If a person spends a few hours out of doors, largely unclothed (think bathing suit, or shirtless) in the summer months (sun high in the sky) will our bodies produce lots and lots of vitamin D? I'm very confused about vitamin D. I supplement in the winter months, but should I in the summer months? I love being outdoors then.

Can we "stockpile" vitamin D in the summer months?

Hypovitaminosis D in British adults at age 45 y:
This is what generally happens in the UK where we measure in nmol/ rather than ng/ml
So at the end of summer average UK level is around 30ng/ml and at the end of winter it's down to 14ng/ml so the average UK adult never really has sufficient vitamin D to meet daily needs let alone store any for the winter.
So as soon as summer is over our 25(OH)D levels head downwards.

The trouble with sun exposure is you cannot be certain how much your skin is making and how much actually gets absorbed.
There are differences according to latitude, time of day, cholesterol level (it's part of the same family) skin pigment, pollution level, ozone level, urban/rural location, altitude. and cloud cover.
What I do is to take sufficient to meet my daily needs with an oral supplement AND get as much sun on as much skin as possible (bear in mind I live in UK and although it's been quite sunny here recently it's still very chilly so midday nude sunbathing is going to be short.
Providing you don't burn I'm all for natural UVB exposure,
Once you've raised your 25(OH)d level you should tan more quickly and the vitamin D itself acts as a photoprotective agent. You may want to increase other iron chelating anti inflammatory antioxidants to further protect your skin. Curcumin, Resveratrol, Astaxanthin, Melatonin, Lycopene, Green Tea Omega 3 all work both together and individually to reduce the chance of burning or skin damage but allow 8~12 weeks to get sufficient into the skin to notice the difference and always stop sunbathing BEFORE the skin even starts to burn.
Most of the skin damage from UV rays is the result of it's effect on iron either excess or inappropriately liganded. If women want to slow the ageing process post menopause then becoming a blood donor to keep losing blood regularly is a smart idea, men should be donating blood from a much earlier age as it's reducing iron status though monthly bleeds that generally gives women longer lives The potential your body has to create it's own vitamin D and melatonin declines with age (above 35) but at every age natural improving secretion of both vitamin D and melatonin is worthwhile but the older you get the harder you have to work at it and the more reliant you become on supplements. Both vitamin D and melatonin help control iron.

The amount of vitamin D your skin will make/absorb does vary with 25(OH)D so the higher your current level the less extra D3 is created so if you are just taking sufficient orally to meet daily needs (approx 5000iu/d) then you can safely leave it up to your DNA to create all it want's/can and not have to worry about overdoing it. I suppose people who live nearer the tropics may have to be a bit more watchful but at latitude 52N I doubt it's possible to reach vitamin D toxicity levels from sun exposure even with a modest 5000iu/d.

I keep my level around 60ng/ml through the year and increase the oral vitamin D in the winter to around 7500iu/daily but really you have to have a 25(OH)d test sometimes because unless you do you can't be certain how your body reacts either to sunshine or supplements. There will be people reading this who require 10,000iu/daily and others for whom 3000iu/d is fine.

Given your weight I wouldn't suggest less than 5000iu orally during the summer even with the Gardening, camping and outdoor exposure and consider going to 3 x 5000iu ever 2 days (7500iu/d) from October through to March next winter.

Thank you for such a wonderfully detailed response! I will certainly read the link you provided carefully. I'm lucky I'm a chemistry teacher...nmol provide no problem for me!

That's not the case if you expose ALL your skin surface area. The more skin you expose the more vitamin D is created but the advantage of full body exposure (as opposed to hands/face only) is that when you put your clothes back on the newly made vitamin D is protected from UVA degradation by the clothes. So it won't get processed on into suprasterols that you don't use.
Do be aware that omega-3 fatty acids in high fat fish oil diet diet have beneficial effects against UVB-induced skin carcinogenesis, and these effects may be associated with an inhibition on UVB-induced inflammatory response
and the damage UVA causes is through iron oxidation.
So reducing the possibility of iron overload by regular blood donation is an obvious choice for everyone particularly before they go on holiday and certainly for men from as young as possible and women from menopause.
As is building up your reserves of antioxidants, Melatonin, vitamin d3, Curcumin, resveratrol, astaxanthin Lycopene but do be aware these are not instant action treatments. It takes time to correct circadian rhythm to increase melatonin secretion, it take time to rebuild your vitamin D reserves so you have vitamin D stored inside cells, it takes time to reduce your omega 6 levels and increase your omega 3 status. etc.
If you start the full program now you will certainly see an improvement is skin sensitivity 12 weeks from now but carry on through the winter and next year you will really appreciate the better ability to enjoy sunlight without damaging your skin. We evolved living mainly outdoors mainly with little clothing, We have become maladapted to our environment but by returning to paleo principles can re-adapt to tolerate sun exposure natural for the latitude at which our skin pigment evolved.