Bradstreet Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bradstreet Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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	16% 6% 2 6% 4 4 4 4 6% 7% 40%	120154
	Night	16% 7% 2 6% 4 4 4 3 5% 6% 43%	118963
Transparency	Day	41% 5% 11% 23% 16% 3	123728
	Night	40% 5% 11% 23% 17% 4	123538
Seeing	Day	20% 14% 28% 22% 12% 3	15866
	Night	19% 11% 31% 28% 10% 1	25141

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% 5% 2 5% 3 4 4 4 5% 6% 49%	3597
	11th to 20th	Day	14% 8% 3 8% 5% 4 5% 3 5% 6% 40%	3690
	21st to end	Day	18% 7% 2 6% 4 4 4 3 4 6% 41%	4466
February	1st to 10th	Day	16% 7% 3 6% 5% 4 5% 4 5% 8% 38%	1800
	11th to 20th	Day	16% 9% 3 6% 4 4 4 4 5% 6% 39%	1894
	21st to end	Day	18% 9% 3 6% 5% 3 4 3 4 6% 39%	1650
March	1st to 10th	Day	20% 5% 2 5% 4 4 4 3 4 6% 45%	4511
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 4 5% 7% 45%	4980
	21st to end	Day	18% 6% 2 6% 4 4 4 3 5% 6% 43%	5780
April	1st to 10th	Day	16% 8% 2 6% 4 5% 4 4 5% 7% 39%	2400
	11th to 20th	Day	15% 7% 2 5% 5% 4 5% 4 5% 7% 41%	2374
	21st to end	Day	16% 5% 2 6% 4 4 5% 5% 6% 8% 39%	2400
May	1st to 10th	Day	26% 8% 2 6% 4 4 5% 4 5% 7% 29%	5824
	11th to 20th	Day	25% 8% 2 6% 4 4 4 3 5% 7% 31%	6318
	21st to end	Day	19% 6% 2 6% 4 4 4 4 5% 9% 36%	6949
June	1st to 10th	Day	13% 6% 2 5% 4 5% 5% 4 7% 9% 39%	2800
	11th to 20th	Day	14% 6% 2 6% 5% 5% 5% 5% 7% 8% 37%	2800
	21st to end	Day	10% 5% 2 6% 4 5% 6% 4 6% 10% 42%	2800
July	1st to 10th	Day	7% 5% 2 6% 5% 6% 7% 6% 9% 11% 36%	4580
	11th to 20th	Day	10% 7% 2 5% 5% 6% 6% 5% 8% 10% 38%	4580
	21st to end	Day	12% 8% 3 8% 6% 5% 5% 5% 6% 9% 33%	4721
August	1st to 10th	Day	19% 5% 2 7% 4 5% 5% 5% 7% 8% 33%	4104
	11th to 20th	Day	12% 5% 2 6% 4 4 4 5% 6% 9% 44%	3739
	21st to end	Day	11% 5% 2 5% 4 4 4 4 6% 8% 48%	4180
September	1st to 10th	Day	15% 5% 1 6% 4 5% 4 3 5% 6% 45%	2280
	11th to 20th	Day	13% 4 2 4 3 3 3 3 5% 8% 51%	2256
	21st to end	Day	19% 8% 3 6% 4 4 4 3 6% 6% 37%	2082
October	1st to 10th	Day	18% 5% 2 5% 3 3 4 3 5% 7% 43%	1900
	11th to 20th	Day	16% 5% 1 4 4 3 3 3 5% 8% 48%	1900
	21st to end	Day	18% 6% 3 4 3 3 4 3 5% 6% 45%	2030
November	1st to 10th	Day	13% 6% 2 4 2 4 3 3 4 7% 52%	1761
	11th to 20th	Day	16% 5% 2 5% 3 3 3 3 6% 6% 48%	1800
	21st to end	Day	16% 6% 2 5% 3 3 4 3 5% 7% 45%	1662
December	1st to 10th	Day	13% 7% 2 6% 4 4 4 4 6% 7% 44%	2600
	11th to 20th	Day	20% 10% 2 4 2 3 3 2 5% 5% 44%	2986
	21st to end	Day	16% 6% 2 5% 4 5% 5% 4 6% 7% 39%	3960
January	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 5% 5% 50%	6349
	11th to 20th	Night	15% 7% 2 5% 3 3 3 3 4 5% 50%	6179
	21st to end	Night	18% 7% 2 5% 4 3 4 3 5% 5% 43%	6490
February	1st to 10th	Night	17% 9% 2 6% 4 4 4 4 4 6% 41%	2600
	11th to 20th	Night	18% 8% 2 6% 4 5% 4 3 5% 5% 41%	2390
	21st to end	Night	17% 8% 2 7% 3 4 4 3 4 6% 42%	1980
