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 2024-11-06.

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	19% 13% 4 9% 5% 5% 5% 4 5% 6% 25%	106544
	Night	17% 15% 4 10% 6% 5% 5% 4 5% 6% 23%	101153
Transparency	Day	57% 12% 16% 11% 4 1	106235
	Night	55% 11% 14% 13% 7% 1	101111
Seeing	Day	19% 8% 19% 27% 21% 6%	18090
	Night	20% 8% 21% 27% 19% 4	22402

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	10% 15% 5% 9% 7% 7% 6% 5% 8% 10% 17%	1283
	11th to 20th	Day	12% 13% 4 8% 6% 5% 5% 4 6% 9% 30%	1676
	21st to end	Day	10% 17% 5% 11% 7% 6% 5% 4 4 5% 27%	2090
February	1st to 10th	Day	11% 18% 4 9% 4 4 4 4 5% 8% 30%	1310
	11th to 20th	Day	16% 22% 5% 10% 6% 5% 4 3 4 4 19%	1479
	21st to end	Day	12% 15% 5% 9% 7% 3 5% 4 5% 7% 29%	1370
March	1st to 10th	Day	9% 19% 5% 12% 6% 5% 4 3 4 4 28%	3068
	11th to 20th	Day	12% 11% 4 10% 6% 6% 6% 5% 6% 7% 29%	3278
	21st to end	Day	17% 23% 5% 9% 4 4 4 3 4 6% 22%	4133
April	1st to 10th	Day	10% 17% 6% 10% 7% 5% 4 4 5% 7% 28%	2323
	11th to 20th	Day	8% 11% 4 8% 6% 5% 5% 5% 6% 5% 36%	2626
	21st to end	Day	11% 12% 3 9% 6% 5% 6% 5% 6% 8% 30%	2679
May	1st to 10th	Day	21% 19% 5% 11% 7% 5% 5% 3 4 4 16%	5023
	11th to 20th	Day	12% 11% 4 9% 6% 5% 5% 4 6% 6% 32%	5023
	21st to end	Day	18% 13% 4 9% 6% 5% 5% 4 6% 7% 23%	5942
June	1st to 10th	Day	15% 12% 3 8% 6% 5% 5% 5% 8% 8% 25%	3420
	11th to 20th	Day	15% 8% 3 8% 6% 6% 6% 5% 7% 8% 29%	3420
	21st to end	Day	17% 9% 3 9% 5% 5% 5% 5% 6% 8% 27%	3420
July	1st to 10th	Day	23% 10% 4 10% 6% 5% 6% 4 6% 6% 20%	5700
	11th to 20th	Day	28% 12% 4 8% 5% 4 5% 3 5% 5% 19%	5528
	21st to end	Day	25% 10% 3 7% 5% 5% 4 4 5% 6% 27%	5509
August	1st to 10th	Day	30% 9% 3 8% 5% 5% 5% 4 5% 6% 21%	5001
	11th to 20th	Day	29% 9% 3 7% 4 4 4 3 5% 6% 25%	4424
	21st to end	Day	27% 10% 3 8% 5% 5% 5% 4 5% 6% 24%	4884
September	1st to 10th	Day	30% 12% 3 9% 6% 5% 4 3 5% 5% 18%	2341
	11th to 20th	Day	33% 11% 4 8% 5% 4 4 3 4 5% 18%	2280
	21st to end	Day	25% 11% 4 9% 5% 4 4 3 4 5% 26%	2020
October	1st to 10th	Day	20% 12% 4 9% 6% 6% 6% 5% 6% 7% 19%	3139
	11th to 20th	Day	14% 10% 3 6% 5% 4 4 3 5% 6% 39%	2804
	21st to end	Day	21% 12% 2 6% 3 2 4 4 5% 6% 35%	2693
November	1st to 10th	Day	16% 17% 5% 11% 6% 4 4 5% 5% 5% 23%	1334
	11th to 20th	Day	13% 12% 2 7% 6% 6% 6% 5% 6% 6% 30%	1140
	21st to end	Day	16% 17% 4 10% 6% 6% 5% 4 6% 7% 19%	993
December	1st to 10th	Day	20% 18% 4 12% 6% 4 3 3 6% 6% 18%	827
	11th to 20th	Day	9% 24% 8% 10% 6% 6% 6% 3 5% 7% 16%	967
	21st to end	Day	7% 16% 5% 11% 8% 7% 11% 6% 8% 10% 11%	1397
January	1st to 10th	Night	11% 15% 5% 11% 8% 7% 5% 4 7% 8% 20%	5672
	11th to 20th	Night	13% 15% 4 9% 5% 5% 5% 5% 7% 7% 26%	5237
	21st to end	Night	12% 13% 5% 10% 5% 4 4 4 6% 8% 29%	5577
February	1st to 10th	Night	12% 17% 4 10% 5% 4 5% 4 5% 6% 29%	2869
	11th to 20th	Night	17% 22% 6% 11% 6% 4 3 3 4 5% 19%	2588
