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 2025-03-27.

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% 4 4 6% 7% 38%	109105
	Night	13% 9% 3 8% 5% 4 4 3 5% 6% 40%	110438
Transparency	Day	43% 4 12% 24% 15% 2	112670
	Night	44% 4 11% 24% 16% 2	115013
Seeing	Day	16% 11% 29% 26% 15% 4	16212
	Night	17% 10% 30% 30% 12% 1	26246

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	11% 11% 6% 16% 6% 6% 5% 3 5% 6% 26%	3617
	11th to 20th	Day	9% 9% 5% 11% 7% 7% 7% 5% 6% 6% 29%	3648
	21st to end	Day	13% 14% 7% 11% 6% 4 5% 3 4 5% 27%	4488
February	1st to 10th	Day	15% 12% 5% 10% 6% 5% 5% 4 4 6% 28%	1800
	11th to 20th	Day	14% 9% 4 8% 7% 6% 4 3 5% 6% 34%	1876
	21st to end	Day	13% 14% 4 10% 5% 4 3 3 5% 7% 34%	1650
March	1st to 10th	Day	16% 12% 4 7% 4 3 3 3 4 5% 38%	3237
	11th to 20th	Day	9% 7% 3 6% 4 4 5% 4 6% 7% 46%	3590
	21st to end	Day	20% 12% 4 10% 5% 4 4 4 5% 6% 27%	4101
April	1st to 10th	Day	15% 12% 3 7% 4 4 4 3 5% 7% 35%	2280
	11th to 20th	Day	15% 10% 3 7% 5% 4 4 3 4 7% 38%	2254
	21st to end	Day	20% 8% 2 8% 5% 4 4 3 5% 5% 35%	2280
May	1st to 10th	Day	16% 7% 2 7% 5% 5% 5% 4 6% 9% 34%	4123
	11th to 20th	Day	17% 7% 3 6% 4 4 4 3 5% 7% 40%	4399
	21st to end	Day	11% 6% 2 6% 4 4 4 4 5% 8% 45%	4824
June	1st to 10th	Day	8% 6% 2 6% 4 4 4 4 6% 8% 46%	2660
	11th to 20th	Day	8% 5% 2 7% 5% 4 4 4 6% 8% 47%	2660
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 4 7% 9% 41%	2660
July	1st to 10th	Day	9% 7% 2 8% 6% 6% 5% 5% 8% 10% 35%	4440
	11th to 20th	Day	6% 4 2 5% 4 5% 5% 4 6% 10% 50%	4440
	21st to end	Day	7% 5% 2 6% 5% 6% 5% 5% 8% 11% 41%	4789
August	1st to 10th	Day	6% 5% 2 6% 4 5% 5% 5% 8% 10% 44%	4104
	11th to 20th	Day	10% 6% 3 6% 4 5% 5% 5% 6% 9% 41%	3802
	21st to end	Day	6% 5% 2 7% 4 4 5% 4 6% 9% 49%	4180
September	1st to 10th	Day	11% 4 2 6% 4 4 4 4 6% 7% 50%	2280
	11th to 20th	Day	10% 4 2 4 3 4 4 3 5% 8% 53%	2256
	21st to end	Day	9% 6% 2 5% 4 4 4 4 6% 8% 47%	2076
October	1st to 10th	Day	12% 6% 2 6% 3 3 4 3 4 6% 51%	1900
	11th to 20th	Day	15% 9% 2 6% 4 4 4 3 3 5% 45%	1900
	21st to end	Day	19% 8% 3 6% 2 4 4 4 5% 6% 40%	2009
November	1st to 10th	Day	11% 8% 3 6% 4 3 4 4 5% 7% 46%	1762
	11th to 20th	Day	17% 10% 3 7% 5% 3 4 3 5% 6% 37%	1800
	21st to end	Day	15% 12% 3 8% 5% 4 4 2 5% 6% 37%	1620
December	1st to 10th	Day	8% 9% 3 8% 4 4 4 3 5% 7% 46%	2610
	11th to 20th	Day	14% 12% 6% 13% 5% 6% 4 4 6% 6% 24%	3008
	21st to end	Day	22% 21% 9% 13% 5% 3 3 2 4 4 15%	3982
January	1st to 10th	Night	12% 12% 6% 13% 4 4 4 3 4 5% 32%	6348
	11th to 20th	Night	10% 13% 6% 13% 7% 6% 5% 3 7% 4 25%	6316
	21st to end	Night	15% 13% 5% 11% 5% 4 4 3 4 4 32%	6490
February	1st to 10th	Night	15% 11% 4 8% 6% 4 5% 3 5% 6% 32%	2600
	11th to 20th	Night	13% 11% 4 7% 5% 4 4 3 6% 5% 38%	2411
	21st to end	Night	15% 9% 3 8% 4 4 3 3 4 5% 42%	1980
