Eielson Visitor Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Eielson Visitor Center clear sky chart from 2005-11-06 to 2024-04-03.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	11% 10% 4 8% 7% 4 6% 6% 7% 8% 28%	77409
	Night	4 7% 4 8% 8% 5% 7% 7% 7% 9% 35%	82718
Transparency	Day	50% 7% 16% 15% 8% 3	77040
	Night	42% 7% 14% 18% 14% 6%	82668
Seeing	Day	17% 8% 21% 26% 20% 8%	17266
	Night	9% 8% 22% 29% 22% 9%	21611

By Month

Percentage Cloud Cover NW (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	3 2 8% 7% 5% 7% 7% 8% 10% 43%	1269
	11th to 20th	Day	1 4 5% 7% 5% 3 5% 7% 10% 9% 45%	1329
	21st to end	Day	3 2 5% 6% 4 7% 8% 10% 10% 44%	1913
February	1st to 10th	Day	4 3 8% 7% 4 5% 4 6% 9% 50%	1230
	11th to 20th	Day	5% 4 11% 7% 4 8% 8% 8% 10% 34%	1405
	21st to end	Day	1 5% 4 12% 9% 5% 8% 8% 7% 9% 32%	1332
March	1st to 10th	Day	1 5% 4 10% 9% 6% 7% 8% 7% 8% 34%	1829
	11th to 20th	Day	4 2 8% 7% 6% 8% 8% 9% 10% 39%	1971
	21st to end	Day	1 6% 5% 11% 11% 5% 6% 6% 6% 9% 34%	2483
April	1st to 10th	Day	1 6% 6% 13% 11% 4 7% 6% 7% 10% 29%	2293
	11th to 20th	Day	2 6% 4 12% 9% 5% 7% 6% 7% 8% 35%	2486
	21st to end	Day	2 11% 5% 8% 6% 4 6% 6% 8% 8% 35%	2657
May	1st to 10th	Day	3 15% 6% 7% 7% 3 6% 7% 8% 10% 28%	2880
	11th to 20th	Day	5% 12% 4 7% 5% 3 7% 6% 8% 8% 35%	2981
	21st to end	Day	9% 11% 4 6% 6% 3 6% 7% 7% 9% 31%	3499
June	1st to 10th	Day	15% 10% 4 8% 7% 5% 8% 7% 8% 10% 19%	3240
	11th to 20th	Day	19% 10% 5% 7% 7% 4 6% 6% 7% 8% 22%	3534
	21st to end	Day	23% 10% 3 7% 6% 4 6% 7% 7% 8% 18%	3508
July	1st to 10th	Day	24% 9% 3 6% 5% 4 6% 6% 6% 7% 23%	3304
	11th to 20th	Day	29% 10% 4 8% 7% 4 6% 6% 6% 7% 13%	3240
	21st to end	Day	24% 12% 4 7% 6% 4 7% 5% 6% 7% 18%	3359
August	1st to 10th	Day	28% 12% 4 6% 6% 4 6% 6% 6% 6% 16%	2865
	11th to 20th	Day	24% 15% 4 8% 7% 4 6% 5% 6% 6% 16%	2654
	21st to end	Day	25% 17% 5% 8% 7% 4 6% 6% 6% 5% 11%	2772
September	1st to 10th	Day	18% 15% 4 9% 8% 5% 5% 6% 6% 8% 18%	2287
	11th to 20th	Day	13% 15% 5% 9% 6% 4 6% 6% 6% 7% 22%	2160
	21st to end	Day	10% 19% 6% 8% 6% 3 5% 5% 6% 8% 24%	1912
October	1st to 10th	Day	5% 17% 6% 9% 7% 3 5% 6% 6% 11% 25%	1776
	11th to 20th	Day	1 9% 4 10% 8% 4 6% 7% 8% 10% 34%	1547
	21st to end	Day	2 8% 6% 11% 8% 5% 7% 7% 7% 9% 31%	1504
November	1st to 10th	Day	1 9% 4 10% 8% 5% 7% 7% 7% 7% 36%	1195
