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 2026-08-15.

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% 5% 5% 4 6% 8% 37%	143904
	Night	14% 8% 3 7% 4 4 4 4 5% 7% 41%	137967
Transparency	Day	42% 8% 11% 20% 15% 3	147628
	Night	40% 7% 11% 20% 17% 4	142305
Seeing	Day	16% 12% 24% 22% 17% 8%	17171
	Night	16% 10% 24% 28% 19% 3	26831

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% 11% 5% 9% 4 4 4 3 4 5% 33%	5258
	11th to 20th	Day	15% 6% 4 11% 3 3 2 2 3 5% 46%	5280
	21st to end	Day	21% 9% 4 7% 5% 3 3 3 4 6% 33%	6352
February	1st to 10th	Day	20% 11% 4 7% 4 4 3 3 5% 5% 35%	2100
	11th to 20th	Day	22% 10% 4 8% 5% 4 3 3 4 5% 33%	2048
	21st to end	Day	19% 10% 3 7% 4 3 3 3 4 5% 38%	1730
March	1st to 10th	Day	19% 10% 3 8% 5% 4 4 4 5% 5% 33%	4580
	11th to 20th	Day	13% 8% 3 6% 4 4 4 3 4 5% 46%	4937
	21st to end	Day	13% 7% 2 7% 4 4 4 4 5% 7% 42%	5788
April	1st to 10th	Day	13% 7% 2 5% 4 4 4 5% 6% 7% 43%	2520
	11th to 20th	Day	13% 8% 3 6% 4 4 4 3 5% 6% 45%	2494
	21st to end	Day	13% 6% 2 7% 5% 4 5% 4 6% 8% 40%	2569
May	1st to 10th	Day	18% 8% 3 6% 4 5% 5% 4 7% 9% 31%	7616
	11th to 20th	Day	14% 9% 3 9% 5% 5% 4 4 6% 7% 34%	7626
	21st to end	Day	20% 10% 3 8% 5% 5% 5% 4 5% 5% 28%	8391
June	1st to 10th	Day	9% 7% 3 7% 5% 6% 5% 4 7% 9% 38%	2730
	11th to 20th	Day	7% 4 2 6% 6% 6% 5% 5% 7% 11% 41%	2730
	21st to end	Day	6% 5% 2 6% 6% 6% 6% 6% 8% 11% 38%	2902
July	1st to 10th	Day	7% 5% 2 7% 6% 6% 7% 6% 8% 12% 33%	8045
	11th to 20th	Day	8% 6% 3 8% 7% 7% 8% 7% 9% 12% 26%	7680
	21st to end	Day	12% 7% 3 8% 6% 6% 7% 6% 8% 12% 26%	8340
August	1st to 10th	Day	4 4 2 6% 4 5% 5% 5% 8% 11% 45%	4162
	11th to 20th	Day	6% 4 2 7% 6% 7% 6% 5% 8% 10% 39%	3897
	21st to end	Day	7% 4 2 5% 5% 5% 5% 4 6% 8% 50%	4312
September	1st to 10th	Day	12% 6% 3 7% 5% 5% 5% 4 7% 8% 40%	2340
	11th to 20th	Day	11% 5% 3 6% 4 4 6% 4 6% 8% 42%	2200
	21st to end	Day	12% 5% 2 5% 4 4 5% 4 6% 7% 45%	2200
October	1st to 10th	Day	11% 5% 2 6% 3 3 2 3 4 6% 55%	2200
	11th to 20th	Day	11% 4 2 5% 3 3 3 3 5% 7% 54%	2200
	21st to end	Day	15% 6% 2 5% 3 4 3 3 4 6% 48%	1997
November	1st to 10th	Day	12% 6% 2 5% 4 4 4 3 5% 5% 52%	1852
	11th to 20th	Day	13% 7% 2 6% 4 3 3 3 5% 7% 48%	1890
	21st to end	Day	15% 9% 2 5% 4 3 4 4 6% 7% 41%	1890
December	1st to 10th	Day	17% 8% 3 6% 5% 4 4 3 4 5% 42%	3020
