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 2025-04-26.

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% 11% 4 10% 6% 5% 5% 4 5% 7% 30%	114309
	Night	13% 11% 4 10% 6% 5% 5% 4 5% 6% 30%	114284
Transparency	Day	52% 6% 12% 19% 10% 1	117945
	Night	52% 6% 13% 19% 10% 1	118812
Seeing	Day	20% 17% 31% 20% 9% 4	15316
	Night	19% 15% 31% 24% 10% 1	24463

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	13% 12% 5% 16% 10% 7% 6% 4 5% 5% 17%	3597
	11th to 20th	Day	10% 17% 7% 19% 10% 8% 6% 3 4 4 11%	3911
	21st to end	Day	15% 18% 8% 15% 7% 5% 5% 3 4 5% 14%	4466
February	1st to 10th	Day	13% 16% 5% 14% 9% 7% 6% 4 5% 4 17%	1800
	11th to 20th	Day	11% 15% 7% 15% 8% 7% 5% 4 5% 6% 19%	1753
	21st to end	Day	16% 16% 4 12% 6% 4 5% 4 4 5% 23%	1537
March	1st to 10th	Day	17% 11% 6% 10% 7% 4 4 3 4 5% 29%	4932
	11th to 20th	Day	9% 15% 5% 10% 5% 5% 4 3 6% 7% 29%	4980
	21st to end	Day	17% 14% 4 9% 5% 4 4 3 4 4 32%	5465
April	1st to 10th	Day	15% 16% 5% 8% 6% 4 5% 4 5% 5% 29%	2352
	11th to 20th	Day	18% 12% 3 6% 4 4 4 4 5% 6% 34%	2476
	21st to end	Day	19% 11% 3 8% 5% 4 4 3 5% 6% 33%	2535
May	1st to 10th	Day	16% 9% 3 8% 4 4 5% 4 5% 8% 35%	4108
	11th to 20th	Day	16% 10% 3 8% 4 5% 5% 4 6% 8% 31%	4362
	21st to end	Day	11% 6% 2 6% 5% 4 4 3 6% 7% 45%	4837
June	1st to 10th	Day	16% 7% 2 6% 4 5% 5% 5% 6% 8% 35%	2660
	11th to 20th	Day	12% 6% 2 5% 5% 5% 5% 5% 7% 9% 40%	2736
	21st to end	Day	14% 6% 3 6% 5% 5% 5% 4 6% 9% 38%	2850
July	1st to 10th	Day	10% 5% 2 7% 5% 6% 5% 6% 7% 10% 36%	4674
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 4 7% 9% 42%	4440
	21st to end	Day	8% 6% 2 7% 6% 5% 6% 5% 7% 8% 41%	4503
August	1st to 10th	Day	13% 8% 3 8% 4 5% 5% 5% 7% 8% 35%	4104
	11th to 20th	Day	10% 8% 3 8% 6% 5% 6% 5% 7% 10% 33%	4044
	21st to end	Day	11% 6% 2 7% 5% 5% 5% 4 6% 9% 41%	4382
September	1st to 10th	Day	13% 7% 3 8% 5% 4 4 3 6% 6% 40%	2280
	11th to 20th	Day	11% 6% 2 7% 5% 5% 5% 3 5% 8% 43%	2152
	21st to end	Day	14% 6% 3 8% 7% 7% 5% 4 6% 8% 34%	2062
October	1st to 10th	Day	18% 8% 4 10% 5% 4 5% 3 4 7% 33%	1900
	11th to 20th	Day	19% 10% 3 10% 6% 6% 4 4 5% 5% 28%	1900
	21st to end	Day	25% 16% 4 9% 5% 4 4 3 4 6% 20%	2090
November	1st to 10th	Day	12% 13% 5% 10% 6% 5% 4 4 5% 8% 28%	1675
	11th to 20th	Day	12% 16% 4 11% 7% 7% 6% 4 6% 6% 22%	1600
	21st to end	Day	16% 18% 6% 12% 7% 6% 5% 4 6% 5% 17%	1600
December	1st to 10th	Day	18% 21% 6% 13% 6% 6% 5% 4 5% 5% 11%	2600
	11th to 20th	Day	16% 20% 6% 13% 5% 5% 4 4 4 4 18%	2986
	21st to end	Day	23% 18% 6% 16% 6% 6% 5% 3 4 4 9%	3960
January	1st to 10th	Night	13% 13% 6% 15% 11% 6% 5% 3 5% 6% 18%	6349
	11th to 20th	Night	11% 14% 7% 19% 12% 7% 6% 4 5% 5% 10%	5969
	21st to end	Night	14% 15% 7% 13% 8% 6% 5% 4 5% 6% 19%	6490
February	1st to 10th	Night	12% 17% 5% 12% 8% 5% 5% 4 6% 6% 21%	2600
	11th to 20th	Night	11% 15% 7% 12% 7% 5% 6% 4 5% 6% 22%	2533
