O'Brien
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the O'Brien clear sky chart from 2005-11-06 to 2025-04-11.

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	9% 6% 2 6% 5% 5% 5% 5% 7% 9% 41%	113897
	Night	9% 6% 2 6% 4 4 5% 4 6% 9% 44%	114385
Transparency	Day	37% 7% 15% 18% 17% 8%	117701
	Night	36% 6% 14% 19% 17% 9%	118719
Seeing	Day	10% 7% 20% 24% 23% 17%	15776
	Night	11% 5% 20% 34% 25% 6%	25898

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	10% 5% 2 5% 3 3 3 3 4 6% 56%	4056
	11th to 20th	Day	18% 8% 2 7% 4 4 4 3 4 7% 39%	4060
	21st to end	Day	9% 6% 2 6% 3 4 4 3 6% 9% 48%	4763
February	1st to 10th	Day	15% 8% 3 6% 5% 3 3 3 5% 6% 44%	2000
	11th to 20th	Day	13% 7% 2 6% 4 4 3 3 4 6% 48%	1948
	21st to end	Day	13% 6% 2 5% 4 4 4 3 5% 5% 50%	1650
March	1st to 10th	Day	15% 6% 3 7% 4 3 4 4 6% 8% 40%	4903
	11th to 20th	Day	7% 4 2 5% 3 4 4 3 6% 9% 54%	4980
	21st to end	Day	11% 4 2 5% 4 4 4 4 5% 9% 49%	5465
April	1st to 10th	Day	8% 5% 2 5% 4 4 4 4 7% 9% 48%	2200
	11th to 20th	Day	9% 6% 3 7% 5% 4 5% 4 5% 7% 46%	2066
	21st to end	Day	5% 4 1 5% 4 4 5% 4 7% 7% 55%	2431
May	1st to 10th	Day	5% 4 1 5% 4 4 5% 5% 6% 10% 51%	4082
	11th to 20th	Day	9% 6% 3 7% 6% 6% 6% 5% 7% 9% 37%	4081
	21st to end	Day	4 4 1 5% 4 5% 6% 5% 7% 11% 48%	4476
June	1st to 10th	Day	9% 6% 3 9% 6% 7% 7% 5% 8% 10% 29%	2470
	11th to 20th	Day	5% 5% 2 7% 7% 8% 8% 7% 9% 12% 31%	2470
	21st to end	Day	7% 7% 3 8% 7% 6% 7% 6% 9% 10% 29%	2470
July	1st to 10th	Day	7% 6% 3 8% 7% 7% 9% 7% 11% 14% 23%	4120
	11th to 20th	Day	4 5% 2 6% 7% 7% 8% 7% 11% 14% 29%	4120
	21st to end	Day	5% 5% 2 8% 8% 8% 8% 7% 11% 15% 23%	4466
August	1st to 10th	Day	11% 4 2 7% 6% 7% 7% 6% 9% 13% 27%	4104
	11th to 20th	Day	7% 5% 2 7% 6% 7% 8% 6% 7% 11% 35%	3743
	21st to end	Day	9% 7% 3 9% 7% 6% 7% 6% 8% 10% 29%	3958
September	1st to 10th	Day	10% 7% 4 10% 8% 8% 7% 6% 8% 9% 24%	2090
	11th to 20th	Day	9% 7% 3 11% 7% 8% 6% 5% 8% 8% 26%	2090
	21st to end	Day	9% 7% 2 9% 6% 5% 5% 4 7% 7% 40%	2090
October	1st to 10th	Day	12% 6% 3 7% 5% 4 4 5% 6% 7% 41%	2090
	11th to 20th	Day	6% 3 2 5% 5% 4 3 3 5% 7% 57%	1953
	21st to end	Day	7% 4 2 5% 4 4 4 4 6% 7% 53%	2090
November	1st to 10th	Day	8% 5% 2 5% 4 3 4 4 6% 7% 52%	1957
	11th to 20th	Day	9% 6% 1 5% 4 4 4 3 5% 6% 51%	1886
	21st to end	Day	9% 6% 2 5% 4 5% 5% 3 6% 6% 49%	1800
December	1st to 10th	Day	5% 4 2 5% 4 5% 4 3 6% 6% 55%	2930
	11th to 20th	Day	12% 6% 2 6% 4 4 4 3 5% 8% 46%	3373
	21st to end	Day	16% 9% 3 7% 4 4 4 3 4 6% 41%	4466
January	1st to 10th	Night	9% 6% 2 4 3 2 3 3 4 7% 57%	5889
	11th to 20th	Night	15% 7% 3 8% 6% 5% 5% 4 5% 7% 34%	5900
	21st to end	Night	13% 7% 2 6% 4 3 3 4 6% 8% 44%	6190
February	1st to 10th	Night	13% 7% 2 7% 5% 3 3 3 5% 6% 47%	2400
	11th to 20th	Night	12% 8% 2 5% 4 4 3 3 5% 6% 48%	2338
	21st to end	Night	11% 8% 3 5% 4 3 3 3 4 6% 50%	1980
