Owens Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Owens Observatory clear sky chart from 2005-11-06 to 2026-10-29.

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	13% 6% 2 6% 4 4 5% 4 6% 8% 40%	148982
	Night	14% 6% 2 6% 4 4 4 4 6% 8% 43%	143094
Transparency	Day	39% 7% 13% 20% 17% 5%	152769
	Night	38% 6% 11% 19% 19% 6%	147378
Seeing	Day	16% 9% 22% 23% 20% 10%	17527
	Night	16% 8% 23% 29% 21% 4	27487

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	19% 10% 3 6% 3 3 3 3 3 4 42%	5258
	11th to 20th	Day	10% 7% 2 5% 3 2 3 2 4 5% 57%	5280
	21st to end	Day	24% 9% 3 6% 3 3 3 2 4 5% 37%	6002
February	1st to 10th	Day	15% 8% 2 6% 4 4 5% 4 5% 6% 40%	2100
	11th to 20th	Day	22% 6% 2 5% 3 3 4 3 4 6% 41%	2048
	21st to end	Day	21% 6% 3 6% 4 3 3 4 4 6% 40%	1730
March	1st to 10th	Day	21% 10% 3 7% 4 4 4 3 5% 5% 34%	4720
	11th to 20th	Day	11% 5% 2 6% 4 4 4 3 5% 7% 50%	5090
	21st to end	Day	16% 5% 2 6% 4 4 4 3 4 5% 45%	5586
April	1st to 10th	Day	12% 5% 2 6% 4 4 4 4 7% 7% 45%	2368
	11th to 20th	Day	11% 5% 2 6% 4 4 4 3 5% 7% 49%	2494
	21st to end	Day	10% 4 2 6% 4 3 4 4 5% 8% 49%	2692
May	1st to 10th	Day	15% 7% 3 7% 4 5% 5% 4 6% 8% 36%	7616
	11th to 20th	Day	14% 7% 2 8% 5% 5% 6% 5% 7% 9% 33%	7626
	21st to end	Day	18% 9% 3 7% 5% 5% 5% 4 6% 7% 31%	8391
June	1st to 10th	Day	10% 7% 2 7% 5% 4 5% 5% 8% 12% 35%	2730
	11th to 20th	Day	5% 4 2 5% 4 5% 6% 6% 9% 12% 41%	2730
	21st to end	Day	7% 4 2 6% 5% 5% 6% 5% 8% 11% 40%	2730
July	1st to 10th	Day	5% 4 2 6% 6% 7% 7% 6% 10% 13% 34%	7680
	11th to 20th	Day	7% 5% 2 7% 5% 6% 7% 6% 9% 13% 32%	7680
	21st to end	Day	8% 7% 3 9% 7% 6% 8% 6% 9% 12% 26%	8340
August	1st to 10th	Day	8% 6% 2 8% 6% 6% 7% 6% 9% 12% 29%	6014
	11th to 20th	Day	6% 4 1 5% 4 4 4 4 7% 12% 50%	5585
	21st to end	Day	6% 4 2 6% 5% 5% 6% 5% 8% 10% 45%	6080
September	1st to 10th	Day	10% 6% 2 6% 5% 4 4 5% 7% 9% 41%	2315
	11th to 20th	Day	14% 6% 3 6% 4 4 4 4 6% 7% 43%	2310
	21st to end	Day	13% 5% 2 6% 4 4 4 4 4 8% 48%	2310
October	1st to 10th	Day	13% 6% 1 4 3 4 3 3 4 5% 54%	2310
	11th to 20th	Day	10% 5% 1 4 2 2 3 3 5% 7% 58%	2190
	21st to end	Day	14% 5% 2 4 3 2 3 2 4 5% 55%	2280
November	1st to 10th	Day	13% 6% 1 4 3 3 3 2 4 5% 55%	1869
	11th to 20th	Day	14% 5% 1 4 3 3 3 2 5% 6% 53%	1890
	21st to end	Day	14% 5% 2 6% 4 3 4 3 6% 7% 47%	1890
December	1st to 10th	Day	20% 7% 2 4 3 3 3 3 5% 6% 45%	3020
	11th to 20th	Day	16% 9% 3 8% 4 4 4 3 4 4 42%	3463
	21st to end	Day	22% 6% 2 5% 3 4 4 3 5% 7% 39%	4565
January	1st to 10th	Night	17% 6% 2 6% 4 3 4 3 5% 6% 44%	7635
	11th to 20th	Night	13% 7% 2 5% 3 3 3 3 4 5% 50%	7680
	21st to end	Night	22% 8% 2 6% 4 3 3 2 4 4 41%	8163
February	1st to 10th	Night	16% 6% 3 5% 4 4 4 3 5% 5% 45%	2520
	11th to 20th	Night	22% 7% 3 6% 3 4 3 3 4 4 41%	2458
	21st to end	Night	20% 7% 2 6% 4 3 4 3 5% 6% 40%	2076
March	1st to 10th	Night	21% 8% 3 6% 4 4 4 3 5% 6% 35%	5352
