Brooks Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Brooks Astronomical Observatory clear sky chart from 2005-11-06 to 2026-01-17.

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 9% 5% 5% 5% 4 5% 7% 34%	126584
	Night	14% 11% 4 9% 5% 5% 4 4 5% 7% 34%	125278
Transparency	Day	47% 6% 12% 21% 12% 1	130111
	Night	48% 6% 12% 21% 12% 1	129811
Seeing	Day	18% 16% 30% 20% 11% 4	17255
	Night	19% 12% 31% 27% 10% 1	27383

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	11% 17% 7% 17% 8% 6% 5% 3 4 5% 18%	3697
	11th to 20th	Day	11% 15% 7% 17% 13% 10% 7% 4 4 4 9%	3668
	21st to end	Day	16% 17% 8% 14% 6% 5% 6% 4 4 5% 16%	4000
February	1st to 10th	Day	11% 13% 4 13% 9% 6% 5% 5% 5% 6% 22%	1800
	11th to 20th	Day	12% 14% 5% 13% 9% 7% 5% 3 4 6% 23%	1943
	21st to end	Day	12% 14% 5% 12% 8% 7% 5% 4 5% 6% 23%	1650
March	1st to 10th	Day	12% 10% 3 9% 6% 5% 5% 3 5% 6% 35%	4480
	11th to 20th	Day	11% 9% 3 9% 6% 5% 5% 4 6% 8% 33%	4928
	21st to end	Day	19% 11% 5% 8% 5% 5% 4 3 5% 6% 30%	5545
April	1st to 10th	Day	17% 14% 3 8% 6% 4 4 3 5% 6% 30%	2400
	11th to 20th	Day	16% 10% 4 7% 4 4 4 4 5% 7% 36%	2487
	21st to end	Day	15% 8% 4 8% 6% 5% 4 4 6% 6% 34%	2600
May	1st to 10th	Day	18% 8% 2 6% 4 4 5% 3 6% 7% 37%	5862
	11th to 20th	Day	14% 7% 2 7% 5% 5% 5% 4 6% 8% 37%	5861
	21st to end	Day	13% 7% 2 7% 4 4 4 3 5% 8% 42%	6878
June	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 4 6% 9% 43%	2800
	11th to 20th	Day	11% 6% 2 6% 4 5% 5% 4 5% 9% 42%	2800
	21st to end	Day	11% 5% 2 6% 4 5% 5% 5% 7% 9% 41%	2800
July	1st to 10th	Day	9% 6% 3 7% 5% 5% 6% 5% 7% 9% 38%	6360
	11th to 20th	Day	6% 5% 2 6% 4 4 4 4 6% 9% 49%	6360
	21st to end	Day	7% 4 2 6% 5% 5% 5% 4 6% 10% 47%	6907
August	1st to 10th	Day	11% 5% 2 7% 5% 5% 5% 5% 6% 8% 42%	4234
	11th to 20th	Day	11% 5% 2 6% 4 4 5% 4 5% 7% 46%	4174
	21st to end	Day	9% 5% 2 7% 5% 5% 6% 4 6% 8% 43%	4495
September	1st to 10th	Day	10% 5% 2 7% 6% 5% 4 4 6% 8% 43%	2334
	11th to 20th	Day	10% 5% 2 6% 4 4 4 3 6% 9% 48%	2200
	21st to end	Day	12% 7% 3 8% 4 5% 4 4 5% 8% 40%	2200
October	1st to 10th	Day	14% 7% 2 6% 4 4 4 3 5% 6% 43%	2199
	11th to 20th	Day	19% 10% 3 8% 5% 5% 5% 4 5% 7% 30%	1945
	21st to end	Day	19% 9% 4 9% 5% 6% 6% 4 5% 7% 26%	1980
November	1st to 10th	Day	13% 9% 4 8% 5% 4 5% 3 6% 7% 36%	1852
	11th to 20th	Day	13% 16% 5% 9% 6% 6% 5% 4 6% 6% 25%	1890
	21st to end	Day	19% 16% 5% 10% 6% 5% 5% 3 5% 5% 23%	1885
December	1st to 10th	Day	12% 18% 5% 15% 7% 6% 5% 4 5% 7% 18%	2482
	11th to 20th	Day	16% 18% 8% 12% 7% 5% 4 4 4 4 18%	2818
	21st to end	Day	26% 20% 6% 11% 5% 5% 4 3 4 4 13%	4070
January	1st to 10th	Night	11% 14% 7% 16% 7% 6% 5% 4 5% 6% 20%	6488
	11th to 20th	Night	11% 14% 7% 17% 11% 9% 5% 4 5% 5% 12%	6445
	21st to end	Night	14% 16% 6% 12% 6% 5% 6% 4 6% 6% 18%	6974
February	1st to 10th	Night	13% 15% 4 12% 8% 6% 6% 3 5% 6% 23%	2600
