Anchorage
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Anchorage clear sky chart from 2005-11-06 to 2025-11-03.

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	18% 13% 4 9% 6% 5% 5% 4 5% 6% 24%	131623
	Night	17% 16% 5% 10% 6% 5% 5% 4 5% 6% 22%	127221
Transparency	Day	57% 12% 16% 11% 4	131266
	Night	56% 11% 14% 12% 7% 1	127158
Seeing	Day	19% 9% 19% 26% 21% 6%	19423
	Night	20% 9% 21% 27% 19% 4	23677

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	13% 17% 5% 9% 6% 7% 5% 5% 7% 8% 17%	1847
	11th to 20th	Day	12% 18% 5% 9% 6% 5% 5% 4 6% 8% 24%	2418
	21st to end	Day	15% 16% 5% 10% 6% 5% 4 3 4 5% 26%	2992
February	1st to 10th	Day	10% 17% 4 8% 4 4 4 4 6% 8% 31%	1379
	11th to 20th	Day	16% 23% 5% 11% 6% 5% 4 3 4 4 19%	1559
	21st to end	Day	12% 15% 5% 9% 7% 4 5% 4 5% 6% 28%	1440
March	1st to 10th	Day	9% 20% 6% 13% 7% 5% 4 3 4 5% 25%	4400
	11th to 20th	Day	9% 16% 6% 12% 6% 6% 6% 4 5% 6% 23%	4694
	21st to end	Day	12% 18% 5% 9% 5% 5% 5% 4 5% 6% 26%	5923
April	1st to 10th	Day	10% 16% 6% 10% 7% 5% 4 4 5% 7% 27%	2445
	11th to 20th	Day	8% 11% 3 8% 6% 5% 5% 5% 6% 5% 35%	2766
	21st to end	Day	12% 12% 4 9% 6% 5% 6% 5% 6% 8% 29%	2820
May	1st to 10th	Day	19% 17% 4 10% 7% 5% 5% 4 5% 6% 17%	7185
	11th to 20th	Day	14% 13% 5% 10% 6% 4 4 3 5% 6% 29%	7213
	21st to end	Day	18% 13% 4 10% 6% 6% 6% 4 6% 7% 22%	8538
June	1st to 10th	Day	16% 12% 3 8% 6% 5% 5% 5% 7% 8% 25%	3600
	11th to 20th	Day	14% 8% 3 8% 6% 6% 6% 5% 7% 8% 30%	3600
	21st to end	Day	16% 9% 4 9% 6% 5% 6% 5% 6% 7% 26%	3600
July	1st to 10th	Day	21% 10% 4 9% 5% 5% 5% 4 6% 6% 23%	8160
	11th to 20th	Day	23% 12% 4 10% 6% 5% 5% 3 6% 6% 20%	7910
	21st to end	Day	25% 9% 3 8% 5% 4 4 4 5% 6% 26%	7909
August	1st to 10th	Day	30% 9% 3 8% 5% 5% 5% 4 5% 6% 21%	5160
	11th to 20th	Day	29% 9% 3 7% 4 4 4 3 5% 6% 25%	4566
	21st to end	Day	27% 10% 3 8% 5% 5% 5% 4 5% 6% 24%	5038
September	1st to 10th	Day	31% 12% 3 9% 6% 4 4 3 5% 5% 17%	2463
	11th to 20th	Day	33% 11% 4 9% 5% 4 4 3 4 5% 18%	2400
	21st to end	Day	25% 11% 4 9% 5% 4 4 3 4 5% 25%	2126
October	1st to 10th	Day	20% 12% 4 9% 6% 6% 6% 5% 5% 7% 19%	3238
	11th to 20th	Day	15% 10% 3 7% 5% 4 4 3 5% 6% 39%	2893
	21st to end	Day	20% 12% 3 7% 3 3 4 4 6% 5% 34%	2779
November	1st to 10th	Day	15% 18% 6% 11% 6% 4 4 5% 5% 6% 22%	1382
	11th to 20th	Day	13% 12% 2 8% 6% 6% 6% 5% 6% 6% 31%	1200
	21st to end	Day	15% 17% 4 10% 7% 6% 5% 4 7% 7% 19%	1045
December	1st to 10th	Day	21% 18% 5% 13% 7% 4 4 3 4 4 17%	1416
	11th to 20th	Day	14% 24% 7% 11% 5% 5% 5% 3 5% 6% 16%	1517
	21st to end	Day	8% 20% 5% 10% 7% 6% 9% 4 6% 8% 16%	2002
January	1st to 10th	Night	13% 17% 5% 10% 7% 6% 5% 3 6% 6% 21%	8119
	11th to 20th	Night	15% 17% 5% 12% 6% 5% 5% 4 6% 6% 20%	7511
	21st to end	Night	14% 14% 4 9% 6% 5% 4 4 5% 7% 28%	7986
February	1st to 10th	Night	11% 16% 4 9% 5% 4 5% 4 6% 6% 29%	3020
	11th to 20th	Night	16% 22% 6% 11% 6% 4 3 3 5% 5% 19%	2728