	11th to 20th	Day	2 5% 4 10% 12% 6% 7% 7% 5% 7% 34%	1045
	21st to end	Day	1 3 5% 8% 9% 5% 8% 7% 8% 10% 36%	826
December	1st to 10th	Day	2 4 3 10% 9% 7% 9% 6% 9% 9% 32%	760
	11th to 20th	Day	1 4 5% 8% 9% 7% 9% 7% 8% 12% 31%	967
	21st to end	Day	1 2 2 6% 5% 3 6% 6% 6% 10% 51%	1397
January	1st to 10th	Night	3 2 7% 8% 5% 7% 7% 8% 10% 43%	5686
	11th to 20th	Night	1 5% 3 7% 9% 5% 7% 7% 7% 9% 41%	5579
	21st to end	Night	2 2 5% 6% 5% 7% 8% 8% 9% 47%	5760
February	1st to 10th	Night	3 4 7% 8% 5% 8% 7% 6% 8% 44%	2949
	11th to 20th	Night	1 5% 4 8% 8% 5% 7% 8% 9% 11% 35%	2662
	21st to end	Night	1 6% 4 12% 13% 7% 8% 7% 6% 7% 29%	2119
March	1st to 10th	Night	3 4 11% 9% 5% 6% 8% 7% 8% 38%	2351
	11th to 20th	Night	3 2 7% 8% 4 5% 8% 8% 11% 41%	2205
	21st to end	Night	4 3 11% 9% 4 5% 8% 8% 10% 37%	2115
April	1st to 10th	Night	5% 6% 12% 9% 4 8% 8% 9% 9% 29%	1709
	11th to 20th	Night	1 7% 5% 8% 7% 4 7% 6% 7% 9% 40%	1423
	21st to end	Night	2 9% 2 7% 5% 3 5% 5% 8% 10% 42%	1300
May	1st to 10th	Night	3 11% 5% 6% 8% 5% 7% 8% 8% 11% 28%	1080
	11th to 20th	Night	6% 10% 5% 6% 6% 3 6% 7% 10% 8% 33%	914
	21st to end	Night	10% 10% 2 6% 4 3 7% 7% 9% 11% 31%	776
June	1st to 10th	Night	17% 11% 3 7% 5% 5% 7% 6% 7% 10% 22%	720
	11th to 20th	Night	17% 12% 5% 5% 7% 5% 6% 7% 7% 8% 22%	420
	21st to end	Night	23% 7% 2 6% 8% 4 7% 8% 9% 9% 16%	448
July	1st to 10th	Night	20% 9% 5% 8% 5% 3 6% 6% 6% 7% 25%	656
	11th to 20th	Night	27% 9% 4 9% 8% 5% 6% 6% 7% 6% 13%	720
	21st to end	Night	25% 12% 3 6% 6% 4 6% 4 5% 6% 22%	958
August	1st to 10th	Night	24% 10% 3 8% 8% 5% 7% 6% 6% 7% 16%	1074
	11th to 20th	Night	21% 13% 3 7% 8% 4 5% 5% 6% 7% 22%	1243
	21st to end	Night	22% 15% 4 8% 8% 4 6% 5% 6% 5% 16%	1584
September	1st to 10th	Night	14% 13% 2 7% 7% 5% 8% 7% 7% 7% 22%	1673
	11th to 20th	Night	12% 14% 5% 8% 8% 4 7% 7% 7% 8% 20%	1800
	21st to end	Night	9% 18% 7% 9% 6% 4 6% 5% 6% 7% 21%	2047
October	1st to 10th	Night	7% 16% 6% 9% 7% 4 7% 6% 7% 8% 25%	2184
	11th to 20th	Night	3 9% 5% 10% 7% 4 7% 6% 7% 11% 32%	2412
	21st to end	Night	2 9% 5% 12% 9% 5% 6% 7% 7% 9% 29%	2852
November	1st to 10th	Night	2 8% 5% 10% 9% 5% 5% 6% 6% 9% 35%	2881
	11th to 20th	Night	2 7% 4 10% 12% 7% 8% 7% 5% 8% 30%	3135
	21st to end	Night	1 5% 4 8% 9% 6% 6% 7% 8% 9% 37%	3353
December	1st to 10th	Night	1 6% 4 8% 7% 4 7% 8% 8% 12% 34%	3420
