Sitka
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sitka clear sky chart from 2005-11-06 to 2026-04-17.

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	33% 22% 4 8% 4 3 3 2 3 4 13%	135175
	Night	30% 23% 5% 8% 4 3 3 2 3 4 15%	135668
Transparency	Day	75% 8% 7% 7% 3	134847
	Night	74% 7% 7% 7% 5% 1	135575
Seeing	Day	42% 14% 18% 16% 8% 1	18619
	Night	44% 12% 16% 16% 10% 2	25741

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	25% 27% 6% 8% 3 4 4 2 3 3 15%	3527
	11th to 20th	Day	30% 26% 4 6% 3 3 3 2 3 3 17%	3522
	21st to end	Day	33% 27% 4 7% 3 3 3 2 2 4 12%	4446
February	1st to 10th	Day	25% 26% 4 9% 4 2 3 2 3 2 21%	1616
	11th to 20th	Day	26% 27% 5% 6% 4 4 2 2 3 3 18%	1713
	21st to end	Day	17% 26% 6% 7% 3 3 2 3 3 5% 24%	1562
March	1st to 10th	Day	13% 34% 6% 9% 3 3 4 3 4 4 16%	4596
	11th to 20th	Day	16% 28% 7% 9% 4 4 3 2 3 3 21%	4861
	21st to end	Day	13% 23% 5% 9% 6% 4 4 3 4 5% 23%	6007
April	1st to 10th	Day	21% 33% 6% 8% 5% 3 3 2 3 2 13%	2520
	11th to 20th	Day	20% 24% 6% 9% 6% 4 4 3 3 4 19%	2734
	21st to end	Day	26% 26% 5% 10% 5% 3 3 3 2 4 13%	2800
May	1st to 10th	Day	32% 26% 4 8% 4 3 3 2 3 4 10%	6423
	11th to 20th	Day	30% 31% 4 7% 4 3 3 2 3 3 10%	7161
	21st to end	Day	30% 27% 5% 10% 6% 4 3 2 3 3 7%	7907
June	1st to 10th	Day	34% 18% 5% 9% 6% 4 3 3 4 4 10%	3200
	11th to 20th	Day	36% 17% 4 9% 5% 4 4 3 4 5% 11%	3200
	21st to end	Day	43% 16% 4 8% 4 4 3 2 3 3 9%	3200
July	1st to 10th	Day	37% 21% 3 7% 4 3 3 3 4 5% 10%	7240
	11th to 20th	Day	52% 16% 3 7% 4 3 3 3 3 3 4	7240
	21st to end	Day	42% 22% 4 7% 4 3 3 2 3 3 8%	7758
August	1st to 10th	Day	31% 9% 3 6% 3 3 3 3 4 5% 30%	4720
	11th to 20th	Day	42% 13% 3 6% 4 4 3 3 4 5% 15%	4480
	21st to end	Day	52% 14% 3 6% 3 3 3 2 3 3 10%	4856
September	1st to 10th	Day	49% 12% 4 7% 3 3 3 3 3 3 9%	2400
	11th to 20th	Day	44% 13% 3 6% 3 3 2 2 3 5% 16%	2400
	21st to end	Day	53% 13% 3 8% 3 3 3 2 3 2 7%	2130
October	1st to 10th	Day	45% 19% 3 8% 4 3 3 2 3 2 8%	3270
	11th to 20th	Day	47% 19% 3 6% 4 3 3 2 3 2 8%	3029
	21st to end	Day	33% 20% 3 5% 3 3 3 3 3 4 20%	2955
November	1st to 10th	Day	35% 21% 3 7% 3 3 2 2 3 3 15%	1576
	11th to 20th	Day	34% 24% 3 6% 3 3 2 2 3 4 16%	1463
	21st to end	Day	32% 28% 5% 7% 4 3 2 2 2 2 13%	1260
December	1st to 10th	Day	33% 21% 4 8% 5% 4 3 3 4 4 12%	2020
	11th to 20th	Day	18% 24% 5% 11% 5% 3 4 2 3 4 20%	2325
	21st to end	Day	26% 31% 6% 10% 4 3 2 2 2 2 11%	3058
January	1st to 10th	Night	26% 27% 5% 9% 3 3 3 2 3 3 14%	9368
	11th to 20th	Night	32% 24% 4 7% 4 4 3 2 3 3 14%	9376
	21st to end	Night	33% 28% 5% 7% 3 3 2 2 3 4 10%	9759
February	1st to 10th	Night	26% 22% 5% 8% 4 4 3 2 3 4 20%	3004
	11th to 20th	Night	25% 29% 5% 7% 3 2 2 2 3 3 18%	2793
	21st to end	Night	18% 27% 6% 8% 4 3 3 2 3 4 24%	2243
