Olin Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Olin 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	12% 9% 3 8% 5% 4 5% 4 5% 7% 38%	131831
	Night	11% 9% 3 9% 5% 4 5% 4 5% 6% 39%	130193
Transparency	Day	45% 4 13% 25% 11% 1	135425
	Night	45% 4 12% 25% 13% 1	134676
Seeing	Day	17% 11% 31% 26% 12% 3	17830
	Night	16% 10% 33% 31% 10% 1	27884

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	7% 15% 7% 19% 9% 7% 5% 3 4 6% 17%	4691
	11th to 20th	Day	6% 12% 6% 13% 7% 7% 9% 8% 8% 7% 18%	4985
	21st to end	Day	4 13% 6% 13% 6% 5% 8% 4 5% 6% 29%	5812
February	1st to 10th	Day	8% 12% 6% 11% 8% 7% 5% 5% 6% 7% 26%	1890
	11th to 20th	Day	8% 12% 4 10% 6% 5% 6% 4 7% 7% 30%	1844
	21st to end	Day	11% 11% 4 9% 6% 6% 4 4 6% 7% 33%	1595
March	1st to 10th	Day	13% 8% 2 7% 4 4 4 3 5% 7% 44%	5046
	11th to 20th	Day	10% 10% 3 6% 3 3 4 4 5% 9% 43%	5090
	21st to end	Day	23% 11% 4 10% 6% 4 4 3 4 6% 24%	5583
April	1st to 10th	Day	17% 10% 3 7% 4 4 3 3 6% 7% 34%	2401
	11th to 20th	Day	19% 13% 3 8% 4 4 4 3 5% 5% 32%	2412
	21st to end	Day	22% 10% 3 7% 4 4 4 4 5% 6% 30%	2600
May	1st to 10th	Day	13% 6% 2 5% 3 4 4 4 5% 7% 47%	5954
	11th to 20th	Day	17% 7% 2 5% 3 3 3 3 5% 6% 46%	6319
	21st to end	Day	18% 6% 2 7% 5% 4 5% 4 5% 7% 37%	6935
June	1st to 10th	Day	11% 5% 2 6% 4 4 5% 3 6% 8% 46%	2800
	11th to 20th	Day	13% 7% 3 6% 5% 5% 4 4 6% 8% 38%	2800
	21st to end	Day	11% 5% 2 6% 5% 5% 5% 4 7% 9% 42%	2800
July	1st to 10th	Day	8% 5% 2 6% 4 4 5% 4 7% 10% 45%	6360
	11th to 20th	Day	7% 4 1 4 3 3 3 3 5% 8% 56%	6360
	21st to end	Day	7% 4 2 6% 5% 4 4 4 7% 11% 46%	6652
August	1st to 10th	Day	11% 6% 2 7% 4 5% 4 4 5% 8% 44%	4234
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 4 6% 9% 41%	4171
	21st to end	Day	8% 5% 3 7% 4 4 4 3 6% 7% 49%	4495
September	1st to 10th	Day	12% 5% 3 6% 3 3 4 4 5% 7% 48%	2400
	11th to 20th	Day	10% 6% 1 5% 5% 4 4 3 5% 7% 49%	2320
	21st to end	Day	13% 6% 3 6% 3 3 5% 3 5% 7% 48%	2134
October	1st to 10th	Day	15% 7% 2 6% 4 3 4 3 5% 6% 45%	2000
	11th to 20th	Day	15% 10% 2 6% 4 4 4 2 4 6% 43%	2000
	21st to end	Day	19% 9% 4 7% 3 3 3 3 4 6% 39%	2200
November	1st to 10th	Day	11% 10% 3 8% 4 3 4 3 5% 8% 40%	1740
	11th to 20th	Day	11% 13% 6% 10% 5% 4 4 4 5% 6% 32%	1680
	21st to end	Day	12% 10% 4 8% 6% 4 4 4 5% 8% 34%	1680
December	1st to 10th	Day	6% 10% 3 14% 10% 7% 5% 4 5% 7% 28%	2690
	11th to 20th	Day	11% 16% 5% 11% 8% 7% 5% 4 5% 6% 23%	3088
	21st to end	Day	19% 18% 7% 19% 5% 4 3 2 3 4 15%	4070
January	1st to 10th	Night	8% 12% 5% 17% 7% 7% 5% 4 5% 5% 25%	8232
	11th to 20th	Night	5% 11% 4 13% 8% 7% 10% 7% 7% 5% 22%	8003
	21st to end	Night	6% 12% 6% 10% 6% 5% 5% 4 5% 6% 35%	8402
February	1st to 10th	Night	8% 13% 5% 10% 7% 6% 5% 3 5% 7% 34%	2730
	11th to 20th	Night	7% 10% 3 8% 5% 6% 5% 3 5% 7% 39%	2663
	21st to end	Night	11% 8% 3 9% 6% 5% 5% 4 5% 7% 37%	2210
