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 2024-10-25.

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 9% 5% 5% 5% 4 6% 7% 31%	101860
	Night	14% 11% 4 9% 6% 5% 5% 4 5% 6% 32%	96940
Transparency	Day	51% 6% 12% 20% 10% 1	101624
	Night	51% 6% 13% 20% 10% 1	96796
Seeing	Day	19% 16% 31% 20% 10% 4	14775
	Night	19% 14% 31% 24% 10% 1	23743

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	16% 15% 6% 11% 7% 5% 6% 3 5% 5% 21%	2517
	11th to 20th	Day	10% 19% 7% 18% 10% 7% 6% 3 4 4 13%	2753
	21st to end	Day	19% 22% 7% 11% 6% 4 4 3 4 4 15%	3124
February	1st to 10th	Day	13% 16% 5% 13% 9% 8% 6% 4 5% 4 17%	1710
	11th to 20th	Day	10% 15% 7% 14% 7% 6% 5% 4 5% 6% 20%	1663
	21st to end	Day	16% 16% 4 12% 6% 4 5% 4 4 5% 24%	1462
March	1st to 10th	Day	16% 13% 6% 10% 6% 4 4 3 4 5% 29%	3432
	11th to 20th	Day	10% 18% 6% 11% 5% 5% 5% 3 5% 5% 27%	3480
	21st to end	Day	14% 12% 4 9% 6% 5% 4 3 5% 4 35%	3815
April	1st to 10th	Day	15% 16% 5% 8% 6% 4 5% 4 5% 5% 28%	2234
	11th to 20th	Day	19% 11% 3 6% 4 4 4 4 5% 6% 35%	2351
	21st to end	Day	19% 11% 3 8% 4 4 4 3 5% 6% 33%	2470
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 8% 5% 5% 4 3 4 6% 20%	2027
November	1st to 10th	Day	12% 13% 5% 9% 6% 5% 4 4 5% 9% 28%	1590
	11th to 20th	Day	11% 16% 4 11% 7% 7% 6% 4 6% 6% 22%	1520
	21st to end	Day	17% 18% 6% 12% 6% 5% 5% 4 6% 5% 17%	1520
December	1st to 10th	Day	15% 19% 6% 15% 8% 6% 5% 4 5% 4 12%	1520
	11th to 20th	Day	16% 16% 5% 14% 6% 5% 5% 5% 6% 5% 16%	1906
	21st to end	Day	25% 20% 7% 16% 6% 4 4 3 3 3 9%	2772
January	1st to 10th	Night	15% 16% 7% 13% 6% 6% 5% 3 5% 5% 18%	4429
	11th to 20th	Night	12% 14% 7% 15% 11% 8% 6% 4 5% 5% 11%	4202
	21st to end	Night	18% 15% 7% 12% 10% 6% 5% 4 5% 5% 14%	4532
February	1st to 10th	Night	12% 17% 5% 12% 8% 5% 5% 4 6% 6% 21%	2470
	11th to 20th	Night	10% 15% 7% 11% 7% 5% 6% 4 5% 6% 23%	2403
	21st to end	Night	12% 13% 4 11% 6% 6% 5% 5% 6% 6% 26%	1986
March	1st to 10th	Night	16% 11% 4 10% 6% 4 5% 3 5% 6% 30%	3408
	11th to 20th	Night	11% 13% 4 8% 5% 4 4 4 5% 6% 35%	3479
	21st to end	Night	14% 8% 4 7% 5% 4 4 3 5% 6% 38%	3817
April	1st to 10th	Night	17% 13% 4 8% 5% 4 4 4 5% 6% 30%	1945
	11th to 20th	Night	17% 9% 3 6% 4 5% 5% 3 5% 7% 37%	1795
	21st to end	Night	14% 11% 3 7% 5% 4 4 4 5% 6% 35%	1710
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 9% 5% 4 5% 5% 5% 5% 25%	2435
November	1st to 10th	Night	16% 10% 3 7% 5% 4 5% 4 6% 6% 34%	2485
	11th to 20th	Night	15% 14% 4 8% 6% 5% 5% 5% 5% 6% 27%	2660
	21st to end	Night	15% 15% 5% 10% 6% 6% 5% 4 6% 6% 21%	2660
December	1st to 10th	Night	18% 16% 6% 11% 7% 5% 5% 4 6% 6% 17%	2660
	11th to 20th	Night	14% 17% 6% 13% 7% 5% 5% 4 6% 6% 17%	3288
	21st to end	Night	25% 19% 7% 14% 6% 5% 4 3 3 4 9%	4884