	21st to end	Night	12% 13% 4 10% 6% 6% 5% 5% 6% 6% 26%	2087
March	1st to 10th	Night	15% 11% 5% 10% 6% 6% 5% 4 5% 6% 27%	4908
	11th to 20th	Night	9% 11% 4 8% 5% 5% 5% 5% 6% 8% 34%	4979
	21st to end	Night	17% 9% 4 9% 6% 4 4 3 5% 6% 33%	5467
April	1st to 10th	Night	17% 12% 4 9% 5% 4 4 4 5% 6% 30%	2047
	11th to 20th	Night	17% 9% 3 6% 4 5% 5% 3 5% 7% 37%	1890
	21st to end	Night	14% 11% 3 7% 5% 4 4 4 5% 6% 35%	1761
May	1st to 10th	Night	16% 8% 3 6% 5% 4 4 3 5% 6% 40%	2833
	11th to 20th	Night	15% 7% 2 7% 5% 4 5% 3 5% 7% 40%	2527
	21st to end	Night	9% 5% 2 5% 3 4 4 3 5% 8% 50%	2738
June	1st to 10th	Night	13% 6% 2 6% 5% 5% 5% 4 5% 9% 41%	1520
	11th to 20th	Night	9% 3 2 7% 4 5% 5% 5% 7% 9% 43%	1436
	21st to end	Night	11% 5% 2 7% 3 5% 4 4 5% 8% 45%	1330
July	1st to 10th	Night	5% 5% 2 6% 5% 4 5% 4 6% 7% 51%	2289
	11th to 20th	Night	11% 5% 2 4 3 4 4 4 4 7% 52%	2520
	21st to end	Night	6% 5% 2 5% 4 4 5% 4 6% 8% 50%	3046
August	1st to 10th	Night	9% 5% 3 6% 4 4 4 3 6% 7% 48%	2825
	11th to 20th	Night	12% 5% 3 5% 3 4 4 4 5% 9% 47%	2785
	21st to end	Night	11% 6% 2 5% 4 4 3 4 6% 8% 47%	3241
September	1st to 10th	Night	9% 6% 3 5% 4 4 4 4 7% 7% 47%	1900
	11th to 20th	Night	11% 5% 2 4 4 3 3 3 5% 6% 54%	2024
	21st to end	Night	15% 7% 2 8% 5% 4 4 3 5% 6% 41%	2118
October	1st to 10th	Night	17% 7% 3 7% 5% 4 4 4 5% 7% 38%	2280
	11th to 20th	Night	17% 9% 3 9% 5% 4 5% 4 5% 6% 32%	2280
	21st to end	Night	20% 13% 4 8% 5% 4 5% 5% 5% 5% 25%	2508
November	1st to 10th	Night	15% 10% 3 8% 5% 4 5% 3 6% 6% 34%	2620
	11th to 20th	Night	15% 14% 4 8% 6% 5% 5% 5% 6% 6% 27%	2800
	21st to end	Night	16% 16% 5% 10% 6% 6% 5% 4 5% 6% 20%	2800
December	1st to 10th	Night	17% 16% 6% 14% 7% 6% 6% 5% 6% 5% 12%	4580
	11th to 20th	Night	13% 18% 7% 13% 7% 5% 5% 4 5% 5% 17%	5208
	21st to end	Night	21% 18% 7% 16% 6% 5% 5% 3 4 4 11%	6996

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	66% 7% 12% 11% 4	3598
	11th to 20th	Day	74% 6% 9% 8% 3 1	3945
	21st to end	Day	71% 4 12% 8% 5%	4466
February	1st to 10th	Day	69% 6% 8% 14% 3	1800
	11th to 20th	Day	65% 6% 9% 14% 5% 1	1762
	21st to end	Day	63% 5% 9% 16% 7% 1	1537
March	1st to 10th	Day	57% 4 13% 13% 12%	4935
	11th to 20th	Day	53% 5% 13% 19% 7% 2	4980
	21st to end	Day	55% 5% 8% 19% 12% 1	5465
April	1st to 10th	Day	57% 5% 9% 19% 9% 1	2352
	11th to 20th	Day	50% 5% 8% 22% 14% 1	2476
	21st to end	Day	52% 6% 11% 18% 12% 2	2535
May	1st to 10th	Day	46% 7% 11% 20% 15% 1	4120
	11th to 20th	Day	49% 8% 13% 21% 8% 1	4366
	21st to end	Day	37% 5% 14% 22% 18% 3	4837
June	1st to 10th	Day	43% 7% 13% 21% 15% 1	2660
	11th to 20th	Day	38% 7% 14% 23% 17% 1	2736
	21st to end	Day	42% 7% 13% 23% 14% 1	2850
July	1st to 10th	Day	40% 7% 15% 21% 14% 1	4602
	11th to 20th	Day	36% 7% 13% 33% 11% 1	4365
	21st to end	Day	38% 8% 13% 24% 17% 1	4359
August	1st to 10th	Day	45% 6% 14% 23% 11% 1	4091
