Begich Boggs Visitor Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Begich Boggs Visitor Center clear sky chart from 2005-11-06 to 2025-05-11.

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	21% 21% 5% 11% 6% 5% 4 3 4 4 16%	116041
	Night	15% 24% 6% 12% 6% 5% 5% 3 4 4 15%	121617
Transparency	Day	68% 8% 10% 9% 4 1	115735
	Night	67% 7% 8% 9% 7% 2	121572
Seeing	Day	36% 12% 16% 19% 14% 3	18444
	Night	34% 12% 18% 19% 14% 2	23239

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	6% 32% 6% 13% 7% 8% 6% 4 6% 4 9%	1847
	11th to 20th	Day	14% 24% 5% 10% 5% 5% 4 4 7% 6% 17%	2627
	21st to end	Day	11% 20% 7% 13% 6% 5% 7% 4 4 4 18%	2992
February	1st to 10th	Day	8% 20% 6% 11% 6% 6% 6% 4 6% 7% 21%	1411
	11th to 20th	Day	14% 28% 7% 12% 7% 5% 5% 3 5% 3 11%	1559
	21st to end	Day	6% 20% 5% 12% 7% 5% 4 4 6% 7% 24%	1418
March	1st to 10th	Day	8% 26% 7% 15% 9% 6% 4 3 3 3 15%	4469
	11th to 20th	Day	5% 23% 9% 16% 8% 6% 6% 4 5% 4 13%	4641
	21st to end	Day	10% 27% 9% 15% 6% 5% 4 3 4 4 14%	5969
April	1st to 10th	Day	9% 26% 7% 13% 6% 5% 4 3 3 4 18%	2440
	11th to 20th	Day	9% 22% 6% 10% 5% 4 5% 3 5% 6% 24%	2748
	21st to end	Day	11% 25% 6% 11% 6% 4 5% 3 4 5% 21%	2835
May	1st to 10th	Day	20% 37% 6% 10% 5% 3 2 2 3 3 9%	5088
	11th to 20th	Day	13% 24% 5% 11% 5% 4 4 3 4 5% 22%	5029
	21st to end	Day	25% 26% 4 9% 5% 4 3 2 3 4 16%	5812
June	1st to 10th	Day	29% 16% 4 7% 6% 4 4 4 5% 4 17%	3420
	11th to 20th	Day	23% 14% 4 10% 5% 5% 5% 4 5% 5% 20%	3420
	21st to end	Day	28% 14% 3 7% 5% 5% 4 3 5% 6% 20%	3420
July	1st to 10th	Day	28% 16% 5% 10% 6% 5% 5% 4 5% 5% 12%	5700
	11th to 20th	Day	32% 18% 4 9% 6% 4 4 4 4 4 12%	5410
	21st to end	Day	30% 13% 4 8% 5% 4 4 4 5% 5% 19%	5499
August	1st to 10th	Day	33% 14% 4 8% 5% 4 4 3 4 4 16%	5026
	11th to 20th	Day	29% 13% 3 9% 6% 5% 6% 3 3 4 18%	4358
	21st to end	Day	37% 12% 3 7% 5% 5% 4 3 4 4 15%	4866
September	1st to 10th	Day	42% 15% 5% 9% 5% 3 2 2 3 3 10%	2337
	11th to 20th	Day	43% 14% 4 7% 4 3 3 2 2 3 13%	2264
	21st to end	Day	34% 17% 4 8% 5% 4 4 2 2 5% 14%	2046
October	1st to 10th	Day	21% 20% 6% 14% 7% 6% 5% 3 3 3 10%	3107
	11th to 20th	Day	17% 19% 5% 12% 4 4 5% 4 6% 4 21%	2834
	21st to end	Day	19% 22% 4 11% 5% 4 4 7% 5% 4 16%	2703
November	1st to 10th	Day	16% 26% 6% 14% 5% 4 5% 3 4 5% 13%	1371
	11th to 20th	Day	13% 20% 5% 9% 7% 4 8% 5% 8% 8% 14%	1249
	21st to end	Day	14% 25% 7% 14% 6% 3 5% 5% 6% 4 12%	1036
December	1st to 10th	Day	19% 30% 6% 12% 7% 6% 7% 3 3 3 4	1571
	11th to 20th	Day	12% 33% 6% 12% 8% 6% 6% 5% 3 3 7%	1517
	21st to end	Day	6% 24% 7% 17% 8% 7% 5% 4 5% 6% 13%	2002
January	1st to 10th	Night	10% 30% 6% 13% 6% 6% 4 3 6% 5% 12%	8106
	11th to 20th	Night	13% 27% 6% 12% 6% 4 3 2 3 3 20%	7291
	21st to end	Night	12% 19% 5% 13% 6% 6% 5% 4 5% 7% 18%	7986
February	1st to 10th	Night	8% 20% 6% 12% 7% 5% 5% 4 7% 5% 20%	2985
