Fainting

My biology book says if you stand still on a hot day for a long time you're more likely to faint.

The standing still part I gather, is due to the leg muscles not squeezing the veins to force the blood upwards towards the heart. But I wonder why the hot day part.

You feint due to low blood pressure, on hot days you sweat more and so are more likely to be dehydrated and dehydration lowers blood pressure.