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 2025-03-27.

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% 9% 3 7% 5% 4 4 4 5% 7% 36%	108752
	Night	16% 9% 3 7% 5% 4 4 3 5% 6% 37%	110587
Transparency	Day	45% 7% 12% 21% 14% 2	112320
	Night	44% 7% 12% 21% 14% 2	115183
Seeing	Day	26% 18% 29% 18% 7% 2	15341
	Night	23% 17% 29% 23% 7% 1	24432

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	12% 9% 5% 14% 7% 6% 4 3 5% 5% 32%	3597
	11th to 20th	Day	12% 9% 4 9% 6% 5% 4 4 5% 6% 37%	3570
	21st to end	Day	14% 11% 5% 8% 4 4 3 3 4 5% 38%	3960
February	1st to 10th	Day	15% 10% 4 7% 6% 5% 5% 6% 5% 7% 30%	1830
	11th to 20th	Day	13% 9% 3 10% 6% 5% 4 3 6% 6% 34%	1948
	21st to end	Day	17% 11% 4 8% 5% 4 4 3 4 5% 34%	1650
March	1st to 10th	Day	20% 14% 5% 9% 6% 5% 3 2 4 4 28%	3216
	11th to 20th	Day	13% 11% 4 7% 6% 4 4 3 5% 6% 37%	3378
	21st to end	Day	20% 10% 3 8% 5% 4 4 3 4 6% 34%	4195
April	1st to 10th	Day	19% 11% 3 7% 6% 3 3 3 4 6% 34%	2280
	11th to 20th	Day	17% 8% 2 7% 5% 5% 5% 4 6% 6% 36%	2254
	21st to end	Day	20% 9% 3 8% 5% 4 4 3 5% 5% 33%	2451
May	1st to 10th	Day	24% 11% 4 7% 4 5% 4 4 4 6% 27%	4106
	11th to 20th	Day	20% 9% 3 8% 5% 4 4 3 5% 6% 33%	4292
	21st to end	Day	14% 6% 2 5% 4 4 4 3 6% 9% 42%	4837
June	1st to 10th	Day	18% 7% 2 5% 6% 6% 5% 5% 6% 9% 31%	2660
	11th to 20th	Day	16% 7% 3 6% 5% 5% 4 4 5% 8% 36%	2660
	21st to end	Day	18% 7% 3 7% 5% 6% 5% 5% 6% 7% 32%	2660
July	1st to 10th	Day	13% 5% 3 6% 5% 5% 5% 5% 7% 9% 36%	4440
	11th to 20th	Day	11% 6% 2 6% 5% 5% 6% 5% 7% 11% 35%	4440
	21st to end	Day	17% 8% 3 7% 5% 4 5% 4 6% 7% 34%	4814
August	1st to 10th	Day	16% 6% 3 7% 5% 5% 5% 4 6% 8% 34%	4269
	11th to 20th	Day	13% 7% 3 7% 5% 5% 6% 4 6% 9% 34%	3917
	21st to end	Day	8% 5% 2 7% 5% 5% 5% 4 8% 9% 42%	4180
September	1st to 10th	Day	13% 6% 2 5% 5% 4 4 4 6% 8% 44%	2280
	11th to 20th	Day	14% 7% 2 5% 4 3 4 2 4 6% 49%	2123
	21st to end	Day	17% 6% 2 5% 4 4 4 3 5% 7% 42%	2090
October	1st to 10th	Day	23% 7% 3 7% 5% 4 4 2 4 5% 36%	2090
	11th to 20th	Day	14% 7% 2 7% 4 3 4 3 5% 6% 43%	1829
	21st to end	Day	21% 10% 2 7% 5% 4 4 3 4 7% 34%	1881
November	1st to 10th	Day	12% 9% 3 6% 4 3 3 3 5% 5% 47%	1761
	11th to 20th	Day	18% 9% 3 6% 4 4 3 3 4 6% 43%	1800
	21st to end	Day	20% 9% 3 5% 4 4 3 3 5% 6% 37%	1748
December	1st to 10th	Day	14% 10% 5% 8% 4 4 4 4 5% 6% 35%	2600
	11th to 20th	Day	19% 11% 4 6% 3 3 4 3 3 4 38%	2986
	21st to end	Day	19% 12% 3 8% 4 4 4 3 4 6% 33%	3960
January	1st to 10th	Night	12% 9% 5% 12% 5% 5% 5% 3 5% 5% 34%	6349
	11th to 20th	Night	12% 11% 3 11% 6% 6% 5% 4 5% 6% 31%	6315
	21st to end	Night	15% 10% 3 8% 5% 5% 5% 4 6% 6% 34%	6996
February	1st to 10th	Night	15% 10% 3 8% 5% 5% 5% 3 5% 6% 35%	2567
	11th to 20th	Night	11% 10% 3 7% 6% 5% 5% 4 7% 6% 35%	2338
	21st to end	Night	15% 10% 3 8% 4 4 4 4 6% 6% 35%	1980