March	1st to 10th	Night	18% 10% 3 6% 4 4 3 3 4 5% 40%	3853
	11th to 20th	Night	11% 7% 3 5% 3 3 3 2 4 6% 51%	3604
	21st to end	Night	20% 12% 5% 10% 5% 5% 4 3 5% 6% 26%	3667
April	1st to 10th	Night	13% 9% 3 8% 4 4 4 3 5% 7% 40%	1900
	11th to 20th	Night	16% 9% 3 6% 3 4 4 3 4 5% 43%	1875
	21st to end	Night	17% 7% 2 7% 6% 3 4 4 5% 7% 39%	1900
May	1st to 10th	Night	17% 8% 3 8% 5% 5% 5% 4 5% 8% 32%	2774
	11th to 20th	Night	14% 6% 3 5% 5% 4 5% 4 7% 7% 41%	2506
	21st to end	Night	8% 5% 2 5% 5% 4 4 4 5% 8% 49%	2743
June	1st to 10th	Night	8% 5% 2 5% 4 4 4 3 6% 9% 50%	1520
	11th to 20th	Night	7% 5% 2 5% 5% 4 4 4 6% 9% 48%	1520
	21st to end	Night	10% 5% 2 6% 5% 4 4 4 6% 7% 48%	1520
July	1st to 10th	Night	13% 5% 2 6% 4 5% 5% 4 6% 9% 40%	2530
	11th to 20th	Night	4 3 2 4 3 4 4 3 5% 9% 59%	2530
	21st to end	Night	6% 4 2 6% 4 5% 7% 4 7% 9% 46%	2771
August	1st to 10th	Night	3 4 1 4 3 4 4 3 6% 10% 55%	2836
	11th to 20th	Night	6% 4 2 5% 4 4 5% 4 5% 8% 53%	3026
	21st to end	Night	6% 3 1 4 5% 4 4 4 6% 9% 54%	3476
September	1st to 10th	Night	12% 4 2 4 3 4 2 2 4 6% 57%	1900
	11th to 20th	Night	10% 5% 2 4 3 4 3 4 5% 7% 53%	1905
	21st to end	Night	9% 4 2 6% 4 4 5% 4 6% 6% 50%	2099
October	1st to 10th	Night	9% 6% 2 5% 3 4 3 3 5% 5% 54%	2280
	11th to 20th	Night	13% 6% 2 6% 4 3 3 3 4 6% 50%	2280
	21st to end	Night	17% 8% 2 5% 4 4 4 3 5% 6% 41%	2589
November	1st to 10th	Night	12% 7% 2 5% 3 3 3 3 5% 7% 51%	2536
	11th to 20th	Night	13% 9% 3 7% 5% 3 4 3 5% 7% 41%	2600
	21st to end	Night	15% 11% 3 7% 5% 4 3 4 4 6% 38%	2778
December	1st to 10th	Night	9% 9% 3 7% 4 4 3 3 4 6% 47%	4580
	11th to 20th	Night	16% 11% 4 11% 5% 4 4 3 4 5% 33%	5199
	21st to end	Night	23% 18% 6% 11% 6% 5% 4 3 4 4 16%	6996

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	60% 3 9% 20% 7% 1	3618
	11th to 20th	Day	55% 3 9% 14% 15% 4	3648
	21st to end	Day	60% 2 10% 19% 8% 1	4488
February	1st to 10th	Day	58% 2 15% 17% 7% 2	1800
	11th to 20th	Day	51% 5% 11% 21% 11% 1	1885
	21st to end	Day	51% 3 10% 23% 10% 2	1650
March	1st to 10th	Day	49% 3 11% 21% 14% 2	3237
	11th to 20th	Day	37% 3 13% 28% 18%	3590
	21st to end	Day	58% 4 14% 20% 3 1	4101
April	1st to 10th	Day	50% 4 12% 23% 12%	2280
	11th to 20th	Day	47% 4 9% 24% 14% 2	2254
	21st to end	Day	51% 5% 9% 21% 13% 2	2280
May	1st to 10th	Day	45% 5% 12% 23% 13% 1	4160
	11th to 20th	Day	44% 4 10% 30% 11%	4403
	21st to end	Day	38% 5% 11% 23% 22% 2	4837
June	1st to 10th	Day	34% 6% 15% 27% 16% 3	2660
	11th to 20th	Day	34% 6% 10% 25% 22% 3	2660
	21st to end	Day	38% 7% 15% 23% 16% 2	2660
July	1st to 10th	Day	42% 6% 13% 25% 12% 2	4371
	11th to 20th	Day	29% 6% 16% 29% 18% 2	4365
	21st to end	Day	35% 8% 19% 20% 17% 1	4638
August	1st to 10th	Day	32% 6% 18% 31% 12% 1	4090
	11th to 20th	Day	38% 7% 14% 26% 15% 1	3839
	21st to end	Day	31% 5% 13% 31% 16% 3	4179
