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Los Angeles county is at the top of the list for natural hazards

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The number of days over 95 degrees is rising fast in Los Angeles!!

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The number of days over 95 degrees is rising fast in Los Angeles!!

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Scientists agree that our climate is changing and that humans are responsible. Climate change is more than just warming temperatures. Throughout Los Angeles County, our state, and the world, we're already seeing the impacts of climate change, including severe storms, temperature extremes, and drought. In LA County, hotter, longer heat waves caused by climate change are one of the major threats to the health and wellbeing of our communities.

All across Los Angeles, we're feeling the effects of climate change, like more very hot days and heat waves later in the summer. Scientists predict that climate change will continue to cause even more extreme heat in the future. Coastal areas and central Los Angeles will experience three times more days of temperatures over 95°F, and the San Fernando and San Gabriel Valleys will have even more extremely hot weather.

This chart developed by UCLA shows predictions for the number of days over 95°F in dark orange (as compared to the current number of days in light orange) if we don't take action to slow climate change:

Figure 1. UCLA Institute of the Environment and Sustainability, Center for Climate Science

Read more about the climate science

See what DPH is doing to reduce temperatures in our neighborhoods.

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Los Angeles County Department of Public Health

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Scientists agree that our climate is changing and that humans are responsible. Climate change is more than just warming temperatures. Throughout Los Angeles County, our state, and the world, we're already seeing the impacts of climate change, including severe storms, temperature extremes, and drought.

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Ultraviolet (UV) radiation is a type of light. Sunlight has UV rays, along with other kinds of rays. Some light bulbs give off UV rays. UV light bulbs are used in tanning machines, some nail dryers, machines used by dermatologists, and more.

Types of UV rays

There are 3 types of UV rays:

UVA. These rays go into the skin more deeply than UVB rays. These play a major part in skin aging and wrinkling. They also contribute to the growth of skin cancer.

UVB. These rays are the main cause of sunburn. They tend to damage the skin's outer layers. These rays play a key role in the growth of skin cancer.

UVC. These rays don't reach our skin. The Earth's atmosphere absorbs UVC rays before they reach the surface.

How UV rays affect the skin

In most cases, UV rays react with a chemical in the skin called melanin. This is the first defense against the sun. Melanin absorbs the dangerous UV rays that can do serious skin damage. But melanin can't absorb all the UV rays, and some people don't have much melanin in their skin. Exposure to UV rays is linked to harmful health conditions such as:

Sunburn. A sunburn happens when the amount of UV rays exceeds the protection that the skin's melanin can provide. Sunburn is damage to the skin. It causes pain, redness, and blistering.

Premature aging of the skin. Premature aging of the skin is also called photoaging. It causes the skin to become thick and leathery over time. Signs of photoaging include earlier-than-normal freckling, wrinkling, loss of collagen, and widening of small blood vessels in the skin. These changes can happen earlier and more quickly in people who sit out in the sun regularly. The skin may also develop brown spots (liver spots) in later years.

Skin cancer. Millions of people are diagnosed with skin cancer in the U.S. each year. Skin cancer is more common as people get older. But skin damage from the sun begins at an early age. Protection should start in childhood to prevent skin cancer later in life.

Cataracts and other eye problems. UV radiation exposure increases the risk of cataracts. With this condition, the lens of the eye becomes cloudy and it becomes hard to see. If not treated, cataracts can lead to blindness over time. Other changes that can decrease vision are also possible.

What is the UV Index?

The UV Index is an official forecast from the National Weather Service. It estimates how much ultraviolet radiation will reach the Earth's surface for most ZIP codes across the U.S. This information can help you plan outdoor events and plan your level of sun exposure. It also gives you tips on how to help prevent sunburns. The UV Index also includes the effects of cloud cover on the forecast UV level. It notes the risk of overexposure to the sun's UV rays on a scale from 0 (low) to 11+ (extreme). The World Health Organization (WHO) uses a system of colors that correspond to UVI level.

