The Stellar Shed
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Stellar Shed clear sky chart from 2005-11-06 to 2026-02-16.

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	12% 8% 3 8% 5% 4 5% 4 6% 7% 38%	131228
	Night	12% 9% 3 8% 5% 4 4 3 5% 7% 40%	129919
Transparency	Day	42% 7% 13% 23% 14% 2	134766
	Night	43% 6% 12% 23% 15% 2	134458
Seeing	Day	15% 11% 29% 26% 15% 5%	17499
	Night	16% 10% 32% 30% 11% 1	27480

By Month

Percentage Cloud Cover SE (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	9% 12% 5% 14% 7% 7% 5% 4 5% 7% 26%	4691
	11th to 20th	Day	7% 9% 4 10% 8% 6% 7% 4 8% 6% 31%	4812
	21st to end	Day	10% 13% 7% 10% 7% 5% 5% 3 4 5% 30%	5812
February	1st to 10th	Day	14% 13% 4 9% 5% 4 4 4 5% 6% 29%	1890
	11th to 20th	Day	14% 10% 4 9% 5% 5% 4 3 5% 6% 36%	1890
	21st to end	Day	12% 12% 4 10% 4 4 4 3 4 5% 37%	1650
March	1st to 10th	Day	17% 11% 3 5% 3 3 3 2 3 5% 44%	4564
	11th to 20th	Day	8% 6% 3 6% 4 5% 4 4 5% 8% 47%	4980
	21st to end	Day	19% 11% 4 9% 5% 4 4 4 5% 7% 27%	5732
April	1st to 10th	Day	14% 12% 3 8% 5% 4 4 3 5% 6% 35%	2400
	11th to 20th	Day	16% 9% 3 7% 5% 3 4 3 4 7% 37%	2374
	21st to end	Day	19% 8% 3 8% 5% 5% 4 4 5% 7% 33%	2400
May	1st to 10th	Day	13% 8% 3 7% 5% 5% 5% 4 6% 9% 36%	5968
	11th to 20th	Day	15% 6% 2 6% 4 4 4 3 5% 7% 43%	6319
	21st to end	Day	13% 7% 3 7% 5% 4 5% 4 6% 7% 39%	6935
June	1st to 10th	Day	9% 6% 2 6% 4 5% 4 5% 6% 9% 45%	2800
	11th to 20th	Day	9% 6% 2 5% 4 5% 5% 4 6% 9% 46%	2800
	21st to end	Day	10% 6% 2 6% 5% 4 5% 5% 7% 9% 42%	2800
July	1st to 10th	Day	7% 7% 3 7% 5% 6% 5% 5% 8% 9% 39%	6360
	11th to 20th	Day	8% 6% 2 7% 5% 5% 5% 4 6% 10% 41%	6360
	21st to end	Day	10% 6% 2 7% 5% 5% 6% 5% 7% 10% 37%	6835
August	1st to 10th	Day	6% 5% 2 6% 4 5% 5% 5% 8% 11% 44%	4234
	11th to 20th	Day	10% 5% 2 6% 4 5% 4 5% 7% 10% 42%	3984
	21st to end	Day	7% 5% 2 6% 4 3 4 3 6% 9% 49%	4312
September	1st to 10th	Day	11% 4 1 6% 3 4 4 4 5% 8% 50%	2400
	11th to 20th	Day	9% 4 2 5% 4 4 4 3 5% 8% 53%	2395
	21st to end	Day	9% 5% 2 6% 4 4 4 4 6% 8% 48%	2150
October	1st to 10th	Day	12% 7% 2 5% 4 3 3 3 5% 6% 51%	2000
	11th to 20th	Day	15% 8% 3 6% 4 3 4 3 4 6% 44%	2000
	21st to end	Day	19% 8% 2 6% 4 4 4 3 5% 6% 41%	2135
November	1st to 10th	Day	11% 9% 2 5% 4 3 4 4 5% 7% 45%	1852
	11th to 20th	Day	15% 11% 3 7% 5% 4 3 3 6% 7% 37%	1866
