Girdwood
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Girdwood clear sky chart from 2005-11-06 to 2026-03-18.

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% 17% 5% 10% 6% 5% 5% 4 5% 6% 20%	134497
	Night	14% 19% 5% 10% 5% 5% 4 4 5% 6% 23%	135830
Transparency	Day	66% 7% 12% 10% 4 1	134144
	Night	66% 7% 9% 9% 8% 2	135764
Seeing	Day	25% 14% 23% 24% 12% 3	19581
	Night	24% 13% 23% 24% 14% 3	24371

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	7% 26% 7% 8% 5% 5% 4 3 4 6% 24%	2399
	11th to 20th	Day	19% 19% 5% 7% 4 4 3 3 4 6% 26%	3327
	21st to end	Day	14% 19% 5% 8% 5% 6% 4 3 5% 6% 24%	3871
February	1st to 10th	Day	7% 17% 5% 8% 6% 5% 4 3 6% 7% 31%	1471
	11th to 20th	Day	12% 22% 6% 13% 7% 6% 5% 3 4 5% 16%	1639
	21st to end	Day	6% 16% 5% 12% 8% 5% 4 4 6% 6% 29%	1494
March	1st to 10th	Day	9% 24% 6% 13% 7% 5% 5% 2 4 5% 19%	4545
	11th to 20th	Day	6% 20% 6% 13% 7% 7% 7% 5% 6% 6% 18%	4717
	21st to end	Day	13% 20% 6% 14% 6% 5% 4 3 5% 5% 18%	5967
April	1st to 10th	Day	7% 23% 7% 14% 7% 6% 4 3 4 6% 18%	2434
	11th to 20th	Day	7% 17% 4 11% 7% 5% 6% 4 6% 7% 26%	2761
	21st to end	Day	8% 19% 5% 10% 7% 7% 5% 4 5% 7% 25%	2830
May	1st to 10th	Day	14% 30% 7% 11% 5% 4 4 3 4 4 14%	7112
	11th to 20th	Day	15% 16% 6% 10% 5% 5% 5% 4 5% 6% 24%	7213
	21st to end	Day	17% 18% 5% 9% 6% 6% 5% 4 5% 5% 18%	8423
June	1st to 10th	Day	15% 16% 4 9% 7% 6% 6% 4 6% 6% 19%	3600
	11th to 20th	Day	12% 13% 5% 9% 6% 6% 6% 5% 7% 7% 23%	3600
	21st to end	Day	15% 14% 4 10% 8% 6% 6% 5% 6% 6% 20%	3600
July	1st to 10th	Day	24% 15% 4 9% 6% 5% 5% 3 5% 6% 19%	8160
	11th to 20th	Day	26% 14% 3 8% 5% 5% 5% 4 5% 7% 17%	7817
	21st to end	Day	23% 10% 3 7% 5% 5% 5% 5% 6% 7% 24%	7899
August	1st to 10th	Day	31% 11% 4 9% 5% 4 4 4 5% 6% 17%	5183
	11th to 20th	Day	30% 11% 3 8% 5% 5% 4 3 5% 6% 20%	4525
	21st to end	Day	29% 14% 3 8% 5% 4 4 3 5% 6% 17%	5038
September	1st to 10th	Day	35% 15% 4 10% 5% 5% 3 3 4 4 12%	2458
	11th to 20th	Day	34% 16% 4 8% 5% 5% 3 4 4 3 14%	2390
	21st to end	Day	26% 17% 5% 9% 5% 5% 5% 3 4 5% 16%	2141
October	1st to 10th	Day	23% 18% 5% 10% 6% 6% 5% 4 4 5% 13%	3211
	11th to 20th	Day	16% 16% 4 9% 5% 4 4 2 4 5% 31%	2924
	21st to end	Day	19% 17% 5% 10% 4 3 4 3 4 5% 27%	2766
November	1st to 10th	Day	12% 19% 7% 11% 5% 5% 5% 4 5% 7% 21%	1441
	11th to 20th	Day	10% 15% 4 9% 5% 7% 5% 4 7% 10% 24%	1291
	21st to end	Day	12% 20% 5% 11% 5% 5% 5% 3 6% 8% 20%	1089
December	1st to 10th	Day	22% 26% 5% 9% 5% 4 4 3 3 5% 13%	1558
	11th to 20th	Day	9% 28% 7% 10% 6% 5% 4 4 5% 6% 16%	1557
	21st to end	Day	7% 20% 6% 10% 6% 6% 5% 3 6% 6% 26%	2046
January	1st to 10th	Night	9% 25% 4 8% 5% 4 3 3 5% 6% 28%	10524
	11th to 20th	Night	17% 21% 5% 9% 5% 4 3 3 4 4 24%	9609
	21st to end	Night	14% 19% 5% 9% 5% 5% 4 3 5% 5% 27%	10342
February	1st to 10th	Night	6% 17% 5% 9% 6% 5% 5% 4 5% 7% 32%	3149
	11th to 20th	Night	13% 22% 5% 11% 7% 6% 5% 4 5% 6% 15%	2868
