Stars Fell Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stars Fell Observatory 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	13% 7% 3 6% 5% 4 5% 4 6% 8% 40%	127003
	Night	13% 7% 2 6% 4 4 4 3 5% 7% 44%	124377
Transparency	Day	42% 3 11% 21% 18% 5%	130765
	Night	40% 3 10% 21% 20% 6%	128685
Seeing	Day	18% 9% 23% 24% 18% 8%	16106
	Night	16% 8% 22% 29% 20% 4	26122

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	17% 10% 5% 5% 3 3 3 3 5% 5% 43%	4065
	11th to 20th	Day	13% 7% 5% 8% 3 3 2 2 4 5% 49%	4060
	21st to end	Day	21% 10% 3 5% 4 3 3 3 4 6% 38%	4972
February	1st to 10th	Day	19% 10% 3 6% 5% 3 3 3 5% 5% 38%	2000
	11th to 20th	Day	22% 9% 2 7% 5% 4 3 3 4 4 37%	1948
	21st to end	Day	19% 10% 3 7% 4 4 3 3 4 5% 38%	1650
March	1st to 10th	Day	18% 10% 3 7% 4 5% 5% 4 5% 5% 34%	4480
	11th to 20th	Day	12% 8% 2 6% 4 3 4 3 6% 6% 45%	4860
	21st to end	Day	15% 8% 2 7% 4 4 4 3 5% 6% 42%	5593
April	1st to 10th	Day	13% 6% 2 5% 4 4 4 3 6% 8% 45%	2400
	11th to 20th	Day	12% 7% 2 6% 4 3 4 3 5% 6% 46%	2374
	21st to end	Day	10% 5% 2 5% 4 4 5% 5% 6% 7% 45%	2430
May	1st to 10th	Day	16% 7% 3 7% 5% 5% 5% 5% 7% 9% 33%	5862
	11th to 20th	Day	16% 9% 3 7% 5% 5% 6% 5% 6% 8% 29%	5861
	21st to end	Day	15% 6% 3 7% 4 4 5% 4 6% 7% 39%	6433
June	1st to 10th	Day	9% 7% 3 8% 5% 6% 6% 5% 8% 9% 34%	2600
	11th to 20th	Day	5% 4 2 6% 5% 4 7% 5% 9% 11% 42%	2600
	21st to end	Day	7% 6% 2 6% 6% 6% 6% 6% 7% 9% 39%	2600
July	1st to 10th	Day	6% 5% 2 8% 6% 7% 7% 6% 8% 11% 34%	5900
	11th to 20th	Day	6% 5% 2 7% 6% 6% 8% 6% 10% 13% 30%	5900
	21st to end	Day	10% 7% 3 9% 7% 7% 8% 7% 9% 11% 22%	6424
August	1st to 10th	Day	5% 4 1 4 4 4 5% 5% 8% 12% 48%	3978
	11th to 20th	Day	5% 5% 2 6% 5% 5% 6% 5% 7% 11% 42%	3849
	21st to end	Day	7% 4 2 5% 4 5% 5% 4 6% 8% 51%	4312
September	1st to 10th	Day	10% 5% 2 7% 4 5% 6% 4 7% 8% 41%	2317
	11th to 20th	Day	13% 5% 2 5% 5% 4 6% 4 6% 8% 42%	2200
	21st to end	Day	12% 5% 2 6% 3 4 4 4 6% 6% 47%	2200
October	1st to 10th	Day	10% 5% 2 5% 4 4 4 3 4 5% 53%	2200
	11th to 20th	Day	9% 4 2 5% 3 3 3 3 5% 7% 56%	2200
	21st to end	Day	14% 6% 2 4 3 4 3 3 3 5% 53%	2055
November	1st to 10th	Day	13% 5% 2 6% 4 4 4 3 4 5% 53%	1852
	11th to 20th	Day	12% 5% 2 6% 3 3 4 3 5% 7% 50%	1890
	21st to end	Day	15% 6% 2 5% 3 4 4 3 4 6% 46%	1890
December	1st to 10th	Day	18% 8% 2 6% 3 3 3 3 4 5% 43%	3020
	11th to 20th	Day	16% 10% 4 8% 4 4 4 3 3 4 38%	3463
	21st to end	Day	19% 9% 4 7% 4 4 4 3 5% 6% 37%	4565
January	1st to 10th	Night	15% 10% 4 8% 3 3 3 3 4 5% 43%	5903
	11th to 20th	Night	16% 8% 3 8% 4 2 3 2 4 5% 45%	5882
	21st to end	Night	19% 10% 2 5% 3 3 3 3 4 4 44%	5984
February	1st to 10th	Night	18% 8% 2 7% 4 4 3 3 4 6% 42%	2400
	11th to 20th	Night	21% 8% 3 7% 4 3 3 3 4 4 40%	2338
	21st to end	Night	20% 7% 3 6% 3 4 5% 3 5% 6% 38%	1980