March	1st to 10th	Night	11% 8% 3 6% 4 3 4 3 4 5% 49%	5060
	11th to 20th	Night	10% 7% 3 6% 4 4 5% 4 6% 8% 42%	5090
	21st to end	Night	23% 11% 4 9% 4 3 3 3 4 5% 30%	5588
April	1st to 10th	Night	14% 11% 2 8% 4 4 4 4 5% 6% 38%	2021
	11th to 20th	Night	19% 9% 3 7% 5% 4 4 3 4 5% 36%	1934
	21st to end	Night	19% 10% 3 7% 4 3 3 4 5% 6% 36%	1800
May	1st to 10th	Night	12% 6% 2 4 4 3 3 3 4 5% 53%	3946
	11th to 20th	Night	15% 6% 2 4 3 3 4 3 5% 6% 51%	3596
	21st to end	Night	19% 7% 2 7% 3 3 3 3 4 7% 41%	3942
June	1st to 10th	Night	10% 5% 2 4 5% 3 4 4 6% 7% 52%	1600
	11th to 20th	Night	10% 6% 2 6% 5% 5% 5% 5% 7% 7% 42%	1600
	21st to end	Night	8% 5% 1 4 4 3 4 3 4 7% 55%	1496
July	1st to 10th	Night	8% 3 2 5% 4 4 5% 4 7% 10% 48%	3618
	11th to 20th	Night	6% 3 2 5% 4 4 4 3 6% 8% 55%	3617
	21st to end	Night	9% 4 2 5% 4 4 4 3 7% 8% 51%	4215
August	1st to 10th	Night	11% 6% 2 5% 4 4 4 3 5% 7% 48%	2926
	11th to 20th	Night	12% 5% 2 5% 3 3 4 4 5% 8% 50%	2888
	21st to end	Night	9% 6% 2 6% 3 4 4 4 5% 8% 51%	3394
September	1st to 10th	Night	9% 5% 2 4 3 3 4 3 5% 7% 56%	2000
	11th to 20th	Night	10% 4 2 5% 4 3 4 4 5% 8% 52%	2075
	21st to end	Night	10% 6% 2 5% 3 4 4 3 4 7% 53%	2245
October	1st to 10th	Night	13% 7% 2 5% 3 3 4 3 4 5% 52%	2400
	11th to 20th	Night	15% 9% 3 5% 3 3 4 3 5% 6% 44%	2400
	21st to end	Night	18% 10% 3 7% 3 3 3 3 5% 6% 40%	2640
November	1st to 10th	Night	13% 8% 3 6% 3 4 4 3 5% 7% 44%	2773
	11th to 20th	Night	12% 13% 4 8% 5% 4 4 3 4 5% 37%	2940
	21st to end	Night	13% 11% 4 8% 5% 4 3 3 5% 7% 38%	2940
December	1st to 10th	Night	6% 9% 4 13% 8% 6% 5% 4 6% 7% 32%	4720
	11th to 20th	Night	11% 14% 6% 13% 7% 5% 5% 3 5% 5% 26%	5339
	21st to end	Night	21% 15% 8% 17% 5% 4 3 3 4 4 17%	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	70% 3 11% 12% 5%	4692
	11th to 20th	Day	63% 2 9% 16% 9%	4985
	21st to end	Day	57% 3 13% 21% 7%	5812
February	1st to 10th	Day	57% 5% 12% 17% 8% 1	1890
	11th to 20th	Day	51% 6% 12% 22% 7% 1	1853
	21st to end	Day	50% 6% 13% 19% 11%	1595
March	1st to 10th	Day	41% 4 15% 28% 10% 2	5046
	11th to 20th	Day	39% 4 14% 28% 12% 2	5090
	21st to end	Day	62% 4 11% 18% 5%	5585
April	1st to 10th	Day	49% 6% 11% 23% 10% 1	2401
	11th to 20th	Day	53% 5% 10% 21% 10% 1	2412
	21st to end	Day	54% 6% 12% 20% 8% 1	2600
May	1st to 10th	Day	36% 5% 11% 30% 16% 1	5992
	11th to 20th	Day	39% 5% 8% 25% 17% 5%	6323
	21st to end	Day	47% 4 9% 30% 9%	6949
June	1st to 10th	Day	35% 6% 15% 28% 15% 1	2800
	11th to 20th	Day	43% 6% 13% 21% 15% 1	2800
	21st to end	Day	37% 7% 16% 26% 12% 2	2800
July	1st to 10th	Day	34% 3 19% 34% 9%	6291
	11th to 20th	Day	28% 3 15% 36% 16% 2	6285
	21st to end	Day	33% 4 16% 31% 14% 1	6463
August	1st to 10th	Day	39% 4 12% 30% 13% 2	4220
	11th to 20th	Day	40% 4 16% 31% 8%	4210
	21st to end	Day	35% 5% 17% 29% 14% 1	4494
