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 2025-05-26.

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	13% 6% 2 6% 4 4 4 4 6% 8% 43%	114360
	Night	13% 6% 2 6% 4 4 4 4 5% 7% 45%	114944
Transparency	Day	38% 5% 11% 21% 19% 6%	118190
	Night	37% 5% 10% 20% 22% 7%	119252
Seeing	Day	16% 9% 21% 23% 20% 12%	15575
	Night	15% 7% 22% 30% 22% 4	25652

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	16% 9% 3 5% 3 3 3 3 4 4 47%	4056
	11th to 20th	Day	11% 8% 3 5% 3 3 3 3 4 6% 53%	4060
	21st to end	Day	23% 9% 2 6% 4 3 3 3 4 5% 39%	4673
February	1st to 10th	Day	15% 8% 3 7% 5% 5% 5% 3 5% 6% 39%	2000
	11th to 20th	Day	22% 7% 3 5% 4 4 4 3 4 5% 40%	1948
	21st to end	Day	20% 6% 2 6% 5% 4 4 3 5% 6% 40%	1650
March	1st to 10th	Day	22% 10% 3 7% 4 4 4 3 5% 5% 34%	4615
	11th to 20th	Day	10% 6% 2 6% 4 4 4 3 5% 6% 50%	4980
	21st to end	Day	17% 6% 2 6% 4 4 4 3 4 5% 44%	5465
April	1st to 10th	Day	12% 6% 2 5% 4 5% 4 4 5% 7% 46%	2254
	11th to 20th	Day	10% 6% 2 6% 5% 4 4 3 4 7% 48%	2374
	21st to end	Day	9% 5% 1 5% 4 4 4 4 6% 8% 49%	2553
May	1st to 10th	Day	10% 6% 3 6% 4 4 4 4 6% 9% 43%	4212
	11th to 20th	Day	16% 8% 2 6% 4 4 5% 4 6% 8% 36%	4211
	21st to end	Day	8% 4 2 5% 4 4 5% 4 7% 10% 46%	4541
June	1st to 10th	Day	9% 6% 3 8% 6% 6% 5% 6% 7% 11% 35%	2470
	11th to 20th	Day	4 4 2 5% 5% 4 6% 5% 9% 11% 45%	2470
	21st to end	Day	6% 5% 2 7% 5% 5% 7% 6% 8% 10% 39%	2470
July	1st to 10th	Day	10% 6% 3 7% 6% 7% 8% 6% 9% 12% 26%	4120
	11th to 20th	Day	8% 5% 3 7% 6% 6% 7% 5% 9% 10% 36%	4120
	21st to end	Day	11% 8% 3 10% 7% 8% 7% 6% 10% 10% 20%	4466
August	1st to 10th	Day	7% 6% 2 6% 5% 5% 6% 5% 8% 12% 39%	4104
	11th to 20th	Day	8% 4 2 5% 4 5% 5% 4 6% 10% 46%	3768
	21st to end	Day	7% 5% 2 5% 3 3 5% 4 6% 9% 50%	4180
September	1st to 10th	Day	10% 6% 2 6% 6% 5% 5% 5% 6% 9% 41%	2095
	11th to 20th	Day	16% 5% 3 6% 4 3 4 4 7% 7% 40%	2090
	21st to end	Day	14% 5% 3 5% 4 4 4 3 4 6% 49%	2090
October	1st to 10th	Day	12% 5% 2 4 3 4 3 3 4 6% 56%	2090
	11th to 20th	Day	10% 4 1 5% 3 3 3 3 5% 7% 55%	1990
	21st to end	Day	13% 4 1 4 4 3 3 3 3 5% 55%	2090
November	1st to 10th	Day	13% 5% 2 5% 3 3 3 3 4 5% 54%	1786
	11th to 20th	Day	14% 5% 2 3 4 4 3 3 5% 5% 52%	1800
	21st to end	Day	12% 7% 2 5% 4 4 5% 3 5% 6% 47%	1800
December	1st to 10th	Day	18% 6% 2 5% 3 3 3 3 5% 7% 45%	2930
	11th to 20th	Day	15% 8% 4 7% 5% 4 4 3 5% 5% 41%	3373
	21st to end	Day	22% 7% 2 5% 3 3 3 3 5% 6% 39%	4466
January	1st to 10th	Night	15% 6% 3 6% 3 3 3 3 5% 6% 48%	5889
	11th to 20th	Night	13% 7% 3 6% 4 3 3 3 5% 5% 48%	5900
	21st to end	Night	18% 10% 2 5% 3 3 3 3 4 5% 43%	6280
February	1st to 10th	Night	16% 7% 2 5% 4 4 4 3 4 6% 44%	2400
	11th to 20th	Night	21% 8% 3 5% 4 3 3 3 5% 5% 41%	2338
	21st to end	Night	19% 6% 2 5% 4 4 3 3 4 6% 42%	1980
March	1st to 10th	Night	21% 9% 2 6% 4 4 4 4 4 6% 36%	5235
