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 2024-02-20.

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 6% 6% 3 5% 5% 6% 7% 38%	79082
	Night	14% 8% 3 5% 5% 3 4 4 5% 7% 42%	80089
Transparency	Day	43% 8% 11% 19% 16% 3	78804
	Night	40% 7% 10% 20% 18% 4	79887
Seeing	Day	16% 11% 23% 23% 18% 9%	13811
	Night	16% 9% 24% 28% 19% 3	23675

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% 12% 4 8% 6% 4 4 5% 5% 6% 29%	2836
	11th to 20th	Day	22% 8% 5% 11% 4 2 4 3 4 6% 32%	2840
	21st to end	Day	24% 11% 6% 5% 4 2 3 3 4 6% 33%	3420
February	1st to 10th	Day	21% 10% 4 6% 4 3 3 3 4 6% 35%	1900
	11th to 20th	Day	21% 10% 4 7% 6% 3 4 3 4 5% 34%	1838
	21st to end	Day	21% 10% 4 6% 5% 2 4 4 4 5% 36%	1480
March	1st to 10th	Day	16% 9% 3 5% 5% 3 4 5% 4 6% 40%	1800
	11th to 20th	Day	20% 9% 3 5% 5% 3 5% 4 5% 6% 35%	1915
	21st to end	Day	16% 8% 3 6% 5% 4 5% 5% 6% 7% 34%	2254
April	1st to 10th	Day	13% 6% 2 5% 4 3 5% 5% 5% 6% 44%	2160
	11th to 20th	Day	14% 7% 3 5% 5% 3 4 3 5% 6% 44%	2134
	21st to end	Day	13% 6% 3 6% 5% 4 4 5% 5% 9% 40%	2201
May	1st to 10th	Day	14% 6% 3 5% 5% 4 5% 4 6% 7% 42%	2340
	11th to 20th	Day	12% 7% 4 6% 7% 4 5% 5% 6% 7% 38%	2301
	21st to end	Day	9% 6% 2 6% 7% 4 7% 7% 8% 8% 38%	2531
June	1st to 10th	Day	9% 7% 3 6% 6% 4 6% 6% 6% 9% 38%	2340
	11th to 20th	Day	6% 3 2 5% 6% 4 6% 7% 7% 11% 42%	2340
	21st to end	Day	6% 4 2 5% 7% 4 6% 7% 7% 11% 39%	2487
July	1st to 10th	Day	9% 5% 3 7% 7% 5% 8% 7% 8% 11% 30%	2453
	11th to 20th	Day	10% 5% 3 6% 7% 4 7% 9% 8% 9% 33%	2340
	21st to end	Day	8% 4 2 7% 8% 5% 8% 8% 7% 11% 33%	2547
August	1st to 10th	Day	6% 6% 3 7% 7% 5% 7% 7% 8% 11% 33%	2221
	11th to 20th	Day	9% 5% 2 8% 7% 5% 7% 6% 7% 10% 33%	2125
	21st to end	Day	12% 5% 2 6% 7% 5% 6% 7% 7% 9% 33%	2376
September	1st to 10th	Day	12% 5% 3 6% 7% 4 5% 5% 7% 8% 39%	2106
	11th to 20th	Day	12% 6% 3 5% 5% 3 5% 5% 5% 8% 43%	1980
	21st to end	Day	11% 4 2 4 5% 3 4 5% 6% 7% 48%	1980
October	1st to 10th	Day	12% 5% 2 4 5% 2 3 3 4 5% 54%	1980
	11th to 20th	Day	12% 4 2 4 4 3 3 4 4 8% 52%	1980
	21st to end	Day	16% 5% 2 5% 4 3 4 4 4 6% 47%	1798
November	1st to 10th	Day	12% 5% 1 4 4 3 3 4 4 5% 55%	1672
	11th to 20th	Day	14% 7% 2 5% 4 3 4 4 4 6% 48%	1710
	21st to end	Day	16% 7% 2 5% 4 2 5% 5% 5% 8% 42%	1710
December	1st to 10th	Day	22% 10% 3 6% 6% 3 4 5% 4 5% 33%	1710
	11th to 20th	Day	17% 10% 3 6% 6% 3 5% 4 4 5% 37%	2153
	21st to end	Day	18% 12% 5% 6% 5% 3 5% 5% 6% 7% 29%	3124
January	1st to 10th	Night	18% 13% 3 6% 4 1 3 4 5% 6% 37%	4109
	11th to 20th	Night	20% 10% 5% 7% 6% 2 3 3 3 4 36%	4120
	21st to end	Night	21% 14% 4 5% 4 2 3 4 4 4 34%	4234
February	1st to 10th	Night	19% 11% 3 6% 5% 2 4 3 5% 6% 37%	2280
	11th to 20th	Night	22% 9% 4 6% 4 3 4 4 3 4 37%	2213
	21st to end	Night	25% 9% 3 6% 5% 3 3 4 4 6% 33%	1776
March	1st to 10th	Night	16% 7% 3 5% 5% 3 4 5% 5% 7% 40%	2160