	11th to 20th	Day	43% 8% 16% 23% 10%	4083
	21st to end	Day	38% 7% 13% 31% 11% 1	4382
September	1st to 10th	Day	43% 7% 12% 26% 10% 2	2280
	11th to 20th	Day	38% 6% 14% 29% 13%	2152
	21st to end	Day	48% 6% 13% 25% 8%	2062
October	1st to 10th	Day	44% 5% 13% 26% 11% 1	3160
	11th to 20th	Day	44% 6% 15% 22% 13%	3140
	21st to end	Day	60% 6% 13% 16% 5%	3428
November	1st to 10th	Day	53% 7% 15% 18% 7%	1675
	11th to 20th	Day	60% 7% 13% 13% 7% 1	1600
	21st to end	Day	69% 6% 13% 11% 2	1600
December	1st to 10th	Day	75% 6% 11% 6% 2	2600
	11th to 20th	Day	68% 8% 10% 8% 6%	2986
	21st to end	Day	77% 6% 5% 7% 4	3960
January	1st to 10th	Night	64% 7% 13% 13% 3	6349
	11th to 20th	Night	73% 7% 10% 8% 2	6010
	21st to end	Night	66% 5% 11% 9% 8% 1	6490
February	1st to 10th	Night	62% 5% 13% 16% 4	2600
	11th to 20th	Night	60% 7% 10% 17% 6%	2535
	21st to end	Night	56% 6% 14% 16% 8%	2087
March	1st to 10th	Night	57% 5% 10% 18% 8% 1	4918
	11th to 20th	Night	44% 6% 18% 21% 8% 2	4980
	21st to end	Night	51% 4 11% 19% 12% 2	5467
April	1st to 10th	Night	55% 5% 10% 18% 12% 1	2047
	11th to 20th	Night	47% 6% 9% 21% 16% 2	1890
	21st to end	Night	49% 5% 12% 17% 15% 2	1761
May	1st to 10th	Night	44% 5% 13% 18% 17% 3	2840
	11th to 20th	Night	42% 7% 13% 23% 12% 2	2528
	21st to end	Night	32% 6% 14% 26% 21% 2	2739
June	1st to 10th	Night	41% 6% 11% 24% 16% 2	1520
	11th to 20th	Night	34% 8% 13% 24% 19% 2	1436
	21st to end	Night	37% 6% 14% 26% 16% 1	1330
July	1st to 10th	Night	32% 5% 15% 26% 19% 3	2252
	11th to 20th	Night	32% 5% 12% 34% 16% 1	2472
	21st to end	Night	31% 8% 14% 31% 15% 2	2949
August	1st to 10th	Night	36% 5% 14% 29% 15% 2	2814
	11th to 20th	Night	35% 6% 17% 29% 13% 1	2812
	21st to end	Night	35% 6% 14% 32% 13% 1	3241
September	1st to 10th	Night	33% 8% 17% 28% 13% 1	1900
	11th to 20th	Night	30% 5% 13% 34% 17% 1	2024
	21st to end	Night	44% 5% 13% 27% 10%	2118
October	1st to 10th	Night	35% 6% 9% 28% 21% 1	3800
	11th to 20th	Night	42% 5% 16% 24% 13%	3779
	21st to end	Night	46% 8% 16% 22% 8%	4120
November	1st to 10th	Night	49% 6% 14% 21% 8% 1	2620
	11th to 20th	Night	56% 7% 14% 15% 7% 1	2800
	21st to end	Night	63% 8% 15% 12% 2 1	2800
December	1st to 10th	Night	69% 8% 11% 9% 2	4580
	11th to 20th	Night	67% 6% 11% 10% 6%	5208
	21st to end	Night	76% 5% 8% 7% 4	6996

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	19% 24% 36% 17% 4 1	200
	11th to 20th	Day	22% 27% 32% 13% 5% 1	275
	21st to end	Day	24% 25% 32% 15% 2	330
February	1st to 10th	Day	27% 31% 28% 10% 3	300
	11th to 20th	Day	25% 29% 39% 5% 1	300
	21st to end	Day	31% 31% 28% 10%	251
March	1st to 10th	Day	23% 21% 37% 15% 4	500
	11th to 20th	Day	21% 24% 34% 16% 4 1	500
	21st to end	Day	30% 20% 28% 16% 5% 1	550
April	1st to 10th	Day	26% 23% 32% 13% 6%	500
	11th to 20th	Day	33% 19% 24% 14% 7% 3	492
	21st to end	Day	26% 18% 30% 17% 7% 2	480