	21st to end	Night	8% 15% 5% 10% 6% 4 5% 5% 6% 6% 30%	2312
March	1st to 10th	Night	7% 29% 6% 11% 5% 5% 3 3 4 4 23%	5529
	11th to 20th	Night	4 18% 7% 14% 6% 5% 5% 4 6% 8% 21%	5394
	21st to end	Night	8% 20% 6% 10% 5% 5% 5% 4 6% 8% 23%	4963
April	1st to 10th	Night	6% 22% 6% 11% 7% 5% 5% 4 6% 5% 23%	1966
	11th to 20th	Night	9% 16% 3 7% 6% 4 6% 5% 7% 7% 29%	1585
	21st to end	Night	7% 16% 6% 10% 6% 6% 5% 4 6% 6% 28%	1570
May	1st to 10th	Night	13% 27% 5% 10% 6% 4 5% 4 4 6% 16%	2868
	11th to 20th	Night	15% 18% 5% 9% 5% 5% 5% 4 5% 6% 24%	2702
	21st to end	Night	16% 19% 5% 8% 5% 5% 5% 5% 6% 7% 18%	2456
June	1st to 10th	Night	16% 15% 5% 10% 5% 5% 6% 4 8% 8% 20%	800
	11th to 20th	Night	13% 16% 4 8% 4 4 5% 6% 9% 7% 26%	800
	21st to end	Night	16% 11% 4 10% 8% 7% 6% 4 6% 8% 20%	800
July	1st to 10th	Night	28% 10% 2 8% 5% 6% 4 4 5% 5% 22%	1820
	11th to 20th	Night	28% 12% 3 8% 4 4 3 5% 5% 6% 22%	2166
	21st to end	Night	24% 9% 3 7% 4 5% 6% 4 6% 7% 25%	2968
August	1st to 10th	Night	32% 10% 3 7% 5% 5% 4 3 4 6% 22%	1976
	11th to 20th	Night	29% 9% 2 6% 4 3 5% 3 5% 8% 26%	2533
	21st to end	Night	30% 11% 3 8% 5% 5% 4 3 5% 5% 20%	2871
September	1st to 10th	Night	33% 13% 5% 10% 5% 5% 5% 3 3 4 14%	1938
	11th to 20th	Night	33% 14% 4 7% 5% 4 4 3 5% 4 17%	2010
	21st to end	Night	26% 14% 4 11% 5% 5% 4 3 5% 4 19%	2256
October	1st to 10th	Night	22% 17% 4 11% 6% 6% 5% 3 4 5% 16%	3971
	11th to 20th	Night	15% 16% 4 8% 5% 4 4 3 4 5% 32%	4224
	21st to end	Night	14% 17% 4 9% 6% 5% 5% 3 4 5% 26%	5033
November	1st to 10th	Night	14% 19% 5% 12% 6% 5% 4 3 5% 6% 20%	3077
	11th to 20th	Night	9% 15% 3 7% 5% 5% 6% 5% 8% 10% 26%	3325
	21st to end	Night	11% 19% 5% 10% 6% 4 5% 4 6% 7% 22%	3530
December	1st to 10th	Night	20% 26% 5% 8% 5% 5% 5% 3 5% 6% 13%	5841
	11th to 20th	Night	8% 26% 7% 11% 6% 5% 5% 4 6% 6% 15%	6880
	21st to end	Night	7% 22% 5% 11% 6% 5% 5% 3 4 6% 27%	9174

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	65% 4 9% 6% 13% 3	2399
	11th to 20th	Day	66% 4 6% 9% 13% 2	3345
	21st to end	Day	62% 7% 8% 7% 13% 4	3872
February	1st to 10th	Day	56% 5% 9% 14% 10% 6%	1471
	11th to 20th	Day	72% 4 5% 10% 7% 2	1646
	21st to end	Day	54% 8% 7% 12% 15% 3	1494
March	1st to 10th	Day	68% 5% 6% 8% 10% 4	4545
	11th to 20th	Day	63% 8% 7% 11% 10% 1	4717
	21st to end	Day	64% 8% 10% 11% 6% 1	5967
April	1st to 10th	Day	69% 7% 9% 10% 5% 1	2434
	11th to 20th	Day	58% 8% 12% 15% 7%	2761
	21st to end	Day	63% 6% 9% 14% 6% 1	2830
May	1st to 10th	Day	75% 6% 6% 12% 2	7112
	11th to 20th	Day	60% 6% 11% 19% 3	7216
	21st to end	Day	68% 7% 13% 11% 1	8424
June	1st to 10th	Day	67% 7% 14% 11% 1	3600
	11th to 20th	Day	60% 9% 15% 14% 1	3600
	21st to end	Day	64% 8% 19% 8%	3600
July	1st to 10th	Day	65% 10% 19% 5%	8071
	11th to 20th	Day	64% 10% 21% 5%	7719
	21st to end	Day	58% 9% 21% 12%	7678
August	1st to 10th	Day	70% 7% 18% 5%	5164
