Brennan Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Brennan Hill Observatory clear sky chart from 2005-11-06 to 2026-07-31.

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	17% 8% 3 7% 5% 4 4 4 5% 7% 35%	137958
	Night	17% 9% 3 7% 5% 4 4 3 5% 6% 36%	135089
Transparency	Day	46% 6% 11% 21% 13% 2	141480
	Night	45% 6% 12% 21% 14% 2	139656
Seeing	Day	26% 18% 29% 17% 7% 2	17125
	Night	23% 17% 30% 23% 7% 1	26015

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	10% 9% 5% 14% 7% 6% 4 3 5% 5% 31%	4661
	11th to 20th	Day	13% 10% 4 8% 6% 5% 4 4 5% 6% 36%	4650
	21st to end	Day	12% 11% 4 7% 4 4 4 3 5% 6% 41%	5124
February	1st to 10th	Day	15% 9% 4 7% 6% 5% 5% 6% 5% 7% 31%	1922
	11th to 20th	Day	13% 9% 3 11% 6% 5% 4 3 6% 6% 34%	2048
	21st to end	Day	17% 11% 5% 8% 5% 4 4 3 4 5% 34%	1730
March	1st to 10th	Day	19% 12% 5% 10% 5% 4 3 3 4 4 31%	4576
	11th to 20th	Day	15% 10% 4 8% 5% 4 4 3 4 6% 37%	4793
	21st to end	Day	24% 10% 3 6% 4 4 4 3 4 6% 33%	6045
April	1st to 10th	Day	19% 11% 3 7% 6% 3 3 3 4 6% 34%	2520
	11th to 20th	Day	17% 8% 2 7% 5% 5% 5% 4 6% 6% 35%	2494
	21st to end	Day	20% 9% 3 7% 5% 4 4 3 5% 5% 35%	2709
May	1st to 10th	Day	28% 9% 3 6% 4 5% 4 4 5% 6% 26%	7639
	11th to 20th	Day	19% 9% 3 8% 5% 4 4 3 4 6% 33%	8032
	21st to end	Day	21% 8% 3 7% 4 5% 4 4 6% 8% 30%	9060
June	1st to 10th	Day	18% 6% 2 5% 6% 5% 5% 5% 6% 9% 32%	2940
	11th to 20th	Day	16% 7% 3 6% 5% 5% 4 4 5% 9% 36%	2940
	21st to end	Day	17% 7% 2 7% 5% 5% 5% 4 6% 8% 33%	2940
July	1st to 10th	Day	15% 6% 2 7% 5% 5% 5% 5% 7% 9% 33%	6500
	11th to 20th	Day	10% 6% 3 7% 5% 5% 6% 5% 7% 11% 36%	6500
	21st to end	Day	16% 6% 2 6% 4 4 4 4 6% 7% 41%	7066
August	1st to 10th	Day	15% 6% 3 7% 5% 5% 5% 4 6% 8% 35%	4404
	11th to 20th	Day	13% 7% 3 7% 5% 5% 6% 4 6% 9% 35%	4043
	21st to end	Day	8% 5% 2 7% 5% 5% 5% 4 8% 9% 42%	4312
September	1st to 10th	Day	13% 6% 2 5% 4 4 4 4 6% 8% 44%	2400
	11th to 20th	Day	13% 6% 2 5% 4 3 4 2 4 7% 49%	2235
	21st to end	Day	17% 6% 2 5% 4 4 4 3 5% 7% 41%	2200
October	1st to 10th	Day	22% 7% 3 6% 4 4 4 2 4 5% 39%	2200
	11th to 20th	Day	15% 8% 2 7% 4 3 4 3 5% 7% 42%	1925
	21st to end	Day	21% 9% 2 7% 5% 4 4 3 4 7% 35%	1980
November	1st to 10th	Day	13% 9% 3 6% 4 3 4 3 4 5% 46%	1851
	11th to 20th	Day	17% 9% 3 6% 4 4 3 3 4 6% 42%	1890
	21st to end	Day	19% 10% 3 6% 4 4 3 3 5% 6% 36%	1835
December	1st to 10th	Day	14% 10% 4 8% 4 4 4 3 5% 6% 36%	2680
	11th to 20th	Day	19% 11% 4 7% 3 3 4 3 3 4 38%	3066
	21st to end	Day	19% 12% 3 8% 5% 4 4 3 4 6% 33%	4048
January	1st to 10th	Night	12% 10% 5% 11% 5% 5% 5% 4 5% 5% 33%	8234
	11th to 20th	Night	13% 12% 3 11% 7% 6% 5% 4 5% 6% 28%	8235
	21st to end	Night	13% 9% 3 7% 5% 5% 5% 4 6% 6% 37%	9057
February	1st to 10th	Night	14% 10% 3 8% 5% 5% 4 3 5% 7% 36%	2695
	11th to 20th	Night	11% 10% 3 7% 6% 5% 5% 4 7% 7% 35%	2458
