If you feel weak in hot weather, consider whether you have heatstroke, which can cause fever, dizziness, weakness, nausea, vomiting and many other symptoms. It is important to get out of the hot environment and into a ventilated and airy place at this time. The heat can also cause heavy sweating, leading to dehydration and electrolyte disturbances, so it is important to rehydrate and take electrolyte-containing drinks to prevent heat stroke.