Rainfall Intensity Categories

Classification Standards

Meteorologists classify rainfall intensity based on accumulation rates measured in millimeters per hour (mm/h). These categories help communicate precipitation severity and associated impacts.

International Standards

Classifications follow World Meteorological Organization (WMO) guidelines, adapted by national weather services including the Bureau of Meteorology (Australia), MeteoSwiss (Switzerland), and USGS (United States).

Trace

< 0.25 mm/h

Barely measurable precipitation

Real-world example: Light mist that barely wets surfaces

Very Light

0.25 - 1.0 mm/h

Light drizzle

Real-world example: Fine droplets, gentle pattering on leaves

Light

1.0 - 2.5 mm/h

Light rain

Real-world example: Comfortable to walk in without umbrella for short periods

Moderate

2.5 - 10 mm/h

Moderate rain

Real-world example: Umbrella recommended, puddles form slowly

Heavy

10 - 50 mm/h

Heavy rain

Real-world example: Rapid puddle formation, reduced visibility while driving

Violent

50+ mm/h

Extreme precipitation

Real-world example: Flash flooding possible, dangerous driving conditions

Experience These Intensities

Use our interactive simulator to visualize and hear these different rainfall intensities:

Unit Conversions

Remember: 1 mm/h = 1 L/m²/h. Converting to other time periods:

10 mm in 1 hour = 10 mm/h
10 mm in 30 minutes = 20 mm/h
10 mm in 15 minutes = 40 mm/h

Regional Rainfall Thresholds

National weather services translate WMO guidance into locally relevant cutoffs for advisories. When you compare bulletins across borders or convert between mm/hr and inches/hr using the calculator, the table below helps you align terminology with the simulator visuals on the rain simulator page.

Agency	Light	Moderate	Heavy	Very Heavy/Violent
US NWS	≤ 2.5 mm/hr	2.6 – 7.6 mm/hr	7.7 – 50 mm/hr	> 50 mm/hr
UK Met Office	≤ 1 mm/hr	1.1 – 4 mm/hr	4.1 – 32 mm/hr	> 32 mm/hr
WMO Reference	≤ 2.5 mm/hr	2.6 – 10 mm/hr	10.1 – 50 mm/hr	> 50 mm/hr

Differences reflect regional hydrology and risk tolerance. Coastal UK catchments saturate quickly, so the Met Office triggers “heavy rain” messaging sooner than the US NWS. Use the shared thresholds to brief teams working across multiple jurisdictions and to align sensor alerts with the simulator’s visual intensity cues.

Safety Guidance for Key Activities

Intensity categories are only meaningful when they trigger timely actions. The guidance below is structured by activity so transport operators, event managers, and construction supervisors can tie response levels to observed mm/hr or inches/hr values. Combine these cues with the resources library for signage and checklists.

Intensity Band	Driving	Outdoor Events	Construction
Trace to Very Light	Maintain normal speeds; monitor for slickness on cold pavement.	Proceed; add non-slip mats near stages and cable ramps.	Continue work; cover sensitive finishes and keep power tools dry.
Light (1 – 2.5 mm/hr)	Increase following distance; use headlights even in daylight.	Have ushers communicate shelter locations and check drainage paths.	Inspect scaffolding boards for slip risk; plan for short pauses during gusts.
Moderate (2.5 – 10 mm/hr)	Slow 10–20% under posted limits; avoid high-speed lane changes.	Shorten program segments; move audio gear under cover.	Suspend elevated work during lightning proximity; reinforce trench shoring.
Heavy (10 – 50 mm/hr)	Restrict heavy vehicles on unpaved routes; expect ponding and splash-back.	Delay or evacuate outdoor crowds; activate preset shelter messaging.	Stop non-essential tasks; deploy pumps and protect electrical runs.
Violent (> 50 mm/hr)	Travel only for emergencies; monitor stream gauges for rapid rises.	Cancel events; shift to indoor contingencies or remote participation.	Cease site operations; evacuate flood-prone areas and secure heavy equipment.

Each row ties directly to the intensity cards above. Use the simulator to rehearse what visibility and spray look like at the thresholds so frontline teams build intuition before an advisory is issued.

FAQ: Applying Intensity Categories

These quick answers reinforce consistent messaging when you reference mm/hr values in briefings or public dashboards. Link to the calculator and resources pages whenever your audience needs supporting visuals or downloadable charts.

Question	Answer
What metric defines intensity?	Accumulation rate (mm/hr), not total rainfall, defines category boundaries.
How do I convert between mm/hr and inches/hr?	Multiply inches/hr by 25.4 to reach mm/hr or use the conversion calculator.
Why do agencies differ on “heavy” rain?	Local drainage capacity and soil saturation levels shift the thresholds—see the regional table above for comparisons.
Can intensity categories predict flooding?	They signal stress on drainage, but combine them with soil moisture and river gauges for accurate flood risk.
How should drivers interpret “moderate” rain?	Expect visibility reduction and hydroplaning risk; the driving guidance table suggests slowing 10–20% under posted speeds.
When should events be paused?	At heavy rain thresholds, pause or move attendees to shelter to protect electronics and footing.
How do construction teams use these categories?	Tie crane, trench, and electrical shutoffs to the activity guidance table; rehearse with the simulator.
Are hail or snow included?	Intensity categories apply to liquid-equivalent rates; frozen precipitation may be measured separately but converted to mm/hr for planning.
Where can I find more reference material?	Browse the resources page for charts, or the units guide for conversion detail.

Authoritative Sources

Primary References:

International Standards:

• World Meteorological Organization (WMO)
• International Association of Meteorology
• National Weather Service Guidelines