Slater Creek Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Slater Creek Observatory clear sky chart from 2005-11-06 to 2026-04-02.

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	3 5% 2 6% 5% 5% 5% 4 6% 8% 51%	140121
	Night	2 4 2 6% 4 4 4 4 6% 8% 57%	139220
Transparency	Day	26% 6% 11% 23% 23% 10%	135089
	Night	23% 6% 12% 26% 25% 8%	135259
Seeing	Day	10% 17% 32% 25% 12% 2	17349
	Night	5% 12% 37% 33% 12% 1	27524

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	4 3 7% 5% 4 3 4 5% 7% 59%	4720
	11th to 20th	Day	2 1 6% 4 4 4 3 4 8% 62%	5058
	21st to end	Day	5% 2 6% 4 4 4 3 5% 7% 60%	5844
February	1st to 10th	Day	6% 3 7% 4 4 2 3 6% 7% 58%	1890
	11th to 20th	Day	3 2 4 4 3 5% 4 7% 9% 59%	2033
	21st to end	Day	5% 3 5% 4 3 3 3 6% 7% 61%	1730
March	1st to 10th	Day	1 6% 3 6% 4 3 3 3 5% 6% 61%	4600
	11th to 20th	Day	1 8% 4 11% 7% 6% 5% 4 5% 7% 43%	5105
	21st to end	Day	1 6% 3 6% 4 4 4 4 7% 9% 51%	5916
April	1st to 10th	Day	2 7% 3 6% 5% 5% 4 4 6% 8% 48%	2413
	11th to 20th	Day	4 9% 3 9% 6% 5% 4 3 6% 7% 45%	2371
	21st to end	Day	4 10% 3 8% 5% 5% 5% 4 7% 8% 41%	2400
May	1st to 10th	Day	4 10% 3 8% 5% 5% 5% 4 6% 9% 42%	5622
	11th to 20th	Day	4 7% 2 6% 5% 6% 5% 5% 7% 8% 45%	6242
	21st to end	Day	3 5% 2 6% 4 4 6% 5% 7% 9% 48%	6925
June	1st to 10th	Day	4 6% 2 6% 5% 4 5% 5% 9% 10% 45%	2800
	11th to 20th	Day	4 4 2 5% 4 4 5% 5% 8% 8% 52%	2800
	21st to end	Day	3 4 2 7% 5% 6% 6% 6% 8% 8% 45%	2800
July	1st to 10th	Day	3 3 1 5% 4 5% 6% 5% 7% 9% 50%	6339
	11th to 20th	Day	4 5% 3 8% 7% 6% 7% 5% 8% 10% 38%	6340
	21st to end	Day	6% 5% 3 8% 6% 6% 6% 6% 7% 7% 39%	6631
August	1st to 10th	Day	5% 5% 2 7% 6% 6% 8% 6% 9% 10% 36%	5745
	11th to 20th	Day	3 3 2 6% 5% 5% 7% 6% 8% 10% 44%	5366
	21st to end	Day	5% 6% 2 7% 6% 6% 6% 5% 7% 9% 39%	5984
September	1st to 10th	Day	5% 5% 2 6% 5% 4 5% 4 5% 6% 50%	2400
	11th to 20th	Day	5% 4 2 5% 4 4 4 4 6% 6% 55%	2385
	21st to end	Day	4 4 2 5% 3 4 3 3 5% 7% 61%	2200
October	1st to 10th	Day	3 3 1 3 3 2 3 3 4 5% 72%	3275
	11th to 20th	Day	2 4 2 5% 4 4 4 4 6% 8% 57%	3250
	21st to end	Day	2 4 2 3 2 3 3 3 4 5% 69%	3485
November	1st to 10th	Day	1 4 2 4 2 2 4 3 5% 7% 67%	1853
	11th to 20th	Day	3 2 4 3 3 2 3 5% 7% 66%	1890
	21st to end	Day	4 2 6% 3 3 4 3 5% 8% 61%	1811
December	1st to 10th	Day	3 2 5% 3 2 3 2 3 6% 71%	2700
	11th to 20th	Day	1 3 2 4 3 3 3 2 5% 8% 66%	3106
	21st to end	Day	3 2 6% 5% 4 5% 4 7% 8% 56%	4092
January	1st to 10th	Night	3 3 9% 5% 3 4 3 6% 8% 57%	8201
	11th to 20th	Night	2 1 5% 4 3 3 3 4 6% 67%	7973
	21st to end	Night	3 2 6% 4 4 4 3 4 6% 65%	8370
February	1st to 10th	Night	3 3 5% 4 3 3 3 6% 8% 62%	2730
	11th to 20th	Night	5% 2 7% 4 4 4 3 5% 8% 58%	2472
	21st to end	Night	5% 2 5% 3 3 3 3 4 7% 65%	2076
March	1st to 10th	Night	1 6% 3 6% 3 3 4 3 5% 6% 59%	5494
	11th to 20th	Night	1 6% 3 7% 5% 4 4 3 6% 8% 54%	5114