	21st to end	Night	15% 11% 3 8% 4 4 4 4 5% 6% 35%	2076
March	1st to 10th	Night	20% 10% 3 7% 5% 5% 5% 3 5% 6% 33%	5484
	11th to 20th	Night	16% 9% 3 7% 5% 4 5% 4 5% 6% 36%	5385
	21st to end	Night	24% 10% 3 7% 4 3 4 3 5% 6% 31%	5124
April	1st to 10th	Night	20% 8% 3 7% 5% 4 4 3 6% 6% 34%	2100
	11th to 20th	Night	19% 10% 3 6% 4 3 4 3 5% 5% 38%	2075
	21st to end	Night	19% 8% 4 7% 4 4 3 3 5% 5% 39%	1911
May	1st to 10th	Night	26% 11% 3 6% 4 4 4 4 6% 6% 26%	5259
	11th to 20th	Night	16% 9% 3 8% 6% 5% 4 4 6% 7% 31%	4882
	21st to end	Night	22% 8% 3 7% 5% 5% 5% 4 5% 7% 30%	5114
June	1st to 10th	Night	18% 8% 3 6% 4 3 4 4 4 7% 39%	1680
	11th to 20th	Night	18% 8% 3 6% 5% 4 5% 3 6% 6% 37%	1680
	21st to end	Night	16% 8% 2 5% 4 4 3 5% 7% 7% 40%	1680
July	1st to 10th	Night	14% 6% 3 6% 5% 4 5% 4 6% 8% 40%	3680
	11th to 20th	Night	13% 7% 3 7% 4 5% 5% 4 6% 8% 37%	3680
	21st to end	Night	12% 6% 2 6% 3 4 4 3 5% 6% 49%	4006
August	1st to 10th	Night	22% 7% 2 6% 4 4 4 3 5% 7% 36%	2713
	11th to 20th	Night	14% 8% 3 9% 4 5% 4 3 5% 8% 37%	3001
	21st to end	Night	8% 4 2 4 3 3 4 3 6% 8% 56%	3586
September	1st to 10th	Night	13% 6% 2 5% 3 3 4 4 5% 7% 48%	2000
	11th to 20th	Night	13% 5% 1 4 3 4 3 3 4 7% 52%	2163
	21st to end	Night	15% 7% 2 6% 4 3 3 3 5% 6% 46%	2200
October	1st to 10th	Night	20% 8% 2 6% 4 3 3 3 4 6% 42%	2200
	11th to 20th	Night	16% 7% 2 6% 3 4 4 3 5% 7% 42%	2473
	21st to end	Night	21% 9% 2 6% 4 3 4 4 4 6% 35%	2860
November	1st to 10th	Night	16% 7% 2 7% 4 4 3 3 4 6% 45%	2667
	11th to 20th	Night	16% 9% 2 6% 4 4 4 4 4 5% 42%	2730
	21st to end	Night	15% 9% 3 7% 5% 4 4 3 5% 6% 39%	2783
December	1st to 10th	Night	17% 11% 4 9% 5% 4 4 3 5% 6% 31%	4720
	11th to 20th	Night	23% 10% 3 7% 4 3 3 3 4 4 36%	5348
	21st to end	Night	19% 12% 4 9% 4 3 3 3 4 5% 34%	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	51% 5% 10% 23% 10% 1	4662
	11th to 20th	Day	45% 5% 11% 20% 16% 3	4680
	21st to end	Day	44% 5% 8% 23% 18% 2	5124
February	1st to 10th	Day	49% 7% 13% 17% 13% 2	1922
	11th to 20th	Day	48% 6% 10% 20% 14% 2	2057
	21st to end	Day	51% 5% 9% 19% 13% 2	1730
March	1st to 10th	Day	55% 4 10% 13% 10% 8%	4579
	11th to 20th	Day	44% 9% 14% 21% 10% 2	4793
	21st to end	Day	50% 4 7% 15% 18% 5%	6045
April	1st to 10th	Day	51% 5% 9% 18% 14% 3	2520
	11th to 20th	Day	46% 8% 10% 19% 16% 2	2494
	21st to end	Day	50% 6% 8% 19% 16% 3	2709
May	1st to 10th	Day	57% 7% 11% 14% 11% 1	7651
	11th to 20th	Day	51% 6% 15% 18% 10% 1	8036
	21st to end	Day	50% 7% 10% 22% 10% 1	9061
June	1st to 10th	Day	46% 8% 13% 22% 12% 1	2940
	11th to 20th	Day	44% 8% 13% 19% 14% 2	2940
	21st to end	Day	47% 8% 13% 20% 12% 1	2940
July	1st to 10th	Day	47% 7% 13% 19% 14% 1	6430
	11th to 20th	Day	40% 8% 16% 23% 13% 1	6425
	21st to end	Day	41% 5% 12% 26% 15% 1	6868
August	1st to 10th	Day	46% 8% 15% 21% 11%	4390
	11th to 20th	Day	43% 8% 15% 25% 8% 1	4082