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% 8% 15% 11% 4	2518
	11th to 20th	Day	73% 7% 8% 8% 3	2753
	21st to end	Day	73% 4 11% 9% 3	3124
February	1st to 10th	Day	69% 6% 8% 13% 3	1710
	11th to 20th	Day	64% 6% 9% 14% 6% 1	1672
	21st to end	Day	62% 5% 9% 15% 7% 1	1462
March	1st to 10th	Day	58% 5% 12% 16% 9%	3435
	11th to 20th	Day	58% 5% 9% 19% 7% 1	3480
	21st to end	Day	50% 5% 10% 19% 13% 2	3815
April	1st to 10th	Day	57% 6% 9% 19% 9% 1	2234
	11th to 20th	Day	50% 5% 8% 22% 15% 1	2351
	21st to end	Day	52% 5% 11% 18% 12% 2	2470
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	52% 5% 12% 20% 11% 1	1900
	11th to 20th	Day	56% 6% 12% 18% 7% 1	1900
	21st to end	Day	66% 5% 12% 12% 5%	2027
November	1st to 10th	Day	52% 7% 15% 18% 7%	1590
	11th to 20th	Day	60% 7% 13% 13% 7% 1	1520
	21st to end	Day	68% 6% 13% 11% 2	1520
December	1st to 10th	Day	74% 5% 12% 8% 1	1520
	11th to 20th	Day	66% 10% 12% 9% 3	1906
	21st to end	Day	80% 5% 6% 7% 1	2772
January	1st to 10th	Night	66% 7% 15% 9% 3	4429
	11th to 20th	Night	72% 7% 10% 9% 2	4202
	21st to end	Night	71% 6% 11% 8% 3 1	4532
February	1st to 10th	Night	63% 5% 12% 16% 4	2470
	11th to 20th	Night	60% 7% 10% 17% 6%	2405
	21st to end	Night	56% 5% 14% 16% 8%	1986
March	1st to 10th	Night	55% 5% 10% 23% 6% 1	3418
	11th to 20th	Night	47% 6% 14% 22% 9% 2	3480
	21st to end	Night	44% 5% 12% 24% 13% 1	3817
April	1st to 10th	Night	55% 4 10% 19% 12% 1	1945
	11th to 20th	Night	47% 6% 9% 21% 16% 2	1795
	21st to end	Night	49% 5% 12% 17% 15% 2	1710
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	46% 7% 11% 21% 15% 1	2280
	11th to 20th	Night	51% 6% 15% 20% 8%	2280
	21st to end	Night	60% 6% 11% 15% 8%	2435
November	1st to 10th	Night	48% 6% 14% 22% 9% 1	2485
	11th to 20th	Night	55% 7% 14% 15% 7% 1	2660
	21st to end	Night	62% 8% 16% 12% 1 1	2660
December	1st to 10th	Night	67% 6% 14% 10% 2	2660
	11th to 20th	Night	66% 7% 14% 11% 2	3288
	21st to end	Night	80% 5% 8% 6% 2	4884

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% 23% 36% 17% 4 1	180
	11th to 20th	Day	23% 26% 32% 13% 4 1	247
	21st to end	Day	25% 23% 33% 17% 2	297
February	1st to 10th	Day	26% 30% 30% 10% 3	270
	11th to 20th	Day	23% 29% 41% 6% 1	270
	21st to end	Day	30% 34% 27% 9%	227
March	1st to 10th	Day	22% 21% 38% 15% 4	460
	11th to 20th	Day	20% 24% 35% 16% 4 1	460
	21st to end	Day	29% 21% 28% 16% 5% 1	506
April	1st to 10th	Day	25% 22% 33% 13% 6%	460
	11th to 20th	Day	33% 19% 24% 14% 7% 3	452
	21st to end	Day	27% 18% 30% 17% 7% 2	460
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	514
November	1st to 10th	Day	23% 23% 29% 20% 4 1	243
	11th to 20th	Day	26% 21% 34% 14% 4 1	180
	21st to end	Day	29% 22% 39% 9% 1	180
December	1st to 10th	Day	23% 32% 31% 12% 2	180
	11th to 20th	Day	27% 29% 28% 12% 2 1	180
	21st to end	Day	25% 27% 36% 10% 3	198
January	1st to 10th	Night	22% 24% 33% 16% 5%	760
	11th to 20th	Night	24% 21% 31% 19% 4	760
	21st to end	Night	19% 15% 36% 22% 6%	836
February	1st to 10th	Night	21% 21% 38% 18% 1	760
	11th to 20th	Night	21% 21% 38% 17% 3	743
	21st to end	Night	21% 21% 36% 19% 2	628
March	1st to 10th	Night	21% 20% 36% 20% 3	760
	11th to 20th	Night	22% 22% 31% 19% 5% 1	760
	21st to end	Night	20% 17% 33% 21% 7%	836
April	1st to 10th	Night	26% 16% 34% 21% 3	760
	11th to 20th	Night	21% 15% 35% 19% 9% 1	671
	21st to end	Night	21% 13% 32% 26% 8%	570
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	30% 16% 31% 18% 5%	812
November	1st to 10th	Night	24% 16% 30% 22% 8% 1	740
	11th to 20th	Night	21% 22% 30% 20% 6% 1	760
	21st to end	Night	24% 20% 32% 19% 4	760
December	1st to 10th	Night	27% 22% 35% 14% 2 1	760
	11th to 20th	Night	22% 24% 34% 17% 3	760
	21st to end	Night	31% 18% 32% 17% 2	836

Page generated at 2024-10-22 20:21:05. Version ac07

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