March	1st to 10th	Night	18% 7% 3 5% 3 3 3 3 4 6% 44%	5326
	11th to 20th	Night	11% 8% 3 5% 3 3 4 3 4 5% 51%	4979
	21st to end	Night	15% 8% 3 6% 4 4 4 3 5% 5% 43%	5151
April	1st to 10th	Night	17% 7% 2 6% 4 4 4 4 5% 6% 40%	2000
	11th to 20th	Night	15% 7% 2 7% 4 4 4 4 5% 7% 41%	1975
	21st to end	Night	17% 5% 2 5% 4 3 4 4 5% 7% 44%	2000
May	1st to 10th	Night	23% 9% 3 8% 4 4 4 4 5% 8% 28%	4159
	11th to 20th	Night	23% 9% 3 7% 4 3 3 3 5% 7% 32%	3575
	21st to end	Night	21% 6% 3 5% 4 3 3 3 6% 8% 38%	3926
June	1st to 10th	Night	15% 7% 2 6% 4 4 4 3 7% 8% 42%	1600
	11th to 20th	Night	12% 6% 2 6% 5% 5% 6% 4 6% 9% 40%	1600
	21st to end	Night	10% 5% 2 5% 5% 4 5% 5% 8% 11% 40%	1600
July	1st to 10th	Night	8% 5% 3 7% 5% 5% 6% 5% 8% 10% 38%	2600
	11th to 20th	Night	11% 7% 2 7% 5% 4 5% 5% 7% 10% 36%	2600
	21st to end	Night	14% 9% 3 6% 4 4 5% 5% 8% 8% 35%	2827
August	1st to 10th	Night	21% 7% 3 6% 4 4 4 4 6% 8% 34%	2825
	11th to 20th	Night	12% 7% 2 7% 4 5% 4 4 5% 7% 42%	3056
	21st to end	Night	11% 5% 2 5% 4 4 4 3 6% 8% 48%	3476
September	1st to 10th	Night	14% 7% 2 6% 4 3 3 3 4 6% 47%	1900
	11th to 20th	Night	14% 5% 2 4 3 2 4 2 5% 6% 53%	1924
	21st to end	Night	19% 6% 2 6% 4 4 4 4 5% 7% 39%	2095
October	1st to 10th	Night	19% 6% 2 6% 4 4 3 3 4 6% 43%	2280
	11th to 20th	Night	15% 5% 1 5% 4 3 3 3 4 6% 51%	2280
	21st to end	Night	17% 6% 2 5% 3 3 3 2 4 5% 49%	2565
November	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 5% 5% 55%	2537
	11th to 20th	Night	17% 7% 2 5% 3 3 4 3 4 5% 48%	2600
	21st to end	Night	13% 6% 2 5% 4 4 4 3 6% 6% 46%	2735
December	1st to 10th	Night	13% 8% 3 5% 4 3 5% 3 5% 6% 47%	4580
	11th to 20th	Night	22% 7% 2 5% 3 3 3 2 5% 7% 41%	5208
	21st to end	Night	19% 7% 3 6% 4 3 4 3 5% 6% 40%	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	35% 4 9% 26% 23% 3	3598
	11th to 20th	Day	43% 5% 8% 22% 16% 5%	3720
	21st to end	Day	44% 5% 11% 24% 14% 1	4466
February	1st to 10th	Day	43% 6% 14% 24% 11% 2	1800
	11th to 20th	Day	44% 5% 10% 24% 15% 2	1903
	21st to end	Day	45% 5% 10% 23% 13% 4	1650
March	1st to 10th	Day	40% 4 9% 24% 16% 7%	4514
	11th to 20th	Day	36% 5% 11% 28% 15% 5%	4980
	21st to end	Day	43% 4 9% 17% 23% 5%	5780
April	1st to 10th	Day	43% 6% 11% 21% 16% 5%	2400
	11th to 20th	Day	41% 5% 13% 21% 16% 4	2374
	21st to end	Day	41% 7% 11% 20% 17% 4	2400
May	1st to 10th	Day	53% 6% 13% 18% 10% 1	5833
	11th to 20th	Day	52% 6% 9% 22% 11% 1	6322
	21st to end	Day	44% 7% 10% 24% 14% 1	6949
June	1st to 10th	Day	39% 7% 14% 23% 15% 1	2800
	11th to 20th	Day	41% 8% 14% 23% 11% 3	2800
	21st to end	Day	37% 9% 13% 24% 17% 1	2800
July	1st to 10th	Day	40% 9% 16% 19% 15% 2	4510
	11th to 20th	Day	41% 6% 16% 23% 13% 1	4505
	21st to end	Day	47% 6% 11% 19% 17% 1	4571
August	1st to 10th	Day	48% 6% 15% 22% 9% 1	4091
	11th to 20th	Day	37% 6% 11% 31% 15% 1	3775
