B. Williams Mem. Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the B. Williams Mem. Observatory clear sky chart from 2005-11-06 to 2026-08-30.

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	14% 10% 4 9% 6% 5% 5% 4 5% 7% 31%	145027
	Night	13% 11% 4 10% 6% 5% 5% 4 5% 6% 31%	137159
Transparency	Day	51% 6% 12% 20% 11% 1	148620
	Night	51% 6% 12% 19% 10% 1	141655
Seeing	Day	20% 17% 31% 20% 9% 3	17134
	Night	19% 15% 31% 25% 10% 1	26026

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% 12% 5% 16% 10% 7% 6% 4 5% 5% 16%	4661
	11th to 20th	Day	9% 19% 8% 19% 10% 8% 6% 3 4 4 10%	5103
	21st to end	Day	13% 16% 7% 15% 8% 7% 6% 4 5% 6% 13%	5780
February	1st to 10th	Day	13% 16% 5% 14% 8% 7% 6% 4 5% 4 18%	1890
	11th to 20th	Day	11% 15% 7% 14% 8% 7% 5% 4 5% 5% 19%	1843
	21st to end	Day	16% 17% 4 12% 6% 4 5% 4 4 5% 23%	1611
March	1st to 10th	Day	16% 11% 6% 10% 7% 4 4 3 4 5% 29%	5042
	11th to 20th	Day	10% 15% 5% 11% 5% 5% 4 3 5% 7% 29%	5090
	21st to end	Day	17% 14% 4 9% 5% 4 4 3 4 4 31%	5586
April	1st to 10th	Day	16% 15% 5% 9% 6% 4 4 4 5% 5% 28%	2470
	11th to 20th	Day	19% 12% 3 7% 4 4 4 4 5% 6% 34%	2601
	21st to end	Day	19% 10% 3 8% 4 4 4 3 5% 6% 33%	2730
May	1st to 10th	Day	16% 9% 4 8% 4 4 5% 4 5% 7% 34%	7642
	11th to 20th	Day	14% 10% 3 8% 4 5% 5% 4 5% 8% 35%	8158
	21st to end	Day	16% 7% 2 6% 4 4 4 3 5% 7% 43%	9060
June	1st to 10th	Day	15% 6% 2 6% 5% 4 5% 5% 6% 8% 37%	2940
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 4 8% 10% 39%	3024
	21st to end	Day	13% 6% 3 6% 5% 5% 5% 4 6% 9% 38%	3150
July	1st to 10th	Day	11% 7% 3 7% 5% 6% 5% 5% 6% 9% 36%	8709
	11th to 20th	Day	7% 4 2 6% 4 4 5% 4 6% 10% 47%	8280
	21st to end	Day	9% 6% 2 6% 5% 5% 5% 4 6% 8% 44%	8405
August	1st to 10th	Day	13% 8% 3 8% 4 5% 5% 4 7% 8% 36%	4364
	11th to 20th	Day	10% 8% 3 8% 6% 5% 5% 5% 8% 10% 33%	4304
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 4 6% 9% 41%	4635
September	1st to 10th	Day	12% 7% 3 8% 5% 4 4 3 7% 7% 40%	2400
	11th to 20th	Day	11% 5% 2 7% 5% 5% 5% 3 5% 8% 44%	2265
	21st to end	Day	14% 6% 3 8% 7% 6% 5% 4 6% 7% 34%	2170
October	1st to 10th	Day	17% 7% 4 9% 5% 4 5% 3 4 7% 35%	2000
	11th to 20th	Day	18% 10% 4 10% 6% 6% 5% 4 5% 5% 29%	2000
	21st to end	Day	24% 15% 4 9% 5% 5% 4 3 5% 6% 21%	2200
November	1st to 10th	Day	14% 13% 5% 9% 6% 5% 4 4 5% 8% 28%	1760
	11th to 20th	Day	11% 16% 4 11% 7% 7% 6% 4 6% 6% 23%	1680
	21st to end	Day	16% 18% 6% 13% 7% 6% 5% 4 6% 5% 16%	1680
December	1st to 10th	Day	18% 21% 6% 13% 6% 6% 5% 4 5% 4 11%	2680
	11th to 20th	Day	16% 20% 6% 14% 5% 5% 4 4 4 4 18%	3066
	21st to end	Day	23% 18% 6% 16% 6% 6% 5% 3 4 4 10%	4048
January	1st to 10th	Night	12% 14% 6% 15% 10% 6% 5% 4 5% 5% 18%	8234
	11th to 20th	Night	10% 17% 7% 18% 12% 7% 6% 4 4 4 10%	7777
	21st to end	Night	11% 13% 6% 13% 9% 7% 6% 5% 6% 6% 18%	8401
February	1st to 10th	Night	11% 16% 5% 12% 7% 5% 5% 4 6% 6% 23%	2730