Understanding the UV Index

Staying safe in the sun

For many people, a small amount of exposure to sunlight is fine. But too much can be dangerous. Keep track of the UV Index. Protect your skin with clothing and sunscreen. Take extra care around sand, water, and snow. They all reflect UV rays and give you more exposure. These steps will help you reduce your risks of cancer, premature aging of the skin, and other harmful effects.

Date Last Reviewed: 4/1/2019

© 2000-2021 The StayWell Company, LLC. All rights reserved. This information is not intended as a substitute for professional medical care. Always follow your healthcare professional's instructions.

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Ultraviolet (UV) Radiation and Skin Cancer

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Ultraviolet (UV) radiation is a type of light. Sunlight has UV rays, along with other kinds of rays. Some light bulbs give off UV rays. UV light bulbs are used in tanning machines, some nail dryers, machines used by dermatologists, and more. There are 3 types of UV rays: UVA.

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The ultraviolet index, or UV index, is an international standard measurement of the strength of sunburn-producing ultraviolet (UV) radiation at a particular place and time. The scale was developed by Canadian scientists in 1992, and then adopted and standardized by the UN's World Health Organization and World Meteorological Organization in 1994. It is primarily used in daily forecasts aimed at the general public, and is increasingly available as an hourly forecast as well.

The UV index is designed as an open-ended linear scale, directly proportional to the intensity of UV radiation that causes sunburn on human skin. For example, if a light-skinned individual (without sunscreen) begins to sunburn in 30 minutes at UV index 6, then that individual should expect to sunburn in about 15 minutes at UV index 12 - twice the UV, twice as fast.

The purpose of the UV index is to help people effectively protect themselves from UV radiation, which has health benefits in moderation but in excess causes sunburn, skin aging, DNA damage, skin cancer, immunosuppression, and eye damage, such as cataracts (see the section Human health-related effects of ultraviolet radiation). Public health organizations recommend that people protect themselves (for example, by applying sunscreen to the skin and wearing a hat and sunglasses) if they spend substantial time outdoors when the UV index is 3 or higher; see the table below for more detailed recommendations.

The recommendations given are for average adults with lightly tanned skin. Those with darker skin are more likely to withstand greater sun exposure, while extra precautions are needed for children, seniors, particularly fair-skinned adults, and those who have greater sun sensitivity for medical reasons or from UV exposure in previous days.

The UV index is a linear[clarification needed ] scale, with higher values representing a greater risk of sunburn (which is correlated with other health risks) due to UV exposure. An index of 0 corresponds to zero UV radiation, as is essentially the case at night. An index of 10 corresponds roughly to midday summer sunlight with a clear sky when the UV index was originally designed; now summertime index values in the tens are common for tropical latitudes, mountainous altitudes, and areas with above-average ozone layer depletion.

While the UV index can be calculated from a direct measurement of the UV spectral power at a given location, as some inexpensive portable devices are able to approximate, the value given in weather reports is usually a prediction based on a computer model. Although this may be in error (especially when cloud conditions are unexpectedly heavy or light), it is usually within ±1 UV index unit as that which would be measured.

When the UV index is presented on a daily basis, it represents UV intensity around the sun's highest point in the day, called solar noon, halfway between sunrise and sunset. This typically occurs around 12:30, or around 13:30 in areas where daylight saving time is being observed. Predictions are made by a computer model that accounts for the effects of Sun elevation and distance, stratospheric ozone, cloud conditions, air pollutants, surface albedo, and ground altitude, all of which influence the amount of UV radiation at the surface. The calculations are weighted in favor of the UV wavelengths to which human skin is most sensitive, according to the CIE-standard McKinlay-Diffey erythemal action spectrum. The resulting UV index cannot be expressed in pure physical units, but is a good indicator of likely sunburn damage.

