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 2025-07-21.

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	10% 11% 4 10% 6% 6% 6% 5% 7% 8% 27%	124545
	Night	4 7% 4 10% 7% 6% 6% 6% 8% 9% 34%	123348
Transparency	Day	49% 8% 15% 17% 9% 2	124224
	Night	41% 7% 13% 19% 14% 6%	123313
Seeing	Day	17% 9% 21% 26% 20% 8%	19236
	Night	9% 8% 22% 29% 22% 9%	23078

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 9% 5% 6% 5% 5% 8% 9% 48%	1819
	11th to 20th	Day	3 3 6% 5% 5% 5% 6% 11% 12% 42%	1921
	21st to end	Day	3 2 7% 5% 6% 12% 10% 11% 9% 35%	2750
February	1st to 10th	Day	5% 3 9% 6% 5% 4 3 7% 8% 50%	1295
	11th to 20th	Day	5% 4 12% 6% 5% 8% 6% 8% 9% 35%	1481
	21st to end	Day	1 5% 3 14% 7% 7% 7% 6% 8% 8% 34%	1400
March	1st to 10th	Day	3 4 13% 9% 9% 8% 6% 9% 9% 30%	4312
	11th to 20th	Day	2 2 8% 7% 8% 8% 6% 9% 11% 38%	4699
	21st to end	Day	7% 5% 14% 8% 6% 5% 5% 8% 10% 33%	5917
April	1st to 10th	Day	1 8% 5% 16% 8% 6% 7% 5% 7% 9% 28%	2521
	11th to 20th	Day	1 8% 4 12% 8% 7% 6% 4 7% 8% 35%	2766
	21st to end	Day	2 11% 4 9% 5% 5% 5% 5% 9% 8% 37%	2955
May	1st to 10th	Day	3 14% 5% 11% 6% 6% 6% 6% 9% 10% 24%	7260
	11th to 20th	Day	3 17% 6% 10% 5% 4 5% 5% 6% 6% 32%	7602
	21st to end	Day	7% 14% 4 9% 6% 6% 7% 6% 8% 9% 24%	8911
June	1st to 10th	Day	14% 11% 4 10% 6% 6% 7% 5% 8% 9% 19%	3600
	11th to 20th	Day	17% 11% 4 8% 6% 5% 6% 4 8% 8% 24%	3929
	21st to end	Day	21% 11% 3 7% 6% 6% 6% 6% 7% 8% 19%	3897
July	1st to 10th	Day	34% 13% 3 7% 4 4 4 4 5% 6% 17%	5986
	11th to 20th	Day	19% 10% 4 9% 6% 6% 6% 5% 6% 8% 20%	5880
	21st to end	Day	27% 12% 3 7% 4 5% 5% 4 6% 6% 21%	5886
August	1st to 10th	Day	27% 12% 3 7% 5% 6% 6% 5% 7% 7% 15%	5046
	11th to 20th	Day	20% 17% 5% 11% 6% 5% 5% 4 6% 6% 16%	4657
	21st to end	Day	20% 21% 5% 11% 5% 5% 5% 4 4 4 16%	4884
September	1st to 10th	Day	17% 16% 4 10% 7% 6% 5% 4 6% 8% 18%	2413
	11th to 20th	Day	14% 17% 5% 10% 5% 5% 5% 5% 6% 7% 21%	2280
	21st to end	Day	9% 19% 6% 10% 4 5% 5% 4 5% 8% 24%	2017
October	1st to 10th	Day	3 12% 5% 12% 6% 4 5% 4 8% 11% 30%	3110
	11th to 20th	Day	1 7% 3 14% 7% 6% 6% 4 7% 10% 35%	2693
	21st to end	Day	3 9% 7% 16% 5% 5% 4 4 5% 7% 35%	2601
November	1st to 10th	Day	1 9% 4 10% 7% 7% 7% 6% 7% 7% 35%	1259
	11th to 20th	Day	2 5% 4 12% 11% 8% 6% 6% 6% 7% 34%	1100
	21st to end	Day	1 4 4 11% 8% 7% 7% 6% 7% 10% 35%	869
