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 2024-11-28.

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	19% 11% 3 8% 5% 4 4 3 5% 6% 31%	101840
	Night	19% 11% 3 8% 5% 4 4 3 5% 6% 32%	97757
Transparency	Day	54% 3 11% 21% 10% 1	105394
	Night	53% 3 11% 21% 10% 1	102293
Seeing	Day	29% 18% 30% 17% 5% 1	16804
	Night	26% 16% 32% 20% 5%	24444

By Month

Percentage Cloud Cover NE (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	14% 14% 5% 11% 5% 4 4 3 4 5% 32%	2207
	11th to 20th	Day	13% 16% 6% 10% 6% 4 3 3 3 4 32%	2376
	21st to end	Day	12% 12% 4 11% 6% 4 4 3 5% 7% 31%	3124
February	1st to 10th	Day	16% 14% 5% 8% 6% 4 4 3 5% 5% 30%	1710
	11th to 20th	Day	16% 15% 5% 10% 6% 5% 3 4 4 5% 28%	1663
	21st to end	Day	12% 14% 3 9% 5% 3 4 4 5% 7% 33%	1446
March	1st to 10th	Day	24% 17% 4 7% 4 4 4 3 4 4 26%	3386
	11th to 20th	Day	17% 23% 5% 9% 5% 5% 4 3 4 4 21%	3480
	21st to end	Day	28% 17% 5% 7% 4 4 3 3 5% 4 21%	3816
April	1st to 10th	Day	22% 16% 4 8% 5% 5% 3 3 4 5% 25%	2237
	11th to 20th	Day	20% 13% 3 7% 5% 4 3 3 4 6% 32%	2409
	21st to end	Day	24% 13% 3 7% 4 4 4 3 5% 6% 29%	2470
May	1st to 10th	Day	22% 19% 3 8% 4 4 4 2 4 6% 24%	4186
	11th to 20th	Day	19% 9% 3 7% 5% 4 4 4 6% 7% 32%	4398
	21st to end	Day	18% 9% 3 8% 6% 5% 5% 4 7% 8% 26%	4836
June	1st to 10th	Day	28% 10% 3 6% 4 4 4 4 5% 6% 26%	2830
	11th to 20th	Day	22% 8% 2 6% 5% 5% 5% 4 5% 7% 31%	2850
	21st to end	Day	28% 11% 3 7% 4 4 4 3 5% 6% 26%	2850
July	1st to 10th	Day	16% 8% 2 7% 5% 6% 6% 4 6% 9% 31%	4760
	11th to 20th	Day	18% 6% 3 8% 5% 4 5% 4 6% 8% 33%	4711
	21st to end	Day	15% 7% 3 8% 5% 5% 5% 5% 7% 8% 34%	4658
August	1st to 10th	Day	21% 9% 3 7% 5% 5% 5% 3 6% 7% 30%	4105
	11th to 20th	Day	15% 6% 3 7% 5% 6% 6% 4 6% 8% 35%	4045
	21st to end	Day	14% 5% 2 5% 4 3 4 4 5% 8% 44%	4450
September	1st to 10th	Day	16% 6% 2 6% 4 4 4 3 5% 7% 43%	2280
	11th to 20th	Day	18% 6% 3 6% 4 3 4 3 4 5% 43%	2150
	21st to end	Day	18% 7% 2 6% 4 3 3 2 4 6% 43%	2066
October	1st to 10th	Day	18% 7% 3 6% 4 3 3 3 6% 7% 40%	1900
	11th to 20th	Day	20% 9% 2 6% 4 4 4 3 4 5% 38%	1900
	21st to end	Day	20% 10% 3 8% 5% 5% 4 3 5% 6% 33%	2042
November	1st to 10th	Day	19% 11% 3 7% 6% 5% 4 3 3 6% 33%	1605
	11th to 20th	Day	20% 14% 4 7% 4 3 4 2 4 6% 32%	1600
	21st to end	Day	20% 16% 4 7% 5% 4 3 3 4 6% 29%	1576