	11th to 20th	Night	15% 28% 5% 15% 7% 5% 4 3 4 3 10%	2728
	21st to end	Night	7% 18% 5% 12% 6% 6% 5% 4 6% 7% 24%	2210
March	1st to 10th	Night	7% 30% 8% 14% 7% 5% 4 3 3 3 16%	5384
	11th to 20th	Night	3 23% 10% 20% 8% 6% 6% 4 5% 4 13%	5314
	21st to end	Night	7% 29% 8% 16% 7% 6% 5% 3 4 3 12%	4962
April	1st to 10th	Night	9% 27% 6% 13% 5% 4 4 3 5% 5% 19%	1960
	11th to 20th	Night	8% 22% 5% 9% 5% 6% 6% 3 6% 7% 23%	1598
	21st to end	Night	11% 23% 7% 10% 5% 4 4 3 4 4 24%	1565
May	1st to 10th	Night	20% 34% 6% 11% 4 3 2 2 3 4 11%	2099
	11th to 20th	Night	14% 26% 5% 9% 5% 5% 4 3 4 4 22%	1882
	21st to end	Night	24% 25% 5% 7% 4 3 4 3 4 4 16%	1762
June	1st to 10th	Night	29% 14% 4 8% 4 5% 5% 4 4 6% 18%	760
	11th to 20th	Night	23% 16% 3 8% 3 3 4 5% 6% 7% 21%	760
	21st to end	Night	27% 14% 2 6% 6% 4 5% 3 5% 7% 21%	760
July	1st to 10th	Night	29% 15% 4 10% 6% 5% 4 3 5% 5% 13%	1270
	11th to 20th	Night	33% 16% 5% 9% 4 3 4 2 3 3 17%	1560
	21st to end	Night	32% 14% 3 7% 4 5% 7% 3 4 6% 15%	2066
August	1st to 10th	Night	36% 14% 3 9% 5% 5% 3 2 3 4 18%	1912
	11th to 20th	Night	30% 12% 3 8% 5% 6% 6% 4 5% 5% 17%	2480
	21st to end	Night	33% 15% 4 9% 5% 4 4 3 3 3 17%	2801
September	1st to 10th	Night	43% 15% 5% 7% 4 4 3 2 2 3 11%	1842
	11th to 20th	Night	42% 15% 4 7% 4 3 3 3 2 2 16%	1916
	21st to end	Night	34% 16% 3 9% 5% 4 3 2 3 3 17%	2133
October	1st to 10th	Night	24% 22% 6% 12% 7% 5% 4 2 3 4 12%	3854
	11th to 20th	Night	18% 20% 4 8% 5% 5% 5% 3 3 4 24%	4094
	21st to end	Night	18% 20% 5% 11% 6% 6% 6% 4 5% 3 14%	4855
November	1st to 10th	Night	18% 25% 6% 13% 6% 5% 4 4 3 3 14%	2927
	11th to 20th	Night	13% 17% 4 11% 6% 6% 7% 4 7% 7% 17%	3150
	21st to end	Night	13% 24% 7% 12% 6% 4 4 5% 5% 6% 14%	3362
December	1st to 10th	Night	15% 35% 6% 13% 6% 5% 5% 3 4 3 6%	5607
	11th to 20th	Night	8% 30% 8% 16% 8% 6% 5% 4 4 3 7%	6700
	21st to end	Night	7% 27% 7% 16% 8% 7% 5% 4 5% 6% 8%	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	67% 8% 5% 9% 9% 3	1847
	11th to 20th	Day	62% 5% 7% 11% 14% 2	2645
	21st to end	Day	59% 8% 5% 6% 16% 6%	2992
February	1st to 10th	Day	58% 6% 6% 14% 10% 6%	1411
	11th to 20th	Day	73% 5% 5% 8% 7% 3	1566
	21st to end	Day	55% 7% 7% 14% 15% 3	1418
March	1st to 10th	Day	67% 6% 7% 8% 7% 5%	4469
	11th to 20th	Day	62% 11% 10% 9% 8% 1	4641
	21st to end	Day	67% 8% 11% 7% 6% 1	5969
April	1st to 10th	Day	68% 6% 11% 9% 5% 1	2440
	11th to 20th	Day	58% 8% 11% 15% 7%	2748
	21st to end	Day	66% 7% 8% 12% 6% 1	2835
May	1st to 10th	Day	82% 5% 6% 5% 2	5088
	11th to 20th	Day	59% 11% 8% 18% 4	5032
	21st to end	Day	69% 7% 11% 11% 2	5813
June	1st to 10th	Day	68% 7% 14% 10% 1	3420
	11th to 20th	Day	65% 9% 12% 11% 2	3420
	21st to end	Day	65% 9% 18% 8%	3420
July	1st to 10th	Day	75% 11% 10% 5%	5611
	11th to 20th	Day	75% 9% 13% 3	5312
	21st to end	Day	66% 10% 17% 7%	5326
August	1st to 10th	Day	72% 8% 15% 4	5007
