Bradley
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bradley clear sky chart from 2005-11-06 to 2025-12-03.

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% 9% 3 8% 5% 5% 4 4 6% 8% 36%	126917
	Night	14% 9% 4 8% 5% 4 4 3 5% 6% 38%	124004
Transparency	Day	45% 6% 11% 23% 14% 2	130452
	Night	46% 5% 11% 22% 15% 2	128543
Seeing	Day	16% 10% 28% 25% 15% 6%	17223
	Night	18% 9% 30% 30% 12% 1	27043

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	12% 12% 8% 14% 6% 5% 4 4 5% 7% 23%	3617
	11th to 20th	Day	11% 13% 6% 14% 7% 7% 4 5% 6% 4 23%	3620
	21st to end	Day	17% 18% 5% 10% 5% 3 4 2 4 6% 26%	4136
February	1st to 10th	Day	18% 11% 4 9% 7% 5% 4 4 5% 6% 26%	1882
	11th to 20th	Day	14% 13% 4 8% 6% 5% 4 3 4 6% 33%	1948
	21st to end	Day	16% 13% 5% 10% 5% 4 4 3 5% 6% 31%	1650
March	1st to 10th	Day	19% 13% 3 7% 4 3 3 2 3 5% 38%	4480
	11th to 20th	Day	8% 8% 4 7% 5% 4 5% 4 6% 9% 40%	4653
	21st to end	Day	18% 13% 5% 11% 5% 5% 5% 3 5% 7% 22%	5967
April	1st to 10th	Day	17% 12% 3 7% 4 5% 3 4 5% 7% 33%	2400
	11th to 20th	Day	15% 10% 3 7% 5% 4 5% 4 5% 6% 37%	2374
	21st to end	Day	19% 8% 3 8% 5% 4 4 4 6% 7% 33%	2505
May	1st to 10th	Day	18% 8% 3 6% 5% 5% 5% 4 6% 8% 31%	5862
	11th to 20th	Day	14% 6% 2 6% 4 5% 5% 4 7% 8% 39%	6041
	21st to end	Day	13% 6% 3 7% 5% 4 5% 4 6% 9% 38%	6935
June	1st to 10th	Day	10% 5% 2 6% 4 4 5% 4 7% 8% 44%	2800
	11th to 20th	Day	9% 5% 2 6% 5% 4 5% 4 7% 8% 44%	2800
	21st to end	Day	10% 6% 3 6% 5% 5% 6% 5% 7% 9% 40%	2800
July	1st to 10th	Day	9% 6% 2 7% 6% 5% 5% 5% 8% 10% 37%	6360
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 5% 7% 11% 39%	6360
	21st to end	Day	11% 6% 2 7% 5% 5% 5% 5% 7% 9% 38%	6924
August	1st to 10th	Day	7% 5% 2 6% 5% 5% 4 4 6% 9% 48%	4403
	11th to 20th	Day	9% 5% 2 6% 4 5% 4 5% 7% 9% 43%	3852
	21st to end	Day	6% 4 2 7% 4 4 5% 4 7% 10% 48%	4312
September	1st to 10th	Day	10% 6% 2 5% 4 4 5% 4 6% 7% 47%	2400
	11th to 20th	Day	11% 4 2 4 3 3 4 4 6% 7% 51%	2257
	21st to end	Day	10% 6% 3 5% 4 5% 4 3 6% 8% 45%	2200
October	1st to 10th	Day	11% 6% 3 6% 4 4 4 4 5% 7% 47%	2175
	11th to 20th	Day	17% 7% 2 6% 5% 4 4 4 5% 7% 39%	1987
	21st to end	Day	18% 10% 3 8% 4 4 4 4 5% 6% 35%	1980
November	1st to 10th	Day	12% 8% 3 5% 4 3 3 3 4 8% 46%	1852
	11th to 20th	Day	17% 10% 4 9% 3 4 4 3 4 6% 36%	1890
	21st to end	Day	16% 12% 3 10% 5% 3 4 3 4 5% 34%	1879
December	1st to 10th	Day	10% 12% 3 9% 4 4 4 3 4 5% 42%	2626
	11th to 20th	Day	13% 15% 5% 11% 6% 5% 5% 4 6% 6% 23%	3008
	21st to end	Day	26% 21% 7% 12% 6% 4 4 3 3 4 11%	3982
January	1st to 10th	Night	15% 13% 7% 12% 6% 5% 4 3 5% 5% 27%	6348
	11th to 20th	Night	13% 16% 7% 14% 7% 5% 5% 3 5% 4 20%	6360
	21st to end	Night	20% 14% 5% 9% 5% 4 4 3 3 4 30%	6840
February	1st to 10th	Night	17% 13% 5% 9% 5% 4 4 4 4 6% 30%	2515
	11th to 20th	Night	15% 11% 3 8% 6% 5% 5% 4 5% 6% 33%	2338