March	1st to 10th	Night	15% 8% 3 8% 4 4 4 3 5% 6% 40%	4947
	11th to 20th	Night	7% 4 1 4 3 3 4 3 6% 10% 54%	4980
	21st to end	Night	10% 5% 2 5% 4 4 4 3 5% 6% 50%	5467
April	1st to 10th	Night	9% 6% 3 6% 4 4 4 4 5% 7% 49%	2200
	11th to 20th	Night	7% 6% 3 6% 4 3 4 4 6% 7% 50%	2070
	21st to end	Night	4 4 2 4 3 4 3 3 6% 8% 58%	1762
May	1st to 10th	Night	6% 5% 2 6% 4 4 4 4 6% 9% 51%	2813
	11th to 20th	Night	8% 6% 3 7% 5% 5% 5% 4 6% 9% 42%	2812
	21st to end	Night	3 3 1 3 3 3 4 3 6% 11% 59%	3079
June	1st to 10th	Night	8% 5% 3 7% 5% 5% 6% 5% 9% 10% 36%	1710
	11th to 20th	Night	5% 5% 2 5% 5% 6% 7% 6% 10% 14% 37%	1710
	21st to end	Night	6% 5% 2 7% 5% 5% 6% 5% 9% 13% 36%	1710
July	1st to 10th	Night	8% 7% 4 9% 7% 7% 8% 6% 9% 12% 23%	2840
	11th to 20th	Night	3 4 2 6% 4 5% 5% 5% 10% 16% 38%	2840
	21st to end	Night	6% 6% 3 9% 7% 7% 7% 6% 9% 14% 27%	3087
August	1st to 10th	Night	9% 5% 2 7% 6% 6% 6% 6% 10% 13% 30%	2825
	11th to 20th	Night	5% 3 1 5% 4 4 6% 5% 8% 14% 44%	3068
	21st to end	Night	9% 8% 3 8% 6% 6% 7% 5% 7% 10% 31%	3663
September	1st to 10th	Night	8% 7% 4 10% 7% 6% 6% 6% 8% 11% 27%	2090
	11th to 20th	Night	9% 8% 3 8% 6% 6% 6% 5% 8% 10% 31%	2090
	21st to end	Night	8% 6% 3 7% 6% 5% 5% 4 6% 8% 42%	2090
October	1st to 10th	Night	9% 7% 2 7% 4 4 5% 4 6% 6% 45%	2090
	11th to 20th	Night	5% 4 1 5% 4 3 4 3 5% 7% 58%	2224
	21st to end	Night	6% 5% 2 5% 5% 4 4 4 5% 7% 53%	2508
November	1st to 10th	Night	7% 6% 2 5% 4 4 5% 4 6% 7% 51%	2341
	11th to 20th	Night	10% 6% 2 5% 4 4 3 3 5% 6% 52%	2510
	21st to end	Night	10% 7% 2 5% 4 4 4 4 6% 7% 48%	2600
December	1st to 10th	Night	10% 5% 2 6% 4 4 3 3 4 6% 54%	4250
	11th to 20th	Night	11% 7% 3 5% 4 4 4 4 5% 6% 48%	4822
	21st to end	Night	13% 9% 3 6% 4 4 4 3 5% 6% 41%	6490

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	30% 3 7% 16% 28% 16%	4057
	11th to 20th	Day	47% 2 5% 13% 20% 12%	4060
	21st to end	Day	34% 3 12% 21% 19% 11%	4763
February	1st to 10th	Day	40% 4 10% 14% 21% 11%	2000
	11th to 20th	Day	39% 3 6% 17% 23% 12%	1958
	21st to end	Day	37% 4 8% 18% 20% 14%	1650
March	1st to 10th	Day	42% 3 10% 15% 19% 11%	4903
	11th to 20th	Day	28% 3 12% 16% 22% 19%	4980
	21st to end	Day	32% 4 10% 25% 18% 12%	5465
April	1st to 10th	Day	33% 5% 12% 18% 19% 13%	2200
	11th to 20th	Day	38% 4 9% 20% 20% 9%	2066
	21st to end	Day	27% 5% 12% 24% 23% 10%	2431
May	1st to 10th	Day	27% 6% 16% 27% 19% 5%	4120
	11th to 20th	Day	42% 5% 15% 22% 15% 2	4085
	21st to end	Day	27% 6% 16% 30% 19% 2	4489
June	1st to 10th	Day	46% 9% 18% 17% 9% 1	2470
	11th to 20th	Day	40% 10% 21% 21% 8%	2470
	21st to end	Day	44% 12% 21% 16% 6% 1	2470
July	1st to 10th	Day	43% 16% 31% 9% 1	4055
	11th to 20th	Day	38% 15% 36% 9% 2	4051
	21st to end	Day	42% 17% 30% 10% 1	4324
August	1st to 10th	Day	43% 12% 26% 16% 4	4091
	11th to 20th	Day	39% 10% 23% 20% 8%	3779