December	1st to 10th	Day	1 3 3 8% 7% 8% 7% 6% 11% 11% 37%	1310
	11th to 20th	Day	1 8% 4 10% 6% 7% 6% 4 8% 11% 34%	1517
	21st to end	Day	3 2 8% 5% 4 6% 5% 6% 10% 50%	2002
January	1st to 10th	Night	3 2 8% 6% 6% 6% 5% 8% 9% 48%	8146
	11th to 20th	Night	4 3 9% 7% 6% 7% 6% 9% 10% 38%	8000
	21st to end	Night	2 2 6% 6% 8% 10% 8% 8% 8% 41%	8239
February	1st to 10th	Night	4 3 8% 7% 6% 7% 5% 7% 7% 44%	3104
	11th to 20th	Night	1 5% 3 9% 7% 6% 7% 6% 9% 11% 36%	2806
	21st to end	Night	1 6% 4 14% 11% 9% 7% 5% 6% 7% 30%	2227
March	1st to 10th	Night	3 3 12% 9% 8% 7% 6% 8% 9% 35%	5542
	11th to 20th	Night	2 2 8% 6% 5% 6% 7% 10% 12% 42%	5257
	21st to end	Night	1 5% 3 10% 7% 6% 6% 6% 9% 11% 36%	5014
April	1st to 10th	Night	7% 5% 13% 9% 7% 7% 6% 9% 9% 28%	1876
	11th to 20th	Night	1 8% 4 10% 7% 6% 6% 4 7% 8% 38%	1583
	21st to end	Night	2 8% 2 7% 4 4 5% 5% 9% 10% 43%	1442
May	1st to 10th	Night	2 12% 6% 12% 7% 6% 6% 7% 9% 11% 22%	2720
	11th to 20th	Night	5% 18% 5% 8% 6% 4 6% 4 6% 5% 32%	2309
	21st to end	Night	10% 16% 4 9% 5% 5% 7% 6% 8% 9% 22%	1986
June	1st to 10th	Night	16% 12% 3 8% 4 6% 6% 5% 9% 9% 22%	800
	11th to 20th	Night	15% 12% 4 6% 6% 7% 5% 5% 8% 8% 23%	465
	21st to end	Night	21% 8% 2 8% 7% 6% 6% 6% 10% 7% 17%	498
July	1st to 10th	Night	28% 15% 5% 10% 5% 5% 5% 4 4 5% 15%	1204
	11th to 20th	Night	16% 11% 3 9% 7% 5% 6% 5% 8% 6% 23%	1310
	21st to end	Night	30% 14% 2 6% 4 5% 4 3 5% 6% 20%	1684
August	1st to 10th	Night	24% 12% 3 7% 5% 6% 6% 4 7% 7% 20%	1894
	11th to 20th	Night	18% 22% 4 8% 6% 5% 5% 4 5% 6% 17%	2178
	21st to end	Night	19% 18% 4 12% 6% 5% 5% 4 6% 4 16%	2783
September	1st to 10th	Night	15% 13% 2 8% 6% 8% 7% 5% 7% 6% 22%	1767
	11th to 20th	Night	12% 16% 5% 9% 8% 5% 6% 5% 7% 8% 19%	1900
	21st to end	Night	9% 18% 7% 10% 6% 6% 5% 4 6% 7% 22%	2161
October	1st to 10th	Night	4 14% 6% 11% 6% 5% 7% 5% 8% 10% 25%	3852
	11th to 20th	Night	2 10% 7% 15% 6% 5% 5% 4 5% 9% 33%	4220
	21st to end	Night	2 10% 6% 16% 7% 6% 5% 5% 6% 7% 30%	4958
November	1st to 10th	Night	2 8% 5% 13% 7% 7% 4 5% 7% 9% 34%	3037
	11th to 20th	Night	2 7% 4 13% 11% 9% 7% 5% 5% 7% 30%	3300
	21st to end	Night	1 6% 4 10% 8% 7% 6% 6% 8% 8% 36%	3530
December	1st to 10th	Night	1 6% 3 8% 6% 5% 6% 6% 9% 12% 38%	5880
	11th to 20th	Night	1 6% 5% 12% 7% 6% 6% 5% 7% 9% 36%	6700
