Sitkum
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sitkum clear sky chart from 2005-11-06 to 2025-04-26.

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	20% 10% 2 6% 4 3 3 3 4 6% 39%	117879
	Night	22% 12% 3 6% 4 3 3 3 4 5% 35%	119592
Transparency	Day	45% 6% 11% 17% 14% 6%	117632
	Night	49% 7% 11% 15% 12% 6%	119429
Seeing	Day	25% 11% 25% 24% 11% 3	15439
	Night	25% 6% 14% 29% 23% 3	24569

By Month

Percentage Cloud Cover SW (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	38% 20% 3 6% 4 3 3 2 4 4 12%	3639
	11th to 20th	Day	27% 12% 3 4 3 3 2 3 3 6% 35%	3934
	21st to end	Day	28% 10% 2 4 3 3 3 3 4 5% 34%	4510
February	1st to 10th	Day	28% 17% 3 7% 4 5% 4 4 4 6% 18%	1800
	11th to 20th	Day	29% 17% 3 7% 4 3 5% 3 4 4 21%	1755
	21st to end	Day	29% 21% 3 6% 4 4 3 3 3 5% 19%	1545
March	1st to 10th	Day	20% 22% 3 7% 4 5% 5% 4 5% 5% 21%	4947
	11th to 20th	Day	29% 18% 3 7% 4 3 3 2 3 3 26%	5000
	21st to end	Day	32% 17% 3 8% 5% 4 5% 3 5% 5% 14%	5486
April	1st to 10th	Day	30% 17% 4 9% 5% 4 4 3 4 4 17%	2387
	11th to 20th	Day	25% 17% 3 8% 6% 3 3 2 4 5% 25%	2373
	21st to end	Day	27% 11% 3 6% 4 4 4 4 5% 6% 25%	2538
May	1st to 10th	Day	22% 12% 3 7% 4 4 4 3 4 5% 32%	4110
	11th to 20th	Day	13% 6% 2 5% 3 3 4 3 5% 7% 51%	4376
	21st to end	Day	16% 8% 3 6% 5% 4 5% 4 6% 8% 35%	4813
June	1st to 10th	Day	23% 11% 2 6% 4 4 3 4 5% 6% 31%	2660
	11th to 20th	Day	17% 9% 2 6% 5% 4 4 4 5% 8% 35%	2660
	21st to end	Day	10% 5% 2 4 3 3 3 3 5% 6% 56%	2660
July	1st to 10th	Day	3 2 1 3 2 2 2 2 3 4 78%	4420
	11th to 20th	Day	4 3 1 3 2 2 2 2 3 4 73%	4420
	21st to end	Day	3 3 1 3 2 3 2 3 4 5% 71%	4500
August	1st to 10th	Day	2 3 2 4 3 3 3 3 5% 7% 65%	4091
	11th to 20th	Day	4 4 1 5% 3 2 3 3 5% 6% 64%	4026
	21st to end	Day	7% 4 1 4 2 2 3 2 4 7% 64%	4314
September	1st to 10th	Day	6% 3 1 3 3 2 3 2 4 7% 66%	2280
	11th to 20th	Day	11% 6% 2 5% 4 4 4 3 6% 6% 48%	2203
	21st to end	Day	13% 5% 2 5% 3 3 3 3 5% 8% 50%	2043
October	1st to 10th	Day	11% 5% 2 5% 3 3 3 2 4 5% 56%	3180
	11th to 20th	Day	18% 8% 2 6% 4 5% 4 4 6% 8% 35%	3161
	21st to end	Day	25% 12% 2 6% 4 4 5% 4 6% 8% 24%	3448
November	1st to 10th	Day	30% 11% 3 9% 4 4 4 3 5% 5% 22%	1748
	11th to 20th	Day	32% 13% 3 9% 4 4 4 2 4 5% 20%	1600
