Weather Contrast

The weather has been bad today in western Alaska, with wind, snow, and rain moving up from the south.  Kotzebue developed a white-out this afternoon with 40-50 mph winds.

2-22-23, 2:27 m, Kotzebue, Alaska, 20 d Fah (-7 C), wind chill 9 d Fah (-18 C), winds E at 37 (59.55 kmh), what we be like pic.twitter.com/0MNph9gjX7

— Tundrabilly ❄☃️🇺🇸 (@tammaq13) February 22, 2023

 

But far to the east on the other side of the ridge axis, it has been a stunning day (after a chilly start) in eastern Alaska and the Yukon, with lots of glorious sunshine.  Here's the webcam view from Dawson:

And in video format:

Here's the 3am 500mb analysis, courtesy of Environment Canada: click to enlarge.  Note that ridge axis right over Alaska.

Warmth for Summer's End

An interesting feature of the interior Alaska climate is that early autumn tends to bring drier and more settled, sunny weather after the typical rains of August, but this year the improving trend seems to have arrived several weeks early.  It's warm, too, with today being the 7th straight day that the temperature has exceeded 70°F in Fairbanks; that hasn't happened this late in the year since 2009 (and that was in early September).  Prior to that, we go back to August 2004, which was very hot and led to a highly unusual late-season extension of that record-breaking fire season.

Of course most places at lower latitudes are still in the throes of summer heat; Death Valley roasted at 130°F just the other day.  The more rapid drop-off in temperatures up north in most years is directly related to the quick decline in solar insolation at high latitudes.  Compare the two figures below: the first showing the annual cycle in solar radiation near Fairbanks, and the second showing the same for my neck of the woods far to the south.  The quick decrease in Fairbanks is partly related to increased cloudiness as summer advances, but it's also a result of the solar geometry.

The summer maximum is even more fleeting at Alaska's northernmost city (see below); but notice how high the peak is - almost as high as the summer peak in Georgia, and that's with much more cloudiness at Utqiaġvik.  (Another interesting feature is that even the cloudiest days in Utqiaġvik still receive a large amount of solar radiation in early summer, owing to 24-hour daylight.)

The late-summer decline in solar energy in Fairbanks means that it's increasingly uncommon for afternoon temperatures to achieve their full "potential" based on temperatures aloft.  Increasing cloudiness of course bears responsibility for this, but even if it's sunny with warm air aloft, the weakening sun in August is increasingly unable to produce a strong surplus of heating at the surface.

For example, here's yesterday's 4pm balloon sounding from Fairbanks; there's certainly a nice well-mixed boundary layer, and there's a small excess of temperature (a super-adiabatic layer) at the surface from the solar heating.  The near-surface temperature of 72°F at the balloon release site is 24°F higher than the 850mb temperature of 48°F; this equates to a lapse rate of 10.0°C/km, which is just slightly above the dry adiabatic (well-mixed) rate of 9.8°C/km.

 

Back on a sunny July 4th, the additional solar heating at that time allowed the 4pm surface temperature to be 27°F higher than at 850mb, for a lapse rate of 10.5°C/km; the sunshine eked out another 2-3°F compared to what it might have done with the same air mass six weeks later.

 

The seasonal changes are more noticeable when we compare the distribution of surface-to-850mb temperature differences by month - see below.  For simplicity, I've used the daily maximum temperature at the surface, even though this does not always occur in the afternoon.

 

As summer advances, the highest lapse rates (like that on July 4) become much less common, and surface temperatures tend more and more to underachieve relative to conditions aloft.  Of course this only means one thing: we're on the long road back to the semi-permanent temperature inversion of the winter months.

 

Winter's Weak Sunshine

Here's a lovely webcam view from UAF's West Ridge at about solar noon today, after clouds cleared out following a lengthy period of light snow in the last couple of days.  At this point on the calendar, the sun rises a mere 2° above the horizon at noon in Fairbanks.

