Weather: Temperature

Temperature

Why is there sometimes snow on the top of a mountain but not at the bottom? And why is the Arctic always cold while the Tropics are always hot? Let’s find out! 

Elevation and Altitude

The distance between sea level and where you are is your elevation. The greater your elevation, the less air there is above you. This means the air pressure is lower. The less pressure there is on the air, the more it expands. And the gases that make up the air get cooler as they expand. So, the air temperature drops about 6.5 °C for each kilometer you climb. 

Did you know?
Elevation refers to how high land is above sea level. Altitude refers to how high an object is above sea level.

You can even feel this without climbing at all! Try using a can of compressed air, or let the air out of a tire. The air that comes out is colder than the air around you. 

Did you know?
An airplane usually flies 11 km above the earth. At that altitude, the pressure is about 25% of what it is on the ground. And the temperature is about -50 °C!

Latitude

Different places on Earth can have very different temperatures and weather all year-round. This is because of the Sun. If more sunlight shines on a place, the temperatures there will be hotter. 

The Earth is a sphere. The places where sunlight hits the Earth at a right angle get a lot of sun in a small area. This area is close to the equator. It’s called the equatorial region. The places where sunlight hits the Earth at a shallow angle get the same amount of sunlight, but it’s spread over a much larger area. Areas like this are close to the north pole or the south pole. They’re called the polar regions.

In the picture below, you can see that the same amount of sunlight covers a large area (a) at a polar region and a small area of (b) at the equatorial region. Because area b is smaller than area a, the sunlight is more concentrated there. This means there are warmer temperatures at b than at a.

Seasonal Temperatures

The Earth spins on its axis. If that axis stood straight up, the equator would always face the Sun as the Earth orbited around it. But this would mean that the Northern Hemisphere would stay cold all year round. We know that that doesn’t happen. In North America, we have colder winter and warmer summer seasons with spring and autumn in between. 

The Earth has seasons because it is tilted at an angle of 23.5°. As the Earth moves around the Sun, the planet is tilted either towards, or away from the Sun. Let’s see how this works in different seasons.

Did You Know?
The northern end of the Earth’s axis always points to basically the same place in space. This explains why the North Star can always be used to find north in the Northern Hemisphere.

In the Northern Hemisphere summer is from June to September. This is when the area north of the Equator is tilted towards the Sun. In North America, our calendars are set so that the Sun is directly overhead at its furthest north position on June 21. This day is called the Summer Solstice. 

The picture below shows the Earth at Summer Solstice. You can see which part of the Earth is lit and which part is in shadow. More of the Northern Hemisphere is in the light half than in the dark half. This is why the days are longer in the summer. In fact, above the Arctic Circle, there are 24 hours of daylight at the Summer Solstice!

The opposite happens in winter in North America. During this part of the Earth’s orbit, the Northern Hemisphere is tilted away from the Sun. The picture below shows the Earth at the Winter Solstice on December 21. More of the northern hemisphere is in the dark half than the light half. This is why the days are shorter in winter, and why the nights are longer. As you may have guessed, above the Arctic Circle, there are 24 hours of darkness at the Winter Solstice!

The picture below shows the Earth at the Autumnal Equinox and at the Spring Equinox, which is also called the Vernal Equinox. The Northern Hemisphere is not tilted towards, or away from the sun. This is when day and night are exactly the same length.  

Equinoxes and solstices are points in time. They are moments when the Earth’s angle is perfectly aligned with the Sun’s rays. But people sometimes use these names for the whole day on which these times fall.

Perceived temperature vs real temperature

The temperature outside is not the only thing that makes you feel cold or warm. This is why weather forecasts sometimes give you a second temperature. This is based on how people feel. You might feel colder on a windy day. This perceived temperature is called wind chill. You might feel hotter on a humid day. This perceived temperature is often called humidex.