	11th to 20th	Night	11% 16% 7% 12% 7% 5% 6% 4 5% 6% 22%	2663
	21st to end	Night	12% 14% 4 11% 6% 7% 5% 5% 6% 6% 25%	2188
March	1st to 10th	Night	15% 11% 5% 10% 6% 6% 5% 4 5% 6% 27%	5018
	11th to 20th	Night	9% 11% 4 8% 5% 5% 5% 5% 6% 8% 34%	5089
	21st to end	Night	17% 9% 4 9% 6% 4 4 3 5% 6% 33%	5588
April	1st to 10th	Night	17% 12% 4 10% 5% 4 4 4 5% 6% 29%	2149
	11th to 20th	Night	17% 9% 3 6% 4 5% 5% 3 5% 7% 37%	1986
	21st to end	Night	14% 10% 3 7% 5% 4 4 4 5% 6% 37%	1890
May	1st to 10th	Night	17% 8% 3 6% 5% 4 4 3 6% 7% 38%	5257
	11th to 20th	Night	14% 7% 3 7% 5% 5% 4 3 6% 7% 40%	4698
	21st to end	Night	17% 6% 2 5% 3 3 3 3 5% 7% 46%	5114
June	1st to 10th	Night	13% 6% 2 6% 4 5% 5% 4 5% 9% 42%	1680
	11th to 20th	Night	9% 4 2 7% 4 5% 5% 5% 7% 9% 42%	1587
	21st to end	Night	11% 5% 3 7% 3 5% 4 4 5% 8% 45%	1470
July	1st to 10th	Night	6% 7% 3 7% 5% 4 5% 4 5% 8% 47%	4241
	11th to 20th	Night	9% 4 2 4 3 4 4 4 5% 8% 52%	4680
	21st to end	Night	10% 5% 2 6% 4 4 4 4 6% 7% 49%	5668
August	1st to 10th	Night	9% 6% 3 6% 4 4 4 3 6% 7% 48%	3005
	11th to 20th	Night	11% 5% 3 5% 3 4 4 4 5% 8% 48%	2965
	21st to end	Night	11% 6% 2 5% 4 4 3 4 6% 8% 47%	3427
September	1st to 10th	Night	8% 5% 2 5% 4 4 4 4 7% 8% 47%	2000
	11th to 20th	Night	10% 4 1 4 4 3 3 3 5% 6% 55%	2131
	21st to end	Night	16% 8% 2 8% 4 4 3 3 4 6% 42%	2230
October	1st to 10th	Night	16% 7% 3 7% 5% 4 4 3 5% 7% 40%	2400
	11th to 20th	Night	17% 9% 3 8% 5% 4 5% 4 6% 6% 33%	2400
	21st to end	Night	19% 12% 4 8% 5% 4 5% 5% 5% 5% 27%	2640
November	1st to 10th	Night	15% 11% 3 8% 5% 4 5% 3 6% 6% 34%	2755
	11th to 20th	Night	15% 14% 4 9% 6% 5% 5% 5% 6% 6% 27%	2940
	21st to end	Night	16% 16% 6% 10% 6% 6% 5% 4 5% 6% 20%	2940
December	1st to 10th	Night	17% 17% 6% 14% 7% 6% 6% 5% 6% 5% 13%	4720
	11th to 20th	Night	13% 18% 6% 14% 7% 5% 5% 4 5% 5% 18%	5348
	21st to end	Night	21% 18% 7% 16% 6% 5% 4 3 3 4 11%	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	67% 7% 12% 10% 4	4662
	11th to 20th	Day	76% 6% 7% 7% 3	5137
	21st to end	Day	70% 4 11% 8% 6%	5780
February	1st to 10th	Day	68% 6% 8% 14% 4	1890
	11th to 20th	Day	66% 6% 9% 14% 5% 1	1852
	21st to end	Day	63% 5% 9% 16% 7% 1	1611
March	1st to 10th	Day	57% 4 13% 13% 12%	5045
	11th to 20th	Day	54% 5% 13% 19% 7% 2	5090
	21st to end	Day	55% 5% 8% 19% 12% 2	5586
April	1st to 10th	Day	57% 5% 9% 19% 9% 1	2470
	11th to 20th	Day	50% 5% 8% 22% 14% 1	2601
	21st to end	Day	51% 5% 11% 19% 12% 2	2730
May	1st to 10th	Day	48% 6% 12% 19% 11% 4	7651
	11th to 20th	Day	47% 7% 10% 19% 16% 1	8162
	21st to end	Day	42% 4 10% 26% 16% 2	9061
June	1st to 10th	Day	42% 7% 12% 21% 16% 1	2940
	11th to 20th	Day	38% 7% 14% 23% 16% 1	3024
	21st to end	Day	42% 7% 13% 24% 14% 1	3150
July	1st to 10th	Day	42% 7% 14% 23% 12% 1	8638
	11th to 20th	Day	31% 6% 12% 36% 15% 1	8205
	21st to end	Day	36% 6% 13% 30% 14% 1	8219
