Bradley Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bradley Observatory clear sky chart from 2005-11-06 to 2026-09-29.

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	13% 6% 2 6% 4 4 5% 4 6% 8% 41%	148691
	Night	13% 6% 2 6% 4 4 4 4 6% 8% 43%	142753
Transparency	Day	39% 7% 13% 20% 17% 5%	152477
	Night	38% 6% 11% 19% 19% 7%	147038
Seeing	Day	16% 9% 21% 23% 20% 12%	17413
	Night	15% 7% 22% 30% 21% 4	27370

By Month

Percentage Cloud Cover SE (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	18% 9% 3 6% 4 3 3 3 3 4 42%	5258
	11th to 20th	Day	10% 7% 2 5% 3 2 2 3 3 5% 58%	5280
	21st to end	Day	24% 9% 3 6% 4 3 3 3 4 5% 36%	6056
February	1st to 10th	Day	14% 7% 3 7% 5% 5% 4 3 5% 6% 40%	2100
	11th to 20th	Day	22% 7% 3 5% 4 4 4 3 4 5% 40%	2048
	21st to end	Day	20% 7% 2 6% 5% 4 4 3 5% 6% 40%	1730
March	1st to 10th	Day	21% 10% 3 7% 4 4 4 3 5% 5% 34%	4718
	11th to 20th	Day	10% 6% 2 6% 4 4 4 3 5% 6% 50%	5090
	21st to end	Day	16% 6% 2 6% 4 4 4 3 4 6% 45%	5586
April	1st to 10th	Day	12% 6% 2 5% 4 5% 4 4 5% 7% 46%	2367
	11th to 20th	Day	10% 5% 2 6% 5% 4 4 3 4 7% 49%	2494
	21st to end	Day	9% 5% 1 6% 4 4 4 4 6% 8% 48%	2681
May	1st to 10th	Day	14% 6% 3 7% 5% 5% 5% 4 6% 8% 37%	7616
	11th to 20th	Day	15% 7% 2 7% 5% 5% 6% 5% 7% 9% 33%	7626
	21st to end	Day	18% 7% 3 7% 5% 5% 5% 4 6% 8% 33%	8391
June	1st to 10th	Day	10% 6% 3 7% 5% 5% 5% 5% 7% 11% 35%	2730
	11th to 20th	Day	5% 4 2 5% 5% 4 6% 5% 9% 12% 43%	2730
	21st to end	Day	6% 4 2 6% 5% 5% 7% 6% 8% 11% 40%	2730
July	1st to 10th	Day	7% 4 2 6% 5% 6% 7% 6% 9% 13% 35%	7680
	11th to 20th	Day	7% 4 2 6% 5% 6% 7% 5% 9% 12% 36%	7680
	21st to end	Day	9% 6% 2 8% 6% 7% 7% 6% 10% 12% 28%	8340
August	1st to 10th	Day	9% 6% 2 7% 6% 6% 7% 6% 9% 12% 31%	6014
	11th to 20th	Day	6% 3 1 4 4 4 5% 4 6% 11% 52%	5554
	21st to end	Day	7% 5% 2 5% 4 4 5% 4 7% 9% 48%	6080
September	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 4 6% 9% 43%	2315
	11th to 20th	Day	14% 5% 2 6% 4 3 4 4 7% 8% 43%	2310
	21st to end	Day	13% 5% 2 6% 4 4 4 3 5% 6% 49%	2288
October	1st to 10th	Day	12% 5% 2 4 3 4 3 3 4 6% 55%	2200
	11th to 20th	Day	10% 4 1 4 3 2 3 3 5% 7% 56%	2094
	21st to end	Day	14% 5% 1 4 4 3 3 3 3 5% 55%	2200
November	1st to 10th	Day	13% 5% 2 5% 3 3 3 3 4 5% 54%	1877
	11th to 20th	Day	13% 5% 2 3 3 3 3 3 5% 5% 53%	1890