Because the index scale is linear (and not logarithmic, as is often the case when measuring things such as brightness or sound level), it is reasonable[vague ] to assume that one hour of exposure at index 5 is approximately equivalent to a half-hour at index 10.[citation needed ]

The UV index is a number linearly related to the intensity of sunburn-producing UV radiation at a given point on the earth's surface. It cannot be simply related to the irradiance (measured in W/) because the UV of greatest concern occupies a spectrum of wavelength from 295 to 325 nm, and shorter wavelengths have already been absorbed a great deal when they arrive at the earth's surface. Skin damage from sunburn, however, is related to wavelength, the shorter wavelengths being much more damaging. The UV power spectrum (expressed as watts per square metre per nanometre of wavelength) is therefore multiplied by a weighting curve known as the erythemal action spectrum, and the result integrated over the whole spectrum. This gave Canadian scientists[who? ] a weighted figure (sometimes called Diffey-weighted UV irradiance, or DUV, or erythemal dose rate) typically around 250 mW/m ² in midday summer sunlight. So, they arbitrarily divided by 25 mW/m ² to generate a convenient index value, essentially a scale of 0 to 11+ (though ozone depletion is now resulting in higher values, as mentioned above).

To illustrate the spectrum weighting principle, the incident power density in midday summer sunlight is typically 0.6 mW/(nm m ²) at 295 nm, 74 mW/(nm m ²) at 305 nm, and 478 mW/(nm m ²) at 325 nm. (Note the huge absorption that has already taken place in the atmosphere at short wavelengths.) The erythemal weighting factors applied to these figures are 1.0, 0.22, and 0.003 respectively. (Also note the huge increase in sunburn damage caused by the shorter wavelengths; e.g., for the same irradiance, 305 nm is 22% as damaging as 295 nm, and 325 nm is 0.3% as damaging as 295 nm.) Integration of these values using all the intermediate weightings over the full spectral range of 290 nm to 400 nm produces a figure of 264 mW/m ² (the DUV), which is then divided by 25 mW/m ² to give a UV index of 10.6.

After sporadic attempts by various meteorologists to define a "sunburn index", and amid growing concern about ozone depletion, Environment Canada scientists James B. Kerr, C. Thomas McElroy, and David I. Wardle invented the modern UV index in Toronto, Ontario. Environment Canada launched it as part of the weather forecast on May 27, 1992, making Canada the first country in the world to issue official predictions of UV levels for the next day. Many other countries followed suit with their own UV indices, among them the United States in 1994. Initially, the methods of calculating and reporting a UV index varied significantly from country to country. A global UV index, first standardized by the World Health Organization and World Meteorological Organization in 1994, gradually replaced the inconsistent regional versions, specifying not only a uniform calculation method (the Canadian definition) but also standard colors and graphics for visual media. In the United States, the WHO standards officially replaced the original US standards in 2004.

On December 29, 2003, a world-record ground-level UV index of 43.3 was detected at Bolivia's Licancabur volcano, though other scientists dispute readings higher than 26.

In 2005, the United States and Australia launched the UV Alert. While the two countries have different baseline UV intensity requirements before issuing an alert, their common goal is to raise awareness of the dangers of over-exposure to the Sun on days with intense UV radiation.

In 2007, the United Nations honored UV index inventors Kerr, McElroy and Wardle with the Innovators Award for their far-reaching work on reducing public health risks from UV radiation. In the same year, a survey among meteorologists ranked the development of the UV index as #11 for The Weather Channel's 100 Biggest Weather Moments.

When the day's predicted UV index is within various numerical ranges, the recommendations for protection are as follows:

UV index

Media graphic color

Risk of harm from unprotected sun exposure, for the average adult

Recommended protection

0 to 2

"Low"

A UV index reading of 0 to 2 means low danger from the Sun's UV rays for the average person.

Wear sunglasses on bright days. If you burn easily, cover up and use broad spectrum SPF 30+ sunscreen. Bright surfaces, sand, water, and snow, will increase UV exposure.

3 to 5

"Moderate"

A UV index reading of 3 to 5 means moderate risk of harm from unprotected sun exposure.

Stay in shade near midday when the Sun is strongest. If outdoors, wear sun-protective clothing, a wide-brimmed hat, and UV-blocking sunglasses. Generously apply broad spectrum SPF 30+ sunscreen every 1.5 hours, even on cloudy days, and after swimming or sweating. Bright surfaces, such as sand, water, and snow, will increase UV exposure.

6 to 7

"High"

A UV index reading of 6 to 7 means high risk of harm from unprotected sun exposure. Protection against skin and eye damage is needed.

