Research Question:

How does the volume of water in a badgir affect the temperature reduction due to evaporative

cooling?

Science:

The sun emits in each second 3.90 × 1026 J of energy in form of electromagnetic radiation.

The sun’s radiation lands on earth’s surface, it is either absorbed, and causing the surface to

get hot or it is reflected.

The energy emitted by the sun spreads out in a sphere. By the time it reaches the Earth at a

distance of 1.50 × 1011 m, the intensity per m2 is:

3.90 × 1026

4𝜋 × (1.50 × 1011 )2= 1380 Wm−2

The intensity of radiation on Earth is related to its distance from the sun. The sun shines

equally at the equator and the poles but the equator is very hot and the poles are very cold

because of the amount of radiation received from the sun is affected by some factors.

The major factor is that while the sun is at right angle over the equator, it is at a slant angle at

the poles. Both the Equator and the poles receive the same amount of radiation, but the area

that receives the radiation at the poles is larger than the area the received the same amount of

radiation at the equator due to the angles. Consequently, the amount of sunshine that falls on a

unit area at the poles is less than the sunshine falling on unit area at the Equator. This can be

explained using the diagram below:

http://www.hko.gov.hk/education/edu06nature/ele_srad_e.htm

Both A and B receive the same amount of radiation. At A, the angle is right but at B the angle

at B is slant, so the total area that received the radiation at B is larger than the total area that

receives the same amount of radiation at A. Mathematically, if we divide the amount of

radiation received from the sun by the total area B and by the total area A, we will find that

the amount of radiation per unit area at A is higher than the amount of radiation per unit area

at B. Figure 1

The other factors include:

a) Absorption of sun’s radiation when passing through the atmosphere: the amount of

absorption depends on the nature and concentration of air molecules and small particles in the

atmosphere. The sun's radiation passes longer path to reach the poles. There are more air

molecules and particles along the way, resulting in greater absorption and scattering.

Therefore, less solar energy reaches poles.

b) Reflection of light waves by the earth's surface. When electromagnetic radiation is incident

on a surface it is either absorbed, causing the surface to get hot, or it is reflected back. The

amount of reflection depends on the nature of the surface. At the poles, the earth is covered by

the snow. Snow is an extreme in terms of reflection so nearly 75 to 95% of the incoming

energy is reflected back. However, with an overall decreasing trend in snow cover in recent

years due to climate change, more and more solar energy gets absorbed.

The Middle-East is situated near to the Equator, that’s why the atmosphere is very warm and

dry in that region. Centuries ago, people living in the Middle-East tried to find out ways to

cool their houses without using electrical energy. They invented simple architectural devices

used to create natural ventilation in buildings called Badgirs.

The energy absorbed depends on the colour of the object. Black/dull objects absorb more

radiation than white/shiny ones.

When the sun heats the atmosphere it causes the air to become less dense and move upwards.

These air movements combined with the rotation of the Earth cause wind. The kinetic energy

in moving air can be used to turn a turbine and produce electricity.

The sun heats the air which becomes less dense and rises, leaving an area of low pressure

close to the earth. Surroundings air will move into this low pressure area and this air

movement is the wind.

Albedo:

When electromagnetic radiation is incident on a surface it is either absorbed, causing the

surface to get hot, or it is reflected. The ratio of reflected to incident radiation is called the

albedo. The average albedo for the Earth including its atmosphere is 30%.

Interaction between electromagnetic radiations and matter:

The light that comes from the sun is made up of photons of many different wavelengths.

When a photon interacts with an atom, it can give energy to the atom by exciting one of its

electrons into higher energy level but this can only happen if the energy of the photon is

exactly the same as the energy needed to excite the electron.

Solids are also made of atoms and once a photon is absorbed, the temperature of the solid

increases.

A molecule is made of several atoms held together by the electromagnetic force. The atoms

that make up the molecule have kinetic energy due to their movement, potential energy due to

their position and internal energy. The molecule therefore oscillate and once it has the same

frequency of the oscillation as the photon of light coming from the sun, then the molecule can

absorb the photon. This causes the molecule to move faster and faster and since have higher

temperature because temperature is the measure of average kinetic energy.

Energy comes from the Sun to the Earth, it warms up the earth and the earth radiates some

energy back into space.

Evaporation:

Water

Evaporation takes place on the surface of water at all temperatures. Temperature of water is

the measure of average kinetic energy of water molecules.

When water evaporates, the fastest-moving particles leave the surface. So the average kinetic

energy of the remaining particles is lower, resulting in a reduction of temperature.

Rate of evaporation can be increased by:

Increasing the surface area so that the number of molecules near the surface increases

and giving them more chance to escape.

Blowing across the surface: when water molecules evaporate, they form a small

vapour cloud above the surface of water, if this cloud is blown away, it allows more

molecules to leave the surface of water more easily.

Raising the temperature so that the kinetic energy of the liquid molecules increases

enabling more molecules to escape.

The latent heat of vaporisation of water is 2.27 × 106 J kg−1. It refers to the amount of heat

required to change the state of 1kg of water from liquid to gas without changing its

temperature.

Evaporation is an endothermic process in which energy is taken from the surroundings to

break hydrogen bounds.