	11th to 20th	Night	11% 5% 2 4 3 3 3 3 5% 7% 53%	4980
	21st to end	Night	14% 6% 2 6% 4 4 4 4 5% 6% 46%	5467
April	1st to 10th	Night	12% 5% 2 5% 4 4 5% 4 6% 7% 47%	2161
	11th to 20th	Night	13% 5% 2 4 3 3 4 3 5% 7% 50%	1975
	21st to end	Night	9% 5% 2 5% 4 3 4 3 5% 8% 52%	1845
May	1st to 10th	Night	8% 6% 2 7% 5% 5% 5% 4 7% 8% 41%	2903
	11th to 20th	Night	16% 7% 2 6% 4 4 5% 3 6% 7% 40%	2902
	21st to end	Night	6% 4 1 6% 3 4 5% 5% 7% 12% 48%	3130
June	1st to 10th	Night	7% 6% 3 6% 5% 5% 6% 5% 9% 12% 36%	1710
	11th to 20th	Night	4 3 1 5% 5% 6% 5% 4 9% 13% 45%	1710
	21st to end	Night	5% 4 2 5% 4 5% 7% 5% 9% 11% 42%	1710
July	1st to 10th	Night	10% 6% 2 9% 6% 6% 6% 5% 8% 11% 31%	2840
	11th to 20th	Night	6% 5% 2 7% 6% 6% 6% 6% 9% 13% 34%	2840
	21st to end	Night	11% 9% 3 10% 6% 6% 7% 6% 9% 11% 23%	3087
August	1st to 10th	Night	7% 4 2 7% 5% 5% 6% 5% 9% 13% 36%	2825
	11th to 20th	Night	8% 5% 2 6% 4 4 5% 5% 6% 9% 48%	3057
	21st to end	Night	8% 5% 2 5% 4 4 4 4 5% 8% 52%	3476
September	1st to 10th	Night	8% 5% 2 6% 4 5% 6% 6% 6% 9% 42%	2065
	11th to 20th	Night	14% 6% 2 6% 4 4 5% 3 6% 7% 43%	2090
	21st to end	Night	12% 5% 2 5% 4 4 4 3 5% 7% 48%	2090
October	1st to 10th	Night	10% 5% 2 5% 2 3 4 3 5% 6% 54%	2090
	11th to 20th	Night	10% 5% 2 4 3 2 3 3 4 5% 58%	2190
	21st to end	Night	12% 4 1 4 3 3 3 3 4 5% 58%	2508
November	1st to 10th	Night	13% 5% 2 5% 3 3 2 2 5% 5% 55%	2509
	11th to 20th	Night	14% 5% 2 5% 3 3 2 2 4 6% 53%	2600
	21st to end	Night	14% 6% 2 4 4 4 4 3 5% 6% 48%	2600
December	1st to 10th	Night	15% 6% 2 5% 3 3 3 2 4 5% 51%	4250
	11th to 20th	Night	18% 7% 3 5% 3 3 3 3 5% 6% 46%	4822
	21st to end	Night	21% 7% 2 5% 4 4 3 3 4 6% 41%	6490

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	41% 3 6% 22% 23% 6%	4057
	11th to 20th	Day	34% 2 5% 23% 29% 7%	4060
	21st to end	Day	49% 2 5% 17% 21% 7%	4673
February	1st to 10th	Day	45% 4 8% 18% 20% 5%	2000
	11th to 20th	Day	47% 3 6% 18% 20% 6%	1958
	21st to end	Day	46% 3 7% 15% 21% 8%	1650
March	1st to 10th	Day	52% 3 8% 11% 19% 6%	4615
	11th to 20th	Day	34% 4 7% 15% 23% 16%	4980
	21st to end	Day	43% 3 7% 22% 22% 4	5465
April	1st to 10th	Day	37% 4 10% 24% 20% 5%	2254
	11th to 20th	Day	36% 4 7% 21% 25% 7%	2374
	21st to end	Day	32% 5% 11% 27% 22% 2	2553
May	1st to 10th	Day	37% 5% 15% 24% 18% 2	4250
	11th to 20th	Day	45% 5% 13% 25% 11% 1	4215
	21st to end	Day	32% 6% 15% 26% 20% 1	4554
June	1st to 10th	Day	41% 7% 17% 23% 10% 2	2470
	11th to 20th	Day	28% 7% 21% 27% 15% 1	2470
	21st to end	Day	35% 9% 18% 22% 13% 3	2470
July	1st to 10th	Day	44% 14% 21% 16% 5% 1	4055
	11th to 20th	Day	39% 11% 21% 22% 5% 1	4051
	21st to end	Day	53% 13% 16% 13% 4	4324
August	1st to 10th	Day	34% 8% 17% 32% 8%	4091
	11th to 20th	Day	31% 8% 17% 27% 14% 2	3805
	21st to end	Day	29% 7% 17% 23% 21% 3	4179
September	1st to 10th	Day	39% 7% 16% 18% 17% 4	2095
