Hi Oscar,

Crudely I would state that the prime variables to be considered are:

1) The speed of the cloud field;

2) How many octants it spans in the hemisphere en hence the frequency of direct and diffuse sunlight and their time spans of irradiation of the Earth's surface.

3) The thickness of the cloud layer(s) and its variation to calculate the magnitude of transmitted sunlight.

4) The thermal inertia of the soil on which solar irradiation direct or diffuse impinges.

Ideally, these variables will allow you for a reference point to calculate how fast and how long a cloud has a full impact on the SST of that point, hence when clear sky conditions occur over the point of reference.

Hi Oscar,

Crudely I would state that the prime variables to be considered are:

1) The speed of the cloud field;

2) How many octants it spans in the hemisphere en hence the frequency of direct and diffuse sunlight and their time spans of irradiation of the Earth's surface.

3) The thickness of the cloud layer(s) and its variation to calculate the magnitude of transmitted sunlight.

4) The thermal inertia of the soil on which solar irradiation direct or diffuse impinges.

Ideally, these variables will allow you for a reference point to calculate how fast and how long a cloud has a full impact on the SST of that point, hence when clear sky conditions occur over the point of reference.

Dear Oscar,

Further to the very pertinent answer provided by Frank, I'd like to add that while Equatorial regions are generally cloud-covered throughout the year, due to the raising branch of the Hadley Cell,

- in most regions, including within the tropics, there are strong seasonal variations, with more clouds during the rainy season and fewer during the dry season, linked to the latitudinal oscillation of the ITCZ, and

- especially in tropical regions, cloudiness exhibits a noticeable variation during the day, whereby clouds build up during the morning and throughout the day, and result in heavy precipitations in late afternoon and early evening.

Hence, if you are looking for the best opportunities to 'find "golden clear sky" conditions' in an area such as Lake Victoria, consider focusing on the early morning of days well within the 2 dry seasons.

Of course, another (complementary) approach might be to consider using data from other sources, and in particular satellites with a finer spatial resolution (e.g., 300 m or less), which will allow you to "peek" through and observe the land in the holes of a broken cloud field, if that is of use for your purposes.

Best regards, Michel.

Dear Oscar

several years ago, we made a simulator (OSIRIS) for simulating landscapes in thermal IR at very high resolution. We used this simulator to study the effect of the shades (that could be created by clouds) and found that the effect may last several hours after the disappearance of the shading object.

See

Article OSIrIS: a physically based simulation tool to improve traini...

orArticle Influence of the three-dimensional effects on the simulation...

Best regards

Lucien

come on of course will impact, it is absolutely like filters

Dear all,

Thank you for your valuable answers. I will comment upon them once I have defined a proper methodology to find the "golden days" !

Kind regards,

Oscar