September	1st to 10th	Day	34% 5% 11% 25% 21% 5%	2280
	11th to 20th	Day	30% 4 10% 28% 24% 4	2256
	21st to end	Day	33% 5% 10% 28% 21% 2	2076
October	1st to 10th	Day	24% 2 6% 26% 40% 2	3160
	11th to 20th	Day	28% 2 9% 24% 33% 4	3140
	21st to end	Day	41% 3 9% 27% 17% 3	3286
November	1st to 10th	Day	38% 3 16% 25% 15% 3	1762
	11th to 20th	Day	48% 4 12% 23% 12% 2	1800
	21st to end	Day	51% 4 14% 22% 9% 1	1620
December	1st to 10th	Day	39% 3 14% 34% 10%	2610
	11th to 20th	Day	60% 3 11% 18% 7% 2	3005
	21st to end	Day	75% 3 8% 10% 4	3982
January	1st to 10th	Night	55% 2 11% 23% 8% 1	6348
	11th to 20th	Night	60% 3 10% 12% 14% 1	6316
	21st to end	Night	57% 3 8% 19% 12% 1	6490
February	1st to 10th	Night	53% 4 11% 20% 12% 1	2600
	11th to 20th	Night	48% 3 11% 24% 12% 2	2412
	21st to end	Night	46% 3 8% 25% 16% 3	1980
March	1st to 10th	Night	48% 4 9% 25% 13% 2	3853
	11th to 20th	Night	35% 3 10% 30% 17% 4	3604
	21st to end	Night	60% 3 13% 17% 6% 1	3667
April	1st to 10th	Night	44% 3 11% 26% 14% 2	1900
	11th to 20th	Night	44% 3 8% 23% 17% 4	1875
	21st to end	Night	45% 5% 11% 20% 15% 3	1900
May	1st to 10th	Night	50% 4 11% 21% 13% 1	2802
	11th to 20th	Night	40% 6% 15% 27% 11% 1	2507
	21st to end	Night	33% 4 12% 25% 24% 2	2751
June	1st to 10th	Night	32% 5% 14% 26% 19% 4	1520
	11th to 20th	Night	31% 5% 11% 25% 22% 5%	1520
	21st to end	Night	34% 7% 14% 21% 20% 3	1520
July	1st to 10th	Night	41% 5% 14% 23% 15% 2	2490
	11th to 20th	Night	23% 5% 13% 33% 24% 1	2482
	21st to end	Night	31% 8% 16% 23% 19% 4	2683
August	1st to 10th	Night	24% 6% 19% 32% 17% 2	2826
	11th to 20th	Night	29% 6% 15% 31% 17% 1	3055
	21st to end	Night	27% 6% 14% 32% 17% 4	3476
September	1st to 10th	Night	29% 4 10% 27% 26% 5%	1900
	11th to 20th	Night	30% 4 10% 28% 21% 6%	1905
	21st to end	Night	33% 4 10% 26% 23% 3	2099
October	1st to 10th	Night	22% 3 7% 28% 36% 4	3800
	11th to 20th	Night	26% 3 8% 23% 35% 5%	3779
	21st to end	Night	41% 3 9% 22% 22% 4	4262
November	1st to 10th	Night	34% 4 12% 27% 19% 4	2536
	11th to 20th	Night	43% 3 11% 25% 16% 2	2600
	21st to end	Night	48% 3 10% 24% 13% 1	2778
December	1st to 10th	Night	39% 3 13% 33% 12% 1	4580
	11th to 20th	Night	54% 3 11% 22% 9% 1	5201
	21st to end	Night	73% 3 8% 12% 4	6996

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	22% 19% 30% 23% 7%	300
	11th to 20th	Day	26% 20% 36% 15% 3	300
	21st to end	Day	20% 21% 35% 18% 5%	330
February	1st to 10th	Day	22% 18% 41% 14% 5%	300
	11th to 20th	Day	23% 19% 35% 18% 4 1	300
	21st to end	Day	23% 15% 47% 14% 1	249
March	1st to 10th	Day	21% 18% 39% 18% 2 1	300
	11th to 20th	Day	24% 20% 33% 16% 6%	300
	21st to end	Day	30% 15% 34% 15% 4 1	446
April	1st to 10th	Day	23% 15% 33% 23% 5% 1	460
	11th to 20th	Day	20% 15% 32% 19% 11% 2	452
	21st to end	Day	24% 15% 31% 20% 8% 2	460