	11th to 20th	Day	67% 10% 10% 12% 1	4400
	21st to end	Day	72% 6% 10% 10% 1	4865
September	1st to 10th	Day	81% 6% 6% 6% 2	2337
	11th to 20th	Day	76% 5% 9% 9% 1	2264
	21st to end	Day	72% 5% 10% 9% 3	2046
October	1st to 10th	Day	76% 10% 7% 6% 1	3107
	11th to 20th	Day	61% 8% 10% 12% 10%	2834
	21st to end	Day	60% 8% 13% 12% 7% 1	2703
November	1st to 10th	Day	70% 3 7% 11% 6% 4	1371
	11th to 20th	Day	57% 5% 8% 19% 11% 1	1249
	21st to end	Day	69% 3 7% 10% 9% 3	1036
December	1st to 10th	Day	78% 6% 7% 4 4 1	1571
	11th to 20th	Day	74% 8% 8% 8% 3	1520
	21st to end	Day	61% 9% 7% 7% 7% 8%	2002
January	1st to 10th	Night	66% 7% 5% 9% 10% 2	8106
	11th to 20th	Night	65% 5% 4 8% 12% 5%	7336
	21st to end	Night	56% 7% 6% 9% 14% 8%	7986
February	1st to 10th	Night	59% 5% 6% 13% 11% 6%	2985
	11th to 20th	Night	74% 5% 6% 7% 6% 2	2732
	21st to end	Night	54% 7% 9% 12% 16% 2	2210
March	1st to 10th	Night	67% 6% 6% 7% 8% 5%	5384
	11th to 20th	Night	63% 11% 9% 8% 8% 1	5316
	21st to end	Night	65% 11% 10% 7% 5% 1	4962
April	1st to 10th	Night	66% 5% 9% 13% 6% 1	1960
	11th to 20th	Night	57% 6% 11% 17% 8%	1598
	21st to end	Night	64% 6% 8% 13% 7% 1	1565
May	1st to 10th	Night	77% 7% 7% 7% 2	2099
	11th to 20th	Night	60% 10% 13% 14% 4	1883
	21st to end	Night	68% 9% 11% 11% 2	1762
June	1st to 10th	Night	69% 9% 10% 10% 1	760
	11th to 20th	Night	62% 9% 16% 11% 2	760
	21st to end	Night	65% 8% 19% 7% 1	760
July	1st to 10th	Night	76% 7% 11% 6%	1250
	11th to 20th	Night	71% 8% 18% 3	1535
	21st to end	Night	69% 11% 17% 3	2000
August	1st to 10th	Night	72% 9% 13% 6%	1906
	11th to 20th	Night	65% 13% 10% 9% 3	2504
	21st to end	Night	73% 6% 9% 10% 2	2801
September	1st to 10th	Night	80% 6% 7% 7% 1	1842
	11th to 20th	Night	75% 5% 8% 9% 3	1916
	21st to end	Night	70% 6% 10% 10% 3	2133
October	1st to 10th	Night	75% 7% 7% 10% 2	3854
	11th to 20th	Night	61% 7% 7% 13% 11%	4094
	21st to end	Night	62% 7% 13% 12% 5% 1	4855
November	1st to 10th	Night	71% 3 6% 10% 7% 3	2927
	11th to 20th	Night	55% 4 9% 19% 11% 1	3150
	21st to end	Night	66% 3 7% 11% 10% 3	3362
December	1st to 10th	Night	78% 5% 6% 5% 4 1	5607
	11th to 20th	Night	73% 8% 8% 6% 4 1	6696
	21st to end	Night	66% 8% 6% 6% 7% 8%	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	19% 17% 22% 18% 20% 4	115
	11th to 20th	Day	33% 14% 23% 19% 9% 2	300
	21st to end	Day	35% 14% 22% 21% 8% 1	330
February	1st to 10th	Day	19% 15% 21% 24% 14% 7%	323
	11th to 20th	Day	31% 16% 23% 17% 10% 2	395
	21st to end	Day	27% 13% 31% 17% 7% 5%	331
March	1st to 10th	Day	23% 13% 27% 20% 15% 2	400
	11th to 20th	Day	19% 20% 23% 18% 16% 4	437
	21st to end	Day	23% 16% 19% 19% 17% 5%	660
April	1st to 10th	Day	27% 22% 23% 16% 11% 1	620
	11th to 20th	Day	25% 12% 15% 25% 19% 5%	690
	21st to end	Day	30% 12% 14% 22% 17% 5%	700
May	1st to 10th	Day	40% 17% 14% 15% 13% 2	703
	11th to 20th	Day	32% 10% 14% 24% 16% 4	677