	21st to end	Night	12% 15% 5% 10% 6% 5% 5% 4 4 5% 30%	2083
March	1st to 10th	Night	9% 20% 6% 11% 5% 4 3 3 5% 5% 30%	3786
	11th to 20th	Night	11% 15% 5% 10% 5% 5% 6% 4 6% 7% 26%	3679
	21st to end	Night	15% 22% 5% 10% 5% 4 3 3 5% 5% 23%	3498
April	1st to 10th	Night	10% 21% 5% 10% 7% 5% 4 4 4 4 25%	1857
	11th to 20th	Night	11% 13% 3 8% 5% 6% 5% 4 6% 6% 32%	1503
	21st to end	Night	10% 13% 4 9% 7% 6% 5% 4 5% 7% 30%	1501
May	1st to 10th	Night	21% 18% 5% 11% 7% 5% 5% 3 5% 4 16%	1946
	11th to 20th	Night	15% 13% 4 10% 7% 5% 5% 4 6% 7% 26%	1882
	21st to end	Night	23% 12% 3 8% 6% 6% 5% 4 6% 7% 21%	1636
June	1st to 10th	Night	19% 13% 4 10% 7% 5% 4 5% 7% 5% 21%	760
	11th to 20th	Night	18% 10% 3 9% 5% 7% 5% 6% 6% 8% 23%	760
	21st to end	Night	20% 8% 4 10% 6% 6% 5% 4 8% 8% 21%	760
July	1st to 10th	Night	29% 11% 4 9% 7% 5% 5% 4 5% 5% 17%	1270
	11th to 20th	Night	36% 9% 3 8% 5% 4 5% 2 4 4 18%	1447
	21st to end	Night	28% 9% 3 8% 5% 3 4 4 5% 7% 23%	2056
August	1st to 10th	Night	34% 9% 4 7% 6% 4 3 3 5% 6% 22%	1937
	11th to 20th	Night	33% 7% 2 6% 4 4 4 3 5% 6% 27%	2414
	21st to end	Night	31% 9% 4 8% 6% 5% 5% 3 4 5% 19%	2783
September	1st to 10th	Night	37% 11% 4 7% 5% 5% 4 3 4 5% 15%	1838
	11th to 20th	Night	35% 11% 3 8% 5% 4 4 3 4 5% 19%	1900
	21st to end	Night	28% 12% 4 7% 4 5% 5% 3 5% 4 24%	2156
October	1st to 10th	Night	22% 14% 4 10% 7% 7% 7% 4 5% 5% 17%	3822
	11th to 20th	Night	18% 10% 2 7% 4 4 5% 3 6% 6% 33%	4120
	21st to end	Night	17% 14% 4 8% 5% 5% 5% 4 5% 6% 28%	4866
November	1st to 10th	Night	19% 17% 5% 9% 4 5% 4 3 5% 6% 23%	2863
	11th to 20th	Night	12% 12% 4 9% 5% 5% 5% 5% 7% 8% 30%	3040
	21st to end	Night	15% 18% 6% 10% 5% 4 4 4 5% 8% 21%	3187
December	1st to 10th	Night	21% 18% 5% 9% 6% 5% 5% 4 4 7% 16%	3350
	11th to 20th	Night	12% 22% 7% 13% 7% 5% 5% 4 5% 6% 16%	4240
	21st to end	Night	9% 16% 5% 13% 8% 6% 8% 7% 7% 7% 14%	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	48% 15% 17% 10% 9% 2	1283
	11th to 20th	Day	43% 11% 12% 19% 12% 3	1691
	21st to end	Day	52% 11% 10% 10% 13% 4	2090
February	1st to 10th	Day	50% 8% 13% 15% 9% 4	1310
	11th to 20th	Day	66% 9% 10% 9% 4 2	1486
	21st to end	Day	51% 10% 11% 17% 10% 1	1370
March	1st to 10th	Day	53% 10% 11% 10% 11% 5%	3068
	11th to 20th	Day	45% 14% 16% 12% 11% 1	3278
	21st to end	Day	63% 10% 12% 8% 6% 1	4133
April	1st to 10th	Day	56% 10% 15% 13% 5% 1	2323
	11th to 20th	Day	46% 12% 17% 19% 7%	2626
	21st to end	Day	51% 9% 14% 18% 7% 1	2679
May	1st to 10th	Day	67% 11% 11% 9% 2	5023
	11th to 20th	Day	49% 10% 19% 18% 3	5026
	21st to end	Day	57% 14% 15% 12% 2	5943
June	1st to 10th	Day	53% 17% 18% 11% 1	3420
	11th to 20th	Day	52% 16% 20% 10% 3	3420
	21st to end	Day	54% 17% 21% 8%	3420
July	1st to 10th	Day	63% 15% 17% 5%	5611
	11th to 20th	Day	65% 14% 17% 3	5430
	21st to end	Day	57% 14% 20% 9%	5335
August	1st to 10th	Day	64% 12% 18% 6%	4982
	11th to 20th	Day	59% 11% 17% 11% 1	4467
