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 2025-04-11.

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	7% 8% 3 7% 4 4 4 3 5% 6% 49%	117672
	Night	7% 9% 3 7% 5% 5% 4 4 5% 6% 45%	119459
Transparency	Day	34% 6% 10% 21% 19% 9%	117425
	Night	38% 8% 11% 20% 16% 7%	119295
Seeing	Day	15% 16% 28% 26% 13% 3	15438
	Night	13% 10% 19% 31% 23% 4	24634

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	12% 15% 6% 12% 7% 7% 5% 4 6% 7% 19%	3639
	11th to 20th	Day	11% 10% 4 7% 4 5% 4 3 4 6% 40%	3944
	21st to end	Day	8% 9% 4 9% 5% 5% 4 3 5% 6% 43%	4510
February	1st to 10th	Day	7% 12% 4 10% 6% 7% 7% 5% 7% 6% 28%	1800
	11th to 20th	Day	8% 12% 3 9% 6% 5% 7% 5% 6% 7% 33%	1755
	21st to end	Day	8% 16% 4 9% 7% 5% 6% 4 5% 7% 29%	1587
March	1st to 10th	Day	7% 16% 4 11% 7% 6% 5% 4 5% 6% 30%	4900
	11th to 20th	Day	7% 13% 4 10% 6% 6% 5% 4 5% 5% 34%	5000
	21st to end	Day	7% 12% 4 12% 8% 6% 7% 5% 7% 8% 24%	5515
April	1st to 10th	Day	10% 13% 3 11% 8% 6% 5% 4 5% 7% 27%	2400
	11th to 20th	Day	13% 13% 4 8% 6% 5% 4 4 5% 7% 31%	2251
	21st to end	Day	9% 11% 3 9% 5% 4 4 4 6% 9% 36%	2408
May	1st to 10th	Day	9% 10% 3 7% 5% 4 4 3 4 6% 45%	4110
	11th to 20th	Day	6% 5% 2 4 3 3 3 2 5% 5% 62%	4372
	21st to end	Day	8% 7% 2 5% 4 3 3 3 5% 8% 51%	4813
June	1st to 10th	Day	10% 8% 2 6% 4 4 4 3 5% 8% 44%	2660
	11th to 20th	Day	7% 6% 2 4 3 4 4 3 5% 6% 54%	2660
	21st to end	Day	4 3 1 3 3 3 2 2 4 5% 70%	2660
July	1st to 10th	Day	1 1 1 1 1 1 1 1 2 3 88%	4420
	11th to 20th	Day	2 2 1 2 2 2 2 2 3 3 80%	4420
	21st to end	Day	1 1 2 1 1 2 2 2 3 86%	4492
August	1st to 10th	Day	2 2 1 2 2 2 2 2 3 4 77%	4091
	11th to 20th	Day	1 1 1 2 2 2 3 2 3 5% 78%	4026
	21st to end	Day	2 2 1 2 2 2 2 2 3 4 77%	4247
September	1st to 10th	Day	3 2 1 2 1 2 2 2 3 5% 76%	2280
	11th to 20th	Day	4 3 2 4 3 4 5% 4 6% 7% 59%	2238
	21st to end	Day	5% 3 1 5% 3 4 3 3 4 6% 61%	2022
October	1st to 10th	Day	5% 3 1 4 2 2 2 2 4 6% 68%	3180
	11th to 20th	Day	4 6% 2 6% 5% 4 5% 4 6% 8% 50%	3161
	21st to end	Day	8% 6% 2 8% 6% 5% 6% 4 5% 9% 41%	3448
November	1st to 10th	Day	13% 10% 3 8% 7% 6% 5% 4 6% 8% 32%	1811
	11th to 20th	Day	11% 10% 3 10% 5% 6% 7% 5% 8% 8% 27%	1600
	21st to end	Day	9% 12% 2 7% 6% 6% 6% 5% 7% 7% 33%	1600
December	1st to 10th	Day	6% 5% 2 7% 5% 5% 5% 5% 7% 8% 46%	2620
	11th to 20th	Day	13% 12% 5% 12% 7% 7% 5% 5% 7% 7% 19%	3028
	21st to end	Day	13% 14% 5% 13% 9% 8% 7% 6% 6% 6% 13%	4004
January	1st to 10th	Night	13% 14% 5% 11% 7% 6% 5% 5% 6% 6% 21%	6306
	11th to 20th	Night	10% 11% 4 9% 5% 5% 4 3 4 6% 40%	6014
	21st to end	Night	9% 10% 4 9% 5% 5% 4 3 5% 5% 39%	6468
February	1st to 10th	Night	9% 12% 4 8% 6% 5% 5% 5% 7% 8% 31%	2600
	11th to 20th	Night	10% 13% 4 8% 5% 5% 6% 4 7% 6% 32%	2532
	21st to end	Night	6% 13% 4 9% 6% 6% 5% 5% 7% 7% 31%	2041
March	1st to 10th	Night	10% 14% 3 9% 6% 6% 5% 4 6% 8% 28%	4989