	11th to 20th	Day	22% 13% 4 7% 4 4 3 3 4 5% 33%	3463
	21st to end	Day	20% 11% 5% 7% 4 4 4 4 6% 7% 30%	4565
January	1st to 10th	Night	18% 11% 3 8% 4 3 3 3 4 5% 39%	7635
	11th to 20th	Night	16% 9% 3 7% 4 3 3 2 3 4 45%	7680
	21st to end	Night	19% 11% 3 7% 4 3 4 3 4 5% 37%	7827
February	1st to 10th	Night	19% 10% 3 7% 5% 3 3 2 5% 6% 36%	2520
	11th to 20th	Night	22% 11% 4 7% 4 4 3 3 3 4 35%	2458
	21st to end	Night	22% 10% 3 6% 4 4 4 3 4 5% 35%	2076
March	1st to 10th	Night	21% 12% 3 7% 4 4 4 3 5% 6% 31%	5493
	11th to 20th	Night	12% 7% 2 5% 4 4 4 3 6% 6% 47%	5242
	21st to end	Night	15% 8% 3 7% 5% 5% 4 3 4 5% 42%	5412
April	1st to 10th	Night	12% 5% 2 6% 4 4 4 3 5% 8% 45%	2100
	11th to 20th	Night	14% 7% 2 4 2 3 3 3 5% 7% 50%	2075
	21st to end	Night	12% 7% 2 6% 4 4 4 4 6% 7% 44%	2048
May	1st to 10th	Night	16% 9% 2 8% 5% 5% 4 4 6% 7% 33%	5237
	11th to 20th	Night	15% 9% 3 8% 5% 4 5% 3 6% 7% 35%	5234
	21st to end	Night	18% 9% 3 8% 5% 5% 5% 4 6% 7% 29%	5761
June	1st to 10th	Night	7% 7% 3 6% 5% 5% 5% 5% 6% 10% 42%	1890
	11th to 20th	Night	4 3 1 5% 3 4 5% 4 8% 12% 49%	1890
	21st to end	Night	5% 4 2 5% 4 4 6% 4 7% 12% 47%	1702
July	1st to 10th	Night	7% 5% 2 6% 5% 5% 6% 5% 8% 11% 39%	4879
	11th to 20th	Night	8% 6% 3 8% 6% 6% 6% 5% 9% 12% 31%	5277
	21st to end	Night	9% 7% 4 8% 6% 5% 6% 5% 8% 12% 30%	5736
August	1st to 10th	Night	3 3 1 4 3 4 4 4 7% 10% 54%	3202
	11th to 20th	Night	5% 3 2 5% 4 4 5% 4 7% 11% 49%	3240
	21st to end	Night	5% 4 1 4 4 4 4 4 6% 9% 55%	3586
September	1st to 10th	Night	10% 5% 2 7% 5% 4 4 4 6% 7% 46%	2040
	11th to 20th	Night	9% 6% 2 6% 5% 4 5% 3 6% 9% 46%	2200
	21st to end	Night	10% 4 2 6% 4 4 3 4 6% 7% 51%	2200
October	1st to 10th	Night	11% 4 1 5% 3 3 3 3 4 6% 58%	2200
	11th to 20th	Night	13% 6% 2 5% 3 4 3 2 4 6% 53%	2200
	21st to end	Night	13% 8% 2 6% 3 3 3 2 4 6% 50%	2836
November	1st to 10th	Night	13% 5% 2 5% 3 3 4 3 4 6% 52%	2666
	11th to 20th	Night	12% 7% 2 5% 3 3 4 3 5% 6% 50%	2730
	21st to end	Night	16% 9% 3 5% 3 3 4 3 5% 6% 43%	2730
December	1st to 10th	Night	19% 10% 2 6% 3 3 3 3 4 6% 40%	4380
	11th to 20th	Night	26% 9% 3 5% 3 3 3 2 4 4 39%	4952
	21st to end	Night	20% 11% 4 8% 4 4 4 3 5% 6% 31%	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	53% 5% 9% 20% 12% 1	5259
	11th to 20th	Day	42% 4 5% 18% 30% 2	5280
