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 2026-08-30.

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	17% 10% 2 6% 4 3 3 3 4 6% 43%	153160
	Night	19% 11% 3 6% 4 4 4 3 4 5% 37%	146478
Transparency	Day	42% 6% 11% 20% 14% 7%	147955
	Night	48% 6% 11% 16% 12% 6%	142683
Seeing	Day	24% 12% 26% 24% 11% 3	17220
	Night	25% 6% 14% 29% 22% 3	26138

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	36% 19% 4 8% 4 3 3 3 4 5% 11%	4722
	11th to 20th	Day	20% 9% 2 3 2 2 2 3 3 6% 47%	5118
	21st to end	Day	24% 11% 3 7% 4 4 4 3 4 4 31%	5843
February	1st to 10th	Day	27% 16% 3 7% 4 5% 4 4 4 6% 19%	1890
	11th to 20th	Day	28% 17% 3 7% 4 3 5% 3 4 4 20%	1845
	21st to end	Day	30% 20% 3 6% 4 4 3 3 3 5% 19%	1620
March	1st to 10th	Day	21% 21% 3 7% 4 5% 5% 4 5% 5% 20%	5057
	11th to 20th	Day	29% 18% 3 7% 4 3 3 2 3 3 26%	5110
	21st to end	Day	32% 17% 3 8% 5% 4 5% 3 5% 5% 14%	5607
April	1st to 10th	Day	29% 17% 4 8% 5% 4 4 4 4 4 17%	2506
	11th to 20th	Day	24% 17% 3 8% 6% 3 3 3 4 5% 24%	2493
	21st to end	Day	26% 11% 3 6% 4 4 4 4 5% 6% 26%	2730
May	1st to 10th	Day	13% 9% 3 7% 4 4 4 3 5% 6% 42%	7627
	11th to 20th	Day	14% 10% 2 6% 4 4 4 3 5% 7% 41%	8159
	21st to end	Day	13% 8% 2 6% 4 4 5% 3 5% 7% 42%	8992
June	1st to 10th	Day	22% 10% 2 6% 4 4 3 4 4 6% 34%	2940
	11th to 20th	Day	16% 9% 2 6% 4 4 5% 4 5% 8% 38%	2940
	21st to end	Day	10% 6% 2 4 3 3 3 3 5% 6% 55%	2940
July	1st to 10th	Day	2 2 1 2 2 2 2 2 3 5% 78%	8219
	11th to 20th	Day	3 3 1 2 2 2 3 2 3 4 76%	8220
	21st to end	Day	3 3 1 3 3 3 2 2 3 5% 71%	8400
August	1st to 10th	Day	2 4 1 4 3 3 3 3 5% 8% 65%	5907
	11th to 20th	Day	5% 5% 1 5% 2 2 3 2 4 6% 64%	5926
	21st to end	Day	5% 3 1 4 3 3 3 2 5% 7% 64%	6284
September	1st to 10th	Day	7% 4 1 3 3 2 3 2 4 7% 65%	2400
	11th to 20th	Day	11% 6% 2 5% 4 4 4 3 6% 6% 49%	2319
	21st to end	Day	13% 5% 2 5% 3 3 3 3 5% 8% 51%	2150
October	1st to 10th	Day	11% 6% 2 5% 3 3 3 2 4 5% 55%	3280
	11th to 20th	Day	17% 8% 2 6% 4 5% 4 4 6% 8% 36%	3261
	21st to end	Day	25% 12% 2 6% 4 4 5% 4 6% 8% 24%	3558
November	1st to 10th	Day	30% 11% 3 9% 4 4 4 3 4 5% 22%	1837
	11th to 20th	Day	32% 13% 3 9% 4 4 3 2 4 5% 19%	1680
	21st to end	Day	27% 11% 3 7% 4 4 3 3 5% 6% 27%	1680
December	1st to 10th	Day	19% 9% 2 5% 4 4 4 4 5% 6% 37%	2700
	11th to 20th	Day	36% 14% 3 8% 4 4 3 3 4 5% 15%	3108
	21st to end	Day	39% 20% 4 7% 4 3 3 2 3 3 11%	4092
January	1st to 10th	Night	39% 17% 3 6% 4 4 3 3 4 4 13%	8172
	11th to 20th	Night	21% 10% 2 4 2 2 2 2 4 6% 44%	7847
	21st to end	Night	29% 11% 3 6% 3 3 3 2 3 4 33%	8371
February	1st to 10th	Night	26% 15% 3 8% 5% 4 4 4 5% 5% 21%	2730
	11th to 20th	Night	27% 15% 4 8% 4 3 3 3 4 4 24%	2662
	21st to end	Night	27% 18% 4 6% 4 4 3 3 4 4 23%	2186
March	1st to 10th	Night	20% 21% 5% 8% 5% 5% 4 3 5% 5% 18%	5056
	11th to 20th	Night	24% 21% 4 7% 3 3 3 2 3 3 27%	5110