	21st to end	Night	18% 11% 4 9% 4 4 3 3 5% 5% 34%	1980
March	1st to 10th	Night	16% 11% 4 7% 4 4 3 3 4 5% 39%	5373
	11th to 20th	Night	11% 7% 2 6% 4 4 5% 3 5% 7% 45%	5336
	21st to end	Night	18% 10% 5% 9% 5% 5% 5% 4 5% 5% 28%	4962
April	1st to 10th	Night	14% 9% 3 7% 5% 4 4 3 5% 7% 40%	2000
	11th to 20th	Night	18% 8% 2 6% 5% 4 4 3 4 7% 39%	1975
	21st to end	Night	18% 7% 3 7% 5% 5% 4 4 6% 6% 36%	1895
May	1st to 10th	Night	18% 9% 4 8% 4 4 4 4 5% 6% 34%	4033
	11th to 20th	Night	13% 7% 2 6% 4 4 4 4 6% 9% 42%	3846
	21st to end	Night	14% 9% 3 7% 4 4 4 4 6% 7% 39%	3942
June	1st to 10th	Night	9% 5% 2 6% 4 4 3 3 5% 7% 51%	1600
	11th to 20th	Night	8% 6% 2 6% 4 4 4 3 7% 9% 47%	1600
	21st to end	Night	9% 6% 2 6% 6% 4 5% 5% 7% 8% 44%	1600
July	1st to 10th	Night	9% 6% 2 7% 5% 5% 5% 5% 7% 9% 41%	3618
	11th to 20th	Night	6% 5% 2 6% 5% 4 5% 4 6% 8% 48%	3617
	21st to end	Night	7% 6% 3 8% 5% 5% 5% 5% 6% 8% 43%	3947
August	1st to 10th	Night	6% 4 2 6% 5% 5% 5% 4 7% 9% 49%	2751
	11th to 20th	Night	6% 4 2 5% 4 4 4 4 6% 9% 51%	3193
	21st to end	Night	7% 4 2 5% 4 3 4 4 5% 8% 53%	3586
September	1st to 10th	Night	10% 4 2 5% 4 3 4 2 4 8% 54%	2000
	11th to 20th	Night	8% 4 1 4 4 3 4 3 5% 8% 56%	2121
	21st to end	Night	10% 6% 2 6% 4 5% 4 4 6% 7% 46%	2200
October	1st to 10th	Night	10% 5% 2 5% 3 4 3 3 5% 6% 53%	2222
	11th to 20th	Night	15% 7% 3 6% 3 3 4 3 5% 6% 47%	2411
	21st to end	Night	19% 9% 2 7% 4 3 4 2 5% 7% 39%	2860
November	1st to 10th	Night	12% 7% 3 5% 3 4 3 4 5% 6% 47%	2666
	11th to 20th	Night	14% 9% 3 7% 4 4 5% 3 5% 6% 39%	2730
	21st to end	Night	18% 11% 3 8% 5% 4 4 2 4 6% 35%	2735
December	1st to 10th	Night	12% 9% 3 7% 4 4 4 3 5% 5% 44%	4609
	11th to 20th	Night	18% 13% 4 9% 5% 4 4 3 4 5% 31%	5199
	21st to end	Night	25% 18% 6% 11% 5% 4 4 3 4 4 16%	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	61% 5% 11% 16% 7% 1	3618
	11th to 20th	Day	62% 3 9% 9% 12% 4	3620
	21st to end	Day	61% 3 12% 17% 7%	4136
February	1st to 10th	Day	58% 6% 11% 16% 8% 1	1882
	11th to 20th	Day	54% 4 9% 22% 9% 2	1958
	21st to end	Day	55% 4 11% 21% 7% 2	1650
March	1st to 10th	Day	50% 4 7% 23% 13% 3	4480
	11th to 20th	Day	39% 5% 10% 33% 11% 1	4653
	21st to end	Day	61% 6% 12% 14% 7% 1	5969
April	1st to 10th	Day	51% 5% 11% 20% 14%	2400
	11th to 20th	Day	48% 6% 10% 22% 12% 3	2374
	21st to end	Day	49% 6% 12% 19% 12% 2	2505
May	1st to 10th	Day	48% 7% 10% 21% 12% 1	5897
	11th to 20th	Day	42% 8% 10% 26% 13% 2	6045
	21st to end	Day	41% 6% 10% 24% 16% 3	6949
June	1st to 10th	Day	36% 8% 11% 24% 18% 3	2800
	11th to 20th	Day	35% 8% 10% 24% 20% 4	2800
	21st to end	Day	38% 8% 14% 21% 16% 3	2800
July	1st to 10th	Day	38% 7% 13% 24% 16% 2	6291
	11th to 20th	Day	36% 8% 15% 26% 14% 1	6285
	21st to end	Day	39% 8% 17% 21% 14% 2	6728
August	1st to 10th	Day	31% 7% 17% 30% 14% 1	4388