	21st to end	Night	3 2 9% 6% 6% 6% 6% 9% 12% 41%	8976

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	27% 6% 13% 22% 17% 15%	1819
	11th to 20th	Day	25% 6% 18% 32% 13% 6%	1933
	21st to end	Day	31% 8% 9% 18% 17% 16%	2750
February	1st to 10th	Day	30% 4 10% 20% 23% 13%	1295
	11th to 20th	Day	38% 6% 16% 20% 13% 6%	1487
	21st to end	Day	39% 7% 13% 17% 18% 5%	1400
March	1st to 10th	Day	42% 5% 10% 21% 15% 7%	4312
	11th to 20th	Day	32% 5% 8% 21% 28% 6%	4699
	21st to end	Day	39% 8% 13% 16% 18% 6%	5917
April	1st to 10th	Day	45% 8% 15% 19% 12% 2	2521
	11th to 20th	Day	43% 6% 16% 20% 10% 5%	2766
	21st to end	Day	42% 6% 13% 22% 16% 2	2955
May	1st to 10th	Day	45% 7% 17% 23% 8%	7260
	11th to 20th	Day	49% 7% 14% 19% 11%	7605
	21st to end	Day	47% 9% 16% 22% 7%	8912
June	1st to 10th	Day	56% 8% 18% 15% 3	3600
	11th to 20th	Day	53% 9% 19% 16% 3	3929
	21st to end	Day	57% 10% 20% 12% 1	3897
July	1st to 10th	Day	66% 7% 17% 10% 1	5897
	11th to 20th	Day	53% 10% 17% 17% 3	5781
	21st to end	Day	59% 8% 18% 13% 1	5700
August	1st to 10th	Day	62% 11% 19% 7% 1	5027
	11th to 20th	Day	63% 10% 12% 12% 2	4705
	21st to end	Day	69% 7% 11% 10% 2	4883
September	1st to 10th	Day	64% 7% 15% 10% 4	2413
	11th to 20th	Day	61% 9% 16% 12% 2	2280
	21st to end	Day	59% 5% 15% 16% 5% 1	2017
October	1st to 10th	Day	46% 8% 15% 19% 12% 1	3110
	11th to 20th	Day	40% 7% 19% 27% 5% 2	2693
	21st to end	Day	48% 6% 11% 19% 15% 1	2601
November	1st to 10th	Day	42% 9% 15% 20% 9% 5%	1259
	11th to 20th	Day	47% 5% 12% 23% 11% 2	1100
	21st to end	Day	39% 7% 16% 19% 13% 6%	869
December	1st to 10th	Day	32% 9% 20% 19% 18% 2	1310
	11th to 20th	Day	38% 5% 11% 25% 15% 5%	1520
	21st to end	Day	25% 6% 15% 16% 20% 18%	2002
January	1st to 10th	Night	26% 6% 12% 24% 17% 14%	8146
	11th to 20th	Night	30% 7% 16% 25% 16% 5%	8050
	21st to end	Night	28% 9% 10% 16% 21% 17%	8239
February	1st to 10th	Night	34% 5% 10% 19% 19% 13%	3104
	11th to 20th	Night	35% 7% 17% 19% 16% 6%	2811
	21st to end	Night	47% 6% 10% 18% 14% 4	2227
March	1st to 10th	Night	37% 5% 8% 23% 18% 9%	5542
	11th to 20th	Night	27% 5% 7% 25% 28% 8%	5259
	21st to end	Night	35% 7% 12% 20% 20% 7%	5014
April	1st to 10th	Night	44% 9% 14% 19% 12% 2	1876
	11th to 20th	Night	40% 6% 16% 20% 14% 3	1583
	21st to end	Night	33% 7% 13% 28% 18% 2	1442