December	1st to 10th	Day	26% 13% 4 7% 3 3 3 2 4 5% 28%	1520
	11th to 20th	Day	19% 19% 5% 10% 5% 3 3 2 4 4 27%	1767
	21st to end	Day	20% 14% 5% 10% 5% 3 3 3 4 4 30%	2431
January	1st to 10th	Night	13% 12% 4 8% 5% 4 4 3 5% 6% 35%	4748
	11th to 20th	Night	16% 14% 4 10% 6% 4 4 3 4 5% 29%	4584
	21st to end	Night	13% 13% 5% 9% 8% 3 3 3 6% 6% 31%	4532
February	1st to 10th	Night	16% 13% 4 9% 5% 4 4 4 5% 6% 29%	2470
	11th to 20th	Night	17% 15% 4 10% 5% 4 4 2 5% 5% 29%	2403
	21st to end	Night	13% 15% 4 9% 5% 4 4 3 6% 5% 33%	2007
March	1st to 10th	Night	26% 14% 3 11% 5% 4 4 3 3 4 24%	3454
	11th to 20th	Night	20% 21% 5% 9% 6% 5% 4 3 4 4 19%	3479
	21st to end	Night	29% 12% 4 9% 4 3 4 3 4 3 24%	3816
April	1st to 10th	Night	23% 15% 4 7% 4 4 4 3 4 4 27%	1943
	11th to 20th	Night	23% 11% 2 6% 4 4 4 3 4 6% 33%	1717
	21st to end	Night	22% 13% 3 6% 4 5% 3 3 4 6% 31%	1710
May	1st to 10th	Night	23% 15% 2 6% 4 4 5% 4 5% 6% 26%	2755
	11th to 20th	Night	19% 8% 2 5% 3 4 5% 3 5% 8% 38%	2506
	21st to end	Night	17% 8% 3 9% 5% 5% 4 4 6% 8% 30%	2749
June	1st to 10th	Night	26% 8% 2 6% 4 4 4 3 6% 8% 29%	1365
	11th to 20th	Night	20% 9% 2 7% 5% 5% 4 3 5% 7% 35%	1330
	21st to end	Night	25% 10% 3 6% 5% 5% 4 4 5% 5% 28%	1330
July	1st to 10th	Night	16% 6% 4 8% 5% 5% 5% 4 7% 7% 32%	2210
	11th to 20th	Night	17% 8% 3 6% 4 5% 5% 4 7% 8% 32%	2210
	21st to end	Night	17% 6% 3 6% 5% 5% 5% 4 6% 7% 37%	2898
August	1st to 10th	Night	20% 9% 4 8% 5% 6% 5% 4 6% 7% 27%	2825
	11th to 20th	Night	12% 7% 3 8% 5% 5% 5% 5% 6% 8% 36%	2782
	21st to end	Night	13% 5% 2 5% 4 3 4 3 5% 7% 50%	3206
September	1st to 10th	Night	17% 6% 3 6% 3 4 4 2 5% 8% 44%	1900
	11th to 20th	Night	17% 7% 2 5% 3 3 3 3 3 7% 46%	2028
	21st to end	Night	17% 8% 2 6% 3 4 3 4 4 5% 44%	2114
October	1st to 10th	Night	20% 8% 3 6% 4 3 3 3 4 8% 39%	2280
	11th to 20th	Night	17% 9% 3 7% 4 4 4 3 5% 6% 38%	2280
	21st to end	Night	23% 10% 2 8% 5% 3 3 3 4 5% 34%	2555
November	1st to 10th	Night	21% 9% 3 6% 4 4 4 4 5% 6% 35%	2692
	11th to 20th	Night	19% 13% 4 8% 5% 4 4 3 4 6% 31%	2800
	21st to end	Night	21% 14% 3 8% 4 5% 3 3 4 5% 31%	2759
December	1st to 10th	Night	24% 12% 4 9% 5% 4 4 3 4 4 27%	2660
	11th to 20th	Night	23% 15% 5% 8% 4 3 4 3 3 4 28%	3424
	21st to end	Night	22% 15% 5% 10% 4 3 3 3 3 4 28%	5236