	11th to 20th	Day	40% 7% 11% 18% 23% 1	2090
	21st to end	Day	36% 5% 9% 21% 23% 6%	2090
October	1st to 10th	Day	25% 3 7% 14% 38% 13%	3480
	11th to 20th	Day	19% 3 5% 17% 30% 27%	3282
	21st to end	Day	27% 3 6% 20% 32% 12%	3428
November	1st to 10th	Day	34% 4 7% 15% 30% 10%	1786
	11th to 20th	Day	34% 3 6% 19% 27% 10%	1800
	21st to end	Day	37% 5% 7% 19% 26% 7%	1800
December	1st to 10th	Day	40% 3 5% 18% 27% 7%	2930
	11th to 20th	Day	46% 4 7% 18% 20% 6%	3370
	21st to end	Day	45% 3 7% 25% 18% 2	4466
January	1st to 10th	Night	37% 3 5% 21% 28% 6%	5889
	11th to 20th	Night	38% 3 5% 20% 26% 8%	5900
	21st to end	Night	44% 3 4 15% 28% 6%	6280
February	1st to 10th	Night	41% 3 8% 19% 19% 9%	2400
	11th to 20th	Night	46% 3 5% 18% 20% 8%	2339
	21st to end	Night	44% 3 7% 17% 18% 11%	1980
March	1st to 10th	Night	49% 4 8% 14% 18% 8%	5235
	11th to 20th	Night	31% 3 8% 15% 25% 18%	4980
	21st to end	Night	39% 4 6% 21% 21% 9%	5467
April	1st to 10th	Night	35% 4 10% 20% 22% 8%	2161
	11th to 20th	Night	34% 2 7% 21% 25% 10%	1975
	21st to end	Night	31% 5% 11% 27% 21% 6%	1845
May	1st to 10th	Night	37% 5% 16% 22% 17% 2	2930
	11th to 20th	Night	43% 4 14% 26% 13% 1	2903
	21st to end	Night	27% 6% 17% 26% 21% 3	3138
June	1st to 10th	Night	37% 10% 18% 21% 13% 2	1710
	11th to 20th	Night	28% 8% 21% 29% 14% 2	1710
	21st to end	Night	31% 11% 21% 19% 15% 4	1710
July	1st to 10th	Night	44% 12% 20% 17% 5% 1	2795
	11th to 20th	Night	35% 14% 23% 18% 7% 2	2786
	21st to end	Night	52% 12% 17% 12% 5% 1	2988
August	1st to 10th	Night	35% 9% 18% 30% 7% 1	2814
	11th to 20th	Night	31% 9% 15% 27% 16% 1	3086
	21st to end	Night	30% 7% 14% 21% 25% 3	3476
September	1st to 10th	Night	35% 8% 17% 18% 17% 5%	2065
	11th to 20th	Night	39% 7% 12% 21% 19% 3	2090
	21st to end	Night	35% 6% 12% 18% 21% 9%	2090
October	1st to 10th	Night	25% 5% 8% 13% 35% 13%	3480
	11th to 20th	Night	17% 2 4 18% 32% 27%	3637
	21st to end	Night	22% 3 4 22% 31% 18%	4120
November	1st to 10th	Night	32% 3 8% 17% 27% 13%	2509
	11th to 20th	Night	34% 3 5% 18% 29% 12%	2600
	21st to end	Night	36% 3 7% 19% 27% 8%	2600
December	1st to 10th	Night	37% 3 6% 19% 28% 8%	4250
	11th to 20th	Night	40% 4 11% 15% 24% 6%	4824
	21st to end	Night	45% 3 5% 27% 18% 2	6490

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	24% 22% 36% 16% 2	300
	11th to 20th	Day	25% 19% 30% 18% 6% 2	300
	21st to end	Day	24% 21% 35% 15% 5% 1	330
February	1st to 10th	Day	19% 18% 35% 22% 5%	300
	11th to 20th	Day	38% 14% 31% 14% 2	300
	21st to end	Day	28% 14% 39% 15% 4	249
March	1st to 10th	Day	25% 17% 28% 22% 7% 1	357
	11th to 20th	Day	21% 15% 31% 25% 7%	500
	21st to end	Day	20% 14% 34% 24% 8% 1	550
April	1st to 10th	Day	18% 15% 30% 21% 14% 2	500
	11th to 20th	Day	12% 13% 32% 25% 16% 2	492
	21st to end	Day	15% 8% 23% 28% 21% 5%	500
May	1st to 10th	Day	12% 10% 27% 26% 19% 7%	503
