Whatever the weather conditions, your fluid requirements will increase significantly when cycling. Losses from exhaled breath and from sweating will serve to reduce your blood volume, resulting in your heart having to work much harder. By the time you feel thirsty you will already be dehydrated, so try to drink small, frequent quantities of water or a sports energy drink throughout the ride.
Running low on energy or ‘bonking’ as cyclists call it, will reduce your cycling enjoyment. Cycling will increase your energy requirements so aim to eat small, frequent meals and snacks on the go in order to maintain energy levels. On the bike carbohydrate drinks will keep you well fuelled or you could try taking a couple of energy bars which are easy to eat in the saddle.