extreme events

Heat Transfer in Weather Systems

Why do you feel cold when you get out of a pool on a hot day?

How does a fire boil water?

Why is a black road hotter than a white sidewalk?

These questions can all be answered by understanding the concept of heat transfer, which impacts our daily life in terms of how we cook, build buildings, dress, experience weather, and so much more! There are also a lot of myths about heat transfer which you can read more about. See Myths & Misconceptions

What is heat?

Heat, a form of energy known as “thermal energy”, is the energy of atoms and molecules in motion. More specifically, it is the energy that flows from something hotter to something cooler – like the energy that flows from a hot mug to your cooler hand. It is NOT the same as temperature, which measures how hot or how cold something is.

Two types of heat include sensible heat and latent heat.

Sensible heat is the heat released or absorbed when there is no phase change. This release or absorption does change the temperature of a substance. You can FEEL this. 

Latent heat is the heat released or absorbed when a substance goes through a phase change – for example, when liquid water evaporates and turns to water vapor, or when ice melts from solid to liquid. Latent heat does not impact the temperature of something. The temperature of boiling liquid water and the resulting gas (water vapor) are both 100° C, even though energy is put into the water to convert it from liquid to gas. 

What is heat transfer?

If heat is a form of energy, then “heat transfer” is the transfer of energy. It is the way in which energy moves from one system to another. This energy transfer explains why many things—including many types of weather—happen.

There are three forms of energy transfer: Conduction, Convection, and Radiation.

What is conduction?

When two objects are touching, and the objects each have a different temperature, then there is a transfer of energy from the hotter to the cooler one. You experience conduction throughout your day. For example, conduction explains why a tile floor feels cold on your feet (the energy from your warm foot is being transferred to the cooler tile floor) or an item taken from a freezer feels cold in your hands (the energy from your warm hand is flowing toward the item you took from the freezer).

Not everything conducts energy the same. Styrofoam, goose down, and wood all conduct energy more slowly than concrete, steel, and aluminum. In other words, if you put a wooden spoon and a metal spoon in the same freezer for the same amount of time, the metal spoon would feel colder than the wooden spoon when you took them out of the freezer because the metal spoon transfers heat to your hand more quickly than the wooden spoon will. The ability to conduct heat is described as “thermal conductivity”.  The amount of heat transferred through conduction also depends on many factors such as:

• The surface area of the two objects that are in contact (more heat transferred is from your whole foot than would be from just your toes if you stood on the cold tiles on your toes).

• The time that they are in contact (less heat is transferred if you step quickly off the tile than if you stand there for a long time).

• The temperature difference between the objects (more heat is transfer from your finger to cold water than cool water, and no transfer takes place between two objects of the same temperature).

• The thickness of material across which heat is being transferred (more heat is transfer through a thin down jacket than a thick down jacket). 

What is convection?

Convection occurs when fluids (liquids or gases) move. When fluids heat up, the heated parcel expands, and gets less dense. When a gas or liquid becomes less dense, it tends to rise, with more dense fluids taking their place. This form of energy transfer helps explain how a lava lamp works: the heat from the light source at the bottom of the lamp heats the colored wax which becomes less dense than the water and rises in the lava lamp. At the top, the wax cools off, gets more dense, and sinks back down to be heated again. Radiators can convect air throughout a room and the convective circulation of the air is a much more efficient mode of heat transfer than conduction which requires objects to be touching.

What is radiation?

Radiation is the transfer of energy in the form of waves through space or some type of material. Light is a type of wave called an “electromagnetic wave”.

The length and speed of an electromagnetic wave determines how people experience those waves and exist on a spectrum of long, slow, low energy waves to short, fast, high energy waves. Long, slow waves help carry sound (as in the sound for your television or radio). Short, fast waves are used for X-rays. Heat and visible light are other examples of the transfer of energy in the form of waves. Solar radiation is the electromagnetic radiation produced by the sun and can be seen as the colors of the rainbow.

Radiation is absorbed differently by different materials, depending on the color and composition of those materials. For example, a dark piece of metal will heat faster than a lighter piece of the same metal. Metal of some color will heat faster than plastic of the same color. When radiation is not absorbed, it is reflected or refracted.

How does heat transfer impact the development of a thunder storm?

The sun’s waves heat the ground, causing it to warm (radiation). The energy flows from the warmer ground to the cooler air (conduction), causing it to rise (convection). When the rising air reaches a certain point, it expands, cools, and the cooler expanded air can no longer hold as much water vapor so it rains. Energy (latent heat) is released when water vapor condenses to rain drops (opposite of the fact that energy is required to evaporate liquid water to water vapor). This boost in energy into the air causes it to rise further, cool and expand more, and lose more water (more rain). The rapid vertical movement bringing very different air masses close to each other allows lightning as charge from one cloud jumps to another nearby one.