What are necessary conditions to form convective clouds with their flat cloud base, like this one:


I know that convective instalibity at the ground is at start of the process, but I have realized that I don't understand what exactly determines the height of the cloud base. Usually, it is told that the rising bubble of air expands, cools and finally its temperature drops below dew point and water droplets form. But the bubble must be warmer than the surrounding air to ascend, so is warmer also at cloud base and even so water condenses inside the bubble and not outside it.

  • $\begingroup$ It is warmer and wetter. $\endgroup$
    – user107153
    Feb 23 '18 at 10:11
  • $\begingroup$ And does this affect already the convective instability condition (that is between clouds the air is steady, no convection) or the end of the process at the cloud base (that is between the clouds is convection too, but without visual effect of condensation)? $\endgroup$
    – Leos Ondra
    Feb 23 '18 at 11:48
  • $\begingroup$ You are right. The short answer would be that the bubble or parcel of ascending air is warmer but also has higher dew point. A cursory look at various meteorogical sources shows that the parcel of air which keeps its water vapor content (adiabatic - no exchange of energy and matter - expansion during the ascent is assumed) from the ground to the CCL where its temperature reaches its dew point (while water content and temperature of the surrounding air is different). $\endgroup$
    – Leos Ondra
    Feb 27 '18 at 18:39

There is no bubble and Convective instability isn't used to explain the cloud base.

It starts with a mixture (wet air) of dry air and water vapor. Hot mixture moves up due to buoyancy, is cooled, and moves down, establishing a vertical convective circular flow.

Temperature above the earth surface decreases over height. When the condition of temperature and water mixture fraction is such that dew occurs, liquid phase of water (droplet) starts to form which is what we call cloud. The droplet grows and may rise until it loses it kinetic energy and falls down due to gravity.

The cloud base is determined by the dew condition (temperature and water mixture fraction). It should be flat in a small region because of the temperature. Its height is determined by local temperature, i.e. in hot weather, the cloud or cloud base is higher in the sky, whereas in cold and wet days, the base is lower.

  • $\begingroup$ This unswer is not useful, in my opinion. There certainly is convection in action and the rising volume of air is usually considered as isolated from the environment (and calculated as adiabatic process). See Convective Condensation Level or CCL (en.wikipedia.org/wiki/Convective_condensation_level) for instance. The problem is that the isolated volume of air ("bubble") has to be warmer than the surrounding air up to the CCL. $\endgroup$
    – Leos Ondra
    Feb 24 '18 at 10:43
  • $\begingroup$ Thanks for pointing me to CCL. In the second paragraph of the wiki paper, to determine the base height of a cloud, a approach is shown . I understand now that your question is about whether it is cooled inside the "bubble". The volume of the "bubble" expands due to pressure decreasing with height. The temperature decreases as well. Note that this change is not due to heat transfer so it doesn't only happen on its boundary surface. The temperature inside the "bubble" drop, everywhere. There is one assumption used here that assume the process is very slow but the above is a good approximation. $\endgroup$
    – user115350
    Feb 24 '18 at 16:38

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