	11th to 20th	Night	16% 9% 3 7% 5% 3 4 4 5% 7% 37%	2044
	21st to end	Night	14% 9% 4 5% 6% 3 3 4 5% 7% 40%	2116
April	1st to 10th	Night	12% 5% 2 5% 4 3 5% 4 5% 9% 47%	1800
	11th to 20th	Night	15% 6% 2 3 3 2 3 4 5% 7% 49%	1775
	21st to end	Night	11% 6% 2 5% 4 3 5% 5% 6% 8% 46%	1756
May	1st to 10th	Night	12% 4 2 3 5% 3 4 4 6% 8% 50%	1620
	11th to 20th	Night	12% 5% 3 5% 5% 3 5% 4 7% 9% 43%	1592
	21st to end	Night	7% 4 2 4 5% 4 6% 5% 6% 11% 45%	1744
June	1st to 10th	Night	6% 6% 3 6% 5% 4 5% 6% 7% 10% 42%	1620
	11th to 20th	Night	3 2 1 4 3 3 4 6% 8% 13% 53%	1620
	21st to end	Night	5% 4 3 4 5% 3 6% 5% 6% 12% 47%	1459
July	1st to 10th	Night	8% 4 3 5% 7% 5% 6% 6% 7% 11% 37%	1494
	11th to 20th	Night	8% 4 2 4 5% 3 5% 5% 8% 11% 44%	1620
	21st to end	Night	6% 5% 4 6% 5% 4 6% 7% 8% 11% 38%	1771
August	1st to 10th	Night	5% 4 2 5% 6% 3 6% 7% 8% 13% 42%	1712
	11th to 20th	Night	8% 4 3 5% 6% 3 6% 6% 6% 11% 42%	1776
	21st to end	Night	8% 6% 2 5% 6% 4 6% 6% 8% 10% 40%	1980
September	1st to 10th	Night	10% 5% 2 5% 6% 3 4 5% 6% 8% 46%	1836
	11th to 20th	Night	9% 6% 3 5% 6% 3 5% 5% 5% 9% 45%	1980
	21st to end	Night	8% 4 2 5% 5% 2 3 5% 6% 7% 54%	1980
October	1st to 10th	Night	11% 4 1 4 4 2 2 3 4 6% 59%	1980
	11th to 20th	Night	14% 6% 3 5% 4 3 3 3 4 6% 49%	1980
	21st to end	Night	14% 8% 2 4 4 2 3 4 4 6% 49%	2552
November	1st to 10th	Night	13% 4 2 4 4 2 4 4 4 6% 53%	2406
	11th to 20th	Night	12% 7% 2 5% 4 2 4 4 5% 7% 50%	2470
	21st to end	Night	16% 8% 3 5% 4 2 4 4 5% 6% 43%	2470
December	1st to 10th	Night	20% 10% 3 5% 5% 3 5% 4 4 6% 35%	2470
	11th to 20th	Night	18% 9% 3 4 5% 2 4 3 5% 5% 43%	3042
	21st to end	Night	18% 11% 4 8% 6% 4 5% 4 5% 7% 28%	4532

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	53% 6% 7% 17% 16% 2	2837
	11th to 20th	Day	54% 6% 6% 13% 20% 1	2840
	21st to end	Day	53% 4 8% 21% 13% 1	3420
February	1st to 10th	Day	52% 4 7% 18% 17% 2	1900
	11th to 20th	Day	53% 4 6% 18% 14% 4	1848
	21st to end	Day	51% 5% 8% 13% 17% 6%	1480
March	1st to 10th	Day	44% 5% 8% 16% 19% 7%	1800
	11th to 20th	Day	51% 6% 10% 15% 15% 3	1915
	21st to end	Day	48% 7% 11% 21% 12% 1	2254
April	1st to 10th	Day	37% 7% 11% 23% 18% 4	2160
	11th to 20th	Day	40% 5% 7% 19% 23% 5%	2134
	21st to end	Day	40% 7% 9% 26% 16% 2	2201
May	1st to 10th	Day	40% 7% 12% 24% 14% 3	2340
	11th to 20th	Day	44% 9% 13% 21% 13%	2305
	21st to end	Day	39% 10% 14% 22% 14% 2	2531
June	1st to 10th	Day	40% 9% 14% 21% 15% 1	2340
	11th to 20th	Day	32% 10% 17% 24% 15% 2	2340
	21st to end	Day	35% 11% 19% 18% 13% 4	2487
July	1st to 10th	Day	43% 12% 16% 17% 9% 3	2387
	11th to 20th	Day	40% 12% 21% 15% 10% 2	2271
	21st to end	Day	40% 13% 17% 21% 9% 1	2405
August	1st to 10th	Day	41% 12% 18% 18% 10% 1	2208
	11th to 20th	Day	43% 12% 14% 17% 13% 2	2125
	21st to end	Day	44% 12% 12% 18% 11% 3	2376