Reduce time in the sun between 10 a.m. and 4 p.m. If outdoors, seek shade and wear sun-protective clothing, a wide-brimmed hat, and UV-blocking sunglasses. Generously apply broad spectrum SPF 30+ sunscreen every 1.5 hours, even on cloudy days, and after swimming or sweating. Bright surfaces, such as sand, water, and snow, will increase UV exposure.

"Very high"

A UV index reading of 8 to 10 means very high risk of harm from unprotected sun exposure. Take extra precautions because unprotected skin and eyes will be damaged and can burn quickly.

Minimize sun exposure between 10 a.m. and 4 p.m. If outdoors, seek shade and wear sun-protective clothing, a wide-brimmed hat, and UV-blocking sunglasses. Generously apply broad spectrum SPF 30+ sunscreen every 1.5 hours, even on cloudy days, and after swimming or sweating. Bright surfaces, such as sand, water, and snow, will increase UV exposure.

11+

"Extreme"

A UV index reading of 11 or more means extreme risk of harm from unprotected sun exposure. Take all precautions because unprotected skin and eyes can burn in minutes.

Try to avoid sun exposure between 10 a.m. and 4 p.m. If outdoors, seek shade and wear sun-protective clothing, a wide-brimmed hat, and UV-blocking sunglasses. Generously apply broad spectrum SPF 30+ sunscreen every 1.5 hours, even on cloudy days, and after swimming or sweating. Bright surfaces, such as sand, water, and snow, will increase UV exposure.

1. ^ Hanneman, K. K.; et al. (January 2006). "Ultraviolet immunosuppression: Mechanisms and consequences". Dermatologic Clinics. 24 (1): 19-25. doi:10.1016/j.det.2005.08.003. PMID 16311164.

2. Dresbach, Sereana Howard & Brown, Wanda (2008). "Ultraviolet Radiation" (PDF). The Invisible Environment Fact Sheet Series. The Ohio State University. CDFS-199-08. Archived from the original (PDF) on January 17, 2009.

3. Fioletov, V.; et al. (July-August 2010). "The UV index: Definition, distribution and factors affecting it". Canadian Journal of Public Health. 101 (4): I5-I9. doi:10.1007/BF03405303. PMC . PMID 21033538.

4. ^ Engelsen, Ola & Kylling, Arve (April 2005). "Fast simulation tool for ultraviolet radiation at the earth's surface". Optical Engineering. 44 (4). 041012. doi:10.1117/1.1885472.

5. McKinlay, A. F. & Diffey, B. L. (1987). "A reference action spectrum for ultraviolet induced erythema in human skin". CIE Journal. 6 (1): 17-22.

6. "UV Spectral Irradiances & Erythemal Action Spectrum". NOAA. 2006.

7. ^ Kerr, J. B.; et al. (1994). "The Canadian Ozone Watch and UV-B advisory programs". Ozone in the Troposphere and Stratosphere, Part 2: Proceedings of the Quadrennial Ozone Symposium 1992. NASA/Goddard Space Flight Center: 794-797. N95-11093.

8. ^ Report of the WMO Meeting of Experts on UV-B Measurements, Data Quality and Standardization of UV Indices, 1994. Global Atmosphere Watch (Report). World Meteorological Organization. 1995. WMO/TD-No. 625.

9. ^ "Global Solar UV Index: A Practical Guide" (PDF). World Health Organization. 2002.

10. ^ Oskin, Becky (July 8, 2014). "Blazing World Record: Strongest UV Rays Measured in South America". LiveScience.com.

11. ^ Cabrol, Nathalie A.; et al. (July 8, 2014). "Record solar UV irradiance in the tropical Andes". Frontiers in Environmental Science. 2. 19. doi:10.3389/fenvs.2014.00019.

12. ^ McKenzie, Richard L.; et al. (April 8, 2015). "Comment on "Record solar UV irradiance in the tropical Andes, by Cabrol et al."". Frontiers in Environmental Science. 3. 26. doi:10.3389/fenvs.2015.00026.

