Air temperature is the first thing that comes to mind when talking about the weather. It is the most important weather parameter. Is it warm today or cold? And tomorrow and the day after tomorrow? This is a question we ask ourselves sometimes several times a day, looking at the weather forecast.
However, even if the temperature outside the window meets your expectations and disposes to do your favorite sport, it is not superfluous to know what it is. After all, the weather can change, disrupting your plans. The weather forecast is only part of the issue. To understand it better, it’s important to understand the weather.
What is the air temperature in general?
Air temperature is the physical quantity expressing how hot or cold the air is.
Why does the air get hot?
It is important to keep in mind that the air is not heated by the Sun itself, rather it absorbs the heat from the surface of the ground.
Meanwhile, the degree to the which the ground can be heated depends on several factors:
- the amount of solar irradiation, which varies with the angle at which the sunlight reaches the Earth;
- the distance between the Sun and the Earth (seasonal differences);
- the type of the surface (plain, rough, dark or light, etc.);
- cloudiness.
Let’s deal with everything:
Solar zenith angle
The closer this angle is to 90 degrees, the more heated is the ground and, consequently, the air. The more acute the angle, the less heat is obtained. This explains why the temperatures are unstable even along the same latitude. They change according to the change in sun angles.
The shape of the Earth is close to spherical and the axis is slightly tilted. We already know, that on the equator the sunbeam strikes the Earth at a 90-degree angle at noon, while only lightly touching the poles. Thereby, the equator receives the most sunlight and heat all year long. The closer to the equator, the higher the temperature of the air. The opposite is true for the poles.
For instance, in the afternoon, when the sun begins to set, the air will continue to receive the heat up until sunset. Then, gradually the surface will start cooling down and the air temperature will drop.
Surface
The water heats up and cools down slower than the ground. The highest air temperature over the World Ocean in the northern hemisphere is registered in August, while the lowest-in February.
Different types of surfaces might not only absorb solar energy but also reflect it. The measure of the reflection of solar radiation is called albedo. Let’s say that a certain surface receives direct sunlight, a 100%. Granite absorbs 53% while reflecting 47%, unlike sand which absorbs 32% of the light and reflects 68%.
One of the highest levels of albedo is characteristic for the snow, 80–85%. It may attribute to the difference in temperatures in cities and suburbs in winter. Snow on the streets of the cities is either removed from the streets or too polluted to reflect a lot of sunlight, hence the low level of albedo. In the suburbs, the snow is clean and has a high level of albedo, which brings the temperatures down.
Color also affects the degree of heat absorption. The darker the surface color, the faster it is heated by the sunlight, which is in a way characteristic of albedo.
During the course of the day, the highest air temperature is reached by 2–3 pm and is the lowest right before sunrise. The hottest month in the northern hemisphere is July, the coldest is January. And vice versa for the southern hemisphere.
Cloudiness
This is a very important factor that affects the temperature. What should be noted:
- Different types of clouds have different modes of interaction with solar radiation, depending on the density and the state of matter in the clouds (ice crystals, water droplets, etc). For example, in winter those clouds that contain ice crystals will be reflecting more sunlight, hence, lowering the air temperature.
- During the night clouds act as a blanket that traps the heat, which “reflects” back down from the clouds. However, if the night is cloudless (often characteristic of anticyclones) then the ground loses a lot of heat and the air temperature in the morning is low.
How the air temperature is measured?
It is measured in several units: Celsius (ºC), Fahrenheit (ºF), and Kelvin (K).
This article was originally published in Windy.app Blog on November, 7th, 2019, as one of the lessons of the weekly newsletter and a textbook “The Basic Meteorology for Wind Sports and Outdoors”, or Windy.app Meteorological Textbook (WMT) on better weather forecasting.
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