Relative Humidity is the key factor in determining cloud formation but i believe prevailing pressure system in the surrounding has to do with how fast the cloud can be formed and how long will it sustain. Low pressure systems are generally associated with bad weather but in a fairly high pressure area (1012hPa) and high temperature (38C) and relative humidity more than 80%, why clouds cannot be formed during mid-day ?
You'll get the best answer by posting in earthscience.se, but I'll give a basic answer based on what I remember from flight school...
In general terms, cloud is formed when moist air is cooled down to its dewpoint. The main methods of cooling are through conduction (touching the cold ground or ocean) or through adiabatic cooling via convection (when the air rises and expands).
High pressure areas means the air is sinking from above. The air converges at altitude, sinks, and diverges at the surface. This limits the ability for convective cooling, making it almost impossible for daytime thunderstorms to develop.
But that doesn't mean you can't get any clouds. In fact fog, which is a cloud, basically needs high pressure in order to form, as well as a cool surface. On a hot day, the ground may be so warm that heated air 'pockets' can break through the sinking air and form cumulus clouds. Any mountain range will also force the air to rise and cool too. In the conditions you've described I wouldn't be surprised to see scattered cloud (especially as 1012hPa isn't that high). You just probably wouldn't see the large towering cumulus clouds that can develop into thunderstorms.
It depends on the amount of water vapour in the air (realative humidity).
Pressure systems within the atmosphere are large and small areas of air that contain rising (low pressure) and descending (high pressure) air; sometimes there is rising and descending air but only in very unstable conditions (thunderstorm).
In general, rising warm-moist air will create clouds as it ascends to a cooler altitude to then condesate to make 'clouds'. This rising warm-moist air is usually caused by solar radiation (convection). The rising air creates a low pressure system. Cold air in the atmosphere will descend and thus create high pressure.
why clouds cannot be formed during mid-day ?
Clouds can be formed during mid-day but they won't suddenly just appear in the sky unless the rising columns of air cool extremely fast. For example: thunderstorms can create new clouds within minutes due to the speed of the updrafts.
There may be evaporation of ground moisture and water, but the time it takes for that water to evaporate could be hindered by high pressure. However a good example of the formation of clouds during mid-day is a fire. Fires release water through combustion and create high speed rising columns of air. The air quickly rises and cools to form pyrocumulus clouds (usually).
Overall pressure does have an influence on the formation of clouds, but you as a pilot should know that low pressure systems will be related to unfavourable weather compared to high pressure systems.