March	1st to 10th	Night	17% 33% 5% 7% 4 4 3 2 3 4 17%	5498
	11th to 20th	Night	17% 26% 7% 9% 4 4 4 3 3 4 20%	5323
	21st to end	Night	16% 22% 4 7% 4 4 4 3 5% 5% 25%	5184
April	1st to 10th	Night	21% 30% 6% 7% 4 4 4 2 3 3 16%	2100
	11th to 20th	Night	18% 23% 6% 10% 5% 4 3 4 3 4 20%	1746
	21st to end	Night	24% 24% 5% 9% 5% 4 3 3 3 4 15%	1600
May	1st to 10th	Night	32% 25% 4 6% 4 3 3 2 3 3 13%	3553
	11th to 20th	Night	25% 27% 5% 10% 4 3 3 3 4 4 12%	2731
	21st to end	Night	23% 27% 7% 11% 5% 5% 3 2 3 4 9%	2968
June	1st to 10th	Night	29% 18% 4 9% 7% 5% 5% 3 4 5% 12%	1200
	11th to 20th	Night	31% 15% 5% 10% 6% 5% 4 3 4 4 11%	1200
	21st to end	Night	36% 17% 4 9% 4 4 4 2 5% 5% 11%	1200
July	1st to 10th	Night	34% 21% 5% 9% 3 3 3 2 4 3 13%	2720
	11th to 20th	Night	47% 18% 6% 8% 3 3 2 1 3 3 7%	2720
	21st to end	Night	40% 23% 6% 8% 3 3 2 2 2 3 7%	3088
August	1st to 10th	Night	29% 12% 2 6% 3 3 4 2 4 5% 30%	2439
	11th to 20th	Night	36% 16% 3 7% 4 3 2 2 3 5% 19%	2578
	21st to end	Night	51% 12% 3 6% 4 2 3 2 2 3 11%	3051
September	1st to 10th	Night	48% 13% 4 7% 4 4 3 2 2 3 10%	2000
	11th to 20th	Night	39% 12% 3 8% 3 4 4 2 3 4 18%	2000
	21st to end	Night	49% 14% 4 8% 4 3 3 2 4 3 8%	2266
October	1st to 10th	Night	43% 20% 3 7% 4 3 3 2 3 2 10%	3920
	11th to 20th	Night	44% 21% 3 7% 3 3 3 3 3 2 8%	4119
	21st to end	Night	32% 22% 4 6% 3 2 3 2 3 4 20%	4845
November	1st to 10th	Night	36% 20% 4 6% 4 3 3 3 3 3 15%	2941
	11th to 20th	Night	29% 24% 4 7% 4 4 4 3 3 4 15%	3155
	21st to end	Night	32% 26% 5% 7% 4 3 3 2 3 3 13%	3360
December	1st to 10th	Night	39% 21% 4 7% 4 3 3 2 3 3 11%	5380
	11th to 20th	Night	21% 24% 4 7% 5% 4 4 2 4 5% 21%	6100
	21st to end	Night	28% 27% 5% 8% 4 3 4 2 3 3 12%	8140

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	76% 6% 5% 5% 6% 2	3527
	11th to 20th	Day	74% 5% 4 7% 10% 1	3540
	21st to end	Day	79% 6% 5% 5% 5% 1	4447
February	1st to 10th	Day	72% 4 6% 8% 8% 3	1616
	11th to 20th	Day	74% 6% 4 6% 8% 2	1721
	21st to end	Day	66% 6% 6% 12% 7% 4	1562
March	1st to 10th	Day	70% 6% 8% 8% 7% 1	4596
	11th to 20th	Day	68% 5% 5% 8% 15%	4861
	21st to end	Day	60% 8% 9% 16% 6%	6007
April	1st to 10th	Day	77% 7% 5% 7% 4	2520
	11th to 20th	Day	70% 7% 6% 9% 7% 1	2734
	21st to end	Day	76% 7% 6% 7% 3	2800
May	1st to 10th	Day	76% 7% 7% 7% 2	6423
	11th to 20th	Day	78% 8% 6% 6% 2	7164
	21st to end	Day	81% 7% 6% 5% 1	7908
June	1st to 10th	Day	78% 9% 7% 4 2	3200
	11th to 20th	Day	75% 10% 9% 5% 1	3200
	21st to end	Day	81% 7% 8% 3 1	3200
July	1st to 10th	Day	69% 13% 11% 6%	7161
	11th to 20th	Day	84% 9% 5% 2	7153
	21st to end	Day	80% 9% 8% 2	7539
August	1st to 10th	Day	54% 10% 13% 15% 7%	4702
	11th to 20th	Day	70% 10% 10% 9% 1	4522
	21st to end	Day	79% 7% 6% 8% 1	4855