	21st to end	Night	30% 20% 3 7% 4 3 3 3 4 5% 17%	5610
April	1st to 10th	Night	27% 16% 3 7% 4 4 5% 3 4 5% 22%	2113
	11th to 20th	Night	22% 16% 4 7% 3 3 4 3 5% 5% 29%	2072
	21st to end	Night	22% 13% 3 5% 5% 5% 4 4 4 6% 28%	1890
May	1st to 10th	Night	14% 11% 4 8% 3 3 4 3 4 5% 40%	5321
	11th to 20th	Night	11% 17% 3 6% 4 4 5% 3 4 6% 38%	4700
	21st to end	Night	12% 12% 2 6% 4 3 3 3 5% 7% 42%	5181
June	1st to 10th	Night	19% 8% 2 6% 5% 4 3 3 4 7% 40%	1680
	11th to 20th	Night	10% 7% 3 6% 4 3 3 4 6% 7% 48%	1680
	21st to end	Night	8% 6% 2 4 3 3 3 3 4 6% 56%	1680
July	1st to 10th	Night	1 3 2 4 5% 4 4 3 4 6% 63%	4740
	11th to 20th	Night	2 3 2 4 3 4 3 2 3 4 70%	4740
	21st to end	Night	4 5% 3 9% 6% 5% 5% 3 4 4 53%	5687
August	1st to 10th	Night	2 5% 3 8% 5% 6% 5% 4 4 6% 51%	4125
	11th to 20th	Night	6% 5% 3 8% 5% 5% 4 4 5% 6% 50%	4142
	21st to end	Night	4 4 1 4 3 4 4 3 4 6% 63%	4905
September	1st to 10th	Night	5% 5% 1 4 3 2 3 3 5% 5% 66%	2000
	11th to 20th	Night	8% 5% 2 5% 4 4 4 3 5% 6% 54%	2078
	21st to end	Night	12% 6% 2 5% 4 3 3 3 3 5% 55%	2249
October	1st to 10th	Night	8% 7% 3 5% 3 2 2 2 3 5% 60%	3920
	11th to 20th	Night	14% 8% 2 7% 4 4 4 4 5% 7% 40%	3897
	21st to end	Night	28% 13% 3 7% 4 4 3 3 4 5% 26%	4253
November	1st to 10th	Night	29% 10% 3 7% 4 4 4 3 5% 7% 26%	2679
	11th to 20th	Night	32% 12% 3 7% 4 4 4 4 4 6% 20%	2940
	21st to end	Night	26% 11% 3 6% 4 4 4 4 4 5% 30%	2940
December	1st to 10th	Night	18% 9% 2 4 3 4 4 4 5% 6% 40%	4700
	11th to 20th	Night	37% 15% 3 7% 4 4 4 3 4 6% 14%	5316
	21st to end	Night	36% 15% 3 7% 4 4 3 2 3 4 16%	7106

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	75% 5% 8% 6% 5% 1	4723
	11th to 20th	Day	41% 3 5% 11% 21% 19%	5118
	21st to end	Day	54% 5% 6% 9% 11% 14%	5843
February	1st to 10th	Day	63% 8% 12% 11% 5% 1	1890
	11th to 20th	Day	66% 7% 11% 10% 5% 2	1854
	21st to end	Day	67% 6% 9% 10% 6% 2	1620
March	1st to 10th	Day	60% 7% 12% 16% 4	5057
	11th to 20th	Day	62% 7% 7% 13% 10% 1	5110
	21st to end	Day	68% 11% 10% 9% 2	5607
April	1st to 10th	Day	68% 8% 10% 11% 4	2506
	11th to 20th	Day	62% 7% 9% 11% 8% 2	2496
	21st to end	Day	55% 9% 12% 16% 7% 1	2730
May	1st to 10th	Day	39% 7% 12% 26% 15% 1	7627
	11th to 20th	Day	40% 7% 17% 27% 9% 1	8162
	21st to end	Day	38% 9% 23% 25% 6%	8994
June	1st to 10th	Day	49% 7% 15% 22% 7% 1	2940
	11th to 20th	Day	42% 8% 16% 22% 10% 2	2940
	21st to end	Day	29% 6% 16% 30% 15% 3	2940
July	1st to 10th	Day	11% 3 10% 32% 25% 19%	8151
	11th to 20th	Day	14% 2 7% 14% 30% 34%	8145
	21st to end	Day	16% 3 12% 35% 28% 6%	8213
August	1st to 10th	Day	18% 4 12% 23% 34% 9%	4336
	11th to 20th	Day	21% 4 14% 37% 21% 2	4325
	21st to end	Day	20% 4 15% 28% 27% 6%	4563
September	1st to 10th	Day	21% 4 10% 28% 26% 12%	2400
	11th to 20th	Day	32% 7% 13% 25% 18% 5%	2319