	11th to 20th	Night	12% 14% 5% 10% 7% 6% 4 4 5% 6% 27%	2340
	21st to end	Night	15% 13% 4 10% 6% 5% 5% 5% 6% 6% 25%	1980
March	1st to 10th	Night	14% 7% 4 7% 5% 4 4 3 5% 7% 40%	5373
	11th to 20th	Night	11% 7% 2 6% 5% 4 4 4 6% 9% 39%	5031
	21st to end	Night	17% 11% 4 8% 4 4 4 3 5% 7% 34%	5427
April	1st to 10th	Night	16% 11% 3 6% 5% 5% 5% 3 5% 6% 36%	2000
	11th to 20th	Night	18% 9% 2 7% 4 4 4 3 5% 6% 37%	1862
	21st to end	Night	16% 9% 3 6% 5% 4 3 4 6% 7% 38%	1800
May	1st to 10th	Night	15% 8% 3 7% 5% 4 4 4 5% 7% 36%	4033
	11th to 20th	Night	17% 7% 3 7% 4 4 4 3 5% 7% 40%	4032
	21st to end	Night	17% 5% 2 5% 3 4 3 3 5% 6% 47%	3994
June	1st to 10th	Night	10% 7% 2 6% 5% 4 4 4 6% 9% 43%	1600
	11th to 20th	Night	10% 5% 2 7% 4 4 4 5% 6% 8% 44%	1600
	21st to end	Night	8% 5% 2 6% 5% 5% 5% 4 5% 9% 47%	1600
July	1st to 10th	Night	8% 7% 2 6% 5% 4 5% 4 6% 8% 44%	3618
	11th to 20th	Night	6% 4 2 6% 4 4 4 4 6% 8% 53%	3617
	21st to end	Night	8% 4 2 6% 4 4 4 4 6% 8% 50%	3960
August	1st to 10th	Night	9% 5% 2 5% 4 4 4 3 6% 7% 50%	2915
	11th to 20th	Night	12% 5% 2 4 3 3 4 3 5% 7% 50%	2875
	21st to end	Night	9% 5% 2 6% 4 4 4 4 5% 8% 49%	3387
September	1st to 10th	Night	9% 4 2 5% 4 3 4 3 5% 7% 52%	2045
	11th to 20th	Night	9% 4 2 4 3 4 4 3 5% 7% 57%	2200
	21st to end	Night	12% 7% 2 6% 4 5% 5% 3 5% 7% 43%	2200
October	1st to 10th	Night	17% 5% 2 4 4 3 3 3 5% 6% 49%	2200
	11th to 20th	Night	18% 11% 3 7% 5% 4 5% 3 4 5% 36%	2452
	21st to end	Night	21% 10% 3 8% 4 4 4 3 5% 7% 32%	2860
November	1st to 10th	Night	13% 10% 3 7% 5% 4 4 4 5% 6% 40%	2666
	11th to 20th	Night	14% 13% 3 8% 6% 5% 5% 4 7% 7% 28%	2730
	21st to end	Night	17% 14% 4 9% 5% 5% 5% 4 5% 7% 25%	2730
December	1st to 10th	Night	12% 15% 5% 12% 6% 6% 5% 4 6% 7% 20%	4920
	11th to 20th	Night	14% 18% 6% 12% 7% 5% 5% 4 5% 5% 18%	5574
	21st to end	Night	26% 21% 5% 10% 5% 3 3 3 4 4 16%	7150

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	66% 7% 13% 11% 3	3698
	11th to 20th	Day	77% 6% 5% 7% 5%	3668
	21st to end	Day	70% 7% 9% 9% 6%	4000
February	1st to 10th	Day	61% 7% 12% 15% 6%	1800
	11th to 20th	Day	63% 6% 10% 14% 7% 1	1953
	21st to end	Day	61% 6% 11% 15% 6% 1	1650
March	1st to 10th	Day	49% 5% 13% 19% 11% 3	4480
	11th to 20th	Day	47% 6% 16% 19% 11% 1	4928
	21st to end	Day	56% 5% 11% 16% 11% 1	5547
April	1st to 10th	Day	55% 5% 11% 18% 11%	2400
	11th to 20th	Day	48% 6% 11% 20% 13% 2	2487
	21st to end	Day	49% 6% 12% 18% 12% 2	2600
May	1st to 10th	Day	44% 7% 12% 19% 17% 1	5897
	11th to 20th	Day	43% 7% 12% 22% 14% 1	5865
	21st to end	Day	40% 6% 10% 23% 19% 2	6892
June	1st to 10th	Day	37% 7% 13% 28% 15% 1	2800
	11th to 20th	Day	38% 6% 15% 23% 17% 1	2800
	21st to end	Day	37% 8% 15% 23% 14% 2	2800
July	1st to 10th	Day	41% 5% 13% 25% 14% 2	6291
	11th to 20th	Day	32% 5% 13% 34% 16% 1	6285
	21st to end	Day	32% 5% 15% 27% 20% 1	6711
August	1st to 10th	Day	40% 5% 12% 27% 15% 1	4221
