Pleasant Hill Starlight Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Pleasant Hill Starlight Observatory clear sky chart from 2005-11-06 to 2025-04-11.

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% 7% 3 7% 5% 4 4 4 6% 7% 39%	114110
	Night	14% 8% 3 6% 4 4 4 3 5% 7% 42%	114178
Transparency	Day	41% 7% 11% 21% 17% 4	117878
	Night	39% 6% 10% 21% 19% 5%	118539
Seeing	Day	17% 12% 24% 22% 17% 8%	15323
	Night	16% 10% 24% 28% 19% 3	25159

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% 11% 5% 10% 4 4 4 3 4 5% 33%	4056
	11th to 20th	Day	18% 7% 5% 12% 3 3 3 2 3 5% 39%	4060
	21st to end	Day	21% 10% 5% 6% 4 3 3 3 4 6% 36%	4902
February	1st to 10th	Day	20% 11% 4 7% 4 4 3 3 5% 5% 35%	2000
	11th to 20th	Day	21% 11% 4 8% 5% 4 3 3 4 5% 33%	1948
	21st to end	Day	20% 10% 3 7% 4 3 3 3 4 5% 38%	1650
March	1st to 10th	Day	19% 10% 4 8% 5% 4 4 4 5% 5% 34%	4480
	11th to 20th	Day	13% 8% 3 6% 4 4 4 3 4 5% 46%	4831
	21st to end	Day	13% 8% 2 7% 4 5% 4 4 5% 7% 42%	5663
April	1st to 10th	Day	13% 7% 2 5% 4 4 5% 4 6% 6% 43%	2400
	11th to 20th	Day	14% 8% 3 6% 4 4 4 3 5% 6% 44%	2254
	21st to end	Day	12% 6% 3 7% 5% 4 4 3 6% 9% 40%	2324
May	1st to 10th	Day	16% 7% 2 6% 5% 5% 5% 4 7% 8% 36%	4082
	11th to 20th	Day	16% 9% 3 7% 5% 4 4 4 6% 7% 35%	4081
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 5% 7% 8% 39%	4476
June	1st to 10th	Day	9% 7% 3 7% 6% 6% 6% 4 7% 9% 37%	2470
	11th to 20th	Day	6% 4 2 5% 6% 6% 6% 6% 7% 11% 43%	2470
	21st to end	Day	6% 5% 2 6% 6% 6% 6% 6% 8% 11% 39%	2626
July	1st to 10th	Day	7% 5% 2 8% 6% 6% 7% 6% 8% 12% 33%	4317
	11th to 20th	Day	9% 6% 3 7% 6% 6% 6% 6% 8% 10% 33%	4120
	21st to end	Day	17% 10% 4 9% 7% 6% 7% 5% 6% 8% 21%	4466
August	1st to 10th	Day	4 4 2 5% 4 5% 5% 5% 8% 11% 47%	3914
	11th to 20th	Day	6% 4 2 7% 6% 7% 6% 5% 8% 10% 39%	3729
	21st to end	Day	7% 4 2 5% 5% 5% 5% 4 6% 8% 51%	4180
September	1st to 10th	Day	12% 6% 3 7% 5% 5% 5% 4 7% 8% 39%	2223
	11th to 20th	Day	12% 6% 3 6% 4 4 6% 4 6% 8% 42%	2090
	21st to end	Day	12% 5% 2 5% 4 3 5% 4 7% 7% 46%	2090
October	1st to 10th	Day	12% 5% 2 5% 3 3 2 3 4 6% 55%	2090
	11th to 20th	Day	12% 4 2 5% 3 3 3 3 5% 8% 53%	2090
	21st to end	Day	15% 6% 2 5% 3 4 3 3 4 6% 49%	1897
November	1st to 10th	Day	12% 5% 2 4 4 4 4 3 5% 5% 52%	1762
	11th to 20th	Day	14% 7% 2 6% 4 3 3 3 5% 6% 47%	1800
	21st to end	Day	15% 9% 2 5% 4 4 4 4 6% 7% 42%	1800
December	1st to 10th	Day	17% 7% 3 6% 4 4 4 3 4 5% 43%	2930
	11th to 20th	Day	22% 13% 4 7% 4 4 3 3 4 5% 32%	3373
	21st to end	Day	20% 11% 5% 7% 4 3 4 4 6% 7% 30%	4466
January	1st to 10th	Night	16% 12% 4 9% 4 3 3 3 4 5% 37%	5889
	11th to 20th	Night	18% 11% 4 7% 4 2 3 2 3 4 42%	5900
	21st to end	Night	18% 11% 3 6% 3 3 3 3 4 5% 39%	6052
February	1st to 10th	Night	19% 11% 3 7% 5% 3 3 2 5% 6% 36%	2400
	11th to 20th	Night	22% 11% 3 7% 4 4 3 3 4 4 35%	2338
	21st to end	Night	23% 10% 3 6% 4 4 3 3 4 5% 35%	1980
