SkyShed Observatories
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the SkyShed Observatories clear sky chart from 2005-11-06 to 2026-01-02.

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	14% 11% 4 10% 5% 5% 5% 4 5% 7% 30%	127388
	Night	14% 13% 5% 11% 6% 5% 4 3 5% 6% 28%	123969
Transparency	Day	52% 6% 12% 19% 10% 1	130968
	Night	55% 6% 11% 17% 10% 1	128478
Seeing	Day	19% 12% 28% 24% 13% 5%	16540
	Night	20% 11% 29% 28% 11% 1	25840

By Month

Percentage Cloud Cover NE (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	11% 17% 10% 26% 7% 4 4 2 4 4 11%	3395
	11th to 20th	Day	9% 19% 11% 26% 9% 9% 5% 2 2 2 5%	3937
	21st to end	Day	15% 18% 7% 21% 9% 6% 5% 3 4 3 8%	4466
February	1st to 10th	Day	15% 18% 7% 16% 11% 6% 4 3 3 4 13%	1800
	11th to 20th	Day	13% 13% 10% 14% 10% 7% 4 4 4 5% 16%	1753
	21st to end	Day	15% 17% 6% 13% 7% 5% 5% 3 5% 5% 20%	1643
March	1st to 10th	Day	15% 12% 6% 14% 7% 4 4 3 4 5% 26%	4794
	11th to 20th	Day	11% 15% 6% 11% 5% 4 4 3 6% 8% 28%	4980
	21st to end	Day	18% 14% 5% 12% 6% 4 3 3 4 5% 26%	5465
April	1st to 10th	Day	17% 16% 5% 7% 4 4 4 3 5% 7% 27%	2275
	11th to 20th	Day	18% 12% 3 6% 4 3 4 3 5% 7% 35%	2569
	21st to end	Day	19% 11% 4 7% 4 3 4 3 5% 7% 33%	2600
May	1st to 10th	Day	17% 10% 3 7% 5% 4 5% 4 6% 7% 34%	5888
	11th to 20th	Day	16% 10% 3 7% 5% 4 5% 4 6% 8% 34%	5921
	21st to end	Day	18% 9% 2 5% 4 4 4 3 4 6% 40%	6949
June	1st to 10th	Day	13% 7% 3 7% 5% 5% 6% 4 6% 8% 38%	2843
	11th to 20th	Day	12% 6% 3 6% 4 4 5% 4 6% 9% 41%	3000
	21st to end	Day	15% 6% 2 6% 4 5% 5% 4 7% 8% 39%	3000
July	1st to 10th	Day	11% 6% 2 6% 4 4 5% 4 6% 9% 42%	6819
	11th to 20th	Day	10% 5% 2 6% 4 4 4 4 6% 9% 45%	6232
	21st to end	Day	8% 5% 2 5% 4 4 5% 4 6% 9% 48%	6424
August	1st to 10th	Day	13% 6% 2 6% 4 5% 5% 5% 8% 10% 37%	4234
	11th to 20th	Day	12% 6% 2 6% 5% 6% 6% 5% 8% 11% 34%	4174
	21st to end	Day	10% 6% 2 6% 4 5% 6% 5% 7% 9% 41%	4624
September	1st to 10th	Day	11% 6% 3 6% 5% 5% 5% 5% 6% 9% 40%	2345
	11th to 20th	Day	11% 5% 2 6% 3 4 5% 4 7% 7% 45%	2200
	21st to end	Day	17% 7% 2 7% 4 5% 5% 4 7% 7% 34%	2200
October	1st to 10th	Day	17% 7% 3 8% 5% 5% 4 4 5% 6% 37%	2042
	11th to 20th	Day	21% 12% 4 8% 6% 5% 4 4 6% 7% 25%	2000
	21st to end	Day	25% 14% 4 8% 5% 5% 5% 4 6% 7% 18%	2145
November	1st to 10th	Day	16% 17% 4 9% 5% 3 4 3 5% 8% 26%	1852
	11th to 20th	Day	15% 18% 4 11% 6% 5% 5% 4 5% 6% 21%	1705
	21st to end	Day	17% 22% 8% 11% 7% 5% 5% 4 4 5% 12%	1680
December	1st to 10th	Day	16% 26% 10% 15% 7% 5% 4 2 4 3 7%	2680
	11th to 20th	Day	21% 20% 7% 15% 9% 4 4 3 4 3 11%	3066
	21st to end	Day	24% 21% 7% 12% 6% 5% 4 4 4 4 8%	3688
January	1st to 10th	Night	12% 17% 8% 25% 8% 6% 4 3 3 4 10%	6576
	11th to 20th	Night	11% 17% 10% 22% 13% 7% 5% 4 4 3 6%	5932
	21st to end	Night	14% 16% 9% 18% 7% 7% 6% 3 4 4 11%	6490
February	1st to 10th	Night	14% 19% 6% 14% 10% 6% 4 3 5% 4 15%	2600