March	1st to 10th	Night	23% 12% 3 7% 4 4 4 3 4 5% 30%	3844
	11th to 20th	Night	12% 10% 3 8% 6% 5% 5% 4 5% 6% 35%	3800
	21st to end	Night	22% 10% 3 7% 4 3 4 3 5% 5% 32%	3571
April	1st to 10th	Night	19% 9% 3 7% 5% 4 4 3 6% 6% 35%	1900
	11th to 20th	Night	19% 9% 3 5% 4 3 4 4 5% 5% 39%	1875
	21st to end	Night	21% 8% 4 7% 4 4 4 3 5% 4 37%	1729
May	1st to 10th	Night	23% 11% 3 7% 5% 4 4 4 6% 6% 28%	2834
	11th to 20th	Night	16% 10% 4 9% 7% 5% 5% 4 6% 7% 26%	2628
	21st to end	Night	16% 6% 3 6% 4 4 4 4 5% 8% 40%	2738
June	1st to 10th	Night	18% 8% 3 7% 4 3 4 4 4 7% 38%	1520
	11th to 20th	Night	18% 7% 3 6% 5% 3 4 3 7% 6% 37%	1520
	21st to end	Night	16% 8% 2 5% 4 4 3 5% 7% 7% 39%	1520
July	1st to 10th	Night	12% 5% 3 7% 6% 5% 6% 6% 6% 8% 38%	2520
	11th to 20th	Night	14% 7% 2 7% 4 5% 4 4 6% 9% 39%	2520
	21st to end	Night	16% 8% 3 6% 4 4 4 3 5% 6% 41%	2739
August	1st to 10th	Night	22% 7% 2 6% 4 4 4 3 5% 7% 35%	2629
	11th to 20th	Night	14% 9% 3 9% 4 5% 4 3 5% 8% 37%	2907
	21st to end	Night	9% 4 2 4 3 3 4 3 6% 8% 56%	3476
September	1st to 10th	Night	13% 6% 2 5% 3 3 4 4 5% 7% 48%	1900
	11th to 20th	Night	13% 6% 1 5% 3 4 3 3 4 7% 51%	2055
	21st to end	Night	15% 7% 2 6% 4 3 3 3 5% 6% 46%	2090
October	1st to 10th	Night	20% 8% 2 6% 4 4 4 3 4 6% 39%	2090
	11th to 20th	Night	16% 7% 2 6% 3 4 4 3 5% 7% 43%	2349
	21st to end	Night	21% 9% 2 6% 4 3 4 4 4 6% 35%	2717
November	1st to 10th	Night	15% 6% 2 7% 4 4 3 3 4 6% 45%	2537
	11th to 20th	Night	16% 9% 2 6% 4 4 4 4 4 5% 43%	2600
	21st to end	Night	15% 9% 3 7% 4 4 4 3 5% 5% 40%	2650
December	1st to 10th	Night	17% 11% 4 9% 5% 4 4 3 5% 6% 31%	4580
	11th to 20th	Night	23% 10% 3 6% 4 3 3 3 4 4 36%	5208
	21st to end	Night	19% 12% 4 8% 4 3 3 3 4 5% 34%	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	51% 5% 8% 24% 11% 1	3598
	11th to 20th	Day	44% 6% 12% 20% 16% 3	3600
	21st to end	Day	46% 6% 8% 20% 17% 3	3960
February	1st to 10th	Day	50% 7% 13% 17% 11% 2	1830
	11th to 20th	Day	48% 6% 10% 19% 14% 2	1957
	21st to end	Day	51% 5% 9% 19% 13% 2	1650
March	1st to 10th	Day	58% 5% 11% 12% 11% 3	3219
	11th to 20th	Day	40% 12% 14% 20% 11% 3	3378
	21st to end	Day	49% 6% 7% 16% 19% 4	4195
April	1st to 10th	Day	50% 6% 9% 19% 13% 3	2280
	11th to 20th	Day	45% 8% 10% 19% 16% 2	2254
	21st to end	Day	51% 6% 8% 17% 15% 3	2451
May	1st to 10th	Day	55% 10% 10% 15% 9% 1	4120
	11th to 20th	Day	51% 7% 15% 16% 9% 2	4296
	21st to end	Day	35% 9% 13% 25% 16% 1	4837
June	1st to 10th	Day	47% 8% 13% 21% 11% 1	2660
	11th to 20th	Day	44% 8% 13% 19% 14% 2	2660
	21st to end	Day	48% 9% 12% 19% 11% 1	2660
July	1st to 10th	Day	43% 8% 13% 20% 16% 1	4370
	11th to 20th	Day	40% 8% 16% 24% 11% 1	4365
	21st to end	Day	47% 7% 12% 19% 13% 1	4661
August	1st to 10th	Day	46% 7% 15% 21% 10%	4255
	11th to 20th	Day	44% 8% 15% 24% 7% 1	3956
	21st to end	Day	35% 7% 13% 29% 14% 2	4180