	21st to end	Night	11% 15% 5% 11% 6% 5% 5% 4 4 5% 29%	2189
March	1st to 10th	Night	10% 20% 6% 11% 5% 4 4 3 5% 5% 26%	5454
	11th to 20th	Night	9% 20% 6% 12% 5% 5% 6% 4 5% 6% 20%	5263
	21st to end	Night	11% 19% 5% 10% 6% 4 5% 4 6% 6% 23%	5008
April	1st to 10th	Night	10% 21% 5% 10% 7% 5% 4 4 4 4 25%	1954
	11th to 20th	Night	11% 14% 3 8% 5% 6% 5% 4 6% 6% 31%	1583
	21st to end	Night	11% 13% 4 9% 7% 6% 5% 4 5% 7% 29%	1580
May	1st to 10th	Night	20% 18% 5% 11% 7% 5% 5% 4 6% 4 15%	2794
	11th to 20th	Night	17% 15% 5% 12% 7% 5% 5% 3 5% 6% 21%	2702
	21st to end	Night	21% 14% 4 10% 6% 5% 5% 4 6% 6% 19%	2347
June	1st to 10th	Night	19% 14% 4 10% 7% 5% 4 5% 6% 5% 21%	800
	11th to 20th	Night	17% 10% 3 10% 5% 7% 6% 6% 6% 7% 25%	800
	21st to end	Night	20% 9% 4 10% 6% 6% 6% 4 8% 8% 20%	800
July	1st to 10th	Night	25% 11% 4 9% 7% 6% 6% 4 6% 6% 16%	1820
	11th to 20th	Night	31% 10% 3 11% 7% 5% 5% 3 4 4 17%	2067
	21st to end	Night	28% 9% 4 9% 5% 4 5% 4 5% 7% 20%	2958
August	1st to 10th	Night	34% 9% 4 7% 6% 4 3 3 5% 5% 22%	1998
	11th to 20th	Night	32% 7% 2 6% 4 4 4 3 5% 6% 26%	2491
	21st to end	Night	32% 9% 4 8% 6% 5% 5% 3 4 5% 19%	2871
September	1st to 10th	Night	38% 11% 4 7% 5% 5% 4 3 4 5% 15%	1935
	11th to 20th	Night	35% 11% 3 9% 5% 4 4 3 4 4 19%	2000
	21st to end	Night	28% 12% 4 7% 4 5% 5% 3 4 4 23%	2270
October	1st to 10th	Night	22% 14% 4 10% 6% 7% 7% 4 5% 5% 17%	3943
	11th to 20th	Night	19% 10% 3 7% 4 4 5% 3 6% 6% 33%	4251
	21st to end	Night	17% 14% 4 9% 5% 5% 5% 4 5% 5% 27%	5022
November	1st to 10th	Night	18% 16% 5% 9% 5% 5% 4 3 5% 6% 22%	2958
	11th to 20th	Night	12% 12% 4 8% 5% 5% 5% 5% 7% 8% 30%	3200
	21st to end	Night	15% 18% 6% 10% 5% 4 4 4 5% 8% 22%	3355
December	1st to 10th	Night	21% 19% 6% 11% 6% 5% 5% 3 4 5% 14%	5768
	11th to 20th	Night	14% 23% 7% 12% 6% 5% 5% 3 5% 6% 15%	6700
	21st to end	Night	11% 17% 5% 13% 7% 6% 7% 6% 6% 7% 15%	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	52% 12% 15% 10% 10% 1	1847
	11th to 20th	Day	50% 11% 12% 16% 9% 2	2433
	21st to end	Day	55% 10% 8% 11% 13% 3	2992
February	1st to 10th	Day	48% 8% 14% 17% 10% 4	1379
	11th to 20th	Day	65% 9% 10% 10% 5% 2	1566
	21st to end	Day	52% 10% 10% 17% 10% 1	1440
March	1st to 10th	Day	55% 9% 9% 11% 13% 3	4400
	11th to 20th	Day	51% 12% 14% 12% 11% 1	4694
	21st to end	Day	56% 9% 13% 13% 8% 1	5923
April	1st to 10th	Day	57% 10% 15% 12% 5% 1	2445
	11th to 20th	Day	47% 11% 17% 18% 6%	2766
	21st to end	Day	53% 9% 14% 17% 7% 1	2820
May	1st to 10th	Day	61% 11% 12% 12% 4	7185
	11th to 20th	Day	54% 10% 17% 16% 3	7216
	21st to end	Day	57% 13% 16% 13% 2	8539
June	1st to 10th	Day	54% 17% 17% 11% 1	3600
	11th to 20th	Day	51% 16% 21% 11% 3	3600
	21st to end	Day	55% 17% 21% 8%	3600
July	1st to 10th	Day	58% 15% 20% 7%	8071
	11th to 20th	Day	62% 15% 19% 3	7812
	21st to end	Day	58% 12% 21% 9%	7687