	21st to end	Day	28% 11% 3 7% 4 4 3 3 5% 6% 27%	1600
December	1st to 10th	Day	18% 9% 2 5% 4 4 4 4 5% 6% 39%	2620
	11th to 20th	Day	36% 14% 4 8% 5% 4 3 3 4 5% 15%	3028
	21st to end	Day	40% 20% 4 7% 4 3 3 2 3 3 11%	4004
January	1st to 10th	Night	41% 16% 3 6% 4 3 3 3 3 3 13%	6306
	11th to 20th	Night	27% 13% 2 5% 3 2 3 2 4 5% 34%	6038
	21st to end	Night	33% 10% 2 4 2 2 2 1 2 4 36%	6468
February	1st to 10th	Night	27% 15% 3 8% 5% 4 4 4 5% 5% 20%	2600
	11th to 20th	Night	27% 15% 3 7% 4 4 3 3 4 5% 25%	2532
	21st to end	Night	27% 18% 4 6% 4 4 3 3 4 4 24%	2085
March	1st to 10th	Night	20% 21% 5% 8% 5% 5% 4 3 5% 5% 19%	4946
	11th to 20th	Night	24% 22% 4 7% 3 3 3 2 2 3 27%	5000
	21st to end	Night	30% 20% 3 7% 4 3 3 3 4 5% 16%	5489
April	1st to 10th	Night	28% 16% 3 7% 4 4 5% 3 4 5% 21%	2012
	11th to 20th	Night	22% 15% 3 7% 3 3 3 2 5% 5% 30%	1972
	21st to end	Night	23% 13% 3 5% 5% 5% 4 5% 4 6% 27%	1758
May	1st to 10th	Night	20% 15% 3 7% 3 2 3 3 4 4 36%	2860
	11th to 20th	Night	10% 8% 2 6% 4 4 4 3 4 6% 49%	2516
	21st to end	Night	16% 11% 3 6% 4 3 3 2 4 7% 41%	2762
June	1st to 10th	Night	20% 9% 2 6% 5% 4 3 3 4 7% 37%	1520
	11th to 20th	Night	11% 8% 3 6% 4 3 3 4 6% 7% 46%	1520
	21st to end	Night	8% 5% 2 4 3 3 4 3 4 6% 57%	1520
July	1st to 10th	Night	2 2 1 3 2 2 2 2 2 4 77%	2540
	11th to 20th	Night	3 2 1 3 3 4 3 2 3 5% 70%	2540
	21st to end	Night	4 5% 3 7% 4 2 3 3 3 4 62%	3045
August	1st to 10th	Night	1 3 3 7% 5% 6% 6% 4 5% 6% 53%	2848
	11th to 20th	Night	3 4 3 9% 5% 5% 4 4 5% 6% 52%	2812
	21st to end	Night	6% 5% 1 3 2 3 2 2 3 4 68%	3359
September	1st to 10th	Night	4 4 1 3 2 2 3 3 4 5% 68%	1900
	11th to 20th	Night	8% 5% 2 5% 4 4 4 3 5% 6% 55%	1974
	21st to end	Night	12% 6% 2 5% 4 3 3 3 4 6% 54%	2136
October	1st to 10th	Night	8% 7% 3 5% 2 2 2 2 3 5% 60%	3800
	11th to 20th	Night	14% 8% 2 7% 4 4 4 4 5% 8% 39%	3777
	21st to end	Night	28% 13% 3 7% 4 3 3 3 4 4 26%	4121
November	1st to 10th	Night	29% 10% 3 7% 3 4 4 3 5% 7% 26%	2548
	11th to 20th	Night	33% 12% 3 7% 4 4 4 4 4 6% 20%	2800
	21st to end	Night	26% 10% 4 6% 4 4 4 4 4 5% 31%	2800
December	1st to 10th	Night	18% 9% 2 4 3 4 4 4 5% 6% 41%	4560
	11th to 20th	Night	37% 15% 3 7% 4 4 4 3 4 6% 14%	5176
	21st to end	Night	37% 15% 3 7% 4 4 3 2 3 4 16%	6952

