melting the ice on the windscreen

 

We have had some sharp frosts again. At least the sun is helping at the moment but how long does it take the sunlight to melt the ice. Windscreen is 0.7m x 1.1m so about 0.8 square metres. Estimate a 2mm thick ice sheet. Density of ice = 0.9 kg per cubic metre approx so mass of ice = 0.8 x 2x10^-3 x 0.9 = 1.5 x 10^-3 kg approx. Say that the ice starts at -2 degrees C. To heat it up to zero needs mc.delta(theta) = 0.0015x2100x2=6.3 Joules. To melt it needs E=-mL = 0.002 x 340000 = 680 Joules. Max solar power = 1400 Watts per square metre. Let's halve that for low sun in spring. So from the sun we get 700 x 0.8 square metres = 560W. If it only needs 686 Joules to melt it, that would take less than 2 seconds full on. It takes longer because the glass is also being heated and the windscreen is heated only a bit at a time.