March	1st to 10th	Night	21% 12% 3 7% 4 4 4 3 4 6% 31%	5373
	11th to 20th	Night	12% 7% 2 5% 4 4 5% 3 6% 6% 47%	5128
	21st to end	Night	15% 8% 3 7% 5% 5% 4 3 4 5% 41%	5294
April	1st to 10th	Night	12% 5% 2 6% 4 4 4 3 5% 8% 45%	2000
	11th to 20th	Night	15% 7% 2 4 2 3 3 3 5% 7% 48%	1881
	21st to end	Night	12% 7% 1 5% 4 4 4 4 6% 7% 46%	1853
May	1st to 10th	Night	14% 6% 2 7% 4 5% 4 4 7% 7% 41%	2813
	11th to 20th	Night	16% 8% 3 7% 4 4 4 3 6% 7% 39%	2812
	21st to end	Night	7% 5% 2 6% 5% 6% 5% 5% 8% 10% 43%	3079
June	1st to 10th	Night	7% 7% 3 7% 5% 5% 4 5% 6% 10% 41%	1710
	11th to 20th	Night	3 3 1 5% 3 3 5% 5% 8% 13% 52%	1710
	21st to end	Night	5% 4 2 5% 5% 4 5% 4 6% 12% 47%	1540
July	1st to 10th	Night	8% 5% 2 7% 6% 6% 7% 5% 7% 11% 36%	2623
	11th to 20th	Night	9% 6% 2 5% 4 5% 5% 4 8% 10% 42%	2840
	21st to end	Night	12% 10% 5% 10% 6% 5% 5% 5% 7% 8% 26%	3087
August	1st to 10th	Night	3 3 1 4 3 4 4 4 7% 11% 56%	3010
	11th to 20th	Night	6% 3 2 5% 4 4 5% 4 7% 11% 49%	3100
	21st to end	Night	5% 4 1 4 4 4 4 4 6% 9% 55%	3476
September	1st to 10th	Night	10% 5% 2 7% 5% 4 4 4 6% 7% 46%	1938
	11th to 20th	Night	9% 6% 3 7% 5% 4 5% 3 6% 8% 45%	2090
	21st to end	Night	10% 4 2 6% 4 3 3 4 6% 7% 52%	2090
October	1st to 10th	Night	11% 4 1 5% 3 3 3 3 4 6% 59%	2090
	11th to 20th	Night	13% 6% 2 5% 4 4 3 2 4 6% 51%	2090
	21st to end	Night	13% 8% 2 5% 3 3 3 3 4 6% 51%	2694
November	1st to 10th	Night	13% 5% 2 5% 3 3 4 3 5% 6% 51%	2536
	11th to 20th	Night	12% 7% 2 5% 3 3 4 3 5% 6% 49%	2600
	21st to end	Night	15% 9% 3 5% 3 3 4 3 5% 6% 43%	2600
December	1st to 10th	Night	19% 10% 2 6% 3 3 3 3 3 6% 41%	4250
	11th to 20th	Night	26% 10% 3 5% 3 2 3 2 4 4 38%	4822
	21st to end	Night	19% 11% 4 8% 4 4 4 3 5% 6% 31%	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	52% 5% 8% 18% 15% 2	4057
	11th to 20th	Day	48% 5% 5% 17% 24% 1	4060
	21st to end	Day	49% 4 8% 21% 15% 3	4902
February	1st to 10th	Day	51% 4 8% 18% 16% 2	2000
	11th to 20th	Day	54% 4 6% 17% 14% 4	1958
	21st to end	Day	50% 5% 7% 14% 19% 6%	1650
March	1st to 10th	Day	49% 6% 8% 13% 16% 9%	4480
	11th to 20th	Day	39% 5% 7% 19% 20% 10%	4831
	21st to end	Day	41% 6% 9% 27% 15% 1	5665
April	1st to 10th	Day	38% 7% 11% 22% 18% 4	2400
	11th to 20th	Day	40% 5% 7% 19% 23% 6%	2254
	21st to end	Day	40% 7% 10% 26% 16% 2	2324
May	1st to 10th	Day	42% 10% 11% 21% 13% 2	4120
	11th to 20th	Day	46% 9% 13% 22% 11%	4085
	21st to end	Day	39% 10% 11% 21% 18% 1	4489
June	1st to 10th	Day	41% 9% 14% 21% 14% 2	2470
	11th to 20th	Day	32% 10% 17% 24% 16% 2	2470
	21st to end	Day	35% 11% 18% 18% 14% 4	2626
July	1st to 10th	Day	38% 12% 21% 21% 6% 1	4251
	11th to 20th	Day	39% 10% 21% 21% 6% 1	4051
	21st to end	Day	58% 10% 12% 15% 6% 1	4324
August	1st to 10th	Day	27% 9% 15% 33% 15% 1	3901
	11th to 20th	Day	35% 10% 15% 25% 13% 2	3765
	21st to end	Day	29% 9% 11% 27% 22% 2	4179
