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 2024-11-02.

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% 42%	100947
	Night	13% 6% 2 6% 4 4 4 4 6% 7% 44%	97987
Transparency	Day	39% 6% 12% 21% 18% 4	100757
	Night	38% 5% 11% 20% 20% 6%	97846
Seeing	Day	15% 8% 20% 23% 21% 13%	14881
	Night	15% 7% 22% 30% 22% 4	24878

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	17% 10% 2 7% 4 4 4 4 5% 5% 37%	2836
	11th to 20th	Day	12% 6% 2 5% 4 3 3 4 4 5% 52%	2840
	21st to end	Day	26% 8% 2 5% 4 3 3 3 4 4 39%	3261
February	1st to 10th	Day	15% 8% 3 7% 5% 5% 4 3 5% 6% 38%	1900
	11th to 20th	Day	21% 7% 3 5% 4 4 4 3 4 5% 40%	1848
	21st to end	Day	21% 7% 2 6% 5% 4 4 3 5% 6% 38%	1570
March	1st to 10th	Day	24% 12% 3 6% 3 3 3 3 4 6% 33%	3213
	11th to 20th	Day	12% 6% 3 6% 4 4 4 3 5% 6% 46%	3480
	21st to end	Day	19% 6% 2 6% 4 4 4 4 5% 7% 40%	3815
April	1st to 10th	Day	12% 6% 2 5% 4 5% 4 4 5% 7% 46%	2141
	11th to 20th	Day	11% 6% 2 7% 5% 4 4 3 4 7% 47%	2254
	21st to end	Day	9% 5% 1 5% 4 4 4 4 6% 8% 50%	2425
May	1st to 10th	Day	10% 6% 3 6% 4 4 4 4 6% 9% 44%	4082
	11th to 20th	Day	16% 8% 2 6% 4 4 5% 4 6% 8% 36%	4081
	21st to end	Day	8% 4 2 5% 4 4 5% 4 7% 10% 46%	4476
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 4 3 3 5% 55%	2090
November	1st to 10th	Day	12% 5% 2 5% 3 3 3 3 5% 5% 56%	1705
	11th to 20th	Day	13% 5% 2 4 4 4 4 3 5% 5% 52%	1710
	21st to end	Day	13% 7% 2 5% 4 4 5% 3 5% 7% 46%	1710
December	1st to 10th	Day	19% 8% 3 6% 4 4 5% 4 5% 7% 37%	1710
	11th to 20th	Day	16% 7% 4 7% 4 4 3 3 5% 5% 43%	2153
	21st to end	Day	22% 8% 3 6% 4 4 4 4 6% 7% 35%	3124
January	1st to 10th	Night	18% 7% 3 5% 4 4 4 3 5% 6% 40%	4109
	11th to 20th	Night	12% 8% 3 6% 4 4 4 3 5% 5% 46%	4120
	21st to end	Night	23% 11% 3 6% 3 3 3 2 3 4 40%	4392
February	1st to 10th	Night	16% 7% 2 5% 4 4 3 3 4 6% 43%	2280
	11th to 20th	Night	20% 8% 3 5% 4 4 3 3 4 5% 42%	2218
	21st to end	Night	20% 7% 3 6% 5% 4 3 3 5% 6% 40%	1884
March	1st to 10th	Night	24% 9% 3 6% 4 3 4 3 4 6% 35%	3637
	11th to 20th	Night	11% 6% 2 4 3 3 4 4 6% 7% 50%	3480
	21st to end	Night	16% 6% 2 7% 4 4 4 4 5% 6% 43%	3817
April	1st to 10th	Night	12% 5% 2 5% 4 4 5% 4 5% 7% 47%	2053
	11th to 20th	Night	14% 5% 2 5% 3 3 4 3 6% 7% 49%	1875
	21st to end	Night	8% 5% 2 5% 4 3 4 3 5% 8% 53%	1753
May	1st to 10th	Night	8% 6% 2 7% 5% 5% 5% 4 7% 8% 42%	2813
	11th to 20th	Night	15% 7% 2 6% 4 4 5% 3 6% 7% 40%	2812
	21st to end	Night	6% 4 1 6% 3 4 5% 5% 7% 12% 48%	3079
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	12% 4 1 4 3 3 2 2 4 5% 57%	2393
	11th to 20th	Night	13% 5% 2 5% 3 3 3 3 4 6% 53%	2470
	21st to end	Night	15% 6% 2 4 4 4 4 3 5% 6% 47%	2470
December	1st to 10th	Night	17% 6% 2 7% 4 4 4 3 6% 6% 41%	2470
	11th to 20th	Night	17% 8% 2 5% 3 3 3 2 4 5% 48%	3042
	21st to end	Night	19% 9% 3 7% 5% 4 4 3 5% 7% 34%	4532