May	1st to 10th	Day	20% 15% 34% 18% 10% 2	463
	11th to 20th	Day	18% 14% 30% 23% 11% 3	487
	21st to end	Day	14% 14% 29% 25% 12% 4	529
June	1st to 10th	Day	18% 14% 29% 27% 9% 2	490
	11th to 20th	Day	14% 12% 33% 25% 12% 2	566
	21st to end	Day	17% 13% 35% 23% 9% 3	680
July	1st to 10th	Day	10% 8% 31% 30% 16% 4	626
	11th to 20th	Day	13% 9% 28% 28% 15% 8%	484
	21st to end	Day	10% 7% 33% 30% 13% 7%	532
August	1st to 10th	Day	10% 9% 27% 29% 16% 10%	486
	11th to 20th	Day	8% 7% 28% 27% 21% 9%	479
	21st to end	Day	15% 9% 27% 26% 16% 7%	539
September	1st to 10th	Day	14% 10% 27% 25% 16% 7%	490
	11th to 20th	Day	10% 9% 25% 22% 19% 15%	490
	21st to end	Day	15% 14% 28% 23% 15% 5%	490
October	1st to 10th	Day	23% 14% 27% 19% 12% 5%	490
	11th to 20th	Day	22% 21% 32% 16% 7% 2	490
	21st to end	Day	30% 21% 29% 16% 4	539
November	1st to 10th	Day	24% 24% 28% 19% 4 1	268
	11th to 20th	Day	26% 21% 35% 14% 5% 1	200
	21st to end	Day	30% 22% 38% 10% 1	200
December	1st to 10th	Day	26% 31% 29% 14% 2	200
	11th to 20th	Day	28% 30% 27% 13% 3 1	200
	21st to end	Day	25% 26% 35% 10% 3 1	220
January	1st to 10th	Night	22% 23% 35% 16% 5%	800
	11th to 20th	Night	24% 21% 32% 19% 4	800
	21st to end	Night	19% 16% 36% 22% 6%	880
February	1st to 10th	Night	21% 21% 38% 18% 1	800
	11th to 20th	Night	21% 21% 38% 17% 3	783
	21st to end	Night	20% 22% 36% 20% 2	660
March	1st to 10th	Night	21% 21% 36% 20% 3	800
	11th to 20th	Night	21% 22% 32% 19% 5% 1	800
	21st to end	Night	21% 18% 33% 21% 7%	880
April	1st to 10th	Night	26% 17% 34% 21% 3	800
	11th to 20th	Night	21% 15% 34% 19% 9% 1	706
	21st to end	Night	20% 13% 32% 26% 8%	587
May	1st to 10th	Night	18% 7% 29% 32% 11% 2	570
	11th to 20th	Night	17% 8% 31% 32% 11% 1	393
	21st to end	Night	10% 8% 32% 36% 14%	411
June	1st to 10th	Night	15% 7% 33% 33% 11%	380
	11th to 20th	Night	12% 8% 29% 36% 14%	380
	21st to end	Night	14% 5% 32% 34% 15% 1	380
July	1st to 10th	Night	6% 5% 26% 42% 20% 1	370
	11th to 20th	Night	12% 4 28% 35% 20% 1	368
	21st to end	Night	8% 4 27% 39% 20% 1	596
August	1st to 10th	Night	11% 3 23% 38% 24% 1	567
	11th to 20th	Night	7% 5% 20% 40% 26% 3	564
	21st to end	Night	13% 5% 26% 32% 22% 2	692
September	1st to 10th	Night	12% 8% 27% 28% 20% 5%	760
	11th to 20th	Night	12% 6% 23% 29% 24% 6%	760
	21st to end	Night	19% 10% 21% 29% 18% 3	760
October	1st to 10th	Night	20% 9% 31% 24% 15% 2	760
	11th to 20th	Night	24% 15% 30% 22% 8% 1	760
	21st to end	Night	29% 16% 31% 19% 5%	836
November	1st to 10th	Night	24% 16% 30% 22% 8% 1	780
	11th to 20th	Night	22% 23% 29% 20% 6% 1	800
	21st to end	Night	25% 21% 32% 19% 4	800
December	1st to 10th	Night	27% 22% 35% 14% 3 1	800
	11th to 20th	Night	21% 24% 34% 17% 3	800
	21st to end	Night	30% 18% 32% 18% 3	880

Page generated at 2025-04-23 09:25:04. Version ac07

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