	21st to end	Day	16% 12% 4 7% 5% 4 4 3 4 6% 37%	1680
December	1st to 10th	Day	8% 10% 3 8% 4 4 4 3 4 6% 45%	2690
	11th to 20th	Day	14% 12% 5% 14% 6% 5% 5% 3 5% 6% 25%	3088
	21st to end	Day	22% 21% 7% 12% 5% 3 3 2 3 4 16%	4070
January	1st to 10th	Night	12% 13% 6% 13% 4 4 4 3 4 5% 32%	8232
	11th to 20th	Night	9% 12% 5% 12% 8% 6% 6% 4 6% 5% 28%	8164
	21st to end	Night	12% 12% 4 11% 5% 4 4 3 4 5% 35%	8402
February	1st to 10th	Night	14% 12% 4 9% 5% 4 4 3 5% 6% 34%	2730
	11th to 20th	Night	12% 11% 4 8% 4 5% 3 4 5% 6% 39%	2506
	21st to end	Night	15% 11% 3 7% 5% 4 3 3 4 5% 41%	1980
March	1st to 10th	Night	18% 9% 4 7% 3 3 3 3 4 5% 42%	5331
	11th to 20th	Night	10% 6% 2 6% 4 4 3 3 5% 7% 50%	4980
	21st to end	Night	17% 11% 5% 10% 6% 4 4 4 5% 6% 28%	5195
April	1st to 10th	Night	16% 9% 3 6% 4 4 3 3 5% 7% 41%	2000
	11th to 20th	Night	16% 9% 3 7% 4 3 3 3 5% 6% 42%	1975
	21st to end	Night	16% 7% 3 7% 5% 4 3 4 6% 8% 38%	2000
May	1st to 10th	Night	17% 9% 3 7% 4 4 4 3 5% 7% 37%	3936
	11th to 20th	Night	12% 5% 2 6% 4 5% 5% 4 6% 9% 42%	3596
	21st to end	Night	12% 8% 2 6% 4 4 4 4 5% 8% 42%	3942
June	1st to 10th	Night	8% 5% 2 5% 5% 4 5% 4 6% 9% 49%	1600
	11th to 20th	Night	7% 5% 3 5% 4 3 5% 5% 7% 10% 46%	1600
	21st to end	Night	9% 6% 1 5% 5% 4 4 4 5% 9% 48%	1600
July	1st to 10th	Night	11% 5% 2 6% 4 5% 5% 5% 7% 9% 41%	3618
	11th to 20th	Night	6% 5% 2 5% 4 4 4 4 6% 10% 50%	3617
	21st to end	Night	9% 7% 3 6% 5% 5% 5% 5% 6% 9% 41%	4032
August	1st to 10th	Night	4 4 1 5% 3 4 5% 4 6% 10% 54%	2926
	11th to 20th	Night	7% 4 2 5% 4 4 5% 3 5% 7% 54%	3069
	21st to end	Night	5% 4 2 5% 4 4 4 3 5% 8% 56%	3586
September	1st to 10th	Night	9% 4 2 4 3 3 3 2 5% 7% 58%	2000
	11th to 20th	Night	9% 4 2 5% 3 4 4 4 5% 8% 51%	2000
	21st to end	Night	7% 5% 2 6% 4 4 3 4 5% 7% 53%	2232
October	1st to 10th	Night	10% 6% 2 6% 4 4 3 3 4 5% 54%	2400
	11th to 20th	Night	14% 7% 2 6% 4 3 3 2 4 6% 48%	2400
	21st to end	Night	17% 7% 3 7% 4 4 5% 3 5% 6% 40%	2704
November	1st to 10th	Night	13% 7% 2 5% 3 2 3 3 4 7% 53%	2666
	11th to 20th	Night	15% 8% 3 6% 4 4 4 3 5% 7% 40%	2751
	21st to end	Night	16% 11% 3 8% 4 4 4 3 4 5% 38%	2940
December	1st to 10th	Night	8% 9% 3 7% 5% 4 4 3 5% 6% 46%	4720
	11th to 20th	Night	15% 12% 4 11% 5% 4 4 3 4 5% 32%	5339
	21st to end	Night	23% 19% 6% 11% 5% 4 4 3 4 4 16%	7150