August	1st to 10th	Day	44% 6% 14% 23% 12% 1	4351
	11th to 20th	Day	42% 8% 16% 24% 9%	4343
	21st to end	Day	38% 6% 14% 32% 10%	4635
September	1st to 10th	Day	42% 7% 12% 27% 10% 2	2400
	11th to 20th	Day	37% 6% 14% 30% 13%	2265
	21st to end	Day	48% 6% 12% 25% 8%	2170
October	1st to 10th	Day	43% 5% 13% 27% 12% 1	3260
	11th to 20th	Day	44% 6% 15% 23% 13%	3240
	21st to end	Day	59% 6% 13% 16% 6%	3538
November	1st to 10th	Day	54% 6% 14% 18% 7%	1760
	11th to 20th	Day	60% 7% 13% 13% 7% 1	1680
	21st to end	Day	69% 6% 12% 11% 2	1680
December	1st to 10th	Day	75% 6% 10% 6% 2	2680
	11th to 20th	Day	68% 8% 10% 8% 6%	3066
	21st to end	Day	77% 6% 6% 7% 4	4048
January	1st to 10th	Night	65% 8% 13% 12% 3	8234
	11th to 20th	Night	75% 6% 8% 8% 3	7818
	21st to end	Night	63% 6% 11% 10% 9%	8401
February	1st to 10th	Night	61% 5% 12% 17% 5%	2730
	11th to 20th	Night	60% 7% 10% 17% 6%	2665
	21st to end	Night	57% 6% 14% 16% 7%	2188
March	1st to 10th	Night	57% 5% 10% 18% 8% 1	5028
	11th to 20th	Night	45% 6% 18% 21% 8% 2	5090
	21st to end	Night	51% 4 11% 19% 12% 2	5588
April	1st to 10th	Night	55% 5% 10% 18% 12% 1	2149
	11th to 20th	Night	48% 6% 9% 21% 15% 2	1986
	21st to end	Night	47% 5% 12% 19% 15% 2	1890
May	1st to 10th	Night	46% 5% 11% 18% 15% 5%	5264
	11th to 20th	Night	43% 7% 11% 23% 15% 1	4699
	21st to end	Night	39% 4 11% 28% 16% 2	5115
June	1st to 10th	Night	39% 6% 11% 24% 18% 2	1680
	11th to 20th	Night	35% 8% 13% 25% 18% 1	1587
	21st to end	Night	37% 6% 14% 26% 16% 1	1470
July	1st to 10th	Night	35% 6% 16% 28% 14% 2	4204
	11th to 20th	Night	29% 5% 11% 34% 19% 1	4632
	21st to end	Night	33% 6% 11% 30% 18% 1	5539
August	1st to 10th	Night	36% 4 14% 29% 15% 2	2994
	11th to 20th	Night	34% 6% 16% 30% 13% 1	2992
	21st to end	Night	34% 6% 14% 32% 12% 1	3427
September	1st to 10th	Night	33% 7% 17% 29% 13% 1	2000
	11th to 20th	Night	29% 5% 14% 35% 17% 1	2131
	21st to end	Night	44% 5% 13% 27% 11% 1	2230
October	1st to 10th	Night	34% 5% 9% 29% 21% 1	3920
	11th to 20th	Night	42% 5% 15% 24% 13%	3899
	21st to end	Night	45% 8% 16% 22% 8%	4252
November	1st to 10th	Night	49% 6% 14% 21% 8% 1	2755
	11th to 20th	Night	56% 7% 14% 15% 7% 1	2940
	21st to end	Night	64% 8% 15% 12% 1	2940
December	1st to 10th	Night	69% 8% 11% 10% 2	4720
	11th to 20th	Night	67% 6% 11% 10% 6%	5348
	21st to end	Night	76% 5% 8% 7% 4	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	20% 23% 38% 15% 3 1	220
	11th to 20th	Day	23% 29% 31% 12% 4 1	303
	21st to end	Day	25% 25% 31% 17% 2	363
February	1st to 10th	Day	26% 29% 30% 11% 3	330
	11th to 20th	Day	25% 28% 40% 6% 1	330
	21st to end	Day	30% 31% 29% 11%	275
March	1st to 10th	Day	24% 21% 37% 15% 4	540
	11th to 20th	Day	24% 24% 32% 15% 4 1	540
	21st to end	Day	30% 21% 28% 15% 5% 1	594
April	1st to 10th	Day	27% 23% 31% 13% 6%	540
	11th to 20th	Day	33% 18% 25% 14% 7% 3	532