The temperature at UAF was about 0°F when the picture was taken, but not far away at the valley-level international airport the temperature has been dropping steadily today as clouds diminish and solar insolation remains effectively zero even at noon.  The chart below shows that nearly constant temperatures prevailed for more than a day and a half while cloud and snow persisted, but the end of the snow presaged a sharp cooling trend.  It's classic interior Alaska weather in the depths of winter.

Seasonal Sunshine

As summer comes to an end on a wet and rather chilly note in the interior, it's interesting to observe that August has not only been very wet in Fairbanks, but cloud cover has been exceptionally high as well.  Two lines of evidence illustrate the remarkable nature of the anomaly:

- The month's average diurnal temperature range (difference between the daily high and low temperatures) is just about the smallest on record for August in Fairbanks (1930-present); this indicates an absence of the clear skies that tend to produce warmer days and cooler nights at this time of year.  If the temperature rises no higher than 53°F today, then this month's diurnal range will be exactly equal to the record low value from August 1998.  Interestingly the third-place year was 2015, and August of last year also had a very small mean diurnal range.

- The solar radiation at the CRN station near Fairbanks has been far below normal and easily the lowest for August in the 16-year history of the site; total solar energy has been only about two-thirds of normal.  Solar panel owners in Fairbanks-land are probably not happy at the moment.

The chart below (click to enlarge) shows the seasonal cycle of solar radiation at the Fairbanks CRN site.  The blue columns indicate the 2003-2017 monthly averages of measured radiation, and the additional gray area in each column shows how much greater the radiation would be if it matched the theoretical "clear sky" input (ignoring any shading effects from topography).  In March and April the Fairbanks CRN site receives about 70% of the theoretical maximum, but increasing cloud cover causes this to drop to only 50% by September.  Consequently, for example, August produces less solar energy than April, even though the sun is higher in the sky in August.  The same contrast is true of July versus May.

Below is the same chart for Utqiaġvik, formerly Barrow, where the CRN site also has a nice 15-year history.  Not only is the seasonal cycle in solar radiation even more pronounced than in Fairbanks because of the higher latitude (notice that Utqiaġvik sees more solar radiation than Fairbanks in May and June), but the seasonal change in cloud cover is also more dramatic.

For context, here's the same chart for Champaign, IL, at 40°N latitude.  The measured solar energy is not all that different from the Alaskan sites in May, but by the end of August there is a pronounced contrast.  And of course, as I discussed recently, this accelerating seasonal difference is related to the quick end of summer in the far north and also the typical seasonal change to wetter weather in August.

Sunshine at CRN Sites

After reviewing the status of Alaska's CRN data last week, curiosity led me to take a look at the solar radiation measurements from these sites.  For example, it's interesting to find out what these instruments have to say about how solar radiation varies across the state - at least for the last few years.

The chart below shows a summary of April-October total solar energy for the 11 sites that have been in operation since at least 2013 and have essentially complete data for these months.  The horizontal black lines show the 5-year mean for each site, and the blue bars indicate the minimum and maximum values within this brief 5-year period.

The sites near Fairbanks and Tok are the only interior sites represented here, and as we would expect they are at the top for solar energy.  Interestingly Metlakatla, on Annette Island in Alaska's far southeast, is in 3rd place and close behind Fairbanks.  It's not surprising to find that rainy Sitka is easily in last place; but note that the difference in solar energy between the cloudiest site and the sunniest site is less than 50%.

The range between minimum and maximum seasonal totals is remarkably high at the Red Dog site, but this is mostly because of a very low total from 2013; this looks suspicious and might be incorrect.  (If we exclude 2013, Red Dog actually comes in above Fairbanks for mean solar energy, and the Red Dog instrument also reported the highest seasonal total of any site in any year.)  More robust, perhaps, is the very low variance of solar radiation at the sites near Barrow (Utqiaġvik) and Tok; it's quite extraordinary, actually, how consistent the solar energy has been at these locations in the past 5 years.  But in the case of Barrow, data from earlier years showed higher values - see below.