May	1st to 10th	Day	19% 11% 34% 26% 8% 2	529
	11th to 20th	Day	20% 11% 30% 25% 12% 2	675
	21st to end	Day	14% 8% 29% 28% 14% 7%	734
June	1st to 10th	Day	8% 8% 26% 32% 20% 6%	680
	11th to 20th	Day	8% 5% 29% 34% 21% 4	680
	21st to end	Day	11% 9% 32% 27% 17% 5%	680
July	1st to 10th	Day	10% 4 17% 31% 24% 14%	671
	11th to 20th	Day	10% 3 23% 33% 22% 9%	668
	21st to end	Day	7% 3 20% 37% 27% 6%	739
August	1st to 10th	Day	6% 4 21% 34% 26% 8%	676
	11th to 20th	Day	10% 6% 19% 30% 24% 12%	528
	21st to end	Day	6% 3 17% 33% 31% 11%	539
September	1st to 10th	Day	12% 6% 16% 31% 27% 7%	490
	11th to 20th	Day	9% 2 19% 32% 28% 10%	490
	21st to end	Day	11% 10% 27% 22% 24% 6%	476
October	1st to 10th	Day	16% 5% 31% 30% 15% 3	300
	11th to 20th	Day	18% 22% 30% 21% 9% 1	300
	21st to end	Day	30% 15% 30% 18% 5% 1	330
November	1st to 10th	Day	12% 14% 39% 23% 9% 3	300
	11th to 20th	Day	23% 17% 31% 20% 7% 1	300
	21st to end	Day	27% 12% 34% 23% 4	300
December	1st to 10th	Day	13% 18% 43% 18% 4 3	300
	11th to 20th	Day	20% 16% 37% 20% 6%	300
	21st to end	Day	26% 21% 35% 15% 2	330
January	1st to 10th	Night	17% 16% 39% 22% 7%	1000
	11th to 20th	Night	18% 16% 39% 21% 5%	1000
	21st to end	Night	19% 14% 38% 26% 4	1100
February	1st to 10th	Night	23% 13% 37% 23% 5%	1000
	11th to 20th	Night	20% 17% 33% 26% 4	856
	21st to end	Night	21% 15% 40% 20% 4	660
March	1st to 10th	Night	21% 14% 38% 23% 5%	800
	11th to 20th	Night	18% 13% 36% 27% 6%	614
	21st to end	Night	25% 15% 31% 24% 5%	647
April	1st to 10th	Night	18% 15% 36% 25% 5%	570
	11th to 20th	Night	22% 12% 31% 27% 8%	563
	21st to end	Night	20% 11% 31% 33% 5%	570
May	1st to 10th	Night	16% 10% 35% 31% 7% 1	570
	11th to 20th	Night	19% 6% 28% 35% 12%	561
	21st to end	Night	12% 5% 29% 35% 18% 1	617
June	1st to 10th	Night	11% 4 24% 44% 16% 1	570
	11th to 20th	Night	10% 4 28% 39% 18% 1	570
	21st to end	Night	11% 4 28% 38% 18% 1	570
July	1st to 10th	Night	10% 2 12% 42% 30% 5%	555
	11th to 20th	Night	6% 3 19% 43% 27% 3	552
	21st to end	Night	7% 1 14% 46% 30% 1	624
August	1st to 10th	Night	6% 2 14% 44% 29% 4	567
	11th to 20th	Night	6% 2 15% 44% 29% 5%	564
	21st to end	Night	8% 1 14% 41% 34% 2	627
September	1st to 10th	Night	12% 5% 16% 37% 26% 4	570
	11th to 20th	Night	10% 3 18% 36% 27% 4	575
	21st to end	Night	12% 6% 24% 32% 23% 3	760
October	1st to 10th	Night	12% 6% 30% 31% 19% 2	760
	11th to 20th	Night	17% 12% 30% 28% 13%	760
	21st to end	Night	23% 11% 35% 24% 8%	836
November	1st to 10th	Night	17% 9% 38% 27% 8% 1	780
	11th to 20th	Night	18% 15% 36% 24% 6%	800
	21st to end	Night	22% 13% 35% 24% 6%	978
December	1st to 10th	Night	19% 14% 38% 24% 5%	1000
	11th to 20th	Night	23% 13% 33% 25% 6%	1000
	21st to end	Night	27% 16% 32% 22% 3	1100

Page generated at 2025-03-24 08:04:26. Version ac07

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