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 2024-11-04.

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% 11% 4 10% 6% 6% 6% 5% 7% 8% 27%	107458
	Night	4 8% 4 10% 7% 6% 6% 5% 8% 9% 33%	100171
Transparency	Day	50% 7% 15% 17% 8% 2	107137
	Night	43% 7% 13% 18% 13% 6%	100139
Seeing	Day	17% 9% 21% 26% 20% 8%	18318
	Night	9% 8% 22% 29% 22% 9%	22140

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 10% 5% 7% 6% 6% 8% 10% 43%	1269
	11th to 20th	Day	1 5% 5% 8% 5% 4 5% 6% 10% 9% 45%	1329
	21st to end	Day	3 2 6% 5% 5% 8% 6% 10% 9% 44%	1913
February	1st to 10th	Day	5% 2 9% 6% 5% 4 3 7% 8% 50%	1230
	11th to 20th	Day	6% 4 12% 6% 5% 8% 7% 8% 9% 34%	1405
	21st to end	Day	1 5% 4 14% 7% 8% 7% 6% 7% 8% 32%	1332
March	1st to 10th	Day	1 4 4 13% 11% 11% 8% 6% 7% 7% 30%	3008
	11th to 20th	Day	3 3 9% 6% 7% 7% 6% 10% 12% 35%	3283
	21st to end	Day	6% 6% 14% 8% 6% 6% 6% 8% 9% 31%	4127
April	1st to 10th	Day	7% 5% 16% 8% 6% 7% 5% 8% 9% 28%	2395
	11th to 20th	Day	1 8% 4 12% 8% 7% 6% 4 7% 8% 36%	2626
	21st to end	Day	2 11% 4 9% 5% 5% 6% 5% 8% 8% 37%	2806
May	1st to 10th	Day	3 13% 4 8% 5% 5% 7% 6% 10% 11% 26%	5070
	11th to 20th	Day	3 13% 4 9% 5% 5% 6% 5% 7% 7% 35%	5305
	21st to end	Day	7% 14% 3 8% 5% 5% 6% 6% 7% 9% 29%	6205
June	1st to 10th	Day	14% 11% 4 10% 6% 6% 7% 6% 8% 10% 19%	3420
	11th to 20th	Day	18% 12% 4 8% 6% 5% 6% 4 7% 7% 22%	3732
	21st to end	Day	21% 11% 3 7% 6% 5% 6% 6% 7% 8% 19%	3702
July	1st to 10th	Day	34% 13% 3 7% 4 4 4 4 5% 5% 16%	5803
	11th to 20th	Day	19% 10% 4 9% 6% 6% 6% 5% 6% 8% 20%	5700
	21st to end	Day	27% 12% 3 7% 4 5% 5% 4 6% 6% 21%	5885
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 10% 5% 10% 7% 7% 7% 6% 7% 6% 35%	1217
	11th to 20th	Day	2 6% 4 12% 10% 8% 6% 6% 6% 6% 34%	1045
	21st to end	Day	1 4 4 10% 8% 7% 7% 5% 8% 10% 35%	826
December	1st to 10th	Day	2 4 3 11% 8% 9% 8% 5% 9% 8% 32%	760
	11th to 20th	Day	1 5% 5% 9% 8% 10% 8% 5% 9% 11% 31%	967
	21st to end	Day	1 3 2 7% 5% 4 6% 5% 6% 10% 51%	1397
January	1st to 10th	Night	3 2 9% 7% 7% 6% 6% 8% 10% 43%	5686
	11th to 20th	Night	1 5% 3 8% 7% 6% 7% 6% 7% 8% 41%	5579
	21st to end	Night	2 2 6% 6% 7% 7% 7% 8% 9% 47%	5760
February	1st to 10th	Night	4 3 8% 7% 6% 7% 5% 7% 7% 44%	2949
	11th to 20th	Night	1 5% 3 9% 7% 6% 7% 6% 9% 10% 35%	2662
	21st to end	Night	1 6% 4 14% 11% 9% 7% 5% 6% 7% 29%	2119
March	1st to 10th	Night	4 4 14% 11% 8% 7% 6% 8% 8% 31%	3846
	11th to 20th	Night	3 3 10% 6% 5% 7% 8% 10% 13% 35%	3673
	21st to end	Night	5% 3 10% 6% 5% 6% 7% 10% 12% 35%	3504
April	1st to 10th	Night	6% 5% 13% 9% 6% 7% 6% 9% 9% 28%	1782
	11th to 20th	Night	1 8% 4 9% 7% 6% 6% 5% 7% 8% 39%	1503
	21st to end	Night	2 9% 2 7% 4 4 5% 5% 9% 9% 43%	1371
May	1st to 10th	Night	2 11% 6% 12% 7% 7% 6% 8% 9% 11% 22%	1900
	11th to 20th	Night	5% 14% 5% 8% 6% 5% 6% 5% 7% 6% 33%	1598
	21st to end	Night	11% 13% 3 7% 4 4 6% 5% 9% 9% 28%	1381
June	1st to 10th	Night	16% 12% 3 8% 4 6% 6% 5% 9% 9% 22%	760
	11th to 20th	Night	16% 12% 5% 6% 6% 7% 5% 6% 7% 8% 22%	442
	21st to end	Night	22% 8% 2 7% 8% 6% 6% 6% 10% 8% 17%	473
July	1st to 10th	Night	29% 15% 5% 10% 5% 5% 4 4 4 5% 15%	1167
	11th to 20th	Night	16% 11% 3 9% 7% 6% 6% 4 8% 7% 23%	1270
	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 9% 5% 12% 7% 7% 5% 5% 6% 8% 34%	2931