September	1st to 10th	Day	80% 8% 7% 6%	2400
	11th to 20th	Day	72% 6% 8% 10% 4	2400
	21st to end	Day	84% 5% 5% 4 1	2130
October	1st to 10th	Day	82% 6% 5% 4 2	3270
	11th to 20th	Day	83% 6% 4 5% 1	3029
	21st to end	Day	67% 7% 10% 13% 3	2955
November	1st to 10th	Day	76% 7% 6% 8% 3	1576
	11th to 20th	Day	75% 5% 6% 8% 5% 1	1463
	21st to end	Day	79% 6% 4 5% 5% 1	1260
December	1st to 10th	Day	77% 9% 8% 4 2	2020
	11th to 20th	Day	68% 8% 7% 8% 8% 1	2328
	21st to end	Day	80% 7% 4 6% 3 1	3058
January	1st to 10th	Night	77% 6% 5% 6% 5% 2	9368
	11th to 20th	Night	75% 5% 5% 5% 8% 2	9420
	21st to end	Night	81% 5% 4 6% 4	9759
February	1st to 10th	Night	72% 5% 5% 7% 8% 3	3004
	11th to 20th	Night	75% 3 4 6% 9% 3	2796
	21st to end	Night	67% 5% 6% 11% 9% 2	2243
March	1st to 10th	Night	72% 5% 6% 9% 7% 1	5498
	11th to 20th	Night	69% 5% 5% 6% 14% 2	5325
	21st to end	Night	60% 7% 10% 16% 7%	5184
April	1st to 10th	Night	74% 6% 9% 6% 5% 1	2100
	11th to 20th	Night	68% 7% 7% 8% 10%	1746
	21st to end	Night	75% 6% 8% 8% 4	1600
May	1st to 10th	Night	75% 6% 7% 9% 3	3550
	11th to 20th	Night	71% 10% 9% 7% 2	2731
	21st to end	Night	77% 8% 8% 5% 1	2968
June	1st to 10th	Night	76% 9% 9% 4 2 1	1200
	11th to 20th	Night	76% 8% 9% 6% 1	1200
	21st to end	Night	78% 7% 11% 3 1	1200
July	1st to 10th	Night	72% 10% 9% 9%	2690
	11th to 20th	Night	81% 10% 7% 2	2684
	21st to end	Night	83% 7% 7% 2	3002
August	1st to 10th	Night	58% 10% 12% 13% 7%	2432
	11th to 20th	Night	67% 9% 17% 6% 1	2602
	21st to end	Night	78% 7% 8% 6%	3051
September	1st to 10th	Night	80% 8% 6% 5% 1	2000
	11th to 20th	Night	70% 7% 8% 9% 5% 1	2000
	21st to end	Night	83% 6% 4 5% 1	2266
October	1st to 10th	Night	81% 6% 5% 6% 2	3920
	11th to 20th	Night	82% 7% 5% 4 2	4119
	21st to end	Night	70% 7% 8% 10% 6%	4845
November	1st to 10th	Night	75% 7% 6% 9% 3	2941
	11th to 20th	Night	75% 7% 6% 7% 4 1	3155
	21st to end	Night	77% 6% 6% 5% 5% 1	3360
December	1st to 10th	Night	80% 7% 6% 4 3	5380
	11th to 20th	Night	66% 7% 8% 9% 10% 1	6096
	21st to end	Night	77% 7% 5% 6% 4 1	8140

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	36% 15% 21% 16% 11% 1	220
	11th to 20th	Day	48% 16% 16% 14% 6%	220
	21st to end	Day	51% 14% 17% 12% 5%	443
February	1st to 10th	Day	40% 17% 22% 15% 5% 1	430
	11th to 20th	Day	40% 18% 20% 15% 6% 1	425
	21st to end	Day	34% 22% 24% 14% 5% 2	355
March	1st to 10th	Day	35% 16% 22% 17% 9% 1	430
	11th to 20th	Day	33% 22% 22% 16% 7% 1	430
	21st to end	Day	31% 19% 21% 16% 10% 2	572
April	1st to 10th	Day	34% 19% 24% 14% 8%	540
	11th to 20th	Day	31% 13% 21% 19% 13% 2	673
	21st to end	Day	42% 16% 19% 14% 8% 1	700
May	1st to 10th	Day	43% 13% 23% 16% 4 1	703
	11th to 20th	Day	35% 10% 20% 21% 12% 2	725
	21st to end	Day	35% 14% 20% 20% 9% 2	790
