Above is a link to a solar elevation calculator based on time of year and where you live.
As you know by now, vitamin D is vital for good immune health. In fact, vitamin D level needed for strong T-cell activation against respiratory virus' (e.g. Covid, flu, colds) needs to be greater than 50-55 ng/ml. And for robust defense against cancer it needs to be even higher ~70 ng/ml.
We all know the sun can create vitamin D in our skin. Light skin produces more vitamin D than darker skin. As white skin tans, however, less vitamin D gets produced.
What is little known and I am sharing here is that the elevation of the sun to produce sun energy sufficient to penetrate the atmosphere and then activate the biochemistry in your skin to produce vitamin D needs to be greater than 50 degrees. In other words, as the sun rises in the sky and you feel more and more warmth on your skin, only after the sun gets above 50 degrees does the chemical reaction begin to occur converting cholesterol to vitamin D (reaction completes in the kidney).
Getting to 50 degrees in the sky follows two paths. One is the everyday sun rise and sun set. The other is time of year. In some locations, such as London, by late August, the sun never gets above 50 degrees in the sky even at noon. In other words, vitamin D from the sun is over by the end of August and your body will begin depleting its vitamin D stores from end of August until later spring when the sun next gets above 50 degrees in the sky - which is later April. Using London as an example, even after the sun crosses over above 50 degrees - the time it spends there depends on how far from noon the sun is located. In late August and late April, for example, the sun barely crosses over 50 degrees for only 15 minutes or so. This means that vitamin D production will be very low and only just starting to happen as the sun moves from Spring into Summer.
I can't stress enough how important it is to supplement with vitamin D3 sufficiently to get blood levels over 50 ng/ml. In ancestral times, our families would be out in the fields all day and in Summer, their skin is busy making a lot of vitamin D gradually increasing, peaking in June (northern hemisphere) and then fading away until their bodies would reverse the storage of vitamin D and use it up during winter. Since many of us work and live indoors now and avoid the sun (i.e. sunscreen), we never get to build up natural vitamin D. I no longer (for the last 10 years) use sunscreen. Instead, I gradually expose my skin beginning March (29 degrees N. Lattitude) when the sun goes above 50 degrees and my skin starts to tan. By June, even though the sun is intense, I never burn (like I used to before I knew better) because my D levels are high and my skin is darker protecting me from the intense rays. This is nature at work.
The calculator above is fun to play with ....