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	74% 6% 8% 6% 6% 1	3639
	11th to 20th	Day	52% 4 6% 13% 21% 4	3934
	21st to end	Day	50% 7% 6% 9% 12% 16%	4510
February	1st to 10th	Day	65% 8% 12% 10% 5% 1	1800
	11th to 20th	Day	65% 7% 11% 10% 5% 2	1764
	21st to end	Day	67% 6% 9% 10% 6% 2	1545
March	1st to 10th	Day	60% 7% 12% 16% 4	4947
	11th to 20th	Day	63% 7% 7% 13% 10% 1	5000
	21st to end	Day	68% 11% 10% 9% 2	5486
April	1st to 10th	Day	69% 8% 9% 10% 4	2387
	11th to 20th	Day	62% 7% 10% 11% 9% 2	2376
	21st to end	Day	56% 9% 12% 15% 7% 1	2538
May	1st to 10th	Day	49% 10% 13% 17% 9% 2	4110
	11th to 20th	Day	30% 7% 17% 29% 17% 1	4379
	21st to end	Day	41% 12% 28% 17% 2	4814
June	1st to 10th	Day	52% 7% 15% 20% 5%	2660
	11th to 20th	Day	45% 9% 17% 21% 7% 1	2660
	21st to end	Day	28% 6% 17% 32% 15% 3	2660
July	1st to 10th	Day	12% 3 8% 17% 23% 36%	4351
	11th to 20th	Day	16% 2 6% 18% 27% 31%	4345
	21st to end	Day	14% 4 11% 23% 38% 10%	4358
August	1st to 10th	Day	18% 4 12% 23% 34% 8%	4076
	11th to 20th	Day	21% 4 14% 37% 22% 3	4065
	21st to end	Day	20% 4 15% 27% 27% 6%	4313
September	1st to 10th	Day	19% 3 9% 28% 28% 13%	2280
	11th to 20th	Day	32% 7% 14% 24% 17% 5%	2203
	21st to end	Day	31% 6% 14% 23% 19% 6%	2043
October	1st to 10th	Day	31% 6% 8% 17% 20% 19%	3180
	11th to 20th	Day	44% 6% 16% 20% 11% 2	3161
	21st to end	Day	55% 7% 10% 13% 12% 3	3448
November	1st to 10th	Day	63% 7% 9% 12% 8% 1	1748
	11th to 20th	Day	67% 5% 10% 8% 7% 4	1600
	21st to end	Day	57% 6% 9% 9% 11% 8%	1600
December	1st to 10th	Day	44% 5% 9% 15% 14% 13%	2620
	11th to 20th	Day	71% 7% 9% 9% 4 1	3028
	21st to end	Day	78% 6% 6% 7% 2 1	4004
January	1st to 10th	Night	74% 7% 5% 6% 6% 2	6306
	11th to 20th	Night	50% 7% 8% 12% 17% 5%	6038
	21st to end	Night	55% 4 4 8% 13% 16%	6468
February	1st to 10th	Night	63% 8% 10% 11% 6% 1	2600
	11th to 20th	Night	61% 7% 10% 13% 7% 2	2534
	21st to end	Night	63% 8% 9% 12% 7% 2	2085
March	1st to 10th	Night	63% 8% 12% 12% 3 2	4946
	11th to 20th	Night	60% 9% 9% 12% 8% 2	5000
	21st to end	Night	66% 11% 12% 10% 2	5489
April	1st to 10th	Night	63% 10% 10% 13% 4	2012
	11th to 20th	Night	54% 9% 13% 14% 6% 3	1972
	21st to end	Night	54% 9% 15% 16% 6% 1	1758
May	1st to 10th	Night	45% 11% 16% 17% 9% 1	2860
	11th to 20th	Night	31% 9% 18% 28% 12% 2	2516
	21st to end	Night	38% 11% 34% 15% 1	2762
June	1st to 10th	Night	45% 9% 17% 25% 4	1520
	11th to 20th	Night	33% 10% 22% 27% 8% 1	1520
	21st to end	Night	23% 8% 18% 32% 15% 4	1520
July	1st to 10th	Night	11% 4 11% 16% 24% 34%	2500
	11th to 20th	Night	15% 2 9% 18% 24% 31%	2492
	21st to end	Night	25% 2 11% 21% 31% 10%	2950
August	1st to 10th	Night	27% 4 14% 22% 25% 7%	2839
	11th to 20th	Night	30% 7% 15% 28% 16% 4	2839
	21st to end	Night	19% 5% 11% 32% 27% 6%	3359
September	1st to 10th	Night	17% 5% 10% 27% 26% 14%	1900
	11th to 20th	Night	28% 8% 15% 26% 18% 6%	1974
	21st to end	Night	31% 6% 14% 22% 18% 10%	2136
October	1st to 10th	Night	28% 5% 8% 19% 23% 17%	3800
	11th to 20th	Night	42% 6% 15% 25% 10% 2	3777
	21st to end	Night	61% 5% 7% 13% 10% 4	4121
November	1st to 10th	Night	56% 8% 12% 12% 11% 2	2548
	11th to 20th	Night	64% 7% 10% 9% 6% 4	2800
	21st to end	Night	54% 7% 9% 11% 12% 7%	2800
December	1st to 10th	Night	43% 5% 8% 15% 19% 10%	4560
	11th to 20th	Night	71% 6% 9% 7% 5% 1	5176
	21st to end	Night	68% 9% 9% 11% 2 1	6952