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	56% 2 14% 16% 11% 1	2208
	11th to 20th	Day	58% 3 4 20% 14% 1	2376
	21st to end	Day	55% 2 12% 15% 15% 1	3124
February	1st to 10th	Day	57% 3 10% 18% 11% 2	1710
	11th to 20th	Day	60% 3 8% 16% 10% 1	1671
	21st to end	Day	50% 3 9% 23% 12% 3	1446
March	1st to 10th	Day	65% 2 7% 17% 7% 3	3390
	11th to 20th	Day	68% 1 7% 15% 7% 1	3480
	21st to end	Day	68% 1 4 15% 9% 2	3816
April	1st to 10th	Day	63% 2 7% 15% 11% 2	2237
	11th to 20th	Day	55% 3 7% 21% 12% 2	2409
	21st to end	Day	57% 3 9% 16% 12% 4	2470
May	1st to 10th	Day	65% 2 6% 19% 8% 1	4200
	11th to 20th	Day	53% 2 12% 24% 8%	4402
	21st to end	Day	55% 4 12% 18% 10%	4836
June	1st to 10th	Day	59% 3 10% 20% 8%	2830
	11th to 20th	Day	53% 3 11% 22% 10% 1	2850
	21st to end	Day	60% 3 10% 17% 8% 1	2850
July	1st to 10th	Day	49% 5% 15% 21% 10%	4685
	11th to 20th	Day	47% 5% 15% 25% 8%	4630
	21st to end	Day	46% 7% 15% 25% 7%	4510
August	1st to 10th	Day	53% 5% 11% 25% 6%	4092
	11th to 20th	Day	46% 5% 13% 25% 11%	4084
	21st to end	Day	37% 4 14% 27% 17% 1	4450
September	1st to 10th	Day	40% 4 15% 27% 14%	2280
	11th to 20th	Day	43% 3 11% 26% 15% 1	2150
	21st to end	Day	44% 2 9% 27% 16% 1	2066
October	1st to 10th	Day	44% 5% 10% 30% 10%	3160
	11th to 20th	Day	47% 3 8% 33% 9%	3140
	21st to end	Day	43% 3 14% 22% 18%	3344
November	1st to 10th	Day	55% 3 9% 22% 11% 1	1605
	11th to 20th	Day	56% 3 12% 17% 11% 1	1600
	21st to end	Day	59% 4 12% 16% 8%	1576
December	1st to 10th	Day	61% 3 11% 18% 7%	1520
	11th to 20th	Day	64% 2 7% 17% 9% 1	1766
	21st to end	Day	62% 2 11% 16% 7% 2	2431
January	1st to 10th	Night	51% 2 11% 27% 8% 1	4748
	11th to 20th	Night	58% 3 8% 21% 9% 1	4584
	21st to end	Night	55% 3 10% 23% 9% 1	4532
February	1st to 10th	Night	56% 4 10% 17% 12% 2	2470
	11th to 20th	Night	58% 3 8% 17% 13% 1	2406
	21st to end	Night	53% 3 9% 19% 14% 2	2007
March	1st to 10th	Night	67% 2 6% 15% 8% 2	3463
	11th to 20th	Night	69% 2 6% 14% 8% 1	3480
	21st to end	Night	66% 2 4 16% 10% 2	3816
April	1st to 10th	Night	61% 2 7% 15% 11% 3	1943
	11th to 20th	Night	54% 3 8% 22% 13% 1	1717
	21st to end	Night	57% 2 8% 16% 15% 2	1710
May	1st to 10th	Night	61% 2 12% 16% 8% 1	2761
	11th to 20th	Night	46% 2 17% 25% 8% 1	2507
	21st to end	Night	53% 3 18% 18% 7%	2750
June	1st to 10th	Night	54% 4 12% 21% 8%	1365
	11th to 20th	Night	50% 3 11% 25% 10% 1	1330
	21st to end	Night	56% 5% 12% 17% 8% 1	1330
July	1st to 10th	Night	49% 4 14% 21% 11%	2175
	11th to 20th	Night	47% 6% 17% 21% 8%	2168
	21st to end	Night	43% 7% 14% 25% 11%	2807
August	1st to 10th	Night	54% 6% 13% 21% 6%	2813
	11th to 20th	Night	44% 6% 17% 23% 10% 1	2809
	21st to end	Night	35% 4 17% 28% 15% 2	3205
September	1st to 10th	Night	40% 4 11% 31% 13%	1900
	11th to 20th	Night	41% 3 13% 28% 15% 1	2028
	21st to end	Night	42% 3 11% 28% 14% 2	2114
October	1st to 10th	Night	49% 4 10% 27% 10%	3800
	11th to 20th	Night	40% 4 11% 31% 14%	3779
	21st to end	Night	45% 3 14% 22% 16%	4203
November	1st to 10th	Night	50% 4 12% 21% 11% 1	2692
	11th to 20th	Night	57% 3 11% 18% 10% 1	2800
	21st to end	Night	57% 4 13% 17% 9%	2759
December	1st to 10th	Night	61% 3 10% 18% 7%	2660
	11th to 20th	Night	63% 3 6% 19% 10%	3426
	21st to end	Night	63% 2 10% 17% 7% 1	5236