September	1st to 10th	Day	40% 9% 14% 19% 15% 4	2106
	11th to 20th	Day	38% 9% 12% 19% 19% 3	1980
	21st to end	Day	33% 7% 11% 22% 20% 6%	1980
October	1st to 10th	Day	32% 6% 8% 22% 26% 6%	1980
	11th to 20th	Day	31% 6% 9% 21% 24% 8%	1980
	21st to end	Day	37% 5% 7% 20% 24% 7%	1798
November	1st to 10th	Day	32% 4 8% 20% 29% 7%	1672
	11th to 20th	Day	37% 4 6% 21% 24% 7%	1710
	21st to end	Day	41% 6% 7% 21% 21% 5%	1710
December	1st to 10th	Day	53% 5% 6% 18% 15% 3	1710
	11th to 20th	Day	48% 5% 9% 18% 17% 3	2150
	21st to end	Day	50% 8% 11% 19% 11% 2	3124
January	1st to 10th	Night	48% 4 7% 20% 18% 2	4109
	11th to 20th	Night	51% 5% 7% 16% 16% 6%	4120
	21st to end	Night	53% 4 6% 18% 17% 2	4234
February	1st to 10th	Night	49% 4 10% 17% 17% 3	2280
	11th to 20th	Night	53% 4 6% 16% 15% 6%	2214
	21st to end	Night	53% 5% 7% 17% 13% 5%	1776
March	1st to 10th	Night	43% 6% 9% 15% 19% 8%	2160
	11th to 20th	Night	47% 7% 8% 19% 17% 2	2044
	21st to end	Night	45% 5% 10% 23% 15% 2	2116
April	1st to 10th	Night	35% 6% 12% 23% 21% 3	1800
	11th to 20th	Night	33% 5% 9% 21% 22% 9%	1775
	21st to end	Night	35% 6% 12% 26% 18% 2	1756
May	1st to 10th	Night	33% 7% 11% 24% 22% 2	1620
	11th to 20th	Night	36% 10% 16% 22% 15% 1	1593
	21st to end	Night	31% 11% 17% 23% 16% 2	1745
June	1st to 10th	Night	36% 9% 13% 23% 16% 3	1620
	11th to 20th	Night	21% 9% 21% 30% 16% 3	1620
	21st to end	Night	28% 10% 17% 25% 16% 4	1459
July	1st to 10th	Night	37% 13% 18% 18% 12% 2	1451
	11th to 20th	Night	31% 10% 20% 22% 12% 5%	1566
	21st to end	Night	35% 14% 16% 24% 11% 1	1672
August	1st to 10th	Night	31% 11% 20% 25% 12% 1	1701
	11th to 20th	Night	34% 12% 20% 18% 15% 2	1776
	21st to end	Night	35% 11% 16% 23% 12% 3	1980
September	1st to 10th	Night	35% 7% 16% 22% 15% 5%	1836
	11th to 20th	Night	34% 8% 13% 19% 23% 3	1980
	21st to end	Night	29% 6% 11% 24% 23% 7%	1980
October	1st to 10th	Night	27% 6% 8% 25% 25% 10%	1980
	11th to 20th	Night	36% 4 8% 22% 20% 9%	1980
	21st to end	Night	36% 4 5% 21% 26% 8%	2552
November	1st to 10th	Night	33% 4 8% 21% 25% 9%	2406
	11th to 20th	Night	34% 4 7% 21% 25% 9%	2470
	21st to end	Night	42% 4 7% 19% 22% 5%	2470
December	1st to 10th	Night	50% 6% 7% 17% 17% 3	2470
	11th to 20th	Night	42% 5% 7% 18% 24% 4	3044
	21st to end	Night	53% 7% 9% 18% 12% 2	4532

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% 28% 30% 11% 6%	270
	11th to 20th	Day	26% 29% 28% 13% 3 1	270
	21st to end	Day	23% 26% 38% 11% 3	297
February	1st to 10th	Day	27% 23% 34% 13% 3	270
	11th to 20th	Day	38% 22% 28% 9% 2 1	267
	21st to end	Day	35% 22% 33% 7% 3	198
March	1st to 10th	Day	25% 20% 38% 14% 3	240
	11th to 20th	Day	24% 22% 29% 18% 6% 1	355
	21st to end	Day	25% 20% 30% 15% 9%	462
April	1st to 10th	Day	19% 16% 30% 22% 11% 2	420
	11th to 20th	Day	19% 14% 31% 25% 9% 2	412
	21st to end	Day	18% 16% 30% 22% 11% 2	420
