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 2024-10-24.

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 7% 5% 5% 6% 5% 7% 9% 39%	100845
	Night	9% 6% 2 6% 5% 4 5% 4 6% 9% 43%	97859
Transparency	Day	39% 7% 16% 18% 14% 6%	100655
	Night	37% 6% 16% 19% 15% 7%	97718
Seeing	Day	10% 6% 19% 24% 23% 17%	15207
	Night	11% 5% 20% 33% 26% 6%	25220

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	13% 7% 3 6% 4 3 3 4 4 6% 46%	2836
	11th to 20th	Day	15% 8% 2 7% 4 4 4 3 5% 8% 39%	2840
	21st to end	Day	10% 7% 3 6% 4 5% 5% 4 6% 7% 44%	3323
February	1st to 10th	Day	15% 8% 3 5% 4 3 3 3 5% 6% 44%	1900
	11th to 20th	Day	13% 7% 2 6% 4 4 4 3 4 6% 49%	1848
	21st to end	Day	13% 7% 2 6% 4 4 4 3 5% 5% 49%	1570
March	1st to 10th	Day	12% 7% 3 8% 5% 4 5% 4 7% 8% 36%	3417
	11th to 20th	Day	7% 5% 2 5% 4 4 5% 4 6% 9% 49%	3480
	21st to end	Day	13% 4 2 5% 3 3 4 4 6% 9% 47%	3815
April	1st to 10th	Day	8% 6% 2 5% 4 4 4 4 7% 9% 47%	2090
	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 5% 4 4 6% 7% 52%	2010
November	1st to 10th	Day	8% 5% 2 5% 4 3 4 4 6% 7% 52%	1857
	11th to 20th	Day	9% 6% 2 5% 4 4 4 3 5% 6% 52%	1792
	21st to end	Day	10% 6% 2 6% 4 5% 5% 3 6% 6% 48%	1710
December	1st to 10th	Day	9% 6% 3 6% 4 5% 4 4 6% 8% 46%	1710
	11th to 20th	Day	14% 7% 3 6% 4 3 4 3 5% 7% 45%	2153
	21st to end	Day	17% 8% 3 7% 5% 4 3 3 4 5% 41%	3124
January	1st to 10th	Night	13% 9% 2 6% 4 3 4 3 5% 7% 43%	4109
	11th to 20th	Night	10% 7% 3 8% 6% 5% 5% 4 5% 8% 39%	4120
	21st to end	Night	13% 9% 3 7% 5% 4 4 4 5% 7% 42%	4330
February	1st to 10th	Night	13% 7% 2 6% 4 3 3 3 5% 6% 47%	2280
	11th to 20th	Night	12% 8% 2 5% 4 4 3 3 5% 6% 48%	2218
	21st to end	Night	11% 8% 3 6% 4 4 3 3 4 5% 50%	1884
March	1st to 10th	Night	14% 10% 3 9% 4 4 5% 4 5% 6% 35%	3433
	11th to 20th	Night	9% 5% 2 5% 4 3 5% 3 7% 11% 46%	3480
	21st to end	Night	12% 5% 2 5% 4 4 4 4 5% 6% 49%	3817
April	1st to 10th	Night	9% 6% 2 5% 4 4 4 4 6% 8% 49%	2090
	11th to 20th	Night	7% 6% 3 6% 4 3 4 4 6% 7% 50%	2063
	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	7% 5% 2 5% 5% 4 4 4 5% 7% 52%	2413
November	1st to 10th	Night	7% 5% 2 5% 4 4 5% 3 6% 7% 52%	2221
	11th to 20th	Night	10% 6% 2 5% 4 4 3 3 4 6% 52%	2384
	21st to end	Night	10% 7% 2 5% 4 4 4 4 6% 7% 47%	2470
December	1st to 10th	Night	12% 5% 2 7% 5% 4 3 3 5% 8% 45%	2470
	11th to 20th	Night	14% 8% 2 5% 3 3 3 3 5% 6% 47%	3042
	21st to end	Night	15% 8% 3 6% 4 4 4 4 6% 7% 40%	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	39% 4 7% 17% 20% 14%	2837
	11th to 20th	Day	45% 3 6% 15% 17% 14%	2840
	21st to end	Day	38% 3 11% 15% 19% 12%	3323
February	1st to 10th	Day	41% 4 10% 14% 19% 12%	1900
	11th to 20th	Day	38% 3 6% 16% 24% 12%	1858
	21st to end	Day	37% 4 9% 17% 20% 13%	1570
March	1st to 10th	Day	44% 4 11% 20% 15% 7%	3417
	11th to 20th	Day	31% 4 16% 16% 22% 10%	3480
	21st to end	Day	33% 3 11% 27% 15% 9%	3815
April	1st to 10th	Day	34% 5% 13% 17% 18% 13%	2090
	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	41% 6% 9% 17% 16% 11%	2090
	11th to 20th	Day	27% 3 8% 21% 30% 11%	1953
