Oakley Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Oakley Observatory clear sky chart from 2005-11-06 to 2026-06-16.

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	14% 8% 3 7% 5% 4 5% 4 6% 8% 37%	137169
	Night	15% 9% 3 7% 4 4 4 3 5% 7% 38%	135085
Transparency	Day	44% 6% 12% 23% 14% 2	140758
	Night	45% 5% 11% 22% 15% 2	139573
Seeing	Day	16% 10% 26% 24% 17% 7%	17764
	Night	18% 8% 27% 31% 14% 1	27617

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	14% 12% 6% 11% 5% 4 4 3 5% 7% 28%	4691
	11th to 20th	Day	10% 12% 6% 14% 7% 6% 5% 4 4 5% 25%	4710
	21st to end	Day	19% 14% 5% 11% 6% 5% 4 4 4 4 25%	5432
February	1st to 10th	Day	17% 12% 7% 8% 5% 4 3 3 5% 6% 31%	2090
	11th to 20th	Day	17% 10% 4 11% 5% 5% 4 3 5% 6% 31%	2048
	21st to end	Day	16% 12% 3 8% 5% 4 3 4 4 5% 35%	1730
March	1st to 10th	Day	19% 13% 4 7% 4 3 3 2 3 4 38%	4580
	11th to 20th	Day	11% 10% 3 6% 4 4 4 3 6% 8% 42%	4779
	21st to end	Day	17% 10% 3 7% 5% 5% 5% 4 5% 6% 32%	6047
April	1st to 10th	Day	17% 10% 3 6% 4 4 3 3 5% 7% 36%	2520
	11th to 20th	Day	17% 8% 2 6% 5% 4 5% 4 5% 5% 39%	2494
	21st to end	Day	19% 7% 2 7% 5% 4 5% 3 5% 7% 36%	2620
May	1st to 10th	Day	18% 8% 3 8% 5% 5% 5% 4 6% 9% 30%	7616
	11th to 20th	Day	16% 7% 3 7% 4 4 4 3 5% 7% 39%	7626
	21st to end	Day	15% 7% 3 9% 5% 5% 6% 5% 7% 9% 29%	8859
June	1st to 10th	Day	11% 5% 2 6% 4 4 4 4 6% 9% 44%	2940
	11th to 20th	Day	9% 5% 2 6% 4 5% 5% 5% 6% 8% 45%	2871
	21st to end	Day	10% 5% 2 6% 5% 5% 4 4 6% 10% 43%	2800
July	1st to 10th	Day	11% 5% 2 5% 4 4 5% 4 6% 12% 43%	6360
	11th to 20th	Day	8% 6% 3 7% 5% 5% 6% 5% 8% 11% 36%	6360
	21st to end	Day	11% 6% 2 7% 5% 5% 6% 5% 8% 10% 34%	6907
August	1st to 10th	Day	7% 5% 2 5% 4 3 4 4 6% 10% 50%	4202
	11th to 20th	Day	9% 5% 2 6% 4 5% 6% 5% 8% 10% 40%	3849
	21st to end	Day	5% 5% 1 5% 4 4 4 4 6% 10% 52%	4312
September	1st to 10th	Day	11% 5% 2 5% 5% 3 5% 4 6% 7% 48%	2400
	11th to 20th	Day	9% 4 2 5% 4 4 5% 5% 7% 7% 49%	2222
	21st to end	Day	12% 5% 2 5% 4 4 4 3 5% 7% 50%	2200
October	1st to 10th	Day	12% 4 2 5% 4 4 3 3 5% 6% 52%	2200
	11th to 20th	Day	15% 5% 2 5% 4 4 5% 4 5% 7% 44%	2028
	21st to end	Day	19% 8% 2 7% 3 4 4 2 5% 5% 40%	1980
November	1st to 10th	Day	11% 6% 2 5% 4 3 4 3 5% 7% 51%	1852
	11th to 20th	Day	18% 10% 2 6% 4 4 3 2 4 6% 40%	1890
	21st to end	Day	19% 11% 3 7% 4 3 4 4 4 6% 36%	1890
December	1st to 10th	Day	13% 11% 3 8% 4 3 3 2 4 4 44%	2906
	11th to 20th	Day	16% 14% 5% 9% 5% 5% 5% 3 4 5% 28%	3088
	21st to end	Day	30% 16% 6% 9% 5% 4 4 3 4 4 17%	4070
January	1st to 10th	Night	16% 13% 7% 11% 4 4 4 3 5% 6% 28%	8232
	11th to 20th	Night	11% 16% 5% 11% 6% 6% 5% 3 4 5% 26%	8280
	21st to end	Night	18% 11% 5% 7% 6% 4 4 3 5% 6% 29%	8778
February	1st to 10th	Night	18% 12% 5% 8% 5% 5% 4 3 5% 5% 30%	2528
	11th to 20th	Night	16% 10% 4 9% 5% 4 4 3 4 6% 34%	2458