	21st to end	Day	34% 6% 10% 28% 21% 2	4180
September	1st to 10th	Day	39% 7% 11% 19% 22% 3	2280
	11th to 20th	Day	32% 5% 10% 25% 24% 4	2256
	21st to end	Day	47% 6% 8% 17% 20% 2	2082
October	1st to 10th	Day	37% 3 10% 18% 28% 5%	3160
	11th to 20th	Day	24% 5% 11% 25% 29% 7%	3140
	21st to end	Day	24% 3 12% 27% 29% 5%	3320
November	1st to 10th	Day	34% 4 13% 25% 21% 4	1761
	11th to 20th	Day	36% 5% 11% 25% 20% 3	1800
	21st to end	Day	37% 4 13% 31% 12% 2	1662
December	1st to 10th	Day	40% 4 16% 26% 13% 2	2600
	11th to 20th	Day	44% 3 12% 30% 10% 2	2986
	21st to end	Day	41% 6% 12% 22% 17% 1	3960
January	1st to 10th	Night	36% 4 8% 25% 21% 6%	6349
	11th to 20th	Night	38% 3 11% 24% 17% 7%	6224
	21st to end	Night	44% 4 9% 23% 18% 3	6490
February	1st to 10th	Night	44% 5% 13% 24% 12% 2	2600
	11th to 20th	Night	45% 4 9% 21% 17% 4	2392
	21st to end	Night	43% 5% 8% 22% 17% 5%	1980
March	1st to 10th	Night	41% 4 9% 24% 13% 9%	5336
	11th to 20th	Night	36% 5% 13% 27% 15% 5%	4980
	21st to end	Night	43% 4 7% 16% 25% 4	5151
April	1st to 10th	Night	43% 5% 11% 20% 16% 4	2000
	11th to 20th	Night	42% 6% 10% 19% 20% 3	1975
	21st to end	Night	39% 5% 11% 21% 20% 5%	2000
May	1st to 10th	Night	54% 6% 11% 17% 11% 2	4166
	11th to 20th	Night	52% 6% 11% 18% 12% 1	3576
	21st to end	Night	44% 7% 12% 21% 14% 2	3927
June	1st to 10th	Night	39% 7% 11% 22% 20% 2	1600
	11th to 20th	Night	39% 9% 13% 25% 14% 1	1600
	21st to end	Night	34% 10% 15% 25% 15% 2	1600
July	1st to 10th	Night	39% 10% 19% 17% 13% 2	2560
	11th to 20th	Night	41% 7% 16% 24% 10% 1	2552
	21st to end	Night	43% 7% 15% 19% 15% 1	2737
August	1st to 10th	Night	48% 6% 18% 17% 9% 1	2814
	11th to 20th	Night	41% 6% 12% 25% 15% 1	3086
	21st to end	Night	33% 6% 10% 29% 20% 2	3476
September	1st to 10th	Night	38% 7% 11% 19% 22% 3	1900
	11th to 20th	Night	32% 5% 9% 22% 27% 4	1924
	21st to end	Night	43% 6% 8% 20% 20% 2	2095
October	1st to 10th	Night	43% 3 10% 21% 21% 2	3800
	11th to 20th	Night	24% 4 10% 29% 29% 5%	3779
	21st to end	Night	27% 3 11% 32% 24% 4	4213
November	1st to 10th	Night	32% 5% 12% 25% 21% 6%	2537
	11th to 20th	Night	40% 4 9% 22% 20% 5%	2600
	21st to end	Night	38% 5% 13% 27% 16% 2	2735
December	1st to 10th	Night	39% 4 11% 31% 13% 2	4580
	11th to 20th	Night	44% 4 10% 24% 13% 4	5208
	21st to end	Night	44% 5% 11% 20% 18% 2	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	26% 28% 31% 12% 2 2	200
	11th to 20th	Day	24% 36% 29% 9% 3	227
	21st to end	Day	24% 29% 28% 14% 5%	330
February	1st to 10th	Day	25% 26% 34% 11% 3	300
	11th to 20th	Day	32% 23% 33% 11% 2	442
	21st to end	Day	28% 23% 31% 14% 3	414
March	1st to 10th	Day	23% 24% 31% 18% 4	500
	11th to 20th	Day	22% 24% 34% 15% 4 1	500
	21st to end	Day	27% 21% 31% 15% 4 1	550
April	1st to 10th	Day	27% 22% 32% 12% 6% 1	500
	11th to 20th	Day	23% 21% 32% 16% 6% 2	492
	21st to end	Day	24% 19% 31% 19% 5% 2	500