May	1st to 10th	Night	47% 7% 13% 25% 9%	2717
	11th to 20th	Night	49% 6% 9% 25% 10%	2309
	21st to end	Night	49% 10% 13% 20% 8%	1986
June	1st to 10th	Night	54% 10% 18% 16% 2	800
	11th to 20th	Night	56% 9% 16% 16% 2	465
	21st to end	Night	58% 14% 16% 11%	498
July	1st to 10th	Night	69% 9% 13% 9% 1	1184
	11th to 20th	Night	52% 10% 15% 21% 2	1286
	21st to end	Night	63% 8% 18% 10% 1	1631
August	1st to 10th	Night	59% 10% 26% 5% 1	1888
	11th to 20th	Night	64% 9% 16% 10% 1	2196
	21st to end	Night	67% 9% 14% 9% 1	2783
September	1st to 10th	Night	60% 8% 17% 11% 4	1767
	11th to 20th	Night	60% 9% 17% 11% 4	1900
	21st to end	Night	61% 5% 12% 14% 7%	2161
October	1st to 10th	Night	50% 9% 12% 19% 7% 3	3852
	11th to 20th	Night	48% 5% 13% 23% 9% 2	4220
	21st to end	Night	49% 7% 12% 18% 14% 1	4958
November	1st to 10th	Night	45% 6% 15% 21% 10% 4	3037
	11th to 20th	Night	51% 5% 11% 17% 13% 2	3300
	21st to end	Night	40% 7% 14% 20% 14% 5%	3530
December	1st to 10th	Night	33% 8% 15% 23% 18% 3	5880
	11th to 20th	Night	39% 6% 14% 19% 15% 6%	6696
	21st to end	Night	29% 6% 16% 15% 18% 16%	8976

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% 24% 25% 7%	100
	11th to 20th	Day	3 9% 26% 38% 17% 7%	148
	21st to end	Day	2 8% 28% 35% 20% 6%	330
February	1st to 10th	Day	3 9% 22% 30% 22% 14%	300
	11th to 20th	Day	5% 11% 28% 28% 19% 9%	355
	21st to end	Day	5% 12% 33% 29% 15% 5%	331
March	1st to 10th	Day	4 7% 29% 36% 19% 6%	400
	11th to 20th	Day	2 7% 26% 31% 25% 8%	475
	21st to end	Day	5% 9% 24% 29% 24% 8%	660
April	1st to 10th	Day	4 13% 33% 28% 16% 6%	623
	11th to 20th	Day	6% 10% 23% 32% 24% 6%	690
	21st to end	Day	6% 5% 20% 35% 30% 6%	700
May	1st to 10th	Day	9% 9% 23% 29% 24% 6%	703
	11th to 20th	Day	8% 8% 23% 31% 22% 8%	789
	21st to end	Day	15% 9% 18% 25% 24% 8%	1006
June	1st to 10th	Day	18% 6% 18% 26% 18% 13%	930
	11th to 20th	Day	19% 6% 16% 24% 24% 12%	1078
	21st to end	Day	22% 6% 14% 22% 22% 14%	1027
July	1st to 10th	Day	27% 7% 18% 26% 16% 6%	916
	11th to 20th	Day	29% 9% 17% 22% 18% 6%	912
	21st to end	Day	30% 5% 19% 21% 17% 7%	910
August	1st to 10th	Day	36% 10% 19% 20% 11% 5%	676
	11th to 20th	Day	32% 13% 22% 20% 10% 3	665
	21st to end	Day	35% 10% 20% 19% 12% 3	748
September	1st to 10th	Day	26% 11% 23% 21% 15% 4	613