September	1st to 10th	Day	40% 9% 13% 18% 15% 5%	2223
	11th to 20th	Day	39% 9% 11% 19% 19% 2	2090
	21st to end	Day	34% 7% 12% 21% 19% 6%	2090
October	1st to 10th	Day	22% 5% 6% 17% 38% 12%	3480
	11th to 20th	Day	21% 4 6% 21% 30% 17%	3458
	21st to end	Day	27% 4 9% 23% 27% 10%	3097
November	1st to 10th	Day	33% 4 8% 20% 28% 6%	1762
	11th to 20th	Day	38% 4 6% 21% 24% 7%	1800
	21st to end	Day	41% 5% 7% 21% 21% 5%	1800
December	1st to 10th	Day	43% 4 5% 23% 22% 3	2930
	11th to 20th	Day	55% 4 7% 18% 12% 3	3370
	21st to end	Day	50% 7% 10% 21% 10% 2	4466
January	1st to 10th	Night	48% 4 9% 19% 18% 2	5889
	11th to 20th	Night	47% 4 6% 16% 21% 5%	5900
	21st to end	Night	47% 4 5% 20% 18% 6%	6052
February	1st to 10th	Night	49% 4 10% 18% 17% 3	2400
	11th to 20th	Night	54% 4 6% 15% 15% 5%	2339
	21st to end	Night	51% 5% 7% 17% 16% 5%	1980
March	1st to 10th	Night	52% 6% 7% 13% 18% 4	5373
	11th to 20th	Night	36% 6% 7% 18% 20% 13%	5128
	21st to end	Night	45% 5% 7% 22% 18% 3	5294
April	1st to 10th	Night	36% 6% 12% 23% 20% 3	2000
	11th to 20th	Night	34% 6% 9% 20% 22% 9%	1881
	21st to end	Night	36% 6% 12% 26% 18% 2	1853
May	1st to 10th	Night	38% 10% 12% 23% 16% 1	2840
	11th to 20th	Night	43% 10% 14% 20% 14%	2813
	21st to end	Night	31% 12% 16% 25% 16% 1	3087
June	1st to 10th	Night	37% 9% 13% 23% 16% 3	1710
	11th to 20th	Night	21% 9% 20% 30% 17% 3	1710
	21st to end	Night	28% 10% 16% 25% 16% 4	1540
July	1st to 10th	Night	38% 12% 20% 22% 8% 1	2580
	11th to 20th	Night	33% 10% 22% 25% 7% 3	2786
	21st to end	Night	54% 11% 12% 17% 6% 1	2988
August	1st to 10th	Night	20% 9% 13% 37% 20% 1	2999
	11th to 20th	Night	26% 10% 22% 26% 15% 1	3130
	21st to end	Night	23% 9% 15% 28% 24% 2	3476
September	1st to 10th	Night	35% 7% 16% 22% 15% 6%	1938
	11th to 20th	Night	35% 8% 12% 19% 23% 3	2090
	21st to end	Night	30% 6% 12% 23% 22% 7%	2090
October	1st to 10th	Night	20% 6% 7% 20% 29% 17%	3480
	11th to 20th	Night	22% 3 5% 20% 30% 20%	3461
	21st to end	Night	24% 3 6% 25% 29% 12%	4432
November	1st to 10th	Night	34% 5% 8% 21% 24% 8%	2536
	11th to 20th	Night	34% 4 7% 20% 26% 9%	2600
	21st to end	Night	42% 4 7% 20% 22% 5%	2600
December	1st to 10th	Night	46% 5% 6% 16% 25% 2	4250
	11th to 20th	Night	49% 4 5% 18% 21% 2	4824
	21st to end	Night	53% 5% 8% 23% 10% 1	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	26% 27% 31% 11% 6%	300
	11th to 20th	Day	26% 28% 29% 13% 3 1	300
	21st to end	Day	23% 25% 38% 11% 2	330
February	1st to 10th	Day	26% 22% 35% 14% 2	300
	11th to 20th	Day	39% 23% 28% 8% 2 1	300
	21st to end	Day	33% 24% 32% 8% 2	249
March	1st to 10th	Day	28% 21% 34% 14% 3	300
	11th to 20th	Day	24% 21% 31% 17% 5% 1	429
	21st to end	Day	24% 21% 32% 15% 8%	550
April	1st to 10th	Day	21% 18% 30% 20% 10% 2	500
	11th to 20th	Day	19% 16% 31% 23% 9% 2	452
	21st to end	Day	17% 15% 32% 22% 12% 2	460