	11th to 20th	Night	14% 15% 7% 15% 8% 6% 5% 4 5% 5% 17%	2533
	21st to end	Night	16% 13% 5% 15% 6% 5% 5% 3 5% 4 22%	1985
March	1st to 10th	Night	13% 12% 5% 10% 6% 5% 4 4 5% 5% 30%	5046
	11th to 20th	Night	10% 10% 6% 10% 5% 5% 5% 4 6% 9% 31%	4979
	21st to end	Night	18% 11% 4 9% 4 4 4 3 4 4 35%	5467
April	1st to 10th	Night	19% 13% 4 7% 5% 4 4 3 5% 6% 30%	2125
	11th to 20th	Night	18% 9% 4 8% 4 4 4 3 5% 7% 34%	1797
	21st to end	Night	15% 12% 3 7% 5% 4 5% 3 5% 7% 34%	1800
May	1st to 10th	Night	17% 9% 3 7% 5% 5% 4 3 4 7% 37%	4053
	11th to 20th	Night	15% 9% 3 8% 5% 5% 4 3 5% 7% 36%	3969
	21st to end	Night	18% 7% 2 4 3 3 3 3 5% 7% 44%	3926
June	1st to 10th	Night	13% 6% 2 6% 5% 5% 5% 4 5% 7% 43%	1551
	11th to 20th	Night	10% 7% 2 6% 4 5% 5% 4 8% 9% 41%	1400
	21st to end	Night	11% 6% 2 6% 5% 4 5% 4 5% 8% 45%	1400
July	1st to 10th	Night	7% 5% 2 6% 4 4 5% 4 7% 9% 47%	3160
	11th to 20th	Night	11% 6% 2 5% 3 3 3 3 5% 8% 50%	3732
	21st to end	Night	9% 6% 2 6% 4 4 4 4 5% 7% 50%	4428
August	1st to 10th	Night	11% 6% 2 7% 5% 4 4 4 5% 7% 46%	2915
	11th to 20th	Night	12% 8% 2 7% 4 3 4 4 5% 7% 44%	2875
	21st to end	Night	10% 5% 3 6% 4 4 4 3 6% 7% 47%	3240
September	1st to 10th	Night	11% 5% 2 6% 5% 5% 5% 5% 5% 6% 46%	2051
	11th to 20th	Night	10% 5% 2 4 3 3 4 3 5% 7% 54%	2200
	21st to end	Night	16% 8% 2 6% 5% 4 4 3 6% 6% 39%	2200
October	1st to 10th	Night	14% 8% 3 8% 4 3 5% 3 5% 6% 40%	2356
	11th to 20th	Night	22% 12% 3 8% 5% 4 4 3 4 6% 30%	2400
	21st to end	Night	24% 10% 3 9% 6% 5% 5% 3 5% 5% 25%	2692
November	1st to 10th	Night	15% 14% 3 9% 4 3 4 4 6% 6% 31%	2666
	11th to 20th	Night	16% 18% 5% 11% 5% 5% 6% 4 4 7% 20%	2914
	21st to end	Night	20% 20% 6% 11% 7% 5% 5% 2 4 4 16%	2940
December	1st to 10th	Night	12% 25% 10% 16% 7% 6% 5% 4 4 4 7%	4720
	11th to 20th	Night	13% 25% 8% 16% 8% 4 4 3 3 4 13%	5348
	21st to end	Night	22% 22% 8% 11% 5% 4 4 3 4 4 13%	7503

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% 5% 8% 7% 3	3396
	11th to 20th	Day	87% 4 4 3 2	3971
	21st to end	Day	80% 5% 8% 5% 3	4466
February	1st to 10th	Day	75% 4 7% 10% 2	1800
	11th to 20th	Day	71% 6% 8% 12% 4	1762
	21st to end	Day	64% 6% 11% 13% 6% 1	1643
March	1st to 10th	Day	61% 5% 14% 15% 5%	4797
	11th to 20th	Day	54% 4 16% 18% 7% 1	4980
	21st to end	Day	59% 5% 11% 16% 8% 1	5465
April	1st to 10th	Day	56% 6% 11% 18% 9%	2275
	11th to 20th	Day	47% 6% 12% 21% 12% 1	2569
	21st to end	Day	51% 6% 11% 19% 11% 2	2600
May	1st to 10th	Day	48% 5% 12% 19% 11% 5%	5897
	11th to 20th	Day	47% 8% 11% 20% 13% 1	5925
	21st to end	Day	45% 6% 10% 18% 17% 4	6949
June	1st to 10th	Day	41% 8% 13% 23% 14% 1	2843
	11th to 20th	Day	40% 7% 11% 24% 16% 1	3000
	21st to end	Day	41% 7% 12% 25% 14% 1	3000
July	1st to 10th	Day	37% 6% 13% 25% 18% 1	6746
	11th to 20th	Day	35% 4 12% 32% 15% 2	6155
	21st to end	Day	33% 5% 15% 27% 19% 1	6240
