Say I heat air that has a relative humidity of 60% near the ground. Let the initial temperature be 10 deg C and suppose I heat it to 25 deg C. Then the relative humidity is about 23.2%. If I use an environmental lapse rate of 6.5 deg C per km and an adiabatic lapse rate of 9.8 deg C then this air could rise (25-10)/(9.8-6.5)=4.5 km before it is at the same temperature as the surrounding air (it will then not rise or fall). The dew point for an RH of 23.2 and temperature of 25 deg C is 2.4. Espy's equation now tells me that the parcel only has to rise 125(25-2.4) = 2825 m before clouds form. It seems that generally one can just heat air near the ground and clouds will form. Is this correct?
Heat air near ground and clouds will form?
Posted 22 September 2016 - 09:40 PM
Posted 24 September 2016 - 05:55 PM
An environmental lapse rate of 6.5c/km implies conditional instablilty (dry adiabatic > lapse rate (envir) > moist adiabatic), so the parcel will rise to the LCL (lifted condensation level) where T= Td. However due to latent heat of condensation the parcel can continue to rise (and remain warmer than) the surrounding environment provided the lapse rate in higher layers of the atmosphere is greater than moist adiabatic.
Yes, if the atmosphere is absolutely unstable (lapse rate ~ dry adiabatic) or conditionally unstable (lapse rate >= moist adiabatic), and provided there's sufficient moisture then surface warming alone can initiate convection.
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