En este vídeo vamos a ver cómo se pueden simular dos situaciones opuestas que tienen lugar en la región atmosférica más próxima a la superficie terrestre. En primer lugar veremos una situación típica diurna en la que la temperatura del aire disminuye con la altura. A continuación trataremos una situación típica de noches despejadas, es decir, sin nubes, en la que la temperatura aumenta con la altura. Esta última situación se denomina inversión térmica.

Fenómenos típicos asociados a las inversiones térmicas de radiación (llamadas también de superficie) son la formación de nieblas radiativas (imagen inferior) y la acumulación de contaminantes atmosféricos en las proximidades de la superficie terrestre.

Material

Para realizar el experimento necesitamos:

1. 2 cajas de metacrilato
2. Cartulina negra
3. Tubos de cartón a modo de chimenea
4. Velas
5. Bolsa de hielos
6. Embudo y tubo
7. Resina de incienso
8. Carbón
9. Cerillas

Generación de Humo

Para llevar a cabo nuestro experimento generamos humo, el cual va a simular una niebla o contaminantes en las proximidades del suelo. Simplemente usamos una pequeña pastilla de carbón que encendemos con cerillas. Y a continuación quemamos resina de incienso sobre el carbón para obtener el humo.

Así nos queda el montaje final

Una vez montado todo debe quedar como en la foto. Podemos observar cómo al generar el humo e introducirlo en la caja para simular la inversión térmica, el humo se acumula en la parte de abajo junto al hielo (frío).

Más profesional

Para hacer el experimento en casa se pueden construir las cajas cortando láminas de metacrilato, haciendo los agujeros para introducir el humo y para permitir que salga, y uniéndolas posteriormente.

Comprar láminas de metacrilato para las caras de las cajas y hacer los orificios con un taladro. Unir las caras usando cloroformo o silicona y dejar que se seque bien. Poner un peso encima puede ayudar. Acabar poniendo unas bisagras para la puerta y la caja está preparada para el experimento. Se puede pegar cartulina negra en las caras de la caja que situemos al fondo de nuestra dirección de observación para así tener un mayor contraste con el humo, que es blanco.

Más sencillo

O bien, usar unas cajas de cartón más sencillas y económicas (en tiempos de crisis) pero igualmente funcionales.

Para hacerlas hay que coger una caja de cartón. En la parte de atrás estará la tapa para poder meter las velas y el hielo y por delante cortar una cara de la caja original y poner forro de plástico transparente para poder ver su interior. También hay que hacer unos agujeros, uno para introducir el humo y otros para que podamos poner las chimeneas de salida del mismo.

What is this all about?

Air temperature can significantly change from one to another location and time. It is sensitive to different factors, such as distance to the equator, proximity to the coast, season of the year or time of the day. Nevertheless, considering a global average -both temporal and spatial-, air temperature tends to diminish with height within the first few kilometres of the atmosphere. For this reason, the term "temperature inversion" indicates an opposite behaviour to the standard vertical variation of the temperature. That means an increase of the air temperature as we ascend.

Figure 1. Variation of temperature with height according to a standard atmosphere

An atmospheric layer with a temperature inversion inhibits the ascending vertical motion of air. Therefore, these are high atmospheric stability zones, considering that the air at the bottom is colder than at the upper part, so that it cannot reach higher levels due to its greater density and weight. Consequently, a thermal inversion close to the ground does not allow an easy vertical dispersion of contaminants coming from the human activities, remaining close to the ground and meaning a risk regarding health issues.

Surface-based temperature inversions frequently occur over the continents during clear nights, with weak or even no wind. These features favour a dramatic cooling of the ground (surface or radiative temperature inversions). In this case, the air adjacent to the Earth's very cold surface gets cooled through this contact. On the other hand, another layer of air which is at an upper level, high enough not to notice this effect, keeps a warmer temperature than the air just below. These temperature inversions are usually eroded during the coming morning: after sunrise, the ground receives again an input of solar energy and progressively gets warmer, and so does the air close to the ground. This kind of inversions is usually observed in steep terrain during the night, where cold air at surface may go slope down due to its greater density. This favours the presence of a colder air layer at surface than the air above.

Moreover, a strong anticyclonic situation can allow the occurrence of temperature inversions also far away of the ground. In this case, they are not surface-based, but at a certain level, usually higher than 500 m. These high pressure situations are linked to slow descending air motions in the central region of the pressure system, with a characteristic warming of the descending mass of air by compression. If this descending process is long enough, i.e. several kilometres, the mass of air may become considerably warmer and drier than the surrounding air, both above and below it. This kind of thermal inversions are known as subsidence thermal inversions, and frequently cover great extensions, being persistent in time.

Another kind of thermal inversion is due to the arrival of a cold front (frontal thermal inversion), so that the cold air mass arrives at a location with warmer air. That arrival makes the warm air ascend, staying therefore over the coming cold air.

Let's see it!

In this video we can watch how to simulate two opposite situations which can occur at the lowest part of the atmosphere. Firstly, a typical diurnal situation in which air temperature decreases with height. Afterwards, the situation corresponds to a typical night with no clouds, in which the air temperature increases with height, called a temperature inversion.

Typical phenomena associated with radiative temperature inversions are the onset and growth of radiative fog (picture below) and the accumulation of pollutants near the ground.

How to do it?

Material

To carry out this experiment we use:

- 2 methacrylate boxes
- black cardboard
- a couple of carton tubes acting as chimneys
- a few small candles
- small bag of ice cubes
- funnel and tube
- frankincense
- carbon tablets
- matches

Smoke generation

To simulate fog or pollutants close to the Earth’s surface we generate smoke in our experiment. We just light with matches a small carbon tablet, and then put the frankincense on it to get it burned and obtain smoke.

 
This is how the experimental design looks like

Once the whole experimental design is prepared, it may look like as in the picture. It can be observed that, when we generate smoke and introduce it into the box to simulate the thermal inversion, the smoke accumulates in the bottom part of the box, placed just on the ice cubes (representing the colder air).

Can I do it at home?

The professional way

You can carry out this experiment at home by building your own boxes. Cut the methacrylate sheets and doing a hole on one side to put the smoke inside the box, and a couple of them at another sheet to let the smoke leaving the box through the chimney. Then, you carefully glue all your sheets.

So the whole process is as follows. Once we have the methacrylate sheets for the box faces, we use a drill to do the appropriate holes. Then the faces need to be joint to each other, for example by using silicone, and letting them dry up. You can also add hinges to have a door in the box. Besides, gluing black cardboard on the back box sides may help to have a better contrast when watching the smoke inside, as it is white.

The simple way

You can also use carton boxes. This is an easier and cheaper option, but fully functional.

To prepare them, take a carton box. The back side can be reserved for the part of the box which can be opened, so that you can put inside the candles or ice cubes. Remove one of its sides and instead, cover this part with transparent plastic, so that you can watch what happens inside. Now you only need to make the holes required: one in order to introduce smoke in the box and the other two to put the carton chimneys.