	11th to 20th	Day	32% 8% 12% 31% 16% 1	3888
	21st to end	Day	30% 7% 13% 28% 18% 3	4311
September	1st to 10th	Day	35% 6% 11% 26% 18% 4	2400
	11th to 20th	Day	31% 6% 10% 26% 23% 4	2257
	21st to end	Day	36% 6% 8% 26% 22% 2	2200
October	1st to 10th	Day	26% 4 7% 28% 32% 2	3539
	11th to 20th	Day	34% 4 11% 24% 26% 1	3215
	21st to end	Day	42% 4 9% 25% 18% 2	3180
November	1st to 10th	Day	37% 5% 14% 26% 16% 3	1852
	11th to 20th	Day	51% 3 11% 22% 12% 1	1890
	21st to end	Day	53% 4 10% 22% 11% 1	1879
December	1st to 10th	Day	46% 4 14% 28% 9%	2626
	11th to 20th	Day	60% 5% 12% 17% 5%	3005
	21st to end	Day	79% 3 6% 7% 4	3982
January	1st to 10th	Night	59% 4 10% 16% 10%	6348
	11th to 20th	Night	67% 3 9% 9% 10% 2	6360
	21st to end	Night	60% 3 9% 17% 10% 1	6840
February	1st to 10th	Night	56% 5% 10% 18% 10% 2	2515
	11th to 20th	Night	51% 5% 10% 21% 12% 1	2339
	21st to end	Night	52% 5% 10% 19% 11% 3	1980
March	1st to 10th	Night	48% 4 9% 26% 10% 3	5373
	11th to 20th	Night	38% 5% 13% 26% 16% 3	5336
	21st to end	Night	56% 6% 9% 21% 7% 1	4962
April	1st to 10th	Night	44% 6% 12% 23% 13% 2	2000
	11th to 20th	Night	46% 5% 8% 20% 17% 4	1975
	21st to end	Night	46% 7% 11% 20% 14% 3	1895
May	1st to 10th	Night	50% 6% 10% 17% 14% 4	4060
	11th to 20th	Night	38% 8% 13% 26% 13% 3	3847
	21st to end	Night	43% 7% 10% 20% 16% 4	3950
June	1st to 10th	Night	32% 7% 10% 26% 21% 4	1600
	11th to 20th	Night	33% 5% 12% 25% 22% 3	1600
	21st to end	Night	37% 7% 15% 23% 15% 4	1600
July	1st to 10th	Night	38% 7% 12% 26% 14% 4	3580
	11th to 20th	Night	33% 7% 14% 25% 20% 1	3572
	21st to end	Night	37% 7% 15% 22% 15% 4	3832
August	1st to 10th	Night	29% 7% 18% 28% 16% 1	2742
	11th to 20th	Night	29% 7% 15% 29% 19% 2	3223
	21st to end	Night	29% 6% 11% 33% 19% 3	3586
September	1st to 10th	Night	30% 6% 11% 25% 23% 5%	2000
	11th to 20th	Night	27% 5% 8% 28% 28% 4	2121
	21st to end	Night	35% 8% 9% 25% 20% 3	2200
October	1st to 10th	Night	21% 4 7% 31% 34% 3	3638
	11th to 20th	Night	32% 4 8% 24% 30% 2	3922
	21st to end	Night	38% 3 9% 24% 21% 4	4610
November	1st to 10th	Night	37% 5% 9% 27% 18% 4	2666
	11th to 20th	Night	46% 4 11% 22% 14% 2	2730
	21st to end	Night	52% 4 10% 23% 11%	2735
December	1st to 10th	Night	42% 3 14% 27% 12% 1	4609
	11th to 20th	Night	57% 4 12% 16% 11% 1	5201
	21st to end	Night	72% 4 8% 10% 5%	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	23% 12% 32% 24% 7% 1	300
	11th to 20th	Day	24% 18% 39% 14% 4 1	300
	21st to end	Day	22% 17% 33% 22% 5% 1	330
February	1st to 10th	Day	23% 19% 37% 17% 4	300
	11th to 20th	Day	26% 15% 35% 20% 4	300
	21st to end	Day	23% 18% 43% 12% 3	249
March	1st to 10th	Day	22% 22% 35% 19% 2	300
	11th to 20th	Day	21% 19% 34% 19% 7%	337
	21st to end	Day	30% 15% 31% 17% 5% 1	550
April	1st to 10th	Day	25% 15% 30% 22% 8% 1	500
	11th to 20th	Day	21% 15% 32% 18% 10% 3	492