	21st to end	Night	1 6% 2 6% 4 4 4 3 5% 8% 56%	5285
April	1st to 10th	Night	2 7% 2 6% 3 3 4 3 5% 7% 59%	2010
	11th to 20th	Night	3 8% 2 7% 5% 5% 4 4 5% 7% 51%	1977
	21st to end	Night	3 10% 3 6% 4 4 4 3 6% 7% 51%	2000
May	1st to 10th	Night	3 8% 2 6% 5% 5% 6% 4 6% 8% 46%	4356
	11th to 20th	Night	4 6% 2 6% 4 4 4 4 6% 8% 52%	3670
	21st to end	Night	4 5% 2 8% 4 3 3 4 6% 7% 54%	3972
June	1st to 10th	Night	3 5% 2 5% 3 4 4 5% 7% 10% 53%	1600
	11th to 20th	Night	3 2 1 3 3 3 4 4 8% 10% 59%	1600
	21st to end	Night	4 3 1 5% 5% 5% 5% 5% 8% 10% 51%	1600
July	1st to 10th	Night	3 3 2 7% 4 5% 5% 4 7% 8% 52%	3640
	11th to 20th	Night	3 3 2 6% 4 6% 6% 5% 9% 13% 44%	3640
	21st to end	Night	3 4 2 7% 6% 6% 6% 6% 9% 10% 42%	4237
August	1st to 10th	Night	4 4 2 7% 5% 5% 5% 5% 8% 10% 44%	4066
	11th to 20th	Night	3 3 2 6% 5% 6% 6% 5% 8% 11% 46%	4483
	21st to end	Night	5% 6% 3 8% 5% 5% 7% 5% 8% 11% 37%	4993
September	1st to 10th	Night	5% 4 2 5% 4 5% 4 4 7% 9% 50%	2000
	11th to 20th	Night	4 4 1 4 3 4 4 4 6% 8% 58%	2014
	21st to end	Night	4 4 2 5% 4 4 4 3 5% 6% 61%	2200
October	1st to 10th	Night	2 3 1 4 3 2 3 3 4 5% 70%	3910
	11th to 20th	Night	2 4 2 5% 4 3 3 3 5% 7% 63%	3898
	21st to end	Night	2 3 1 3 2 2 2 3 4 5% 72%	4281
November	1st to 10th	Night	4 2 3 3 2 3 3 5% 7% 68%	2665
	11th to 20th	Night	2 1 3 4 3 4 3 5% 7% 68%	2730
	21st to end	Night	1 5% 2 4 3 4 3 3 5% 7% 62%	2804
December	1st to 10th	Night	2 1 3 2 2 2 2 4 7% 72%	4710
	11th to 20th	Night	2 1 3 3 3 3 4 6% 8% 67%	5321
	21st to end	Night	5% 3 7% 4 5% 5% 4 6% 7% 55%	7128

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	21% 5% 10% 31% 25% 7%	4723
	11th to 20th	Day	17% 5% 10% 25% 30% 12%	5058
	21st to end	Day	21% 4 9% 25% 27% 13%	5844
February	1st to 10th	Day	23% 4 14% 27% 20% 11%	1890
	11th to 20th	Day	20% 4 12% 26% 30% 8%	2043
	21st to end	Day	21% 5% 9% 22% 28% 15%	1730
March	1st to 10th	Day	20% 5% 7% 16% 33% 20%	4600
	11th to 20th	Day	34% 6% 10% 22% 22% 5%	5105
	21st to end	Day	24% 7% 12% 38% 16% 3	5916
April	1st to 10th	Day	29% 7% 13% 22% 20% 9%	2413
	11th to 20th	Day	35% 7% 10% 22% 19% 7%	2374
	21st to end	Day	35% 8% 13% 24% 17% 3	2400
May	1st to 10th	Day	32% 8% 12% 24% 17% 7%	5622
	11th to 20th	Day	30% 9% 11% 19% 21% 10%	6246
	21st to end	Day	24% 9% 14% 22% 23% 8%	6926
June	1st to 10th	Day	29% 11% 16% 19% 18% 8%	2800
	11th to 20th	Day	25% 9% 10% 17% 23% 17%	2800
	21st to end	Day	29% 9% 12% 20% 20% 10%	2800
July	1st to 10th	Day	25% 7% 7% 16% 21% 23%	6271
	11th to 20th	Day	36% 5% 15% 25% 13% 6%	6265
	21st to end	Day	39% 7% 14% 15% 15% 11%	6440
August	1st to 10th	Day	40% 8% 18% 16% 13% 3	4185
	11th to 20th	Day	33% 7% 15% 15% 19% 12%	3882
	21st to end	Day	35% 6% 15% 17% 21% 6%	4311
September	1st to 10th	Day	30% 7% 10% 19% 23% 11%	2400