	11th to 20th	Day	67% 8% 12% 12% 1	4567
	21st to end	Day	68% 7% 13% 10% 1	5037
September	1st to 10th	Day	80% 5% 7% 6% 2	2458
	11th to 20th	Day	77% 6% 8% 8% 1	2390
	21st to end	Day	72% 6% 9% 11% 2	2141
October	1st to 10th	Day	75% 8% 9% 7% 1	3211
	11th to 20th	Day	61% 6% 14% 10% 9%	2924
	21st to end	Day	64% 6% 10% 14% 5% 1	2766
November	1st to 10th	Day	67% 6% 7% 10% 6% 3	1441
	11th to 20th	Day	61% 5% 8% 17% 8% 1	1291
	21st to end	Day	70% 3 8% 9% 7% 2	1089
December	1st to 10th	Day	81% 6% 5% 4 3 1	1558
	11th to 20th	Day	76% 7% 5% 7% 4 1	1560
	21st to end	Day	60% 10% 7% 7% 9% 7%	2046
January	1st to 10th	Night	62% 5% 9% 6% 14% 4	10524
	11th to 20th	Night	69% 4 5% 7% 10% 3	9654
	21st to end	Night	62% 6% 7% 6% 15% 4	10342
February	1st to 10th	Night	58% 5% 8% 12% 12% 6%	3149
	11th to 20th	Night	72% 5% 6% 8% 8% 2	2872
	21st to end	Night	53% 7% 9% 12% 16% 3	2312
March	1st to 10th	Night	66% 4 5% 8% 11% 5%	5529
	11th to 20th	Night	61% 8% 9% 11% 10% 1	5396
	21st to end	Night	62% 11% 10% 10% 5% 1	4963
April	1st to 10th	Night	64% 7% 10% 14% 6% 1	1966
	11th to 20th	Night	55% 7% 14% 17% 8%	1585
	21st to end	Night	60% 6% 10% 16% 8% 1	1570
May	1st to 10th	Night	72% 8% 7% 12% 2	2865
	11th to 20th	Night	61% 6% 14% 13% 5%	2703
	21st to end	Night	68% 9% 14% 9% 1	2456
June	1st to 10th	Night	67% 7% 14% 11% 1	800
	11th to 20th	Night	57% 10% 19% 13% 2	800
	21st to end	Night	63% 9% 22% 6% 1	800
July	1st to 10th	Night	66% 9% 20% 5%	1800
	11th to 20th	Night	64% 10% 22% 4	2141
	21st to end	Night	59% 8% 24% 8%	2884
August	1st to 10th	Night	69% 7% 18% 6%	1970
	11th to 20th	Night	62% 9% 16% 10% 3	2557
	21st to end	Night	69% 7% 12% 10% 2	2871
September	1st to 10th	Night	79% 5% 8% 7% 1	1938
	11th to 20th	Night	74% 6% 9% 8% 3	2010
	21st to end	Night	70% 6% 10% 11% 3	2256
October	1st to 10th	Night	73% 6% 9% 10% 2	3971
	11th to 20th	Night	60% 8% 10% 13% 9%	4224
	21st to end	Night	65% 7% 12% 11% 5% 1	5033
November	1st to 10th	Night	69% 4 8% 9% 7% 3	3077
	11th to 20th	Night	60% 6% 7% 16% 10% 1	3325
	21st to end	Night	68% 5% 6% 10% 8% 3	3530
December	1st to 10th	Night	80% 5% 5% 6% 4 1	5841
	11th to 20th	Night	73% 8% 6% 6% 7% 1	6876
	21st to end	Night	62% 10% 6% 7% 9% 7%	9174

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	23% 12% 18% 20% 20% 6%	122
	11th to 20th	Day	24% 12% 30% 21% 11% 3	320
	21st to end	Day	24% 15% 26% 20% 11% 4	352
February	1st to 10th	Day	17% 14% 24% 24% 15% 6%	337
	11th to 20th	Day	31% 16% 25% 20% 7% 2	425
	21st to end	Day	21% 17% 28% 22% 7% 6%	355
March	1st to 10th	Day	20% 10% 32% 20% 14% 4	430
	11th to 20th	Day	15% 16% 25% 25% 15% 4	470
	21st to end	Day	17% 18% 27% 22% 13% 4	660
April	1st to 10th	Day	18% 24% 26% 23% 8% 1	618
	11th to 20th	Day	16% 17% 20% 30% 14% 3	690
	21st to end	Day	20% 14% 22% 29% 12% 3	700
May	1st to 10th	Day	27% 17% 24% 21% 9% 2	703
	11th to 20th	Day	21% 12% 26% 26% 14% 2	725