	11th to 20th	Night	5% 10% 3 9% 6% 6% 6% 4 6% 7% 39%	5000
	21st to end	Night	7% 12% 5% 12% 8% 7% 6% 5% 6% 7% 26%	5458
April	1st to 10th	Night	9% 14% 3 8% 6% 6% 5% 4 7% 8% 28%	2000
	11th to 20th	Night	9% 13% 3 8% 6% 5% 4 5% 7% 7% 34%	1883
	21st to end	Night	11% 11% 3 7% 3 4 4 5% 7% 8% 37%	1768
May	1st to 10th	Night	9% 12% 3 8% 4 4 4 3 4 6% 44%	2860
	11th to 20th	Night	5% 6% 2 4 3 4 4 3 4 5% 62%	2517
	21st to end	Night	10% 8% 2 5% 3 3 3 2 4 6% 53%	2762
June	1st to 10th	Night	9% 8% 2 5% 4 4 3 4 5% 7% 47%	1520
	11th to 20th	Night	8% 5% 2 5% 3 3 3 2 4 6% 58%	1520
	21st to end	Night	4 2 1 3 2 3 2 1 3 4 75%	1520
July	1st to 10th	Night	1 1 1 1 1 1 2 1 2 3 86%	2540
	11th to 20th	Night	2 1 1 2 1 2 2 2 3 3 83%	2540
	21st to end	Night	1 1 1 1 1 1 1 2 3 87%	3067
August	1st to 10th	Night	2 1 1 2 2 2 2 2 3 3 80%	2848
	11th to 20th	Night	1 1 2 2 2 3 2 4 6% 78%	2812
	21st to end	Night	2 2 1 3 2 3 2 2 3 4 76%	3429
September	1st to 10th	Night	2 1 2 2 1 2 1 3 5% 80%	1900
	11th to 20th	Night	4 3 1 4 3 2 4 3 5% 7% 64%	1938
	21st to end	Night	6% 5% 2 4 3 3 3 3 4 6% 61%	2157
October	1st to 10th	Night	4 3 1 3 3 3 3 2 2 4 72%	3800
	11th to 20th	Night	6% 5% 2 5% 4 4 4 3 5% 7% 54%	3777
	21st to end	Night	7% 8% 3 9% 5% 6% 7% 5% 6% 6% 38%	4121
November	1st to 10th	Night	11% 10% 4 9% 5% 7% 5% 4 5% 7% 33%	2484
	11th to 20th	Night	11% 9% 3 8% 9% 6% 7% 5% 7% 8% 27%	2800
	21st to end	Night	10% 10% 2 7% 5% 5% 5% 5% 6% 7% 37%	2800
December	1st to 10th	Night	6% 7% 2 6% 4 4 5% 4 6% 7% 48%	4560
	11th to 20th	Night	10% 12% 5% 12% 8% 7% 7% 5% 6% 8% 20%	5176
	21st to end	Night	13% 12% 4 10% 7% 6% 6% 5% 6% 7% 23%	6952

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	59% 10% 8% 15% 7% 1	3639
	11th to 20th	Day	45% 6% 7% 21% 18% 4	3944
	21st to end	Day	40% 8% 8% 12% 20% 12%	4510
February	1st to 10th	Day	51% 10% 13% 14% 10% 2	1800
	11th to 20th	Day	48% 9% 13% 19% 9% 2	1764
	21st to end	Day	55% 8% 8% 17% 9% 2	1587
March	1st to 10th	Day	53% 7% 8% 21% 9% 1	4900
	11th to 20th	Day	47% 8% 11% 15% 17% 2	5000
	21st to end	Day	52% 9% 14% 20% 5%	5515
April	1st to 10th	Day	55% 8% 13% 14% 7% 2	2400
	11th to 20th	Day	52% 7% 15% 15% 8% 3	2254
	21st to end	Day	44% 9% 15% 20% 12% 1	2408
May	1st to 10th	Day	39% 7% 11% 25% 16% 2	4110
	11th to 20th	Day	24% 5% 9% 29% 30% 3	4375
	21st to end	Day	31% 7% 20% 35% 7%	4814
June	1st to 10th	Day	39% 7% 16% 28% 11%	2660
	11th to 20th	Day	29% 7% 12% 34% 16% 2	2660
	21st to end	Day	18% 4 10% 33% 27% 8%	2660
July	1st to 10th	Day	6% 2 5% 16% 23% 48%	4351
	11th to 20th	Day	11% 2 4 14% 30% 38%	4345
	21st to end	Day	7% 2 4 20% 45% 22%	4350
August	1st to 10th	Day	12% 2 5% 20% 40% 20%	4076
	11th to 20th	Day	10% 3 6% 35% 35% 11%	4065
	21st to end	Day	12% 4 8% 31% 37% 8%	4246
September	1st to 10th	Day	11% 4 4 24% 34% 23%	2280
	11th to 20th	Day	23% 7% 9% 26% 27% 9%	2238
