Why April 15th is D-Day in the south of England

Read Time: 6 min

[Updated 07/04/18]

  • If you live in the South of England you can start making vitamin D from April 15th, close to midday (1pm)
  • To create vitamin D your skin needs UVB which only reaches the surface of the Earth when the sun is more than 50º above the horizon
  • Unless you live in the tropics this means knowing the months of the year and times of day when the sun is higher than 50º
  • Outside this vitamin D window you will only be exposed to UVA, which causes skin aging, and you will not be making vitamin D

When can we make vitamin D?

After the long English winter, we are all at our lowest levels of vitamin D.

The good news is that from mid April (in the south of England) the sun climbs high enough to allow sufficient ultraviolet light (specifically UVB) to reach the surface of the earth which enables the skin to start producing vitamin D. However, on D-Day there will only be sufficient UVB for a few minutes either side of mid-day, as shown in by the yellow vitamin-D ‘window’ below:

The sun’s angle above the horizon (zenith angle) needs to be above 50º before there is sufficient UVB to produce vitamin D in the skin. In the south of the UK this limits vitamin D production to between April 15th and August 30th, in a narrow window either side of mid day. Data source: Solar Topo

When the sun is lower than 50º, UVB radiation is almost entirely absorbed by the atmosphere. So to maximise your vitamin D levels you should get outside when the sun is higher than 50º degrees above the horizon – either side of mid-day.

The vitamin D window gradually expands each day until mid-summer, when vitamin D can be synthesised from 10:30 in the morning until 3:30 in the afternoon. Beyond June 21st this window begins to narrow again, until by late August the sun no longer rises high enough to let through any UVB, so vitamin D synthesis stops.

Why it matters

Data source: Vitamin D deficiency: a worldwide problem with health consequences. Michael F Holick, 2008

As the graphic above shows the more frequently individuals get sun exposure the better their vitamin D status.

You can see the distinct jump in vitamin D at the start of summer (day120), but it begins to fall towards the end of August (day 270) as UVB disappears from the sunlight.

Also notice that individuals who get more sunlight in the summer (the frequent exposure line) do not drop as low in winter and spring as those having less summer sun exposure.

Only those who have frequent sun exposure remain with sufficient vitamin D or better all year round. Those who avoided the sun spend most of the year with insufficient vitamin D, and all the increased risk of the diseases that go with deficiency:

The major cause of vitamin D deficiency is the lack of appreciation that sun exposure in moderation is the major source of vitamin D for most humans. Vitamin D deficiency causes rickets in children and will precipitate and exacerbate osteopenia, osteoporosis, and fractures in adults. Vitamin D deficiency has been associated with increased risk of common cancers, autoimmune diseases, hypertension, and infectious diseases. A circulating level of 25-hydroxyvitamin D of >75 nmol/L, or 30 ng/mL, is required to maximize vitamin D’s beneficial effects for health. –  American Journal of Clinical Nutrition, 2008

How to check the vitamin D window where you are

The figures above apply to the south of England. If you live somewhere else you can check the sun zenith graphs using a sun position calculator.

Remember, the sun needs to be above 50 degrees before you have a chance of making significant vitamin D.

Alternatively, you can use the following rule of thumb.

Rule of thumb

If your shadow is shorter than your height then the sun is above 45 degrees, so you can probably make vitamin D.

You can use any object, for example, a pencil. Stand it vertically on a flat surface outside, so you can see its shadow. If the shadow is shorter than the pencil then there is probably enough UVB to make vitamin D.


Animated UV map

[Edit 07/04/18: Sorry, but UV awareness.com is now defunct. If I find a suitable alternative I will update the following information]

The limitations of the above methods are that high level cloud, haze and pollution can reduce the UVB reaching the ground, even within the vitamin D window. Apart from resorting to a UV meter (expensive), you can use UV forecast maps that take haze and pollution into account.

One such resource is provided by UV Awareness: an animated hour-by-hour UV map for any location in the world. It has the benefit of incorporating weather data, so you can see if UV levels are high in your part of the world during your vitamin D window.

