Knox College Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Knox College Observatory clear sky chart from 2005-11-06 to 2026-07-31.

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% 5% 5% 4 6% 7% 38%	138598
	Night	12% 9% 3 8% 5% 4 4 3 5% 6% 40%	134714
Transparency	Day	44% 4 12% 24% 15% 2	142121
	Night	44% 4 11% 24% 16% 2	139270
Seeing	Day	16% 11% 29% 25% 14% 4	18184
	Night	17% 11% 31% 29% 12% 1	27974

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	10% 11% 5% 14% 6% 6% 5% 4 5% 7% 27%	4691
	11th to 20th	Day	7% 10% 5% 11% 8% 6% 6% 5% 7% 6% 29%	4752
	21st to end	Day	11% 13% 6% 11% 6% 5% 5% 3 5% 6% 29%	5812
February	1st to 10th	Day	14% 12% 5% 10% 6% 5% 4 4 4 6% 29%	1890
	11th to 20th	Day	14% 10% 4 9% 6% 5% 4 3 5% 6% 34%	1972
	21st to end	Day	13% 13% 4 10% 5% 4 3 3 5% 6% 34%	1730
March	1st to 10th	Day	17% 11% 3 6% 3 3 3 2 3 5% 43%	4597
	11th to 20th	Day	8% 6% 2 6% 5% 4 5% 4 6% 8% 45%	5090
	21st to end	Day	19% 11% 4 10% 4 5% 4 4 5% 6% 28%	5907
April	1st to 10th	Day	16% 12% 3 7% 4 4 4 3 5% 6% 34%	2520
	11th to 20th	Day	15% 10% 3 7% 5% 4 4 3 5% 7% 37%	2494
	21st to end	Day	19% 8% 2 8% 5% 4 4 3 5% 6% 36%	2520
May	1st to 10th	Day	14% 8% 2 7% 5% 5% 5% 4 6% 8% 36%	7670
	11th to 20th	Day	14% 6% 3 6% 4 4 4 4 5% 7% 43%	8222
	21st to end	Day	13% 7% 3 8% 6% 5% 5% 4 6% 8% 36%	9047
June	1st to 10th	Day	9% 6% 2 7% 4 4 5% 4 6% 8% 46%	2940
	11th to 20th	Day	8% 6% 2 7% 5% 4 4 4 6% 8% 47%	2940
	21st to end	Day	10% 7% 2 7% 5% 5% 5% 4 7% 9% 40%	2940
July	1st to 10th	Day	8% 6% 2 7% 5% 6% 5% 4 7% 10% 39%	6500
	11th to 20th	Day	9% 7% 2 6% 4 5% 5% 4 7% 10% 41%	6500
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 4 8% 10% 37%	7027
August	1st to 10th	Day	6% 5% 2 6% 4 5% 5% 5% 8% 10% 44%	4234
	11th to 20th	Day	10% 6% 3 6% 5% 5% 5% 5% 7% 9% 42%	3925
	21st to end	Day	6% 4 2 7% 4 5% 5% 4 6% 9% 49%	4312
September	1st to 10th	Day	11% 4 2 6% 4 4 4 4 6% 7% 50%	2400
	11th to 20th	Day	10% 4 2 4 3 4 4 3 5% 8% 53%	2375
	21st to end	Day	9% 5% 2 5% 4 4 4 4 6% 8% 48%	2185
October	1st to 10th	Day	12% 6% 2 6% 3 3 4 3 5% 6% 51%	2000
	11th to 20th	Day	15% 9% 2 6% 4 4 4 3 3 5% 45%	2000
	21st to end	Day	18% 8% 3 7% 3 4 4 4 5% 6% 40%	2115
November	1st to 10th	Day	10% 8% 3 6% 4 3 4 4 5% 7% 46%	1852
	11th to 20th	Day	16% 10% 3 7% 5% 3 4 3 5% 6% 37%	1890
	21st to end	Day	15% 12% 4 8% 5% 4 3 2 5% 5% 37%	1701
December	1st to 10th	Day	7% 9% 3 8% 5% 4 4 3 5% 7% 45%	2690
	11th to 20th	Day	14% 12% 6% 13% 5% 6% 4 4 6% 6% 24%	3088
	21st to end	Day	22% 21% 9% 13% 5% 4 3 2 4 4 15%	4070
January	1st to 10th	Night	12% 12% 5% 13% 4 4 4 3 4 5% 34%	8232
	11th to 20th	Night	8% 11% 5% 12% 8% 7% 6% 4 7% 5% 28%	8222
	21st to end	Night	13% 12% 4 11% 5% 4 4 3 4 5% 35%	8402
February	1st to 10th	Night	15% 11% 4 8% 6% 4 4 3 5% 6% 34%	2730
	11th to 20th	Night	13% 11% 4 7% 5% 4 4 3 5% 6% 38%	2535