	21st to end	Day	12% 6% 2 5% 4 4 5% 3 5% 6% 48%	1890
December	1st to 10th	Day	18% 6% 2 5% 3 3 3 3 5% 7% 44%	3020
	11th to 20th	Day	15% 8% 4 7% 5% 4 4 3 4 5% 42%	3463
	21st to end	Day	22% 7% 2 5% 3 3 3 3 5% 6% 40%	4565
January	1st to 10th	Night	16% 6% 3 6% 4 4 4 3 5% 6% 45%	7635
	11th to 20th	Night	13% 7% 3 6% 3 3 3 2 5% 5% 50%	7680
	21st to end	Night	21% 10% 2 6% 3 3 3 3 3 4 41%	8122
February	1st to 10th	Night	15% 7% 2 6% 4 4 4 3 4 6% 44%	2520
	11th to 20th	Night	21% 8% 3 5% 4 4 3 3 5% 5% 41%	2458
	21st to end	Night	19% 7% 3 5% 5% 4 3 3 4 6% 42%	2076
March	1st to 10th	Night	21% 9% 2 6% 4 4 4 4 4 7% 36%	5352
	11th to 20th	Night	11% 5% 2 4 3 3 3 3 5% 7% 53%	5090
	21st to end	Night	14% 6% 2 6% 4 4 4 4 4 6% 46%	5588
April	1st to 10th	Night	11% 5% 2 5% 4 4 5% 4 6% 7% 47%	2269
	11th to 20th	Night	13% 5% 2 4 3 3 4 2 5% 7% 52%	2075
	21st to end	Night	9% 5% 2 5% 4 3 4 3 5% 8% 51%	1937
May	1st to 10th	Night	13% 7% 3 8% 5% 5% 5% 3 7% 8% 37%	5237
	11th to 20th	Night	16% 6% 2 7% 4 4 5% 3 7% 8% 38%	5234
	21st to end	Night	16% 7% 2 7% 4 5% 5% 4 6% 9% 34%	5761
June	1st to 10th	Night	7% 6% 3 6% 4 5% 5% 5% 9% 12% 37%	1890
	11th to 20th	Night	5% 3 1 5% 5% 6% 6% 4 9% 13% 43%	1890
	21st to end	Night	5% 4 2 5% 4 5% 7% 5% 9% 11% 43%	1890
July	1st to 10th	Night	7% 5% 2 7% 5% 6% 7% 5% 8% 13% 35%	5278
	11th to 20th	Night	7% 5% 2 7% 5% 6% 6% 5% 10% 14% 33%	5277
	21st to end	Night	10% 6% 3 9% 5% 6% 6% 6% 8% 12% 30%	5736
August	1st to 10th	Night	10% 4 2 8% 6% 6% 6% 6% 9% 12% 30%	4135
	11th to 20th	Night	6% 4 1 5% 3 4 5% 5% 7% 12% 48%	4487
	21st to end	Night	8% 5% 2 6% 5% 4 5% 4 6% 9% 47%	5046
September	1st to 10th	Night	8% 5% 2 6% 4 5% 6% 6% 6% 10% 42%	2283
	11th to 20th	Night	13% 6% 2 6% 4 4 5% 3 6% 7% 45%	2310
	21st to end	Night	12% 5% 2 6% 4 4 4 3 6% 8% 47%	2289
October	1st to 10th	Night	11% 5% 2 6% 2 3 4 4 4 6% 53%	2200
	11th to 20th	Night	9% 5% 2 4 3 2 3 3 4 5% 59%	2305
	21st to end	Night	13% 4 1 4 3 3 3 3 4 5% 58%	2640
November	1st to 10th	Night	13% 5% 2 5% 3 3 3 2 4 5% 56%	2638
	11th to 20th	Night	13% 5% 2 5% 3 3 2 2 4 6% 54%	2730
	21st to end	Night	14% 6% 2 5% 4 4 4 3 5% 6% 48%	2730
December	1st to 10th	Night	15% 6% 2 5% 3 3 3 2 4 5% 51%	4380
	11th to 20th	Night	17% 6% 2 5% 3 3 3 3 5% 6% 47%	4952
	21st to end	Night	21% 7% 2 5% 4 4 3 3 4 5% 41%	6633

