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 2024-10-24.

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 6% 4 4 4 3 5% 6% 51%	101311
	Night	8% 9% 3 7% 5% 4 4 4 5% 6% 46%	97635
Transparency	Day	34% 7% 10% 21% 19% 9%	101064
	Night	38% 8% 11% 20% 16% 7%	97471
Seeing	Day	14% 15% 28% 26% 13% 3	14940
	Night	13% 10% 19% 31% 24% 4	23956

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	9% 17% 6% 13% 7% 7% 5% 4 5% 7% 19%	2539
	11th to 20th	Day	16% 14% 4 10% 6% 6% 5% 4 5% 6% 23%	2746
	21st to end	Day	11% 13% 5% 12% 7% 6% 6% 4 6% 7% 25%	3146
February	1st to 10th	Day	6% 12% 4 10% 7% 7% 7% 6% 7% 7% 29%	1710
	11th to 20th	Day	8% 11% 3 8% 6% 5% 6% 5% 6% 7% 33%	1665
	21st to end	Day	8% 17% 4 10% 7% 6% 6% 4 5% 6% 28%	1510
March	1st to 10th	Day	9% 18% 5% 12% 7% 6% 5% 3 5% 5% 25%	3403
	11th to 20th	Day	6% 10% 3 7% 5% 5% 5% 4 5% 5% 46%	3490
	21st to end	Day	9% 14% 5% 12% 7% 6% 6% 5% 7% 8% 21%	3841
April	1st to 10th	Day	10% 13% 3 11% 8% 6% 5% 4 5% 7% 27%	2280
	11th to 20th	Day	13% 13% 4 8% 6% 5% 4 4 5% 7% 31%	2249
	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	8% 5% 2 5% 2 3 3 3 5% 6% 58%	1900
	11th to 20th	Day	6% 6% 2 6% 4 4 5% 4 6% 7% 50%	1900
	21st to end	Day	9% 6% 2 8% 5% 5% 5% 3 4 8% 44%	2011
November	1st to 10th	Day	13% 10% 3 8% 6% 6% 5% 4 6% 8% 31%	1720
	11th to 20th	Day	11% 10% 3 9% 5% 6% 7% 6% 8% 8% 28%	1520
	21st to end	Day	8% 12% 2 7% 6% 6% 6% 5% 7% 7% 33%	1520
December	1st to 10th	Day	8% 8% 3 9% 6% 5% 6% 6% 7% 7% 37%	1520
	11th to 20th	Day	12% 11% 4 11% 6% 7% 5% 5% 7% 8% 24%	1928
	21st to end	Day	10% 13% 5% 12% 8% 8% 7% 6% 6% 7% 18%	2794
January	1st to 10th	Night	12% 15% 5% 12% 7% 7% 6% 6% 7% 6% 19%	4406
	11th to 20th	Night	14% 14% 4 11% 6% 6% 5% 4 5% 7% 25%	4205
	21st to end	Night	13% 14% 5% 12% 7% 6% 5% 4 5% 6% 23%	4521
February	1st to 10th	Night	8% 12% 4 8% 6% 5% 5% 5% 7% 8% 32%	2470
	11th to 20th	Night	10% 13% 4 8% 5% 5% 6% 4 7% 6% 33%	2402
	21st to end	Night	6% 14% 4 9% 6% 6% 6% 5% 7% 7% 30%	1942
March	1st to 10th	Night	11% 14% 3 9% 7% 7% 5% 4 7% 7% 26%	3466
	11th to 20th	Night	5% 10% 2 7% 5% 4 5% 3 5% 6% 48%	3490
	21st to end	Night	7% 12% 4 11% 8% 8% 6% 6% 6% 7% 24%	3810
April	1st to 10th	Night	10% 14% 3 8% 7% 6% 5% 5% 7% 8% 28%	1900
	11th to 20th	Night	9% 13% 3 8% 6% 5% 4 5% 7% 7% 34%	1879
	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	7% 4 1 4 3 2 3 3 4 5% 62%	2280
	11th to 20th	Night	7% 6% 1 6% 4 4 4 3 5% 6% 54%	2280
	21st to end	Night	9% 7% 2 7% 4 4 5% 4 6% 7% 46%	2412
November	1st to 10th	Night	11% 10% 4 9% 5% 7% 5% 4 5% 7% 33%	2356
	11th to 20th	Night	11% 9% 3 8% 8% 6% 6% 5% 7% 8% 28%	2660
	21st to end	Night	10% 11% 2 7% 5% 5% 6% 5% 6% 7% 36%	2660
December	1st to 10th	Night	9% 9% 3 8% 5% 5% 6% 4 7% 8% 36%	2660
	11th to 20th	Night	9% 13% 5% 12% 8% 6% 6% 4 6% 8% 23%	3276
	21st to end	Night	11% 14% 4 10% 7% 6% 6% 4 6% 7% 25%	4862