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	46% 3 8% 16% 21% 6%	2837
	11th to 20th	Day	34% 3 7% 25% 24% 6%	2840
	21st to end	Day	49% 2 4 18% 21% 6%	3261
February	1st to 10th	Day	46% 4 7% 17% 21% 5%	1900
	11th to 20th	Day	46% 2 6% 18% 21% 6%	1858
	21st to end	Day	48% 3 8% 14% 20% 8%	1570
March	1st to 10th	Day	54% 2 9% 12% 14% 8%	3213
	11th to 20th	Day	38% 4 9% 18% 24% 8%	3480
	21st to end	Day	44% 3 9% 21% 19% 4	3815
April	1st to 10th	Day	38% 4 9% 23% 20% 6%	2141
	11th to 20th	Day	37% 4 8% 20% 25% 7%	2254
	21st to end	Day	31% 5% 11% 28% 23% 2	2425
May	1st to 10th	Day	36% 4 14% 24% 19% 2	4120
	11th to 20th	Day	44% 5% 13% 25% 11% 1	4085
	21st to end	Day	32% 6% 15% 26% 20% 1	4489
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	31% 3 9% 18% 30% 7%	2090
	11th to 20th	Day	28% 4 7% 24% 26% 11%	1990
	21st to end	Day	33% 3 6% 18% 31% 10%	2090
November	1st to 10th	Day	32% 4 7% 15% 31% 11%	1705
	11th to 20th	Day	34% 3 7% 20% 26% 10%	1710
	21st to end	Day	38% 4 7% 19% 25% 7%	1710
December	1st to 10th	Day	47% 4 7% 19% 17% 6%	1710
	11th to 20th	Day	45% 2 5% 17% 24% 7%	2150
	21st to end	Day	47% 3 8% 20% 19% 3	3124
January	1st to 10th	Night	43% 3 5% 17% 26% 6%	4109
	11th to 20th	Night	39% 4 6% 22% 21% 8%	4120
	21st to end	Night	50% 2 4 12% 25% 6%	4392
February	1st to 10th	Night	42% 3 8% 18% 20% 9%	2280
	11th to 20th	Night	46% 3 5% 17% 21% 8%	2219
	21st to end	Night	46% 3 8% 16% 17% 10%	1884
March	1st to 10th	Night	51% 3 9% 16% 14% 7%	3637
	11th to 20th	Night	33% 3 9% 16% 27% 12%	3480
	21st to end	Night	41% 4 6% 19% 20% 9%	3817
April	1st to 10th	Night	35% 3 10% 20% 23% 8%	2053
	11th to 20th	Night	35% 3 7% 20% 24% 11%	1875
	21st to end	Night	30% 5% 10% 28% 22% 6%	1753
May	1st to 10th	Night	37% 5% 16% 22% 17% 2	2840
	11th to 20th	Night	42% 4 13% 26% 13% 1	2813
	21st to end	Night	27% 6% 17% 27% 21% 3	3087
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	30% 5% 10% 18% 28% 9%	2090
	11th to 20th	Night	28% 4 6% 22% 28% 12%	2190
	21st to end	Night	29% 3 5% 19% 31% 13%	2508
November	1st to 10th	Night	30% 3 8% 17% 28% 13%	2393
	11th to 20th	Night	33% 3 5% 18% 28% 13%	2470
	21st to end	Night	37% 3 7% 19% 26% 8%	2470
December	1st to 10th	Night	43% 4 9% 19% 20% 6%	2470
	11th to 20th	Night	39% 2 8% 16% 28% 7%	3044
	21st to end	Night	49% 3 6% 20% 18% 3	4532

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	22% 21% 37% 18% 2	270
	11th to 20th	Day	25% 18% 30% 19% 7% 1	270
	21st to end	Day	24% 21% 35% 15% 5% 1	297
February	1st to 10th	Day	21% 19% 34% 21% 5%	270
	11th to 20th	Day	36% 14% 33% 14% 3	270
	21st to end	Day	31% 14% 38% 14% 3	225
March	1st to 10th	Day	23% 16% 29% 23% 7% 2	324
	11th to 20th	Day	20% 15% 30% 26% 8%	460
	21st to end	Day	20% 14% 33% 24% 8% 1	506
April	1st to 10th	Day	18% 15% 28% 21% 15% 2	460
	11th to 20th	Day	12% 13% 31% 25% 17% 2	452
	21st to end	Day	15% 8% 24% 28% 21% 5%	460
May	1st to 10th	Day	11% 9% 26% 26% 21% 7%	463
	11th to 20th	Day	13% 4 19% 25% 24% 15%	487
	21st to end	Day	13% 4 13% 25% 26% 19%	529
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% 30% 22% 5%	539
November	1st to 10th	Day	20% 6% 24% 26% 19% 5%	297
	11th to 20th	Day	17% 14% 33% 24% 10% 2	270
	21st to end	Day	12% 13% 32% 29% 11% 2	270
December	1st to 10th	Day	28% 14% 33% 16% 9%	270
	11th to 20th	Day	23% 16% 29% 24% 8% 1	270
	21st to end	Day	31% 13% 31% 19% 6% 1	297
January	1st to 10th	Night	23% 14% 34% 24% 5% 1	760
	11th to 20th	Night	19% 15% 31% 26% 8%	760
	21st to end	Night	21% 13% 36% 26% 4	836
February	1st to 10th	Night	19% 10% 32% 33% 6%	760
	11th to 20th	Night	24% 11% 37% 25% 3	743
	21st to end	Night	23% 14% 35% 25% 4	628
March	1st to 10th	Night	17% 13% 32% 33% 6%	760
	11th to 20th	Night	18% 10% 35% 26% 10%	760
	21st to end	Night	17% 12% 29% 32% 11%	836
April	1st to 10th	Night	14% 10% 30% 33% 13% 1	723
	11th to 20th	Night	16% 8% 29% 31% 15% 1	563
	21st to end	Night	11% 5% 26% 35% 21% 2	570
May	1st to 10th	Night	10% 7% 22% 35% 24% 1	570
	11th to 20th	Night	12% 4 20% 34% 26% 5%	561
	21st to end	Night	12% 2 11% 34% 34% 8%	617
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	15% 6% 22% 34% 22% 2	745
	11th to 20th	Night	16% 9% 30% 30% 14% 1	760
	21st to end	Night	17% 8% 27% 33% 14% 1	760
December	1st to 10th	Night	19% 10% 29% 32% 9% 1	760
	11th to 20th	Night	22% 8% 30% 30% 10%	760
	21st to end	Night	27% 10% 27% 29% 7%	836

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