The decrease of solar energy at the Barrow CRN site, evident in the chart below, is highly statistically significant even over such a short period.  Presumably this is related to the increase in open water area in the Arctic Ocean and therefore higher evaporation and increased cloudiness in recent years.

Autumn Cooling

There's a slight chill in the air today across interior Alaska, with afternoon temperatures around Fairbanks only in the mid-40s at valley-level thanks to clouds and some light rain.  Temperatures are in the mid-upper 30s in the higher hills; and a glance at the calendar reveals the reason.  Here are some of the observations as of about 6pm (click to enlarge).

Yesterday was the first day with a high temperature below 50°F in Fairbanks, and this is about a week late compared to normal.  Fairbanks usually sees its first sub-50°F day more than a week earlier than Anchorage, which illustrates the contrast between the rapid cooling of the interior and the slower cooling that occurs closer to the waters of the North Pacific.  Anchorage has never seen a sub-50°F day in August during the modern historical record, but it's not too uncommon in Fairbanks and happened just two years ago.

Here are a couple of webcam views of Fairbanks; autumn colors look to be a little past peak.

At this time of year it's always interesting to be reminded of the rapid drop-off in solar radiation at northern latitudes during autumn.  Thanks to 15 years of high-quality data from the Fairbanks CRN site (actually 11 miles northeast of town), we know what the normal incoming solar radiation curve looks like - see below.  I've added the normal cloud cover from Fairbanks airport to give an idea of relative changes in cloudiness through the year, although cloudiness is presumably a bit greater at the CRN site.

Notice how quickly the solar radiation diminishes in mid to late September; the average amount of incoming solar energy drops by 50% in the last 3 weeks of the month.  Of course, at the beginning of September it's already down by about 50% from the peak in June.

Here's the equivalent chart for Barrow, also from 15 years of CRN data; there's little available solar energy by this date in Barrow, and the very high cloud cover in late summer and autumn only exacerbates the rate of decline.

Just for fun, here's the equivalent solar data from the most southerly CRN site in the continental U.S., at Everglades City in Florida, along with the normal cloud cover from nearby Fort Myers.  By September 20 the normal solar radiation has decreased by only about 20% from the solstice, although the annual peak in south Florida occurs in May prior to the wet season.  The slow decline of the solar input, along with the maritime environment, explains why September is basically still high summer in Florida.

But the most interesting aspect of the comparison may be that Barrow receives more solar energy on average in June than south Florida - even though Barrow is very much more cloudy!  This is the result of 24-hour daylight; the noon sun in Barrow is nothing like the noon sun in Florida, but the hours of sunshine really add up in the far north.

Low-Elevation Sunshine

Colder than normal conditions have settled over the interior, and so far the RAWS thermometers at Norutak Lake and at Prospect Creek/Jim River have measured the coldest conditions: -45°F this morning.  A report of -49°F from the Bettles SNOTEL site seems suspect as that site always runs cold.

Norutak Lake is located just north of the Arctic Circle and just to the east of the upper Kobuk River where it drains south out of the Brooks Range.

It is interesting to note that yesterday's hourly temperature observations from Norutak Lake showed a rise of 8°F in association with some very weak solar radiation during the day (see below).

 

The sun only rose to an elevation of 2° above the horizon yesterday at Norutak Lake, so I wouldn't have expected to see such a large diurnal temperature change.  However, a closer look at the topographic map reveals that the RAWS site is located on a south-facing slope at the north side of the lake.  The RAWS elevation is given as 800', but Norutak Lake itself is at 712', so the thermometer is almost 100' above lake level.  It seems that the south-facing exposure allowed for a bit more solar heating than would occur at lake level, and the daytime warming may even reflect the warm bias that RAWS thermometers are known to encounter on calm, sunny days.

With a surface-based temperature inversion undoubtedly in place over Norutak Lake, it's likely that temperatures near the lake surface were at least a couple of degrees lower than at the RAWS site, and perhaps substantially lower.  At Fairbanks airport it is normal to see an inversion of 4-5°F per 100 feet at the surface during winter, and 10°F per 100 feet is not uncommon.