June	1st to 10th	Day	40% 16% 16% 17% 9% 2	730
	11th to 20th	Day	39% 13% 17% 22% 8% 2	730
	21st to end	Day	46% 10% 13% 19% 9% 3	730
July	1st to 10th	Day	35% 10% 19% 23% 11% 3	721
	11th to 20th	Day	49% 13% 15% 17% 5% 1	718
	21st to end	Day	47% 10% 13% 18% 9% 2	794
August	1st to 10th	Day	41% 10% 19% 20% 9% 2	726
	11th to 20th	Day	51% 11% 16% 15% 6% 1	715
	21st to end	Day	50% 11% 15% 16% 7% 1	778
September	1st to 10th	Day	45% 12% 17% 16% 8% 2	530
	11th to 20th	Day	42% 13% 18% 16% 9% 2	530
	21st to end	Day	57% 18% 12% 9% 3 1	483
October	1st to 10th	Day	48% 13% 17% 14% 7% 2	420
	11th to 20th	Day	57% 16% 17% 7% 3	420
	21st to end	Day	43% 16% 19% 15% 6% 1	462
November	1st to 10th	Day	52% 17% 17% 10% 5%	354
	11th to 20th	Day	51% 10% 15% 14% 8% 2	220
	21st to end	Day	50% 19% 15% 15% 2	220
December	1st to 10th	Day	49% 19% 12% 13% 6% 2	220
	11th to 20th	Day	38% 16% 20% 17% 8% 1	220
	21st to end	Day	46% 13% 23% 9% 5% 4	242
January	1st to 10th	Night	41% 15% 19% 16% 7% 1	950
	11th to 20th	Night	51% 16% 18% 10% 5% 1	950
	21st to end	Night	50% 16% 14% 12% 6% 1	1045
February	1st to 10th	Night	41% 16% 19% 17% 6% 1	950
	11th to 20th	Night	42% 16% 18% 17% 8%	934
	21st to end	Night	37% 16% 23% 17% 7% 1	779
March	1st to 10th	Night	36% 14% 21% 17% 10% 2	840
	11th to 20th	Night	35% 17% 21% 16% 9% 1	735
	21st to end	Night	31% 10% 20% 19% 14% 4	693
April	1st to 10th	Night	37% 14% 17% 17% 12% 3	630
	11th to 20th	Night	29% 12% 22% 19% 15% 3	611
	21st to end	Night	38% 13% 16% 16% 13% 4	600
May	1st to 10th	Night	48% 7% 15% 15% 13% 3	565
	11th to 20th	Night	29% 8% 13% 25% 22% 4	394
	21st to end	Night	30% 10% 13% 23% 22% 2	433
June	1st to 10th	Night	38% 8% 13% 16% 22% 4	400
	11th to 20th	Night	36% 5% 15% 26% 17% 2	400
	21st to end	Night	40% 8% 15% 17% 18% 3	400
July	1st to 10th	Night	36% 7% 11% 23% 22% 1	390
	11th to 20th	Night	47% 5% 11% 19% 15% 4	388
	21st to end	Night	43% 4 12% 23% 16% 2	484
August	1st to 10th	Night	44% 7% 12% 17% 16% 4	597
	11th to 20th	Night	51% 6% 9% 15% 15% 4	594
	21st to end	Night	50% 6% 12% 18% 13% 3	660
September	1st to 10th	Night	51% 9% 9% 16% 13% 2	600
	11th to 20th	Night	43% 9% 11% 18% 15% 4	600
	21st to end	Night	55% 11% 14% 11% 6% 2	784
October	1st to 10th	Night	49% 8% 12% 19% 11% 1	800
	11th to 20th	Night	57% 11% 16% 11% 3 1	800
	21st to end	Night	46% 10% 18% 17% 9% 1	960
November	1st to 10th	Night	48% 16% 19% 12% 5%	930
	11th to 20th	Night	44% 15% 18% 15% 8% 1	950
	21st to end	Night	49% 16% 19% 13% 4	950
December	1st to 10th	Night	48% 15% 17% 13% 6% 1	950
	11th to 20th	Night	41% 16% 19% 16% 7% 1	950
	21st to end	Night	46% 16% 17% 15% 6% 2	1045

Page generated at 2026-04-13 22:44:29. Version ac07

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