	21st to end	Day	31% 6% 14% 23% 20% 7%	2150
October	1st to 10th	Day	31% 6% 8% 17% 20% 18%	3280
	11th to 20th	Day	44% 6% 16% 21% 11% 2	3261
	21st to end	Day	55% 7% 10% 13% 11% 3	3558
November	1st to 10th	Day	63% 7% 9% 13% 8% 1	1837
	11th to 20th	Day	68% 5% 9% 8% 6% 4	1680
	21st to end	Day	57% 6% 10% 9% 10% 8%	1680
December	1st to 10th	Day	45% 5% 9% 15% 14% 12%	2700
	11th to 20th	Day	70% 7% 8% 9% 4 1	3108
	21st to end	Day	77% 6% 6% 7% 2 1	4092
January	1st to 10th	Night	74% 6% 7% 7% 5% 1	8172
	11th to 20th	Night	41% 5% 6% 11% 17% 20%	7847
	21st to end	Night	57% 3 4 8% 13% 15%	8371
February	1st to 10th	Night	62% 8% 10% 11% 7% 2	2730
	11th to 20th	Night	62% 7% 10% 13% 6% 2	2664
	21st to end	Night	63% 7% 9% 11% 7% 2	2186
March	1st to 10th	Night	63% 9% 12% 12% 3 2	5056
	11th to 20th	Night	60% 9% 9% 12% 8% 2	5110
	21st to end	Night	66% 10% 12% 10% 2	5610
April	1st to 10th	Night	62% 10% 10% 14% 4	2113
	11th to 20th	Night	55% 9% 13% 14% 5% 3	2072
	21st to end	Night	54% 8% 14% 17% 5% 1	1890
May	1st to 10th	Night	42% 7% 13% 22% 14% 1	5319
	11th to 20th	Night	44% 8% 17% 23% 6% 1	4700
	21st to end	Night	38% 8% 29% 22% 3	5182
June	1st to 10th	Night	43% 8% 17% 25% 7%	1680
	11th to 20th	Night	32% 10% 20% 27% 9% 2	1680
	21st to end	Night	25% 7% 18% 30% 16% 3	1680
July	1st to 10th	Night	20% 5% 12% 22% 22% 18%	4700
	11th to 20th	Night	18% 2 7% 15% 28% 30%	4692
	21st to end	Night	33% 3 13% 23% 22% 6%	5563
August	1st to 10th	Night	27% 4 14% 22% 25% 8%	3019
	11th to 20th	Night	31% 7% 15% 27% 16% 4	3019
	21st to end	Night	19% 5% 11% 31% 27% 6%	3550
September	1st to 10th	Night	18% 5% 11% 27% 25% 13%	2000
	11th to 20th	Night	28% 8% 15% 26% 18% 5%	2078
	21st to end	Night	30% 6% 14% 22% 18% 10%	2249
October	1st to 10th	Night	28% 5% 8% 19% 23% 17%	3920
	11th to 20th	Night	42% 6% 15% 24% 10% 2	3897
	21st to end	Night	61% 5% 7% 13% 10% 4	4253
November	1st to 10th	Night	57% 7% 12% 12% 10% 2	2679
	11th to 20th	Night	64% 7% 10% 9% 6% 4	2940
	21st to end	Night	54% 7% 9% 11% 12% 7%	2940
December	1st to 10th	Night	44% 5% 8% 15% 18% 10%	4700
	11th to 20th	Night	71% 6% 9% 7% 5% 1	5316
	21st to end	Night	68% 8% 9% 11% 2 1	7106

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	43% 20% 19% 10% 7% 1	220
	11th to 20th	Day	35% 11% 19% 17% 15% 4	351
	21st to end	Day	33% 11% 21% 20% 14% 2	473
February	1st to 10th	Day	36% 8% 25% 21% 10% 1	430
	11th to 20th	Day	41% 16% 21% 16% 4 1	425
	21st to end	Day	44% 15% 17% 18% 5% 1	388
March	1st to 10th	Day	39% 16% 22% 17% 6%	540
	11th to 20th	Day	37% 12% 23% 19% 7% 1	540
	21st to end	Day	40% 16% 22% 17% 5% 1	594
April	1st to 10th	Day	37% 16% 22% 16% 7% 2	540
	11th to 20th	Day	32% 18% 25% 17% 6% 2	532
	21st to end	Day	32% 15% 24% 19% 8% 2	540
May	1st to 10th	Day	23% 17% 29% 21% 8% 3	543
	11th to 20th	Day	32% 15% 26% 17% 8% 2	568