Alaska Brightness

** It's been a while since I posed here but I thought the Deep Cold readers might like this. **

Did you know that interior Alaska actually receives more sun and twilight than any other place in the U.S.

Alaska is known for long summer days and long winter nights. However, if you average all 365 days together, everyone ends up with 12 hours of daylight and 12 hours of darkness throughout the course of the year no matter where in the world you are, right? Actually, that is not correct. 

Looking at the chart that accompanies this post (see Figure 1), the red line at the bottom of the chart shows that daylight at the equator averages about 12 hours and 15 minutes per day over the course of the year. Remember that the sun is a circle (not a point) and we receive daylight from the top of the sun's disc before the middle of the disk reaches the horizon. The same is true at sunset. This makes an average day over 12 hours everywhere.

Figure 1. Hours of daylight and daylight plus Civil Twilight by latitude. All data obtained from the U.S. Naval Observatory's Astronomical Observations Department.

At the equator, the sun moves nearly straight up and down with respect to the horizon. This means the sun rises quickly and sets quickly. As we move to higher latitudes, the path of the sun is more oblique; i.e., the sun moves more and more diagonally with respect to the horizon. It therefore takes longer for the entire disk of the sun to make is across the horizon at both sunrise and sunset. 

If the sun were a point and not a circle, the average length of a day would be 12 hours everywhere. Since the sun is a 2-dimensional circle from our perspective, the relative speed of the rising and setting dramatically changes the amount of light we receive. This effect is greatest at the Arctic and Antarctic Circles due to the effective ground speed of the sub-solar point near the solstices. The difference in cumulative day lengths between the equator and the Arctic Circle (for all 365 days) is 225 hours per year – or 37 minutes per day on average. 

Figure 2 and 3 show the length of daylight (Figure 2) and the combined length of daylight and Civil Twilight on the summer solstice.

 Figure 2. Length of daylight on the summer solstice.

Figure 3. Combined length of daylight and Civil Twilight on the summer solstice.

If we include Civil Twilight, which is when the entire sun's disk is below the horizon but by no more than 6°, the extra light for Alaska dramatically increases. At 69°N latitude, the 365-day average for daylight plus Civil Twilight is 15 hours and 6 minutes. At the equator, the 365-day average is only 12 hours and 52 minutes. The average difference in light (daylight plus Civil Twilight) is a shocking 2 hours and 16 minutes per day. Places just north of the Brooks Range (e.g., Umiat)  therefore receive the most usable light of any place in the U.S. Table 1 shows the cumulative length of daylight and Civil Twilight for Fairbanks, Barrow, Anchorage, and Juneau measured in hours.

Table 1. Annual hours of daylight and Civil Twilight for Fairbanks, Barrow, Anchorage, and Juneau.

The final two maps (Figure 4 and 5) show the length of daylight plus Civil Twilight for Alaska and the Lower 48. Again, note how much more light Alaska receives than the Lower 48 over the course of the year.

Figure 4. Average annual length of daylight plus Civil Twilight. The average is for all 365 days of the year. The map perspective is Alaska-centric.

Figure 5. Average annual length of daylight plus Civil Twilight. The average is for all 365 days of the year. The map perspective is Lower 48–centric.

The next time you hear someone complain about how dark it gets in Alaska during the winter, just remind them that interior Alaska is the light champion on the U.S.

The Long Night

Polar night is now advancing south across far northern Alaska, having reached Barrow a few days ago.  The edge of the long night is currently moving south at a rate of about 15 miles per day and will cross the 70th parallel before noon on Monday.  The webcam images below show the view to the south at solar noon today at Barrow, Wainwright, and Arctic Village.

 

Arctic Village is only at 68.1°N, but is already in permanent shadow because of topography to the south.  The view to the north shows the Brooks Range mountains illuminated by the low sun.  Temperature at the time of the photo: -22 °F.

