Skip to main content

Revisiting the AQI

Revisiting the AQI:

With persistent Air Quality Alerts this week, I wanted to revisit this topic which we talked about last August when we had persistent air quality alerts due to smoke from Western USA and Canada wildfires. In fact we had 11 Air Quality Alerts last summer, a larger than normal number.

This week the Air Quality Alerts have been provoked due to high particulate matter trapped in the lower atmosphere by persistent inversions (warmer temperatures with height). The Air Quality Index (AQI) is calculated by the EPA as a mandate from the Clean Air Act (first passed by Congress in 1963 and amended several times since). The EPA meteorologists partner with the NOAA National Weather Service in issuing Air Quality Alerts. The EPA regularly monitors for five pollutants: ground-level ozone (O3); particulate matter (microscopic); carbon monoxide (CO); sulfur dioxide (SO2); and nitrogen dioxide (NO2). For each of these pollutants there are air quality standards (thresholds) used to protect public health.

The AQI uses six color-coded levels: the first two (green-good, and yellow-moderate) are not worrisome; orange is unhealthy for sensitive groups, senior citizens, children, and those with heart and lung diseases; the remaining levels, red-purple-maroon designate progressively more serious levels for pollutants for all people. We rarely seen purple or maroon levels in Minnesota, but these have become more of a concern in some Asian Cities like Beijing, China, where daily AQI measurements have sometimes exceeded even the purple category. Air Quality Alerts in Minnesota are almost always associated with stagnant high pressure systems. More information can be found from theEPA-AirNow web site or the MPCA web site.

Remarks on Seasonal Affective Disorder (SAD):

I am aware that a number of people suffer from this, and it is especially so in recent months.

Many people have remarked about the absence of sun in the past several months and the Minnesota StateClimatology Office has produced a summary on the gloominess of the autumn weather.

Although I think that the absence of sunny skies, greatly compounded by daytime cloudiness, has provoked SAD, there are other weather conditions that have compounded the dreariness. If we look at the number of days with fog, mist, dust, smoke, or haze there has been an unusual high frequency. Listed below are the number of days by month in the Twin Cities observations where fog, mist, dust, smoke, or haze were noted:
October 18 days
November 17 days
December (1-12) 11 days

This is by far the highest frequency in many years, and certainly a factor in holding daytime high temperatures down. This week we also experienced some driving difficulties due to ice-fog as well.

Weekly Weather Potpourri:

This week NOAA scientists released another report card on the ArcticRegion. This is the 13th year in a row for such a report. Temperatures continue to rise sharply there, both in the air and the ocean. In fact a higher frequency of red tide is observed along with a continued decline in reindeer and caribou populations.

Cyclone Owen was bringing high seas and strong winds to portions of northern Australia this week, mostly southeast of Dawin. It was expected to bring heavy rains to northern Queensland (Carins) this weekend, perhaps 10-15 inch amounts in places.

In recent years the World Meteorological Organization has promoted the value of long term climate histories by designating Global Centennial Observing Stations, basically climate stations which have kept a daily record of measurements for 100 years or longer. Though the WMO has yet to recognize the such stations in Minnesota we have several locations that have measurement histories of 100 years or more, including Morris, Crookston, and Milan among many others.

The BBC Weather Centre had an interesting story (with video) about sea form (called spume) which was whipped up by the wind and blown into the town of Dawlish (in Devonshire) this week. The sea form results from high winds churning sea waters that are full of algae which creates huge amounts of foam.

MPR listener question:

What is the record for number of continuous days with snow cover in the Twin Cities? Also if you have it, what is the same record on a statewide basis?


For the Twin Cities, the record is 146 days, from November 7, 1921 to April 1, 1922. The most recent such longevity of Twin Cities snow cover was November 16, 2000 to April 4, 2001, a period of 141 days.

The longest continuous snow cover season I can find in the state climate database is 197 days at Gunflint Lake (Cook County) from November 8, 1965 to May 23, 1966. That’s 6 ½ months of snow cover!

Twin Cities Almanac for December 14th:

The average MSP high temperature for this date is 27 degrees F (plus or minus 13 degrees F standard deviation), while the average low is 12 degrees F (plus or minus 15 degrees F standard deviation).

MSP Local Records for December 14th:

MSP records for this date include: highest daily maximum temperature of 55 degrees F in 1998; lowest daily maximum temperature of -14 degree F in 1901; lowest daily minimum temperature of -27 degrees F in 1901; highest daily minimum temperature of 43 degrees F in 2014; record precipitation of 1.50 inches in 1891. Record snowfall is 5.2 inches in 1996.

Average dew point for December 14th is 10°F; the maximum dew point on this date is 39°F in 1928; and the minimum dew point on this date is -22°F in 1985.

All-time state records for December 14th:

The state record high temperature for this date is 60 degrees F at Preston (Fillmore County) in 1912. The state record low temperature for this date is -48 degrees F at Detroit Lakes (Becker County) in 1901. The state record precipitation for this date is 2.38 inches at Red Wing (Goodhue County) in 1891. Record snowfall for this date is 20.8 inches at Two Harbors (Lake County) in 2005.

Past Weather Features:

Following a December 11-12 snow storm, Arctic High Pressure gripped the state on December 14, 1901, the coldest in Minnesota history. Over 30 Minnesota communities reported a morning low of -30 degrees F or colder. Wind Chill readings were not common then, but clearly based on measurements many areas saw Wind Chill values of -50 F or colder.

On December 14, 1998 a number of golf courses were open for business as the absence of snow combined with afternoon temperatures in the 50s F under sunny skies prompting many to take a Monday afternoon off. Over 50 Minnesota cities reported highs of 50°F or greater.

A double-barreled storm brought a significant blanket of snow across much of the eastern half of Minnesota over December 13-14, 2005. It was an unusual event with two areas of low pressure, one over Nebraska and another along the North Dakota/Canadian Border. Both areas drifted to the east during the day on Wednesday, December 14. This set the stage for at least a half foot of snow to fall across much of eastern Minnesota. Some northeastern locations received 10-20 inches of snow.


A respite from the dreariness comes this weekend with plenty of sunshine and warmer than normal temperatures around the state. Temperatures will generally range from 5-10 degrees above normal. The sun and warmth will last until at least Thursday of next week.

Print Friendly and PDF


Unknown said…
Hi Mark,

Thanks for reminding folks about the Air Quality Index (AQI) -- a great way to easily understand the current air quality. Minnesota typically resides in the Green (good) category and people appreciate and expect it here. Occasionally Mother Nature has a different opinion and presents the area with 'dirty air' due to wildfire smoke, low-level ozone, or winter stagnation. With the latest air quality alert we have officially issued 10 in 2018 - a record for Minnesota since they began in 2008. These alerts have covered a total of 15 days for at least some portion of the state. Today's blue skies and much cleaner atmosphere is a welcome break for us all here in the North Star state!

Daniel Dix
Air Quality Meteorologist
Minnesota Pollution Control Agency