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	45% 3 7% 22% 18% 5%	5259
	11th to 20th	Day	30% 2 5% 21% 31% 11%	5280
	21st to end	Day	52% 2 4 15% 19% 7%	6056
February	1st to 10th	Day	44% 4 8% 18% 22% 5%	2100
	11th to 20th	Day	47% 3 6% 19% 20% 6%	2058
	21st to end	Day	46% 3 7% 15% 21% 8%	1730
March	1st to 10th	Day	52% 3 9% 12% 19% 6%	4718
	11th to 20th	Day	34% 4 8% 15% 23% 16%	5090
	21st to end	Day	42% 3 7% 22% 21% 4	5586
April	1st to 10th	Day	37% 4 10% 23% 20% 5%	2367
	11th to 20th	Day	35% 4 7% 21% 26% 7%	2494
	21st to end	Day	33% 5% 11% 27% 22% 2	2681
May	1st to 10th	Day	44% 6% 14% 21% 14% 1	7651
	11th to 20th	Day	44% 8% 16% 20% 10% 1	7630
	21st to end	Day	49% 7% 10% 19% 14% 1	8405
June	1st to 10th	Day	41% 7% 17% 23% 11% 2	2730
	11th to 20th	Day	29% 8% 22% 26% 14% 1	2730
	21st to end	Day	33% 10% 18% 23% 13% 3	2730
July	1st to 10th	Day	35% 14% 25% 21% 5% 1	7615
	11th to 20th	Day	36% 15% 28% 17% 3 1	7611
	21st to end	Day	44% 13% 19% 18% 6%	8156
August	1st to 10th	Day	40% 13% 19% 23% 5%	6001
	11th to 20th	Day	25% 9% 23% 29% 13% 2	5591
	21st to end	Day	29% 8% 17% 24% 19% 3	6079
September	1st to 10th	Day	36% 7% 17% 19% 17% 4	2315
	11th to 20th	Day	38% 7% 12% 21% 21% 1	2310
	21st to end	Day	36% 5% 11% 21% 22% 6%	2288
October	1st to 10th	Day	25% 4 7% 14% 37% 13%	3590
	11th to 20th	Day	19% 3 5% 17% 30% 27%	3386
	21st to end	Day	27% 3 6% 20% 32% 12%	3538
November	1st to 10th	Day	33% 4 7% 15% 31% 10%	1877
	11th to 20th	Day	33% 3 7% 20% 27% 10%	1890
	21st to end	Day	37% 4 8% 19% 26% 7%	1890
December	1st to 10th	Day	41% 3 5% 18% 26% 7%	3020
	11th to 20th	Day	45% 3 8% 18% 20% 6%	3460
	21st to end	Day	44% 3 8% 26% 18% 2	4565
January	1st to 10th	Night	39% 4 8% 22% 22% 5%	7636
	11th to 20th	Night	36% 3 5% 19% 28% 9%	7680
	21st to end	Night	48% 2 4 15% 24% 8%	8122
February	1st to 10th	Night	41% 3 8% 19% 20% 8%	2520
	11th to 20th	Night	46% 3 6% 18% 20% 8%	2459
	21st to end	Night	45% 3 7% 17% 18% 11%	2076
March	1st to 10th	Night	48% 4 8% 15% 18% 8%	5352
	11th to 20th	Night	31% 4 8% 15% 25% 18%	5090
	21st to end	Night	39% 4 6% 21% 21% 9%	5588
April	1st to 10th	Night	35% 4 11% 21% 22% 8%	2269
	11th to 20th	Night	33% 2 7% 22% 26% 10%	2075
	21st to end	Night	32% 5% 11% 26% 21% 5%	1937
May	1st to 10th	Night	44% 6% 16% 17% 15% 3	5264
	11th to 20th	Night	43% 5% 14% 25% 12% 1	5235
	21st to end	Night	46% 7% 13% 18% 14% 2	5771
June	1st to 10th	Night	36% 10% 17% 20% 13% 3	1890
	11th to 20th	Night	29% 8% 21% 27% 13% 1	1890
	21st to end	Night	29% 11% 21% 20% 15% 4	1890
July	1st to 10th	Night	36% 13% 25% 21% 4 1	5235
	11th to 20th	Night	36% 17% 28% 13% 4 1	5226
	21st to end	Night	44% 12% 22% 14% 6% 2	5606
August	1st to 10th	Night	41% 12% 19% 23% 5%	4124
	11th to 20th	Night	26% 9% 23% 27% 13% 1	4516
	21st to end	Night	32% 9% 14% 23% 21% 2	5046
September	1st to 10th	Night	35% 8% 17% 19% 17% 4	2283
	11th to 20th	Night	37% 7% 12% 22% 19% 3	2310
	21st to end	Night	35% 6% 13% 18% 20% 8%	2289
October	1st to 10th	Night	26% 5% 8% 13% 35% 13%	3590
	11th to 20th	Night	17% 2 4 18% 32% 27%	3752
	21st to end	Night	23% 3 4 22% 31% 17%	4252
November	1st to 10th	Night	32% 3 8% 17% 28% 12%	2638
	11th to 20th	Night	32% 3 5% 19% 29% 12%	2730
	21st to end	Night	36% 3 7% 19% 26% 8%	2730
December	1st to 10th	Night	38% 3 6% 19% 28% 8%	4380
	11th to 20th	Night	39% 4 11% 15% 25% 6%	4954
	21st to end	Night	45% 3 5% 27% 18% 2	6633