	21st to end	Night	14% 10% 4 8% 4 4 3 3 4 5% 42%	2076
March	1st to 10th	Night	17% 10% 3 7% 3 4 3 3 3 5% 41%	5493
	11th to 20th	Night	11% 7% 3 6% 3 4 3 3 5% 6% 50%	5118
	21st to end	Night	16% 12% 5% 10% 5% 5% 4 3 5% 6% 28%	5263
April	1st to 10th	Night	14% 8% 3 8% 4 4 4 3 5% 7% 39%	2100
	11th to 20th	Night	16% 10% 3 6% 3 4 4 3 4 5% 42%	2075
	21st to end	Night	16% 7% 2 7% 5% 4 4 3 5% 7% 39%	2100
May	1st to 10th	Night	15% 8% 3 7% 5% 5% 5% 4 5% 7% 38%	5165
	11th to 20th	Night	11% 5% 2 5% 4 4 5% 4 7% 9% 42%	4670
	21st to end	Night	12% 8% 2 7% 5% 4 4 4 5% 8% 41%	5139
June	1st to 10th	Night	9% 5% 2 5% 4 4 4 3 6% 9% 49%	1680
	11th to 20th	Night	8% 5% 2 5% 4 4 4 4 6% 9% 48%	1680
	21st to end	Night	10% 5% 2 6% 5% 4 4 4 6% 7% 47%	1680
July	1st to 10th	Night	10% 6% 2 6% 4 5% 5% 4 7% 10% 42%	3698
	11th to 20th	Night	6% 5% 2 5% 4 5% 4 4 6% 9% 50%	3697
	21st to end	Night	10% 6% 2 7% 4 5% 6% 4 6% 8% 41%	4062
August	1st to 10th	Night	4 4 1 4 3 4 4 4 6% 10% 55%	2926
	11th to 20th	Night	6% 4 2 5% 4 4 5% 4 5% 8% 53%	3123
	21st to end	Night	6% 3 1 4 5% 4 4 4 6% 9% 54%	3586
September	1st to 10th	Night	11% 4 2 4 3 4 3 3 4 6% 58%	2000
	11th to 20th	Night	9% 5% 2 4 3 4 3 4 6% 7% 53%	2005
	21st to end	Night	9% 4 2 6% 4 4 5% 4 6% 7% 50%	2210
October	1st to 10th	Night	9% 6% 2 6% 3 4 3 3 5% 5% 55%	2400
	11th to 20th	Night	13% 6% 2 6% 4 3 4 3 4 6% 49%	2400
	21st to end	Night	17% 8% 2 6% 5% 4 4 3 5% 5% 41%	2725
November	1st to 10th	Night	12% 7% 2 5% 3 3 3 3 4 6% 52%	2666
	11th to 20th	Night	13% 9% 3 7% 5% 3 4 3 5% 7% 41%	2730
	21st to end	Night	16% 11% 3 7% 5% 4 3 3 4 5% 38%	2917
December	1st to 10th	Night	9% 9% 3 7% 4 4 4 3 4 6% 46%	4720
	11th to 20th	Night	15% 11% 4 11% 5% 4 4 3 4 5% 33%	5339
	21st to end	Night	23% 18% 6% 11% 6% 5% 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	57% 3 13% 19% 7% 1	4692
	11th to 20th	Day	53% 3 8% 19% 14% 3	4752
	21st to end	Day	57% 2 11% 21% 7% 1	5812
February	1st to 10th	Day	57% 2 15% 18% 7% 1	1890
	11th to 20th	Day	51% 5% 11% 21% 10% 2	1981
	21st to end	Day	51% 3 10% 24% 10% 2	1730
March	1st to 10th	Day	47% 3 8% 24% 16% 2	4597
	11th to 20th	Day	36% 3 10% 32% 17%	5090
	21st to end	Day	56% 4 13% 21% 5% 1	5909
April	1st to 10th	Day	50% 4 11% 22% 12%	2520
	11th to 20th	Day	47% 4 10% 24% 13% 2	2494
	21st to end	Day	50% 5% 9% 22% 13% 2	2520
May	1st to 10th	Day	45% 5% 10% 23% 16% 1	7707
	11th to 20th	Day	41% 4 11% 26% 15% 3	8226
	21st to end	Day	46% 5% 9% 21% 18% 2	9061
June	1st to 10th	Day	35% 6% 14% 27% 16% 2	2940
	11th to 20th	Day	34% 6% 10% 25% 22% 2	2940
	21st to end	Day	38% 6% 15% 23% 15% 2	2940
July	1st to 10th	Day	38% 6% 13% 27% 15% 2	6431
	11th to 20th	Day	37% 6% 15% 26% 15% 1	6425
	21st to end	Day	40% 7% 18% 22% 12% 1	6831
August	1st to 10th	Day	32% 6% 18% 31% 12% 1	4220
	11th to 20th	Day	37% 7% 14% 27% 15% 1	3962