	21st to end	Night	18% 12% 3 8% 3 4 4 3 5% 4 36%	2076
March	1st to 10th	Night	20% 14% 4 6% 3 3 3 2 4 4 36%	5493
	11th to 20th	Night	14% 7% 2 5% 4 3 3 3 5% 8% 46%	5387
	21st to end	Night	17% 9% 3 8% 5% 5% 4 4 6% 6% 33%	5168
April	1st to 10th	Night	15% 9% 2 6% 4 4 4 3 6% 6% 41%	2100
	11th to 20th	Night	17% 8% 2 6% 4 3 4 3 4 7% 43%	2075
	21st to end	Night	17% 7% 3 7% 4 5% 5% 4 6% 6% 36%	2000
May	1st to 10th	Night	19% 9% 3 7% 5% 4 4 3 5% 8% 32%	5237
	11th to 20th	Night	14% 6% 3 6% 4 4 4 4 6% 8% 41%	5234
	21st to end	Night	14% 7% 3 8% 5% 6% 5% 4 6% 8% 32%	5313
June	1st to 10th	Night	10% 6% 3 6% 3 3 4 3 4 8% 48%	1680
	11th to 20th	Night	9% 6% 1 6% 4 4 4 4 5% 8% 50%	1640
	21st to end	Night	7% 4 2 6% 4 4 5% 5% 7% 10% 46%	1600
July	1st to 10th	Night	9% 5% 2 5% 4 4 5% 3 6% 9% 48%	3618
	11th to 20th	Night	7% 5% 2 7% 5% 5% 5% 4 8% 9% 44%	3617
	21st to end	Night	8% 7% 3 8% 5% 5% 5% 5% 7% 10% 38%	3961
August	1st to 10th	Night	6% 5% 2 5% 4 4 4 3 4 9% 54%	2943
	11th to 20th	Night	6% 5% 2 7% 4 3 5% 4 6% 8% 50%	3200
	21st to end	Night	5% 4 1 4 4 4 4 3 6% 9% 57%	3586
September	1st to 10th	Night	10% 5% 2 5% 4 4 4 3 5% 8% 51%	2000
	11th to 20th	Night	7% 4 2 4 4 3 3 3 4 8% 59%	2157
	21st to end	Night	13% 6% 2 5% 4 3 4 4 5% 6% 49%	2200
October	1st to 10th	Night	8% 4 1 5% 3 4 3 3 5% 5% 57%	2200
	11th to 20th	Night	16% 5% 2 5% 3 3 3 3 4 6% 51%	2367
	21st to end	Night	18% 8% 2 5% 4 4 4 4 5% 7% 40%	2860
November	1st to 10th	Night	10% 8% 3 5% 4 3 4 3 4 5% 51%	2666
	11th to 20th	Night	13% 9% 3 6% 4 3 4 4 5% 7% 43%	2730
	21st to end	Night	21% 11% 4 6% 4 4 3 3 4 5% 36%	2730
December	1st to 10th	Night	16% 10% 3 8% 5% 3 3 3 4 4 41%	4482
	11th to 20th	Night	18% 11% 4 9% 5% 4 4 3 5% 5% 32%	5339
	21st to end	Night	29% 17% 5% 9% 5% 4 3 3 3 4 19%	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	56% 4 12% 21% 7% 1	4692
	11th to 20th	Day	61% 4 10% 13% 11% 2	4710
	21st to end	Day	64% 3 7% 18% 6% 2	5432
February	1st to 10th	Day	54% 4 12% 19% 9% 2	2090
	11th to 20th	Day	55% 4 10% 21% 9% 1	2058
	21st to end	Day	51% 6% 8% 23% 9% 2	1730
March	1st to 10th	Day	52% 3 6% 19% 16% 3	4580
	11th to 20th	Day	39% 5% 11% 32% 12% 1	4779
	21st to end	Day	50% 7% 12% 21% 9% 1	6049
April	1st to 10th	Day	47% 7% 11% 22% 13% 1	2520
	11th to 20th	Day	45% 6% 11% 25% 10% 3	2494
	21st to end	Day	47% 5% 10% 24% 10% 3	2620
May	1st to 10th	Day	49% 7% 14% 19% 12%	7651
	11th to 20th	Day	44% 6% 11% 23% 15%	7630
	21st to end	Day	48% 7% 13% 18% 13% 2	8873
June	1st to 10th	Day	35% 8% 13% 23% 17% 4	2940
	11th to 20th	Day	33% 8% 12% 22% 21% 3	2871
	21st to end	Day	36% 8% 17% 19% 18% 3	2800
July	1st to 10th	Day	34% 6% 11% 30% 17% 2	6291
	11th to 20th	Day	38% 10% 14% 24% 13% 1	6285
	21st to end	Day	42% 8% 16% 23% 10% 1	6711
August	1st to 10th	Day	29% 6% 15% 33% 15% 1	4189
	11th to 20th	Day	36% 8% 14% 27% 15% 1	3885
