Heat: Outdoors and Indoors

What is heat?

Heat is when thermal energy moves between objects with different temperatures. Heat moves from things with higher temperatures to things with lower temperatures. Heat and temperature are related, but they are not the same thing.

• Temperature is the amount of thermal energy something has.
• Heat is the transfer of thermal energy between things.

Image - Text Version

Shown is a colour illustration of a pot on a burner over flames. The pot is bright red. Its lid is open and steam is escaping. Across the front are the words, "Temperature is how hot or cold an object is." The pot sits on a black bracket over a burner with five flames. Below, a grey box contains the words, "Heat is transferred to an object. For example how the stove "heats" the pot.

Why is heat important to living things?

Living things need heat to survive. But why is this?

Many life processes involve chemical reactions. Photosynthesis in plants is one example. It works best between 10 and 34 degrees Celsius. Beyond this range the rate of the reaction is much lower.

Each type of living thing has an optimal temperature range. Most cannot survive long in temperatures below 0 °C. At temperatures lower than this, water in cells begins to freeze. Most also cannot survive long in temperatures above 45 °C. At temperatures higher than this, the proteins in cells start to break down.

Try this!

When you cook an egg, you can see the egg white change from transparent to white. This is evidence that the protein has broken down.

Some living things generate their own heat. We call these endotherms. Humans and other mammals are endotherms. Others get much of their heat from the environment. We call these ectotherms. Reptiles and amphibians are ectotherms. Both endotherms and ectotherms are affected by changes in the temperatures of their environments.

Heat Outdoors

On Earth, heat comes from many different sources of thermal energy. These include the Sun, Earth’s core, and chemical reactions.

The Sun

The Sun is the main source of thermal energy on Earth. It releases thermal energy when hydrogen atoms combine to form helium atoms in its core. We call this process nuclear fusion. Some of the energy travels through the solar system to Earth. The energy heats up the atmosphere causing weather. It also heats up oceans causing ocean currents. People convert energy from the Sun into other forms of energy such as electricity.

Image - Text Version

Shown is a colour illustration of loops of yellow light bursting up from the surface of the Sun. The Sun’s curves down the left edge of the image. Its surface is mottled red and orange. Yellow glows from underneath. In a few spots, bright yellow breaks through the surface. Above these, bursts of wispy yellow light can be seen against the blackness of space.

 Did you know?

The Sun provides most of the heat on Earth. Only about 0.03% comes from other sources.

Earth’s Interior

Earth also has its own source of energy. It comes from its core. The core contains radioactive materials. These unstable materials release heat as they become stable. We call energy from inside the Earth geothermal energy. Geothermal energy is what causes earthquakes. It is also what causes volcanoes and mountains to form.

Image - Text version

Shown is a colour illustration of the Earth sliced open to show the hot core.

The Earth is shown in black space. A triangular wedge is cut out, down into the centre. The cut reveals the different layers within the Earth. The layers range from deep red near the surface to bright yellowish white at the core.

Chemical Reactions

Energy is stored in the bonds of molecules. It gets released through chemical reactions. Some chemical reactions produce thermal energy. We call these exothermic chemical reactions. For example, when wood burns it releases heat. Burning is a type of chemical reaction that we call combustion.

Image - Text version

Shown is a colour photograph of flames licking around charred logs.

The only light in the photograph comes from the fire. The flames are warm and glowing against a dark background. They show the shapes of a pile of logs, charred black and grey.

Try this!

This hands-on activity lets you experience an exothermic chemical reaction.

What is the connection between heat and climate change?

Everything on Earth absorbs and reflects energy from the Sun. Light-coloured surfaces, like snow, ice, and clouds, reflect more energy than they absorb. Dark-coloured surfaces, like forests and oceans absorb more energy than they reflect. We call how Earth systems absorb and reflect Earth's energy budget.