	21st to end	Day	27% 17% 31% 16% 7% 1	540
May	1st to 10th	Day	20% 16% 35% 18% 9% 2	543
	11th to 20th	Day	17% 16% 31% 22% 10% 4	567
	21st to end	Day	17% 15% 30% 23% 11% 4	617
June	1st to 10th	Day	17% 14% 32% 26% 9% 2	570
	11th to 20th	Day	15% 13% 35% 24% 11% 2	654
	21st to end	Day	16% 12% 36% 24% 9% 3	780
July	1st to 10th	Day	10% 9% 30% 31% 16% 4	720
	11th to 20th	Day	12% 10% 27% 28% 15% 7%	564
	21st to end	Day	10% 8% 33% 30% 13% 7%	620
August	1st to 10th	Day	10% 8% 25% 31% 16% 10%	566
	11th to 20th	Day	8% 8% 30% 26% 21% 8%	559
	21st to end	Day	15% 9% 28% 26% 16% 7%	619
September	1st to 10th	Day	15% 11% 26% 25% 16% 8%	530
	11th to 20th	Day	10% 8% 24% 23% 20% 16%	530
	21st to end	Day	15% 13% 27% 24% 15% 5%	530
October	1st to 10th	Day	22% 13% 27% 20% 14% 5%	530
	11th to 20th	Day	21% 22% 33% 16% 7% 2	530
	21st to end	Day	29% 21% 29% 17% 4	583
November	1st to 10th	Day	25% 24% 29% 17% 4 1	293
	11th to 20th	Day	27% 20% 35% 13% 5% 1	220
	21st to end	Day	29% 23% 37% 10% 1	220
December	1st to 10th	Day	26% 29% 30% 15% 1	220
	11th to 20th	Day	29% 28% 27% 13% 2 1	220
	21st to end	Day	26% 27% 35% 10% 2 1	242
January	1st to 10th	Night	22% 23% 34% 16% 4	840
	11th to 20th	Night	24% 22% 31% 19% 4	840
	21st to end	Night	19% 17% 37% 22% 6%	924
February	1st to 10th	Night	20% 21% 38% 19% 2	840
	11th to 20th	Night	21% 21% 38% 18% 3	823
	21st to end	Night	21% 21% 36% 19% 2	692
March	1st to 10th	Night	21% 21% 35% 20% 3	840
	11th to 20th	Night	22% 22% 31% 19% 5%	840
	21st to end	Night	21% 18% 34% 21% 7%	924
April	1st to 10th	Night	26% 17% 34% 21% 2	840
	11th to 20th	Night	21% 15% 35% 19% 8% 1	742
	21st to end	Night	20% 13% 31% 28% 8%	630
May	1st to 10th	Night	18% 8% 30% 32% 10% 2	630
	11th to 20th	Night	18% 8% 32% 31% 10%	435
	21st to end	Night	13% 8% 31% 36% 12%	455
June	1st to 10th	Night	15% 7% 33% 34% 11%	420
	11th to 20th	Night	12% 8% 30% 36% 14%	420
	21st to end	Night	13% 5% 30% 36% 15% 1	420
July	1st to 10th	Night	6% 5% 25% 44% 19% 1	410
	11th to 20th	Night	11% 4 28% 36% 19% 1	408
	21st to end	Night	8% 4 26% 40% 20% 1	660
August	1st to 10th	Night	11% 2 21% 39% 25% 1	627
	11th to 20th	Night	7% 6% 18% 42% 25% 2	624
	21st to end	Night	13% 5% 26% 32% 23% 2	758
September	1st to 10th	Night	12% 8% 27% 29% 21% 4	800
	11th to 20th	Night	12% 6% 22% 29% 24% 6%	800
	21st to end	Night	19% 10% 21% 29% 18% 3	800
October	1st to 10th	Night	19% 9% 30% 25% 16% 2	800
	11th to 20th	Night	24% 15% 31% 22% 8% 1	800
	21st to end	Night	28% 15% 31% 21% 5%	880
November	1st to 10th	Night	24% 17% 30% 22% 8% 1	820
	11th to 20th	Night	22% 23% 28% 20% 6% 1	840
	21st to end	Night	25% 20% 33% 18% 4	840
December	1st to 10th	Night	26% 21% 35% 14% 3 1	840
	11th to 20th	Night	21% 24% 35% 17% 3	840
	21st to end	Night	31% 18% 32% 17% 2	924

Page generated at 2026-08-27 04:07:14. Version ac07

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