September	1st to 10th	Day	35% 5% 14% 31% 15%	2400
	11th to 20th	Day	35% 6% 16% 31% 11% 1	2320
	21st to end	Day	36% 5% 12% 33% 13% 1	2134
October	1st to 10th	Day	31% 2 6% 35% 25% 1	3260
	11th to 20th	Day	39% 3 11% 25% 21% 1	3240
	21st to end	Day	43% 3 14% 30% 11%	3538
November	1st to 10th	Day	43% 5% 14% 21% 15% 3	1740
	11th to 20th	Day	51% 5% 13% 20% 10% 1	1680
	21st to end	Day	48% 4 18% 22% 7% 1	1680
December	1st to 10th	Day	56% 5% 15% 18% 6%	2690
	11th to 20th	Day	64% 4 15% 13% 4	3085
	21st to end	Day	75% 3 9% 10% 3	4070
January	1st to 10th	Night	62% 3 13% 17% 5%	8233
	11th to 20th	Night	61% 4 10% 16% 9% 1	8003
	21st to end	Night	50% 3 12% 25% 10%	8402
February	1st to 10th	Night	52% 3 12% 22% 9% 1	2730
	11th to 20th	Night	46% 5% 13% 25% 10% 2	2664
	21st to end	Night	47% 5% 12% 23% 11% 2	2210
March	1st to 10th	Night	37% 4 10% 32% 14% 2	5060
	11th to 20th	Night	38% 3 12% 30% 15% 1	5090
	21st to end	Night	58% 3 11% 18% 9% 1	5588
April	1st to 10th	Night	46% 5% 12% 23% 13% 1	2021
	11th to 20th	Night	50% 4 9% 19% 16% 3	1934
	21st to end	Night	48% 5% 9% 22% 14% 2	1800
May	1st to 10th	Night	34% 4 10% 29% 21% 2	3973
	11th to 20th	Night	35% 5% 9% 22% 20% 9%	3597
	21st to end	Night	45% 3 9% 29% 12% 1	3950
June	1st to 10th	Night	31% 6% 13% 31% 17% 2	1600
	11th to 20th	Night	38% 6% 13% 22% 18% 3	1600
	21st to end	Night	28% 5% 15% 30% 19% 3	1496
July	1st to 10th	Night	31% 3 20% 30% 14% 1	3580
	11th to 20th	Night	28% 3 14% 35% 17% 2	3572
	21st to end	Night	32% 3 17% 30% 16% 2	4093
August	1st to 10th	Night	36% 5% 12% 28% 15% 4	2916
	11th to 20th	Night	34% 4 16% 31% 14% 1	2915
	21st to end	Night	32% 4 18% 28% 16% 2	3394
September	1st to 10th	Night	28% 5% 13% 33% 20% 1	2000
	11th to 20th	Night	31% 6% 15% 31% 16% 1	2075
	21st to end	Night	32% 4 12% 30% 20% 2	2245
October	1st to 10th	Night	24% 3 6% 34% 30% 4	3920
	11th to 20th	Night	31% 3 13% 27% 23% 4	3899
	21st to end	Night	43% 3 11% 30% 14% 1	4252
November	1st to 10th	Night	41% 4 14% 24% 14% 2	2773
	11th to 20th	Night	49% 6% 12% 20% 12% 1	2940
	21st to end	Night	48% 4 16% 23% 8% 1	2940
December	1st to 10th	Night	51% 4 18% 20% 6%	4720
	11th to 20th	Night	60% 4 13% 18% 4	5341
	21st to end	Night	72% 4 8% 13% 3	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	15% 15% 35% 23% 9% 2	330
	11th to 20th	Day	19% 14% 40% 22% 4 1	330
	21st to end	Day	13% 13% 39% 26% 8% 1	363
February	1st to 10th	Day	15% 16% 44% 20% 4 1	330
	11th to 20th	Day	13% 14% 42% 25% 6%	330
	21st to end	Day	19% 15% 41% 23% 2	273
March	1st to 10th	Day	20% 14% 42% 22% 3	330
	11th to 20th	Day	26% 13% 32% 23% 5% 1	330
	21st to end	Day	29% 14% 30% 18% 7% 1	363
April	1st to 10th	Day	27% 15% 32% 21% 4	493
	11th to 20th	Day	29% 15% 26% 21% 8% 1	530
	21st to end	Day	28% 14% 28% 20% 7% 2	700