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	44% 19% 18% 12% 8% 2	200
	11th to 20th	Day	38% 11% 18% 16% 14% 4	325
	21st to end	Day	35% 10% 20% 20% 13% 2	440
February	1st to 10th	Day	37% 8% 24% 20% 10% 1	400
	11th to 20th	Day	42% 15% 20% 17% 5% 1	395
	21st to end	Day	43% 15% 17% 19% 5% 1	361
March	1st to 10th	Day	40% 16% 22% 17% 6%	500
	11th to 20th	Day	38% 12% 22% 20% 8% 2	500
	21st to end	Day	41% 14% 22% 17% 5% 1	550
April	1st to 10th	Day	38% 16% 22% 16% 7% 1	500
	11th to 20th	Day	32% 17% 25% 18% 6% 2	492
	21st to end	Day	33% 15% 25% 18% 7% 2	481
May	1st to 10th	Day	24% 17% 28% 20% 8% 3	463
	11th to 20th	Day	31% 13% 26% 18% 9% 3	488
	21st to end	Day	25% 13% 28% 23% 9% 3	529
June	1st to 10th	Day	24% 12% 28% 23% 9% 4	490
	11th to 20th	Day	20% 22% 30% 20% 7% 1	490
	21st to end	Day	9% 8% 36% 32% 12% 3	490
July	1st to 10th	Day	7% 7% 35% 34% 15% 2	486
	11th to 20th	Day	5% 7% 36% 33% 15% 4	484
	21st to end	Day	2 5% 35% 40% 14% 5%	531
August	1st to 10th	Day	5% 7% 28% 38% 16% 6%	487
	11th to 20th	Day	4 6% 27% 36% 21% 6%	477
	21st to end	Day	8% 7% 31% 32% 17% 6%	539
September	1st to 10th	Day	8% 7% 29% 31% 19% 6%	490
	11th to 20th	Day	15% 8% 28% 30% 16% 3	490
	21st to end	Day	15% 11% 26% 31% 14% 3	490
October	1st to 10th	Day	19% 11% 22% 28% 17% 4	490
	11th to 20th	Day	21% 11% 22% 23% 19% 4	490
	21st to end	Day	29% 11% 25% 19% 12% 4	539
November	1st to 10th	Day	32% 15% 18% 20% 12% 3	332
	11th to 20th	Day	37% 8% 22% 18% 11% 5%	200
	21st to end	Day	36% 10% 17% 22% 14% 2	200
December	1st to 10th	Day	32% 10% 18% 20% 15% 6%	200
	11th to 20th	Day	45% 8% 17% 21% 7% 3	200
	21st to end	Day	42% 14% 16% 16% 11%	220
January	1st to 10th	Night	43% 8% 18% 18% 11% 2	800
	11th to 20th	Night	35% 8% 15% 24% 15% 4	800
	21st to end	Night	33% 6% 11% 28% 19% 3	880
February	1st to 10th	Night	35% 6% 14% 26% 17% 2	800
	11th to 20th	Night	34% 8% 15% 27% 15% 2	784
	21st to end	Night	34% 10% 16% 26% 13% 1	660
March	1st to 10th	Night	34% 10% 17% 25% 13% 1	800
	11th to 20th	Night	31% 11% 13% 28% 17% 1	800
	21st to end	Night	38% 9% 15% 24% 12% 1	880
April	1st to 10th	Night	33% 11% 17% 22% 17% 1	800
	11th to 20th	Night	27% 10% 18% 26% 17% 2	784
	21st to end	Night	27% 6% 15% 28% 22% 2	586
May	1st to 10th	Night	24% 7% 16% 29% 22% 3	570
	11th to 20th	Night	21% 5% 13% 32% 27% 2	374
	21st to end	Night	21% 5% 16% 28% 27% 1	411
June	1st to 10th	Night	21% 5% 11% 36% 26% 2	380
	11th to 20th	Night	12% 5% 15% 44% 21% 3	380
	21st to end	Night	11% 2 9% 40% 37% 1	380
July	1st to 10th	Night	5% 1 9% 40% 44% 1	370
	11th to 20th	Night	5% 1 11% 48% 33% 1	368
	21st to end	Night	3 1 8% 41% 43% 4	586
August	1st to 10th	Night	4 10% 35% 43% 7%	567
	11th to 20th	Night	4 2 7% 32% 49% 7%	564
	21st to end	Night	6% 2 14% 38% 36% 5%	749
September	1st to 10th	Night	6% 2 12% 35% 40% 4	760
	11th to 20th	Night	9% 3 13% 30% 38% 8%	760
	21st to end	Night	14% 4 12% 34% 31% 4	760
October	1st to 10th	Night	15% 3 17% 31% 30% 4	760
	11th to 20th	Night	19% 4 11% 31% 29% 6%	760
	21st to end	Night	27% 7% 14% 29% 17% 6%	836
November	1st to 10th	Night	30% 6% 15% 29% 18% 1	780
	11th to 20th	Night	37% 7% 13% 28% 13% 2	800
	21st to end	Night	29% 6% 15% 30% 17% 3	800
December	1st to 10th	Night	28% 9% 15% 27% 19% 4	800
	11th to 20th	Night	42% 6% 14% 23% 14% 2	800
	21st to end	Night	43% 10% 14% 22% 9% 1	880

Page generated at 2025-04-23 06:25:07. Version ac07

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