Wednesday, January 4, 2017


The first two graphs are for Cape Town, South Africa (lat 33.9 deg S). These two graphs show 
1) the solar energy falling per day on one square metre, maximum (always facing the sun - upper graph) and for a horizontal surface (lower graph). Solar energy in kWh 

2) the theoretical volume of air per day (cubic metres) that could be heated 5 deg C by a solar air heater of dimensions 1m by 1m always facing the sun (upper graph) and on a horizontal surface (lower graph).

The graph below is for Delhi, India. Lat 28.4 deg N. The graph shows the theoretical volume of air per day (cubic metres) that could be heated 5 deg C by a solar air heater of dimensions 1m by 1m on a horizontal surface. The x-axis shows 1 July, 1 Aug, etc. So on 1 July, in a day, the 1m by 1m solar heater on a horizontal surface in Delhi could heat 5241 cubic metres of air by 5 deg C.
Looking at temperatures and relative humidities for Delhi (India) and taking a low rainfall month of November, with an average RH of 55% and daily average temperature of 20.8 deg C, the graph uses figures as follows: The surrounding air temperature is 20.8 deg C and air is heated to the temperature shown on the T-axis (parcel of heated hotter air is at temperature T deg C), using solar air heaters. The line with the steeper slope shows the height to which the parcel will rise, using a dry adiabatic lapse rate of 9.8 deg per 1000 m rise and an environmental lapse rate of 6.5 deg C every 1000 m (fairly standard sort of figures). The line with less steep slope shows how high the heated air parcel must rise before clouds start to form (uses Espy's equation). When the parcel is heated to 28 deg C it will rise further than it needs to before clouds start to form. Before about 28 deg C it will not rise far enough for clouds to form. Actual lapse rates for Delhi would have to be taken into consideration for accurate conclusions. You can also work this out yourselves.I will tell you the near ground dew point for the parcel - it is 11.43 deg C. Espy's equation says, for clouds to form, the air parcel must rise 125(T-Tdew) where T is the near ground level temperature of the parcel and Tdew is the near ground level dew point of the parcel. As for how high it can rise, after it has risen 1 km, starting at T=27 deg C, say, the temperature of the parcel is 27-9.8 deg and the surrounding air is at 20.8-6.5 deg, etc. When the parcel and surrounding air are at the same temperature the parcel will stop rising (this is modified a bit because water vapour is less dense than air)

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