Image - Text version

Shown is a colour infographic showing the ways energy is absorbed and reflected on Earth.
The title, “Earth’s Energy Budget” is in large white font across the top. To the left, the Sun is shown as an orange circle. Below, Earth is shown as a beige curve. In the background, the sky goes from dark blue at the top to light blue near Earth. Two fluffy white clouds float above Earth’s surface. A note on the left reads “100 units is equivalent to 342 wm-2.”
A wide yellow arrow points down from the Sun to Earth. This is labelled “Incoming solar energy 100.” This arrow is divided into four points. The largest point goes into Earth’s surface. This is labelled “Absorbed by surface 49.” A smaller point points up toward the top of the illustration. This is labelled “Reflected by atmosphere and clouds 22.” A slightly smaller point stretches horizontally from the main arrow to the clouds beside it. This is orange and labelled “Absorbed by atmosphere 20.” An even smaller point bounces off Earth and back up. This is labelled “Reflected by surface 9.”
From the clouds, a wide pink arrow points down to Earth. This is labelled “Heat radiated to surface from clouds and atmosphere 95.” A narrower pink arrow points up. This is labelled “Heat radiated to space from clouds and atmosphere 57.”
From Earth, a short orange arrow points up towards the clouds. This is labelled “Convection and formation of water vapour.” Two wide, overlapping pink arrows point up. The shorter one is labelled “Heat radiated from the surface 114.” The longer one is darker pink and turns to point into the clouds. This is labelled “Heat absorbed by greenhouse gases and clouds 102.” Finally, a tall, narrow pink arrow points up from Earth to the top of the illustration. This is labelled “Heat radiated to space from surface 12.”

About half of the energy from the Sun is absorbed by the land and oceans. Of the rest of the Sun’s energy, some is reflected by the atmosphere and clouds. Some is reflected by the surface. And some is absorbed by the atmosphere.

The Earth’s surface transfers some energy into the air through convection. It also transforms some liquid water into water vapour. Most energy radiates as heat from the surface back towards space. Most heat is absorbed by greenhouse gases and clouds. The heat is then radiated back down to Earth. We call the cycling of heat up to the atmosphere and back down to Earth the Greenhouse Effect. The Greenhouse Effect is an important part of Global Warming and Climate Change.

Did you know?

The average temperature on Earth in 2023 was 14.98°C. Temperatures on Earth can vary a lot depending on location.

Heat Indoors

Many living things need protection from the environment. We often call this shelter. People use shelters to keep them comfortable. People often need to heat or cool the air inside shelters. There are many different ways of doing this.

In colder climates, shelters often need additional heat to keep people warm enough. This heat can come from a number of different sources. And it can be transferred in different ways.

Radiant Heating

Many buildings use radiant heat. This is heat that is transferred through electromagnetic waves. When you feel the heat coming off of a fire, this is radiant heat. In buildings, you can feel the heating coming off baseboard heaters and radiant ceiling panels. These radiate heat from resistors in their electrical circuits. Their source of energy is electricity. Unlike some types of heating systems, these only produce heat, they do not remove heat.

Hot water and steam radiators radiate heat using water that passes through them. The water is heated in a boiler in one part of a building and sent through pipes to other parts of the building. The source of energy used to heat the water is usually a fossil fuel such as oil or natural gas.

Windows allow energy from the Sun to come into buildings. Some energy is reflected back outdoors. The rest is absorbed by the people and things in the room. This makes these things feel warmer.

Image - Text Version

Shown is a colour infographic of students indoors being warmed by different systems.
A group of students is shown in the centre of a green square labelled “Inside the Room.” A vertical rectangle on the right is labelled “Outside the Room.”
Inside the room, on the top left, is a white rectangle with a grey vent along the top. This is labelled “Radiant Ceiling Panels.” Next to it is an electric plug icon with a lightning bolt on it. Squiggly orange lines point down into the room. A label indicates these represent radiation.


On the top right is a grid of white lines with two squiggly orange lines crossing through it. One line points up to an orange Sun outside the room. The other points from the Sun down into the room. This is labelled “Sunlight through a window.”


On the lower left is a white rectangle with a grey vent along the bottom. This is labelled “Electric baseboard heater.” Next to it is an electric plug icon with a lightning bolt on it. Squiggly orange lines point up from it, into the room.


In the lower right corner is a rectangular structure made of vertical grey structures joined along the top and bottom. This is labelled “Radiators.” One orange squiggly line points from this into the room. Another line points from the room back to it. Two pipes connect the radiator to a white cylinder outside the room. This is labelled “Water boiler.” An orange flame icon next to it is labelled “Oil. Natural gas.” One of the pipes is pink and labelled “Warm” with a red arrow pointing from the boiler to the radiator. The other pipe is blue and labelled cool, with a blue arrow pointing from the radiator to the boiler. A label between the pipes reads, “Water or Steam.”

Did you know?

People radiate heat all of the time! We can “see” this heat using thermal imaging technology.