	11th to 20th	Night	10% 4 2 4 3 3 4 4 6% 8% 52%	5090
	21st to end	Night	14% 6% 2 7% 4 4 4 3 5% 6% 45%	5588
April	1st to 10th	Night	11% 4 2 5% 5% 4 5% 4 6% 7% 47%	2268
	11th to 20th	Night	13% 4 1 4 3 3 3 4 5% 7% 52%	2075
	21st to end	Night	9% 6% 2 5% 5% 4 4 3 5% 8% 49%	1927
May	1st to 10th	Night	13% 8% 3 8% 5% 5% 5% 4 6% 8% 35%	5237
	11th to 20th	Night	15% 7% 2 6% 5% 5% 5% 4 6% 8% 36%	5234
	21st to end	Night	16% 8% 3 7% 5% 5% 5% 4 6% 8% 33%	5761
June	1st to 10th	Night	8% 5% 2 7% 5% 5% 7% 6% 9% 10% 36%	1890
	11th to 20th	Night	5% 5% 1 6% 5% 5% 6% 5% 8% 13% 42%	1890
	21st to end	Night	6% 5% 1 6% 4 5% 5% 5% 7% 12% 44%	1890
July	1st to 10th	Night	6% 6% 2 8% 6% 6% 6% 5% 9% 12% 34%	5278
	11th to 20th	Night	6% 5% 2 7% 6% 6% 7% 7% 10% 14% 31%	5277
	21st to end	Night	9% 8% 3 9% 6% 6% 6% 5% 8% 10% 30%	5736
August	1st to 10th	Night	9% 6% 2 8% 5% 5% 6% 6% 9% 13% 31%	4135
	11th to 20th	Night	6% 4 2 6% 4 4 4 4 6% 12% 48%	4443
	21st to end	Night	7% 5% 2 6% 4 5% 4 4 6% 9% 47%	5046
September	1st to 10th	Night	9% 5% 2 7% 5% 5% 6% 4 7% 8% 42%	2281
	11th to 20th	Night	14% 5% 2 5% 4 4 5% 4 5% 9% 42%	2310
	21st to end	Night	12% 4 2 6% 4 4 5% 3 6% 8% 46%	2310
October	1st to 10th	Night	12% 5% 2 5% 3 3 4 3 5% 6% 53%	2310
	11th to 20th	Night	10% 5% 2 4 3 3 3 2 3 5% 60%	2427
	21st to end	Night	12% 4 1 4 3 2 3 3 4 5% 57%	2737
November	1st to 10th	Night	13% 5% 2 4 4 2 3 3 4 6% 55%	2645
	11th to 20th	Night	14% 5% 2 4 3 3 3 2 4 6% 55%	2730
	21st to end	Night	15% 5% 2 5% 4 3 4 3 5% 6% 48%	2730
December	1st to 10th	Night	16% 6% 2 6% 3 3 3 2 4 6% 49%	4380
	11th to 20th	Night	18% 6% 2 5% 3 3 3 3 4 6% 45%	4952
	21st to end	Night	21% 7% 3 5% 3 4 3 3 4 6% 41%	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	47% 3 6% 21% 18% 5%	5259
	11th to 20th	Day	31% 2 6% 19% 31% 12%	5280
	21st to end	Day	51% 2 5% 15% 18% 8%	6002
February	1st to 10th	Day	44% 4 8% 19% 21% 4	2100
	11th to 20th	Day	46% 3 7% 19% 19% 6%	2058
	21st to end	Day	45% 4 8% 15% 21% 8%	1730
March	1st to 10th	Day	51% 4 8% 12% 18% 7%	4720
	11th to 20th	Day	34% 4 7% 15% 24% 16%	5090
	21st to end	Day	42% 4 8% 23% 20% 4	5586
April	1st to 10th	Day	37% 5% 10% 23% 20% 5%	2368
	11th to 20th	Day	35% 5% 9% 20% 25% 7%	2494
	21st to end	Day	32% 6% 13% 28% 20% 2	2692
May	1st to 10th	Day	44% 6% 13% 21% 13% 2	7651
	11th to 20th	Day	45% 8% 16% 21% 10% 1	7630
	21st to end	Day	50% 8% 10% 18% 14% 1	8405
June	1st to 10th	Day	38% 11% 17% 23% 10% 2	2730
	11th to 20th	Day	29% 11% 21% 26% 12% 1	2730
	21st to end	Day	33% 11% 16% 25% 12% 3	2730
July	1st to 10th	Day	34% 13% 24% 22% 5%	7615
	11th to 20th	Day	37% 14% 28% 17% 3 1	7611
	21st to end	Day	46% 12% 18% 18% 6%	8156
August	1st to 10th	Day	40% 12% 17% 24% 6%	6001
	11th to 20th	Day	26% 8% 24% 29% 13% 1	5623
	21st to end	Day	31% 9% 14% 25% 19% 2	6079