	21st to end	Day	21% 7% 9% 25% 23% 14%	2022
October	1st to 10th	Day	18% 5% 10% 16% 25% 26%	3180
	11th to 20th	Day	28% 7% 16% 30% 16% 4	3161
	21st to end	Day	36% 8% 11% 22% 17% 6%	3448
November	1st to 10th	Day	49% 8% 12% 16% 12% 3	1811
	11th to 20th	Day	50% 10% 15% 11% 9% 4	1600
	21st to end	Day	47% 9% 9% 14% 14% 8%	1600
December	1st to 10th	Day	31% 6% 10% 21% 24% 8%	2620
	11th to 20th	Day	57% 10% 13% 12% 6% 1	3028
	21st to end	Day	62% 12% 8% 13% 4 1	4004
January	1st to 10th	Night	59% 9% 9% 14% 7% 1	6306
	11th to 20th	Night	45% 7% 7% 20% 16% 4	6014
	21st to end	Night	46% 7% 6% 11% 16% 14%	6468
February	1st to 10th	Night	48% 9% 14% 15% 11% 3	2600
	11th to 20th	Night	50% 9% 11% 18% 10% 2	2534
	21st to end	Night	49% 12% 14% 15% 9% 2	2041
March	1st to 10th	Night	51% 9% 15% 17% 7% 1	4989
	11th to 20th	Night	41% 9% 11% 21% 15% 3	5000
	21st to end	Night	51% 12% 18% 15% 3	5458
April	1st to 10th	Night	52% 10% 15% 15% 6% 1	2000
	11th to 20th	Night	47% 11% 14% 17% 8% 3	1883
	21st to end	Night	42% 11% 19% 21% 7% 1	1768
May	1st to 10th	Night	39% 8% 14% 25% 11% 2	2860
	11th to 20th	Night	25% 7% 12% 38% 16% 2	2517
	21st to end	Night	33% 7% 28% 28% 4	2762
June	1st to 10th	Night	37% 9% 17% 30% 7%	1520
	11th to 20th	Night	29% 5% 17% 34% 14% 1	1520
	21st to end	Night	17% 5% 11% 38% 23% 7%	1520
July	1st to 10th	Night	8% 2 6% 17% 23% 44%	2500
	11th to 20th	Night	9% 2 4 17% 36% 32%	2492
	21st to end	Night	5% 2 6% 23% 47% 18%	2971
August	1st to 10th	Night	11% 3 5% 22% 48% 12%	2839
	11th to 20th	Night	9% 3 10% 40% 26% 12%	2839
	21st to end	Night	14% 4 7% 33% 34% 8%	3429
September	1st to 10th	Night	9% 3 6% 25% 37% 20%	1900
	11th to 20th	Night	19% 7% 12% 29% 26% 7%	1938
	21st to end	Night	25% 5% 11% 24% 19% 16%	2157
October	1st to 10th	Night	19% 4 8% 19% 27% 23%	3800
	11th to 20th	Night	29% 7% 14% 33% 11% 6%	3777
	21st to end	Night	40% 9% 11% 20% 14% 7%	4121
November	1st to 10th	Night	49% 9% 11% 14% 11% 5%	2484
	11th to 20th	Night	50% 12% 12% 12% 8% 6%	2800
	21st to end	Night	44% 9% 11% 13% 15% 8%	2800
December	1st to 10th	Night	32% 6% 8% 21% 23% 8%	4560
	11th to 20th	Night	55% 12% 11% 14% 7% 1	5176
	21st to end	Night	54% 10% 11% 17% 6% 1	6952

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% 26% 26% 16% 6% 2	200
	11th to 20th	Day	23% 14% 27% 18% 14% 3	335
	21st to end	Day	20% 16% 26% 21% 15% 2	440
February	1st to 10th	Day	20% 21% 28% 21% 11%	400
	11th to 20th	Day	25% 23% 25% 20% 7% 1	395
	21st to end	Day	25% 22% 25% 19% 8% 1	383
March	1st to 10th	Day	25% 24% 25% 19% 6%	500
	11th to 20th	Day	22% 24% 22% 21% 9% 1	500
	21st to end	Day	26% 22% 27% 19% 5% 1	550
April	1st to 10th	Day	20% 24% 28% 19% 7% 1	500
	11th to 20th	Day	22% 24% 27% 18% 7% 2	453
	21st to end	Day	18% 22% 30% 18% 9% 2	460
May	1st to 10th	Day	21% 16% 32% 19% 10% 3	463
