Defining rainfall: light, moderate, heavy

[Rev Green]

Can anyone provide a reference for defining rainfall as light, moderate or heavy by visibility please? Ie is moderate rain say 4000m and heavy 2000m?

I have found definitions by rainfall rates per hour but this is obviously harder to determine during periods of high workload.

The reason for the request is due to the TALPA ARC requirements for landing.

Many thanks.

[Jonnyknoxville]

On approach , if i can see the RW lights but its a blurry picture , then its moderate
If i can't see the lights properly , and i go around , then its heavy .
If its raining and my vis is still perfect , then its light .

[HamishMcBush]

From Wikipedia:

Rainfall intensity is classified according to the rate of precipitation:

• Light rain — when the precipitation rate is < 2.5 mm (0.098 in) per hour
• Moderate rain — when the precipitation rate is between 2.5 mm (0.098 in) - 7.6 mm (0.30 in) or 10 mm (0.39 in) per hour
• Heavy rain — when the precipitation rate is > 7.6 mm (0.30 in) per hour, or between 10 mm (0.39 in) and 50 mm (2.0 in) per hour
• Violent rain — when the precipitation rate is > 50 mm (2.0 in) per hour

Nothing about visibility - could depend on how "clean" the rainwater is; possibly you can still see for over 1km in violent rain consisting of clear droplets, but dirty light rain could alter the visibility more dramatically.

However you could try here:
https://www.google.co.uk/url?sa=t&rc...25596728,d.ZGg

[safetypee]

RG, if there is a relationship between visibility and rainfall rate it probably involves droplet size, e.g. when does light rain or drizzle transition to cloud/fog. This data is unlikely to be available for normal operations.

I cannot find any relationship between rainfall rate /visibility and the TALPA recommendations for landing.
However, IIRC there have been discussions re rainfall rate and runway drainage involving wet vs contaminated runway braking conditions. Again IIRC a minimum 15 min delay is recommended before landing after heavy rainfall - tower report or red WXR; or a longer time on runways known to have poor drainage, no grooves, or crosswind and other 'damming' effects.

... Then again how does ATC assess rainfall rate?

[Wageslave]

Surely rainfall "rate" has nothing to do with landing decisions, it is the runway state that matters as that is the only thing that can be quantified.- at least, I've never seen figures or recommendations relating to rate.

Drizzle is defined as having droplets small enough that they leave no impression on a still water surface. Light rain disturbs the surface.

[MarkerInbound]

From the US Federal Meteorological Handbook-

Light

From scattered drops that, regardless of duration, do not completely wet an exposed surface up to a condition where individual drops are easily seen.

Moderate

Individual drops are not clearly identifiable; spray is observable just above pavements and other hard surfaces.

Heavy

Rain seemingly falls in sheets; individual drops are not identifiable; heavy spray to height of several inches is observed over hard surfaces.

Rather hard to apply while you are flying.

[Rev Green]

Thanks for the responses, I had seen the rainfall rate per hour definition and the other general definition of rainfall but they're not pilot friendly IMHO. Surely a visibility definition tied to rainfall rates would be much clearer. It appears it does not exist though. Cheers.

[Derfred]

Surely rainfall "rate" has nothing to do with landing decisions, it is the runway state that matters as that is the only thing that can be quantified.

Yes, but rainfall "rate" may give an indication of the likely runway condition.

Standing water is obviously more likely to be a runway condition in heavy rain.

My Company has it's own requirements involving rainfall "rate" for that reason, despite TALPA ARC not having any requirements other than runway condition. I'm guessing the OP may work for the same airline, hence the question.

[HamishMcBush]

Can anyone provide a reference for defining rainfall as light, moderate or heavy by visibility please?

In summary..... relative humidity will affect visibility in rainfall..... just think of "wet" fog and light drizzle being very similar in terms of "wetness" but vastly different in terms of visibility. There is no easy formula to calculate visibility based solely on the rainfall "density"

[roulishollandais]

Is it any risk of engine flame out depending on the flow rate of water ? I remember some approach to Bombay.