Terrestrial radiation

This short article written for www.bassing.co.za is aimed at helping our fellow anglers expel fisherman’s tales and supply the scientific facts regarding weather phenomena and our environment. It may not affect our fishing directly but will most certainly affect it indirectly as it affects us the fisherman.

A scientist I am not. An enquiring mind and a craving for knowledge have led me to research, the findings of this research I am sharing with you.

As bass fisherman we have often watched the sun rise and indeed we often aim to be on the water as the first rays of sun light break the horizon and enable us to do what we love to do, hunt for bass. I have and I am sure we have all asked the question why there is a sudden drop in temperature as soon as day light breaks.

It is indeed true the coldest part of the day is directly after the first rays of sun appears, this in general occurs 15mins before sunrise. For obvious reasons your location and prevailing conditions control the extremity of this phenomena and the temperature can drop at this time by 0.1 degrees to up to 10 degrees in extreme conditions.

There are several reasons why this sudden drop in temperature happens, namely terrestrial radiation which results in solar winds, deposition and sublimation. So what is terrestrial radiation? In short it is “Electromagnetic radiation originating from Earth and its atmosphere.

Electromagnetic radiation you say! Where does this come from? All matter with a temperature greater than 0 K (Kelvin) emits electromagnetic energy; the amount of energy and wavelengths at which the energy is emitted are dependent on the temperature of the body.

In simple bassing terms, during the course of the day the sun heats up the earth. The earth absorbs this heat and at night time when the sun disappears beyond the horizon the absorbed heat radiates from the earth back into the atmosphere. Gradually as the earth dissipates more and more heat the cooler the earth becomes making the atmosphere progressively cooler as time goes by. This drop in temperature is progressive through out the night and is not the sudden drop in temperature we experience as the sun starts to rise. However it does set the stage for other factors to play their part.

The earth is a better radiator than the air surrounding it. Once the earth has dissipated as much radiation as it can before the sun rises, the earth and a very thin layer of atmosphere around the earth is much colder than the rest of the atmosphere. This cold layer of air could be as thin as a few millimeters thin to several meters thick. The greater the height of this layer of cold air the less the sudden drop will be.

When the sun starts to rise, remembering that daylight breaks before the sun rises. The angle of the light hitting the earth’s surface is too great for the earth to absorb the radiation. However the angle that this light and radiation is hitting the earth causes it to bounce of the earth surface, this reflected radiation has enough energy to stir up the cold layer of air (solar wind in it’s simplest form) mentioned earlier causing a major drop in temperature.

Now you can also add in deposition and sublimation into the equation. Deposition is the transformation of gas into a solid (frost) without an in-between stage. This occurs as soon as the earth starts to absorb the suns radiation and starts to radiate heat itself. Frost is only produced from the earth’s radiation and not the suns, if the moisture in the air starts to build too early and is bombarded by the suns radiation before the earths then dew is formed. Dew is also formed by the sublimated frost being thawed by the suns radiation.

In extreme circumstances sublimation can occur almost immediately after deposition further dropping the temperature. Sublimation is the transfer of a solid (frost) into a gas (water vapor) without an in-between stage (dew) which occurs as soon as the suns radiation and earth’s radiation act together simultaneously.

Atmospheric pressure also plays a part, but only as to the extent of the temperature drop and is not responsible for it. The greater the atmospheric pressure the less the temperature drop and visa versa.

Now whilst sitting around the fire and someone asks why there is a temperature drop as the sun starts to rise, as bassing.co.za readers you now know how to give them an educated answer. Or you can listen to all the myths and assumptions and chuckle to yourself before enlightening everyone with your superior knowledge.

Some interesting facts discovered whilst researching the above:

1. The mett office takes temperature readings and gives predictions of temperatures at a height of 2m. This could be to high for the temperature drop or at least the worst of the temperature drop. As such when you hear on the radio or read on the net a low of -1 is expected, at ground level it could be as low as -4.
2. Whilst camping, the greater the height you are off the floor whilst sleeping the better you are, and it is better to have more blankets underneath you than on top of you.

Reproduced with the permission of the author
pomerob