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	56% 7% 14% 17% 7% 1	4692
	11th to 20th	Day	51% 5% 10% 19% 14% 2	4812
	21st to end	Day	57% 4 12% 20% 7% 1	5812
February	1st to 10th	Day	54% 6% 15% 17% 6% 1	1890
	11th to 20th	Day	49% 6% 12% 22% 10% 1	1899
	21st to end	Day	49% 5% 12% 22% 10% 2	1650
March	1st to 10th	Day	43% 5% 9% 24% 17% 3	4564
	11th to 20th	Day	33% 7% 11% 31% 17% 1	4980
	21st to end	Day	54% 8% 14% 18% 5% 1	5734
April	1st to 10th	Day	49% 6% 12% 22% 11%	2400
	11th to 20th	Day	47% 6% 10% 23% 13% 2	2374
	21st to end	Day	49% 7% 12% 18% 13% 2	2400
May	1st to 10th	Day	43% 8% 12% 19% 17% 1	6004
	11th to 20th	Day	39% 7% 11% 27% 12% 3	6323
	21st to end	Day	41% 8% 12% 22% 15% 1	6949
June	1st to 10th	Day	34% 9% 15% 24% 16% 2	2800
	11th to 20th	Day	34% 8% 11% 24% 20% 3	2800
	21st to end	Day	36% 9% 15% 22% 15% 2	2800
July	1st to 10th	Day	37% 7% 13% 27% 14% 1	6291
	11th to 20th	Day	37% 8% 15% 26% 14% 1	6285
	21st to end	Day	37% 10% 19% 21% 12% 1	6640
August	1st to 10th	Day	30% 9% 20% 29% 11% 1	4220
	11th to 20th	Day	35% 8% 15% 26% 14% 1	4023
	21st to end	Day	31% 6% 14% 30% 16% 2	4311
September	1st to 10th	Day	31% 7% 11% 26% 20% 4	2400
	11th to 20th	Day	28% 6% 12% 28% 22% 3	2395
	21st to end	Day	32% 7% 11% 27% 21% 2	2150
October	1st to 10th	Day	23% 4 6% 26% 39% 2	3260
	11th to 20th	Day	28% 3 10% 23% 30% 5%	3240
	21st to end	Day	38% 4 9% 28% 17% 3	3425
November	1st to 10th	Day	38% 6% 15% 25% 14% 3	1852
	11th to 20th	Day	46% 5% 13% 22% 12% 1	1866
	21st to end	Day	49% 5% 15% 22% 9%	1680
December	1st to 10th	Day	40% 4 14% 34% 8% 1	2690
	11th to 20th	Day	58% 6% 12% 16% 7% 2	3085
	21st to end	Day	73% 5% 9% 10% 3	4070
January	1st to 10th	Night	53% 6% 15% 19% 7%	8233
	11th to 20th	Night	55% 4 12% 14% 13% 1	8164
	21st to end	Night	51% 4 13% 20% 11% 1	8402
February	1st to 10th	Night	50% 5% 13% 20% 11% 1	2730
	11th to 20th	Night	46% 5% 11% 24% 12% 1	2507
	21st to end	Night	45% 4 11% 23% 15% 2	1980
March	1st to 10th	Night	45% 5% 8% 25% 16% 2	5331
	11th to 20th	Night	34% 5% 10% 29% 18% 4	4980
	21st to end	Night	55% 9% 12% 16% 7% 1	5195
April	1st to 10th	Night	43% 5% 11% 25% 14% 2	2000
	11th to 20th	Night	43% 5% 10% 22% 16% 4	1975
	21st to end	Night	43% 7% 12% 21% 13% 3	2000
May	1st to 10th	Night	45% 8% 11% 18% 16% 2	3965
	11th to 20th	Night	36% 9% 14% 23% 15% 3	3597
	21st to end	Night	38% 8% 12% 24% 17% 1	3950
June	1st to 10th	Night	31% 8% 15% 26% 18% 4	1600
	11th to 20th	Night	30% 9% 11% 23% 22% 4	1600
	21st to end	Night	33% 8% 15% 21% 20% 3	1600
July	1st to 10th	Night	36% 8% 13% 26% 15% 2	3580
	11th to 20th	Night	29% 7% 15% 29% 19% 1	3572
	21st to end	Night	36% 10% 16% 22% 14% 2	3916
August	1st to 10th	Night	23% 8% 18% 31% 18% 2	2916
	11th to 20th	Night	28% 7% 16% 31% 17% 1	3096
	21st to end	Night	26% 7% 14% 31% 17% 4	3586
September	1st to 10th	Night	27% 6% 10% 27% 26% 4	2000
	11th to 20th	Night	30% 6% 12% 27% 20% 5%	2000
	21st to end	Night	30% 6% 11% 25% 25% 3	2232
October	1st to 10th	Night	23% 4 7% 27% 37% 4	3920
	11th to 20th	Night	27% 4 9% 22% 33% 5%	3899
	21st to end	Night	42% 5% 8% 21% 20% 4	4364
November	1st to 10th	Night	33% 5% 13% 27% 19% 4	2666
	11th to 20th	Night	43% 6% 11% 25% 14% 2	2751
	21st to end	Night	48% 5% 11% 23% 12% 1	2940
December	1st to 10th	Night	39% 5% 12% 32% 11% 1	4720
	11th to 20th	Night	53% 5% 12% 22% 8% 1	5341
	21st to end	Night	69% 7% 10% 10% 4	7150

