Williams Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Williams Observatory clear sky chart from 2005-11-06 to 2026-08-15.

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	6% 7% 2 6% 4 4 4 3 5% 6% 53%	152911
	Night	7% 8% 3 7% 4 4 4 3 5% 6% 48%	146391
Transparency	Day	30% 6% 10% 24% 21% 9%	147730
	Night	35% 7% 11% 22% 17% 8%	142569
Seeing	Day	14% 15% 28% 26% 13% 4	17225
	Night	13% 10% 19% 31% 23% 3	26217

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	10% 15% 5% 12% 7% 7% 6% 5% 6% 7% 20%	4722
	11th to 20th	Day	8% 8% 3 6% 4 4 4 3 4 5% 51%	5142
	21st to end	Day	6% 8% 4 9% 6% 6% 6% 4 6% 6% 40%	5843
February	1st to 10th	Day	7% 12% 4 10% 6% 6% 6% 5% 7% 7% 29%	1890
	11th to 20th	Day	8% 11% 4 9% 6% 6% 7% 5% 6% 7% 31%	1845
	21st to end	Day	9% 16% 4 9% 7% 5% 6% 4 5% 7% 28%	1664
March	1st to 10th	Day	7% 15% 4 11% 7% 6% 5% 4 5% 6% 30%	5009
	11th to 20th	Day	7% 12% 4 10% 6% 6% 5% 4 5% 5% 34%	5110
	21st to end	Day	7% 12% 4 12% 7% 6% 6% 5% 7% 8% 25%	5636
April	1st to 10th	Day	10% 13% 3 11% 8% 6% 5% 4 5% 7% 27%	2520
	11th to 20th	Day	12% 12% 4 8% 6% 5% 4 4 5% 7% 33%	2489
	21st to end	Day	9% 10% 3 8% 4 4 4 4 6% 9% 37%	2662
May	1st to 10th	Day	6% 6% 2 5% 4 3 4 4 5% 7% 55%	7627
	11th to 20th	Day	4 5% 2 5% 4 4 4 4 6% 7% 56%	8155
	21st to end	Day	7% 6% 2 4 3 3 3 3 5% 7% 57%	8992
June	1st to 10th	Day	9% 7% 2 5% 4 4 4 3 5% 8% 48%	2940
	11th to 20th	Day	6% 5% 2 4 3 4 4 3 5% 6% 57%	2940
	21st to end	Day	4 3 1 3 3 3 3 3 4 5% 68%	2940
July	1st to 10th	Day	1 1 1 1 1 1 1 1 3 3 84%	8219
	11th to 20th	Day	2 1 1 2 1 2 2 2 3 3 82%	8220
	21st to end	Day	1 1 1 2 1 1 2 2 3 3 84%	8367
August	1st to 10th	Day	1 2 1 2 1 2 2 2 3 5% 77%	5907
	11th to 20th	Day	3 2 1 2 2 2 2 2 3 5% 77%	5849
	21st to end	Day	2 1 1 2 2 2 3 3 4 5% 74%	6077
September	1st to 10th	Day	3 2 1 3 2 2 2 2 4 6% 75%	2400
	11th to 20th	Day	4 3 2 4 3 3 5% 4 6% 7% 59%	2356
	21st to end	Day	5% 3 1 5% 3 4 3 3 4 6% 62%	2128
October	1st to 10th	Day	5% 4 1 4 2 2 2 2 4 6% 67%	3280
	11th to 20th	Day	4 6% 2 6% 5% 4 5% 4 6% 8% 50%	3261
	21st to end	Day	8% 6% 2 8% 6% 5% 5% 4 5% 9% 41%	3558
November	1st to 10th	Day	12% 10% 3 8% 6% 6% 5% 4 6% 8% 32%	1903
	11th to 20th	Day	11% 10% 3 10% 5% 6% 7% 5% 8% 8% 26%	1680
	21st to end	Day	9% 11% 2 7% 6% 6% 6% 5% 7% 7% 33%	1680
December	1st to 10th	Day	6% 5% 2 6% 5% 5% 5% 5% 7% 8% 46%	2700
	11th to 20th	Day	13% 12% 5% 12% 7% 7% 6% 5% 6% 7% 20%	3108
	21st to end	Day	13% 14% 5% 13% 9% 8% 7% 5% 6% 6% 14%	4092
January	1st to 10th	Night	12% 14% 5% 11% 7% 6% 6% 5% 7% 7% 21%	8172
	11th to 20th	Night	7% 8% 3 7% 4 4 3 3 4 6% 49%	7823
	21st to end	Night	7% 9% 4 9% 6% 6% 6% 4 5% 6% 37%	8371
February	1st to 10th	Night	9% 12% 4 8% 6% 5% 5% 5% 7% 8% 32%	2730
	11th to 20th	Night	9% 13% 4 8% 6% 5% 6% 4 7% 6% 32%	2662
	21st to end	Night	7% 13% 4 9% 6% 6% 5% 5% 7% 7% 30%	2140