	21st to end	Day	30% 5% 9% 16% 25% 15%	2010
November	1st to 10th	Day	30% 4 10% 20% 20% 15%	1857
	11th to 20th	Day	34% 4 8% 18% 25% 12%	1792
	21st to end	Day	35% 4 9% 20% 23% 8%	1710
December	1st to 10th	Day	36% 4 13% 22% 18% 7%	1710
	11th to 20th	Day	40% 4 7% 20% 21% 9%	2150
	21st to end	Day	46% 3 5% 19% 19% 7%	3124
January	1st to 10th	Night	41% 3 8% 14% 19% 16%	4109
	11th to 20th	Night	43% 3 7% 15% 21% 11%	4120
	21st to end	Night	43% 3 11% 13% 19% 10%	4330
February	1st to 10th	Night	38% 4 8% 16% 20% 13%	2280
	11th to 20th	Night	38% 3 7% 19% 20% 14%	2219
	21st to end	Night	37% 4 9% 19% 18% 13%	1884
March	1st to 10th	Night	49% 4 11% 15% 13% 7%	3433
	11th to 20th	Night	33% 4 15% 20% 17% 11%	3480
	21st to end	Night	36% 3 10% 20% 20% 10%	3817
April	1st to 10th	Night	34% 4 13% 18% 18% 12%	2090
	11th to 20th	Night	32% 5% 9% 23% 22% 9%	2063
	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	38% 6% 12% 16% 16% 11%	2090
	11th to 20th	Night	26% 4 9% 24% 29% 9%	2224
	21st to end	Night	31% 5% 9% 17% 23% 15%	2413
November	1st to 10th	Night	31% 5% 9% 22% 19% 15%	2221
	11th to 20th	Night	34% 4 8% 18% 22% 14%	2384
	21st to end	Night	36% 4 10% 20% 20% 10%	2470
December	1st to 10th	Night	38% 4 12% 24% 16% 7%	2470
	11th to 20th	Night	39% 4 9% 16% 22% 10%	3044
	21st to end	Night	43% 4 9% 20% 19% 5%	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	10% 13% 48% 21% 7% 1	270
	11th to 20th	Day	9% 15% 36% 28% 10% 3	270
	21st to end	Day	16% 19% 44% 16% 4	297
February	1st to 10th	Day	15% 16% 35% 24% 7% 2	270
	11th to 20th	Day	17% 12% 40% 20% 10%	270
	21st to end	Day	8% 16% 36% 29% 12%	225
March	1st to 10th	Day	10% 15% 31% 29% 15% 1	442
	11th to 20th	Day	13% 8% 34% 27% 16% 2	460
	21st to end	Day	11% 10% 32% 28% 17% 2	506
April	1st to 10th	Day	13% 12% 30% 24% 18% 3	460
	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% 22% 5%	507
November	1st to 10th	Day	13% 4 22% 34% 21% 6%	455
	11th to 20th	Day	15% 7% 29% 33% 13% 4	352
	21st to end	Day	8% 10% 33% 33% 14% 1	270
December	1st to 10th	Day	10% 11% 34% 25% 14% 5%	270
	11th to 20th	Day	17% 9% 35% 28% 10% 1	270
	21st to end	Day	13% 11% 31% 30% 12% 2	297
January	1st to 10th	Night	16% 8% 33% 34% 8% 1	760
	11th to 20th	Night	12% 8% 28% 38% 14% 1	760
	21st to end	Night	17% 10% 34% 32% 7%	836
February	1st to 10th	Night	16% 8% 29% 34% 13%	760
	11th to 20th	Night	16% 9% 32% 35% 8%	743
	21st to end	Night	14% 6% 33% 41% 6%	628
March	1st to 10th	Night	11% 8% 31% 38% 11%	760
	11th to 20th	Night	13% 7% 28% 37% 15% 1	760
	21st to end	Night	10% 7% 30% 38% 14%	836
April	1st to 10th	Night	11% 6% 26% 41% 14% 1	760
	11th to 20th	Night	10% 6% 26% 41% 15% 1	751
	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% 4 20% 41% 24% 2	805
November	1st to 10th	Night	9% 4 25% 40% 20% 1	740
	11th to 20th	Night	12% 6% 25% 40% 16% 1	760
	21st to end	Night	12% 5% 27% 42% 14% 1	760
December	1st to 10th	Night	13% 6% 26% 37% 18% 1	760
	11th to 20th	Night	14% 6% 27% 35% 18%	760
	21st to end	Night	15% 6% 26% 37% 15%	836

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