September	1st to 10th	Day	37% 9% 15% 18% 18% 4	2315
	11th to 20th	Day	39% 8% 12% 20% 20% 1	2310
	21st to end	Day	36% 5% 12% 19% 22% 6%	2310
October	1st to 10th	Day	26% 4 6% 13% 38% 13%	3700
	11th to 20th	Day	19% 3 5% 16% 29% 29%	3482
	21st to end	Day	28% 3 6% 22% 29% 13%	3618
November	1st to 10th	Day	33% 3 8% 15% 31% 10%	1869
	11th to 20th	Day	33% 3 7% 19% 28% 9%	1890
	21st to end	Day	37% 5% 7% 20% 26% 6%	1890
December	1st to 10th	Day	41% 4 5% 18% 25% 7%	3020
	11th to 20th	Day	46% 4 8% 17% 19% 6%	3460
	21st to end	Day	44% 4 9% 24% 17% 2	4565
January	1st to 10th	Night	40% 3 9% 21% 22% 5%	7636
	11th to 20th	Night	37% 3 5% 18% 28% 9%	7680
	21st to end	Night	48% 2 4 15% 23% 8%	8163
February	1st to 10th	Night	40% 4 9% 18% 21% 8%	2520
	11th to 20th	Night	46% 4 5% 18% 20% 7%	2459
	21st to end	Night	45% 4 7% 16% 18% 10%	2076
March	1st to 10th	Night	49% 3 9% 14% 18% 7%	5352
	11th to 20th	Night	30% 5% 7% 15% 27% 17%	5090
	21st to end	Night	39% 4 7% 22% 20% 9%	5588
April	1st to 10th	Night	35% 5% 11% 19% 23% 7%	2268
	11th to 20th	Night	31% 4 9% 21% 25% 10%	2075
	21st to end	Night	33% 6% 10% 26% 21% 5%	1927
May	1st to 10th	Night	45% 7% 14% 18% 14% 3	5264
	11th to 20th	Night	44% 6% 14% 23% 12% 1	5235
	21st to end	Night	47% 8% 12% 18% 13% 2	5771
June	1st to 10th	Night	37% 12% 17% 19% 13% 2	1890
	11th to 20th	Night	30% 9% 21% 25% 13% 1	1890
	21st to end	Night	30% 11% 19% 21% 15% 3	1890
July	1st to 10th	Night	38% 13% 24% 21% 5%	5235
	11th to 20th	Night	38% 17% 27% 14% 4 1	5226
	21st to end	Night	46% 11% 20% 15% 6% 2	5606
August	1st to 10th	Night	40% 13% 17% 24% 6%	4124
	11th to 20th	Night	29% 8% 23% 26% 13% 1	4471
	21st to end	Night	32% 8% 13% 23% 21% 2	5046
September	1st to 10th	Night	37% 9% 14% 20% 17% 4	2281
	11th to 20th	Night	38% 8% 11% 22% 19% 3	2310
	21st to end	Night	35% 8% 12% 17% 21% 8%	2310
October	1st to 10th	Night	26% 4 9% 13% 37% 11%	3700
	11th to 20th	Night	17% 3 4 17% 33% 26%	3874
	21st to end	Night	24% 3 4 22% 30% 18%	4349
November	1st to 10th	Night	32% 4 8% 17% 27% 12%	2645
	11th to 20th	Night	32% 3 6% 18% 30% 12%	2730
	21st to end	Night	36% 4 7% 20% 25% 8%	2730
December	1st to 10th	Night	38% 3 5% 18% 28% 7%	4380
	11th to 20th	Night	40% 4 10% 16% 24% 6%	4954
	21st to end	Night	45% 3 7% 27% 17% 2	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	25% 22% 36% 15% 2	330
	11th to 20th	Day	24% 23% 30% 17% 5% 1	330
	21st to end	Day	25% 20% 37% 13% 3 1	363
February	1st to 10th	Day	19% 20% 37% 21% 4	330
	11th to 20th	Day	35% 22% 28% 14% 2	330
	21st to end	Day	29% 17% 36% 14% 4	273
March	1st to 10th	Day	23% 16% 31% 20% 9% 1	392
	11th to 20th	Day	20% 19% 31% 23% 7% 1	540
	21st to end	Day	18% 15% 35% 24% 8% 1	594
April	1st to 10th	Day	19% 12% 31% 25% 12% 2	540
	11th to 20th	Day	13% 12% 34% 24% 16% 1	532
	21st to end	Day	14% 11% 22% 31% 18% 4	540
May	1st to 10th	Day	14% 10% 30% 26% 15% 5%	543
	11th to 20th	Day	14% 5% 23% 28% 20% 11%	567
	21st to end	Day	17% 5% 16% 26% 24% 12%	617
June	1st to 10th	Day	12% 3 16% 24% 29% 15%	570
	11th to 20th	Day	7% 3 14% 28% 24% 24%	570
	21st to end	Day	8% 2 13% 27% 30% 20%	570
July	1st to 10th	Day	9% 2 5% 22% 34% 27%	565
	11th to 20th	Day	9% 1 8% 18% 34% 30%	564
	21st to end	Day	7% 2 8% 23% 36% 24%	620
August	1st to 10th	Day	14% 3 10% 23% 27% 23%	566
	11th to 20th	Day	10% 3 7% 22% 38% 20%	559
	21st to end	Day	12% 2 10% 25% 29% 22%	627
September	1st to 10th	Day	10% 3 12% 27% 29% 19%	570
	11th to 20th	Day	15% 4 11% 21% 32% 16%	570
	21st to end	Day	15% 3 16% 24% 29% 13%	570
October	1st to 10th	Day	18% 4 16% 32% 23% 8%	570
	11th to 20th	Day	11% 9% 24% 26% 25% 6%	570
	21st to end	Day	18% 7% 23% 28% 21% 4	615
November	1st to 10th	Day	20% 9% 26% 24% 15% 5%	347
	11th to 20th	Day	16% 15% 35% 22% 9% 3	330
	21st to end	Day	11% 18% 36% 24% 11% 1	330
December	1st to 10th	Day	28% 20% 32% 14% 5% 1	330
	11th to 20th	Day	22% 15% 30% 24% 8% 1	330
	21st to end	Day	30% 12% 33% 18% 6% 1	363
January	1st to 10th	Night	24% 17% 33% 21% 5%	840
	11th to 20th	Night	20% 14% 34% 24% 7%	840
	21st to end	Night	20% 15% 37% 24% 3	924
February	1st to 10th	Night	19% 14% 32% 30% 5%	840
	11th to 20th	Night	25% 15% 33% 25% 2	823
	21st to end	Night	24% 15% 35% 24% 3	692
March	1st to 10th	Night	19% 10% 34% 31% 6%	840
	11th to 20th	Night	16% 12% 36% 27% 9% 1	840
	21st to end	Night	17% 13% 29% 32% 9%	924
April	1st to 10th	Night	14% 13% 30% 31% 13% 1	798
	11th to 20th	Night	15% 8% 27% 35% 15% 1	623
	21st to end	Night	10% 7% 25% 37% 20% 1	630
May	1st to 10th	Night	11% 7% 23% 36% 22% 1	630
	11th to 20th	Night	13% 4 22% 33% 24% 4	621
	21st to end	Night	14% 2 14% 32% 32% 6%	683
June	1st to 10th	Night	10% 1 10% 38% 35% 6%	630
	11th to 20th	Night	7% 1 12% 31% 37% 12%	630
	21st to end	Night	6% 1 10% 33% 40% 10%	630
July	1st to 10th	Night	9% 2 7% 21% 47% 15%	615
	11th to 20th	Night	8% 1 7% 17% 53% 13%	612
	21st to end	Night	8% 2 8% 24% 48% 10%	690
August	1st to 10th	Night	11% 1 8% 26% 43% 10%	627
	11th to 20th	Night	10% 1 9% 25% 46% 9%	624
	21st to end	Night	15% 1 9% 27% 39% 9%	693
September	1st to 10th	Night	10% 2 10% 28% 40% 9%	811
	11th to 20th	Night	15% 2 10% 28% 41% 4	840
	21st to end	Night	14% 4 12% 34% 33% 3	840
October	1st to 10th	Night	14% 3 17% 35% 25% 6%	840
	11th to 20th	Night	12% 5% 23% 33% 22% 3	840
	21st to end	Night	14% 7% 22% 36% 19% 2	913
November	1st to 10th	Night	15% 7% 24% 33% 19% 2	820
	11th to 20th	Night	17% 9% 29% 30% 14% 1	840
	21st to end	Night	18% 10% 29% 29% 14%	840
December	1st to 10th	Night	21% 12% 31% 29% 7% 1	840
	11th to 20th	Night	23% 10% 30% 28% 9% 1	840
	21st to end	Night	26% 12% 26% 29% 6%	924

Page generated at 2026-10-26 04:37:53. Version ac07

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