March	1st to 10th	Night	22% 11% 3 7% 5% 4 4 3 4 5% 31%	5373
	11th to 20th	Night	12% 6% 1 5% 3 3 4 3 5% 7% 49%	5085
	21st to end	Night	15% 9% 3 7% 4 4 4 3 4 4 44%	5377
April	1st to 10th	Night	11% 5% 2 5% 4 3 4 4 6% 7% 48%	2000
	11th to 20th	Night	13% 7% 3 5% 3 3 4 3 4 6% 50%	1975
	21st to end	Night	10% 5% 2 6% 5% 4 4 4 5% 7% 49%	1968
May	1st to 10th	Night	12% 8% 3 8% 5% 5% 5% 4 6% 8% 36%	4033
	11th to 20th	Night	19% 8% 3 9% 6% 5% 5% 4 5% 6% 31%	4032
	21st to end	Night	11% 7% 3 6% 4 4 4 5% 7% 8% 40%	4421
June	1st to 10th	Night	6% 6% 2 7% 6% 6% 6% 4 7% 10% 41%	1800
	11th to 20th	Night	4 4 1 4 4 4 4 5% 7% 12% 50%	1800
	21st to end	Night	5% 4 2 5% 5% 4 6% 5% 7% 12% 46%	1800
July	1st to 10th	Night	5% 5% 3 8% 5% 6% 6% 5% 8% 12% 37%	4058
	11th to 20th	Night	6% 4 2 5% 5% 5% 6% 5% 9% 14% 40%	4057
	21st to end	Night	7% 8% 3 10% 7% 7% 7% 6% 8% 11% 27%	4427
August	1st to 10th	Night	3 3 1 5% 3 4 4 4 7% 12% 54%	3167
	11th to 20th	Night	6% 5% 2 5% 4 4 4 3 6% 9% 52%	3200
	21st to end	Night	6% 4 1 5% 3 4 4 3 5% 8% 56%	3586
September	1st to 10th	Night	7% 5% 2 6% 5% 5% 5% 5% 6% 7% 46%	2060
	11th to 20th	Night	12% 5% 2 6% 5% 4 5% 3 5% 8% 44%	2200
	21st to end	Night	8% 5% 3 6% 3 4 4 3 5% 8% 51%	2200
October	1st to 10th	Night	9% 4 2 4 3 3 3 3 4 6% 59%	2200
	11th to 20th	Night	10% 5% 2 5% 3 3 3 3 4 6% 56%	2200
	21st to end	Night	14% 5% 2 5% 4 3 3 3 3 4 54%	2780
November	1st to 10th	Night	12% 5% 2 5% 3 3 3 3 5% 5% 53%	2666
	11th to 20th	Night	12% 6% 2 4 3 2 3 3 4 6% 55%	2730
	21st to end	Night	14% 8% 2 6% 4 3 3 3 4 5% 47%	2730
December	1st to 10th	Night	20% 8% 2 5% 3 3 3 2 4 5% 45%	4380
	11th to 20th	Night	17% 10% 3 7% 4 3 3 3 4 5% 42%	4952
	21st to end	Night	19% 7% 3 7% 4 4 4 3 5% 6% 36%	6633

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	45% 1 4 24% 23% 3	4066
	11th to 20th	Day	41% 2 4 24% 27% 2	4060
	21st to end	Day	49% 1 6% 17% 21% 6%	4972
February	1st to 10th	Day	48% 2 6% 20% 20% 3	2000
	11th to 20th	Day	51% 2 4 19% 18% 6%	1958
	21st to end	Day	50% 2 5% 15% 22% 6%	1650
March	1st to 10th	Day	54% 2 6% 13% 18% 7%	4480
	11th to 20th	Day	40% 2 5% 17% 22% 13%	4860
	21st to end	Day	45% 2 8% 23% 19% 3	5595
April	1st to 10th	Day	38% 3 12% 23% 18% 6%	2400
	11th to 20th	Day	39% 2 6% 21% 24% 6%	2374
	21st to end	Day	37% 3 10% 28% 20% 3	2430
May	1st to 10th	Day	48% 2 15% 22% 12% 2	5897
	11th to 20th	Day	53% 2 14% 22% 9%	5865
	21st to end	Day	44% 3 10% 24% 17% 2	6447
June	1st to 10th	Day	44% 5% 15% 21% 14% 1	2600
	11th to 20th	Day	33% 5% 18% 27% 16% 2	2600
	21st to end	Day	39% 6% 16% 24% 13% 3	2600
July	1st to 10th	Day	41% 5% 21% 23% 10%	5835
	11th to 20th	Day	42% 6% 29% 18% 5% 1	5831
	21st to end	Day	52% 6% 21% 16% 4	6240
August	1st to 10th	Day	27% 4 14% 39% 15% 1	3965
	11th to 20th	Day	34% 4 17% 26% 16% 2	3885
	21st to end	Day	32% 3 15% 23% 23% 4	4311