	21st to end	Day	33% 11% 18% 21% 14% 3	745
June	1st to 10th	Day	34% 11% 11% 20% 19% 5%	870
	11th to 20th	Day	29% 9% 13% 24% 21% 5%	870
	21st to end	Day	34% 10% 11% 18% 21% 6%	870
July	1st to 10th	Day	36% 9% 12% 23% 16% 4	856
	11th to 20th	Day	40% 12% 11% 18% 15% 3	786
	21st to end	Day	43% 11% 11% 16% 14% 4	739
August	1st to 10th	Day	47% 10% 12% 17% 11% 3	676
	11th to 20th	Day	48% 12% 15% 14% 8% 2	665
	21st to end	Day	48% 9% 16% 16% 10% 1	748
September	1st to 10th	Day	49% 13% 14% 13% 10% 1	610
	11th to 20th	Day	48% 11% 12% 15% 11% 2	564
	21st to end	Day	45% 12% 14% 15% 13% 2	390
October	1st to 10th	Day	45% 12% 16% 13% 11% 2	390
	11th to 20th	Day	40% 15% 18% 15% 9% 3	390
	21st to end	Day	40% 11% 18% 16% 13% 2	429
November	1st to 10th	Day	34% 13% 19% 18% 13% 3	395
	11th to 20th	Day	30% 11% 23% 24% 11% 1	324
	21st to end	Day	32% 13% 21% 21% 11% 3	136
December	1st to 10th	Day	39% 9% 16% 24% 11% 1	100
	11th to 20th	Day	33% 13% 26% 16% 10% 2	100
	21st to end	Day	35% 14% 25% 17% 9%	110
January	1st to 10th	Night	26% 14% 19% 23% 16% 2	1000
	11th to 20th	Night	34% 16% 21% 17% 10% 2	917
	21st to end	Night	33% 12% 24% 21% 8% 1	990
February	1st to 10th	Night	23% 13% 22% 25% 14% 2	900
	11th to 20th	Night	36% 15% 21% 19% 9%	884
	21st to end	Night	23% 15% 28% 21% 11% 3	643
March	1st to 10th	Night	20% 16% 24% 24% 13% 3	700
	11th to 20th	Night	17% 16% 22% 27% 15% 2	696
	21st to end	Night	29% 12% 17% 21% 17% 5%	660
April	1st to 10th	Night	29% 17% 18% 19% 15% 2	600
	11th to 20th	Night	25% 11% 15% 21% 26% 2	593
	21st to end	Night	28% 11% 17% 19% 21% 3	600
May	1st to 10th	Night	34% 15% 12% 17% 19% 3	487
	11th to 20th	Night	32% 10% 9% 19% 26% 4	374
	21st to end	Night	32% 8% 10% 21% 25% 4	249
June	1st to 10th	Night	34% 13% 7% 18% 23% 4	190
	11th to 20th	Night	33% 7% 8% 17% 31% 4	190
	21st to end	Night	35% 8% 12% 14% 27% 5%	190
July	1st to 10th	Night	37% 6% 10% 17% 26% 3	185
	11th to 20th	Night	39% 6% 10% 17% 24% 4	281
	21st to end	Night	47% 7% 11% 13% 18% 3	416
August	1st to 10th	Night	50% 8% 11% 12% 16% 2	378
	11th to 20th	Night	49% 8% 13% 17% 11% 2	556
	21st to end	Night	47% 9% 11% 17% 13% 4	627
September	1st to 10th	Night	54% 8% 11% 12% 15% 1	570
	11th to 20th	Night	54% 9% 11% 13% 12% 2	570
	21st to end	Night	45% 8% 11% 17% 16% 3	593
October	1st to 10th	Night	47% 10% 13% 16% 14% 1	698
	11th to 20th	Night	44% 9% 15% 17% 13% 2	860
	21st to end	Night	39% 8% 18% 18% 14% 2	946
November	1st to 10th	Night	38% 13% 19% 20% 10% 1	880
	11th to 20th	Night	25% 13% 26% 23% 11% 2	900
	21st to end	Night	31% 12% 23% 21% 10% 2	900
December	1st to 10th	Night	38% 13% 19% 18% 9% 3	916
	11th to 20th	Night	29% 14% 25% 23% 7% 2	1000
	21st to end	Night	34% 17% 18% 19% 10% 2	1100

Page generated at 2025-05-08 05:34:00. Version ac07

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