	11th to 20th	Night	4 4 10% 9% 5% 8% 8% 8% 9% 33%	4240
	21st to end	Night	1 3 2 6% 7% 5% 6% 7% 9% 12% 42%	6270

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	30% 7% 14% 25% 17% 7%	1269
	11th to 20th	Day	30% 5% 11% 29% 15% 9%	1341
	21st to end	Day	27% 5% 7% 18% 19% 22%	1913
February	1st to 10th	Day	30% 5% 10% 20% 23% 13%	1230
	11th to 20th	Day	39% 6% 17% 20% 13% 5%	1411
	21st to end	Day	41% 7% 14% 16% 17% 5%	1332
March	1st to 10th	Day	41% 6% 15% 16% 15% 7%	1829
	11th to 20th	Day	33% 4 12% 23% 20% 7%	1971
	21st to end	Day	43% 7% 15% 15% 11% 9%	2483
April	1st to 10th	Day	44% 8% 15% 18% 12% 1	2293
	11th to 20th	Day	43% 6% 17% 19% 9% 5%	2486
	21st to end	Day	45% 5% 14% 18% 16% 2	2657
May	1st to 10th	Day	48% 7% 19% 19% 6%	2880
	11th to 20th	Day	44% 9% 18% 21% 9%	2984
	21st to end	Day	46% 8% 18% 22% 5%	3500
June	1st to 10th	Day	56% 8% 19% 14% 3	3240
	11th to 20th	Day	56% 9% 17% 15% 3	3534
	21st to end	Day	59% 10% 20% 11%	3508
July	1st to 10th	Day	54% 8% 22% 14% 2	3215
	11th to 20th	Day	68% 7% 19% 5%	3141
	21st to end	Day	61% 10% 21% 7% 1	3172
August	1st to 10th	Day	67% 9% 17% 7% 1	2846
	11th to 20th	Day	67% 8% 15% 9% 1	2654
	21st to end	Day	72% 9% 11% 6% 1	2772
September	1st to 10th	Day	64% 7% 16% 10% 4	2287
	11th to 20th	Day	60% 9% 16% 12% 2	2160
	21st to end	Day	59% 5% 15% 15% 5% 1	1912
October	1st to 10th	Day	55% 8% 19% 13% 5% 2	1776
	11th to 20th	Day	43% 8% 23% 16% 6% 4	1547
	21st to end	Day	46% 8% 16% 21% 7% 2	1504
November	1st to 10th	Day	43% 10% 15% 19% 9% 5%	1195
	11th to 20th	Day	46% 5% 12% 24% 11% 2	1045
	21st to end	Day	38% 7% 17% 20% 14% 5%	826
December	1st to 10th	Day	43% 9% 15% 15% 15% 3	760
	11th to 20th	Day	38% 6% 10% 25% 18% 3	970
	21st to end	Day	24% 5% 12% 16% 21% 21%	1397
January	1st to 10th	Night	30% 8% 14% 26% 16% 6%	5686
	11th to 20th	Night	31% 6% 14% 23% 19% 7%	5629
	21st to end	Night	27% 5% 8% 15% 21% 24%	5760
February	1st to 10th	Night	34% 5% 10% 19% 20% 13%	2949
	11th to 20th	Night	36% 7% 17% 18% 16% 6%	2667
	21st to end	Night	49% 6% 11% 17% 14% 4	2119
March	1st to 10th	Night	38% 6% 13% 18% 18% 7%	2351
	11th to 20th	Night	32% 5% 9% 24% 21% 9%	2207
	21st to end	Night	37% 7% 15% 19% 11% 11%	2115
April	1st to 10th	Night	43% 9% 14% 19% 12% 2	1709
	11th to 20th	Night	38% 6% 17% 21% 14% 4	1423