August	1st to 10th	Day	40% 8% 13% 24% 14% 1	4221
	11th to 20th	Day	41% 7% 17% 25% 9%	4213
	21st to end	Day	37% 5% 15% 30% 12% 1	4624
September	1st to 10th	Day	39% 8% 14% 26% 11% 1	2345
	11th to 20th	Day	35% 7% 13% 30% 14% 1	2200
	21st to end	Day	46% 7% 14% 23% 9% 1	2200
October	1st to 10th	Day	40% 6% 12% 25% 16% 1	3328
	11th to 20th	Day	44% 6% 12% 21% 16%	3240
	21st to end	Day	54% 6% 17% 17% 6%	3447
November	1st to 10th	Day	56% 8% 11% 18% 7% 1	1852
	11th to 20th	Day	63% 6% 13% 11% 6% 1	1705
	21st to end	Day	73% 7% 10% 9% 2	1680
December	1st to 10th	Day	82% 4 8% 5% 1	2680
	11th to 20th	Day	76% 5% 7% 8% 3	3066
	21st to end	Day	78% 7% 7% 6% 2	3688
January	1st to 10th	Night	76% 6% 7% 9% 2	6576
	11th to 20th	Night	82% 7% 6% 3 2	5973
	21st to end	Night	75% 6% 8% 7% 3	6490
February	1st to 10th	Night	72% 6% 9% 11% 2	2600
	11th to 20th	Night	68% 5% 12% 12% 4	2535
	21st to end	Night	63% 6% 11% 14% 6%	1985
March	1st to 10th	Night	54% 5% 13% 18% 8% 1	5056
	11th to 20th	Night	47% 5% 19% 19% 8% 1	4980
	21st to end	Night	52% 5% 11% 18% 13% 1	5467
April	1st to 10th	Night	55% 6% 9% 17% 12% 1	2125
	11th to 20th	Night	49% 5% 10% 18% 16% 2	1797
	21st to end	Night	49% 6% 10% 17% 15% 3	1800
May	1st to 10th	Night	47% 5% 12% 15% 13% 8%	4060
	11th to 20th	Night	48% 7% 10% 20% 13% 3	3970
	21st to end	Night	39% 6% 12% 23% 17% 3	3927
June	1st to 10th	Night	39% 6% 10% 26% 16% 3	1551
	11th to 20th	Night	37% 7% 13% 24% 16% 3	1400
	21st to end	Night	37% 6% 13% 25% 18% 2	1400
July	1st to 10th	Night	33% 5% 14% 25% 19% 4	3125
	11th to 20th	Night	33% 4 11% 28% 20% 2	3687
	21st to end	Night	34% 5% 10% 26% 22% 2	4297
August	1st to 10th	Night	37% 6% 12% 27% 17% 1	2904
	11th to 20th	Night	39% 6% 15% 25% 14% 1	2902
	21st to end	Night	35% 7% 11% 32% 14% 1	3240
September	1st to 10th	Night	36% 8% 12% 25% 16% 2	2051
	11th to 20th	Night	29% 6% 14% 29% 20% 1	2200
	21st to end	Night	44% 7% 12% 23% 13% 1	2200
October	1st to 10th	Night	34% 6% 9% 27% 23% 1	3850
	11th to 20th	Night	47% 5% 14% 19% 14%	3899
	21st to end	Night	49% 6% 14% 21% 10%	4340
November	1st to 10th	Night	52% 7% 11% 20% 9%	2666
	11th to 20th	Night	64% 7% 10% 13% 5% 1	2914
	21st to end	Night	70% 7% 12% 9% 2	2940
December	1st to 10th	Night	81% 6% 7% 5% 1	4720
	11th to 20th	Night	77% 4 7% 8% 3	5348
	21st to end	Night	75% 4 9% 9% 3	7503

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	19% 23% 35% 16% 6% 2	252
	11th to 20th	Day	21% 19% 43% 12% 4 1	300
	21st to end	Day	25% 14% 37% 19% 3 1	330
February	1st to 10th	Day	23% 23% 36% 15% 3	300
	11th to 20th	Day	22% 24% 40% 12% 2	300
	21st to end	Day	26% 26% 31% 16% 1	249
March	1st to 10th	Day	22% 15% 38% 19% 6%	435
	11th to 20th	Day	20% 16% 33% 23% 6% 1	500
	21st to end	Day	25% 15% 31% 23% 5% 1	550
April	1st to 10th	Day	23% 18% 32% 20% 5% 1	500
	11th to 20th	Day	25% 18% 25% 19% 10% 3	492