Disappearing Sun; Barrow Update

Today marks the day when the sun's angle above the horizon at solar noon has declined half way from the equinox to the winter solstice; or equivalently we have traveled three-quarters of the way from the summer to winter solstice in terms of the sun's elevation at noon.

What does this mean?  It means we hereby enter the dark third of the year in the northern hemisphere; and of course this fact is more inescapable the farther north you go.  We can illustrate the lack of solar energy across Alaska in winter by calculating the theoretical solar insolation under clear skies.  From this theoretical standpoint, the total solar energy received in Fairbanks over the next 4 months is less than is received in 6 days in the height of summer.  In Bettles the dark third of the year receives less radiation than in 4 days in summer.  However, in Anchorage the winter sun provides the equivalent of about 10 days in summer.

Here's the view at close to solar noon yesterday from the Alaska Climate Research Center webcam on UAF West Ridge.  The weakness of the sun is illustrated by the fact that some snow remains on the ground despite most days getting above freezing in the past two weeks; the official snow depth in Fairbanks has been at 1 inch for 10 days now.

On another note, Barrow has had a chilly and very windy time of it in the past several days, with a very strong pressure gradient importing cold Arctic air from the northeast.  Here's the surface analysis from Monday afternoon when winds were sustained at about 40 mph for a time.

The high temperature on Tuesday was only 17 °F in Barrow, which is the coldest day so early in the season since 2002.  As we've mentioned many times before, Octobers since 2002 have been extremely warm in Barrow compared to previous decades, and so this kind of chill would have been completely normal in the last century.  For example, the 1930-2000 normal for coldest high temperature to have occurred by October 21 in Barrow was 10 °F; and a high temperature of 17 °F would normally have been observed by October 12.  In 1996, when sea ice was firmly established from Barrow eastward by late September, the high temperature was -7 °F on October 11!

So, it's been a little cooler in Barrow in the past few days - but even this is only approaching normal from earlier decades.  October 2014 is still running well above both the 1981-2010 and 1971-2000 normals.

Advancing Night

The long summer day is now fading in Arctic Alaska, as a daily period of darkness advances north across Alaska with the change of seasons.  Last night was the first night that civil twilight was observed at Barrow, i.e. the sun briefly dipped more than 6° below the northern horizon.  However, it will be another 2-3 weeks before it starts to get reasonably "dark" at night in Barrow.

Here are a couple of "night" pictures from the Barrow sea-ice webcam in the past few days: the first, a beautiful scene, taken about an hour and a half before sunrise on Monday, and the second taken a little before solar midnight last night.

Evaporation Calculations

As a follow-up to my recent post about potential evaporation rates in Fairbanks last summer, I decided to go one step further and include the effect of wind speed and solar radiation.  Higher wind speeds and higher solar radiation both increase the rate of evaporation from land, water, and vegetative surfaces, and so it's of interest to see whether these factors also contributed to high moisture loss last summer.  Hourly radiation, wind, temperature, and humidity data are recorded (among other variables) at the Fairbanks Climate Reference Network (CRN) site, which is located at 1140' elevation about 10 miles northeast of Fairbanks.  Based on this data, and with a requirement for no missing hourly reports in each 24-hour average, I calculated the theoretical daily evaporation rate from an evaporation pan (formula supplied on page 11-12 of this document - thanks Brian).

The chart below shows the monthly mean calculated evaporation rate during the growing season months since 2011.  Unfortunately the humidity measurements did not come online at the Fairbanks CRN until August 2010, so it was not possible to include a larger number of years.  Nevertheless, the results confirm the very high evaporation rate in June 2013, and interestingly the evaporation was higher in each month from May through August 2013 than in the same months of the previous two years.  The theoretical evaporation from June 2013 amounted to well over 8 inches for the month, which is not much less than the precipitation total for the entire year in Fairbanks.

