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 2026-02-01.

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%	127786
	Night	19% 12% 3 8% 5% 4 4 3 5% 6% 32%	124366
Transparency	Day	54% 3 11% 21% 10% 1	131321
	Night	54% 3 11% 21% 11% 1	128919
Seeing	Day	29% 18% 30% 17% 5% 1	18321
	Night	27% 16% 32% 20% 5%	26077

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% 22% 5% 12% 5% 4 4 2 4 4 25%	3227
	11th to 20th	Day	13% 17% 7% 13% 7% 4 3 2 3 4 27%	3469
	21st to end	Day	10% 11% 4 10% 6% 4 4 4 5% 7% 36%	4565
February	1st to 10th	Day	16% 14% 5% 9% 6% 4 4 3 5% 5% 31%	1800
	11th to 20th	Day	16% 14% 5% 10% 6% 5% 3 4 4 5% 28%	1753
	21st to end	Day	12% 14% 3 9% 5% 3 4 4 5% 7% 33%	1520
March	1st to 10th	Day	23% 16% 4 13% 4 4 3 2 3 4 24%	4858
	11th to 20th	Day	20% 17% 4 7% 4 4 4 3 5% 6% 26%	4980
	21st to end	Day	26% 17% 4 7% 4 4 3 3 5% 5% 22%	5466
April	1st to 10th	Day	22% 16% 4 8% 5% 5% 3 3 4 5% 25%	2355
	11th to 20th	Day	20% 13% 3 7% 5% 4 3 3 4 6% 31%	2537
	21st to end	Day	24% 13% 3 7% 3 4 4 3 5% 6% 29%	2600
May	1st to 10th	Day	27% 16% 3 8% 4 4 4 3 4 6% 22%	5994
	11th to 20th	Day	24% 10% 3 7% 5% 4 4 4 5% 6% 30%	6318
	21st to end	Day	22% 11% 3 7% 5% 4 4 3 5% 7% 27%	6948
June	1st to 10th	Day	27% 10% 3 6% 4 4 4 4 5% 6% 27%	2979
	11th to 20th	Day	22% 8% 2 6% 5% 5% 5% 4 5% 7% 31%	3000
	21st to end	Day	27% 11% 3 7% 4 4 4 3 5% 6% 26%	3000
July	1st to 10th	Day	17% 9% 3 7% 5% 6% 6% 5% 7% 9% 26%	6820
	11th to 20th	Day	15% 6% 2 7% 4 4 4 4 6% 8% 39%	6757
	21st to end	Day	13% 6% 3 7% 5% 5% 6% 4 6% 8% 37%	6700
August	1st to 10th	Day	20% 9% 3 7% 4 5% 5% 3 6% 7% 31%	4235
	11th to 20th	Day	15% 6% 3 7% 5% 6% 5% 4 6% 8% 35%	4175
	21st to end	Day	14% 5% 2 5% 4 4 5% 4 5% 8% 44%	4591
September	1st to 10th	Day	16% 6% 2 6% 4 4 4 3 5% 8% 44%	2400
	11th to 20th	Day	18% 6% 2 6% 4 3 4 3 4 5% 44%	2263
	21st to end	Day	18% 7% 3 6% 4 3 4 2 4 6% 43%	2175
October	1st to 10th	Day	18% 6% 3 6% 4 3 3 3 6% 6% 41%	2000
	11th to 20th	Day	19% 10% 3 7% 4 4 4 3 4 5% 37%	2000
	21st to end	Day	20% 10% 3 8% 5% 5% 4 3 5% 6% 32%	2149
November	1st to 10th	Day	19% 12% 3 7% 5% 5% 4 3 3 5% 33%	1687
	11th to 20th	Day	19% 14% 4 7% 4 3 4 2 4 6% 32%	1680
	21st to end	Day	19% 16% 4 8% 5% 4 4 3 5% 6% 28%	1680
December	1st to 10th	Day	19% 13% 4 8% 4 3 4 3 5% 7% 30%	2690
	11th to 20th	Day	20% 15% 4 9% 4 3 3 2 3 3 33%	2862
	21st to end	Day	17% 13% 5% 11% 5% 4 4 3 4 4 31%	3553
January	1st to 10th	Night	14% 17% 5% 10% 5% 4 4 3 4 5% 28%	6958
	11th to 20th	Night	15% 15% 6% 12% 6% 4 4 3 4 5% 26%	6641
	21st to end	Night	10% 12% 4 8% 6% 4 4 3 5% 6% 39%	6633
February	1st to 10th	Night	16% 13% 4 9% 6% 4 4 4 5% 6% 29%	2601
	11th to 20th	Night	17% 15% 4 9% 5% 4 4 2 5% 5% 29%	2533
	21st to end	Night	13% 15% 4 8% 5% 4 4 3 6% 4 34%	2109
