Springtime UV Dilemma?
Resolved (hopefully) ...
When is a spring not a spring? Some clarification is needed to my last post. A colleague (thanks Sasha) reminded me that for most people (at least in the USA, and therefore promoted by Google), spring is taken as starting at the spring equinox: September 21 in the Southern Hemisphere and March 21 in the North. But there’s not universal agreement. Others, including meteorologists and mainstream New Zealand media, give the start date as the first day of the month that the equinox occurs, on September 1 in the south and March 1 in the north. The same duplicate conventions apply for other seasons. I’ve just learnt that our New Zealand Maori have yet another definition. Their lunar calendar had spring starting this year on 27 August. That date will obviously vary from year-to-year depending on the phase of the moon. No doubt other cultures have their own definitions.
In my post I confused things even more by drawing a plot showing spring centered at the spring equinox and so on. Using the more usual definitions, my broad-brush statement from last week isn’t strictly true. What remains true is that near the spring equinox, sunburning UV in southern New Zealand (Lauder) can be double that at the autumn equinox.
An explanatory note. Some could call it an excuse. To all intents and purposes, we can think of the spring equinox as:
the date that the hours of daylight after the winter solstice first exceeds 12 hours, and
the date when peak daily sun elevation angle is changing most rapidly, and therefore
the date when the daily solar radiation (including UV) is increasing most rapidly), and
the date when Sun’s angle from zenith direction (i.e., the SZA = 1 - Elevation Angle) first becomes larger than the latitude angle
Where I live near 45S, the spring equinox is the day the Sun first rises above 45 degrees elevation at noon, and at the South Pole it’s the day it first peeps above the horizon after the ‘polar night’ that began six months earlier - at the autumn equinox.
In terms of UV exposure, I think of ‘summer’ as the period between the spring equinox and the autumn equinox - centered on the summer solstice. And ‘winter’ as the corresponding period centered on the winter solstice. That’s essentially what I showed in the figure, except I further split out equal periods which I called ‘spring’ and ‘autumn’ between. But there’s a lag of several weeks between this solar forcing and daily mean temperatures at the Earth’s surface. That’s why the official seasons - as defined above - are delayed by 3 to 6 weeks, depending on which definition is used. I just confused things more by introducing another definition!
Sorry about that. I’ve modified that last post accordingly. It would have been far less confusing if instead of inventing those non-standard ‘seasons’, I’d just labelled my figure by the dates of the equinoxes and solstices. I’ve done that in the version below. I’ve also included the corresponding plot for Boulder Colorado at latitude 40N. It shows the same spring-autumn asymmetry and is probably easier to comprehend with “summer” near the centre of the picture.
Apart from that obvious phase-difference between the two sites, the absolute UVI values are rather different. The winter UVI is much greater in Boulder than at Lauder, primarily due to its lower latitude (40N) compared with Lauder (45S), which means that the solar elevations in winter are 5 degrees larger. It’s also about 1.3 km higher above sea level (and in clean air UVI increases by about 5 percent per kilometer). But despite those factors, the peak summertime UVI is still significantly larger at Lauder compared with Boulder, for reasons that have been discussed previously. In both cases - but more so for Lauder with its larger seasonal variation in ozone - the UVI is significantly larger at the autumn equinox than the spring equinox.
The bottom line is that at near the spring equinox, when the daily rate of increase in UV is its largest and our skin are skins are at their most sensitive, the higher seasonal ozone amounts keep UV levels much lower than at the autumn equinox when the skin is better sun-adapted through natural tanning.
That’s for everybody except for those living in Antarctica. More on that next week …
But take care. Even at the spring equinox, the UVI can comfortably exceed the alert threshold of 3, even at the southernmost tip of New Zealand. And it increases rapidly going into the summer. It can exceed 14 around the middle of the day near the summer solstice in the north. Use one of our free smartphone apps (UVNZ, or GlobalUV) to be sure of the risk and what you should do about it at any place, date, or time. UVNZ works only in New Zealand but has more detailed cloud forecasts, while GLOBALUV (no spaces there) works anywhere in the world.