September	1st to 10th	Day	38% 4 16% 19% 16% 6%	2317
	11th to 20th	Day	40% 4 12% 22% 19% 3	2200
	21st to end	Day	37% 4 11% 20% 22% 7%	2200
October	1st to 10th	Day	25% 3 4 14% 39% 16%	3590
	11th to 20th	Day	21% 1 4 19% 30% 25%	3568
	21st to end	Day	27% 2 5% 23% 32% 11%	3292
November	1st to 10th	Day	36% 2 6% 18% 28% 10%	1852
	11th to 20th	Day	34% 2 6% 22% 26% 9%	1890
	21st to end	Day	40% 3 6% 19% 26% 6%	1890
December	1st to 10th	Day	44% 2 5% 18% 26% 5%	3020
	11th to 20th	Day	51% 2 7% 18% 16% 5%	3460
	21st to end	Day	49% 1 7% 25% 16% 2	4565
January	1st to 10th	Night	46% 1 4 23% 22% 3	5903
	11th to 20th	Night	44% 2 3 20% 24% 6%	5882
	21st to end	Night	46% 1 3 18% 26% 6%	5984
February	1st to 10th	Night	45% 2 5% 21% 22% 6%	2400
	11th to 20th	Night	49% 2 5% 18% 18% 8%	2339
	21st to end	Night	47% 3 5% 16% 20% 8%	1980
March	1st to 10th	Night	55% 2 5% 14% 19% 5%	5373
	11th to 20th	Night	35% 2 7% 17% 23% 16%	5085
	21st to end	Night	46% 1 5% 23% 19% 6%	5377
April	1st to 10th	Night	35% 4 11% 23% 21% 7%	2000
	11th to 20th	Night	37% 2 5% 22% 23% 11%	1975
	21st to end	Night	35% 3 10% 27% 20% 4	1968
May	1st to 10th	Night	46% 4 15% 20% 14% 1	4060
	11th to 20th	Night	54% 3 13% 21% 9% 1	4033
	21st to end	Night	41% 3 13% 25% 16% 2	4429
June	1st to 10th	Night	39% 4 15% 23% 17% 2	1800
	11th to 20th	Night	27% 4 20% 28% 18% 3	1800
	21st to end	Night	30% 7% 20% 23% 16% 5%	1800
July	1st to 10th	Night	37% 6% 22% 23% 10% 1	4015
	11th to 20th	Night	33% 6% 32% 22% 5% 1	4006
	21st to end	Night	50% 8% 23% 16% 4	4297
August	1st to 10th	Night	24% 4 15% 40% 15% 2	3156
	11th to 20th	Night	30% 4 19% 29% 16% 2	3230
	21st to end	Night	28% 4 15% 20% 29% 4	3586
September	1st to 10th	Night	36% 5% 18% 20% 16% 6%	2060
	11th to 20th	Night	39% 5% 11% 21% 21% 4	2200
	21st to end	Night	33% 3 10% 23% 24% 8%	2200
October	1st to 10th	Night	21% 3 5% 18% 35% 17%	3590
	11th to 20th	Night	20% 1 3 19% 31% 26%	3571
	21st to end	Night	26% 2 3 25% 31% 13%	4493
November	1st to 10th	Night	33% 3 8% 21% 24% 12%	2666
	11th to 20th	Night	33% 2 4 21% 29% 11%	2730
	21st to end	Night	40% 2 5% 22% 24% 7%	2730
December	1st to 10th	Night	44% 2 5% 21% 26% 3	4380
	11th to 20th	Night	47% 2 6% 17% 24% 4	4954
	21st to end	Night	49% 1 5% 25% 17% 2	6633

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	27% 26% 33% 13% 2	303
	11th to 20th	Day	30% 21% 31% 13% 4 1	300
	21st to end	Day	25% 23% 37% 12% 2 1	330
February	1st to 10th	Day	25% 22% 35% 16% 2 1	300
	11th to 20th	Day	42% 17% 28% 11% 1	300
	21st to end	Day	31% 24% 35% 9% 1	249
March	1st to 10th	Day	30% 21% 32% 13% 3 1	300
	11th to 20th	Day	25% 18% 33% 18% 6% 1	445
	21st to end	Day	23% 17% 33% 21% 5% 1	550
April	1st to 10th	Day	20% 16% 32% 20% 11% 1	500
	11th to 20th	Day	19% 15% 31% 22% 11% 1	492
	21st to end	Day	15% 11% 31% 28% 12% 2	500