	21st to end	Day	34% 7% 13% 29% 15% 2	4312
September	1st to 10th	Day	38% 6% 12% 25% 17% 1	2400
	11th to 20th	Day	35% 5% 9% 25% 23% 2	2235
	21st to end	Day	42% 6% 9% 23% 17% 3	2200
October	1st to 10th	Day	45% 5% 7% 23% 17% 2	3590
	11th to 20th	Day	36% 4 10% 26% 20% 4	3115
	21st to end	Day	37% 4 10% 30% 15% 3	3180
November	1st to 10th	Day	39% 6% 13% 25% 15% 2	1851
	11th to 20th	Day	42% 5% 10% 20% 19% 3	1890
	21st to end	Day	47% 5% 15% 21% 10% 2	1835
December	1st to 10th	Day	46% 6% 13% 26% 9% 1	2680
	11th to 20th	Day	48% 5% 11% 18% 17% 1	3066
	21st to end	Day	51% 6% 8% 17% 13% 6%	4048
January	1st to 10th	Night	46% 6% 14% 21% 11% 2	8234
	11th to 20th	Night	50% 6% 13% 15% 13% 4	8280
	21st to end	Night	46% 5% 10% 18% 18% 4	9057
February	1st to 10th	Night	46% 7% 12% 21% 12% 2	2695
	11th to 20th	Night	44% 7% 13% 20% 13% 2	2460
	21st to end	Night	47% 6% 10% 22% 11% 4	2076
March	1st to 10th	Night	48% 5% 11% 17% 15% 5%	5494
	11th to 20th	Night	45% 9% 16% 18% 10% 2	5386
	21st to end	Night	50% 5% 8% 13% 21% 4	5124
April	1st to 10th	Night	49% 6% 11% 18% 14% 2	2100
	11th to 20th	Night	46% 7% 10% 19% 16% 2	2075
	21st to end	Night	46% 6% 10% 18% 16% 3	1911
May	1st to 10th	Night	56% 8% 12% 13% 10% 1	5265
	11th to 20th	Night	50% 8% 13% 19% 9% 1	4883
	21st to end	Night	53% 7% 11% 20% 9% 1	5115
June	1st to 10th	Night	45% 7% 11% 25% 11% 1	1680
	11th to 20th	Night	46% 8% 13% 20% 12% 1	1680
	21st to end	Night	41% 9% 14% 23% 13% 1	1680
July	1st to 10th	Night	42% 6% 14% 24% 13%	3640
	11th to 20th	Night	43% 8% 15% 19% 15%	3632
	21st to end	Night	37% 5% 13% 30% 15% 1	3889
August	1st to 10th	Night	49% 7% 12% 21% 10% 1	2703
	11th to 20th	Night	45% 7% 15% 23% 9%	3028
	21st to end	Night	26% 5% 16% 34% 17% 1	3586
September	1st to 10th	Night	36% 6% 13% 26% 19% 1	2000
	11th to 20th	Night	33% 5% 11% 27% 23% 2	2163
	21st to end	Night	40% 6% 9% 26% 16% 2	2200
October	1st to 10th	Night	40% 5% 14% 24% 14% 2	3590
	11th to 20th	Night	32% 4 12% 25% 22% 4	4022
	21st to end	Night	38% 5% 11% 31% 13% 2	4610
November	1st to 10th	Night	40% 6% 12% 22% 18% 3	2667
	11th to 20th	Night	42% 5% 12% 22% 17% 3	2730
	21st to end	Night	42% 7% 15% 24% 11% 1	2783
December	1st to 10th	Night	49% 5% 12% 26% 8%	4720
	11th to 20th	Night	48% 6% 13% 13% 19% 1	5348
	21st to end	Night	49% 6% 8% 18% 15% 4	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	30% 24% 30% 13% 3	220
	11th to 20th	Day	36% 23% 30% 8% 3	220
	21st to end	Day	28% 31% 28% 10% 2	242
February	1st to 10th	Day	32% 30% 26% 10% 1	382
	11th to 20th	Day	33% 25% 30% 11% 1	536
	21st to end	Day	34% 26% 28% 11% 2	446
March	1st to 10th	Day	34% 20% 32% 11% 3	540
	11th to 20th	Day	33% 28% 26% 9% 4 1	540
	21st to end	Day	34% 23% 29% 7% 4 2	594
April	1st to 10th	Day	37% 22% 24% 10% 5% 1	540
	11th to 20th	Day	32% 26% 26% 10% 5% 2	532
	21st to end	Day	31% 19% 31% 12% 6% 1	540