	21st to end	Day	59% 14% 16% 10% 1	4883
September	1st to 10th	Day	68% 13% 10% 7% 1	2341
	11th to 20th	Day	68% 10% 13% 8% 1	2280
	21st to end	Day	63% 10% 14% 12% 2	2020
October	1st to 10th	Day	59% 12% 15% 13% 1	3139
	11th to 20th	Day	44% 10% 19% 16% 11%	2804
	21st to end	Day	48% 11% 17% 19% 4	2693
November	1st to 10th	Day	58% 10% 12% 11% 7% 1	1334
	11th to 20th	Day	48% 11% 20% 15% 6%	1140
	21st to end	Day	60% 10% 12% 10% 6% 2	993
December	1st to 10th	Day	64% 10% 14% 8% 4 1	827
	11th to 20th	Day	64% 10% 14% 9% 3	970
	21st to end	Day	45% 15% 19% 10% 10% 2	1397
January	1st to 10th	Night	48% 18% 15% 9% 8% 2	5672
	11th to 20th	Night	45% 10% 11% 20% 11% 2	5285
	21st to end	Night	45% 9% 11% 14% 16% 5%	5577
February	1st to 10th	Night	50% 9% 12% 15% 9% 5%	2869
	11th to 20th	Night	65% 9% 10% 10% 6% 1	2592
	21st to end	Night	54% 9% 10% 16% 10% 1	2083
March	1st to 10th	Night	52% 8% 10% 15% 8% 7%	3786
	11th to 20th	Night	44% 13% 14% 15% 14% 1	3681
	21st to end	Night	57% 11% 13% 10% 7% 2	3498
April	1st to 10th	Night	59% 9% 13% 15% 4 1	1857
	11th to 20th	Night	48% 9% 17% 18% 7%	1503
	21st to end	Night	50% 9% 14% 18% 9%	1501
May	1st to 10th	Night	64% 11% 11% 12% 2	1946
	11th to 20th	Night	55% 11% 14% 17% 3	1883
	21st to end	Night	54% 15% 16% 14% 2	1636
June	1st to 10th	Night	60% 13% 16% 10% 1	760
	11th to 20th	Night	54% 16% 17% 10% 3	760
	21st to end	Night	58% 16% 19% 7%	760
July	1st to 10th	Night	63% 14% 18% 6%	1250
	11th to 20th	Night	66% 11% 19% 4	1422
	21st to end	Night	58% 11% 24% 7%	1991
August	1st to 10th	Night	63% 10% 19% 7%	1931
	11th to 20th	Night	57% 12% 17% 11% 4	2437
	21st to end	Night	64% 10% 14% 9% 2	2783
September	1st to 10th	Night	70% 13% 8% 8% 2	1838
	11th to 20th	Night	67% 11% 12% 9% 2	1900
	21st to end	Night	62% 9% 15% 12% 2	2156
October	1st to 10th	Night	62% 9% 12% 14% 2	3822
	11th to 20th	Night	47% 10% 17% 16% 11%	4120
	21st to end	Night	54% 8% 13% 19% 7%	4866
November	1st to 10th	Night	59% 11% 12% 10% 7% 1	2863
	11th to 20th	Night	48% 10% 18% 17% 8%	3040
	21st to end	Night	58% 10% 14% 11% 6% 1	3187
December	1st to 10th	Night	63% 10% 13% 8% 5%	3350
	11th to 20th	Night	63% 10% 13% 10% 4	4236
	21st to end	Night	49% 13% 19% 10% 7% 2	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	5% 9% 22% 29% 28% 8%	104
	11th to 20th	Day	15% 8% 25% 30% 15% 7%	280
	21st to end	Day	18% 12% 19% 30% 13% 7%	308
February	1st to 10th	Day	14% 8% 19% 31% 16% 13%	280
	11th to 20th	Day	21% 14% 22% 28% 12% 3	365
	21st to end	Day	17% 9% 28% 27% 10% 8%	307
March	1st to 10th	Day	16% 9% 21% 27% 20% 6%	370
	11th to 20th	Day	9% 6% 22% 31% 24% 8%	436
	21st to end	Day	12% 10% 22% 26% 22% 7%	616
April	1st to 10th	Day	11% 9% 26% 30% 19% 5%	569
	11th to 20th	Day	10% 10% 14% 29% 30% 7%	640
	21st to end	Day	14% 6% 19% 30% 26% 5%	650
May	1st to 10th	Day	18% 13% 23% 24% 20% 2	653