	21st to end	Day	46% 11% 20% 19% 4	3958
September	1st to 10th	Day	52% 10% 18% 12% 6% 2	2090
	11th to 20th	Day	51% 9% 17% 14% 8% 1	2090
	21st to end	Day	42% 6% 13% 20% 15% 5%	2090
October	1st to 10th	Day	50% 5% 7% 12% 16% 10%	3480
	11th to 20th	Day	18% 2 6% 17% 37% 21%	3219
	21st to end	Day	20% 3 7% 18% 40% 12%	3428
November	1st to 10th	Day	30% 5% 10% 20% 20% 14%	1957
	11th to 20th	Day	34% 4 8% 17% 24% 13%	1886
	21st to end	Day	34% 4 10% 20% 23% 9%	1800
December	1st to 10th	Day	29% 3 11% 19% 24% 13%	2930
	11th to 20th	Day	36% 5% 7% 23% 21% 8%	3370
	21st to end	Day	45% 3 7% 17% 23% 5%	4466
January	1st to 10th	Night	29% 3 6% 15% 32% 15%	5889
	11th to 20th	Night	49% 3 6% 15% 20% 8%	5900
	21st to end	Night	40% 3 11% 18% 19% 10%	6190
February	1st to 10th	Night	39% 4 8% 16% 21% 13%	2400
	11th to 20th	Night	38% 3 8% 19% 19% 13%	2339
	21st to end	Night	37% 4 9% 19% 19% 13%	1980
March	1st to 10th	Night	46% 3 9% 15% 16% 10%	4947
	11th to 20th	Night	26% 3 12% 20% 21% 19%	4980
	21st to end	Night	35% 3 9% 21% 21% 12%	5467
April	1st to 10th	Night	35% 4 12% 19% 18% 12%	2200
	11th to 20th	Night	32% 5% 9% 23% 22% 9%	2070
	21st to end	Night	24% 4 12% 30% 22% 9%	1762
May	1st to 10th	Night	28% 5% 18% 27% 16% 6%	2840
	11th to 20th	Night	37% 5% 20% 24% 10% 3	2813
	21st to end	Night	18% 6% 18% 35% 20% 2	3087
June	1st to 10th	Night	37% 9% 22% 24% 7%	1710
	11th to 20th	Night	33% 11% 28% 22% 7%	1710
	21st to end	Night	36% 10% 27% 20% 6% 1	1710
July	1st to 10th	Night	50% 14% 25% 10% 2	2795
	11th to 20th	Night	30% 15% 44% 10% 2	2786
	21st to end	Night	43% 14% 32% 9% 2	2988
August	1st to 10th	Night	40% 13% 30% 14% 4	2814
	11th to 20th	Night	27% 11% 32% 23% 7%	3097
	21st to end	Night	46% 10% 24% 16% 4 1	3663
September	1st to 10th	Night	46% 12% 21% 14% 6% 1	2090
	11th to 20th	Night	45% 10% 20% 17% 7% 1	2090
	21st to end	Night	38% 7% 17% 19% 15% 4	2090
October	1st to 10th	Night	49% 6% 10% 12% 13% 10%	3480
	11th to 20th	Night	19% 3 6% 21% 32% 20%	3697
	21st to end	Night	20% 3 7% 21% 35% 13%	4120
November	1st to 10th	Night	31% 5% 9% 23% 19% 14%	2341
	11th to 20th	Night	34% 3 8% 18% 22% 14%	2510
	21st to end	Night	35% 4 10% 20% 20% 11%	2600
December	1st to 10th	Night	33% 3 8% 19% 20% 17%	4250
	11th to 20th	Night	37% 6% 9% 20% 19% 10%	4824
	21st to end	Night	43% 4 9% 19% 21% 6%	6490

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	9% 13% 51% 19% 7% 1	300
	11th to 20th	Day	10% 15% 37% 27% 9% 2	300
	21st to end	Day	15% 18% 45% 18% 4	330
February	1st to 10th	Day	14% 15% 33% 27% 9% 2	300
	11th to 20th	Day	17% 13% 39% 20% 9%	300
	21st to end	Day	8% 14% 37% 29% 11%	249
March	1st to 10th	Day	12% 15% 31% 28% 14% 1	481
	11th to 20th	Day	13% 9% 33% 27% 15% 2	500
	21st to end	Day	11% 10% 32% 29% 16% 1	550
April	1st to 10th	Day	12% 12% 29% 26% 18% 3	500
	11th to 20th	Day	11% 10% 27% 29% 19% 5%	452
