O'Brien Dark Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the O'Brien Dark Site 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	12% 6% 2 6% 5% 4 5% 4 6% 8% 42%	127331
	Night	13% 7% 2 5% 4 3 4 3 5% 7% 47%	124611
Transparency	Day	38% 7% 12% 19% 17% 7%	131006
	Night	36% 6% 11% 20% 18% 9%	128998
Seeing	Day	13% 9% 23% 28% 21% 6%	16449
	Night	15% 9% 26% 33% 16% 2	26383

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	17% 9% 3 5% 4 3 3 3 4 5% 43%	4167
	11th to 20th	Day	15% 6% 3 5% 3 3 3 3 3 5% 49%	4134
	21st to end	Day	29% 8% 3 6% 3 2 2 2 3 6% 36%	4488
February	1st to 10th	Day	21% 8% 3 6% 4 3 3 3 5% 6% 38%	1915
	11th to 20th	Day	21% 8% 3 7% 4 3 3 2 4 5% 39%	1949
	21st to end	Day	18% 8% 3 6% 4 4 3 2 5% 5% 41%	1793
March	1st to 10th	Day	15% 8% 3 8% 5% 4 4 3 5% 6% 38%	4936
	11th to 20th	Day	14% 6% 2 6% 4 3 4 3 5% 7% 47%	4980
	21st to end	Day	17% 8% 2 6% 5% 4 4 3 5% 6% 40%	5462
April	1st to 10th	Day	14% 9% 3 9% 5% 4 4 4 5% 6% 38%	2200
	11th to 20th	Day	15% 7% 3 6% 5% 5% 4 4 5% 6% 39%	2309
	21st to end	Day	11% 7% 2 6% 5% 5% 4 4 5% 7% 43%	2400
May	1st to 10th	Day	19% 7% 2 7% 5% 5% 5% 4 7% 7% 32%	5679
	11th to 20th	Day	11% 6% 2 7% 5% 4 5% 4 6% 8% 44%	5862
	21st to end	Day	10% 6% 3 7% 5% 5% 6% 5% 8% 10% 34%	6434
June	1st to 10th	Day	6% 5% 2 5% 5% 6% 7% 5% 8% 10% 42%	2600
	11th to 20th	Day	5% 4 1 5% 5% 5% 7% 5% 8% 10% 43%	2600
	21st to end	Day	5% 5% 2 7% 9% 7% 9% 6% 9% 10% 32%	2600
July	1st to 10th	Day	8% 5% 2 7% 6% 6% 7% 6% 9% 11% 34%	5900
	11th to 20th	Day	5% 5% 2 7% 5% 6% 6% 6% 9% 14% 35%	5900
	21st to end	Day	9% 7% 3 7% 5% 5% 6% 4 8% 11% 36%	6422
August	1st to 10th	Day	2 1 1 3 3 4 5% 4 6% 10% 61%	4234
	11th to 20th	Day	3 2 1 3 4 4 5% 5% 9% 14% 48%	4077
	21st to end	Day	4 4 2 7% 6% 6% 7% 6% 8% 11% 39%	4159
September	1st to 10th	Day	6% 5% 2 7% 6% 6% 6% 5% 7% 8% 43%	2200
	11th to 20th	Day	10% 7% 3 8% 5% 5% 5% 5% 6% 7% 39%	2200
	21st to end	Day	6% 3 2 7% 4 5% 5% 5% 6% 7% 49%	2200
October	1st to 10th	Day	5% 3 2 4 4 4 4 3 5% 6% 60%	2101
	11th to 20th	Day	11% 5% 2 5% 3 4 4 3 4 5% 55%	2000
	21st to end	Day	13% 6% 2 5% 3 2 3 2 5% 5% 55%	2200
November	1st to 10th	Day	10% 6% 3 6% 4 4 4 4 6% 6% 48%	2058
	11th to 20th	Day	11% 6% 2 5% 4 3 3 3 5% 6% 51%	2100
	21st to end	Day	16% 7% 2 7% 4 4 4 4 5% 5% 43%	1974
December	1st to 10th	Day	22% 8% 2 6% 3 2 3 3 4 5% 40%	3030
	11th to 20th	Day	16% 9% 3 6% 4 4 3 3 4 6% 43%	3481
	21st to end	Day	15% 7% 3 6% 3 3 3 3 4 5% 47%	4587
January	1st to 10th	Night	18% 7% 2 5% 3 3 3 3 4 5% 48%	6018
	11th to 20th	Night	14% 6% 2 4 2 3 3 2 3 5% 54%	5979
	21st to end	Night	29% 9% 2 4 3 2 2 2 3 5% 37%	6490
February	1st to 10th	Night	19% 9% 3 7% 4 3 3 2 4 5% 40%	2500
	11th to 20th	Night	21% 9% 3 5% 3 3 2 3 4 4 44%	2338
	21st to end	Night	20% 8% 2 6% 4 3 3 3 5% 5% 41%	1835
March	1st to 10th	Night	19% 9% 3 6% 4 3 4 3 4 6% 39%	4919