	21st to end	Day	23% 11% 29% 25% 11% 1	831
June	1st to 10th	Day	22% 13% 20% 27% 16% 3	930
	11th to 20th	Day	15% 9% 21% 37% 15% 3	930
	21st to end	Day	18% 12% 20% 31% 14% 3	930
July	1st to 10th	Day	22% 14% 22% 29% 12% 1	916
	11th to 20th	Day	27% 12% 20% 27% 11% 2	869
	21st to end	Day	29% 13% 20% 24% 13% 2	794
August	1st to 10th	Day	31% 14% 22% 23% 8% 2	726
	11th to 20th	Day	35% 16% 21% 18% 8% 2	715
	21st to end	Day	36% 12% 21% 21% 9%	803
September	1st to 10th	Day	40% 17% 17% 16% 8% 1	647
	11th to 20th	Day	42% 14% 17% 16% 10% 2	610
	21st to end	Day	35% 16% 17% 18% 11% 3	420
October	1st to 10th	Day	39% 16% 18% 16% 8% 2	420
	11th to 20th	Day	28% 14% 24% 22% 9% 3	420
	21st to end	Day	31% 13% 23% 18% 11% 3	462
November	1st to 10th	Day	24% 14% 24% 19% 15% 5%	425
	11th to 20th	Day	17% 12% 25% 25% 15% 6%	336
	21st to end	Day	21% 12% 19% 31% 12% 5%	149
December	1st to 10th	Day	25% 14% 21% 20% 15% 5%	110
	11th to 20th	Day	23% 15% 29% 25% 5% 2	110
	21st to end	Day	21% 17% 28% 19% 10% 4	121
January	1st to 10th	Night	16% 16% 21% 25% 17% 4	1060
	11th to 20th	Night	25% 15% 26% 20% 10% 3	988
	21st to end	Night	23% 13% 27% 21% 14% 3	1045
February	1st to 10th	Night	15% 12% 27% 30% 13% 4	950
	11th to 20th	Night	28% 17% 28% 19% 7% 1	934
	21st to end	Night	17% 15% 33% 20% 11% 4	673
March	1st to 10th	Night	17% 11% 28% 27% 14% 2	740
	11th to 20th	Night	12% 12% 24% 34% 15% 3	728
	21st to end	Night	17% 11% 25% 24% 17% 5%	660
April	1st to 10th	Night	19% 16% 23% 25% 15% 3	600
	11th to 20th	Night	17% 11% 19% 28% 22% 3	593
	21st to end	Night	15% 15% 22% 29% 17% 3	600
May	1st to 10th	Night	20% 14% 23% 24% 17% 2	470
	11th to 20th	Night	19% 10% 15% 30% 23% 3	394
	21st to end	Night	18% 11% 22% 26% 20% 3	260
June	1st to 10th	Night	22% 13% 15% 29% 19% 4	200
	11th to 20th	Night	20% 10% 12% 34% 24% 2	200
	21st to end	Night	22% 9% 14% 35% 18% 4	200
July	1st to 10th	Night	22% 8% 17% 28% 23% 3	195
	11th to 20th	Night	27% 8% 15% 27% 20% 2	294
	21st to end	Night	29% 7% 16% 28% 16% 3	438
August	1st to 10th	Night	34% 12% 18% 20% 13% 2	398
	11th to 20th	Night	35% 11% 23% 18% 12% 1	571
	21st to end	Night	35% 9% 15% 26% 13% 2	660
September	1st to 10th	Night	39% 13% 21% 16% 11% 1	600
	11th to 20th	Night	40% 12% 15% 18% 13% 1	600
	21st to end	Night	33% 11% 15% 23% 16% 3	631
October	1st to 10th	Night	34% 14% 19% 19% 12% 1	735
	11th to 20th	Night	28% 12% 21% 23% 13% 3	910
	21st to end	Night	27% 11% 22% 23% 14% 3	1001
November	1st to 10th	Night	24% 13% 24% 24% 13% 2	930
	11th to 20th	Night	17% 11% 25% 28% 15% 3	950
	21st to end	Night	24% 12% 22% 24% 13% 5%	950
December	1st to 10th	Night	28% 13% 23% 20% 13% 4	987
	11th to 20th	Night	20% 13% 28% 25% 12% 3	1060
	21st to end	Night	24% 19% 22% 19% 12% 4	1166

Page generated at 2026-03-14 22:23:56. Version ac07

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