March	1st to 10th	Night	28% 14% 3 10% 5% 4 4 2 3 4 23%	4982
	11th to 20th	Night	22% 16% 4 8% 5% 4 3 4 4 5% 24%	4979
	21st to end	Night	29% 13% 4 8% 4 3 3 3 4 4 25%	5466
April	1st to 10th	Night	23% 15% 4 7% 4 4 4 3 4 4 27%	2045
	11th to 20th	Night	24% 11% 2 6% 4 4 4 3 4 6% 33%	1808
	21st to end	Night	22% 13% 3 6% 4 5% 4 3 4 6% 30%	1800
May	1st to 10th	Night	28% 15% 2 6% 4 4 5% 3 5% 5% 23%	3949
	11th to 20th	Night	24% 8% 2 6% 4 4 5% 3 5% 7% 34%	3596
	21st to end	Night	19% 10% 3 7% 4 4 4 3 5% 7% 34%	3948
June	1st to 10th	Night	25% 8% 2 6% 4 4 4 3 6% 8% 29%	1437
	11th to 20th	Night	20% 8% 2 7% 5% 5% 4 3 5% 7% 35%	1400
	21st to end	Night	24% 10% 3 6% 5% 5% 4 4 5% 5% 29%	1400
July	1st to 10th	Night	18% 7% 3 8% 5% 6% 5% 4 7% 8% 29%	3160
	11th to 20th	Night	15% 7% 3 6% 4 5% 5% 4 6% 8% 36%	3160
	21st to end	Night	17% 6% 2 6% 4 4 4 4 6% 8% 38%	4161
August	1st to 10th	Night	19% 8% 3 8% 5% 6% 5% 4 6% 7% 29%	2915
	11th to 20th	Night	12% 7% 3 8% 5% 5% 5% 5% 6% 8% 37%	2872
	21st to end	Night	13% 5% 2 5% 4 4 4 3 5% 7% 49%	3307
September	1st to 10th	Night	17% 5% 2 5% 3 4 4 2 5% 8% 44%	2000
	11th to 20th	Night	16% 7% 2 5% 3 3 3 3 4 7% 47%	2135
	21st to end	Night	17% 8% 2 5% 3 4 3 4 5% 6% 44%	2225
October	1st to 10th	Night	19% 7% 3 6% 4 3 3 3 4 8% 40%	2400
	11th to 20th	Night	17% 10% 3 7% 4 4 5% 3 5% 6% 37%	2400
	21st to end	Night	23% 10% 2 8% 5% 3 3 3 4 5% 33%	2690
November	1st to 10th	Night	21% 10% 3 6% 4 4 4 4 5% 6% 34%	2830
	11th to 20th	Night	19% 13% 4 8% 5% 4 4 3 4 5% 31%	2940
	21st to end	Night	21% 14% 3 8% 4 4 3 3 3 5% 31%	2940
December	1st to 10th	Night	16% 11% 4 9% 5% 4 4 3 5% 5% 34%	4720
	11th to 20th	Night	22% 13% 4 7% 3 3 3 2 4 4 35%	5559
	21st to end	Night	20% 15% 5% 11% 4 3 3 3 3 4 28%	7667

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	65% 2 11% 13% 8% 1	3228
	11th to 20th	Day	63% 3 8% 16% 10% 1	3496
	21st to end	Day	50% 2 9% 17% 19% 3	4565
February	1st to 10th	Day	57% 3 10% 18% 12% 2	1800
	11th to 20th	Day	60% 3 9% 17% 10% 1	1761
	21st to end	Day	50% 3 9% 23% 12% 3	1520
March	1st to 10th	Day	67% 1 7% 13% 11% 2	4862
	11th to 20th	Day	60% 3 13% 16% 6% 2	4980
	21st to end	Day	66% 2 4 17% 9% 1	5466
April	1st to 10th	Day	63% 2 6% 15% 12% 2	2355
	11th to 20th	Day	56% 3 7% 21% 12% 2	2537
	21st to end	Day	57% 3 9% 16% 12% 3	2600
May	1st to 10th	Day	66% 2 6% 16% 9%	6005
	11th to 20th	Day	57% 3 10% 23% 6%	6322
	21st to end	Day	57% 4 10% 18% 11%	6948
June	1st to 10th	Day	58% 3 9% 20% 9%	2979
	11th to 20th	Day	52% 4 11% 22% 10% 1	3000
	21st to end	Day	59% 3 10% 17% 9% 1	3000
July	1st to 10th	Day	52% 5% 15% 19% 8%	6745
	11th to 20th	Day	42% 5% 15% 28% 9% 1	6676
	21st to end	Day	43% 7% 15% 28% 7%	6509
August	1st to 10th	Day	52% 5% 11% 26% 7%	4222
	11th to 20th	Day	45% 4 13% 25% 12%	4214
	21st to end	Day	37% 4 14% 27% 17% 1	4591
