We need to be aware that thermal stratification can occur in summer. This happens in extreme heat and calm conditions when the water has little chance to be sufficiently mixed. Thermal stratification refers to a difference in water temperatures at various depths. As colder water is denser than warm water, a band of warmer water (epilimnion) sits on top of deeper lying colder water (hypolimnion). These are separated by the thermocline (diagram 2). When thermal stratification is present, trout tend to become widespread throughout the epilimnion zone, often pushing away from the margins and into the tarn centre where they feed on daphnia and the like. Obviously, this takes them well out of our casting range when bank fishing. Remember though that most hill tarns are exposed where wind and wave action churn up water to mix both warm surface currents and underlying colder parts together, resulting in acceptable temperatures for trout to function, even in high summer.