	11th to 20th	Night	17% 7% 3 5% 4 3 3 3 4 5% 46%	4980
	21st to end	Night	19% 9% 3 7% 4 3 4 3 3 4 41%	5467
April	1st to 10th	Night	14% 7% 2 6% 4 4 4 3 5% 8% 43%	2200
	11th to 20th	Night	13% 6% 2 6% 4 4 4 3 5% 9% 45%	2040
	21st to end	Night	15% 7% 2 6% 3 4 4 3 5% 7% 45%	2000
May	1st to 10th	Night	22% 9% 3 7% 5% 4 4 3 6% 8% 30%	4212
	11th to 20th	Night	11% 6% 3 7% 4 4 5% 4 6% 8% 42%	4053
	21st to end	Night	11% 7% 2 6% 5% 5% 6% 5% 7% 10% 35%	4443
June	1st to 10th	Night	6% 4 1 4 3 4 4 4 7% 10% 53%	1800
	11th to 20th	Night	3 3 1 4 3 3 4 4 6% 12% 56%	1800
	21st to end	Night	4 4 1 6% 5% 4 6% 5% 8% 12% 46%	1800
July	1st to 10th	Night	6% 5% 2 6% 5% 5% 6% 5% 9% 14% 38%	4078
	11th to 20th	Night	4 4 2 5% 4 4 5% 4 7% 12% 51%	4077
	21st to end	Night	7% 7% 2 6% 4 5% 5% 4 6% 9% 45%	4449
August	1st to 10th	Night	1 1 1 2 2 2 3 2 5% 10% 72%	2926
	11th to 20th	Night	2 2 1 3 2 3 4 4 8% 14% 59%	2977
	21st to end	Night	4 3 1 5% 4 4 6% 5% 8% 12% 47%	3732
September	1st to 10th	Night	5% 6% 2 5% 4 4 5% 4 7% 9% 50%	2200
	11th to 20th	Night	8% 6% 2 7% 5% 5% 5% 4 6% 8% 45%	2200
	21st to end	Night	8% 5% 1 4 3 3 3 4 5% 9% 56%	2200
October	1st to 10th	Night	4 3 1 4 3 3 4 2 5% 8% 63%	2277
	11th to 20th	Night	9% 4 2 3 3 2 3 3 4 6% 61%	2400
	21st to end	Night	12% 5% 2 5% 2 3 2 2 3 4 60%	2640
November	1st to 10th	Night	10% 6% 2 4 4 3 4 3 4 6% 54%	2460
	11th to 20th	Night	10% 5% 2 5% 4 3 3 3 4 6% 52%	2520
	21st to end	Night	15% 7% 2 6% 4 4 4 3 5% 6% 45%	2642
December	1st to 10th	Night	20% 10% 3 7% 4 3 3 3 4 6% 37%	4380
	11th to 20th	Night	17% 10% 3 6% 3 3 3 3 4 5% 44%	4946
	21st to end	Night	15% 8% 2 5% 3 3 3 2 3 4 52%	6633

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	44% 4 5% 20% 20% 7%	4168
	11th to 20th	Day	38% 3 4 17% 27% 11%	4134
	21st to end	Day	52% 2 3 12% 16% 15%	4488
February	1st to 10th	Day	48% 4 6% 13% 17% 12%	1915
	11th to 20th	Day	48% 4 7% 14% 16% 10%	1959
	21st to end	Day	46% 5% 8% 14% 15% 12%	1793
March	1st to 10th	Day	45% 6% 8% 12% 18% 10%	4936
	11th to 20th	Day	37% 5% 8% 11% 19% 20%	4980
	21st to end	Day	43% 6% 8% 20% 17% 5%	5464
April	1st to 10th	Day	47% 6% 7% 15% 15% 10%	2200
	11th to 20th	Day	45% 6% 9% 15% 17% 9%	2309
	21st to end	Day	39% 6% 10% 19% 20% 5%	2400
May	1st to 10th	Day	48% 8% 10% 20% 11% 3	5714
	11th to 20th	Day	37% 7% 15% 19% 21% 2	5866
	21st to end	Day	40% 9% 13% 25% 12% 1	6447
June	1st to 10th	Day	33% 10% 17% 23% 16% 1	2600
	11th to 20th	Day	31% 10% 17% 23% 15% 4	2600
	21st to end	Day	42% 12% 16% 20% 9% 1	2600
July	1st to 10th	Day	38% 12% 26% 21% 3	5836
	11th to 20th	Day	33% 12% 23% 24% 7%	5831
	21st to end	Day	40% 9% 17% 24% 9% 1	6240
August	1st to 10th	Day	17% 7% 17% 37% 23%	4220
	11th to 20th	Day	23% 9% 31% 29% 6% 1	4116
	21st to end	Day	34% 15% 16% 21% 13% 2	4158