	21st to end	Day	26% 5% 17% 31% 16% 4	4311
September	1st to 10th	Day	34% 6% 12% 26% 19% 4	2400
	11th to 20th	Day	30% 7% 13% 24% 23% 3	2222
	21st to end	Day	34% 6% 10% 23% 25% 3	2200
October	1st to 10th	Day	23% 3 8% 27% 37% 3	3590
	11th to 20th	Day	29% 4 9% 25% 31% 1	3282
	21st to end	Day	39% 3 9% 23% 25% 1	3180
November	1st to 10th	Day	34% 3 13% 28% 17% 5%	1852
	11th to 20th	Day	46% 4 9% 21% 15% 4	1890
	21st to end	Day	50% 5% 9% 21% 14% 1	1890
December	1st to 10th	Day	45% 4 11% 27% 12%	2906
	11th to 20th	Day	58% 5% 9% 21% 6% 1	3085
	21st to end	Day	71% 5% 10% 10% 4	4070
January	1st to 10th	Night	58% 4 12% 20% 6%	8233
	11th to 20th	Night	61% 3 9% 13% 11% 3	8280
	21st to end	Night	55% 3 10% 21% 9% 1	8778
February	1st to 10th	Night	56% 4 13% 17% 8% 1	2528
	11th to 20th	Night	51% 4 11% 19% 12% 2	2459
	21st to end	Night	50% 5% 10% 21% 11% 2	2076
March	1st to 10th	Night	52% 3 6% 24% 11% 4	5493
	11th to 20th	Night	37% 5% 9% 27% 20% 1	5387
	21st to end	Night	50% 6% 11% 25% 7% 1	5168
April	1st to 10th	Night	43% 6% 11% 24% 14% 2	2100
	11th to 20th	Night	41% 6% 10% 20% 17% 5%	2075
	21st to end	Night	46% 7% 9% 23% 12% 3	2000
May	1st to 10th	Night	50% 7% 11% 22% 9% 1	5264
	11th to 20th	Night	40% 7% 12% 23% 19%	5235
	21st to end	Night	47% 8% 13% 18% 12% 3	5323
June	1st to 10th	Night	34% 6% 10% 25% 21% 4	1680
	11th to 20th	Night	32% 7% 11% 23% 21% 6%	1640
	21st to end	Night	29% 8% 15% 22% 20% 5%	1600
July	1st to 10th	Night	33% 7% 10% 26% 21% 3	3580
	11th to 20th	Night	33% 8% 15% 26% 16% 1	3572
	21st to end	Night	38% 8% 17% 24% 11% 2	3846
August	1st to 10th	Night	28% 6% 16% 36% 14% 2	2932
	11th to 20th	Night	30% 6% 16% 27% 20% 1	3230
	21st to end	Night	24% 6% 13% 30% 22% 4	3586
September	1st to 10th	Night	32% 6% 11% 26% 20% 6%	2000
	11th to 20th	Night	25% 5% 10% 27% 29% 5%	2157
	21st to end	Night	35% 6% 10% 21% 24% 5%	2200
October	1st to 10th	Night	19% 4 7% 23% 43% 3	3590
	11th to 20th	Night	25% 3 8% 26% 35% 3	3852
	21st to end	Night	35% 4 8% 23% 25% 5%	4610
November	1st to 10th	Night	35% 4 9% 26% 21% 5%	2666
	11th to 20th	Night	41% 4 10% 24% 18% 3	2730
	21st to end	Night	51% 4 10% 22% 13% 1	2730
December	1st to 10th	Night	47% 4 11% 24% 13%	4482
	11th to 20th	Night	54% 5% 12% 18% 11%	5341
	21st to end	Night	70% 4 9% 11% 5% 1	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	24% 17% 32% 19% 7%	330
	11th to 20th	Day	24% 19% 38% 15% 4	330
	21st to end	Day	23% 21% 34% 18% 3 1	363
February	1st to 10th	Day	25% 17% 34% 19% 5%	330
	11th to 20th	Day	26% 20% 35% 14% 4 1	330
	21st to end	Day	26% 15% 40% 16% 2	273
March	1st to 10th	Day	22% 21% 38% 16% 3	330
	11th to 20th	Day	27% 17% 36% 14% 5% 1	398
	21st to end	Day	27% 17% 32% 18% 5% 1	594
April	1st to 10th	Day	21% 16% 33% 21% 7% 1	540
	11th to 20th	Day	19% 16% 30% 22% 10% 2	532
	21st to end	Day	21% 15% 31% 23% 9% 2	540