September	1st to 10th	Day	39% 6% 12% 25% 17% 1	2280
	11th to 20th	Day	36% 5% 9% 25% 22% 2	2123
	21st to end	Day	41% 6% 9% 23% 18% 3	2090
October	1st to 10th	Day	47% 5% 7% 22% 16% 2	3480
	11th to 20th	Day	36% 4 10% 26% 20% 4	3019
	21st to end	Day	36% 4 10% 30% 15% 3	3081
November	1st to 10th	Day	39% 6% 13% 25% 15% 2	1761
	11th to 20th	Day	42% 5% 11% 20% 20% 3	1800
	21st to end	Day	46% 5% 16% 21% 10% 2	1748
December	1st to 10th	Day	47% 6% 13% 26% 8% 1	2600
	11th to 20th	Day	48% 5% 11% 17% 17% 1	2986
	21st to end	Day	50% 6% 8% 17% 13% 6%	3960
January	1st to 10th	Night	45% 6% 13% 23% 11% 2	6349
	11th to 20th	Night	48% 7% 14% 17% 12% 3	6360
	21st to end	Night	48% 5% 11% 16% 16% 4	6996
February	1st to 10th	Night	47% 8% 13% 20% 11% 2	2567
	11th to 20th	Night	44% 7% 13% 20% 14% 3	2340
	21st to end	Night	47% 6% 10% 22% 11% 4	1980
March	1st to 10th	Night	53% 7% 10% 19% 10% 3	3854
	11th to 20th	Night	43% 11% 13% 20% 12% 2	3801
	21st to end	Night	47% 7% 10% 13% 19% 5%	3571
April	1st to 10th	Night	48% 7% 11% 18% 14% 2	1900
	11th to 20th	Night	45% 7% 11% 20% 16% 2	1875
	21st to end	Night	47% 7% 10% 17% 15% 4	1729
May	1st to 10th	Night	53% 10% 11% 16% 8% 1	2840
	11th to 20th	Night	53% 10% 14% 13% 8% 2	2629
	21st to end	Night	40% 8% 14% 22% 14% 2	2739
June	1st to 10th	Night	46% 7% 11% 24% 10% 1	1520
	11th to 20th	Night	45% 8% 13% 20% 13% 1	1520
	21st to end	Night	41% 9% 14% 21% 14% 1	1520
July	1st to 10th	Night	41% 8% 14% 21% 15% 1	2480
	11th to 20th	Night	41% 8% 13% 23% 14%	2472
	21st to end	Night	44% 6% 12% 22% 15% 1	2651
August	1st to 10th	Night	50% 7% 12% 21% 10%	2619
	11th to 20th	Night	45% 7% 15% 23% 9%	2934
	21st to end	Night	26% 5% 16% 34% 18% 1	3476
September	1st to 10th	Night	36% 6% 13% 26% 18% 1	1900
	11th to 20th	Night	33% 5% 11% 26% 22% 2	2055
	21st to end	Night	39% 6% 9% 27% 17% 2	2090
October	1st to 10th	Night	41% 5% 15% 24% 14% 2	3480
	11th to 20th	Night	32% 4 12% 25% 23% 4	3898
	21st to end	Night	37% 5% 12% 31% 13% 2	4467
November	1st to 10th	Night	39% 6% 12% 22% 18% 3	2537
	11th to 20th	Night	42% 5% 12% 22% 17% 3	2600
	21st to end	Night	42% 7% 15% 23% 12% 1	2650
December	1st to 10th	Night	49% 5% 12% 26% 8%	4580
	11th to 20th	Night	48% 6% 13% 12% 19% 1	5208
	21st to end	Night	49% 6% 9% 18% 15% 4	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	32% 24% 29% 13% 3	200
	11th to 20th	Day	36% 20% 32% 9% 3	200
	21st to end	Day	26% 30% 30% 11% 3	220
February	1st to 10th	Day	35% 31% 24% 9% 1	350
	11th to 20th	Day	35% 26% 27% 11% 1	496
	21st to end	Day	33% 26% 28% 11% 2	414
March	1st to 10th	Day	34% 20% 33% 10% 3	500
	11th to 20th	Day	31% 28% 26% 10% 4 1	500
	21st to end	Day	33% 22% 31% 8% 4 2	530
April	1st to 10th	Day	35% 21% 26% 11% 6% 2	460
	11th to 20th	Day	30% 26% 26% 10% 6% 2	452
	21st to end	Day	32% 17% 31% 12% 6% 1	460