May	1st to 10th	Day	38% 20% 24% 11% 5% 1	543
	11th to 20th	Day	23% 21% 35% 14% 5% 2	567
	21st to end	Day	22% 17% 31% 23% 6% 1	617
June	1st to 10th	Day	22% 14% 35% 20% 8% 2	570
	11th to 20th	Day	22% 16% 33% 21% 6% 1	570
	21st to end	Day	21% 17% 35% 17% 9% 1	570
July	1st to 10th	Day	18% 11% 32% 25% 12% 2	565
	11th to 20th	Day	16% 10% 33% 29% 10% 2	564
	21st to end	Day	17% 12% 34% 25% 9% 3	616
August	1st to 10th	Day	15% 10% 30% 29% 12% 4	526
	11th to 20th	Day	16% 9% 28% 27% 15% 5%	519
	21st to end	Day	14% 13% 31% 27% 12% 5%	583
September	1st to 10th	Day	19% 11% 27% 25% 11% 6%	530
	11th to 20th	Day	17% 13% 26% 21% 14% 9%	530
	21st to end	Day	19% 12% 35% 17% 11% 5%	530
October	1st to 10th	Day	28% 13% 23% 21% 11% 4	530
	11th to 20th	Day	24% 18% 28% 20% 7% 2	489
	21st to end	Day	35% 13% 27% 19% 4 1	462
November	1st to 10th	Day	27% 20% 26% 17% 9% 1	425
	11th to 20th	Day	31% 24% 24% 17% 3 1	430
	21st to end	Day	36% 20% 27% 13% 4 1	375
December	1st to 10th	Day	30% 27% 28% 11% 3	220
	11th to 20th	Day	40% 25% 25% 9% 2	220
	21st to end	Day	34% 21% 31% 10% 4	242
January	1st to 10th	Night	22% 27% 33% 16% 3	840
	11th to 20th	Night	28% 23% 30% 16% 2	840
	21st to end	Night	21% 24% 33% 19% 3	924
February	1st to 10th	Night	23% 25% 33% 17% 2	840
	11th to 20th	Night	25% 24% 35% 15% 1	823
	21st to end	Night	28% 25% 30% 15% 2	692
March	1st to 10th	Night	26% 21% 33% 17% 2	840
	11th to 20th	Night	25% 25% 28% 18% 3	840
	21st to end	Night	27% 20% 31% 17% 5%	924
April	1st to 10th	Night	30% 20% 28% 19% 3	840
	11th to 20th	Night	26% 20% 29% 19% 5%	831
	21st to end	Night	27% 18% 29% 21% 4	651
May	1st to 10th	Night	30% 12% 32% 21% 4	630
	11th to 20th	Night	21% 15% 35% 24% 5%	503
	21st to end	Night	21% 7% 32% 31% 8% 1	455
June	1st to 10th	Night	21% 8% 28% 33% 10%	420
	11th to 20th	Night	21% 12% 30% 29% 7% 1	420
	21st to end	Night	18% 8% 27% 39% 9%	420
July	1st to 10th	Night	16% 5% 31% 37% 10%	410
	11th to 20th	Night	14% 4 30% 35% 14% 2	408
	21st to end	Night	13% 4 33% 37% 13%	458
August	1st to 10th	Night	15% 4 26% 36% 19%	473
	11th to 20th	Night	16% 8% 25% 31% 17% 2	669
	21st to end	Night	16% 5% 25% 34% 18% 2	880
September	1st to 10th	Night	17% 9% 27% 31% 15% 2	800
	11th to 20th	Night	17% 7% 25% 29% 18% 4	800
	21st to end	Night	18% 9% 27% 30% 14% 2	800
October	1st to 10th	Night	25% 11% 23% 26% 13% 2	800
	11th to 20th	Night	22% 14% 29% 24% 10% 1	800
	21st to end	Night	27% 18% 27% 22% 6% 1	880
November	1st to 10th	Night	22% 19% 30% 22% 7% 1	820
	11th to 20th	Night	23% 23% 27% 20% 6% 1	840
	21st to end	Night	23% 21% 32% 18% 5% 1	840
December	1st to 10th	Night	28% 20% 30% 18% 4	840
	11th to 20th	Night	28% 22% 31% 15% 4	840
	21st to end	Night	29% 21% 33% 15% 3	924

Page generated at 2026-07-28 03:25:24. Version ac07

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