The all-singing-and-dancing UV4You

A new free smartphone app that provide UV information and tailored advice at your fingertips for any time and place ...

It’s a bit late. This was going to be a summer-solstice treat for all you northern hemisphere readers. But your UV levels will still be close to their annual peaks. For we antipodeans, it’s a reminder of how comparatively little UV there is in winter compared with summer, because the typical summer maximum at any site is now also available.

A few months back we announced an update of our old GlobalUV app, which we’d modified to use average monthly ozone amounts rather than daily forecasts from satellites. This had the advantage that it would work off-line. Unfortunately, those climatological values sometimes differ too much from reality, so my friend Jerry Burke has bitten the bullet and written an improved app that can use both. We call it UV4You. I’m chuffed that he agreed to give it a name that’s so closely related to my posts here at UV&You. He wrote it in a new (to me) programming language called Flutter, which is compatible across both iPhone and Android devices. As a result, unlike earlier apps, this one has exactly the same functionality across those platforms.

Our previous apps used global ozone forecasts from NIWA via NOAA in the USA. But, with tRump-induced uncertainties in that chain we’re steering clear of that route. We now use global ozone forecasts from the European Space Agency’s ‘TEMIS’ website that’s maintained by the (Royal) Dutch Meteorological Institute (KNMI). We’re grateful to Jos van Geffen and Michiel van Weele there for their assistance. If no WiFi is available, the app reverts to its off-line ‘climatological’ mode.

We’re no longer using cloud forecasts, as they sometimes over-predict their blocking effect, and that can lead to potentially harmful under-predictions of the UVI. The required downloads to represent the entire world any more than for one time each day would be prohibitively large anyway (but we do still use higher-resolution cloud forecasts in our UVNZ app, which is limited to the New Zealand region). In UV4You we leave it to the users to specify their current cloud conditions.

The basic UV information you need is all available on the startup Map screen, which defaults to your current location. But the interactive Graph screen is much more interesting and contains a wealth of extra information. In both, you can get more details about the calculation parameters by touching the coloured boxes. Their colours match the internationally agreed standards for UV levels.

A big improvement in the new app is that a touch of the ‘Graph’ screen allows you to compare UV on the current day with that for a typical summer day at the same site. The original idea there was to fill the empty space on the graph in winter when the UV is boringly low, and to remind you much it changes with the seasons. In the summer it also allows you to see how much it changes due to day-to-day changes in ozone. Usefully, such comparisons can be made for any location in the world, either by clicking on different places on the map, or selecting specific sites from a drop-down menu. The ‘typical’ summer values you see there are calculated from a table of monthly mean ozone amounts as a function of latitude. Because of ozone’s day-to-day variability and its longitudinal patterns, absolute peaks can sometimes exceed these by more than 20 percent. For example, at Lauder, it gives a summer UV of 10.7 whereas in reality, the all-time absolute peak can exceed 13. To calculate the effects of extinctions by aerosols, the app makes use of a satellite-derived 10-year climatology of aerosol optical depths at 1-degree steps in latitude and longitude. Because of day-to-day differences in air quality, uncertainties are larger at polluted sites.

Behavioural advice and information on how much skin damaging UV accrues over the course of the day are also provided. That dose is expressed as the number of MEDs (Minimum Erythemal Doses). That’s the number of doses that can cause a perceptible reddening of the skin, an indicator of skin damage. The scale for that is on the right. The numbers get quite scary, sometimes exceeding 30 MEDs in summer, which means that if you were outdoors all day with a correctly applied sunscreen of SPF30, you’d still receive a ‘harmful’ dose of UV. The specified times for skin-damage all depend on your skin type, which is specified from a drop-down menu. On touching the graph screen, the app also gives the (very approximate) number of minutes of outdoor exposure that would be required to maintain healthy levels of vitamin D for previously specified attire.

Messages for other times of the day can be seen by sliding your finger over the graph. Fortunately, when the UV is high, the time for vitamin-D sufficiency is much shorter than the time for skin damage.

Some examples are shown below, all from late June 2026.

The first is from Lauder, New Zealand, where it’s a few days short of the winter solstice.

The second example below is from De Bilt in the Netherlands. That’s where our helpful colleagues at KNMI are based. For them, it’s close to the summer solstice. Their peak UVI is close to their expected summer maximum, and over the course of the day the dose will be about 13 MEDs. Also shown on this is the solar elevation and the calculated number of minutes of exposure for a selected time that would be needed to receive enough UV to maintain healthy levels of vitamin D. That’s about 4 minutes in this example (for skin type 2, assuming you’re wearing a t-shirt and shorts). In comparison, the calculated time for skin damage is about 30 minutes.

The third example, below, is from the high-altitude Mauna Loa Observatory in Hawaii, where the peak UVI and daily dose are much higher.

The app has a couple of cute educational features too.

First, there’s an interactive ‘UV Map’ page that shows the current distribution of UVI over the entire globe. You can also see how that pattern changes with month of the year and time of day. It’s pretty cool. I sometimes think the phone might get overheated with the number of calculations involved, but it doesn’t seem to overly drain the battery 😊.

Finally, there is an aptly named ‘Geeks’ screen, written by geeks for the use of geeks. You geekily can play around there to your heart’s content to learn more about what makes the UV tick.

Thinking of taking a holiday abroad or moving to another region? Wherever that might be, if you’re concerned about skin damage, the app lets you to check out the UVI there today, its expected summer peak, and how it varies with seasons. What more could you ask for?

If you want to know more about the physics behind the app, just follow this link.

Enjoy.

As always, the app is free. It’s available for iPhone here and for Android here.

Tell your friends.

PS. Sorry about the incorrect link to Greg Bodeker’s 1996 paper in my last post. The correct link has now been added.