	21st to end	Day	22% 14% 34% 20% 8% 2	500
May	1st to 10th	Day	21% 11% 33% 21% 11% 3	503
	11th to 20th	Day	19% 8% 31% 22% 15% 5%	603
	21st to end	Day	14% 8% 33% 22% 15% 8%	789
June	1st to 10th	Day	11% 8% 28% 28% 19% 5%	730
	11th to 20th	Day	11% 6% 28% 32% 18% 5%	730
	21st to end	Day	10% 8% 32% 30% 14% 6%	730
July	1st to 10th	Day	9% 4 20% 29% 23% 15%	721
	11th to 20th	Day	9% 3 24% 31% 20% 13%	718
	21st to end	Day	8% 4 21% 34% 26% 8%	794
August	1st to 10th	Day	7% 2 16% 33% 27% 14%	631
	11th to 20th	Day	9% 4 19% 28% 27% 13%	519
	21st to end	Day	7% 3 16% 35% 27% 12%	583
September	1st to 10th	Day	11% 5% 18% 28% 27% 10%	530
	11th to 20th	Day	9% 4 15% 24% 32% 15%	530
	21st to end	Day	12% 7% 27% 23% 21% 9%	530
October	1st to 10th	Day	12% 6% 25% 31% 19% 7%	505
	11th to 20th	Day	18% 20% 26% 27% 9% 1	330
	21st to end	Day	29% 13% 33% 18% 6% 1	363
November	1st to 10th	Day	15% 14% 38% 22% 8% 4	330
	11th to 20th	Day	23% 15% 37% 16% 8% 2	330
	21st to end	Day	27% 12% 35% 20% 5%	330
December	1st to 10th	Day	17% 18% 38% 18% 8% 1	306
	11th to 20th	Day	18% 20% 36% 21% 5%	300
	21st to end	Day	28% 20% 33% 16% 2	330
January	1st to 10th	Night	21% 15% 38% 21% 5%	1000
	11th to 20th	Night	22% 15% 36% 22% 4 1	1000
	21st to end	Night	22% 13% 36% 25% 4	1100
February	1st to 10th	Night	25% 12% 37% 23% 3	915
	11th to 20th	Night	20% 13% 36% 25% 5%	783
	21st to end	Night	24% 15% 38% 21% 3	660
March	1st to 10th	Night	21% 14% 36% 24% 5%	800
	11th to 20th	Night	18% 12% 38% 27% 5% 1	799
	21st to end	Night	25% 10% 34% 24% 6%	660
April	1st to 10th	Night	17% 12% 40% 25% 5% 1	600
	11th to 20th	Night	22% 12% 32% 26% 7%	593
	21st to end	Night	21% 11% 33% 28% 7% 1	600
May	1st to 10th	Night	18% 6% 35% 31% 10%	600
	11th to 20th	Night	17% 6% 29% 34% 13% 1	591
	21st to end	Night	12% 5% 28% 36% 18% 1	650
June	1st to 10th	Night	12% 4 25% 45% 14%	600
	11th to 20th	Night	12% 4 28% 42% 14% 1	600
	21st to end	Night	11% 3 26% 40% 20% 1	600
July	1st to 10th	Night	8% 2 16% 42% 28% 4	585
	11th to 20th	Night	7% 2 20% 38% 29% 3	582
	21st to end	Night	6% 1 17% 46% 29% 1	657
August	1st to 10th	Night	6% 1 10% 50% 28% 4	597
	11th to 20th	Night	7% 2 11% 43% 33% 4	594
	21st to end	Night	7% 2 16% 38% 35% 3	660
September	1st to 10th	Night	11% 3 15% 39% 29% 4	600
	11th to 20th	Night	9% 3 15% 39% 30% 4	721
	21st to end	Night	12% 5% 23% 33% 24% 3	800
October	1st to 10th	Night	12% 5% 24% 39% 19% 2	800
	11th to 20th	Night	17% 13% 32% 28% 10%	800
	21st to end	Night	23% 13% 32% 24% 7%	880
November	1st to 10th	Night	16% 10% 39% 25% 9% 1	820
	11th to 20th	Night	18% 16% 35% 24% 6%	840
	21st to end	Night	24% 13% 33% 23% 7%	845
December	1st to 10th	Night	23% 12% 36% 24% 5%	1011
	11th to 20th	Night	24% 11% 34% 25% 5%	1000
	21st to end	Night	29% 13% 34% 22% 2	1100

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