March	1st to 10th	Night	10% 14% 4 9% 6% 6% 5% 4 6% 8% 28%	5100
	11th to 20th	Night	5% 10% 3 8% 6% 6% 6% 4 6% 7% 39%	5110
	21st to end	Night	7% 12% 5% 12% 8% 7% 6% 5% 6% 7% 26%	5579
April	1st to 10th	Night	9% 14% 3 8% 6% 6% 5% 4 7% 8% 29%	2100
	11th to 20th	Night	9% 13% 3 8% 5% 5% 4 5% 6% 7% 35%	2079
	21st to end	Night	11% 11% 3 7% 3 4 4 4 6% 7% 40%	1954
May	1st to 10th	Night	6% 8% 2 6% 3 3 3 3 5% 7% 53%	5321
	11th to 20th	Night	4 9% 2 5% 3 4 4 3 4 6% 55%	4701
	21st to end	Night	10% 6% 2 4 2 3 3 2 4 6% 58%	5181
June	1st to 10th	Night	8% 7% 2 5% 4 4 3 4 5% 7% 49%	1680
	11th to 20th	Night	8% 5% 2 5% 3 3 3 2 4 6% 60%	1680
	21st to end	Night	4 3 1 3 2 3 3 2 3 4 73%	1680
July	1st to 10th	Night	1 1 1 2 1 2 2 2 3 4 81%	4740
	11th to 20th	Night	1 1 1 2 1 1 1 1 2 2 85%	4740
	21st to end	Night	1 1 1 2 1 1 2 1 2 3 84%	5723
August	1st to 10th	Night	1 1 1 2 1 2 2 2 3 3 81%	4125
	11th to 20th	Night	4 2 1 2 2 2 2 2 3 5% 76%	4088
	21st to end	Night	2 2 2 2 3 3 2 5% 6% 73%	4918
September	1st to 10th	Night	2 2 2 2 1 2 1 3 5% 79%	2000
	11th to 20th	Night	4 3 1 4 3 2 4 3 5% 8% 64%	2040
	21st to end	Night	6% 5% 2 4 3 3 3 3 4 6% 62%	2271
October	1st to 10th	Night	4 3 1 3 3 3 3 2 3 4 71%	3920
	11th to 20th	Night	6% 5% 2 5% 4 4 4 4 5% 7% 54%	3897
	21st to end	Night	7% 8% 2 8% 5% 6% 6% 5% 6% 7% 38%	4253
November	1st to 10th	Night	11% 10% 4 9% 5% 7% 5% 4 6% 7% 33%	2611
	11th to 20th	Night	11% 10% 3 8% 8% 6% 7% 5% 7% 8% 27%	2940
	21st to end	Night	10% 10% 2 7% 5% 5% 5% 5% 6% 7% 37%	2940
December	1st to 10th	Night	6% 7% 2 6% 4 4 5% 4 6% 7% 48%	4700
	11th to 20th	Night	10% 12% 4 12% 8% 7% 7% 5% 6% 8% 21%	5316
	21st to end	Night	13% 12% 4 10% 7% 6% 6% 5% 6% 7% 23%	7106

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	55% 9% 11% 18% 6% 1	4723
	11th to 20th	Day	36% 4 5% 16% 22% 17%	5142
	21st to end	Day	39% 7% 9% 15% 19% 11%	5843
February	1st to 10th	Day	50% 10% 12% 15% 11% 2	1890
	11th to 20th	Day	49% 10% 12% 18% 9% 2	1854
	21st to end	Day	56% 8% 9% 17% 9% 2	1664
March	1st to 10th	Day	53% 7% 9% 21% 9% 1	5009
	11th to 20th	Day	46% 8% 11% 15% 17% 2	5110
	21st to end	Day	51% 9% 14% 20% 5%	5636
April	1st to 10th	Day	54% 8% 14% 15% 7% 2	2520
	11th to 20th	Day	50% 7% 14% 16% 10% 3	2492
	21st to end	Day	42% 9% 14% 21% 13% 1	2662
May	1st to 10th	Day	27% 5% 11% 30% 24% 2	7627
	11th to 20th	Day	24% 7% 13% 33% 22% 2	8158
	21st to end	Day	25% 5% 16% 42% 12%	8994
June	1st to 10th	Day	36% 6% 14% 29% 13% 1	2940
	11th to 20th	Day	27% 7% 11% 34% 18% 3	2940
	21st to end	Day	18% 4 10% 33% 26% 7%	2940
July	1st to 10th	Day	7% 2 5% 28% 31% 27%	8151
	11th to 20th	Day	10% 1 4 15% 37% 34%	8145
	21st to end	Day	7% 2 4 32% 37% 17%	8180
August	1st to 10th	Day	12% 2 5% 21% 39% 21%	4336
	11th to 20th	Day	10% 3 7% 34% 35% 11%	4248
	21st to end	Day	11% 4 8% 31% 37% 9%	4380
September	1st to 10th	Day	12% 4 4 24% 33% 22%	2400