May	1st to 10th	Day	19% 12% 34% 23% 9% 2	703
	11th to 20th	Day	21% 13% 27% 25% 11% 4	725
	21st to end	Day	19% 8% 28% 29% 13% 2	789
June	1st to 10th	Day	11% 10% 27% 31% 17% 4	730
	11th to 20th	Day	16% 8% 36% 28% 11% 2	730
	21st to end	Day	13% 8% 34% 28% 13% 4	730
July	1st to 10th	Day	8% 5% 23% 31% 26% 7%	721
	11th to 20th	Day	8% 6% 27% 33% 19% 6%	718
	21st to end	Day	7% 4 24% 40% 20% 5%	794
August	1st to 10th	Day	11% 3 24% 32% 26% 5%	726
	11th to 20th	Day	13% 4 22% 31% 23% 6%	715
	21st to end	Day	10% 4 28% 33% 18% 5%	701
September	1st to 10th	Day	15% 8% 28% 29% 17% 3	530
	11th to 20th	Day	11% 6% 30% 33% 16% 4	450
	21st to end	Day	15% 13% 27% 25% 14% 6%	330
October	1st to 10th	Day	18% 11% 29% 26% 13% 3	330
	11th to 20th	Day	22% 14% 39% 18% 6% 1	330
	21st to end	Day	26% 16% 37% 15% 6%	363
November	1st to 10th	Day	21% 16% 37% 18% 8% 1	330
	11th to 20th	Day	19% 20% 41% 16% 4	330
	21st to end	Day	21% 19% 37% 19% 4	330
December	1st to 10th	Day	16% 11% 42% 24% 7%	330
	11th to 20th	Day	19% 15% 40% 20% 5%	330
	21st to end	Day	28% 15% 36% 17% 4	363
January	1st to 10th	Night	12% 11% 38% 29% 8% 1	1050
	11th to 20th	Night	16% 13% 35% 30% 6% 1	1050
	21st to end	Night	17% 12% 36% 29% 6%	1155
February	1st to 10th	Night	15% 13% 38% 28% 5%	1050
	11th to 20th	Night	13% 13% 40% 28% 5% 1	1029
	21st to end	Night	14% 12% 36% 30% 7% 1	826
March	1st to 10th	Night	18% 11% 39% 24% 8% 1	648
	11th to 20th	Night	19% 11% 38% 26% 5% 1	630
	21st to end	Night	23% 11% 33% 24% 9% 1	693
April	1st to 10th	Night	17% 15% 36% 27% 5%	603
	11th to 20th	Night	24% 14% 30% 25% 7% 1	593
	21st to end	Night	22% 12% 34% 25% 7% 1	600
May	1st to 10th	Night	17% 11% 33% 30% 9% 1	600
	11th to 20th	Night	17% 9% 33% 29% 11% 1	591
	21st to end	Night	18% 7% 28% 34% 13%	650
June	1st to 10th	Night	11% 5% 29% 40% 14% 1	600
	11th to 20th	Night	13% 5% 33% 41% 8%	600
	21st to end	Night	10% 5% 29% 41% 15% 1	600
July	1st to 10th	Night	6% 3 21% 41% 27% 2	585
	11th to 20th	Night	4 4 25% 45% 20% 1	582
	21st to end	Night	6% 2 22% 50% 18% 2	657
August	1st to 10th	Night	8% 2 20% 46% 23% 2	597
	11th to 20th	Night	11% 2 20% 41% 24% 3	594
	21st to end	Night	9% 2 24% 41% 22% 2	660
September	1st to 10th	Night	12% 4 24% 41% 16% 3	600
	11th to 20th	Night	10% 6% 32% 34% 16% 3	600
	21st to end	Night	11% 9% 32% 32% 15% 1	645
October	1st to 10th	Night	16% 8% 33% 29% 13% 1	800
	11th to 20th	Night	21% 11% 35% 27% 6% 1	800
	21st to end	Night	25% 12% 30% 26% 6%	880
November	1st to 10th	Night	17% 13% 36% 26% 7% 1	961
	11th to 20th	Night	17% 15% 38% 24% 6%	1050
	21st to end	Night	18% 14% 38% 23% 6%	1050
December	1st to 10th	Night	16% 11% 38% 28% 6% 1	1050
	11th to 20th	Night	20% 10% 38% 26% 6%	1050
	21st to end	Night	25% 10% 34% 25% 5% 1	1155

Page generated at 2026-03-30 01:18:45. Version ac07

Copyright © 2026 Attilla Danko.