	21st to end	Day	52% 4 7% 20% 13% 4	6352
February	1st to 10th	Day	51% 4 8% 19% 16% 2	2100
	11th to 20th	Day	54% 4 6% 18% 15% 4	2058
	21st to end	Day	49% 5% 7% 14% 19% 5%	1730
March	1st to 10th	Day	49% 6% 8% 13% 16% 8%	4580
	11th to 20th	Day	39% 5% 7% 20% 20% 10%	4937
	21st to end	Day	41% 6% 10% 27% 15% 1	5790
April	1st to 10th	Day	37% 7% 11% 23% 17% 4	2520
	11th to 20th	Day	38% 5% 7% 21% 23% 5%	2494
	21st to end	Day	41% 7% 10% 25% 16% 2	2569
May	1st to 10th	Day	46% 9% 13% 19% 12% 1	7651
	11th to 20th	Day	47% 8% 13% 20% 12% 1	7630
	21st to end	Day	55% 8% 8% 16% 12% 2	8405
June	1st to 10th	Day	40% 9% 14% 21% 15% 2	2730
	11th to 20th	Day	34% 9% 16% 23% 15% 2	2730
	21st to end	Day	35% 11% 19% 19% 13% 3	2902
July	1st to 10th	Day	37% 13% 20% 23% 7% 1	7979
	11th to 20th	Day	43% 15% 22% 16% 3 1	7611
	21st to end	Day	46% 12% 19% 17% 6%	8156
August	1st to 10th	Day	28% 10% 15% 32% 14% 1	4149
	11th to 20th	Day	35% 10% 15% 26% 13% 2	3933
	21st to end	Day	30% 9% 11% 27% 22% 2	4311
September	1st to 10th	Day	39% 9% 14% 19% 15% 5%	2340
	11th to 20th	Day	38% 9% 11% 20% 19% 2	2200
	21st to end	Day	35% 7% 12% 21% 19% 6%	2200
October	1st to 10th	Day	23% 5% 6% 17% 38% 12%	3590
	11th to 20th	Day	21% 4 6% 22% 30% 18%	3568
	21st to end	Day	28% 4 8% 23% 27% 10%	3197
November	1st to 10th	Day	34% 5% 8% 20% 28% 6%	1852
	11th to 20th	Day	37% 4 7% 22% 23% 7%	1890
	21st to end	Day	41% 6% 7% 21% 21% 4	1890
December	1st to 10th	Day	44% 4 5% 22% 21% 3	3020
	11th to 20th	Day	55% 4 7% 17% 13% 3	3460
	21st to end	Day	50% 7% 10% 21% 10% 2	4565
January	1st to 10th	Night	46% 4 14% 19% 15% 1	7636
	11th to 20th	Night	43% 4 6% 17% 26% 4	7680
	21st to end	Night	50% 3 7% 17% 16% 6%	7827
February	1st to 10th	Night	49% 4 10% 18% 17% 2	2520
	11th to 20th	Night	54% 4 6% 16% 14% 5%	2459
	21st to end	Night	51% 5% 6% 16% 16% 5%	2076
March	1st to 10th	Night	52% 6% 7% 13% 17% 4	5493
	11th to 20th	Night	36% 6% 7% 19% 20% 13%	5242
	21st to end	Night	44% 5% 8% 23% 17% 3	5412
April	1st to 10th	Night	36% 6% 13% 23% 20% 3	2100
	11th to 20th	Night	33% 5% 9% 22% 23% 8%	2075
	21st to end	Night	37% 6% 12% 25% 17% 2	2048
May	1st to 10th	Night	46% 9% 11% 21% 11% 1	5264
	11th to 20th	Night	46% 8% 13% 18% 14% 1	5235
	21st to end	Night	51% 10% 11% 18% 10% 1	5771
June	1st to 10th	Night	36% 9% 14% 22% 16% 3	1890