	11th to 20th	Day	19% 18% 27% 22% 10% 3	488
	21st to end	Day	18% 17% 27% 23% 11% 4	529
June	1st to 10th	Day	15% 18% 28% 22% 12% 4	490
	11th to 20th	Day	12% 21% 32% 23% 8% 4	490
	21st to end	Day	4 9% 32% 36% 13% 5%	490
July	1st to 10th	Day	3 10% 32% 34% 15% 5%	486
	11th to 20th	Day	2 8% 37% 32% 16% 5%	484
	21st to end	Day	1 5% 31% 39% 18% 7%	531
August	1st to 10th	Day	3 7% 29% 34% 19% 8%	487
	11th to 20th	Day	2 8% 25% 37% 19% 9%	477
	21st to end	Day	4 8% 29% 35% 19% 6%	539
September	1st to 10th	Day	5% 7% 27% 36% 18% 7%	490
	11th to 20th	Day	9% 11% 28% 32% 17% 3	490
	21st to end	Day	10% 12% 27% 29% 19% 3	490
October	1st to 10th	Day	11% 14% 23% 29% 19% 3	490
	11th to 20th	Day	12% 12% 24% 27% 21% 3	490
	21st to end	Day	17% 16% 27% 24% 13% 4	539
November	1st to 10th	Day	19% 14% 25% 23% 16% 1	359
	11th to 20th	Day	22% 13% 24% 26% 15% 2	200
	21st to end	Day	17% 17% 22% 28% 15% 2	200
December	1st to 10th	Day	18% 13% 25% 25% 14% 7%	200
	11th to 20th	Day	25% 13% 26% 26% 9% 3	200
	21st to end	Day	27% 17% 24% 24% 7% 1	220
January	1st to 10th	Night	19% 15% 29% 25% 11% 2	800
	11th to 20th	Night	17% 14% 25% 27% 13% 4	800
	21st to end	Night	17% 11% 22% 30% 17% 2	880
February	1st to 10th	Night	17% 14% 20% 35% 14% 1	800
	11th to 20th	Night	18% 15% 23% 28% 15% 2	784
	21st to end	Night	14% 16% 24% 32% 14% 1	660
March	1st to 10th	Night	20% 18% 22% 26% 12% 2	800
	11th to 20th	Night	17% 17% 20% 29% 15% 2	800
	21st to end	Night	16% 19% 25% 26% 13% 1	880
April	1st to 10th	Night	18% 14% 25% 26% 15% 1	800
	11th to 20th	Night	14% 16% 22% 26% 18% 4	753
	21st to end	Night	17% 10% 20% 31% 21% 2	628
May	1st to 10th	Night	13% 10% 20% 30% 24% 3	570
	11th to 20th	Night	13% 7% 16% 31% 29% 4	375
	21st to end	Night	16% 9% 16% 30% 27% 3	411
June	1st to 10th	Night	12% 5% 19% 29% 32% 3	380
	11th to 20th	Night	11% 6% 20% 37% 25% 2	380
	21st to end	Night	6% 3 11% 36% 43% 1	380
July	1st to 10th	Night	3 1 6% 38% 47% 4	370
	11th to 20th	Night	2 1 8% 44% 42% 3	368
	21st to end	Night	2 1 4 32% 53% 9%	595
August	1st to 10th	Night	3 4 32% 51% 10%	567
	11th to 20th	Night	2 1 5% 30% 52% 10%	564
	21st to end	Night	3 1 11% 37% 41% 6%	793
September	1st to 10th	Night	3 3 12% 33% 43% 6%	760
	11th to 20th	Night	5% 5% 11% 33% 38% 9%	760
	21st to end	Night	9% 6% 12% 34% 31% 8%	760
October	1st to 10th	Night	10% 4 16% 35% 31% 4	760
	11th to 20th	Night	10% 8% 15% 33% 29% 5%	760
	21st to end	Night	13% 9% 18% 33% 21% 6%	836
November	1st to 10th	Night	18% 9% 22% 31% 16% 3	780
	11th to 20th	Night	17% 14% 25% 32% 12% 2	800
	21st to end	Night	16% 12% 23% 30% 18% 1	800
December	1st to 10th	Night	15% 14% 21% 33% 16% 2	800
	11th to 20th	Night	18% 15% 29% 25% 11% 2	800
	21st to end	Night	29% 15% 25% 24% 6% 1	880

Page generated at 2025-04-08 06:17:23. Version ac07

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