	21st to end	Night	35% 7% 14% 25% 17% 2	1300
May	1st to 10th	Night	49% 7% 16% 21% 6%	1080
	11th to 20th	Night	42% 9% 15% 26% 8%	914
	21st to end	Night	44% 11% 17% 23% 4	776
June	1st to 10th	Night	55% 8% 19% 15% 2	720
	11th to 20th	Night	60% 9% 14% 16% 2	420
	21st to end	Night	60% 14% 16% 9%	448
July	1st to 10th	Night	56% 10% 20% 14% 1	636
	11th to 20th	Night	70% 8% 17% 4	696
	21st to end	Night	61% 9% 24% 5% 1	905
August	1st to 10th	Night	67% 10% 17% 6% 1	1068
	11th to 20th	Night	63% 6% 19% 11% 1	1243
	21st to end	Night	68% 10% 15% 6% 2	1584
September	1st to 10th	Night	59% 8% 18% 11% 4	1673
	11th to 20th	Night	59% 9% 17% 11% 4	1800
	21st to end	Night	62% 5% 13% 13% 7%	2047
October	1st to 10th	Night	56% 8% 15% 14% 4 3	2184
	11th to 20th	Night	45% 7% 21% 17% 7% 4	2412
	21st to end	Night	48% 7% 16% 18% 9% 1	2852
November	1st to 10th	Night	45% 6% 15% 20% 10% 4	2881
	11th to 20th	Night	50% 5% 12% 18% 13% 2	3135
	21st to end	Night	39% 7% 14% 21% 14% 5%	3353
December	1st to 10th	Night	40% 7% 16% 18% 16% 4	3420
	11th to 20th	Night	40% 7% 11% 18% 19% 4	4236
	21st to end	Night	29% 6% 12% 15% 22% 16%	6270

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	2 14% 28% 23% 26% 7%	90
	11th to 20th	Day	2 8% 24% 39% 19% 8%	135
	21st to end	Day	2 7% 25% 37% 22% 7%	308
February	1st to 10th	Day	3 10% 23% 29% 22% 14%	280
	11th to 20th	Day	6% 12% 29% 28% 17% 9%	329
	21st to end	Day	6% 13% 34% 28% 15% 4	307
March	1st to 10th	Day	4 7% 28% 36% 19% 6%	370
	11th to 20th	Day	2 7% 27% 32% 25% 7%	441
	21st to end	Day	6% 9% 25% 29% 24% 7%	616
April	1st to 10th	Day	3 13% 33% 28% 17% 6%	547
	11th to 20th	Day	5% 8% 21% 33% 27% 7%	590
	21st to end	Day	6% 5% 20% 34% 29% 6%	600
May	1st to 10th	Day	9% 7% 22% 29% 26% 7%	603
	11th to 20th	Day	8% 7% 22% 32% 22% 8%	682
	21st to end	Day	16% 8% 19% 26% 24% 7%	874
June	1st to 10th	Day	19% 6% 16% 25% 19% 15%	810
	11th to 20th	Day	20% 6% 17% 24% 22% 12%	942
	21st to end	Day	24% 5% 14% 22% 21% 14%	898
July	1st to 10th	Day	26% 7% 18% 26% 17% 7%	796
	11th to 20th	Day	31% 7% 16% 22% 17% 6%	792
	21st to end	Day	29% 6% 19% 21% 18% 7%	845
August	1st to 10th	Day	35% 9% 19% 20% 11% 5%	626
	11th to 20th	Day	32% 12% 22% 20% 10% 3	615
	21st to end	Day	36% 10% 20% 19% 13% 2	693
September	1st to 10th	Day	26% 11% 23% 21% 15% 4	570
