4] On the equinox, the sun sets at your location at 6PM and rises at 6AM. At sun

Question

4] On the equinox, the sun sets at your location at 6PM and rises at 6AM. At sunset the temperature is 25°C and the relative humidity is 70%. The air cools between sunset and sunrise at the rate of 1°C per hour, while the amount of water vapor in the air remains constant. a-How much water vapor (in g/kg) is in the air at sunset? b - Find the relative humidity for all the hours between sunset and sunrise. c- At approximately what time will fog first form? Recall that once the relative humidity reaches 100%, if the temperature decreases further, then water condenses and the relative humidity stays at 100%. For this problem you will need a more precise version of saturation mixing ratios (than figure 4.1). You can find an online calculator for this by Googling "noaa saturation mixing ratio calculator". Assume that the atmospheric pressure is 760 mm Hg.

Explanation / Answer

Ans:

a) At 25 degree CWater vapor should be =20 g/kg (Specific humidity of a kilogram of air)

However, RH=70 given

So,  water vapor (in g/kg) is in the air at sunset => 70=(x/20)*100 or x =(70*20)/100=140/100=14 gm/kg

b) temperature decrease is 1 degree per hour

C) 12 midnight or after 11 PM we can say

TIME 6 7 8 9 10 11 12 1 2 3 4 5 6 TEMP C 25 24 23 22 21 20 19 18 17 16 15 14 13 RH 70 74.34944 79.09605 84.18521 89.62868 95.43286 101.7442

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