	11th to 20th	Day	36% 6% 13% 30% 15% 1	4213
	21st to end	Day	38% 5% 14% 29% 13% 1	4494
September	1st to 10th	Day	37% 8% 15% 27% 13% 1	2334
	11th to 20th	Day	33% 6% 15% 29% 15% 1	2200
	21st to end	Day	41% 7% 13% 26% 13%	2200
October	1st to 10th	Day	30% 4 8% 33% 21% 3	3576
	11th to 20th	Day	41% 4 12% 30% 13%	3148
	21st to end	Day	52% 5% 12% 22% 7% 1	3180
November	1st to 10th	Day	45% 7% 14% 22% 11% 1	1852
	11th to 20th	Day	58% 7% 13% 16% 6% 1	1890
	21st to end	Day	62% 6% 13% 14% 4	1885
December	1st to 10th	Day	66% 5% 17% 10% 2	2482
	11th to 20th	Day	69% 5% 10% 13% 3	2814
	21st to end	Day	74% 6% 7% 9% 5%	4070
January	1st to 10th	Night	62% 7% 10% 14% 7% 1	6488
	11th to 20th	Night	72% 7% 8% 8% 5%	6445
	21st to end	Night	64% 6% 12% 11% 7%	6974
February	1st to 10th	Night	61% 7% 10% 15% 6% 1	2600
	11th to 20th	Night	57% 6% 11% 17% 8% 1	2341
	21st to end	Night	56% 7% 13% 16% 8% 1	1980
March	1st to 10th	Night	44% 5% 15% 24% 11% 1	5373
	11th to 20th	Night	39% 6% 18% 24% 12% 1	5031
	21st to end	Night	49% 6% 13% 20% 11% 1	5427
April	1st to 10th	Night	48% 5% 11% 20% 15% 1	2000
	11th to 20th	Night	46% 6% 9% 21% 16% 2	1862
	21st to end	Night	44% 7% 11% 21% 14% 3	1800
May	1st to 10th	Night	45% 7% 12% 15% 18% 3	4060
	11th to 20th	Night	43% 5% 11% 24% 15% 2	4033
	21st to end	Night	38% 5% 8% 24% 20% 4	4002
June	1st to 10th	Night	36% 6% 11% 27% 17% 2	1600
	11th to 20th	Night	36% 7% 15% 22% 20% 1	1600
	21st to end	Night	33% 7% 13% 29% 16% 2	1600
July	1st to 10th	Night	36% 4 12% 27% 16% 5%	3580
	11th to 20th	Night	29% 4 13% 31% 21% 1	3572
	21st to end	Night	32% 5% 16% 27% 19% 1	3845
August	1st to 10th	Night	31% 6% 15% 30% 18% 1	2904
	11th to 20th	Night	32% 5% 13% 33% 16% 1	2902
	21st to end	Night	34% 4 12% 32% 16% 1	3387
September	1st to 10th	Night	31% 7% 13% 31% 18% 1	2045
	11th to 20th	Night	27% 6% 14% 34% 18% 1	2200
	21st to end	Night	40% 7% 14% 25% 13% 1	2200
October	1st to 10th	Night	27% 4 11% 36% 22% 1	3590
	11th to 20th	Night	43% 4 11% 24% 17% 1	3988
	21st to end	Night	45% 4 10% 29% 11%	4610
November	1st to 10th	Night	43% 6% 13% 24% 14% 1	2666
	11th to 20th	Night	52% 8% 15% 18% 7% 1	2730
	21st to end	Night	57% 7% 15% 16% 5% 1	2730
December	1st to 10th	Night	60% 6% 18% 12% 3	4920
	11th to 20th	Night	65% 6% 9% 12% 7%	5576
	21st to end	Night	73% 5% 8% 8% 7%	7150

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% 21% 38% 13% 4 1	330
	11th to 20th	Day	24% 24% 39% 10% 3	318
	21st to end	Day	26% 20% 36% 15% 3	330
February	1st to 10th	Day	25% 30% 30% 13% 1 1	300
	11th to 20th	Day	24% 28% 37% 9% 1	300
	21st to end	Day	34% 30% 29% 6% 1	249
March	1st to 10th	Day	26% 29% 36% 9%	300
	11th to 20th	Day	26% 19% 30% 20% 5%	474
	21st to end	Day	28% 19% 30% 17% 5% 1	550
April	1st to 10th	Day	26% 20% 32% 17% 5% 1	500
	11th to 20th	Day	29% 22% 29% 11% 6% 3	492
	21st to end	Day	21% 19% 30% 21% 8% 1	500