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	25% 20% 36% 17% 2	330
	11th to 20th	Day	26% 21% 29% 17% 5% 2	330
	21st to end	Day	25% 21% 35% 15% 4 1	363
February	1st to 10th	Day	19% 19% 36% 20% 5%	330
	11th to 20th	Day	37% 16% 32% 13% 2	330
	21st to end	Day	30% 13% 38% 15% 4	273
March	1st to 10th	Day	23% 17% 28% 23% 7% 1	390
	11th to 20th	Day	21% 15% 31% 26% 7% 1	540
	21st to end	Day	19% 14% 33% 25% 8% 1	594
April	1st to 10th	Day	17% 14% 29% 23% 14% 2	540
	11th to 20th	Day	11% 12% 31% 26% 17% 2	532
	21st to end	Day	14% 9% 23% 28% 21% 5%	540
May	1st to 10th	Day	12% 10% 27% 26% 18% 6%	543
	11th to 20th	Day	13% 6% 20% 25% 23% 13%	567
	21st to end	Day	15% 6% 15% 24% 24% 16%	617
June	1st to 10th	Day	12% 4 13% 25% 30% 16%	570
	11th to 20th	Day	7% 3 14% 26% 22% 28%	570
	21st to end	Day	9% 2 10% 27% 29% 22%	570
July	1st to 10th	Day	7% 2 5% 20% 35% 31%	565
	11th to 20th	Day	7% 2 5% 18% 32% 35%	564
	21st to end	Day	6% 2 8% 20% 38% 26%	620
August	1st to 10th	Day	12% 4 12% 21% 27% 24%	566
	11th to 20th	Day	10% 3 8% 22% 34% 24%	559
	21st to end	Day	15% 2 8% 25% 28% 22%	627
September	1st to 10th	Day	8% 3 13% 25% 30% 20%	570
	11th to 20th	Day	14% 5% 9% 20% 35% 16%	570
	21st to end	Day	15% 3 15% 24% 30% 12%	562
October	1st to 10th	Day	16% 3 19% 33% 20% 9%	530
	11th to 20th	Day	11% 9% 22% 26% 25% 7%	530
	21st to end	Day	18% 8% 21% 28% 20% 4	583
November	1st to 10th	Day	21% 8% 24% 24% 18% 4	355
	11th to 20th	Day	16% 14% 36% 23% 10% 2	330
	21st to end	Day	15% 15% 31% 27% 9% 2	330
December	1st to 10th	Day	29% 16% 33% 14% 7%	330
	11th to 20th	Day	23% 15% 28% 25% 7% 1	330
	21st to end	Day	29% 14% 31% 20% 5% 1	363
January	1st to 10th	Night	23% 15% 32% 24% 5%	840
	11th to 20th	Night	20% 15% 32% 26% 8%	840
	21st to end	Night	21% 14% 36% 25% 3	924
February	1st to 10th	Night	19% 12% 31% 33% 6%	840
	11th to 20th	Night	25% 12% 36% 25% 2	823
	21st to end	Night	22% 14% 35% 25% 3	692
March	1st to 10th	Night	18% 13% 31% 33% 6%	840
	11th to 20th	Night	17% 11% 34% 28% 9%	840
	21st to end	Night	16% 12% 30% 32% 10%	924
April	1st to 10th	Night	14% 10% 30% 33% 12% 1	799
	11th to 20th	Night	15% 8% 29% 32% 15% 1	623
	21st to end	Night	11% 6% 26% 35% 21% 2	630
May	1st to 10th	Night	11% 7% 23% 36% 22% 1	630
	11th to 20th	Night	12% 4 21% 34% 24% 4	621
	21st to end	Night	14% 2 12% 32% 32% 7%	683
June	1st to 10th	Night	10% 2 9% 38% 35% 6%	630
	11th to 20th	Night	6% 2 13% 30% 37% 13%	630
	21st to end	Night	6% 2 7% 32% 43% 10%	630
July	1st to 10th	Night	9% 1 7% 22% 47% 15%	615
	11th to 20th	Night	7% 2 4 20% 51% 16%	612
	21st to end	Night	6% 2 6% 29% 45% 12%	690
August	1st to 10th	Night	9% 2 8% 26% 43% 12%	627
	11th to 20th	Night	11% 1 7% 26% 46% 10%	624
	21st to end	Night	14% 3 7% 27% 40% 9%	693
September	1st to 10th	Night	9% 3 9% 27% 40% 12%	813
	11th to 20th	Night	14% 2 10% 27% 43% 4	840
	21st to end	Night	15% 3 12% 32% 34% 4	833
October	1st to 10th	Night	13% 3 17% 35% 26% 7%	800
	11th to 20th	Night	12% 6% 23% 34% 22% 3	800
	21st to end	Night	15% 6% 22% 35% 20% 3	880
November	1st to 10th	Night	16% 6% 22% 34% 21% 1	820
	11th to 20th	Night	16% 9% 31% 30% 14% 1	840
	21st to end	Night	17% 10% 28% 32% 13% 1	840
December	1st to 10th	Night	19% 11% 30% 31% 9% 1	840
	11th to 20th	Night	22% 8% 29% 30% 10%	840
	21st to end	Night	26% 10% 27% 30% 7%	924

Page generated at 2026-09-26 04:18:39. Version ac07

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