Drinking excessively in warmer climates is particularly dangerous due to the dehydrating effects of alcohol.
Alcohol is a diuretic, which means that it forces your kidneys to excrete water. It can force them to excrete more water than you have taken in, leading to dehydration. The inevitable by-product of sitting in the sun is sweating, which in some cases, can potentially be dangerous as this extra loss of water can, in extreme cases, lead to a heart attack or even organ failure.
It is possible to drink in the sun safely though, as long as you ensure that you drink within the responsible drinking guidelines (2-3 units for women, 3-4 units for men) and alternate alcoholic beverages with plenty of water to keep hydrated.
For more information on the effects of alcohol on the body please see our factsheets.