In general, we know that temperature decreases with height. This variation is known by the name of vertical thermal gradient, and it is because the source of heat that is radiating the atmosphere comes from the ground. Thus, the further away from the source, the air will be colder.
This gradient can be altered by several processes: sudden drop or rise in soil temperature or strong winds. To understand it better, in this special we will see what is the structure of the atmosphere and why does the temperature change as we go up.
Structure of the atmosphere
The atmosphere is composed of 5 layers: the troposphere, the stratosphere, the mesosphere, the thermosphere and exosphere.
- Troposphere: it is where we find ourselves, and has an altitude of about 12km. This is where clouds form, plants and animals live, we find the ocean and deserts, etc.
- Stratosphere: located between the 12 and 50km of altitude, there we will see the supersonic planes.
- Mesosphere: located between 50 and 80km of altitude. This is where radio waves 'sail', and where cosmic rays arrive.
- Thermosphere: among 80 and 690km of altitude the northern lights will appear, in addition to the spacecraft in Earth orbit.
- Exosphere: and lastly, from 690km altitude we will find Sputnik I.
Vertical thermal gradient
As we said, the temperature normally decreases with height. In the troposphere it has an approximate value of six degrees per kilometer. This means that if, for example, the temperature at sea level is 15 degrees, at an altitude of approximately five kilometers, it will reach the value of -15 degrees (a decrease of 30 degrees).
The sun's rays do not reach all parts of the globe in the same way, and neither do they reach the seasons. Thus, in temperate zones the thermal gradient is much greater than in the tropical zone, 1ºC for every 155m of altitude, due to the less insolation it receives and the less thickness of the atmosphere. Also in these same areas different variations occur as a result of the orientation of the relief and the distance it is from the equator, as well as from the poles.
In the intertropical zone the temperature decreases one degree for every 180m of altitude approximately, since the atmosphere is thicker and is very close to the equator. To this, added to the planet's own rotational movement, a warm climate is generated.
But in some regions of the atmosphere the opposite happens, that is, that the temperature increases with the height. In this case the vertical thermal gradient is said to be negative. For example: if the temperature increases by 21 degrees for a slope of 1 km, the vertical thermal gradient is said to be equal to -2ºC per km. Even in some layers of the troposphere it can happen, producing what is called a temperature inversion.
Temperature inversions also occur in the upper part of the stratosphere. On the contrary, in the mesosphere the temperature decreases on average when it rises, that is, the vertical thermal gradient is positive.
In the thermosphere, temperature increases with height and, therefore, the vertical thermal gradient becomes negative again in this region of the atmosphere.
What is thermal inversion?
This is a phenomenon that occurs when the ground is rapidly cooled by radiation, which in turn cools the air that is in contact with it. And in turn, the colder, heavier air in the top layer gets even colder. In this way, the speed with which the two layers of air mix decreases sharply.
It usually occurs especially in winter, leading to persistent fogs and frost. Although the thermal inversion tends to break after a few hours, in unfavorable conditions it can remain for several days until the air in contact with the ground heats up and restores circulation in the troposphere.
A very clear example of investment can be seen in Lima, due to the Humboldt current. This oceanic current cools the coast, and the upper layers, which are warmer, make the sky very cloudy and that the area has a cooler and drier climate than it should have taking into account its latitude.
Still, if there are no changes in the air masses, that is, if there is no instability in the atmosphere or there are no active fronts, the temperature will increase in relation to the height, in some places more than others.
Did you know what the vertical thermal gradient was and what it consisted of? Has it been useful to you?
thank you it has helped me a lot
Good info. Although I would have liked to know more about it.
There is something that I doubt or they were wrong when they say "the ground cools quickly by radiation" the ground can be cooled by convection, by coming into contact with a cold air mass. By radiation it would be solar radiation and in that case it would heat up, would it be like my comment? Thank you very much!
The Sun is not the only body that emits radiation. All bodies just because they are at a temperature emit radiation. The earth's surface receives more radiation during the day than it emits and heats up, and at night the opposite happens, it emits more radiation than it receives and cools. Air is a poor conductor of heat and is generally a thermal insulator. When the air moves, it carries heat better (convection) but this mechanism only works when the earth's surface is hot and the air near it heats up and, being lighter than the air above, it tends to rise.
I still don't understand why the temperature increases in the troposphere and then why it decreases again in the mesosphere
I don't understand why the «Vertical Thermal Gradient is positive when the T decreases with height». Could you help me understand it, please?
Example 1:
(T2-T1)/(h2-h1)=(-10-5)/(100-10)=-15/90; GTV < 0
Example 2:
(T2-T1)/(h2-h1)=(-10-(-8))/(100-10)=-2/90; GTV < 0
Example 3:
(T2-T1)/(h2-h1)=(15-20)/(100-10)=-5/90; GTV < 0
Greetings,