	11th to 20th	Day	26% 5% 11% 19% 27% 11%	2385
	21st to end	Day	23% 5% 8% 23% 32% 9%	2200
October	1st to 10th	Day	15% 5% 7% 17% 36% 20%	3275
	11th to 20th	Day	24% 3 8% 32% 26% 8%	3250
	21st to end	Day	16% 4 6% 25% 40% 10%	3485
November	1st to 10th	Day	16% 3 12% 23% 34% 13%	1853
	11th to 20th	Day	15% 4 9% 33% 29% 10%	1890
	21st to end	Day	20% 4 11% 26% 28% 12%	1811
December	1st to 10th	Day	16% 3 10% 32% 28% 10%	2700
	11th to 20th	Day	16% 4 11% 30% 31% 8%	3104
	21st to end	Day	19% 5% 12% 26% 27% 10%	4092
January	1st to 10th	Night	23% 5% 8% 33% 27% 5%	8202
	11th to 20th	Night	15% 5% 10% 28% 26% 15%	7973
	21st to end	Night	19% 5% 10% 22% 32% 13%	8370
February	1st to 10th	Night	20% 4 13% 30% 24% 9%	2730
	11th to 20th	Night	25% 5% 11% 27% 24% 7%	2472
	21st to end	Night	18% 4 9% 25% 31% 13%	2076
March	1st to 10th	Night	21% 7% 7% 16% 37% 11%	5494
	11th to 20th	Night	26% 6% 10% 21% 29% 8%	5114
	21st to end	Night	24% 5% 15% 37% 18% 2	5285
April	1st to 10th	Night	24% 5% 13% 28% 24% 6%	2010
	11th to 20th	Night	30% 6% 12% 26% 20% 6%	1977
	21st to end	Night	31% 7% 15% 28% 17% 4	2000
May	1st to 10th	Night	30% 6% 18% 28% 11% 7%	4353
	11th to 20th	Night	27% 7% 12% 29% 19% 7%	3671
	21st to end	Night	27% 7% 14% 22% 23% 6%	3972
June	1st to 10th	Night	24% 9% 22% 23% 14% 10%	1600
	11th to 20th	Night	17% 9% 14% 20% 22% 17%	1600
	21st to end	Night	25% 9% 13% 21% 20% 11%	1600
July	1st to 10th	Night	27% 7% 8% 20% 27% 10%	3600
	11th to 20th	Night	29% 6% 23% 29% 11% 3	3592
	21st to end	Night	33% 8% 19% 20% 11% 10%	4116
August	1st to 10th	Night	41% 8% 21% 16% 10% 4	2950
	11th to 20th	Night	29% 9% 22% 18% 17% 5%	3243
	21st to end	Night	28% 8% 23% 21% 17% 2	3597
September	1st to 10th	Night	29% 7% 13% 24% 20% 7%	2000
	11th to 20th	Night	23% 6% 14% 23% 23% 12%	2014
	21st to end	Night	23% 6% 10% 25% 28% 8%	2200
October	1st to 10th	Night	16% 5% 9% 24% 34% 12%	3910
	11th to 20th	Night	21% 3 12% 32% 24% 8%	3898
	21st to end	Night	15% 3 6% 29% 37% 10%	4281
November	1st to 10th	Night	15% 5% 12% 27% 30% 12%	2665
	11th to 20th	Night	15% 4 9% 32% 29% 11%	2730
	21st to end	Night	20% 4 10% 25% 29% 11%	2804
December	1st to 10th	Night	13% 4 10% 36% 30% 7%	4710
	11th to 20th	Night	14% 4 11% 31% 36% 6%	5322
	21st to end	Night	23% 5% 12% 23% 30% 7%	7128

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	12% 21% 34% 24% 8%	330
	11th to 20th	Day	13% 22% 39% 22% 3 1	330
	21st to end	Day	5% 23% 41% 23% 6% 1	363
February	1st to 10th	Day	12% 34% 34% 17% 1 2	330
	11th to 20th	Day	13% 30% 41% 13% 3	330
	21st to end	Day	18% 27% 36% 16% 3	273
March	1st to 10th	Day	15% 23% 34% 22% 6%	330
	11th to 20th	Day	15% 27% 35% 17% 6%	535
	21st to end	Day	17% 30% 36% 14% 3 1	594
April	1st to 10th	Day	15% 32% 34% 15% 3 1	505
	11th to 20th	Day	20% 33% 32% 12% 3 1	492
	21st to end	Day	17% 32% 31% 16% 4 1	500