	11th to 20th	Day	23% 6% 9% 27% 27% 8%	2356
	21st to end	Day	21% 7% 9% 25% 24% 15%	2128
October	1st to 10th	Day	19% 5% 10% 16% 25% 25%	3280
	11th to 20th	Day	28% 7% 16% 30% 16% 4	3261
	21st to end	Day	36% 7% 11% 22% 17% 6%	3558
November	1st to 10th	Day	48% 8% 12% 17% 12% 3	1903
	11th to 20th	Day	50% 10% 15% 12% 9% 4	1680
	21st to end	Day	46% 9% 10% 15% 13% 7%	1680
December	1st to 10th	Day	31% 7% 10% 21% 23% 8%	2700
	11th to 20th	Day	57% 10% 13% 12% 6% 2	3108
	21st to end	Day	61% 12% 8% 13% 4 1	4092
January	1st to 10th	Night	56% 9% 13% 15% 6% 1	8172
	11th to 20th	Night	36% 6% 6% 16% 18% 18%	7823
	21st to end	Night	44% 6% 8% 14% 17% 12%	8371
February	1st to 10th	Night	47% 9% 13% 16% 12% 3	2730
	11th to 20th	Night	50% 9% 10% 18% 10% 2	2664
	21st to end	Night	49% 12% 14% 14% 9% 2	2140
March	1st to 10th	Night	52% 9% 15% 17% 7% 1	5100
	11th to 20th	Night	40% 9% 11% 22% 15% 3	5110
	21st to end	Night	51% 12% 18% 16% 3	5579
April	1st to 10th	Night	52% 10% 15% 16% 6% 1	2100
	11th to 20th	Night	45% 11% 14% 19% 8% 3	2079
	21st to end	Night	40% 10% 18% 23% 8% 1	1954
May	1st to 10th	Night	29% 6% 13% 31% 18% 2	5319
	11th to 20th	Night	28% 7% 14% 36% 13% 1	4701
	21st to end	Night	28% 4 21% 37% 9%	5182
June	1st to 10th	Night	36% 9% 16% 31% 9%	1680
	11th to 20th	Night	28% 5% 15% 34% 15% 3	1680
	21st to end	Night	17% 5% 11% 38% 23% 7%	1680
July	1st to 10th	Night	10% 3 6% 32% 26% 24%	4700
	11th to 20th	Night	8% 2 3 16% 42% 29%	4692
	21st to end	Night	8% 2 6% 35% 36% 13%	5598
August	1st to 10th	Night	10% 3 5% 22% 47% 13%	3019
	11th to 20th	Night	9% 3 10% 40% 26% 12%	2965
	21st to end	Night	14% 4 7% 33% 34% 8%	3537
September	1st to 10th	Night	10% 3 7% 26% 35% 19%	2000
	11th to 20th	Night	20% 7% 12% 30% 25% 7%	2040
	21st to end	Night	24% 5% 11% 25% 19% 16%	2271
October	1st to 10th	Night	20% 4 8% 19% 27% 23%	3920
	11th to 20th	Night	29% 7% 14% 33% 11% 6%	3897
	21st to end	Night	39% 9% 12% 21% 14% 6%	4253
November	1st to 10th	Night	48% 9% 11% 15% 11% 5%	2611
	11th to 20th	Night	51% 11% 13% 12% 9% 5%	2940
	21st to end	Night	43% 9% 11% 14% 16% 8%	2940
December	1st to 10th	Night	32% 6% 10% 21% 23% 8%	4700
	11th to 20th	Night	55% 12% 11% 14% 7% 1	5316
	21st to end	Night	54% 10% 11% 17% 6% 1	7106

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	25% 25% 25% 17% 6% 2	220
	11th to 20th	Day	22% 13% 25% 19% 17% 4	362
	21st to end	Day	19% 15% 27% 22% 15% 2	473
February	1st to 10th	Day	19% 20% 28% 21% 11% 1	430
	11th to 20th	Day	24% 23% 25% 21% 7%	425
	21st to end	Day	27% 21% 25% 19% 7%	412
March	1st to 10th	Day	24% 24% 26% 19% 6%	540
	11th to 20th	Day	21% 23% 24% 22% 9% 1	540
	21st to end	Day	26% 22% 28% 18% 5% 1	594
April	1st to 10th	Day	19% 23% 28% 20% 8% 2	540
	11th to 20th	Day	21% 24% 27% 17% 8% 2	532
	21st to end	Day	17% 20% 32% 19% 10% 2	540
May	1st to 10th	Day	19% 15% 33% 19% 11% 3	543
