Lower Atmosphere

Basics

The Troposphere - variations with height and temperature

The lower layer of our atmosphere, which is next to the Earth's surface, is called 'troposphere'. It is nothing else than the air surrounding us, from the ground to the highest clouds. If we look into the sky we tend to overestimate the thickness of this layer.

The atmosphere protects us from the intensive sunlight, provides the oxygen for breathing and makes life possible on Earth. But it is only a very thin cover.

Dimension of the troposphere

Although the troposphere is the thinnest layer of the atmosphere, about 11 km around a planet of 12.800 km in diameter, it contains about 90% of its mass, this means 90% of the molecules of the air.
The troposphere goes from the ground up to an altitude of about 7 km at the poles and 17 km at the equator. The next atmospheric layer above the troposphere is the stratosphere. The border between both layers is called tropopause. But how can we know, where the troposphere ends? Changes in the temperature trend give an answer.

Temperature profile and transport of air

Temperature falls with increasing altitude. It becomes colder in the troposphere, the higher we go. You feel it if you hike in the mountains. But there is a point in the atmosphere where this trend changes. This is the tropopause - a temperature minimum in the atmosphere. Some scientists call it a cold trap, because this is a point where ascending air cannot go higher. Imagine a parcel of air as shown on the right. You see, that it is stopped on the way upwards. This is important for the dynamics and the chemistry in the troposphere, for the cloud formation and the weather.
What is the reason? Warm air is lighter than cold air. We know this phenomenon from our daily life. If you open the door in winter, you always feel the cold air first at the feet, since it is heavier and sinks to the floor. Similarly, when the Earth warms up at the ground in the warm sunlight of the morning, the air at the ground becomes warmer and therefore lighter than the air farer away from the ground. The air at the ground starts ascending like a light baloon. As long as the air around is colder (= heavier) it can go on raising. At the tropopause this travel stops, because the air above is warmer and lighter. This is why it is rather difficult for water (clouds) and for the chemical compounds to cross this invisible temperature barrier at the tropopause. Therefore, most of the air chemistry and the weather take place in the troposphere. If water cannnot go higher than the troposphere, clouds cannot form anymore in higher layers, because clouds consist of water droplets.

The animation shows average temperatures at the ground (15°C) and at the tropopause (-50°C) and a rather simple temperature profile.

The real world however is more complicated. We do not have the same temperature everywhere at the ground and we don't have everywhere -50°C at the tropopause round the Earth. Moreover temperature changes with the seasons. The picture gives you an idea of different temperature profiles in different latitudes in summer and winter (which is nearly the same in the tropics).
tropics: green profile; tropopause > 15 km
temperate regions: light red = summer; dark red = winter; tropopause > 10 km
polar regions: light blue = summer; dark blue = winter; tropopause < 10 km

Sometimes the tropospheric layer next to the ground behaves a bit different. We call it "planetary boundary layer" (PBL). In the PBL influences such as friction, heat transport, evaporation, and air pollution lead to changes within one hour. The thickness of this layer can vary between a few hundred meters and about two kilometers. The part above is called free troposphere. The most important process in the PBL is a mixing of air due to raising air in the morning.

Related pages:

More about the planetary boundary layer PBL in
Lower Atmosphere - More - Unit 1 - Night and Nitrate

About the layers in the atmosphere:
Higher Atmosphere - Basics - Unit 1 - Layers

About this page:

author: Dr. Elmar Uherek - Max Planck Institute for Chemistry -  Mainz / Germany
1. sci. reviewing: Dr. Katja Mannschreck - GAW Hohenpeissenberg - 2003-08-07
2. sci. reviewing: Dr. Gerd Folberth - Meteorological Service of Canada / Univ. of Victoria - 2003-08-10
educational reviewing:  Michael Seesing - Uni Duisburg - 2003-07-02
revised and last published: 2004-06-12