May	1st to 10th	Day	35% 20% 27% 12% 5% 2	463
	11th to 20th	Day	22% 19% 36% 15% 6% 2	487
	21st to end	Day	20% 15% 31% 26% 7% 1	529
June	1st to 10th	Day	23% 14% 35% 19% 9% 2	490
	11th to 20th	Day	20% 15% 33% 23% 7% 1	490
	21st to end	Day	21% 17% 34% 17% 9% 2	490
July	1st to 10th	Day	18% 10% 31% 26% 13% 2	485
	11th to 20th	Day	16% 8% 33% 30% 10% 2	484
	21st to end	Day	16% 12% 36% 26% 9% 2	532
August	1st to 10th	Day	15% 10% 29% 30% 12% 4	486
	11th to 20th	Day	16% 10% 29% 26% 15% 4	479
	21st to end	Day	14% 12% 29% 27% 13% 5%	539
September	1st to 10th	Day	20% 11% 26% 26% 12% 6%	490
	11th to 20th	Day	18% 14% 26% 19% 13% 9%	490
	21st to end	Day	19% 13% 35% 17% 11% 6%	490
October	1st to 10th	Day	29% 14% 22% 21% 10% 4	490
	11th to 20th	Day	23% 18% 28% 21% 8% 2	453
	21st to end	Day	35% 12% 28% 20% 4 1	429
November	1st to 10th	Day	25% 19% 27% 17% 10% 1	395
	11th to 20th	Day	31% 24% 24% 17% 3 1	400
	21st to end	Day	35% 19% 27% 14% 4 1	348
December	1st to 10th	Day	29% 27% 29% 12% 3 1	200
	11th to 20th	Day	40% 24% 26% 9% 2	200
	21st to end	Day	31% 22% 31% 11% 5%	220
January	1st to 10th	Night	22% 27% 33% 16% 3	800
	11th to 20th	Night	27% 23% 31% 17% 3	800
	21st to end	Night	20% 24% 33% 19% 3	880
February	1st to 10th	Night	24% 25% 32% 17% 2	800
	11th to 20th	Night	25% 24% 35% 15% 1	783
	21st to end	Night	27% 25% 30% 16% 2	660
March	1st to 10th	Night	26% 22% 32% 18% 2	800
	11th to 20th	Night	24% 26% 28% 19% 3	800
	21st to end	Night	26% 20% 31% 18% 5%	863
April	1st to 10th	Night	28% 19% 29% 21% 4	760
	11th to 20th	Night	25% 21% 29% 20% 6%	751
	21st to end	Night	28% 18% 29% 20% 5%	589
May	1st to 10th	Night	29% 12% 31% 23% 5%	570
	11th to 20th	Night	21% 15% 34% 24% 5%	455
	21st to end	Night	19% 7% 31% 32% 9% 1	411
June	1st to 10th	Night	21% 8% 28% 33% 10%	380
	11th to 20th	Night	21% 11% 30% 30% 7% 2	380
	21st to end	Night	18% 8% 27% 38% 8%	380
July	1st to 10th	Night	15% 5% 31% 37% 12% 1	370
	11th to 20th	Night	14% 3 29% 36% 15% 2	368
	21st to end	Night	13% 4 31% 38% 13%	416
August	1st to 10th	Night	15% 4 26% 36% 18%	449
	11th to 20th	Night	17% 8% 25% 32% 16% 2	635
	21st to end	Night	17% 5% 24% 34% 19% 2	836
September	1st to 10th	Night	17% 8% 28% 30% 15% 2	760
	11th to 20th	Night	18% 7% 25% 28% 18% 4	760
	21st to end	Night	18% 9% 28% 29% 14% 2	760
October	1st to 10th	Night	26% 12% 22% 26% 12% 2	760
	11th to 20th	Night	21% 14% 29% 24% 10% 1	760
	21st to end	Night	27% 18% 27% 21% 6% 1	836
November	1st to 10th	Night	20% 19% 30% 23% 7% 1	780
	11th to 20th	Night	23% 24% 26% 20% 6% 1	800
	21st to end	Night	24% 21% 33% 17% 5% 1	800
December	1st to 10th	Night	28% 20% 30% 18% 4	800
	11th to 20th	Night	28% 22% 31% 15% 4	800
	21st to end	Night	27% 21% 33% 15% 3	880

Page generated at 2025-03-24 09:04:32. Version ac07

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