Lightning prediction vs detection
Flash-to-bang: The seconds between seeing a flash and hearing its thunder. Sound covers about a mile in five seconds, so dividing the count by five gives a rough distance in miles.; The 30-30 rule, explained

G

Globe temperature: The temperature inside a black globe in the sun, standing in for radiant heat from the sun and hot surfaces. It is one of the three readings combined into WBGT.; WBGT forecast vs on-site reading
Graupel: Soft, small pellets of rimed ice inside a storm. Collisions between graupel and ice crystals are a main way thunderstorm clouds become charged.

H

Hail swath: The path on the ground where a storm dropped hail. Flash resolves a swath to 1×1 km cells, each with a predicted size class.; Hail prediction
Heat acclimatization: The period at the start of a season when practice length, intensity and equipment are increased step by step, so athletes adapt to the heat. State associations set the schedule.; State heat policies
Heat index: How hot it feels when relative humidity is combined with air temperature, devised for shade and light wind. Full sun can raise it by up to 15°F; WBGT also accounts for sun and wind.; Heat & WBGT

I

Intra-cloud (IC) lightning: A flash that stays inside the cloud, or passes between clouds, without reaching the ground. It is far more common than cloud-to-ground lightning and often comes before a storm’s first ground strike.; Prediction vs sensors vs detection

L

Lead time: How long before an event the warning arrives. For lightning, the minutes between the alert for a location and the first strike there; detection-based alerts have no lead time on a storm’s first strike.; Accuracy method
Lightning detection network: A network of ground sensors that record the radio signal of each discharge and locate it by comparing arrival times at several stations. It reports lightning that has already happened.; Lightning prediction vs detection
Lightning detection vs prediction: Detection reports where lightning has already struck; prediction forecasts where it is likely to strike next. A detection alert fires after a strike inside your radius, while a prediction can flag a location before its first strike.; Lightning prediction vs detection

M

MESH: Maximum Expected (or Estimated) Size of Hail: a radar-derived estimate of the largest hail a storm is producing, published on MRMS grids. It is an estimate, best checked against ground reports.; Hail size classes
MRMS: Multi-Radar Multi-Sensor: the NOAA system that blends data from many radars with other observations into national grids of precipitation, hail and storm products.; Hail prediction

N

Natural wet-bulb temperature: The temperature of a wetted thermometer exposed to the open air, reflecting how well sweat can evaporate. It is the largest part of the outdoor WBGT formula.; WBGT forecast vs on-site reading
NLDN: The National Lightning Detection Network: ground sensors across the continental U.S. that locate strikes by triangulation. The reference record most U.S. safety programmes use.; Accuracy method
Nowcast: A very short-range forecast, typically the next hour or two, built from the latest radar, satellite and surface observations. Lightning and hail alerts for a site are nowcasts.; Lightning prediction

P

Positive lightning: A cloud-to-ground strike that lowers positive charge, often from the upper part or anvil of a storm. It is less common than negative lightning and can strike well away from the rain.; How far can lightning strike?
Probability of detection (POD): The share of events that were warned for: hits divided by hits plus misses. Read it beside the false-alarm ratio, because warning for everything gives a perfect POD.; False-alarm ratio and lightning metrics

R

Radar reflectivity (dBZ): The strength of the radar signal returned by rain, hail and other targets, measured in dBZ. Higher values mean heavier precipitation, and very high values often mean hail.; Hail prediction
Refresh rate: How often a forecast is re-run with new data. Flash refreshes its lightning forecast every 2 minutes and its hail forecast every 5 minutes.; Lightning prediction
Return-to-play: The wait after the last detected strike before activity resumes. Its length is set by your policy; Flash shows detected strikes on the same map so the clock starts from the right one.; The all-clear rule

S

Safe shelter: A substantial, fully enclosed building with plumbing or electrical wiring, or a hard-topped metal vehicle with the windows closed. Dugouts, tents, pavilions, rain shelters and golf shelters do not count.; Lightning safety policies
Severe hail: Hail 1 inch (quarter size) in diameter or larger: the size at which the National Weather Service counts a storm as severe.; Hail size classes
Severe thunderstorm warning: An NWS warning for a storm detected or reported with wind gusts of 58 mph or more, hail 1 inch or larger, or both. Lightning alone does not make a storm severe.; Lightning safety policies

T

Total lightning: Intra-cloud and cloud-to-ground lightning counted together. Some detection networks report both; safety rules are usually written around strikes that reach the ground.; Prediction vs sensors vs detection

U

Updraft: The column of rising air that builds a thunderstorm. Stronger updrafts can hold hailstones aloft longer, letting them grow larger.; Hail prediction

W

Watch vs warning (NWS): A watch means conditions favour the hazard, but the exact time and place are not yet known. A warning means the hazard is occurring or imminent in the warned area.; Lightning safety policies
WBGT: Wet-bulb globe temperature: a heat-stress index combining temperature, humidity, wind and solar load. Some state policies require an on-site reading.; Heat & WBGT

Sources

NWS-defined terms (heat index, severe hail, warnings and watches) are checked against the National Weather Service pages below. Flash figures are those published on the accuracy method page.

Weather-safety glossary: lightning, hail and heat terms

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A

B

C

E

F

G

H

I

L

M

N

P

R

S

T

U

W

Sources

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