May	1st to 10th	Day	16% 15% 29% 23% 13% 5%	463
	11th to 20th	Day	10% 10% 26% 27% 18% 8%	487
	21st to end	Day	12% 6% 22% 31% 16% 13%	529
June	1st to 10th	Day	12% 7% 22% 26% 20% 13%	490
	11th to 20th	Day	6% 3 17% 27% 28% 19%	490
	21st to end	Day	8% 4 19% 28% 27% 14%	646
July	1st to 10th	Day	9% 3 16% 29% 24% 19%	603
	11th to 20th	Day	10% 4 12% 21% 31% 22%	484
	21st to end	Day	10% 5% 10% 25% 30% 20%	532
August	1st to 10th	Day	10% 4 11% 29% 29% 18%	486
	11th to 20th	Day	9% 3 14% 29% 32% 12%	479
	21st to end	Day	14% 3 8% 22% 34% 19%	539
September	1st to 10th	Day	10% 5% 12% 25% 34% 14%	490
	11th to 20th	Day	11% 5% 15% 25% 26% 18%	490
	21st to end	Day	16% 6% 16% 28% 29% 7%	490
October	1st to 10th	Day	13% 4 22% 28% 25% 8%	490
	11th to 20th	Day	12% 12% 30% 27% 17% 3	490
	21st to end	Day	19% 11% 28% 28% 12% 2	345
November	1st to 10th	Day	20% 8% 33% 28% 10% 2	300
	11th to 20th	Day	18% 14% 38% 25% 4 1	300
	21st to end	Day	21% 16% 35% 16% 10% 2	300
December	1st to 10th	Day	27% 21% 30% 16% 6%	300
	11th to 20th	Day	22% 23% 31% 14% 9%	300
	21st to end	Day	27% 18% 34% 16% 4 1	330
January	1st to 10th	Night	24% 18% 34% 20% 5%	800
	11th to 20th	Night	27% 19% 30% 20% 4	800
	21st to end	Night	20% 16% 39% 23% 3	880
February	1st to 10th	Night	25% 15% 33% 24% 4	800
	11th to 20th	Night	28% 18% 33% 19% 2	783
	21st to end	Night	27% 20% 30% 20% 2	660
March	1st to 10th	Night	22% 17% 31% 26% 4	800
	11th to 20th	Night	20% 16% 34% 23% 7% 1	800
	21st to end	Night	19% 15% 32% 25% 9%	810
April	1st to 10th	Night	15% 15% 33% 26% 10% 1	600
	11th to 20th	Night	18% 9% 33% 28% 10% 1	565
	21st to end	Night	15% 9% 31% 32% 12% 1	570
May	1st to 10th	Night	13% 6% 30% 31% 18% 2	570
	11th to 20th	Night	15% 5% 23% 32% 21% 4	561
	21st to end	Night	8% 2 18% 38% 29% 6%	617
June	1st to 10th	Night	8% 4 18% 33% 31% 5%	570
	11th to 20th	Night	4 1 11% 37% 37% 9%	570
	21st to end	Night	6% 2 12% 36% 37% 7%	570
July	1st to 10th	Night	8% 2 10% 27% 41% 12%	555
	11th to 20th	Night	8% 3 8% 26% 43% 12%	552
	21st to end	Night	7% 2 8% 31% 41% 11%	624
August	1st to 10th	Night	6% 2 7% 25% 51% 9%	567
	11th to 20th	Night	8% 2 10% 33% 40% 7%	564
	21st to end	Night	9% 2 8% 28% 43% 11%	627
September	1st to 10th	Night	10% 3 13% 31% 37% 7%	608
	11th to 20th	Night	10% 3 16% 28% 36% 6%	760
	21st to end	Night	11% 3 17% 32% 34% 3	760
October	1st to 10th	Night	11% 6% 18% 31% 28% 6%	760
	11th to 20th	Night	15% 8% 24% 35% 18% 1	760
	21st to end	Night	18% 8% 24% 31% 17% 2	836
November	1st to 10th	Night	16% 9% 21% 33% 18% 3	780
	11th to 20th	Night	16% 12% 32% 28% 12% 1	800
	21st to end	Night	20% 13% 27% 29% 11% 1	800
December	1st to 10th	Night	25% 14% 28% 27% 6%	800
	11th to 20th	Night	26% 11% 29% 26% 8% 1	800
	21st to end	Night	25% 17% 28% 25% 5%	880

Page generated at 2025-04-08 08:17:30. Version ac07

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