While it's always important to stay hydrated, in hot weather it's even more important to stave off dehydration and take on fluids before you reach a point of thirst.
An expert on the matter, Dr Irvin Sulapas, says: "The rule of thumb is, if you’re thirsty, you’re already dehydrated. So keep well hydrated by drinking plenty of water, even before you begin your outdoor activity."
It's widely reported that while a human may go without food for 40 days, you won't survive more than a week without drinking fluids.
An average adult body is 60-75% water, with it found in every cell, tissue and organ, and almost every body process needs water to function.
Water intake and loss should balance out, and to check if you’re drinking enough water, check your urine. Generally speaking, if it looks yellow or orange you aren’t drinking enough water, while clearer urine means you're OK.
Your body may trigger signs of thirst, signalling the need to drink, but you actually need water long before this point. During hot weather this is especially important for older people, children, and for the unwell.
A medical perspective:
Some thoughts on what to drink in hot weather: