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 2025-07-06.

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% 6% 2 6% 4 4 4 4 6% 8% 41%	119994
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 44%	118807
Transparency	Day	39% 6% 11% 20% 18% 5%	123823
	Night	37% 6% 10% 19% 21% 7%	123114
Seeing	Day	16% 9% 22% 23% 19% 10%	15733
	Night	16% 8% 23% 29% 21% 4	25770

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	18% 10% 3 5% 3 3 2 3 4 4 46%	4056
	11th to 20th	Day	12% 7% 2 5% 3 3 3 3 4 5% 52%	4060
	21st to end	Day	22% 9% 3 6% 3 3 3 2 4 6% 40%	4632
February	1st to 10th	Day	16% 8% 2 6% 4 4 5% 4 5% 6% 39%	2000
	11th to 20th	Day	23% 6% 2 5% 3 3 4 3 5% 5% 41%	1948
	21st to end	Day	21% 6% 3 6% 4 3 3 4 4 6% 40%	1650
March	1st to 10th	Day	22% 10% 3 7% 4 4 4 3 5% 5% 34%	4617
	11th to 20th	Day	11% 5% 2 6% 4 4 4 3 5% 7% 50%	4980
	21st to end	Day	16% 5% 2 6% 4 4 4 3 4 5% 44%	5465
April	1st to 10th	Day	12% 5% 2 6% 4 4 4 4 7% 7% 45%	2255
	11th to 20th	Day	12% 5% 2 6% 4 4 4 3 5% 7% 48%	2374
	21st to end	Day	10% 4 2 5% 4 3 4 4 5% 8% 49%	2564
May	1st to 10th	Day	14% 7% 3 7% 4 5% 5% 4 6% 9% 36%	5862
	11th to 20th	Day	18% 7% 2 8% 5% 5% 6% 5% 7% 8% 29%	5861
	21st to end	Day	14% 7% 2 6% 5% 4 4 4 6% 8% 40%	6433
June	1st to 10th	Day	9% 7% 3 8% 5% 4 5% 5% 8% 12% 35%	2600
	11th to 20th	Day	5% 4 2 5% 4 4 6% 6% 9% 12% 42%	2600
	21st to end	Day	7% 4 2 6% 5% 5% 6% 5% 8% 11% 40%	2600
July	1st to 10th	Day	8% 5% 3 8% 6% 8% 8% 6% 10% 12% 27%	4185
	11th to 20th	Day	8% 5% 2 8% 5% 6% 6% 6% 8% 12% 34%	4120
	21st to end	Day	10% 9% 4 10% 8% 7% 8% 7% 9% 9% 19%	4466
August	1st to 10th	Day	7% 5% 2 6% 5% 6% 6% 5% 8% 12% 38%	4104
	11th to 20th	Day	8% 5% 2 6% 4 5% 5% 5% 7% 11% 43%	3798
	21st to end	Day	7% 4 2 6% 5% 4 5% 4 7% 9% 47%	4180
September	1st to 10th	Day	11% 7% 2 7% 5% 5% 5% 5% 7% 8% 40%	2095
	11th to 20th	Day	16% 6% 3 6% 4 4 5% 4 6% 7% 40%	2090
	21st to end	Day	14% 5% 2 6% 4 4 4 4 4 7% 48%	2090
October	1st to 10th	Day	12% 5% 1 4 3 4 3 3 4 4 57%	2090
	11th to 20th	Day	11% 5% 2 4 2 2 3 3 5% 7% 56%	1982
	21st to end	Day	13% 5% 2 4 3 3 3 2 4 5% 55%	2090
November	1st to 10th	Day	14% 6% 2 4 3 3 3 2 4 5% 55%	1778
	11th to 20th	Day	14% 6% 1 4 3 3 3 3 5% 6% 52%	1800
	21st to end	Day	14% 5% 2 6% 4 3 4 3 6% 7% 47%	1800
December	1st to 10th	Day	20% 6% 2 4 3 3 3 3 5% 6% 46%	2930
	11th to 20th	Day	16% 9% 3 8% 4 4 4 3 4 4 41%	3373
	21st to end	Day	23% 6% 2 5% 3 3 4 3 5% 6% 39%	4466
January	1st to 10th	Night	16% 6% 2 5% 3 3 3 3 5% 6% 48%	5889
	11th to 20th	Night	14% 8% 3 6% 3 3 3 3 4 5% 48%	5900
	21st to end	Night	20% 7% 2 6% 3 3 3 3 4 5% 43%	6308
February	1st to 10th	Night	16% 6% 3 5% 4 4 4 3 5% 5% 44%	2400
	11th to 20th	Night	22% 7% 3 6% 3 4 3 3 4 4 42%	2338
	21st to end	Night	20% 6% 2 6% 4 3 4 3 5% 6% 41%	1980
March	1st to 10th	Night	21% 8% 3 6% 4 4 4 3 5% 6% 35%	5235
	11th to 20th	Night	10% 4 2 4 3 3 4 4 6% 8% 52%	4980
	21st to end	Night	15% 6% 2 7% 4 4 4 3 5% 6% 45%	5467
April	1st to 10th	Night	11% 4 2 5% 5% 3 5% 4 6% 7% 48%	2160
	11th to 20th	Night	14% 5% 1 4 3 3 3 4 5% 7% 51%	1975
	21st to end	Night	9% 6% 2 5% 5% 4 4 3 5% 9% 50%	1835
May	1st to 10th	Night	11% 7% 3 8% 6% 5% 5% 4 7% 9% 36%	4033
	11th to 20th	Night	19% 8% 2 7% 5% 5% 5% 4 6% 7% 31%	4032
	21st to end	Night	12% 6% 2 5% 4 4 5% 4 7% 9% 42%	4421
June	1st to 10th	Night	7% 5% 2 7% 5% 5% 7% 6% 9% 10% 36%	1800
	11th to 20th	Night	5% 5% 1 6% 5% 5% 6% 6% 8% 13% 42%	1800
	21st to end	Night	6% 5% 1 6% 4 5% 5% 6% 7% 12% 44%	1800
July	1st to 10th	Night	9% 7% 3 9% 6% 6% 6% 5% 8% 9% 31%	2885
	11th to 20th	Night	6% 5% 2 7% 5% 6% 7% 7% 8% 14% 33%	2840
	21st to end	Night	11% 9% 4 10% 6% 7% 6% 6% 9% 9% 22%	3087
August	1st to 10th	Night	7% 5% 2 7% 4 5% 6% 5% 8% 13% 38%	2825
	11th to 20th	Night	8% 6% 3 7% 4 4 4 4 5% 8% 47%	3027
	21st to end	Night	7% 5% 2 5% 4 4 4 4 5% 8% 51%	3476
September	1st to 10th	Night	10% 5% 2 7% 5% 5% 6% 4 7% 8% 42%	2063
	11th to 20th	Night	15% 6% 2 5% 4 4 5% 4 5% 9% 42%	2090
	21st to end	Night	13% 4 2 5% 4 4 5% 3 6% 8% 46%	2090
October	1st to 10th	Night	11% 4 2 4 3 3 3 3 5% 7% 55%	2090
	11th to 20th	Night	10% 5% 2 5% 3 3 3 2 3 6% 58%	2195
	21st to end	Night	12% 4 2 4 3 2 3 3 4 6% 57%	2508
November	1st to 10th	Night	13% 5% 2 4 4 3 3 3 3 6% 55%	2516
	11th to 20th	Night	15% 5% 2 4 3 3 3 2 4 6% 54%	2600
	21st to end	Night	16% 5% 2 5% 4 3 4 3 5% 6% 48%	2600
December	1st to 10th	Night	16% 6% 2 6% 4 3 3 2 4 6% 49%	4250
	11th to 20th	Night	19% 6% 2 5% 3 3 3 3 4 6% 45%	4822
	21st to end	Night	21% 7% 3 5% 3 4 3 3 4 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	42% 3 5% 20% 23% 6%	4057
	11th to 20th	Day	34% 2 7% 21% 28% 7%	4060
	21st to end	Day	47% 2 6% 16% 21% 7%	4632
February	1st to 10th	Day	45% 4 8% 19% 20% 5%	2000
	11th to 20th	Day	46% 4 7% 19% 19% 6%	1958
	21st to end	Day	45% 4 8% 14% 21% 8%	1650
March	1st to 10th	Day	52% 4 7% 12% 18% 7%	4617
	11th to 20th	Day	34% 4 7% 14% 24% 16%	4980
	21st to end	Day	42% 4 7% 22% 20% 4	5465
April	1st to 10th	Day	36% 5% 10% 23% 20% 5%	2255
	11th to 20th	Day	36% 5% 8% 19% 24% 7%	2374
	21st to end	Day	31% 6% 13% 28% 20% 3	2564
May	1st to 10th	Day	43% 7% 15% 20% 14% 1	5897
	11th to 20th	Day	48% 9% 14% 19% 8% 1	5865
	21st to end	Day	40% 7% 11% 23% 18% 1	6447
June	1st to 10th	Day	38% 11% 17% 23% 10% 2	2600
	11th to 20th	Day	28% 11% 20% 26% 12% 1	2600
	21st to end	Day	34% 11% 16% 24% 12% 3	2600
July	1st to 10th	Day	42% 14% 20% 18% 5% 1	4120
	11th to 20th	Day	38% 12% 23% 20% 6% 1	4051
	21st to end	Day	55% 14% 13% 13% 4	4324
August	1st to 10th	Day	34% 10% 15% 32% 9%	4091
	11th to 20th	Day	33% 9% 17% 25% 14% 2	3836
	21st to end	Day	31% 9% 13% 24% 20% 3	4179
September	1st to 10th	Day	39% 9% 14% 16% 18% 4	2095
	11th to 20th	Day	41% 8% 11% 17% 21% 1	2090
	21st to end	Day	36% 5% 11% 19% 22% 7%	2090
October	1st to 10th	Day	24% 4 6% 13% 39% 13%	3480
	11th to 20th	Day	19% 3 5% 16% 29% 28%	3274
	21st to end	Day	28% 3 6% 22% 29% 13%	3428
November	1st to 10th	Day	33% 3 8% 15% 31% 10%	1778
	11th to 20th	Day	34% 3 7% 17% 29% 10%	1800
	21st to end	Day	37% 5% 7% 19% 26% 6%	1800
December	1st to 10th	Day	40% 4 5% 18% 26% 7%	2930
	11th to 20th	Day	47% 4 8% 17% 19% 6%	3370
	21st to end	Day	45% 3 9% 24% 17% 2	4466
January	1st to 10th	Night	37% 3 6% 20% 29% 6%	5889
	11th to 20th	Night	38% 3 5% 19% 26% 8%	5900
	21st to end	Night	45% 2 5% 15% 28% 6%	6308
February	1st to 10th	Night	41% 4 9% 18% 20% 8%	2400
	11th to 20th	Night	46% 4 5% 17% 20% 7%	2339
	21st to end	Night	44% 4 7% 16% 18% 10%	1980
March	1st to 10th	Night	49% 3 8% 14% 19% 7%	5235
	11th to 20th	Night	30% 5% 7% 14% 27% 17%	4980
	21st to end	Night	40% 4 6% 21% 20% 9%	5467
April	1st to 10th	Night	35% 5% 10% 19% 23% 7%	2160
	11th to 20th	Night	32% 4 9% 20% 25% 10%	1975
	21st to end	Night	32% 6% 10% 27% 20% 5%	1835
May	1st to 10th	Night	42% 9% 16% 20% 13% 2	4060
	11th to 20th	Night	49% 7% 15% 20% 9% 1	4033
	21st to end	Night	36% 7% 14% 24% 17% 2	4429
June	1st to 10th	Night	37% 12% 17% 20% 12% 2	1800
	11th to 20th	Night	30% 10% 21% 25% 13% 1	1800
	21st to end	Night	31% 11% 19% 21% 15% 4	1800
July	1st to 10th	Night	45% 12% 18% 18% 6% 1	2840
	11th to 20th	Night	35% 15% 22% 19% 7% 2	2786
	21st to end	Night	53% 14% 15% 12% 4 1	2988
August	1st to 10th	Night	34% 11% 16% 31% 7% 1	2814
	11th to 20th	Night	34% 8% 16% 25% 16% 1	3055
	21st to end	Night	30% 8% 13% 21% 25% 3	3476
September	1st to 10th	Night	37% 9% 13% 19% 18% 4	2063
	11th to 20th	Night	39% 8% 11% 20% 19% 3	2090
	21st to end	Night	35% 7% 12% 17% 21% 8%	2090
October	1st to 10th	Night	24% 4 9% 13% 38% 12%	3480
	11th to 20th	Night	18% 3 4 17% 33% 25%	3642
	21st to end	Night	23% 3 4 22% 30% 18%	4120
November	1st to 10th	Night	32% 4 8% 17% 27% 12%	2516
	11th to 20th	Night	33% 3 6% 17% 29% 12%	2600
	21st to end	Night	36% 4 7% 19% 26% 8%	2600
December	1st to 10th	Night	38% 3 5% 18% 28% 7%	4250
	11th to 20th	Night	40% 4 10% 16% 24% 6%	4824
	21st to end	Night	45% 3 7% 27% 17% 2	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	25% 22% 38% 13% 2	300
	11th to 20th	Day	25% 20% 31% 17% 6% 1	300
	21st to end	Day	25% 21% 37% 13% 3 1	330
February	1st to 10th	Day	20% 19% 35% 21% 4	300
	11th to 20th	Day	37% 21% 27% 14% 2	300
	21st to end	Day	28% 17% 37% 15% 3	249
March	1st to 10th	Day	25% 17% 32% 18% 8% 1	359
	11th to 20th	Day	20% 19% 31% 23% 7%	500
	21st to end	Day	19% 16% 36% 22% 7% 1	550
April	1st to 10th	Day	19% 13% 32% 23% 11% 1	500
	11th to 20th	Day	14% 12% 34% 24% 14% 1	492
	21st to end	Day	15% 11% 22% 31% 18% 3	500
May	1st to 10th	Day	14% 10% 30% 26% 15% 6%	503
	11th to 20th	Day	14% 5% 23% 28% 18% 11%	527
	21st to end	Day	15% 5% 16% 26% 24% 13%	573
June	1st to 10th	Day	11% 3 17% 25% 29% 15%	530
	11th to 20th	Day	6% 3 12% 28% 25% 26%	530
	21st to end	Day	9% 2 13% 26% 30% 21%	530
July	1st to 10th	Day	10% 2 6% 22% 33% 27%	505
	11th to 20th	Day	9% 1 8% 19% 31% 32%	484
	21st to end	Day	8% 2 8% 20% 37% 26%	532
August	1st to 10th	Day	12% 3 11% 23% 27% 24%	486
	11th to 20th	Day	11% 3 8% 22% 37% 19%	479
	21st to end	Day	13% 1 9% 24% 29% 23%	539
September	1st to 10th	Day	11% 3 12% 26% 29% 18%	490
	11th to 20th	Day	17% 4 12% 22% 31% 13%	490
	21st to end	Day	16% 3 17% 23% 29% 12%	490
October	1st to 10th	Day	16% 4 17% 31% 24% 9%	490
	11th to 20th	Day	11% 9% 25% 24% 24% 6%	490
	21st to end	Day	18% 7% 22% 28% 21% 4	539
November	1st to 10th	Day	22% 8% 26% 24% 15% 6%	316
	11th to 20th	Day	18% 17% 34% 21% 9% 2	300
	21st to end	Day	11% 17% 35% 25% 11% 1	300
December	1st to 10th	Day	27% 20% 33% 14% 6% 1	300
	11th to 20th	Day	22% 16% 30% 24% 7% 1	300
	21st to end	Day	32% 13% 32% 17% 5% 1	330
January	1st to 10th	Night	24% 17% 33% 20% 5%	800
	11th to 20th	Night	21% 14% 34% 24% 8%	800
	21st to end	Night	20% 15% 37% 24% 3	880
February	1st to 10th	Night	19% 13% 31% 31% 6%	800
	11th to 20th	Night	26% 15% 33% 25% 2	783
	21st to end	Night	23% 15% 35% 25% 3	660
March	1st to 10th	Night	19% 11% 35% 30% 5%	800
	11th to 20th	Night	16% 12% 36% 26% 10% 1	800
	21st to end	Night	17% 13% 29% 32% 8%	880
April	1st to 10th	Night	14% 13% 30% 30% 12% 1	760
	11th to 20th	Night	16% 8% 28% 34% 13% 1	593
	21st to end	Night	10% 8% 25% 37% 20% 1	600
May	1st to 10th	Night	11% 7% 23% 35% 23% 1	600
	11th to 20th	Night	14% 4 23% 31% 24% 4	591
	21st to end	Night	13% 2 13% 34% 32% 7%	650
June	1st to 10th	Night	9% 1 10% 39% 35% 7%	600
	11th to 20th	Night	6% 1 12% 32% 38% 12%	600
	21st to end	Night	6% 1 10% 33% 40% 10%	600
July	1st to 10th	Night	10% 2 7% 22% 45% 14%	570
	11th to 20th	Night	9% 1 7% 17% 53% 13%	552
	21st to end	Night	8% 2 8% 24% 48% 11%	624
August	1st to 10th	Night	10% 1 9% 25% 45% 11%	567
	11th to 20th	Night	10% 1 10% 27% 45% 7%	564
	21st to end	Night	15% 1 9% 26% 40% 10%	627
September	1st to 10th	Night	11% 2 11% 27% 40% 10%	733
	11th to 20th	Night	17% 1 11% 28% 39% 4	760
	21st to end	Night	14% 3 13% 33% 34% 3	760
October	1st to 10th	Night	13% 3 18% 35% 25% 7%	760
	11th to 20th	Night	13% 5% 24% 33% 21% 3	760
	21st to end	Night	14% 7% 22% 36% 19% 3	836
November	1st to 10th	Night	15% 6% 24% 32% 20% 2	780
	11th to 20th	Night	17% 10% 29% 29% 14% 1	800
	21st to end	Night	18% 10% 28% 29% 14%	800
December	1st to 10th	Night	20% 13% 31% 29% 7% 1	800
	11th to 20th	Night	23% 10% 30% 28% 9% 1	800
	21st to end	Night	27% 12% 26% 29% 6%	880

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