	11th to 20th	Day	16% 9% 19% 31% 20% 6%	675
	21st to end	Day	19% 9% 19% 30% 19% 4	825
June	1st to 10th	Day	12% 11% 17% 24% 27% 8%	870
	11th to 20th	Day	13% 5% 17% 32% 26% 6%	870
	21st to end	Day	15% 7% 14% 28% 27% 9%	870
July	1st to 10th	Day	17% 7% 16% 27% 29% 5%	856
	11th to 20th	Day	24% 4 15% 28% 23% 6%	852
	21st to end	Day	23% 6% 16% 23% 23% 8%	749
August	1st to 10th	Day	26% 7% 15% 26% 21% 5%	676
	11th to 20th	Day	27% 9% 20% 23% 17% 4	665
	21st to end	Day	30% 8% 14% 23% 19% 6%	748
September	1st to 10th	Day	28% 12% 17% 21% 20% 3	596
	11th to 20th	Day	32% 9% 16% 20% 19% 5%	580
	21st to end	Day	28% 7% 17% 18% 24% 7%	400
October	1st to 10th	Day	29% 8% 20% 23% 14% 5%	390
	11th to 20th	Day	21% 9% 18% 29% 16% 7%	390
	21st to end	Day	21% 10% 18% 25% 19% 7%	429
November	1st to 10th	Day	21% 9% 26% 19% 18% 6%	379
	11th to 20th	Day	16% 5% 25% 24% 24% 5%	280
	21st to end	Day	16% 8% 29% 25% 16% 7%	133
December	1st to 10th	Day	20% 11% 16% 26% 22% 6%	90
	11th to 20th	Day	17% 7% 26% 36% 10% 6%	90
	21st to end	Day	14% 12% 32% 25% 13% 3	99
January	1st to 10th	Night	10% 7% 25% 33% 21% 4	940
	11th to 20th	Night	17% 10% 26% 28% 16% 4	909
	21st to end	Night	19% 9% 22% 31% 16% 4	935
February	1st to 10th	Night	15% 9% 21% 34% 17% 4	850
	11th to 20th	Night	22% 13% 26% 24% 13% 1	834
	21st to end	Night	15% 7% 30% 31% 12% 5%	588
March	1st to 10th	Night	13% 8% 25% 30% 19% 3	660
	11th to 20th	Night	9% 6% 22% 34% 23% 5%	660
	21st to end	Night	14% 8% 21% 26% 23% 9%	636
April	1st to 10th	Night	16% 10% 18% 31% 21% 4	570
	11th to 20th	Night	12% 8% 14% 28% 30% 8%	563
	21st to end	Night	13% 5% 19% 29% 30% 4	570
May	1st to 10th	Night	18% 7% 19% 30% 24% 3	413
	11th to 20th	Night	16% 9% 13% 29% 30% 3	374
	21st to end	Night	19% 5% 15% 34% 24% 3	252
June	1st to 10th	Night	23% 8% 14% 24% 29% 3	190
	11th to 20th	Night	18% 6% 15% 29% 27% 4	190
	21st to end	Night	20% 3 14% 28% 32% 3	190
July	1st to 10th	Night	24% 8% 12% 19% 34% 3	185
	11th to 20th	Night	33% 5% 9% 22% 27% 5%	282
	21st to end	Night	30% 5% 15% 19% 22% 9%	416
August	1st to 10th	Night	33% 6% 15% 24% 19% 5%	378
	11th to 20th	Night	34% 7% 18% 22% 15% 4	514
	21st to end	Night	33% 7% 12% 21% 20% 6%	627
September	1st to 10th	Night	35% 10% 16% 21% 14% 4	570
	11th to 20th	Night	36% 8% 13% 17% 20% 5%	570
	21st to end	Night	29% 8% 14% 21% 21% 6%	610
October	1st to 10th	Night	32% 9% 17% 23% 17% 3	679
	11th to 20th	Night	21% 8% 20% 27% 19% 6%	860
	21st to end	Night	21% 9% 20% 28% 19% 3	946
November	1st to 10th	Night	22% 10% 23% 26% 16% 3	859
	11th to 20th	Night	15% 7% 24% 32% 19% 3	850
	21st to end	Night	19% 10% 25% 24% 17% 5%	850
December	1st to 10th	Night	22% 11% 21% 26% 16% 4	908
	11th to 20th	Night	14% 9% 26% 31% 15% 4	940
	21st to end	Night	18% 10% 29% 24% 15% 5%	1034

Page generated at 2024-11-03 09:19:02. Version ac07

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