Dear Dr. Towe

Thankyou very much for your valuable assistance and kind reply

Regards

A dry bulb (DB) thermometer will measure the ambient air temperature.  A wet bulb thermometer will measure the temperature of the air which gets to be saturated with water vapor.  There are two relations:

(1)  Because the air loses heat to vaporize the water in the cloth of the wet bulb, the wet bulb is cooler than the dry bulb.

(2)  The difference between DB and WB, (I mean, DB - WB) is approximately proportional to the (100% - RH).  Here, RH stands for "relative humidity"  (in %).  So, the  (DB-WB) in a shinny day is big, because the RH is low.  On the other end, the rainy day  the (DB-WB) is small.

These two relations are always true (100%) unless the measurement is not done properly.

Dear Sir, Thanks for the information.

But the thing is that how one should calculate the mean coincident wet bulb temperature for the outdoor weather design conditions with occurrence frequency of 99% in winter season?

With higher RH say above 98% the WB & DB temperatures are almost equal and this occurs in rainy days with maximum and minimum temperature difference of the day less than 40 C. 

Dear Muduli Sir

Thanks for your reply

This means that the DBT during the period or record (POR) will be 99% of the time more than the recorded value. Or, only 1% of the period of record will be less than this value. This is in Winter.

The MCWB is the mean coincident wet-bulb temperature, you should consider in your design.

For example, if we talk about the winter in Sharjah City in UAE

The DP is more than 10.2 C (99% of the time)

Thanks @ MKM for your reply on this.

I think you can first find 99% DBs, and for the times that the 99% DB occurs, you can average corresponding WBs.