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	20% 15% 36% 22% 6% 1	330
	11th to 20th	Day	18% 20% 44% 15% 3 1	330
	21st to end	Day	19% 20% 39% 17% 4 1	363
February	1st to 10th	Day	21% 18% 40% 16% 4	330
	11th to 20th	Day	23% 17% 36% 18% 4 1	315
	21st to end	Day	20% 18% 44% 16% 2 1	249
March	1st to 10th	Day	19% 19% 39% 20% 3	300
	11th to 20th	Day	21% 20% 32% 20% 5% 2	300
	21st to end	Day	27% 20% 33% 15% 4 1	445
April	1st to 10th	Day	23% 16% 35% 21% 6%	500
	11th to 20th	Day	21% 18% 30% 20% 10% 1	492
	21st to end	Day	20% 13% 35% 23% 7% 2	500
May	1st to 10th	Day	17% 11% 37% 25% 9% 2	617
	11th to 20th	Day	19% 14% 25% 26% 14% 2	725
	21st to end	Day	15% 7% 31% 26% 14% 7%	789
June	1st to 10th	Day	7% 8% 26% 32% 20% 8%	730
	11th to 20th	Day	8% 5% 29% 32% 21% 5%	730
	21st to end	Day	10% 9% 30% 31% 16% 3	730
July	1st to 10th	Day	8% 4 19% 31% 24% 14%	721
	11th to 20th	Day	9% 4 22% 36% 21% 8%	718
	21st to end	Day	7% 2 21% 35% 28% 6%	794
August	1st to 10th	Day	6% 3 22% 36% 24% 8%	726
	11th to 20th	Day	9% 5% 17% 31% 26% 12%	606
	21st to end	Day	6% 3 18% 34% 28% 11%	583
September	1st to 10th	Day	11% 7% 18% 32% 25% 8%	530
	11th to 20th	Day	7% 2 18% 33% 29% 11%	530
	21st to end	Day	11% 9% 24% 26% 21% 9%	480
October	1st to 10th	Day	16% 7% 27% 28% 17% 6%	330
	11th to 20th	Day	15% 20% 30% 23% 12%	330
	21st to end	Day	29% 14% 31% 21% 5% 1	363
November	1st to 10th	Day	13% 13% 41% 22% 7% 3	330
	11th to 20th	Day	22% 17% 32% 22% 6% 2	330
	21st to end	Day	24% 13% 34% 23% 5%	330
December	1st to 10th	Day	12% 17% 42% 22% 4 2	330
	11th to 20th	Day	18% 16% 37% 21% 8%	330
	21st to end	Day	26% 21% 34% 17% 3	363
January	1st to 10th	Night	16% 14% 39% 24% 6%	1050
	11th to 20th	Night	18% 15% 39% 23% 5%	1050
	21st to end	Night	18% 14% 38% 26% 4	1155
February	1st to 10th	Night	20% 12% 40% 23% 4	1050
	11th to 20th	Night	17% 16% 35% 28% 3	912
	21st to end	Night	21% 14% 40% 22% 2 1	660
March	1st to 10th	Night	20% 12% 39% 24% 4	784
	11th to 20th	Night	16% 13% 37% 28% 7%	600
	21st to end	Night	21% 16% 36% 22% 4	660
April	1st to 10th	Night	19% 15% 38% 25% 3	600
	11th to 20th	Night	19% 14% 32% 28% 6%	593
	21st to end	Night	18% 11% 35% 30% 7%	600
May	1st to 10th	Night	18% 8% 37% 30% 7% 1	600
	11th to 20th	Night	17% 6% 30% 36% 11%	591
	21st to end	Night	11% 6% 30% 35% 16% 1	650
June	1st to 10th	Night	9% 4 25% 46% 15% 1	600
	11th to 20th	Night	7% 4 28% 42% 19%	600
	21st to end	Night	10% 5% 27% 40% 17% 1	600
July	1st to 10th	Night	9% 2 11% 42% 31% 4	585
	11th to 20th	Night	5% 3 20% 41% 27% 3	582
	21st to end	Night	8% 1 17% 44% 29% 2	657
August	1st to 10th	Night	5% 3 14% 46% 30% 2	597
	11th to 20th	Night	6% 2 14% 43% 29% 5%	594
	21st to end	Night	8% 1 15% 45% 29% 3	660
September	1st to 10th	Night	11% 4 18% 38% 27% 3	600
	11th to 20th	Night	10% 3 20% 36% 26% 6%	600
	21st to end	Night	8% 7% 23% 35% 24% 2	784
October	1st to 10th	Night	12% 6% 30% 32% 19% 2	800
	11th to 20th	Night	17% 12% 32% 29% 11%	800
	21st to end	Night	21% 11% 34% 26% 7%	880
November	1st to 10th	Night	17% 8% 39% 27% 8% 1	820
	11th to 20th	Night	19% 13% 38% 24% 6%	861
	21st to end	Night	22% 12% 36% 25% 5%	1050
December	1st to 10th	Night	18% 12% 41% 24% 4 1	1050
	11th to 20th	Night	21% 13% 35% 25% 6%	1050
	21st to end	Night	27% 14% 34% 23% 3	1155

Page generated at 2026-02-13 00:42:15. Version ac07

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