help! humidity?

[Susan Evans Garlinghouse <suendavid@worldnet.att.net>]

Help!  I have to give a talk tomorrow morning on energetics and heat
dissipation in exercising horses (which means 90% of the talk is on
endurance) and I can't remember what the formula is for adding up air
temperature and relative humidity to decide when you more or less cancel
a ride.  Isn't there some rule of thumb about that---150, 160...?

Any help would be greatly appreciated, if anyone is still up (nothing
like waiting til the last minute to get my notes together, huh?) ;-D

Thanks,

Susan Garlinghouse