13. ^ "What is UV?". SunSmart. Cancer Council Victoria. Archived from the original on January 26, 2016.

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The ultraviolet index, or UV index, is an international standard measurement of the strength of sunburn-producing ultraviolet (UV) radiation at a particular place and time. The scale was developed by Canadian scientists in 1992, and then adopted and standardized by the UN's World Health Organization and World Meteorological Organization in 1994.

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Climate Change is Fueling Southern CA Fires!

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Climate Change is Fueling Southern CA Fires!

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Institute of the Environment and Sustainability at UCLA

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By Alison Hewitt and Karen Lefkowitz
Soaring temperatures triggered by a serious increase in greenhouse gases in the atmosphere will intensify wildfires by the middle of this century in Southern California according to a new study by researchers from three University of California campuses and partner institutions. IoES' Alex Hall and other colleagues from UCLA, along with scientists from UC Davis, UC Irvine, the U.S. Forest Service, and NASA's Jet Propulsion Laboratory, teamed up to quantify the drivers and impacts of wildfire in the current climate, and to project future impacts given climate change. Their paper, "Identification of Two Distinct Fire Regimes in Southern California: Implications for Economic Impact and Future Change," was published on September 4 in the scientific journal Environmental Research Letters. The projections revealed in the study can help fire agencies better prepare for coming seasons of more frequent, stronger blazes. Hall spoke with UCLA Newsroom's Alison Hewitt on the relevance of these findings. Q: What will "fuel" the increased burning you're projecting in Southern California by mid-century?
Alex Hall: There are two types of fires. The Santa Ana-driven fires happen mainly in the fall, after the summertime dryness and before the rain, when the vegetation is very vulnerable to fire. The very low moisture of the Santa Ana winds has a desiccating effect on the vegetation, drying it out still further. By mid-century, climate change will have driven down the relative humidity of the Santa Ana winds even more, increasing their ability to spread fire.

The other type of fires is the non-Santa Ana fires, which are much more common in the summertime. They're driven not so much by the wind but by the extreme heat, which makes the vegetation much more susceptible to ignition.
Q: What did your lab provide for the study?
Alex Hall: My lab provided all the current climate data for the fire models, and we generated the future-climate predictions by downscaling global climate models.

Our climate predictions assume a "business as usual" scenario, where there are no significant reductions in greenhouse gas emissions. However, in previous studies, we've found that even when we use a scenario of significant reduction in greenhouse gases, climate models show mid-century warming increases to about 70 percent of the "business as usual" scenario. We can infer that even if we mitigate climate change, the growth in fire damage would be smaller, but not dramatically so.

"Even if we mitigate climate change, the growth in fire damage would be smaller, but not dramatically so."

Q: Why do fires driven by the Santa Anas seem to cause more economic damage than others? And why do you think other wildfires will start to cause similar levels of damage?
Alex Hall: The Santa Ana fires are much larger currently, and tend to spread into the urban-wildland interface. Non-Santa Ana fires tend to occur in places like the National Forest, where there are fewer structures. But as they get bigger, they could start to intrude into inhabited areas.

We found that non-Santa Ana fires will play a larger role in the overall fire risk for Southern California. Those are the types of fires that will change more in the future, and we will become more vulnerable to the non-Santa Ana types of fires. This is a new way of seeing fire and fire risk in Southern California, and in a sense it extends the fire season. Right now, Santa Anas really dominate the sense of there being a "fire season." The non-Santa Anas will come to really dominate in the summer.
Q: Why is this research important? Why should people care about how wildfires could grow?
Alex Hall: Wildfires are obviously very destructive of property. We live in a region where people have built into the wild, and that urban-wildland interface is vulnerable to fire.

These wildfires have an impact on people and on our water resources. For example, the 2009 Station Fire deposited a large amount of ash, and when the rains came, a lot of it got washed into the reservoirs. There's still a huge build-up of ash in these reservoirs, making it difficult to use the water.

Finally, wildfire is a really critical element of the ecosystem in Southern California. It's a natural aspect of the ecology, and a change in wildfire leads to a change in the ecology. So if we care about conserving our natural resources, we have to understand how these changes in wildfires would impact our ecosystems.