	21st to end	Day	31% 5% 13% 31% 16% 3	4311
September	1st to 10th	Day	33% 5% 11% 27% 21% 4	2400
	11th to 20th	Day	29% 4 10% 28% 23% 4	2375
	21st to end	Day	33% 5% 10% 28% 22% 2	2185
October	1st to 10th	Day	24% 2 6% 26% 40% 2	3260
	11th to 20th	Day	29% 2 9% 24% 32% 4	3240
	21st to end	Day	41% 3 9% 27% 17% 3	3392
November	1st to 10th	Day	38% 3 16% 25% 15% 3	1852
	11th to 20th	Day	47% 4 11% 24% 12% 2	1890
	21st to end	Day	51% 3 13% 22% 9% 1	1701
December	1st to 10th	Day	40% 3 14% 33% 9% 1	2690
	11th to 20th	Day	60% 3 11% 18% 6% 2	3085
	21st to end	Day	75% 3 8% 10% 4	4070
January	1st to 10th	Night	54% 3 14% 22% 7%	8233
	11th to 20th	Night	56% 3 10% 15% 14% 1	8222
	21st to end	Night	53% 3 11% 21% 11% 1	8402
February	1st to 10th	Night	52% 4 11% 21% 11% 1	2730
	11th to 20th	Night	47% 3 11% 24% 12% 2	2536
	21st to end	Night	46% 3 8% 25% 15% 3	2076
March	1st to 10th	Night	48% 3 7% 25% 15% 2	5493
	11th to 20th	Night	36% 3 9% 30% 18% 5%	5118
	21st to end	Night	57% 4 12% 19% 8% 1	5263
April	1st to 10th	Night	45% 3 10% 26% 14% 2	2100
	11th to 20th	Night	44% 4 9% 23% 17% 4	2075
	21st to end	Night	45% 5% 11% 22% 14% 3	2100
May	1st to 10th	Night	46% 4 9% 22% 17% 2	5193
	11th to 20th	Night	37% 6% 15% 23% 15% 4	4671
	21st to end	Night	42% 4 10% 22% 20% 2	5149
June	1st to 10th	Night	32% 5% 14% 26% 19% 4	1680
	11th to 20th	Night	31% 5% 11% 25% 23% 4	1680
	21st to end	Night	34% 7% 14% 22% 20% 3	1680
July	1st to 10th	Night	37% 5% 13% 26% 16% 2	3660
	11th to 20th	Night	30% 6% 13% 30% 20% 1	3652
	21st to end	Night	38% 7% 15% 23% 15% 3	3947
August	1st to 10th	Night	24% 6% 19% 32% 17% 2	2916
	11th to 20th	Night	29% 6% 15% 31% 17% 1	3152
	21st to end	Night	27% 6% 14% 32% 17% 4	3586
September	1st to 10th	Night	28% 4 10% 29% 25% 5%	2000
	11th to 20th	Night	29% 4 11% 29% 22% 5%	2005
	21st to end	Night	32% 4 10% 26% 24% 3	2210
October	1st to 10th	Night	22% 3 6% 28% 37% 4	3920
	11th to 20th	Night	27% 3 8% 23% 35% 5%	3899
	21st to end	Night	41% 3 8% 22% 22% 4	4398
November	1st to 10th	Night	34% 4 11% 27% 19% 4	2666
	11th to 20th	Night	43% 3 11% 25% 16% 2	2730
	21st to end	Night	49% 3 10% 24% 13% 1	2917
December	1st to 10th	Night	40% 3 12% 32% 12% 1	4720
	11th to 20th	Night	54% 3 11% 22% 9% 1	5341
	21st to end	Night	73% 3 9% 12% 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% 20% 31% 22% 7%	330
	11th to 20th	Day	25% 21% 38% 14% 3	330
	21st to end	Day	19% 22% 37% 17% 5%	363
February	1st to 10th	Day	22% 19% 41% 14% 5%	330
	11th to 20th	Day	24% 19% 35% 18% 3 1	330
	21st to end	Day	22% 18% 45% 13% 1	273
March	1st to 10th	Day	22% 19% 39% 18% 2 1	330
	11th to 20th	Day	25% 20% 35% 15% 6%	330
	21st to end	Day	28% 17% 35% 14% 4 1	506
April	1st to 10th	Day	25% 17% 31% 21% 5% 1	540
	11th to 20th	Day	23% 16% 32% 18% 9% 2	532
	21st to end	Day	23% 15% 32% 21% 8% 1	540