May	1st to 10th	Day	24% 19% 31% 16% 7% 3	503
	11th to 20th	Day	19% 14% 30% 21% 11% 5%	527
	21st to end	Day	15% 13% 31% 24% 13% 5%	758
June	1st to 10th	Day	11% 14% 37% 26% 9% 4	730
	11th to 20th	Day	14% 12% 35% 22% 11% 5%	730
	21st to end	Day	14% 11% 34% 25% 13% 3	730
July	1st to 10th	Day	8% 7% 25% 30% 22% 8%	721
	11th to 20th	Day	9% 10% 27% 28% 17% 8%	718
	21st to end	Day	8% 8% 30% 30% 17% 7%	789
August	1st to 10th	Day	8% 6% 25% 25% 21% 13%	526
	11th to 20th	Day	10% 5% 20% 32% 21% 11%	519
	21st to end	Day	12% 8% 23% 28% 20% 8%	583
September	1st to 10th	Day	11% 11% 27% 23% 16% 11%	530
	11th to 20th	Day	9% 7% 19% 26% 29% 11%	530
	21st to end	Day	18% 13% 22% 24% 18% 6%	530
October	1st to 10th	Day	18% 15% 27% 21% 16% 4	530
	11th to 20th	Day	23% 17% 33% 20% 7% 1	475
	21st to end	Day	25% 20% 36% 14% 4 1	363
November	1st to 10th	Day	24% 20% 36% 14% 5%	330
	11th to 20th	Day	25% 25% 36% 9% 4 1	330
	21st to end	Day	25% 27% 33% 12% 3	327
December	1st to 10th	Day	29% 25% 29% 10% 6% 1	220
	11th to 20th	Day	26% 29% 33% 10% 1	250
	21st to end	Day	33% 25% 31% 9% 2 1	363
January	1st to 10th	Night	22% 19% 34% 20% 5% 1	1050
	11th to 20th	Night	23% 20% 32% 20% 4 1	1031
	21st to end	Night	19% 14% 37% 23% 6%	1091
February	1st to 10th	Night	21% 16% 35% 24% 4	800
	11th to 20th	Night	22% 18% 34% 22% 4	783
	21st to end	Night	23% 22% 35% 18% 2	660
March	1st to 10th	Night	19% 17% 34% 24% 6%	800
	11th to 20th	Night	18% 17% 36% 23% 6% 1	800
	21st to end	Night	23% 15% 33% 23% 5% 1	865
April	1st to 10th	Night	21% 15% 37% 22% 4 1	600
	11th to 20th	Night	23% 16% 30% 24% 7% 1	593
	21st to end	Night	21% 12% 33% 26% 6% 1	600
May	1st to 10th	Night	17% 10% 37% 30% 7% 1	600
	11th to 20th	Night	19% 8% 32% 30% 11% 1	591
	21st to end	Night	14% 6% 31% 32% 16% 1	650
June	1st to 10th	Night	11% 8% 31% 37% 13% 1	600
	11th to 20th	Night	14% 6% 29% 36% 15% 1	600
	21st to end	Night	10% 5% 31% 37% 17% 1	600
July	1st to 10th	Night	8% 4 21% 42% 22% 3	585
	11th to 20th	Night	8% 5% 21% 41% 21% 3	582
	21st to end	Night	8% 2 28% 41% 20% 1	657
August	1st to 10th	Night	8% 3 22% 44% 21% 3	597
	11th to 20th	Night	9% 3 21% 39% 25% 3	594
	21st to end	Night	12% 5% 21% 36% 24% 2	660
September	1st to 10th	Night	10% 5% 26% 33% 22% 4	645
	11th to 20th	Night	11% 5% 21% 35% 25% 3	800
	21st to end	Night	14% 8% 27% 29% 18% 3	800
October	1st to 10th	Night	19% 8% 27% 29% 15% 1	800
	11th to 20th	Night	25% 14% 29% 24% 7% 1	800
	21st to end	Night	26% 14% 33% 22% 5%	880
November	1st to 10th	Night	20% 15% 33% 23% 9%	820
	11th to 20th	Night	22% 19% 32% 20% 6%	840
	21st to end	Night	24% 18% 32% 22% 4	840
December	1st to 10th	Night	24% 17% 33% 22% 4	964
	11th to 20th	Night	24% 18% 37% 18% 3	1050
	21st to end	Night	32% 15% 32% 18% 2	1155

Page generated at 2026-01-14 01:14:29. Version ac07

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