September	1st to 10th	Day	36% 10% 14% 18% 15% 6%	2200
	11th to 20th	Day	42% 8% 13% 18% 16% 4	2200
	21st to end	Day	32% 8% 12% 19% 16% 13%	2200
October	1st to 10th	Day	16% 4 6% 13% 23% 38%	3426
	11th to 20th	Day	21% 4 6% 16% 33% 20%	3240
	21st to end	Day	25% 4 5% 16% 35% 15%	3538
November	1st to 10th	Day	35% 6% 10% 16% 23% 11%	2058
	11th to 20th	Day	33% 5% 10% 16% 23% 13%	2100
	21st to end	Day	42% 6% 7% 16% 19% 10%	1974
December	1st to 10th	Day	46% 4 7% 16% 22% 5%	3030
	11th to 20th	Day	42% 5% 7% 12% 22% 11%	3479
	21st to end	Day	40% 5% 5% 14% 27% 10%	4587
January	1st to 10th	Night	39% 4 6% 22% 21% 9%	6018
	11th to 20th	Night	34% 3 5% 19% 28% 12%	5979
	21st to end	Night	52% 3 4 12% 17% 12%	6490
February	1st to 10th	Night	47% 4 6% 12% 17% 15%	2500
	11th to 20th	Night	45% 4 7% 15% 17% 12%	2338
	21st to end	Night	46% 4 7% 13% 15% 14%	1835
March	1st to 10th	Night	46% 5% 7% 13% 20% 9%	4919
	11th to 20th	Night	40% 5% 7% 14% 15% 20%	4980
	21st to end	Night	47% 4 7% 14% 19% 8%	5467
April	1st to 10th	Night	41% 6% 10% 17% 16% 12%	2200
	11th to 20th	Night	38% 5% 11% 17% 16% 12%	2040
	21st to end	Night	39% 6% 10% 19% 20% 6%	2000
May	1st to 10th	Night	51% 7% 10% 20% 9% 3	4241
	11th to 20th	Night	37% 7% 13% 20% 19% 3	4054
	21st to end	Night	40% 10% 15% 25% 9% 1	4452
June	1st to 10th	Night	24% 9% 19% 30% 19% 1	1800
	11th to 20th	Night	20% 8% 21% 29% 18% 4	1800
	21st to end	Night	29% 11% 21% 26% 13% 1	1800
July	1st to 10th	Night	32% 11% 32% 21% 3	4035
	11th to 20th	Night	25% 9% 26% 32% 8%	4026
	21st to end	Night	34% 9% 18% 28% 10%	4320
August	1st to 10th	Night	10% 6% 17% 43% 23% 1	2916
	11th to 20th	Night	15% 8% 34% 33% 9% 1	3004
	21st to end	Night	27% 14% 21% 26% 10% 1	3732
September	1st to 10th	Night	29% 10% 21% 20% 14% 7%	2200
	11th to 20th	Night	36% 8% 14% 20% 16% 6%	2200
	21st to end	Night	25% 7% 15% 22% 19% 13%	2200
October	1st to 10th	Night	15% 3 7% 18% 18% 40%	3719
	11th to 20th	Night	15% 3 6% 22% 31% 23%	3899
	21st to end	Night	22% 3 5% 18% 36% 16%	4252
November	1st to 10th	Night	32% 5% 11% 20% 21% 12%	2460
	11th to 20th	Night	34% 4 9% 18% 21% 14%	2520
	21st to end	Night	41% 5% 9% 16% 19% 11%	2642
December	1st to 10th	Night	50% 3 6% 15% 20% 6%	4380
	11th to 20th	Night	44% 5% 8% 8% 21% 14%	4947
	21st to end	Night	37% 4 5% 14% 29% 10%	6633

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% 34% 18% 3 1	330
	11th to 20th	Day	25% 17% 33% 20% 3 2	318
	21st to end	Day	26% 22% 32% 18% 2	330
February	1st to 10th	Day	27% 16% 38% 15% 4	300
	11th to 20th	Day	34% 21% 28% 16% 1	300
	21st to end	Day	28% 27% 32% 11% 2	249
March	1st to 10th	Day	25% 25% 33% 14% 3	300
	11th to 20th	Day	18% 29% 37% 13% 3	300
	21st to end	Day	22% 17% 41% 17% 4	330
April	1st to 10th	Day	21% 22% 38% 16% 4	300
	11th to 20th	Day	23% 19% 29% 20% 7% 1	427
	21st to end	Day	17% 15% 38% 23% 7% 1	500
