Sunday, October 16, 2011

What is a Pyrometer Measuring When You Aim it at the Sky?

Temperature based on the infrared spectrum of the device. It i not directly measuring DWLR due to the "Greenhouse" effect, it is measuring temperature which is energy.

Why would it measure about 320Wm-2 or 275K which is 1 degree C? Because there is potential energy in the atmosphere. The weight of the atmophere that is held up against the force of gravity by out going energy, mainly conductive flux assisted by radiant flux leaving the surface to space that is creating the potential energy.

At night does the tropopause fall hundreds of meters? No, it slowly sinks, so slowly there is little change in altitude. The energy flow through the atmosphere changes by nearly two hundred Wm-2 between day and night, more from season to season. Why doesn't the altitude of the tropopause constantly move up and down with the change? Because the tropopause regulates the flow of energy by changing temperature. The Tropopause can change by more than 30C faster than its altitude can change. This is because conductive flux from the surface maintains the lapse rate along with radiant energy interacting with water vapor.

In the day, solar enrgy is absorbed both at the surface and in the atmosphere. The average ratio is 70 atmosphere/170 surface. This average ratio, 0.41 times the surface flux is 160Wm-2. Which happens to be approximately atmospheric effect at the top of the troposphere. That is why the atmopheric effect is roughly in equilibrium. Clouds, Greenhouse gases, dust can change that equilibrium ratio. Latent flux change attempts to balance changes in that equilibrium.

The change in solar cycles change the ratio. High energy short wave, UV changes more than low energy near infrared. It is a push versus pull effect on the lapse rate. The surface convection pushing, the upper troposphere convection pulling. That amplifies the solar change slightly.

The Pyrometer or infrared thermometer is measuring the net down welling energy of all this dynamic energy transfer. A large portion of which is the response to the conductive flux, the potential energy of the atmophere. You could measure at the surface and subtract the temperature at the end of the lapse rate. Why bother? You have the temperature at the surface and the temperature at the top end of the lapse rate, calculate the DWLR. It is about 288-(-28C)or 288K-246K = 42K on average.

The Earth's atmosphere is in a remarkable balance of competing energy flux effects. It is easy to think you are measuring one, when you are in fact measuring several.

What is the significance of the 42K? It would be the approximate change in surface temperature due to the "Greenhouse" gas portion of the atmospheric effect. Remember, latent flux cools the surface.

If that is the case? 216/42=5.16 Wm-2/K is the climate sensitivity at the top of the tropopause and 216/33=6.54Wm-2/K the sensitivity at the surface. There is an inverse relationship between energy at the surface and energy at the top of the tropopause. A doubling of CO2, if it equals 3.7Wm-2 of forcing, would produce 3.7/6.54=0.8 degrees at the surface. At the top of the troposphere, 5.16/3.7=1.4 degrees at the top of the troposphere. Where the change in forcing is felt is very important to know. If you consider the conductive and latent flux response, the ratio changes slightly.

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