Example UV map for Europe, Tuesday 14th April 2015. Source: uvawareness.com

Whilst these UV maps only show the overall UV level, rather than UVB, we know that UVB is highest when overall UV levels are highest – either side of mid-day, in the vitamin D window. So it would be reasonable to assume that you can make vitamin D when the UV levels on the map are high (mid orange) or above.

Remember – these maps take into account ozone pollution and high level cloud that reduce UV, so can help you decide when to get some sun exposure each day.

You can see in the image above, which is for 14th April 2015, that the mid-ornage ‘high’ UV patch passes over Europe, but just misses the UK.

Why not build up a tan through sun exposure outside of the vitamin D window?

Morning and evening sun contain almost no UVB, but still significant levels of UVA.

UVA causes skin aging with no appreciable vitamin D synthesis. A UVA tan is caused by melanin becoming oxidised, leading to cellular oxidative stress.

UVB on the other hand, leads to tanning via increased melanin production and migration away from the cell nuclei so produces a more protective tan.

How much time should be spent in the sun?

Build up your tolerance gradually by not overdoing it to start with. Try to get a few minutes every lunchtime on sunny days throughout April if possible, gradually increase this throughout May. If you start to burn skip a day or two.

The key is to aim for the maximum midday exposure you can get without reddening (burning). This amount of UV is called a sub-erythemal dose,and maximises your vitamin D production time whilst avoiding the skin damage associated with burning.

You don’t ever need more than 40 minutes of mid-day exposure as your body makes all of the vitamin D it can in that time.

Embed from Getty Images


What about sun protection?

When aiming for vitamin D production the larger the area of your body exposed the better. Be aware that parts of the body that are usually covered will burn more easily – so don’t overdo it. You can extend the time you spend in the sun by turning over, increasing the surface of your body that will make vitamin D.

A wide brimmed hat will keep excess sun off your face as this is the place our skin ages most rapidly as it has the highest regular sun exposure.

I don’t recommend sunscreens (but that’s a discussion for another post). Instead, once you have had enough potosynthesising in the vitamin D window, simply cover up to minimise UVA photo-aging. Thin white cotton shirts, trousers and wide brimmed hats  are cool and comfortable.

Can foods protect the skin from UV damage?

Good news! Daily dark chocolate consumption (70% cocoa or higher) might increase your sun tolerance.

study in 2009 found that subjects who ate just 20g of high flavanol dark chocolate per day for 12 weeks could take more than twice the amount of sun exposure before burning compared to those who ate 20g of milk chocolate per day.

Unfortunately a more recent study failed to replicate this effect, but other health benefits of dark chocolate mean it is probably worth persisting with – Oh, the hardship!

Apart from chocolate, there are many other skin-protective foods which work primarily by boosting anti-oxidants in the skin. Plant polyphenols have some potential to reduce skin photo-aging and protect from UV damage (see our list of the top polyphenol containing foods). Unsurprisingly perhaps, humans – being the only ‘naked’ mammals – have some unique biological mechanisms for transporting sun-protective nutrients into the skin, see our related posts:

What if I get a sunburn?

If you inadvertently get sunburn apply aloe vera liberally. I use the fresh plant, snapping off a leaf, trimming off the serrated edges, then slitting it open length-ways and applying the jelly directly to the affected area. There is nothing like aloe vera for burns – it is invaluable. Everyone should have a plant or two in their house. If you can’t keep a plant, then use a proprietary aloe vera gel product.

So make sure you know when D-Day is where you live and start photosynthesising!






2 thoughts on “Why April 15th is D-Day in the south of England”

  1. Brilliant~! (No pun intended …) Especially I love that shadow/height thing, a simplistic old dog myself I like simple rules.
    I found myself wondering if it is a universal, but of course it must be.

    And I use any excuse for wolfing chocolate, you’ve just added to my arsenal, thanks again.

    • Thanks Argus. I think you will appreciate our other articles just as much. We try to keep it real, practical and in line with current research. All comments welcome.


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