May	1st to 10th	Day	15% 15% 39% 23% 7%	621
	11th to 20th	Day	14% 10% 31% 32% 12% 1	725
	21st to end	Day	10% 9% 32% 33% 14% 2	789
June	1st to 10th	Day	6% 3 21% 37% 27% 7%	730
	11th to 20th	Day	8% 2 19% 40% 25% 6%	730
	21st to end	Day	6% 2 15% 33% 31% 12%	730
July	1st to 10th	Day	8% 2 11% 33% 33% 13%	721
	11th to 20th	Day	4 1 6% 38% 41% 10%	718
	21st to end	Day	6% 1 6% 35% 42% 11%	794
August	1st to 10th	Day	2 1 6% 29% 49% 13%	726
	11th to 20th	Day	5% 4 26% 50% 15%	660
	21st to end	Day	7% 2 6% 21% 39% 24%	495
September	1st to 10th	Day	7% 1 11% 31% 39% 11%	330
	11th to 20th	Day	8% 3 9% 30% 37% 14%	330
	21st to end	Day	7% 2 11% 38% 31% 10%	330
October	1st to 10th	Day	6% 3 18% 39% 28% 5%	330
	11th to 20th	Day	12% 7% 22% 35% 20% 5%	330
	21st to end	Day	22% 10% 26% 22% 16% 4	363
November	1st to 10th	Day	16% 8% 25% 34% 15% 2	330
	11th to 20th	Day	14% 11% 29% 37% 8% 1	330
	21st to end	Day	18% 22% 32% 19% 6% 2	330
December	1st to 10th	Day	23% 10% 39% 18% 6% 4	330
	11th to 20th	Day	24% 18% 32% 20% 4 2	330
	21st to end	Day	23% 15% 33% 21% 7% 1	363
January	1st to 10th	Night	22% 14% 32% 25% 6%	1050
	11th to 20th	Night	21% 16% 32% 24% 6%	1031
	21st to end	Night	23% 14% 34% 25% 5%	1100
February	1st to 10th	Night	26% 14% 35% 22% 3	900
	11th to 20th	Night	27% 14% 33% 21% 4	784
	21st to end	Night	26% 17% 39% 16% 2	515
March	1st to 10th	Night	21% 17% 38% 21% 4	600
	11th to 20th	Night	19% 13% 39% 25% 5%	600
	21st to end	Night	17% 12% 45% 20% 6%	660
April	1st to 10th	Night	18% 14% 40% 26% 3	600
	11th to 20th	Night	16% 15% 34% 28% 7%	593
	21st to end	Night	18% 12% 37% 25% 8%	600
May	1st to 10th	Night	15% 11% 39% 27% 6% 1	600
	11th to 20th	Night	14% 7% 33% 34% 12% 1	591
	21st to end	Night	8% 5% 34% 37% 15% 1	650
June	1st to 10th	Night	7% 2 18% 48% 23% 3	600
	11th to 20th	Night	3 2 24% 52% 18% 2	600
	21st to end	Night	5% 1 15% 44% 30% 5%	600
July	1st to 10th	Night	4 2 11% 44% 33% 5%	585
	11th to 20th	Night	4 1 9% 51% 32% 4	582
	21st to end	Night	5% 1 9% 53% 28% 5%	657
August	1st to 10th	Night	2 1 6% 56% 31% 3	597
	11th to 20th	Night	4 5% 49% 37% 4	594
	21st to end	Night	6% 1 8% 35% 43% 7%	660
September	1st to 10th	Night	6% 2 8% 38% 39% 7%	600
	11th to 20th	Night	10% 3 9% 35% 40% 5%	600
	21st to end	Night	10% 2 14% 35% 35% 5%	600
October	1st to 10th	Night	4 3 15% 43% 29% 5%	677
	11th to 20th	Night	10% 4 25% 40% 19% 2	800
	21st to end	Night	16% 7% 27% 31% 17% 2	880
November	1st to 10th	Night	12% 7% 27% 35% 18% 1	820
	11th to 20th	Night	13% 10% 29% 34% 13%	840
	21st to end	Night	19% 12% 31% 27% 11% 1	962
December	1st to 10th	Night	24% 9% 31% 26% 10%	1050
	11th to 20th	Night	24% 14% 29% 26% 6%	1050
	21st to end	Night	22% 13% 29% 29% 7%	1155

Page generated at 2026-01-14 00:14:31. Version ac07

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