	21st to end	Day	25% 13% 29% 22% 9% 3	617
June	1st to 10th	Day	23% 13% 28% 23% 8% 5%	570
	11th to 20th	Day	18% 21% 31% 22% 7% 2	570
	21st to end	Day	11% 8% 36% 31% 12% 2	570
July	1st to 10th	Day	6% 6% 35% 34% 15% 3	566
	11th to 20th	Day	5% 9% 36% 32% 14% 4	564
	21st to end	Day	2 6% 35% 38% 14% 5%	619
August	1st to 10th	Day	4 7% 30% 37% 16% 6%	567
	11th to 20th	Day	5% 6% 29% 34% 20% 6%	557
	21st to end	Day	8% 7% 31% 31% 17% 6%	620
September	1st to 10th	Day	9% 8% 29% 30% 18% 6%	530
	11th to 20th	Day	15% 8% 28% 30% 16% 3	530
	21st to end	Day	15% 11% 26% 30% 14% 3	530
October	1st to 10th	Day	21% 11% 23% 26% 15% 4	530
	11th to 20th	Day	22% 12% 22% 23% 18% 4	530
	21st to end	Day	30% 10% 26% 19% 11% 4	583
November	1st to 10th	Day	32% 15% 19% 19% 12% 2	361
	11th to 20th	Day	39% 8% 21% 18% 10% 4	220
	21st to end	Day	35% 10% 17% 22% 15% 1	220
December	1st to 10th	Day	33% 10% 20% 19% 14% 5%	220
	11th to 20th	Day	45% 8% 16% 21% 7% 3	220
	21st to end	Day	42% 14% 16% 17% 10% 1	242
January	1st to 10th	Night	43% 9% 18% 18% 11% 2	840
	11th to 20th	Night	33% 8% 15% 25% 15% 4	840
	21st to end	Night	32% 7% 12% 27% 18% 3	924
February	1st to 10th	Night	34% 6% 16% 27% 16% 2	840
	11th to 20th	Night	34% 8% 15% 27% 15% 1	824
	21st to end	Night	35% 10% 16% 25% 12% 1	692
March	1st to 10th	Night	33% 10% 17% 26% 13% 1	840
	11th to 20th	Night	31% 10% 14% 28% 16% 1	840
	21st to end	Night	38% 9% 16% 24% 12% 1	924
April	1st to 10th	Night	32% 10% 16% 23% 17% 1	840
	11th to 20th	Night	28% 10% 18% 26% 17% 2	824
	21st to end	Night	27% 6% 15% 28% 22% 2	630
May	1st to 10th	Night	22% 6% 18% 29% 22% 3	630
	11th to 20th	Night	22% 5% 13% 33% 25% 2	414
	21st to end	Night	21% 5% 16% 30% 27% 2	455
June	1st to 10th	Night	20% 5% 12% 36% 25% 2	420
	11th to 20th	Night	11% 4 15% 45% 22% 2	420
	21st to end	Night	12% 3 9% 39% 36% 1	420
July	1st to 10th	Night	4 1 9% 41% 43% 1	410
	11th to 20th	Night	4 1 12% 50% 32% 1	408
	21st to end	Night	4 1 8% 41% 42% 5%	648
August	1st to 10th	Night	4 11% 35% 42% 7%	627
	11th to 20th	Night	5% 2 7% 33% 48% 6%	624
	21st to end	Night	6% 2 14% 37% 36% 5%	820
September	1st to 10th	Night	7% 3 12% 35% 40% 4	800
	11th to 20th	Night	9% 3 13% 31% 36% 8%	800
	21st to end	Night	14% 4 13% 34% 31% 4	800
October	1st to 10th	Night	15% 3 17% 32% 29% 4	800
	11th to 20th	Night	20% 5% 11% 31% 28% 5%	800
	21st to end	Night	27% 7% 15% 28% 17% 6%	880
November	1st to 10th	Night	31% 6% 16% 29% 17% 1	820
	11th to 20th	Night	36% 8% 14% 27% 13% 2	840
	21st to end	Night	29% 6% 15% 30% 17% 3	840
December	1st to 10th	Night	29% 9% 15% 26% 18% 4	840
	11th to 20th	Night	42% 6% 14% 23% 13% 2	840
	21st to end	Night	43% 11% 14% 22% 9% 1	924

Page generated at 2026-08-27 01:07:10. Version ac07

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