September	1st to 10th	Day	39% 4 15% 27% 15%	2400
	11th to 20th	Day	43% 3 11% 27% 15% 1	2263
	21st to end	Day	44% 2 9% 27% 16% 1	2175
October	1st to 10th	Day	43% 5% 10% 30% 11%	3260
	11th to 20th	Day	47% 3 8% 32% 9%	3240
	21st to end	Day	43% 3 14% 22% 18%	3451
November	1st to 10th	Day	55% 3 9% 22% 10% 1	1687
	11th to 20th	Day	56% 3 13% 18% 11% 1	1680
	21st to end	Day	58% 4 12% 17% 8%	1680
December	1st to 10th	Day	55% 3 13% 22% 6%	2690
	11th to 20th	Day	58% 2 8% 20% 10% 2	2861
	21st to end	Day	59% 2 10% 17% 10% 1	3553
January	1st to 10th	Night	60% 3 9% 22% 6% 1	6958
	11th to 20th	Night	62% 4 9% 18% 7% 1	6689
	21st to end	Night	47% 2 9% 23% 16% 2	6633
February	1st to 10th	Night	56% 3 10% 17% 12% 2	2601
	11th to 20th	Night	57% 3 9% 17% 13% 1	2536
	21st to end	Night	53% 3 8% 19% 14% 2	2109
March	1st to 10th	Night	68% 2 5% 11% 12% 2	4991
	11th to 20th	Night	62% 3 11% 14% 9% 1	4980
	21st to end	Night	65% 2 4 16% 12% 1	5466
April	1st to 10th	Night	61% 2 6% 15% 12% 3	2045
	11th to 20th	Night	54% 2 8% 22% 13% 1	1808
	21st to end	Night	57% 3 8% 16% 14% 2	1800
May	1st to 10th	Night	64% 2 9% 14% 9% 1	3955
	11th to 20th	Night	52% 3 13% 22% 8% 2	3597
	21st to end	Night	51% 4 18% 19% 8%	3949
June	1st to 10th	Night	53% 4 11% 22% 9%	1437
	11th to 20th	Night	49% 3 11% 25% 10% 1	1400
	21st to end	Night	56% 5% 12% 17% 9% 1	1400
July	1st to 10th	Night	52% 5% 13% 21% 9%	3125
	11th to 20th	Night	44% 6% 17% 25% 8%	3118
	21st to end	Night	42% 7% 15% 25% 11%	4039
August	1st to 10th	Night	53% 5% 12% 22% 7%	2903
	11th to 20th	Night	43% 6% 17% 23% 10% 1	2899
	21st to end	Night	35% 4 17% 28% 15% 2	3306
September	1st to 10th	Night	40% 4 11% 32% 14% 1	2000
	11th to 20th	Night	40% 3 13% 28% 15% 1	2135
	21st to end	Night	42% 3 10% 28% 14% 2	2225
October	1st to 10th	Night	48% 4 10% 27% 11%	3920
	11th to 20th	Night	41% 4 11% 30% 14%	3899
	21st to end	Night	46% 3 14% 22% 16%	4338
November	1st to 10th	Night	51% 4 12% 21% 11% 1	2830
	11th to 20th	Night	57% 3 11% 19% 9% 1	2940
	21st to end	Night	56% 4 13% 18% 10%	2940
December	1st to 10th	Night	51% 4 10% 25% 9%	4720
	11th to 20th	Night	55% 3 8% 20% 13% 2	5561
	21st to end	Night	62% 3 9% 17% 9% 1	7667

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	33% 25% 31% 10% 1	220
	11th to 20th	Day	41% 21% 23% 12% 3	267
	21st to end	Day	24% 21% 37% 15% 3 1	473
February	1st to 10th	Day	28% 25% 31% 13% 4 1	400
	11th to 20th	Day	33% 30% 24% 9% 3	396
	21st to end	Day	27% 25% 31% 14% 3	349
March	1st to 10th	Day	37% 25% 26% 11% 1 1	500
	11th to 20th	Day	30% 23% 30% 13% 3 1	500
	21st to end	Day	35% 24% 27% 9% 3 1	550
April	1st to 10th	Day	44% 21% 23% 9% 2	500
	11th to 20th	Day	37% 22% 26% 12% 3	492
	21st to end	Day	35% 23% 25% 12% 5% 1	500
May	1st to 10th	Day	40% 19% 25% 13% 3 1	546
