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Life Sciences => The Environment => Topic started by: dentstudent on 26/05/2009 07:37:46

Title: Why does it often begin to rain very heavily after lightning?
Post by: dentstudent on 26/05/2009 07:37:46

Perhaps this is observer bias, but I have been in a couple of heavy thunderstorms recently, and noticed that often when there is a lightning bolt rather close (say below a count of 6 seconds away, and therefore less than a mile), after perhaps 20 seconds or so, there is a dramatic increase in the rainfall. Is it just a coincidence that this is the heaviest part of the storm and so the rain was coming anyway, or is there a mechanism by which perhaps rain that was in suspension in the cloud could be released as a result of the lightning? Or could the lightning cause more nucleation points for saturated air to condense into raindrops?

What do you think?

Title: Why does it often begin to rain very heavily after lightning?
Post by: Chemistry4me on 26/05/2009 07:41:34

Can't say I've noticed this. Perhaps I should pay more attention next time. We've hardly had any lighning in the past 6 months.

That's my two cents worth.

Title: Why does it often begin to rain very heavily after lightning?
Post by: paul.fr on 26/05/2009 08:22:46

This is an electrostatic precipitation effect, or more commonly called a rain gush, that you are noticing.

The ``Rain gush'', lightning, and the lower positive charge center in thunderstorms (http://adsabs.harvard.edu/abs/1984JGR....8911816J)
Laboratory studies of charge transfer, when vapor-grown ice crystals bounce off a hailstone in simulated thunderstorm conditions, have shown that the sign and magnitude of the charge separated is a sensitive function of the cloud and cloud particle properties. In general, at temperatures colder than -20°C, hail becomes negatively charged, but at warmer temperatures it becomes positively charged. The formation of the lower positive charge center in thunderstorms and the initiation of a lightning flash which precedes the commonly observed gush of rain on the ground may be explained in terms of this charging process.

electrostatic precipitation effect (http://www.springerlink.com/content/2443p3387513r4wr/)
A. K. Kamra1
(1)    Indian Institute of Tropical Meteorology, Ramdurg House, University Road, Pune-411 005 Shivajinagar, India

Received: 15 November 1984 
Summary  Changes in the velocity of charged cloud and precipitation particles due to large electrical forces acting on them, cause them to accumulate in small regions of intense electrification in thunderclouds. These accumulations of particles can modify the microphysical properties of such regions. Here we have calculated the changes in size-distribution of the cloud and precipitation particles, the rain water content and the rainfall intensity in these regions for different values of electric field, drop charges and updraft speed. Modified size-distributions of precipitation particles show a general increase in concentrations of large particles and a maxima for particle sizes whose velocity under the influence of gravitational and electrical forces is equal and opposite to the updraft speed. Rain water content increases and the rainfall generally decreases in such regions. Size-range of the particles contributing the maximum to the rain water content is higher in higher electric fields and for large particle charges. However, size-range of particles contribute the maximum to the rainfall intensity. For any appreciable change in size-distribution of cloud particles, the electric field and the particle charges have to be very large. In view of the above results, it is suggested that levitation of the particles in regions of intense electrification may cause or contribute to the rain gush phenomenon in thunderclouds.

Full text of AMS journal article Gushes of Rain and Hail After Lightning (http://ams.allenpress.com/archive/1520-0469/21/6/pdf/i1520-0469-21-6-646.pdf)

EDIT:
A quick search including the word "hydrometeors", gives this

http://www.newton.dep.anl.gov/askasci/wea00/wea00121.htm

The heavier rain after or just about the time of more frequent lightning is probably not a coincidence.  Research on lightning frequency and rainfall suggests that the action of hydrometeors (rain and hail) being carried around in the thunderstorm (in updrafts as well as downdrafts) creates electrical charge buildup in the clouds.  The more active the storm and the more hydrometeors there are, the more electric charge is built up and the more frequent the lightning is.  The more hydrometeors there are, the greater the likelihood of heavy precipitation, although it may occur after most of the lightning, as a downdraft has to set up or the updrafts decline to allow the hydrometeors to fall towards the ground.

David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory

Title: Why does it often begin to rain very heavily after lightning?
Post by: dentstudent on 26/05/2009 08:34:46

Haha! Thanks Paul. That explains it nicely!

Title: Why does it often begin to rain very heavily after lightning?
Post by: reasonmclucus on 29/05/2009 08:19:25

Could the situation be comparable to what happens with avalanches?  The water droplets have sufficient mass to fall, but are  held in place by other factors including the upward motion of warmer air below the cloud (other than updrafts).  A large electrical discharge disrupts these factors tipping the balance so that the gravitational attraction of the earth is higher than the forces preventing rainfall.