	11th to 20th	Day	27% 13% 20% 23% 13% 4	540
	21st to end	Day	24% 11% 16% 27% 16% 6%	383
October	1st to 10th	Day	21% 13% 26% 21% 16% 5%	360
	11th to 20th	Day	8% 6% 21% 26% 26% 15%	360
	21st to end	Day	5% 9% 23% 28% 22% 14%	396
November	1st to 10th	Day	5% 8% 25% 28% 27% 8%	314
	11th to 20th	Day	5% 13% 19% 26% 24% 12%	185
	21st to end	Day	4 12% 21% 32% 21% 9%	90
December	1st to 10th	Day	1 10% 32% 28% 20% 9%	90
	11th to 20th	Day	3 9% 32% 28% 21% 7%	90
	21st to end	Day	9% 15% 25% 26% 19% 5%	99
January	1st to 10th	Night	4 9% 22% 28% 26% 11%	940
	11th to 20th	Night	4 7% 26% 36% 18% 9%	940
	21st to end	Night	3 6% 28% 33% 23% 7%	985
February	1st to 10th	Night	3 8% 25% 32% 24% 10%	850
	11th to 20th	Night	4 10% 31% 31% 19% 5%	834
	21st to end	Night	7% 16% 27% 27% 19% 5%	624
March	1st to 10th	Night	2 8% 25% 38% 22% 6%	660
	11th to 20th	Night	2 5% 21% 33% 27% 12%	660
	21st to end	Night	3 8% 21% 29% 22% 17%	638
April	1st to 10th	Night	4 9% 23% 34% 21% 10%	546
	11th to 20th	Night	4 7% 18% 27% 29% 15%	533
	21st to end	Night	6% 3 14% 36% 29% 12%	479
May	1st to 10th	Night	8% 3 18% 27% 32% 12%	360
	11th to 20th	Night	8% 8% 16% 32% 27% 8%	263
	21st to end	Night	13% 5% 20% 21% 35% 7%	195
June	1st to 10th	Night	22% 8% 12% 28% 19% 11%	180
	11th to 20th	Night	21% 3 18% 25% 25% 8%	180
	21st to end	Night	26% 5% 12% 21% 23% 13%	180
July	1st to 10th	Night	27% 5% 13% 28% 19% 8%	175
	11th to 20th	Night	29% 5% 18% 22% 20% 6%	174
	21st to end	Night	29% 5% 15% 23% 18% 10%	331
August	1st to 10th	Night	31% 4 22% 22% 16% 5%	358
	11th to 20th	Night	27% 9% 21% 23% 15% 5%	410
	21st to end	Night	28% 9% 18% 20% 16% 7%	594
September	1st to 10th	Night	25% 8% 20% 24% 14% 9%	540
	11th to 20th	Night	22% 10% 22% 19% 18% 9%	540
	21st to end	Night	20% 8% 17% 26% 20% 9%	585
October	1st to 10th	Night	18% 12% 21% 22% 18% 9%	654
	11th to 20th	Night	8% 6% 21% 26% 26% 12%	810
	21st to end	Night	7% 7% 20% 34% 23% 10%	891
November	1st to 10th	Night	7% 8% 22% 32% 23% 9%	830
	11th to 20th	Night	5% 9% 22% 34% 20% 10%	850
	21st to end	Night	4 11% 22% 29% 26% 8%	908
December	1st to 10th	Night	5% 11% 26% 28% 21% 9%	940
	11th to 20th	Night	4 6% 29% 31% 23% 7%	940
	21st to end	Night	7% 11% 21% 30% 20% 11%	1034

Page generated at 2024-03-31 02:08:55. Version ac07

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