Image - Text Version

Shown is a colour image showing heat from a person’s hand.
A scale along the right edge shows that blue and green are the coldest temperatures, yellow is medium, and red and white are the warmest. The person’s shirt sleeve appears bright blue and green. Their hand appears in shades of red and white, the warmest parts of the scale. The hand is surrounded by a glow. Near the skin this is yellow. Further away it is bright blue. The rest of the air around the hand is dark blue, the very coldest part of the scale.

Convection Heating

Other systems use convection heating. Convection is the movement of a gas or liquid due to differences in temperature. In forced air heating systems, air is warmed by a furnace. This is similar to the water in a boiler. The air is transported via air ducts to rooms. Cool air returns through a different set of air ducts.

Image - Text Version

Shown is a colour infographic of students being warmed and cooled by two different systems.
A group of students is shown in the lower left of a green square labelled “Inside the Room.” A vertical rectangle on the right is labelled “Outside the Room.” White space on the far right is labelled “Outdoors.” At the top of the green square is a circle with arrows pointing clockwise. Its colour changes from pink on the left to blue on the right. This is labelled “Convection.”
At the top right of the room are two grey, vented boxes. One is labelled “Warm Air,” the other is labelled “Cool Air.” A pink pipe leads from the warm box and a blue pipe from the cold, These connect to a larger grey rectangle labelled “Forced Air Furnace.” An orange flame icon next to it is labelled “Oil, Natural gas.”
At the bottom left of the room is a vertical, vented box. Above it are arrows labelled “Winter.” One of these is pink and points from outside to inside. The other is blue and points from inside to outside. Below it are arrows labelled “Summer.” One of these is blue and points from outside to inside. The other is pink and points from inside to outside. Two pipes connect the box to another grey rectangle outside the room labelled “Indoor Unit.” A loop of pipes lead from here to a grey cylinder labelled “Outdoor Unit.” Next to this is a pair of arrows labelled “Summer.” One of these is pink and points from the unit to the air.. The other is blue and points from the air to the unit. Below, another pair of arrows is labelled “Winter.” One of these is pink and points from the air to the unit. The other is blue and points from the unit to the air. This system is labelled “Air Source Heat Pump.” Next to it is an electric plug icon with a lightning bolt on it.

Air Source Heat Pumps work in a different way. They have two main parts.

1. Outdoor Unit
   The outdoor unit pulls in outdoor air with a fan. The air blows past a heat exchanger. In it, a liquid refrigerant with a low boiling point becomes a gas. The gas is squeezed by a compressor to increase its temperature. The hot gas passes through a second heat exchanger. It transfers the heat to the indoor unit. The now cooler refrigerant begins the cycle again.
2. Indoor Unit
   Similar to the forced air system, this unit pushes air into rooms. It then returns air outdoors.

Unlike furnaces and boilers, these heat pumps run on electricity.

In addition to using outside air, some heat pumps transfer heat to and from the Earth. We call these geothermal heat pumps. Geothermal heat pumps need less energy than electric heating systems or furnaces.

Schools in Canada use a number of different sources of energy for heating. The main two are natural gas (56%) and electricity (39%).

Image -Text Version

Shown is a colour pie chart illustrating the percentage use of eight different fuels. The information is as follows:
Electricity 39%, On-site Electricity <1%, Natural Gas 56%, Fuel Oil 4%, District Steam <1%, District Chilled Water <1%, Propane 1%, Other <1%.

Air Conditioning

Sometimes people want the inside of a building to be cooler. That is where air conditioners come in. Air conditioners are basically one-way heat pumps. They are only designed to take heat out of a building. They are not designed to put heat into buildings.

How do people control heat indoors?

Heat control happens as part of a building’s HVAC system. HVAC stands for heating, ventilation and air conditioning. HVAC systems let people control the climate indoors.

An indoor climate control system has 3 parts.

1. A source of heat.
2. A system for moving the air around.
3. A way to control the system.

We call the device that controls the system a thermostat. A thermostat is a device that switches a heating system on and off. It works by sensing air temperature and comparing it to a set value. If the temperature is higher than the value, it turns the system off. If it is lower than the value, it turns the system on. Many thermostats can be programmed to turn the temperature up or down at specific times.

There are other, simpler ways of controlling the temperature indoors. These include:

• Opening doors or windows to let air in or out
• Using blinds, shades or curtains to let light in or block light out
• Using fans
• Having good insulation in the walls

Indoor heating and cooling often comes with a cost. This includes a cost to people for electricity or fuel. It also often comes at a cost to the environment. Burning of fossil fuels for energy releases greenhouse gases. These contribute to climate change. This is why people continue to look for ways to heat and cool indoor spaces that cost less to people and the environment.