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Alex Hall: How Climate Change is Fueling Southern California Wildfires

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By Alison Hewitt and Karen Lefkowitz Soaring temperatures triggered by a serious increase in greenhouse gases in the atmosphere will intensify wildfires by the middle of this century in Southern California according to a new study by researchers from three University of California campuses and partner institutions.

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Projections of the rising temperture in Los Angeles

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Projections of the rising temperture in Los Angeles

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Institute of the Environment and Sustainability at UCLA

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Research Project | 2015

The most comprehensive study of climate change in LA to date, the Climate Change in the Los Angeles Region Project was conducted by Center for Climate Science Faculty Director Alex Hall and his research group between 2010 and 2015. Dr. Hall and his team developed a novel method for bringing global climate model projections to high spatial resolution, creating neighborhood-by-neighborhood projections of future climate over the greater Los Angeles region under different scenarios of greenhouse gas emissions.

Context

People making decisions about adapting to future climate change impacts and preventing further climate change need to know how climate change will affect their communities. But until recently, the most robust climate change research has focused on large regions the globe, and information about climate change in our own backyards has been hard to come by.

The scientific challenge

The best tools scientists have for projecting future climate are global climate models, state-of-the-art computer models that simulate the climate system. While global climate models are incredibly useful for understanding climate change on global and continental scales, they are too low in spatial resolution to help us understand climate impacts on smaller scales, particularly in areas like the Los Angeles region, whose complex topography creates microclimates that a global climate model cannot account for. Various techniques exist to "downscale" global climate models to finer spatial resolutions, but they have limitations: Those that are physically realistic are highly computationally expensive, limiting how many different global climate model outcomes can be considered in any particular study, whereas those that are computationally efficient are not necessarily physically realistic.

The breakthrough

For this project, Dr. Hall and his team developed a novel "hybrid downscaling" technique that combines the strengths of the various downscaling methods while avoiding their limitations. The researchers used this technique to downscale about three dozen latest-generation global climate models at 1.2-mile resolution over the greater Los Angeles region. Focusing on two future periods, 2041-2060 and 2081-2100, they analyzed changes in various aspects of climate-temperature, extreme heat, precipitation, snowfall, and runoff from precipitation in the region's mountains-under two different scenarios of greenhouse gas emissions. The "business as usual" scenario represents a continued rise in emissions of heat-trapping greenhouse gases, and the "mitigation" scenario represents aggressive action to curb emissions over the coming decades.

Key findings

At mid-century under the Business As Usual scenario, average temperatures over the region's land areas rise by 4.3°F, compared with a reference period of 1981-2000.

Warming is not uniform across the LA region. Valleys and inland areas warm more than areas near the coast.

The number of days hotter than 95°F increases across the region, but to a greater extent in the interior compared with coastal areas.

At mid-century, temperature changes in the Mitigation scenario are 70% of those in Business As Usual scenario. That warming is doesn't differ greatly between the two scenarios means significant effects of climate change are inevitable.

At the end of the century, there's a much larger difference between the two scenarios. In the Mitigation scenario, temperatures level off, and by end-century, average temperatures are about 3°F warmer than in 1981-2000. Under Business As Usual, end-century average temperatures will be 8.2°F warmer than they were in 1981-2000. This stark difference indicates that global action to reduce greenhouse gas emissions would have significant benefits.

Average annual precipitation totals do not change significantly in either time period or scenario. (Note: Further studies are required for a holistic analysis of precipitation changes. In California, precipitation varies greatly from year to year, so changes to the average are just one part of the story. Other projects by the Center for Climate Science are assessing changes to the distribution of precipitation events and the effects of climate change on drought.)

Because temperature increases cause a greater share of winter precipitation to fall as rain instead of snow, snowfall in the region's mountains will be reduced. At mid-century under Business As Usual, elevations below about 6,500 feet lose half their snowfall compared with 1981-2000, while higher elevations lose up to 30%. At the end of the century under Business As Usual, lower elevations stand to lose 80% of the snowfall they received in 1981-2000.