	21st to end	Day	6% 5% 22% 34% 24% 9%	460
May	1st to 10th	Day	6% 6% 21% 27% 26% 14%	463
	11th to 20th	Day	6% 5% 16% 27% 29% 17%	487
	21st to end	Day	7% 4 10% 21% 32% 26%	529
June	1st to 10th	Day	12% 2 9% 22% 31% 24%	490
	11th to 20th	Day	7% 3 11% 20% 27% 32%	490
	21st to end	Day	6% 3 5% 19% 34% 33%	490
July	1st to 10th	Day	6% 1 5% 15% 25% 48%	485
	11th to 20th	Day	6% 2 2 9% 31% 50%	484
	21st to end	Day	7% 1 5% 15% 35% 37%	532
August	1st to 10th	Day	12% 1 5% 17% 27% 38%	486
	11th to 20th	Day	8% 2 4 12% 32% 42%	479
	21st to end	Day	9% 3 6% 20% 29% 33%	539
September	1st to 10th	Day	13% 2 8% 17% 31% 29%	490
	11th to 20th	Day	12% 4 9% 24% 32% 19%	490
	21st to end	Day	10% 3 9% 22% 35% 21%	490
October	1st to 10th	Day	16% 4 16% 31% 24% 9%	490
	11th to 20th	Day	4 6% 19% 33% 30% 9%	490
	21st to end	Day	7% 6% 21% 39% 24% 4	539
November	1st to 10th	Day	12% 4 22% 34% 22% 6%	495
	11th to 20th	Day	15% 6% 29% 33% 13% 4	386
	21st to end	Day	8% 12% 33% 31% 14% 2	300
December	1st to 10th	Day	9% 12% 36% 25% 13% 5%	300
	11th to 20th	Day	17% 9% 35% 28% 11% 1	300
	21st to end	Day	13% 12% 32% 30% 11% 2	330
January	1st to 10th	Night	15% 9% 34% 34% 8% 1	800
	11th to 20th	Night	13% 9% 28% 38% 13% 1	800
	21st to end	Night	17% 10% 35% 32% 7%	880
February	1st to 10th	Night	16% 8% 29% 35% 13%	800
	11th to 20th	Night	15% 9% 33% 35% 7%	783
	21st to end	Night	14% 6% 34% 40% 6%	660
March	1st to 10th	Night	11% 9% 32% 37% 11%	800
	11th to 20th	Night	12% 7% 29% 37% 14% 1	800
	21st to end	Night	10% 7% 30% 39% 14%	880
April	1st to 10th	Night	11% 7% 26% 40% 15% 1	800
	11th to 20th	Night	10% 6% 26% 41% 15% 1	754
	21st to end	Night	6% 3 24% 42% 23% 2	589
May	1st to 10th	Night	5% 3 16% 49% 24% 3	570
	11th to 20th	Night	6% 3 15% 42% 30% 4	561
	21st to end	Night	5% 2 9% 36% 42% 6%	617
June	1st to 10th	Night	9% 2 9% 33% 40% 8%	570
	11th to 20th	Night	6% 3 5% 27% 41% 17%	570
	21st to end	Night	7% 2 4 21% 49% 16%	570
July	1st to 10th	Night	7% 2 3 17% 48% 23%	555
	11th to 20th	Night	6% 1 2 11% 52% 29%	552
	21st to end	Night	6% 1 2 15% 58% 18%	624
August	1st to 10th	Night	8% 1 3 14% 53% 20%	567
	11th to 20th	Night	5% 1 4 11% 56% 24%	564
	21st to end	Night	11% 2 5% 22% 43% 18%	736
September	1st to 10th	Night	10% 2 8% 22% 42% 15%	760
	11th to 20th	Night	13% 3 8% 29% 37% 9%	760
	21st to end	Night	10% 3 11% 31% 38% 6%	760
October	1st to 10th	Night	13% 4 17% 38% 25% 3	760
	11th to 20th	Night	7% 3 19% 42% 27% 2	760
	21st to end	Night	9% 3 20% 42% 24% 2	836
November	1st to 10th	Night	9% 4 24% 40% 21% 1	780
	11th to 20th	Night	12% 6% 26% 39% 16% 2	800
	21st to end	Night	12% 5% 28% 41% 14% 1	800
December	1st to 10th	Night	13% 6% 26% 37% 17% 1	800
	11th to 20th	Night	14% 6% 27% 35% 17%	800
	21st to end	Night	15% 6% 27% 37% 14%	880

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