May	1st to 10th	Day	17% 11% 27% 26% 14% 5%	503
	11th to 20th	Day	15% 6% 25% 29% 17% 8%	527
	21st to end	Day	15% 5% 18% 29% 20% 12%	573
June	1st to 10th	Day	14% 2 19% 32% 23% 11%	530
	11th to 20th	Day	8% 4 17% 26% 27% 19%	530
	21st to end	Day	12% 2 13% 28% 28% 18%	530
July	1st to 10th	Day	10% 1 8% 28% 31% 22%	525
	11th to 20th	Day	9% 2 6% 24% 32% 27%	524
	21st to end	Day	11% 2 9% 23% 35% 21%	576
August	1st to 10th	Day	13% 2 11% 23% 31% 20%	526
	11th to 20th	Day	11% 2 9% 31% 31% 16%	519
	21st to end	Day	16% 2 9% 26% 32% 15%	583
September	1st to 10th	Day	10% 3 14% 31% 32% 11%	530
	11th to 20th	Day	15% 2 13% 28% 30% 12%	530
	21st to end	Day	15% 4 16% 29% 28% 8%	530
October	1st to 10th	Day	14% 3 24% 32% 18% 8%	530
	11th to 20th	Day	12% 9% 26% 34% 17% 2	530
	21st to end	Day	20% 10% 24% 32% 14% 1	428
November	1st to 10th	Day	22% 8% 32% 25% 10% 2	330
	11th to 20th	Day	16% 18% 40% 21% 5%	330
	21st to end	Day	19% 18% 38% 17% 7% 1	330
December	1st to 10th	Day	32% 17% 32% 14% 4	330
	11th to 20th	Day	27% 18% 33% 17% 5% 1	330
	21st to end	Day	30% 15% 34% 17% 4 1	363
January	1st to 10th	Night	23% 17% 33% 22% 4	805
	11th to 20th	Night	23% 15% 31% 25% 6%	800
	21st to end	Night	21% 14% 37% 25% 3	880
February	1st to 10th	Night	23% 12% 32% 28% 4	800
	11th to 20th	Night	27% 15% 32% 24% 3	783
	21st to end	Night	25% 15% 35% 22% 3	660
March	1st to 10th	Night	21% 15% 31% 28% 5%	800
	11th to 20th	Night	21% 12% 32% 25% 9% 1	800
	21st to end	Night	20% 14% 30% 27% 9%	841
April	1st to 10th	Night	16% 11% 33% 29% 12%	600
	11th to 20th	Night	17% 9% 33% 27% 13% 1	593
	21st to end	Night	14% 6% 27% 36% 15% 1	600
May	1st to 10th	Night	13% 5% 24% 33% 22% 3	600
	11th to 20th	Night	15% 2 21% 34% 23% 5%	591
	21st to end	Night	10% 2 13% 36% 32% 6%	650
June	1st to 10th	Night	11% 1 12% 36% 34% 6%	600
	11th to 20th	Night	5% 2 11% 30% 40% 12%	600
	21st to end	Night	8% 1 8% 30% 43% 12%	600
July	1st to 10th	Night	9% 1 5% 24% 46% 16%	585
	11th to 20th	Night	8% 1 4 23% 43% 20%	582
	21st to end	Night	8% 2 7% 24% 46% 13%	657
August	1st to 10th	Night	7% 1 4 30% 45% 14%	597
	11th to 20th	Night	11% 1 8% 26% 43% 10%	594
	21st to end	Night	13% 1 5% 31% 40% 10%	660
September	1st to 10th	Night	11% 2 9% 31% 37% 10%	660
	11th to 20th	Night	14% 2 9% 31% 38% 6%	800
	21st to end	Night	11% 2 15% 31% 37% 4	800
October	1st to 10th	Night	11% 4 20% 35% 25% 6%	800
	11th to 20th	Night	13% 7% 24% 35% 19% 2	800
	21st to end	Night	16% 8% 20% 36% 18% 3	880
November	1st to 10th	Night	15% 7% 24% 33% 19% 2	820
	11th to 20th	Night	16% 8% 31% 32% 12% 1	840
	21st to end	Night	19% 10% 28% 30% 11% 1	840
December	1st to 10th	Night	25% 12% 28% 26% 8%	840
	11th to 20th	Night	22% 11% 31% 27% 8%	840
	21st to end	Night	26% 12% 27% 28% 6%	924

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