More about the project

The Climate Change in the Los Angeles Region project was commissioned by the City of Los Angeles in partnership with the Los Angeles Regional Collaborative for Climate Action, with support from the US Department of Energy. Additional funding was provided by the National Science Foundation and the Southwest Climate Science Center.

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Climate Change in the Los Angeles Region

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Research Project | 2015 The most comprehensive study of climate change in LA to date, the Climate Change in the Los Angeles Region Project was conducted by Center for Climate Science Faculty Director Alex Hall and his research group between 2010 and 2015.

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Los Angeles Rainfall varies extremely from year to year

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Los Angeles Rainfall varies extremely from year to year

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Most of Los Angeles county is still in Extreme Drought!

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Most of Los Angeles county is still in Extreme Drought!

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This is the 10th driest year in the last 127 years!!

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The U.S. Drought Monitor (USDM) is a national map released every Thursday, showing parts of the U.S. that are in drought. The USDM relies on drought experts to synthesize the best available data and work with local observers to interpret the information. The USDM also incorporates ground truthing and information about how drought is affecting people, via a network of more than 450 observers across the country, including state climatologists, National Weather Service staff, Extension agents, and hydrologists. Learn more.

The Standardized Precipitation Index (SPI) is an index to characterize meteorological drought on a range of timescales, ranging from 1 to 72 months. The SPI is the number of standard deviations that observed cumulative precipitation deviates from the climatological average. NOAA's National Centers for Environmental Information produce the 9-month SPI values below on a monthly basis, going back to 1895. Learn more.

Tree-rings are used to extend the instrumental record of drought to over 2000 years. The Living Blended Drought Product (LBDP) is a recalibrated data series of June-July-August Palmer Modified Drought Index (PMDI) values in the lower 48 U.S. states. This dataset blends tree-ring reconstructions and instrumental data to estimate the average summer PMDI values, which extend over 2000 years in some parts of the U.S. Learn more.

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The U.S. Drought Monitor (USDM) is a national map released every Thursday, showing parts of the U.S. that are in drought. The USDM relies on drought experts to synthesize the best available data and work with local observers to interpret the information.

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UV radiation increases in Northern Hemisphere due to Artic Ozone hole that has appeared several times in a decade.  

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UV radiation increases in Northern Hemisphere due to Artic Ozone hole that has appeared several times in a decade.  

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Along with the mild winter across much of the eastern United States and the return of something more like real winter in Alaska, here's something else we can blame on the polar vortex: a rare "hole" in the ozone layer over the Arctic in February and March 2020.

The stratospheric ozone layer is a natural barrier to high-energy ultraviolet (UV) light that can cause skin cancer and other DNA damage in people, other animals, and plants. In the late 1970s, scientists discovered that CFCs-short for chlorofluorocarbons-being used in cooling systems and aerosol sprays were damaging the ozone layer. The discovery led to an international treaty to phase out and eventually eliminate their use.

While it isn't on par with the ozone hole that develops each year over the South Pole, this year's severe event is a record for the Northern Hemisphere. Since ozone observations began in the 1970s, events like this have only happened two other times, during the winters of 1996-97 and 2010-11. In each case, the extreme ozone loss was the result of unusual weather that kept the Arctic colder and more isolated than normal in late winter.

What's cold got to do with it?

In the lower atmosphere, CFCs are inert, but exposure to ultraviolet light in the stratosphere breaks them down into more reactive gases. The process is accelerated within polar stratospheric clouds. Commonly known as noctilucent ("night-shining") clouds, they only form at temperatures below -78°C. It rarely gets that cold in the Arctic, even in the winter. That's why when we talk about "the ozone hole," we generally mean the one that happens every spring over the South Pole, where that kind of extreme cold is widespread.

But the winter of 2019-20 was highly unusual, explained Craig Long of NOAA's Climate Prediction Center via email. "The cold temperatures in the Northern Hemisphere polar region were present all winter long without 'weather' disrupting the circulation pattern," he wrote. "There have been previous years where part of the winter has been cold, 2010-2011 for example, but not for the entire winter. This winter is also interesting in that the stratosphere and troposphere [lower atmosphere] were coupled throughout most of the winter. By this I mean that the polar region (60°-90°N) had cold anomalies throughout the troposphere and stratosphere."