	11th to 20th	Day	28% 13% 20% 23% 13% 4	580
	21st to end	Day	22% 10% 15% 26% 18% 7%	414
October	1st to 10th	Day	20% 12% 25% 20% 17% 6%	390
	11th to 20th	Day	7% 7% 20% 27% 25% 14%	390
	21st to end	Day	6% 9% 24% 27% 22% 13%	429
November	1st to 10th	Day	5% 8% 24% 28% 26% 9%	338
	11th to 20th	Day	5% 12% 20% 26% 25% 13%	200
	21st to end	Day	4 12% 22% 32% 22% 8%	100
December	1st to 10th	Day	2 9% 33% 27% 21% 8%	100
	11th to 20th	Day	4 9% 32% 29% 20% 6%	100
	21st to end	Day	8% 14% 24% 27% 21% 6%	110
January	1st to 10th	Night	4 10% 23% 29% 25% 11%	1000
	11th to 20th	Night	5% 8% 27% 36% 17% 8%	1000
	21st to end	Night	3 7% 29% 32% 22% 6%	1045
February	1st to 10th	Night	3 8% 24% 33% 23% 9%	900
	11th to 20th	Night	4 10% 30% 32% 19% 5%	884
	21st to end	Night	7% 15% 26% 27% 21% 5%	660
March	1st to 10th	Night	2 8% 24% 39% 21% 6%	700
	11th to 20th	Night	2 5% 21% 32% 28% 12%	700
	21st to end	Night	4 8% 20% 28% 24% 17%	672
April	1st to 10th	Night	4 9% 24% 35% 20% 10%	600
	11th to 20th	Night	5% 9% 20% 27% 26% 13%	593
	21st to end	Night	6% 3 14% 37% 29% 11%	531
May	1st to 10th	Night	8% 4 20% 28% 31% 11%	400
	11th to 20th	Night	9% 9% 18% 30% 26% 7%	292
	21st to end	Night	13% 5% 20% 21% 35% 6%	217
June	1st to 10th	Night	22% 9% 14% 27% 19% 10%	200
	11th to 20th	Night	20% 4 17% 25% 27% 9%	200
	21st to end	Night	24% 6% 12% 23% 24% 12%	200
July	1st to 10th	Night	27% 5% 13% 29% 18% 7%	195
	11th to 20th	Night	27% 6% 19% 23% 20% 6%	194
	21st to end	Night	30% 5% 14% 23% 19% 10%	350
August	1st to 10th	Night	31% 5% 21% 22% 16% 5%	378
	11th to 20th	Night	27% 10% 21% 23% 14% 5%	433
	21st to end	Night	28% 9% 19% 19% 17% 7%	627
September	1st to 10th	Night	25% 8% 20% 24% 14% 9%	570
	11th to 20th	Night	22% 10% 22% 20% 17% 8%	570
	21st to end	Night	19% 8% 16% 26% 23% 9%	620
October	1st to 10th	Night	17% 12% 20% 22% 19% 9%	696
	11th to 20th	Night	8% 7% 21% 27% 25% 12%	860
	21st to end	Night	8% 7% 21% 34% 22% 9%	946
November	1st to 10th	Night	7% 8% 22% 32% 23% 9%	880
	11th to 20th	Night	5% 9% 22% 34% 21% 10%	900
	21st to end	Night	4 11% 23% 30% 25% 8%	965
December	1st to 10th	Night	5% 11% 26% 27% 22% 9%	1000
	11th to 20th	Night	4 7% 29% 32% 22% 7%	1000
	21st to end	Night	7% 11% 21% 31% 20% 10%	1100

Page generated at 2025-07-17 21:10:05. Version ac07

Copyright © 2025 Attilla Danko.