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	62% 10% 10% 14% 3 2	2539
	11th to 20th	Day	60% 8% 7% 12% 8% 5%	2746
	21st to end	Day	54% 11% 11% 13% 8% 3	3146
February	1st to 10th	Day	50% 11% 13% 14% 11% 2	1710
	11th to 20th	Day	48% 10% 12% 19% 9% 3	1674
	21st to end	Day	57% 8% 8% 16% 8% 2	1510
March	1st to 10th	Day	61% 8% 8% 15% 8%	3403
	11th to 20th	Day	38% 8% 11% 18% 23% 2	3490
	21st to end	Day	58% 10% 13% 14% 4	3841
April	1st to 10th	Day	56% 8% 13% 13% 8% 2	2280
	11th to 20th	Day	52% 7% 15% 15% 8% 3	2252
	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	27% 6% 12% 18% 23% 14%	1900
	11th to 20th	Day	31% 8% 12% 20% 22% 7%	1900
	21st to end	Day	38% 7% 12% 18% 16% 9%	2011
November	1st to 10th	Day	49% 8% 12% 15% 13% 4	1720
	11th to 20th	Day	49% 10% 15% 12% 10% 5%	1520
	21st to end	Day	48% 9% 9% 14% 13% 8%	1520
December	1st to 10th	Day	41% 9% 12% 18% 17% 3	1520
	11th to 20th	Day	55% 10% 13% 12% 8% 2	1928
	21st to end	Day	58% 14% 7% 14% 5% 2	2794
January	1st to 10th	Night	61% 11% 10% 11% 4 2	4406
	11th to 20th	Night	57% 9% 8% 12% 8% 5%	4205
	21st to end	Night	60% 8% 7% 12% 8% 4	4521
February	1st to 10th	Night	47% 9% 14% 16% 12% 3	2470
	11th to 20th	Night	50% 9% 10% 18% 10% 2	2404
	21st to end	Night	50% 12% 14% 14% 8% 2	1942
March	1st to 10th	Night	54% 10% 13% 17% 6%	3466
	11th to 20th	Night	38% 8% 11% 21% 19% 4	3490
	21st to end	Night	55% 14% 16% 13% 3	3810
April	1st to 10th	Night	53% 10% 15% 15% 6% 1	1900
	11th to 20th	Night	47% 11% 14% 17% 8% 3	1879
	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	25% 6% 11% 19% 25% 13%	2280
	11th to 20th	Night	31% 7% 12% 22% 18% 10%	2280
	21st to end	Night	36% 9% 12% 18% 14% 11%	2412
November	1st to 10th	Night	49% 9% 11% 15% 11% 5%	2356
	11th to 20th	Night	50% 11% 12% 13% 9% 6%	2660
	21st to end	Night	45% 9% 11% 12% 15% 8%	2660
December	1st to 10th	Night	43% 10% 9% 19% 15% 4	2660
	11th to 20th	Night	55% 11% 8% 14% 10% 1	3276
	21st to end	Night	55% 11% 10% 15% 7% 2	4862

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	24% 27% 24% 17% 5% 2	180
	11th to 20th	Day	25% 15% 27% 17% 14% 2	308
	21st to end	Day	21% 16% 27% 21% 14% 1	407
February	1st to 10th	Day	18% 21% 28% 22% 11%	370
	11th to 20th	Day	24% 22% 24% 21% 8% 1	365
	21st to end	Day	25% 23% 25% 19% 7%	354
March	1st to 10th	Day	27% 24% 25% 18% 7%	460
	11th to 20th	Day	20% 23% 23% 23% 10% 1	460
	21st to end	Day	25% 22% 29% 19% 5%	506
April	1st to 10th	Day	20% 24% 28% 19% 8% 1	460
	11th to 20th	Day	22% 24% 27% 18% 7% 2	452
	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	16% 16% 27% 24% 13% 5%	508
November	1st to 10th	Day	20% 15% 24% 24% 16% 1	328
	11th to 20th	Day	16% 13% 24% 28% 17% 2	180
	21st to end	Day	16% 17% 23% 29% 16%	180
December	1st to 10th	Day	19% 14% 25% 25% 12% 4	180
	11th to 20th	Day	24% 13% 26% 24% 10% 3	180
	21st to end	Day	25% 16% 25% 25% 8% 2	198
January	1st to 10th	Night	18% 14% 29% 25% 11% 2	760
	11th to 20th	Night	18% 14% 25% 27% 12% 4	760
	21st to end	Night	18% 12% 22% 30% 16% 2	836
February	1st to 10th	Night	16% 14% 20% 35% 14% 1	760
	11th to 20th	Night	18% 15% 22% 28% 15% 2	744
	21st to end	Night	13% 16% 24% 31% 14% 1	628
March	1st to 10th	Night	20% 19% 22% 25% 13% 2	760
	11th to 20th	Night	15% 17% 20% 30% 16% 2	760
	21st to end	Night	16% 19% 26% 26% 12% 1	836
April	1st to 10th	Night	19% 14% 25% 26% 16% 1	760
	11th to 20th	Night	14% 16% 22% 26% 18% 4	751
	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% 17% 33% 21% 7%	804
November	1st to 10th	Night	18% 9% 21% 32% 16% 4	740
	11th to 20th	Night	17% 12% 25% 33% 12% 2	760
	21st to end	Night	16% 12% 24% 31% 17% 1	760
December	1st to 10th	Night	16% 14% 22% 32% 15% 2	760
	11th to 20th	Night	18% 14% 29% 26% 11% 2	760
	21st to end	Night	28% 15% 24% 24% 7% 2	836

Page generated at 2024-10-20 23:20:06. Version ac07

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