Abundant amounts of polar stratospheric clouds throughout the dark winter months created a much bigger reservoir of reactive CFC byproducts than usual. As the Sun returned through late February and early March, ozone destruction occurred rapidly.

Unusual for the Arctic, but not as bad as the real ozone hole

According to NOAA satellite observations, the lowest ozone values over the Arctic in March were on the order of 200 Dobson Units (DU) according to NOAA satellite analysis. A NASA analysis found similarly low values. That's significant for the Arctic, but far less severe than the hole that forms each spring over Antarctica. There, experts classify any value below 220 DU to be part of the ozone hole, and the area meeting that definition reaches 21million square kilometers on average. In places, ozone levels can drop below 100 DU.

Blame the polar vortex

The key to understanding why an ozone hole forms every year in the Antarctic but rarely in the Arctic is the polar vortex. The polar vortex is large pool of extremely cold air enclosed by a ring of fiercely strong westerly winds called the Polar Night Jet. The jet emerges in the upper stratosphere at each pole during winter. Inside the vortex, air temperatures plummet. The stronger the polar vortex, and the longer it lasts, the colder the air inside becomes.

The polar vortex weakens when it is jostled from below, which happens a lot in the Northern Hemisphere. Below the stratosphere, the atmosphere in the Northern Hemisphere is flowing over land surfaces with rugged topography. Like boulders in a river, these obstacles in the atmosphere's flow can generate downstream waves that break upward into the stratosphere. The waves slow or sometimes even stop polar vortex. As the vortex relaxes, the air inside sinks and warms.

In the Southern Hemisphere, warming disruptions are rare because the South Pole is surrounded by the relatively smooth surface of vast ocean. The differences in physical geography mean that the Southern Hemisphere polar vortex is stronger, larger, and more stable than the Northern one, and temperatures there get far colder.

In the lower stratosphere, where ozone destruction happens, the Northern Hemisphere polar vortex reaches an average size of about 14 million square kilometers in late January or early February. Throughout March it drops rapidly, and in an average year, it has disappeared altogether by April 1. This winter, it peaked at close to 19 million square kilometers in mid-February and remained at more than 15 million square kilometers through the middle of April. (Higher up, the final stratospheric warming event of the season usually comes in mid-April.)

A surprising prediction: long-range models anticipated this year's exceptional polar vortex

According to polar vortex expert Amy Butler of CIRES/NOAA Chemical Sciences Laboratory, the size and strength of this year's polar vortex were indeed remarkable. Even as of mid-April, winds surrounding the vortex were so strong that the air inside remains ozone-depleted because it is cut off from ozone-rich air from the mid-latitudes.

But the unusual behavior of this year's Northern Hemisphere polar vortex may not even be the most interesting thing about the event.

"Surprisingly," Butler says, "long-range models were predicting this event as far back as last fall. They even predicted the strong coupling between the stratosphere and the troposphere, and the strongly positive Arctic Oscillation," which led to the extremely mild winter across the Eastern U.S.

"Whatever it was the models saw," Butler says, "it has to be something with a long-lasting influence for the models to have picked up on it so early, something like SSTs [sea surface temperatures] in the tropics. People are already starting attribution studies to see if we can figure out why the models were able to capture this event so successfully."

In the meantime, stratospheric climate experts will be watching the Arctic and wondering: when will this winter's polar vortex finally break?

Links

Ozone page at NOAA Climate Prediction Center

Twenty questions about the ozone layer from NOAA Chemical Science Lab

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Spring 2020 brings rare ozone "hole" to the Arctic | NOAA Climate.gov

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Along with the mild winter across much of the eastern United States and the return of something more like real winter in Alaska, here's something else we can blame on the polar vortex: a rare "hole" in the ozone layer over the Arctic in February and March 2020.

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Here's a 10 minute video about heat waves in Phoenix.

https://www.youtube.com/watch?v=Q2RQjtucG3M&ab_channel=PBSTerra