How the Sun Powers Earth's Weather, Water and Lights

A question to start

Why is it warm in Windsor and freezing in Iqaluit on the same day in March? Both cities get sunlight. Both are on the same planet. The difference comes from how the Sun's energy reaches different parts of Earth. In this lesson you will follow that energy as it powers winds, rain, ocean currents and even the northern lights.

How the Sun's energy reaches Earth

The Sun sends out energy as electromagnetic radiation. Most of it is visible light, infrared (which we feel as heat) and some ultraviolet (UV). Radiation can travel through the near-empty space between the Sun and Earth, which is why sunlight reaches us across 150 million km.

When sunlight is absorbed by land, water and air, it turns into thermal energy, and things warm up. Some sunlight is reflected straight back to space by clouds, snow and ice.

Uneven heating: the key idea

Earth is a sphere, so sunlight does not hit every place the same way.

• Near the equator, sunlight strikes the surface nearly straight on (at a high angle). The energy is concentrated on a small area, so it heats strongly.
• Near the poles, sunlight strikes at a low, slanting angle. The same amount of energy spreads over a much bigger area, so each square metre gets less. The light also travels through more atmosphere.
• Different surfaces heat differently. Land heats up and cools down faster than water. Snow and ice reflect most sunlight; dark forest and ocean absorb most of it.

This uneven heating is the engine behind most of Earth's weather.

Phenomenon 1: Winds

Air over a warm surface heats up, expands, becomes less dense and rises. Cooler, denser air moves in to replace it. That moving air is wind.

• Local winds: on a summer afternoon at a lake, the land heats faster than the water. Warm air rises over the land, and cool air from over the lake flows in: a lake breeze. At night the pattern can reverse.
• Global winds: warm air rises near the equator and cooler air from higher latitudes moves toward it. Earth's rotation bends these flows into large, steady wind belts. Canada is mostly in the zone of westerly winds, which is why our weather usually moves from west to east.

Phenomenon 2: The water cycle

The Sun evaporates water from oceans, lakes and plants. The water vapour rises, cools and condenses into clouds, then falls as precipitation (rain or snow). Gravity carries it back downhill in rivers. Without the Sun, there would be no evaporation and no rain or snow.

Phenomenon 3: Ocean currents

Winds push on the ocean surface and drive surface currents. Differences in water temperature and saltiness also make cold, dense water sink near the poles and warm water flow toward them. Currents carry heat around the planet. For example, the warm Gulf Stream carries heat from the tropics into the North Atlantic.

Phenomenon 4: Weather and storms

When warm, moist air meets cold, dry air, the warm air is pushed upward, its water vapour condenses, and clouds and storms form. Hurricanes form over warm tropical ocean water and get their energy from water that the Sun evaporated.

Phenomenon 5: Photosynthesis and the carbon cycle

Plants capture sunlight and store its energy in sugar. When you eat, you use energy that started in the Sun. You will connect this to ecosystems and climate change in the Biology strand of this course.

When the Sun reaches out: the aurora

The solar wind and coronal mass ejections send charged particles toward Earth. Earth's magnetic field creates a protective region called the magnetosphere that deflects most of them. But some particles are guided along magnetic field lines toward the regions around the North and South Poles.

There, at heights of roughly 100 to 300 km, the particles crash into oxygen and nitrogen in the upper atmosphere. The gases absorb energy and then give it off as light:

• oxygen produces the common green glow (and sometimes red, very high up);
• nitrogen can produce blue and purple tints.

The result is the aurora borealis (northern lights) in the north and the aurora australis in the south. Canada is one of the best places on Earth to see the aurora, especially in the North, and during strong solar storms it can be seen in southern Ontario too. Many First Nations, Métis and Inuit communities have their own names, stories and teachings about the northern lights, which differ from nation to nation.

Space weather: when the Sun affects technology

Strong solar flares and CMEs cause geomagnetic storms. These can:

• create electric currents in long power lines and overload transformers;
• disturb GPS signals and radio communication, including for airplanes;
• damage satellites and increase radiation risk for astronauts.

A famous example happened in Canada. On March 13, 1989, a strong geomagnetic storm caused the Hydro-Québec power grid to collapse, leaving millions of people in Quebec without electricity for about nine hours. Today, space-weather forecasters, including at Natural Resources Canada, watch the Sun so that power companies and satellite operators can prepare.