August	1st to 10th	Day	64% 12% 18% 6%	5141
	11th to 20th	Day	59% 11% 17% 11% 1	4609
	21st to end	Day	60% 14% 16% 9% 1	5037
September	1st to 10th	Day	69% 13% 9% 7% 1	2463
	11th to 20th	Day	69% 9% 13% 8% 1	2400
	21st to end	Day	63% 10% 13% 12% 2	2126
October	1st to 10th	Day	59% 12% 15% 13% 1	3238
	11th to 20th	Day	45% 10% 19% 15% 11%	2893
	21st to end	Day	49% 11% 16% 19% 4	2779
November	1st to 10th	Day	59% 11% 13% 10% 7% 1	1382
	11th to 20th	Day	48% 11% 19% 15% 6%	1200
	21st to end	Day	60% 10% 11% 10% 6% 2	1045
December	1st to 10th	Day	65% 10% 11% 12% 2	1416
	11th to 20th	Day	64% 10% 15% 9% 2	1520
	21st to end	Day	50% 13% 17% 13% 7% 1	2002
January	1st to 10th	Night	49% 15% 13% 10% 12% 1	8119
	11th to 20th	Night	53% 11% 11% 15% 8% 2	7559
	21st to end	Night	48% 9% 9% 13% 17% 4	7986
February	1st to 10th	Night	48% 9% 13% 17% 9% 5%	3020
	11th to 20th	Night	64% 8% 10% 10% 6% 1	2732
	21st to end	Night	54% 9% 10% 16% 9% 1	2189
March	1st to 10th	Night	55% 8% 9% 13% 10% 5%	5454
	11th to 20th	Night	50% 12% 12% 14% 12% 1	5265
	21st to end	Night	53% 11% 12% 16% 7% 1	5008
April	1st to 10th	Night	58% 9% 13% 15% 4 1	1954
	11th to 20th	Night	49% 9% 17% 18% 7%	1583
	21st to end	Night	51% 9% 14% 17% 9%	1580
May	1st to 10th	Night	61% 11% 10% 14% 3	2791
	11th to 20th	Night	63% 9% 11% 12% 5%	2703
	21st to end	Night	57% 13% 16% 13% 1	2347
June	1st to 10th	Night	61% 13% 15% 10% 1	800
	11th to 20th	Night	53% 16% 17% 11% 3	800
	21st to end	Night	58% 17% 19% 7%	800
July	1st to 10th	Night	62% 14% 18% 7%	1800
	11th to 20th	Night	67% 11% 18% 4	2042
	21st to end	Night	61% 9% 23% 7%	2875
August	1st to 10th	Night	63% 10% 19% 7%	1992
	11th to 20th	Night	57% 12% 17% 11% 4	2514
	21st to end	Night	65% 10% 14% 9% 2	2871
September	1st to 10th	Night	70% 12% 8% 8% 2	1935
	11th to 20th	Night	67% 11% 11% 9% 2	2000
	21st to end	Night	62% 9% 14% 12% 2	2270
October	1st to 10th	Night	62% 9% 12% 14% 2	3943
	11th to 20th	Night	47% 10% 17% 15% 11%	4251
	21st to end	Night	54% 8% 13% 18% 7%	5022
November	1st to 10th	Night	60% 10% 12% 10% 6% 1	2958
	11th to 20th	Night	47% 10% 18% 17% 8%	3200
	21st to end	Night	57% 10% 14% 11% 7% 1	3355
December	1st to 10th	Night	66% 10% 11% 10% 3	5768
	11th to 20th	Night	63% 10% 16% 8% 3	6696
	21st to end	Night	51% 12% 19% 12% 5% 1	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	7% 10% 23% 26% 26% 9%	115
	11th to 20th	Day	17% 9% 25% 29% 14% 6%	300
	21st to end	Day	19% 12% 20% 28% 13% 7%	330
February	1st to 10th	Day	13% 7% 18% 31% 18% 13%	300
	11th to 20th	Day	20% 15% 22% 28% 12% 3	395
	21st to end	Day	16% 9% 28% 27% 11% 8%	331
March	1st to 10th	Day	16% 10% 22% 27% 19% 6%	400
	11th to 20th	Day	9% 7% 22% 32% 23% 8%	470
	21st to end	Day	11% 10% 23% 26% 22% 8%	660
April	1st to 10th	Day	12% 10% 27% 29% 17% 4	610
	11th to 20th	Day	10% 11% 14% 30% 28% 7%	690
	21st to end	Day	16% 6% 19% 29% 25% 5%	700