	11th to 20th	Day	30% 16% 29% 18% 6% 1	724
	21st to end	Day	30% 18% 32% 16% 4	790
June	1st to 10th	Day	35% 15% 28% 16% 6%	730
	11th to 20th	Day	27% 16% 33% 18% 6% 1	730
	21st to end	Day	30% 13% 30% 22% 5% 1	730
July	1st to 10th	Day	23% 14% 36% 20% 8% 1	720
	11th to 20th	Day	19% 8% 37% 27% 8% 1	718
	21st to end	Day	20% 12% 37% 24% 7% 1	695
August	1st to 10th	Day	18% 11% 38% 25% 7% 1	526
	11th to 20th	Day	19% 11% 32% 27% 9% 2	519
	21st to end	Day	18% 16% 32% 24% 7% 2	583
September	1st to 10th	Day	14% 16% 40% 18% 8% 2	530
	11th to 20th	Day	23% 14% 29% 23% 8% 2	530
	21st to end	Day	23% 16% 34% 16% 7% 3	516
October	1st to 10th	Day	23% 15% 29% 20% 11% 1	420
	11th to 20th	Day	26% 20% 27% 17% 8% 1	420
	21st to end	Day	31% 21% 28% 13% 6% 1	462
November	1st to 10th	Day	30% 20% 30% 12% 5% 1	425
	11th to 20th	Day	36% 21% 24% 13% 5% 1	430
	21st to end	Day	35% 23% 26% 12% 3 1	430
December	1st to 10th	Day	37% 20% 26% 13% 3	430
	11th to 20th	Day	42% 23% 24% 9% 2	328
	21st to end	Day	34% 21% 32% 11% 2	242
January	1st to 10th	Night	28% 23% 31% 14% 3	950
	11th to 20th	Night	29% 21% 28% 18% 5%	911
	21st to end	Night	24% 21% 35% 16% 4 1	924
February	1st to 10th	Night	26% 22% 32% 16% 3	801
	11th to 20th	Night	32% 22% 31% 12% 3	783
	21st to end	Night	24% 20% 34% 18% 3	660
March	1st to 10th	Night	29% 22% 31% 15% 2	800
	11th to 20th	Night	25% 19% 32% 18% 6%	800
	21st to end	Night	28% 18% 32% 17% 4	880
April	1st to 10th	Night	32% 21% 27% 17% 2	645
	11th to 20th	Night	30% 19% 30% 17% 5%	594
	21st to end	Night	30% 18% 29% 19% 5% 1	600
May	1st to 10th	Night	34% 15% 27% 21% 4 1	600
	11th to 20th	Night	31% 14% 29% 21% 5%	592
	21st to end	Night	22% 15% 41% 18% 4	649
June	1st to 10th	Night	28% 9% 34% 24% 5%	437
	11th to 20th	Night	21% 12% 36% 26% 5% 1	400
	21st to end	Night	26% 7% 36% 27% 5% 1	400
July	1st to 10th	Night	21% 6% 41% 25% 7%	390
	11th to 20th	Night	17% 7% 38% 31% 7% 1	388
	21st to end	Night	20% 8% 37% 30% 6%	656
August	1st to 10th	Night	16% 9% 36% 30% 9%	598
	11th to 20th	Night	18% 8% 34% 29% 11%	592
	21st to end	Night	14% 10% 36% 29% 10% 1	660
September	1st to 10th	Night	19% 10% 37% 27% 7%	600
	11th to 20th	Night	20% 9% 31% 30% 9% 1	735
	21st to end	Night	21% 12% 30% 27% 9% 1	800
October	1st to 10th	Night	23% 10% 33% 25% 8% 1	800
	11th to 20th	Night	24% 15% 33% 21% 7% 1	800
	21st to end	Night	31% 19% 28% 19% 4	880
November	1st to 10th	Night	30% 16% 30% 18% 5%	907
	11th to 20th	Night	31% 17% 29% 16% 5% 1	950
	21st to end	Night	31% 19% 30% 16% 3	950
December	1st to 10th	Night	31% 19% 28% 18% 4	950
	11th to 20th	Night	33% 23% 27% 13% 3	950
	21st to end	Night	33% 22% 26% 16% 3	1045

Page generated at 2026-01-29 02:21:40. Version ac07

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