Ozone is a UV filter you cannot see and cannot feel. A thinner column lets more UVB through. The sky can look like yesterday’s sky. The index does not have to.

That is a different lever from heat, and a different one from altitude. Cool air is infrared. Thin mountain air is less atmosphere. Column ozone is a gas overhead. None of those write a sensation on your skin while the dose is arriving.

A filter you cannot see

WHO’s split is blunt: all UVC and most UVB is absorbed by ozone (and other gases) before it reaches the ground. UVA is not filtered as significantly. The UV index is an erythemal meter—weighted toward the burn band that ozone actually blocks. When the column thins, the index rises, especially the UVB slice.

How much, if nothing else changes:

• UNEP’s Environmental Effects Assessment Panel: for a small ozone drop, the UV index rises about 1.1% for each 1% less ozone (radiation amplification factor for erythema ≈ 1.1). Later assessments and McKenzie et al. put that near 1.2% at high sun.
• NOAA’s 2022 ozone-assessment FAQ: a 50% decline in total ozone is associated with about a factor of two increase in the UV index. Same sun elevation. Cloud-free. The column is the variable.

A 10% thinner column is then on the order of an 11–12% higher index. That is the same order as WHO’s altitude step of about 10–12% per 1,000 m. You feel neither as heat.

NASA’s picture of how little gas this is: squeeze the typical column to sea-level pressure and it is about 300 Dobson units—3 mm, the height of two stacked pennies. The Antarctic ozone hole averages about 100 DU, a dime’s thickness. The everyday mid-latitude swing is not a hole. It is still invisible.

Same sun height, different ozone

Sun angle still dominates the day. Ozone moves the number at that angle.

McKenzie and colleagues hold solar zenith angle fixed and watch the column. At mid-latitudes the year’s minimum ozone can be about half the year’s maximum—lowest in autumn, highest in spring. At the same sun height, autumn UV-index values can then nearly double spring. The disc looks the same height in the sky. The filter does not.

Two other maps, same rule:

• Tropics. EPA: ozone is naturally thinner there than at mid- and high latitudes. NOAA: that low column plus a high midday sun is why the largest index values sit in the tropics. You do not get a warning glow from the missing millimetres.
• Polar spring. NOAA: at Palmer Station (64°S), spring peaks after the ozone hole developed can equal or exceed spring peaks in San Diego (32°N)—a much lower latitude. The hole is the loud version of the same fact. It is not the only version.

WHO: ozone levels vary over the year and even across the day. EPA: thickness also moves with weather patterns. Yesterday’s peak is not a measurement of today’s column.

Southern-summer contrast—closer sun, seasonal ozone, cleaner air—is a different stack: same latitude is not the same UV. Climbing a ridge leaves air below you: higher elevation means stronger UV. A thin ozone column can sit over a beach. This piece is the gas over your head this week.

The trap is reading the sky

Three everyday misses:

1. "It looks like yesterday." Blue is visible light. UVB is not. NOAA: UV-B is invisible to the human eye. Matching skies do not mean matching columns.
2. "I would feel it if the sun were worse." You feel infrared. UV peaks before the heat is that lag. Ozone does not close it. A mild morning under a thin column is still a UV morning.
3. "Ozone alert" as a UV cue. Ground-level ozone is the smog pollutant in the troposphere. UNEP: about 10% of atmospheric ozone sits there; about 90% is the stratospheric filter. EPA treats those as different gases for different jobs. An air-quality ozone warning is not a UV forecast. A day without one is not an all-clear.

Do not try to guess Dobson units from the color of the air. The live UV index is already the erythemal rate for this location. Ozone is inside that number. It is not a second meter you can eyeball.

Read today’s peak

Use the homepage the way how to use a live UV index describes—current, today’s peak, next few hours:

1. Take today’s peak, not last weekend’s, not the sky’s hue. A High or Extreme peak is still cover weather if the breeze is easy.
2. Recheck the next few hours when a front is moving. Column ozone travels with weather. So does cloud. Neither writes a feeling on your arms.
3. Do not import a remembered number into the tropics, or into a high-latitude spring, because the sun “looks the same height.” The filter may not be.
4. Feed the SPF calculator today’s index, not a typical-for-this-month guess. The calculator multiplies skin type by the rate you give it. A stale rate hides the column.
5. Keep altitude and ozone separate. A high trail plus a thin column stacks. Cool air still explains neither.

The millimetres overhead do not announce themselves. The index does.

Sources

WHO on ozone absorbing most UVB, UVA passing more freely, and ozone varying over the year and even across the day: Radiation: ultraviolet (UV). Erythemal RAF ≈ 1.1 (1% less ozone → ~1.1% higher UV index) is the UNEP Environmental Effects Assessment Panel, 2002. 50% less ozone → about 2× UV index, UV-B as invisible, tropics as low-ozone + high sun, and Palmer vs San Diego spring: NOAA / WMO–UNEP Twenty Questions Q16 (2022). Mid-latitude ozone min ≈ half the year’s max; autumn UVI at the same sun height can nearly double spring: McKenzie et al., 2022 (RAF for erythema ~1.2 in that paper). Typical column ~300 DU (3 mm) and hole ~100 DU: NASA Ozone Watch, Dobson Unit facts. Ozone thinner in the tropics, thickness moving with season and weather, UVA vs UVB vs the ozone layer, and tropospheric “bad” ozone vs the stratospheric filter: EPA, The Sun, UV, and You. About 90% of ozone in the stratosphere, 10% in the troposphere: UNEP EEAP FAQ, 2010 (Q1). Altitude’s 10–12% per 1,000 m is WHO UV-index Q&A—a different mechanism, cited only as a size check. Those percentages are all-else-equal planning ranges, not a forecast for your street.