	11th to 20th	Day	13% 5% 20% 25% 22% 14%	527
	21st to end	Day	13% 5% 14% 24% 25% 18%	549
June	1st to 10th	Day	12% 3 11% 25% 31% 18%	490
	11th to 20th	Day	7% 2 12% 24% 22% 32%	490
	21st to end	Day	11% 2 10% 27% 27% 23%	490
July	1st to 10th	Day	8% 2 5% 19% 34% 32%	485
	11th to 20th	Day	8% 2 5% 17% 30% 39%	484
	21st to end	Day	6% 2 7% 17% 39% 29%	532
August	1st to 10th	Day	11% 3 11% 21% 27% 27%	486
	11th to 20th	Day	11% 3 8% 21% 33% 24%	479
	21st to end	Day	16% 2 5% 24% 29% 24%	539
September	1st to 10th	Day	9% 4 12% 24% 30% 21%	490
	11th to 20th	Day	16% 5% 11% 20% 33% 15%	490
	21st to end	Day	16% 3 16% 21% 32% 12%	490
October	1st to 10th	Day	16% 3 18% 32% 21% 10%	490
	11th to 20th	Day	11% 9% 23% 24% 25% 8%	490
	21st to end	Day	17% 7% 19% 29% 22% 5%	539
November	1st to 10th	Day	22% 8% 23% 23% 19% 5%	324
	11th to 20th	Day	17% 14% 33% 23% 10% 2	300
	21st to end	Day	13% 15% 31% 29% 10% 2	300
December	1st to 10th	Day	28% 15% 34% 15% 8%	300
	11th to 20th	Day	23% 15% 29% 25% 7% 1	300
	21st to end	Day	31% 14% 29% 19% 6% 1	330
January	1st to 10th	Night	23% 16% 33% 23% 5% 1	800
	11th to 20th	Night	20% 15% 32% 26% 8%	800
	21st to end	Night	20% 14% 36% 26% 4	880
February	1st to 10th	Night	19% 11% 31% 33% 6%	800
	11th to 20th	Night	26% 11% 36% 25% 2	783
	21st to end	Night	22% 14% 35% 26% 3	660
March	1st to 10th	Night	18% 13% 32% 32% 5%	800
	11th to 20th	Night	18% 11% 34% 27% 10%	800
	21st to end	Night	17% 12% 29% 31% 10%	880
April	1st to 10th	Night	15% 11% 30% 32% 12% 1	761
	11th to 20th	Night	16% 8% 29% 31% 15% 1	593
	21st to end	Night	11% 6% 26% 35% 21% 2	600
May	1st to 10th	Night	10% 7% 23% 36% 23% 1	600
	11th to 20th	Night	13% 4 21% 33% 25% 4	591
	21st to end	Night	11% 2 12% 34% 33% 8%	634
June	1st to 10th	Night	10% 1 8% 39% 36% 7%	570
	11th to 20th	Night	4 2 12% 28% 39% 15%	570
	21st to end	Night	6% 2 7% 32% 43% 10%	570
July	1st to 10th	Night	10% 1 7% 21% 46% 14%	555
	11th to 20th	Night	7% 2 4 18% 51% 17%	552
	21st to end	Night	7% 2 5% 27% 46% 13%	624
August	1st to 10th	Night	7% 2 7% 25% 45% 13%	567
	11th to 20th	Night	11% 1 7% 27% 45% 9%	564
	21st to end	Night	13% 3 5% 27% 42% 10%	627
September	1st to 10th	Night	10% 2 9% 26% 40% 12%	735
	11th to 20th	Night	16% 2 11% 26% 42% 4	760
	21st to end	Night	16% 3 11% 31% 36% 4	760
October	1st to 10th	Night	12% 3 16% 35% 27% 7%	760
	11th to 20th	Night	12% 6% 23% 34% 22% 3	760
	21st to end	Night	14% 5% 21% 36% 21% 3	836
November	1st to 10th	Night	16% 6% 22% 33% 21% 2	780
	11th to 20th	Night	17% 9% 30% 30% 14% 1	800
	21st to end	Night	17% 9% 27% 32% 13% 1	800
December	1st to 10th	Night	19% 10% 30% 32% 9% 1	800
	11th to 20th	Night	22% 8% 29% 30% 10%	800
	21st to end	Night	27% 11% 27% 29% 7%	880

Page generated at 2025-05-23 09:20:58. Version ac07

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