Before explaining the reasons let me ask one simple question. Do we play or continue playing a match when there is a slight drizzle. I know your answer is NO. Umpires will call for covers and players will walk out. We all know that moisture in any form is harmful to a cricket pitch as it will change the behavior of the pitch surface dramatically.
There are many forms of moisture that appear in the atmosphere which will affect the pitch behavior. It can be in the form of fog, mist, a light shower (or a drizzle) and a heavy shower. All these forms are visible to us so we can stop play and protect the pitch. But there is one more atmospheric factor which is very dangerous for the cricket pitch but unfortunately as we cannot physically see it in the air or atmosphere we simply neglect it. That is the DEW factor or Dew Formation. Why it effects only during the second session? There are two factors for that. They are; Dew Point (DP) and Relative Humidity (RH).
When the air is saturated with water vapor we say it is too humid. When the RH increases beyond 80% it is really uncomfortable for us. When the air temperature drops below the Dew Point of the air this water vapor will condense and droplets of water will be formed on the grass or pitch surface (or cooler part of the ground). That is called the Dew. In theory if the RH increases up to 100% Dew will form at any current temperature. Suppose the RH is lesser than 100% still water condensation will take place when the air cools down to its Dew point. So it will be a non playable situation if the RH is close to 100% or more than 90 % after 800 PM. (This time will vary according to the DP and RH values). Now how this will affect the pitch?
When the match starts at say at 300 PM air is warm RH is low so no moisture will be deposited on the pitch surface and pitch is ideal for batting (except the first few overs). When the air cools down in the night, RH will increase and the air temp will approach the DP as well as the Dew formation point.
However if the RH in the air is low, amount of dew formed will be very little and will not affect the pitch behavior that much. At high RH levels like 90% and above more water droplets will be deposited on the pitch and the surface will become softer than how it was during the first session. Seamers who can maintain a good seam position can exploit these favorable conditions and will get lot of seam movement off the pitch as the seam of the fresh cherry will penetrate more in to the softer pitch surface. (surface has become softer due to the absorption of dew in to the soil). Even after 20 to 30 overs since the air is heavy with high levels of humidity and if the bowlers can maintain a difference in surface smoothness on two halves of the ball, they can achieve more swing in the air or more reverse swing.
SLC should give top priority to this problem and solve it with a scientific approach. First of all they must study the technical side before finding solutions. Our fans should also realize why some countries still prefer to play day matches when D/N matches are commercially viable and popular among spectators. If it is really required to play D/N matches in Kettarama we can change timings on both ODI and T20 matches by starting the play little early and finish it the latest at 730 PM ( on humid nights). If the RH is less than 80% during the night we can continue the play until late night.
In summary I can say that D/N matches should not be played in any part of the country (mainly in Colombo) where it is very hot and low RH during the first session and much cooler and has a very high RH level (greater than 90%) in the second session.

Tags: Affects, Cricket, Factor, Match, Much, Night, Results