Heat can flow by
. When we're talking about people, heat can also be transferred by
. This mechanism uses the latent heat of vaporization, incidentally. People sweat to cool their bodies. Sweat glands create moisture on the surface of the skin, so the moisture will evaporate into the air around it. As the water is evaporated, there is a loss of heat from the body. This makes us feel cooler. The more humid it is outside, the less moisture will be evaporated from your skin. With less evaporation, there is less cooling and you feel warmer. This helps explain why dry heat is more comfortable than hot, humid weather.
Let's take a closer look at the three conventional types of heat transfer:
is the transfer of heat between two bodies by direct contact. Putting a metal rod in the flame of a fireplace will heat up the rod by conduction. Putting your cold hands on someone's cheeks will cool their cheeks by conduction. Heat flows from the cheeks into your hands.
On a hot summer day, the sand on the beach can feel like it's burning your feet. It is conduction that transfers heat from the sand to your feet.
is the transfer of heat through invisible light waves from one body to another. Radiant heat can move through air or through a vacuum. Heat travels in a straight line from the source to a solid body that receives the heat. The sun and the earth are good examples. The sun radiates its heat through about 93 million miles to warm the earth. Infrared lamps in restaurants radiate heat onto the food to keep it warm. Overhead radiant heaters in arenas, factories and warehouses at the ceiling level radiate heat to the people and objects below. These heaters don't warm the air directly.
Let's go back to the beach. How did the sand get hot in the first place? The sun heated the sand by radiation. The sand heats your feet by conduction.
is the movement of heat through liquids or gases that will flow, such as air or water. Warmer air or water is lighter than cool air or cool water. If a bundle of air is heated by conduction, for example, it will rise. As it rises, cool, dense bundles of air will move in to take its place. As the warmed air that has risen cools because it is no longer close to a heat source, it will fall. This sets up what are known as convective loops. It is this concept of convection that has led to the phrase heat rises.
Let's go back to the hot summer day at the beach. The sun heats up the sand (radiation) much more quickly than it does the water (convection), because of differences in specific heat. As the sand gets hot, it warms the air above it. The air above the beach rises (convection) and the cooler air over the water moves in to take its place. This results in an onshore breeze during a hot summer day. This breeze is caused by convection. An
breeze often feels cool because the air over the water is not heated as much as the air over the sand.
Air moving across your skin cools you. On a calm day, as droplets of moisture are evaporated off your skin, they saturate the air immediately adjacent to your skin. This slows the rate of evaporation because the air can't hold any more moisture. If that air is quickly replaced by relatively dry air, evaporation of moisture from your skin is faster and you feel cooler. When it's hot, you feel cooler if there is a breeze or a fan, because of the increased rate of evaporation of perspiration.
On a cold winter day, a similar process takes place. It doesn't necessarily involve evaporation (a latent heat), but it does involve the carrying away of heat from the body (sensible heat). Heat moves from warm bodies to cold bodies faster if there is a big temperature difference between the two bodies. Your skin temperature may be considerably warmer than the outdoor air. Heat will flow from your skin to the air. The colder it is, the faster the heat will be lost. On a calm day, your skin transfers heat into the air next to your skin. As this air is warmed, the rate of heat transfer slows. When it's windy, the air that is warmed by your skin is quickly removed and replaced by cold air. This accelerates the rate of heat loss from your body. That's why you feel colder on a windy winter day.
We've talked about heat moving from our skin into the air around us. Someone sleeping might release heat at a rate of 300 BTUs per hour. Someone sitting might release 400 BTUs per hour. Someone walking three miles per hour might release about 1,000 BTUs per hour. Walking upstairs quickly can release about 4,000 BTUs per hour. By comparison, an average house might lose 60,000 BTUs per hour when the outdoor temperature is 30F.