May	1st to 10th	Day	22% 12% 28% 26% 9% 3	543
	11th to 20th	Day	17% 9% 30% 25% 14% 5%	567
	21st to end	Day	12% 6% 29% 26% 18% 9%	773
June	1st to 10th	Day	12% 6% 23% 31% 23% 6%	780
	11th to 20th	Day	11% 4 23% 33% 22% 6%	756
	21st to end	Day	12% 4 26% 28% 22% 7%	730
July	1st to 10th	Day	11% 3 14% 28% 24% 20%	721
	11th to 20th	Day	7% 3 18% 31% 28% 13%	718
	21st to end	Day	11% 4 13% 29% 32% 11%	789
August	1st to 10th	Day	9% 2 14% 26% 32% 16%	526
	11th to 20th	Day	14% 2 14% 30% 27% 13%	519
	21st to end	Day	5% 2 13% 29% 34% 17%	583
September	1st to 10th	Day	9% 4 16% 28% 32% 11%	530
	11th to 20th	Day	7% 2 12% 28% 32% 18%	530
	21st to end	Day	14% 6% 23% 22% 21% 14%	530
October	1st to 10th	Day	10% 4 23% 32% 21% 9%	530
	11th to 20th	Day	13% 14% 30% 23% 15% 4	373
	21st to end	Day	24% 19% 31% 19% 7%	363
November	1st to 10th	Day	12% 14% 34% 25% 12% 2	330
	11th to 20th	Day	22% 17% 31% 18% 10% 2	330
	21st to end	Day	25% 13% 33% 22% 6%	330
December	1st to 10th	Day	16% 21% 38% 18% 7% 2	330
	11th to 20th	Day	24% 13% 38% 19% 5% 1	330
	21st to end	Day	35% 13% 33% 15% 4	363
January	1st to 10th	Night	22% 14% 33% 27% 5%	1050
	11th to 20th	Night	24% 13% 38% 21% 3	1050
	21st to end	Night	22% 12% 38% 25% 2	1155
February	1st to 10th	Night	24% 11% 36% 26% 2	848
	11th to 20th	Night	22% 14% 37% 24% 3	823
	21st to end	Night	25% 11% 38% 24% 2	692
March	1st to 10th	Night	24% 14% 34% 25% 3	840
	11th to 20th	Night	23% 11% 35% 26% 5%	840
	21st to end	Night	23% 9% 31% 31% 5%	737
April	1st to 10th	Night	17% 13% 33% 32% 5%	630
	11th to 20th	Night	20% 12% 29% 29% 10%	623
	21st to end	Night	19% 9% 34% 31% 7% 1	630
May	1st to 10th	Night	18% 8% 32% 32% 10%	630
	11th to 20th	Night	14% 6% 27% 35% 17% 2	621
	21st to end	Night	12% 3 22% 38% 23% 2	683
June	1st to 10th	Night	11% 4 19% 44% 21% 1	630
	11th to 20th	Night	11% 3 18% 45% 22%	615
	21st to end	Night	8% 3 20% 40% 28% 1	600
July	1st to 10th	Night	10% 2 9% 36% 36% 6%	585
	11th to 20th	Night	6% 3 14% 39% 34% 4	582
	21st to end	Night	6% 2 10% 44% 34% 3	657
August	1st to 10th	Night	8% 1 9% 43% 37% 3	597
	11th to 20th	Night	7% 1 12% 39% 37% 4	594
	21st to end	Night	8% 1 11% 35% 40% 6%	660
September	1st to 10th	Night	11% 2 13% 38% 31% 5%	600
	11th to 20th	Night	7% 2 14% 36% 35% 5%	757
	21st to end	Night	16% 5% 17% 34% 25% 4	800
October	1st to 10th	Night	10% 4 21% 39% 23% 4	800
	11th to 20th	Night	19% 9% 26% 30% 15% 1	800
	21st to end	Night	21% 10% 33% 28% 7%	880
November	1st to 10th	Night	15% 7% 33% 33% 11% 1	820
	11th to 20th	Night	18% 12% 33% 29% 8%	840
	21st to end	Night	25% 11% 30% 26% 7%	840
December	1st to 10th	Night	23% 10% 37% 24% 5%	903
	11th to 20th	Night	25% 10% 33% 26% 6% 1	1050
	21st to end	Night	31% 13% 31% 22% 3	1155

Page generated at 2026-06-13 02:31:38. Version ac07

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