We’re rapidly gaining daylength in Colorado... and we’ll get plenty of it with high pressure in place this week. With that gain, we’re also seeing jumps in our peak UV index, which is now peaking regularly at an eight for the first time this year.
DAYLIGHT GAINED IN COLORADO IN APRIL
Colorado gains 65-75 minutes of daylight in April. The biggest gain is in the northern part of the state and least in the south (this is a typical variance trend when talking about day length variability year-round: northerly latitudes gain and lose more light than southerly latitudes). April's gain is Colorado's second biggest of the year after March. By the end of the month, sunset in Colorado Springs will be at 7:51 PM.
SOLAR ELEVATION AND UV INDEX
At the same time that day length is increasing, solar altitude is too. Altitude is an astronomy term that refers to the angle of a celestial body - in this case the Sun - above the horizon. At mid-day, the sun rises to an altitude of 56 degrees on April 1st in Colorado Springs. On April 30th it’s up to 66 degrees.
Solar altitude is not just an esoteric factoid. It's very important specifically for the amount of UV-B light reaching the ground. The amount of UV-B reaching the ground depends on the sun's altitude. The common statistic is 50 degrees. This is not fully accurate, but it's a fair approximation. Below this angle, the atmosphere largely absorbs UV-B radiation. This has human health implications - both good and bad.
SOLAR RADIATION BASICS
The sun produces visible and "invisible" light. More technically: it produces light we can see, and light we cannot see. UV rays are in the invisible light spectrum. They contain a lot of energy. Because they contain a lot of energy, they are able to damage and burn our skin. They can also provide energy to the skin which can produce chemical reactions.
There are three types of UV rays: A, B, and C. The ozone layer absorbs most of the UV-C rays. UV-B rays are the most affected by changes in our sun angle. The higher the sun is in the sky, the more UV-B rays pass through. UV-B rays are the ones that cause sunburn. They're also the only type of solar radiation that produces a chemical reaction in your skin that produces Vitamin D. The U.S. National Institutes of Health indicate that over a third of the U.S. adult population has a deficiency of Vitamin D. This is largely due to a lack of sun exposure. Critically though: even getting adequate sun exposure will not produce vitamin D, if the sun is not high enough in the sky.
UV-A rays pass easily through clouds, the ozone layer, and will still be present at low sun angles in winter. UV-A rays cause skin aging, wrinkles, and can contribute to the development of skin cancer - less so than UV-B or UV-C however.
COLORADO'S ELEVATION IMPACT ON UV RADIATION EXPOSURE
This is where Colorado's elevation comes in. The atmosphere absorbs UV light. At higher elevations, there is lower air pressure. In other words, there is less atmosphere between the ground, and space.
With less atmosphere to block UV rays, we get more UV radiation for the same sun angle than our sea-level friends do.
According to the Environmental Protection Agency, UV radiation increases 2 percent for every 1,000 feet of elevation gain. In Colorado Springs, that makes the UV level about 12% higher than at sea level. In Pueblo, it's about 10%. And on top of Pikes Peak, it’s nearly 30% higher!
This also has an impact on UV-B radiation. The typical statistic of 50 degrees at sea level is not fully accurate, but as sun angle decreases, the distance light travels through the sky before reaching your skin increases exponentially. As a result, UV-B rapidly declines the lower you go below 50 degrees. It's effectively zero by around 30-35 degrees.
In Colorado more UV, including UV-B light passes through the atmosphere for the same solar angle compared to sea level. So - on the positive side, Coloradans can produce meaningful Vitamin D for slightly longer than our sea level peers in the shoulder months of the year. On the negative side - this increases risk of negative impacts from the sun like sunburn and skin cancer.
THE UV INDEX SCALE
We classify the amount of UV hitting the ground using the UV index scale. The UV index scale technically has no limit, but we classify it from one to eleven-plus. Anything above 11 is considered extreme. The scale is based on impacts to our skin. At a UV index of 6 or 7, burn times on unprotected skin are about 20 minutes. At an 8, it can take as little as 10 minutes. The UV index doesn't specifically measure UV-B. It measures the combined intensity of UV-A and UV-B. However, as discussed above - UV-A passes through the atmosphere largely unimpeded. Therefore, it remains fairly constant throughout the year. UV-B is highly variable. Therefore the UV index largely quantifies the amount of UV-B light reaching the ground and thus the potential to produce vitamin D - and get sunburned.
The UV index corresponds to sunburn risk. For example, a 6 has twice the sunburn potential of a 3. The higher the number, the faster you can get burned.
UV INDEX FORECAST
So let's talk about the UV index this week. UV is affected by cloud cover. Mainly by thick clouds. Thin clouds still let UV rays pass through.
On Tuesday, notice the highest index values are over the mountain peaks. There will be some clouds over the plains.
On Wednesday, the index peaks at eight along I-25 and parts of the southeast plains.
Thursday, which should be almost completely sunny, sees that index eight cover all of southern Colorado - and the vast majority of the state writ large.
We won't cover how sunscreen works in this report. But, here's the upshot: if you’re getting outside to enjoy the great weather this week and will be out for awhile, pack SPF 30+ (or ideally 50+) sunscreen to protect your skin from the high UV index.
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