	21st to end	Day	24% 14% 32% 21% 8% 2	500
May	1st to 10th	Day	22% 11% 28% 27% 9% 3	503
	11th to 20th	Day	20% 7% 29% 24% 16% 5%	527
	21st to end	Day	17% 8% 25% 27% 17% 7%	573
June	1st to 10th	Day	12% 9% 32% 29% 13% 4	573
	11th to 20th	Day	14% 9% 30% 30% 13% 3	730
	21st to end	Day	15% 8% 28% 33% 13% 3	730
July	1st to 10th	Day	9% 6% 25% 30% 22% 7%	721
	11th to 20th	Day	15% 4 25% 29% 20% 7%	628
	21st to end	Day	8% 4 24% 36% 20% 8%	576
August	1st to 10th	Day	11% 4 21% 27% 25% 12%	526
	11th to 20th	Day	13% 4 17% 33% 21% 13%	519
	21st to end	Day	16% 5% 21% 25% 23% 10%	583
September	1st to 10th	Day	14% 6% 23% 25% 18% 13%	530
	11th to 20th	Day	9% 5% 17% 25% 30% 15%	530
	21st to end	Day	17% 11% 25% 24% 15% 8%	530
October	1st to 10th	Day	18% 10% 26% 23% 18% 5%	530
	11th to 20th	Day	22% 16% 32% 21% 7% 1	530
	21st to end	Day	28% 17% 31% 19% 5%	528
November	1st to 10th	Day	26% 20% 27% 19% 7% 1	330
	11th to 20th	Day	26% 21% 32% 15% 6% 1	233
	21st to end	Day	25% 22% 38% 13% 2	220
December	1st to 10th	Day	26% 25% 31% 14% 5%	220
	11th to 20th	Day	29% 26% 28% 12% 4	220
	21st to end	Day	34% 20% 30% 13% 2	242
January	1st to 10th	Night	22% 17% 37% 18% 5% 1	1006
	11th to 20th	Night	27% 17% 32% 20% 4	834
	21st to end	Night	19% 14% 39% 22% 5%	880
February	1st to 10th	Night	24% 16% 36% 22% 2	800
	11th to 20th	Night	22% 16% 33% 24% 5%	783
	21st to end	Night	23% 17% 36% 22% 3	660
March	1st to 10th	Night	19% 14% 36% 23% 7%	800
	11th to 20th	Night	19% 16% 36% 23% 6% 1	800
	21st to end	Night	23% 12% 32% 25% 8% 1	880
April	1st to 10th	Night	25% 15% 30% 26% 4	800
	11th to 20th	Night	21% 14% 32% 23% 10% 1	613
	21st to end	Night	20% 13% 30% 29% 8%	600
May	1st to 10th	Night	19% 8% 31% 30% 11% 1	600
	11th to 20th	Night	18% 5% 29% 34% 12% 1	567
	21st to end	Night	14% 4 27% 38% 16% 1	433
June	1st to 10th	Night	15% 4 31% 35% 15%	400
	11th to 20th	Night	11% 7% 28% 43% 13%	400
	21st to end	Night	11% 4 25% 46% 13% 1	400
July	1st to 10th	Night	6% 2 18% 51% 21% 1	390
	11th to 20th	Night	11% 2 21% 41% 24% 2	540
	21st to end	Night	9% 3 17% 47% 23% 1	657
August	1st to 10th	Night	9% 2 14% 42% 29% 3	597
	11th to 20th	Night	11% 2 15% 39% 28% 4	594
	21st to end	Night	13% 3 19% 37% 27% 2	664
September	1st to 10th	Night	13% 5% 22% 34% 22% 5%	800
	11th to 20th	Night	11% 4 17% 37% 27% 4	800
	21st to end	Night	19% 6% 22% 33% 16% 4	800
October	1st to 10th	Night	17% 8% 28% 31% 15% 2	800
	11th to 20th	Night	25% 14% 26% 25% 9% 1	800
	21st to end	Night	29% 11% 32% 25% 4	880
November	1st to 10th	Night	24% 13% 28% 28% 7% 1	820
	11th to 20th	Night	25% 16% 35% 18% 5% 1	840
	21st to end	Night	30% 13% 31% 21% 4	840
December	1st to 10th	Night	27% 19% 31% 18% 5%	840
	11th to 20th	Night	23% 21% 36% 16% 3	840
	21st to end	Night	30% 15% 36% 17% 2	1082

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