May	1st to 10th	Day	30% 17% 28% 15% 9% 2	503
	11th to 20th	Day	24% 12% 30% 23% 9% 2	527
	21st to end	Day	22% 7% 24% 26% 17% 4	573
June	1st to 10th	Day	14% 14% 28% 26% 15% 3	530
	11th to 20th	Day	17% 10% 28% 27% 15% 3	530
	21st to end	Day	12% 10% 28% 30% 15% 5%	530
July	1st to 10th	Day	14% 3 20% 33% 23% 7%	525
	11th to 20th	Day	9% 3 22% 33% 27% 6%	524
	21st to end	Day	14% 4 26% 34% 18% 5%	532
August	1st to 10th	Day	12% 3 25% 31% 21% 6%	486
	11th to 20th	Day	13% 4 19% 35% 20% 8%	479
	21st to end	Day	13% 7% 23% 29% 18% 9%	539
September	1st to 10th	Day	18% 10% 21% 27% 18% 7%	490
	11th to 20th	Day	15% 5% 25% 28% 17% 9%	490
	21st to end	Day	26% 12% 20% 25% 12% 4	490
October	1st to 10th	Day	25% 7% 24% 22% 14% 7%	490
	11th to 20th	Day	16% 16% 29% 25% 13% 2	490
	21st to end	Day	21% 14% 32% 20% 10% 3	506
November	1st to 10th	Day	17% 13% 32% 24% 12% 3	395
	11th to 20th	Day	20% 19% 34% 22% 6% 1	400
	21st to end	Day	23% 19% 34% 19% 5% 1	262
December	1st to 10th	Day	25% 24% 37% 9% 6%	200
	11th to 20th	Day	32% 22% 32% 11% 5%	200
	21st to end	Day	25% 20% 33% 12% 9%	220
January	1st to 10th	Night	21% 19% 37% 20% 4	800
	11th to 20th	Night	25% 17% 31% 24% 4	800
	21st to end	Night	20% 18% 39% 22% 2	880
February	1st to 10th	Night	22% 15% 38% 24% 1	800
	11th to 20th	Night	26% 19% 38% 16% 1	783
	21st to end	Night	22% 19% 38% 18% 3	660
March	1st to 10th	Night	21% 19% 35% 23% 3	800
	11th to 20th	Night	21% 18% 37% 19% 5% 1	800
	21st to end	Night	20% 14% 40% 21% 5%	880
April	1st to 10th	Night	22% 19% 37% 18% 5%	800
	11th to 20th	Night	20% 13% 35% 26% 6%	791
	21st to end	Night	21% 12% 33% 29% 6%	800
May	1st to 10th	Night	24% 10% 32% 28% 6%	654
	11th to 20th	Night	20% 11% 33% 30% 6%	394
	21st to end	Night	19% 6% 21% 39% 13% 2	433
June	1st to 10th	Night	18% 5% 28% 37% 11% 1	400
	11th to 20th	Night	13% 6% 29% 41% 11% 1	400
	21st to end	Night	11% 4 25% 46% 14%	400
July	1st to 10th	Night	13% 4 21% 42% 19% 2	390
	11th to 20th	Night	9% 3 20% 45% 22% 2	388
	21st to end	Night	10% 4 24% 41% 19% 2	468
August	1st to 10th	Night	13% 2 17% 43% 23% 2	567
	11th to 20th	Night	15% 2 18% 38% 25% 2	721
	21st to end	Night	14% 4 18% 39% 23% 2	836
September	1st to 10th	Night	17% 4 25% 34% 17% 3	760
	11th to 20th	Night	15% 4 19% 36% 22% 4	760
	21st to end	Night	23% 4 22% 32% 18% 1	760
October	1st to 10th	Night	23% 7% 23% 26% 17% 4	760
	11th to 20th	Night	17% 10% 28% 32% 12% 1	760
	21st to end	Night	21% 10% 32% 27% 10%	836
November	1st to 10th	Night	15% 8% 36% 28% 11% 1	780
	11th to 20th	Night	21% 14% 30% 26% 8% 1	800
	21st to end	Night	17% 17% 33% 25% 7% 1	800
December	1st to 10th	Night	25% 16% 33% 21% 6% 1	800
	11th to 20th	Night	21% 15% 36% 24% 6%	800
	21st to end	Night	22% 14% 34% 27% 3	880

Page generated at 2025-07-18 01:10:00. Version ac07

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