May	1st to 10th	Day	16% 14% 29% 24% 13% 5%	423
	11th to 20th	Day	10% 9% 27% 27% 19% 9%	447
	21st to end	Day	12% 6% 19% 31% 18% 15%	485
June	1st to 10th	Day	11% 6% 21% 26% 21% 14%	450
	11th to 20th	Day	6% 3 16% 27% 27% 20%	450
	21st to end	Day	8% 4 19% 28% 26% 15%	597
July	1st to 10th	Day	10% 3 16% 28% 24% 20%	557
	11th to 20th	Day	10% 4 12% 20% 32% 23%	444
	21st to end	Day	8% 4 9% 25% 32% 21%	488
August	1st to 10th	Day	11% 4 10% 28% 28% 19%	446
	11th to 20th	Day	10% 2 12% 29% 34% 13%	439
	21st to end	Day	15% 3 9% 22% 33% 18%	495
September	1st to 10th	Day	11% 4 11% 25% 35% 15%	450
	11th to 20th	Day	10% 5% 15% 26% 25% 19%	450
	21st to end	Day	14% 4 16% 28% 30% 7%	450
October	1st to 10th	Day	14% 4 21% 27% 26% 8%	450
	11th to 20th	Day	14% 12% 30% 26% 15% 3	450
	21st to end	Day	21% 12% 27% 26% 12% 2	312
November	1st to 10th	Day	19% 8% 32% 30% 9% 2	270
	11th to 20th	Day	17% 13% 40% 26% 4	270
	21st to end	Day	20% 16% 37% 15% 10% 2	270
December	1st to 10th	Day	29% 20% 28% 17% 6%	270
	11th to 20th	Day	20% 24% 33% 14% 10%	270
	21st to end	Day	27% 19% 35% 16% 3 1	297
January	1st to 10th	Night	24% 18% 33% 20% 5%	760
	11th to 20th	Night	27% 19% 29% 19% 5%	760
	21st to end	Night	21% 16% 38% 23% 3	836
February	1st to 10th	Night	24% 15% 34% 23% 4	760
	11th to 20th	Night	27% 16% 35% 20% 2	742
	21st to end	Night	29% 19% 31% 19% 2	592
March	1st to 10th	Night	21% 17% 31% 27% 4	720
	11th to 20th	Night	20% 17% 33% 22% 7% 1	720
	21st to end	Night	19% 15% 32% 25% 9%	730
April	1st to 10th	Night	14% 13% 33% 28% 11% 1	540
	11th to 20th	Night	18% 8% 32% 29% 11% 2	533
	21st to end	Night	15% 9% 30% 33% 12% 1	540
May	1st to 10th	Night	13% 5% 31% 31% 18% 2	540
	11th to 20th	Night	15% 4 24% 32% 22% 4	531
	21st to end	Night	8% 2 17% 37% 31% 6%	584
June	1st to 10th	Night	8% 3 18% 33% 33% 5%	540
	11th to 20th	Night	4 1 11% 38% 36% 10%	540
	21st to end	Night	5% 1 13% 37% 37% 8%	540
July	1st to 10th	Night	8% 2 10% 26% 41% 13%	525
	11th to 20th	Night	9% 2 8% 25% 43% 13%	522
	21st to end	Night	6% 2 7% 31% 43% 11%	591
August	1st to 10th	Night	6% 2 6% 25% 51% 10%	537
	11th to 20th	Night	8% 2 8% 33% 42% 7%	534
	21st to end	Night	10% 2 8% 28% 42% 10%	594
September	1st to 10th	Night	10% 2 11% 30% 39% 7%	576
	11th to 20th	Night	10% 3 17% 28% 36% 7%	720
	21st to end	Night	9% 3 17% 33% 35% 3	720
October	1st to 10th	Night	12% 6% 17% 30% 28% 6%	720
	11th to 20th	Night	16% 7% 24% 34% 17% 1	720
	21st to end	Night	19% 8% 23% 31% 16% 2	792
November	1st to 10th	Night	16% 9% 21% 34% 19% 3	740
	11th to 20th	Night	15% 12% 32% 29% 11% 1	760
	21st to end	Night	21% 11% 27% 29% 11% 1	760
December	1st to 10th	Night	26% 14% 28% 27% 6%	760
	11th to 20th	Night	24% 11% 29% 26% 8% 1	760
	21st to end	Night	25% 17% 28% 26% 4	836

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