May	1st to 10th	Day	18% 12% 34% 25% 8% 2	616
	11th to 20th	Day	20% 13% 29% 24% 12% 2	775
	21st to end	Day	15% 9% 30% 27% 13% 6%	844
June	1st to 10th	Day	9% 8% 27% 32% 19% 6%	780
	11th to 20th	Day	9% 7% 30% 32% 19% 3	780
	21st to end	Day	11% 8% 32% 28% 16% 4	780
July	1st to 10th	Day	10% 4 17% 33% 23% 13%	771
	11th to 20th	Day	10% 3 23% 32% 21% 10%	768
	21st to end	Day	9% 3 21% 37% 25% 5%	845
August	1st to 10th	Day	6% 3 21% 36% 26% 8%	726
	11th to 20th	Day	9% 6% 19% 29% 25% 12%	571
	21st to end	Day	5% 3 18% 33% 30% 10%	583
September	1st to 10th	Day	12% 6% 17% 32% 26% 7%	530
	11th to 20th	Day	8% 2 17% 32% 30% 10%	530
	21st to end	Day	10% 9% 27% 22% 24% 8%	515
October	1st to 10th	Day	15% 6% 30% 29% 17% 3	330
	11th to 20th	Day	18% 21% 30% 21% 9% 1	330
	21st to end	Day	29% 15% 32% 18% 5% 1	363
November	1st to 10th	Day	13% 15% 39% 22% 8% 3	330
	11th to 20th	Day	23% 18% 32% 20% 7% 1	330
	21st to end	Day	27% 13% 33% 22% 5% 1	330
December	1st to 10th	Day	13% 18% 42% 20% 4 3	330
	11th to 20th	Day	21% 15% 38% 20% 5%	330
	21st to end	Day	26% 21% 36% 14% 2	363
January	1st to 10th	Night	17% 16% 39% 22% 6%	1050
	11th to 20th	Night	18% 17% 40% 20% 5%	1050
	21st to end	Night	19% 15% 38% 25% 3	1155
February	1st to 10th	Night	22% 13% 37% 22% 5%	1050
	11th to 20th	Night	20% 17% 33% 26% 4	900
	21st to end	Night	21% 16% 39% 20% 3	692
March	1st to 10th	Night	21% 15% 38% 22% 5%	840
	11th to 20th	Night	19% 13% 35% 27% 6%	645
	21st to end	Night	25% 16% 32% 22% 5%	693
April	1st to 10th	Night	20% 16% 35% 24% 5%	630
	11th to 20th	Night	22% 13% 32% 26% 8%	623
	21st to end	Night	20% 11% 31% 33% 5%	630
May	1st to 10th	Night	15% 9% 37% 30% 7% 1	630
	11th to 20th	Night	19% 7% 29% 33% 11%	621
	21st to end	Night	13% 5% 29% 35% 17% 1	683
June	1st to 10th	Night	11% 5% 24% 44% 16% 1	630
	11th to 20th	Night	10% 5% 28% 38% 17%	630
	21st to end	Night	10% 5% 30% 37% 17% 1	630
July	1st to 10th	Night	9% 2 13% 42% 29% 4	615
	11th to 20th	Night	7% 2 18% 43% 27% 2	612
	21st to end	Night	8% 2 14% 45% 29% 1	687
August	1st to 10th	Night	6% 2 15% 44% 28% 4	597
	11th to 20th	Night	6% 2 15% 44% 29% 5%	594
	21st to end	Night	8% 1 14% 42% 34% 2	660
September	1st to 10th	Night	12% 5% 17% 36% 26% 4	600
	11th to 20th	Night	10% 3 18% 36% 28% 4	605
	21st to end	Night	12% 6% 23% 33% 24% 3	800
October	1st to 10th	Night	11% 6% 30% 32% 18% 2	800
	11th to 20th	Night	17% 12% 31% 29% 12%	800
	21st to end	Night	23% 11% 35% 23% 7%	880
November	1st to 10th	Night	17% 10% 38% 26% 7% 1	820
	11th to 20th	Night	18% 15% 36% 24% 6%	840
	21st to end	Night	23% 13% 35% 23% 6%	1027
December	1st to 10th	Night	19% 14% 38% 24% 5%	1050
	11th to 20th	Night	23% 14% 32% 25% 6%	1050
	21st to end	Night	27% 16% 32% 22% 2	1155

Page generated at 2026-07-28 02:25:24. Version ac07

Copyright © 2026 Attilla Danko.