May	1st to 10th	Day	18% 13% 23% 24% 19% 3	703
	11th to 20th	Day	16% 8% 19% 30% 19% 6%	725
	21st to end	Day	19% 9% 19% 30% 18% 4	885
June	1st to 10th	Day	15% 11% 17% 24% 26% 7%	930
	11th to 20th	Day	13% 5% 16% 32% 27% 7%	930
	21st to end	Day	14% 8% 15% 28% 27% 8%	930
July	1st to 10th	Day	17% 7% 16% 28% 28% 5%	916
	11th to 20th	Day	23% 5% 16% 28% 23% 6%	912
	21st to end	Day	24% 6% 16% 23% 22% 8%	804
August	1st to 10th	Day	26% 8% 15% 26% 21% 5%	726
	11th to 20th	Day	26% 10% 20% 23% 17% 4	715
	21st to end	Day	30% 8% 13% 24% 18% 5%	803
September	1st to 10th	Day	29% 12% 17% 20% 19% 3	637
	11th to 20th	Day	33% 9% 16% 20% 18% 4	620
	21st to end	Day	28% 7% 17% 18% 23% 7%	430
October	1st to 10th	Day	30% 9% 20% 22% 14% 5%	420
	11th to 20th	Day	22% 9% 19% 27% 16% 7%	420
	21st to end	Day	21% 10% 18% 25% 19% 6%	462
November	1st to 10th	Day	19% 10% 26% 20% 18% 6%	399
	11th to 20th	Day	16% 6% 25% 25% 24% 5%	300
	21st to end	Day	15% 8% 27% 28% 17% 6%	145
December	1st to 10th	Day	23% 11% 16% 24% 20% 6%	100
	11th to 20th	Day	18% 7% 27% 32% 11% 5%	100
	21st to end	Day	14% 14% 32% 24% 14% 4	110
January	1st to 10th	Night	11% 8% 25% 31% 22% 3	1000
	11th to 20th	Night	18% 11% 26% 27% 15% 4	965
	21st to end	Night	19% 9% 23% 30% 15% 4	990
February	1st to 10th	Night	14% 8% 20% 34% 19% 5%	900
	11th to 20th	Night	21% 13% 26% 25% 13% 1	884
	21st to end	Night	15% 7% 29% 31% 13% 5%	622
March	1st to 10th	Night	13% 9% 25% 31% 19% 3	700
	11th to 20th	Night	9% 7% 22% 34% 23% 5%	700
	21st to end	Night	13% 8% 20% 26% 24% 9%	670
April	1st to 10th	Night	16% 10% 19% 32% 20% 4	600
	11th to 20th	Night	13% 8% 15% 28% 29% 7%	593
	21st to end	Night	15% 5% 19% 29% 29% 4	600
May	1st to 10th	Night	18% 7% 19% 30% 23% 3	435
	11th to 20th	Night	17% 9% 13% 29% 29% 3	394
	21st to end	Night	19% 6% 15% 33% 23% 3	265
June	1st to 10th	Night	23% 9% 15% 23% 28% 3	200
	11th to 20th	Night	17% 6% 15% 30% 29% 4	200
	21st to end	Night	20% 4 14% 28% 33% 3	200
July	1st to 10th	Night	24% 8% 12% 20% 33% 3	195
	11th to 20th	Night	31% 5% 9% 23% 27% 5%	297
	21st to end	Night	29% 5% 15% 21% 21% 9%	438
August	1st to 10th	Night	33% 6% 15% 23% 19% 5%	398
	11th to 20th	Night	34% 7% 18% 22% 15% 4	541
	21st to end	Night	34% 7% 12% 21% 20% 6%	660
September	1st to 10th	Night	36% 10% 16% 22% 14% 3	600
	11th to 20th	Night	36% 9% 13% 17% 20% 5%	600
	21st to end	Night	29% 8% 15% 21% 22% 6%	644
October	1st to 10th	Night	31% 9% 18% 22% 16% 3	720
	11th to 20th	Night	21% 8% 20% 27% 18% 6%	910
	21st to end	Night	20% 9% 20% 27% 20% 4	1001
November	1st to 10th	Night	22% 9% 24% 26% 16% 3	890
	11th to 20th	Night	15% 7% 23% 32% 20% 3	900
	21st to end	Night	19% 10% 24% 26% 17% 5%	900
December	1st to 10th	Night	22% 12% 22% 26% 15% 4	965
	11th to 20th	Night	14% 9% 27% 31% 15% 4	1000
	21st to end	Night	17% 10% 28% 24% 15% 5%	1100

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