May	1st to 10th	Day	15% 25% 34% 18% 6% 1	581
	11th to 20th	Day	12% 17% 34% 28% 9% 2	725
	21st to end	Day	10% 19% 29% 28% 13% 2	789
June	1st to 10th	Day	6% 14% 30% 29% 18% 3	730
	11th to 20th	Day	9% 12% 33% 29% 14% 4	730
	21st to end	Day	4 10% 37% 28% 17% 4	730
July	1st to 10th	Day	6% 6% 25% 34% 25% 5%	721
	11th to 20th	Day	7% 2 20% 39% 26% 7%	718
	21st to end	Day	10% 3 21% 34% 27% 5%	675
August	1st to 10th	Day	11% 6% 28% 35% 16% 4	527
	11th to 20th	Day	7% 6% 29% 35% 16% 7%	517
	21st to end	Day	9% 6% 32% 33% 16% 3	583
September	1st to 10th	Day	8% 10% 29% 32% 17% 5%	530
	11th to 20th	Day	7% 12% 38% 28% 14% 2	515
	21st to end	Day	7% 12% 22% 33% 21% 5%	330
October	1st to 10th	Day	8% 12% 29% 26% 18% 6%	330
	11th to 20th	Day	10% 21% 32% 22% 13% 2	330
	21st to end	Day	10% 20% 34% 25% 10% 1	363
November	1st to 10th	Day	12% 18% 36% 23% 10% 2	330
	11th to 20th	Day	10% 16% 38% 27% 8% 1	330
	21st to end	Day	6% 18% 43% 24% 8% 1	330
December	1st to 10th	Day	9% 24% 38% 21% 8%	330
	11th to 20th	Day	10% 21% 47% 16% 6%	330
	21st to end	Day	12% 26% 37% 19% 5% 1	363
January	1st to 10th	Night	4 18% 41% 28% 8% 1	1050
	11th to 20th	Night	4 21% 43% 24% 7% 1	955
	21st to end	Night	2 12% 51% 30% 5% 1	924
February	1st to 10th	Night	3 16% 56% 20% 4	840
	11th to 20th	Night	4 18% 51% 24% 3	824
	21st to end	Night	3 21% 50% 23% 2	692
March	1st to 10th	Night	3 16% 48% 28% 4	840
	11th to 20th	Night	5% 13% 43% 33% 7%	840
	21st to end	Night	5% 16% 48% 27% 4	780
April	1st to 10th	Night	7% 18% 44% 28% 3	603
	11th to 20th	Night	8% 18% 38% 29% 5% 1	593
	21st to end	Night	10% 15% 40% 30% 6%	600
May	1st to 10th	Night	11% 11% 35% 35% 8% 1	600
	11th to 20th	Night	8% 8% 31% 39% 11% 1	591
	21st to end	Night	7% 10% 34% 34% 14% 1	650
June	1st to 10th	Night	6% 4 26% 43% 18% 3	600
	11th to 20th	Night	4 6% 30% 46% 14% 1	600
	21st to end	Night	5% 3 21% 45% 24% 2	600
July	1st to 10th	Night	5% 3 15% 45% 30% 2	585
	11th to 20th	Night	5% 1 12% 44% 35% 3	582
	21st to end	Night	7% 2 10% 40% 38% 4	657
August	1st to 10th	Night	8% 3 13% 44% 29% 3	597
	11th to 20th	Night	6% 2 14% 45% 29% 3	594
	21st to end	Night	8% 3 15% 42% 30% 3	660
September	1st to 10th	Night	6% 2 19% 44% 25% 4	600
	11th to 20th	Night	7% 3 24% 44% 20% 3	600
	21st to end	Night	5% 7% 29% 40% 18% 2	600
October	1st to 10th	Night	5% 7% 34% 39% 13% 2	790
	11th to 20th	Night	4 7% 41% 34% 13% 2	800
	21st to end	Night	6% 9% 39% 32% 12% 2	897
November	1st to 10th	Night	4 12% 43% 32% 9% 1	1025
	11th to 20th	Night	4 15% 44% 30% 7% 1	1050
	21st to end	Night	5% 14% 42% 31% 8%	1050
December	1st to 10th	Night	4 18% 41% 30% 6%	1050
	11th to 20th	Night	4 16% 50% 24% 6%	1050
	21st to end	Night	5% 20% 47% 23% 5%	1155

Page generated at 2026-03-30 00:18:39. Version ac07

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