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High pressure systems are normally correlated with stable atmospheric conditions with little to no cloud cover, but due to this minimal cloud cover, doesn't that open up the ocean / ground moisture to evaporate to then later condensate to form clouds?

I understand there are other forms of cloud formation like subsidence or frontal uplift, but some days here in Perth are very still and there hasn't been any fronts recently. For example during today (27/12/2021), it has been very still yet the heat has reached 40°C. For the previous two days there has also been temperatures reaching up to 45°C for prolonged periods of time with clear skies. I haven't felt much wind or any down drafts within my vicinity.

Today, there has been altostratus, altocumulus and cirrus clouds. Most of the clouds are isolated but some have formed extensive 'clumps' of cloud coverage with other clouds. So despite the high pressure system is it possible for there to be a decent amount of cloud generation? Can the ocean evaporate despite there being descending air? enter image description here

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  • $\begingroup$ I am not sure you can associate high pressure systems to high altitude cloud patterns. my understanding is that a high-pressure area is a region where the atmospheric pressure at the surface of the planet is greater than its surrounding environment. $\endgroup$
    – Juan Jimenez
    Dec 30, 2021 at 17:58
  • $\begingroup$ High pressure means that there is descending air / 'the air above a given parcel of air is heavier than it' $\endgroup$
    – JandyPilot
    Dec 31, 2021 at 2:50
  • $\begingroup$ But that has nothing to do with the formation of high altitude clouds. $\endgroup$
    – Juan Jimenez
    Dec 31, 2021 at 22:04
  • $\begingroup$ If you read the answer, that is why I asked the question. $\endgroup$
    – JandyPilot
    Jan 1, 2022 at 8:15

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As always you could probably get a better answer on EarthScience.SE, but:

The centre of a high pressure system is less likely to see significant cloud yes, but the same is not true for the areas with closer isobars. For example, I was on the NSW southern coast that same day where we had miserably low temperatures and overcast low cloud. This is due to the same high pressure system affecting WA that was sitting south of the continent. In my case, because the wind flows anti-clockwise around a high in the southern hemisphere, this bad weather is essentially the ocean evaporation being blown onto us.

In Perth the story is a little different. The high is blowing hot desert air over your city, but the isobar spacing is reduced so therefore it has less force to it. There is not enough force to overcome the impact of that trough from the north. Whilst the trough officially ends slightly north of the city, it is still bringing significant moisture with it which can carry further south due to momentum. Furthermore, that isobar is actually the last associated with the high pressure system, and the gradient wind becomes an oceanic north-westerly just north of Perth.

The isobar also tells us the QNH around Perth is about 1012hPa which is neither high pressure nor low pressure. It is absolutely feasible in those conditions for there to be vertical uplift and instability to form clouds provided there is sufficient moisture - which the trough and the gentle north-westerly bring.

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  • $\begingroup$ I already tried earth science, but there's been no people answering the question. $\endgroup$
    – JandyPilot
    Dec 31, 2021 at 2:43

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