Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

Defining rainfall: light, moderate, heavy

Tech Log The very best in practical technical discussion on the web

Defining rainfall: light, moderate, heavy

Old 27th Jun 2016, 02:52
  #1 (permalink)  
Thread Starter
 
Join Date: Jan 2006
Location: Sydney
Posts: 15
Defining rainfall: light, moderate, heavy

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.
Rev Green is offline  
Old 27th Jun 2016, 17:47
  #2 (permalink)  
 
Join Date: Apr 2016
Location: Europe
Posts: 51
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 .
Jonnyknoxville is offline  
Old 28th Jun 2016, 08:01
  #3 (permalink)  
 
Join Date: Jul 2009
Location: Surrey, UK
Posts: 130
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

Last edited by HamishMcBush; 28th Jun 2016 at 08:06. Reason: The dreaded typos strike again, and added link to article
HamishMcBush is offline  
Old 28th Jun 2016, 10:41
  #4 (permalink)  
 
Join Date: Dec 2002
Location: UK
Posts: 2,071
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?
safetypee is offline  
Old 28th Jun 2016, 11:38
  #5 (permalink)  
 
Join Date: May 2011
Location: uk
Posts: 580
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.
Wageslave is offline  
Old 29th Jun 2016, 07:37
  #6 (permalink)  
 
Join Date: Nov 2007
Location: Texas
Posts: 1,840
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.
MarkerInbound is offline  
Old 30th Jun 2016, 02:34
  #7 (permalink)  
Thread Starter
 
Join Date: Jan 2006
Location: Sydney
Posts: 15
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.
Rev Green is offline  
Old 4th Jul 2016, 05:30
  #8 (permalink)  
 
Join Date: Jun 2006
Location: Brisbane
Posts: 117
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.
Derfred is offline  
Old 4th Jul 2016, 08:30
  #9 (permalink)  
 
Join Date: Jul 2009
Location: Surrey, UK
Posts: 130
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"
HamishMcBush is offline  
Old 6th Jul 2016, 07:19
  #10 (permalink)  
 
Join Date: Jun 2011
Location: france
Posts: 759
Snoop

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

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information -

Copyright 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.