	11th to 20th	Day	19% 21% 27% 22% 9% 3	568
	21st to end	Day	17% 17% 28% 23% 11% 4	617
June	1st to 10th	Day	14% 18% 29% 24% 11% 4	570
	11th to 20th	Day	12% 19% 31% 25% 9% 4	570
	21st to end	Day	4 11% 31% 35% 13% 5%	570
July	1st to 10th	Day	3 10% 33% 34% 16% 5%	566
	11th to 20th	Day	3 8% 37% 31% 16% 5%	564
	21st to end	Day	2 6% 32% 37% 17% 7%	619
August	1st to 10th	Day	3 6% 28% 35% 19% 9%	567
	11th to 20th	Day	2 8% 26% 36% 19% 9%	534
	21st to end	Day	4 8% 29% 34% 19% 7%	583
September	1st to 10th	Day	5% 8% 27% 36% 17% 7%	530
	11th to 20th	Day	9% 10% 28% 33% 17% 4	530
	21st to end	Day	10% 12% 26% 29% 19% 4	530
October	1st to 10th	Day	12% 14% 23% 29% 18% 4	530
	11th to 20th	Day	12% 12% 25% 28% 20% 3	530
	21st to end	Day	17% 15% 27% 24% 13% 4	583
November	1st to 10th	Day	20% 15% 26% 23% 16% 1	391
	11th to 20th	Day	21% 12% 25% 26% 15% 2	220
	21st to end	Day	15% 16% 23% 28% 15% 2	220
December	1st to 10th	Day	17% 12% 27% 25% 14% 6%	220
	11th to 20th	Day	25% 12% 25% 24% 10% 4	220
	21st to end	Day	27% 17% 24% 22% 7% 3	242
January	1st to 10th	Night	19% 15% 28% 25% 10% 2	840
	11th to 20th	Night	16% 13% 24% 28% 15% 4	840
	21st to end	Night	16% 11% 23% 31% 17% 2	924
February	1st to 10th	Night	16% 13% 20% 35% 14% 1	840
	11th to 20th	Night	17% 15% 24% 28% 14% 1	824
	21st to end	Night	15% 16% 24% 31% 13% 1	692
March	1st to 10th	Night	20% 18% 22% 27% 12% 2	840
	11th to 20th	Night	16% 16% 22% 30% 15% 2	840
	21st to end	Night	16% 19% 26% 27% 12% 1	924
April	1st to 10th	Night	19% 13% 24% 28% 15% 1	840
	11th to 20th	Night	14% 15% 22% 27% 17% 4	831
	21st to end	Night	16% 9% 19% 31% 23% 2	694
May	1st to 10th	Night	12% 9% 19% 32% 24% 3	630
	11th to 20th	Night	13% 9% 16% 32% 27% 4	415
	21st to end	Night	15% 8% 16% 31% 27% 2	455
June	1st to 10th	Night	12% 6% 20% 30% 30% 2	420
	11th to 20th	Night	10% 6% 19% 38% 25% 2	420
	21st to end	Night	6% 4 11% 37% 41% 1	420
July	1st to 10th	Night	3 1 7% 39% 47% 3	410
	11th to 20th	Night	2 1 9% 45% 39% 3	408
	21st to end	Night	2 5% 32% 52% 8%	658
August	1st to 10th	Night	3 5% 33% 50% 9%	627
	11th to 20th	Night	2 1 5% 31% 51% 10%	606
	21st to end	Night	3 1 12% 37% 41% 6%	835
September	1st to 10th	Night	3 3 13% 33% 43% 6%	800
	11th to 20th	Night	5% 5% 11% 34% 37% 9%	800
	21st to end	Night	9% 6% 13% 34% 31% 8%	800
October	1st to 10th	Night	10% 5% 17% 35% 30% 4	800
	11th to 20th	Night	10% 8% 16% 33% 28% 5%	800
	21st to end	Night	13% 9% 18% 34% 21% 6%	880
November	1st to 10th	Night	19% 10% 21% 31% 16% 3	820
	11th to 20th	Night	18% 13% 25% 31% 11% 1	840
	21st to end	Night	16% 12% 23% 31% 18% 1	840
December	1st to 10th	Night	15% 13% 22% 33% 15% 2	840
	11th to 20th	Night	19% 14% 29% 25% 12% 2	840
	21st to end	Night	28% 15% 25% 24% 6% 2	924

Page generated at 2026-08-12 00:37:18. Version ac07

Copyright © 2026 Attilla Danko.