Looking at the solar radiation and wind speed in isolation (see below), the solar radiation was higher last year in May and June than in the same months of any of the previous 10 years.  The wind speed was also a little higher than normal in June and July.  (The reported wind speed looks much too low for May and June of 2011, but I double-checked the data and this is what the supposedly high-quality CRN platform reported.  There is no evidence of lower wind speeds in these months at Fairbanks airport.)

In conclusion, the CRN data confirm that Fairbanks-area land-surface moisture loss in summer 2013 was caused not only by dry air but also by abnormally sunny conditions.  Of course we would expect these two anomalies to go together; both were quite extreme last summer, leading to highly elevated moisture removal rates.  Fortunately, fire activity was not catastrophic as in 2004 - probably because of lack of lightning.  Here's what the state 2013 fire report had to say: "2013 will be remembered as one of the shortest, but hottest summers on record... Fuel conditions reached near record dryness at some locations... Despite many record-setting hot temperatures, the stable high pressure kept thunderstorms at bay, and lightning to a minimum... With many comparisons between the hot, dry summers of 2004 and 2013, it seems that the lack of lightning, and therefore the lack of natural fire starts, is what kept the 2013 fire season from becoming catastrophic."

Sunrise in Barrow

The sun rose in Barrow today for the first time since November 19th last year (64 days). It was up from 1:29 p.m. to 1:51 p.m. Below is the FAA webcam picture looking south at 1:31 p.m. – 2 minutes after sunrise. Here is the only METAR from when the sun was up: 

Site M/A Day Time Sky Conditions           VIS Weather Temp DP Wind(kt)  Alt  RH  Chill Peak

PABR  MP 22 1349  OVC055                     3 BS-       0  -8 06018     997  68% -22    

(PABR 222249Z 06018KT 3SM BLSN OVC055 M18/M22 A2997=)

Solar Heating Minimum

** Updated on 12/30 **

As Richard noted in a recent post, there is little to no solar energy for much of Alaska this time of year. In fact, there is no functional solar heating anywhere north of Fairbanks for the next several weeks. In looking at hourly data for Bettles from NCDC, I looked at the average hourly temperature for every day of the year to see when solar energy stopped influencing daily temperatures (n=45 years). Bettles was chosen due to its location far away from any alternate heat source (e.g., an ocean).

Though trial and error, it appears that when the sun is 3° above the horizon or less, there is no perceptible diurnal temperature difference. At 3° above the horizon, the theoretical solar energy is 9 watts/square meter. At the latitude of Bettles, that translates to a 55-day period of no solar heating. The next two charts show the annual solar elevation for Bettles and for Barrow.

The chart immediately below shows the average hourly temperature for the 55-days at Bettles where the sun is less than 3° above the horizon. Interestingly, there is a very slight bump at the equivalent of solar noon – even on days when there is no sunrise at all!

The final graphic (map) shows the number of days with no effective solar heating; i.e., to maximum solar elevation is 3° or less. The 835 GHCN stations are shown as green dots.

** Updated Section **

Here are some charts showing the hourly temperatures and sea level pressure readings for the days with no sunrise and no sunset at Bettles (Dec. 12 to Dec. 31 & June 12 to July 2 respectively). Note: Due to Leap Years and the procession of the equinoxes, the dates will be slightly different from year to year.

The final (?) chart shows the hourly temperatures for Barrow between December 1st and January 27th. December 1st was chosen to account for the variability in sea ice pack closure around Barrow. January 27th was chosen due to that being the date that the sun is still below the horizon. The local effect at Bettles disappears. When only those days when the sun was within 2 degrees of the horizon were chosen, the line was still effectively flat. Interestingly, the point at which a daily insolation pattern was established in the Spring wasn't until the sun was 5° above the horizon (~Feb. 14th).

Also, the 3-hour pattern that Eric noted is highly exaggerated for Barrow. It turns out that this is due to some days recording temperatures in 3-hour increments instead of hourly. Those occurrences are much more frequent in the earlier part of the climate record when the temperatures at Barrow were significantly colder. It will take more work to tease out those obs.