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	32% 22% 34% 11% 1	180
	11th to 20th	Day	41% 21% 24% 11% 2	222
	21st to end	Day	23% 19% 38% 16% 3 1	407
February	1st to 10th	Day	28% 24% 30% 13% 4 1	370
	11th to 20th	Day	33% 30% 24% 10% 4	366
	21st to end	Day	26% 27% 31% 13% 4	323
March	1st to 10th	Day	37% 25% 26% 10% 1 1	460
	11th to 20th	Day	30% 22% 31% 12% 3 1	460
	21st to end	Day	35% 24% 27% 9% 4 1	506
April	1st to 10th	Day	43% 21% 23% 10% 2	460
	11th to 20th	Day	35% 22% 27% 13% 3	452
	21st to end	Day	34% 22% 25% 13% 5% 1	460
May	1st to 10th	Day	40% 18% 25% 13% 3 1	504
	11th to 20th	Day	30% 16% 28% 19% 6% 1	674
	21st to end	Day	29% 19% 32% 16% 4	735
June	1st to 10th	Day	37% 15% 28% 15% 6%	680
	11th to 20th	Day	28% 15% 32% 19% 6% 1	680
	21st to end	Day	30% 12% 30% 23% 5% 1	680
July	1st to 10th	Day	23% 13% 35% 20% 8% 1	670
	11th to 20th	Day	19% 8% 37% 26% 8% 1	668
	21st to end	Day	21% 11% 35% 24% 7% 1	645
August	1st to 10th	Day	20% 11% 37% 26% 6% 1	486
	11th to 20th	Day	20% 11% 32% 26% 9% 1	479
	21st to end	Day	19% 14% 32% 25% 8% 2	539
September	1st to 10th	Day	15% 16% 42% 18% 8% 2	490
	11th to 20th	Day	24% 14% 29% 22% 8% 1	490
	21st to end	Day	23% 16% 34% 17% 8% 3	477
October	1st to 10th	Day	23% 16% 27% 19% 12% 2	390
	11th to 20th	Day	27% 19% 27% 18% 7% 1	390
	21st to end	Day	30% 21% 30% 12% 6% 1	429
November	1st to 10th	Day	29% 20% 31% 13% 6% 1	395
	11th to 20th	Day	36% 21% 25% 13% 5% 1	400
	21st to end	Day	37% 22% 26% 11% 3	391
December	1st to 10th	Day	38% 20% 24% 14% 3 1	370
	11th to 20th	Day	41% 23% 25% 8% 2	278
	21st to end	Day	35% 21% 32% 10% 2	198
January	1st to 10th	Night	28% 23% 32% 15% 3	850
	11th to 20th	Night	29% 21% 29% 17% 4	819
	21st to end	Night	23% 21% 34% 17% 4 1	836
February	1st to 10th	Night	27% 22% 33% 16% 3 1	760
	11th to 20th	Night	31% 22% 32% 12% 3	743
	21st to end	Night	23% 21% 34% 18% 4	628
March	1st to 10th	Night	28% 22% 33% 15% 2	760
	11th to 20th	Night	25% 19% 32% 18% 6%	760
	21st to end	Night	28% 18% 32% 18% 4	836
April	1st to 10th	Night	32% 21% 27% 17% 3	613
	11th to 20th	Night	30% 18% 30% 17% 5%	564
	21st to end	Night	29% 18% 29% 19% 5% 1	570
May	1st to 10th	Night	34% 14% 27% 21% 4 1	570
	11th to 20th	Night	30% 13% 29% 22% 6%	562
	21st to end	Night	22% 15% 41% 18% 4	616
June	1st to 10th	Night	28% 9% 35% 23% 5%	415
	11th to 20th	Night	21% 12% 35% 27% 6% 1	380
	21st to end	Night	26% 7% 36% 26% 5% 1	380
July	1st to 10th	Night	21% 6% 42% 24% 7%	370
	11th to 20th	Night	17% 7% 37% 31% 7% 1	368
	21st to end	Night	19% 8% 36% 30% 6%	623
August	1st to 10th	Night	17% 9% 37% 29% 8%	568
	11th to 20th	Night	18% 9% 34% 28% 11%	562
	21st to end	Night	15% 10% 36% 30% 10% 1	627
September	1st to 10th	Night	20% 10% 38% 26% 6%	570
	11th to 20th	Night	21% 9% 31% 30% 9% 1	698
	21st to end	Night	22% 13% 29% 27% 9% 1	760
October	1st to 10th	Night	24% 10% 31% 26% 9% 1	760
	11th to 20th	Night	23% 16% 34% 20% 7%	760
	21st to end	Night	30% 20% 28% 18% 3	836
November	1st to 10th	Night	29% 16% 31% 18% 6%	859
	11th to 20th	Night	31% 18% 29% 16% 6% 1	900
	21st to end	Night	32% 19% 30% 16% 3	886
December	1st to 10th	Night	32% 19% 27% 17% 4	850
	11th to 20th	Night	33% 22% 28% 13% 3	850
	21st to end	Night	34% 21% 25% 16% 3 1	935

Page generated at 2024-11-25 01:22:22. Version ac07

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