The king of all winter swimmers.
- Thermoregulation
Humans, like most mammals, can maintain a constant core body temperature. This is known as being warm blooded or homiothermic. The core temperature of the human body remains constant at around 37c. Heat is produced in the body as a by-product of metabolic reactions and as a result the amount of heat produced depends on the metabolic rate. However, the way we control our body temperature is by increasing or decreasing heat loss to the environment, according to our needs. In humans, the basic mechanism that underlies temperature control or thermoregulation, involves part of the brain called the hypothalamus. This acts as a thermostat. It can detect the temperature of the blood that passes through it and if the temperature increases or decreases even slightly the hypothalamus initiates corrective responses such as sweating or shivering.
When the Titanic famously sank the air and water temp were both around 0 but those in lifeboats had a far greater chance of survival. The high conductivity of the water drew the heat out of those in the sea, most of whom died of hypothermia.
Heat loss must be equal to heat gain in order to maintain a stable body temperature.
Heat can be gained or lost in four ways :
- Conduction
- Convection
- Evaporation
- Radiation
Conduction
This involves the transfer of heat between two objects that are in contact with each other. Heat is always conducted from an area of higher temperature to an area of lower temperature. For example when you sit on a cold seat heat is conducted from your body into the seat until both are the same temperature. Different materials have different thermal conductivities. Air has a low thermal conductivity, water has a much higher one. A clothed human walking in air at 15c could maintain body temperature comfortably. If immersed in water at 15c it would not be long before their core temperature dropped and hypothermia began to set in. The heat loss into the water would be much greater – water can draw out heat around 30 times faster than air.
Convection
This is heat transfer due to currents of air or water. In a river or sea heat is lost quickly as the water moves over the body. The same applies with air. A cold windy day feels colder than a cold calm day even though the air temperature may be the same. This is known a the wind chill factor. Our clothes help heat loss by convection as a layer of air is trapped next to the skin. In water, divers use wetsuits and drysuits to reduce heat lost through convection. (but in winter swimming we don’t generally use wetsuits)
Evaporation
Evaporation is the change in state from liquid to gas. The evaporation of water uses up a lot of energy, meaning this has a great cooling effect. You feel the power of this coolin when you step out of a hot bath and stand in a draught. It is also why sweating is an effective way of losing heat. Even hot air can have a cooling effect provided that the skin is wet so that evaporation can take place.
Radiation
This is the loss of infra-red heat into the surroundings. A human sitting in a room at 20c radiates heat into the air, mainly from exposed areas of skin. Radiation is different from the other three as it does not occur as a result of the movement of molecules.