	11th to 20th	Night	2 7% 4 13% 10% 9% 7% 5% 6% 7% 30%	3135
	21st to end	Night	1 6% 3 9% 8% 8% 6% 6% 8% 8% 37%	3353
December	1st to 10th	Night	1 7% 4 9% 7% 5% 6% 7% 9% 12% 34%	3420
	11th to 20th	Night	5% 3 12% 8% 7% 7% 7% 8% 9% 33%	4240
	21st to end	Night	1 4 2 7% 6% 6% 6% 6% 9% 11% 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	45% 4 12% 15% 14% 10%	3008
	11th to 20th	Day	33% 4 10% 26% 22% 5%	3283
	21st to end	Day	41% 9% 17% 19% 9% 5%	4127
April	1st to 10th	Day	44% 8% 15% 19% 12% 1	2395
	11th to 20th	Day	43% 6% 17% 19% 10% 5%	2626
	21st to end	Day	43% 5% 13% 20% 16% 2	2806
May	1st to 10th	Day	43% 6% 19% 25% 8%	5070
	11th to 20th	Day	44% 7% 15% 23% 11%	5308
	21st to end	Day	45% 7% 14% 24% 9%	6206
June	1st to 10th	Day	56% 8% 19% 14% 3	3420
	11th to 20th	Day	55% 9% 17% 16% 3	3732
	21st to end	Day	57% 10% 20% 12% 1	3702
July	1st to 10th	Day	66% 7% 17% 9% 1	5714
	11th to 20th	Day	53% 10% 17% 17% 3	5601
	21st to end	Day	59% 8% 18% 13% 1	5699
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	44% 9% 15% 19% 8% 5%	1217
	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	42% 4 9% 15% 20% 9%	3846
	11th to 20th	Night	32% 5% 9% 26% 23% 6%	3675
	21st to end	Night	34% 7% 16% 23% 14% 7%	3504
April	1st to 10th	Night	44% 9% 14% 19% 12% 2	1782
	11th to 20th	Night	38% 7% 16% 21% 14% 4	1503
	21st to end	Night	34% 7% 13% 27% 18% 2	1371
May	1st to 10th	Night	50% 6% 14% 23% 7%	1900
	11th to 20th	Night	48% 7% 10% 25% 10%	1598
	21st to end	Night	46% 9% 14% 20% 11%	1381
June	1st to 10th	Night	54% 9% 19% 15% 2	760
	11th to 20th	Night	58% 9% 15% 16% 2	442
	21st to end	Night	58% 14% 16% 10%	473
July	1st to 10th	Night	70% 9% 12% 8% 1	1147
	11th to 20th	Night	53% 9% 15% 21% 2	1246
	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% 20% 10% 4	2931
	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% 32% 29% 17% 6%	581
	11th to 20th	Day	5% 9% 22% 33% 25% 6%	640
	21st to end	Day	6% 4 20% 35% 30% 5%	650
May	1st to 10th	Day	9% 8% 23% 30% 25% 7%	653
	11th to 20th	Day	8% 8% 23% 32% 22% 8%	736
	21st to end	Day	15% 9% 19% 26% 24% 7%	940
June	1st to 10th	Day	18% 6% 18% 26% 18% 14%	870
	11th to 20th	Day	20% 6% 16% 23% 23% 12%	1010
	21st to end	Day	23% 5% 14% 22% 22% 14%	962
July	1st to 10th	Day	28% 7% 18% 25% 16% 6%	856
	11th to 20th	Day	30% 8% 17% 22% 17% 6%	852
	21st to end	Day	30% 5% 19% 21% 17% 7%	909
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% 27% 8%	324
	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%	570
	11th to 20th	Night	4 8% 19% 27% 27% 14%	563
	21st to end	Night	6% 3 14% 38% 28% 11%	505
May	1st to 10th	Night	7% 3 19% 28% 31% 11%	380
	11th to 20th	Night	8% 9% 18% 31% 26% 8%	277
	21st to end	Night	13% 5% 21% 21% 34% 6%	206
June	1st to 10th	Night	21% 9% 14% 27% 18% 11%	190
	11th to 20th	Night	21% 4 17% 25% 25% 9%	190
	21st to end	Night	25% 5% 12% 22% 25% 13%	190
July	1st to 10th	Night	27% 5% 14% 29% 18% 8%	185
	11th to 20th	Night	28% 5% 18% 23% 19% 6%	184
	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% 21% 32% 23% 9%	847
	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 2024-11-01 18:47:44. Version ac07

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