	11th to 20th	Night	24% 9% 20% 29% 16% 2	1890
	21st to end	Night	28% 11% 16% 26% 16% 3	1702
July	1st to 10th	Night	35% 12% 23% 21% 8% 1	4838
	11th to 20th	Night	39% 15% 23% 18% 4 2	5226
	21st to end	Night	43% 12% 23% 15% 7%	5606
August	1st to 10th	Night	22% 9% 14% 35% 19% 1	3191
	11th to 20th	Night	26% 10% 22% 26% 15% 1	3270
	21st to end	Night	23% 8% 15% 28% 24% 2	3586
September	1st to 10th	Night	35% 7% 16% 21% 15% 6%	2040
	11th to 20th	Night	34% 8% 12% 21% 23% 3	2200
	21st to end	Night	31% 7% 12% 22% 22% 7%	2200
October	1st to 10th	Night	21% 6% 7% 20% 29% 17%	3590
	11th to 20th	Night	21% 3 5% 20% 30% 20%	3571
	21st to end	Night	25% 3 6% 25% 29% 12%	4574
November	1st to 10th	Night	34% 5% 8% 21% 24% 8%	2666
	11th to 20th	Night	34% 4 6% 21% 25% 8%	2730
	21st to end	Night	42% 4 7% 20% 22% 5%	2730
December	1st to 10th	Night	47% 5% 6% 16% 24% 2	4380
	11th to 20th	Night	49% 4 6% 18% 21% 2	4954
	21st to end	Night	53% 5% 8% 22% 10% 1	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	28% 25% 31% 11% 5%	330
	11th to 20th	Day	25% 29% 30% 12% 3 1	330
	21st to end	Day	24% 26% 38% 10% 2	363
February	1st to 10th	Day	26% 23% 36% 12% 2	330
	11th to 20th	Day	38% 24% 28% 8% 2 1	330
	21st to end	Day	33% 24% 32% 8% 3	273
March	1st to 10th	Day	27% 21% 35% 13% 3	330
	11th to 20th	Day	25% 22% 30% 18% 5% 1	465
	21st to end	Day	23% 21% 34% 15% 7%	594
April	1st to 10th	Day	20% 18% 31% 21% 9% 1	540
	11th to 20th	Day	18% 17% 32% 23% 9% 2	532
	21st to end	Day	17% 16% 31% 22% 11% 2	540
May	1st to 10th	Day	17% 16% 29% 23% 11% 4	543
	11th to 20th	Day	11% 12% 29% 25% 17% 7%	567
	21st to end	Day	15% 9% 22% 28% 14% 12%	617
June	1st to 10th	Day	12% 7% 22% 27% 19% 13%	570
	11th to 20th	Day	8% 5% 19% 25% 26% 17%	570
	21st to end	Day	8% 5% 18% 27% 27% 15%	742
July	1st to 10th	Day	9% 3 15% 28% 25% 19%	695
	11th to 20th	Day	10% 4 13% 20% 30% 22%	564
	21st to end	Day	10% 5% 11% 26% 31% 18%	620
August	1st to 10th	Day	10% 4 11% 28% 30% 17%	566
	11th to 20th	Day	9% 3 13% 28% 33% 14%	535
	21st to end	Day	13% 3 10% 23% 33% 18%	583
September	1st to 10th	Day	11% 5% 13% 26% 32% 14%	530
	11th to 20th	Day	10% 5% 15% 26% 27% 17%	530
	21st to end	Day	15% 5% 16% 28% 28% 7%	530
October	1st to 10th	Day	13% 4 22% 28% 25% 7%	530
	11th to 20th	Day	12% 11% 29% 27% 18% 3	530
	21st to end	Day	20% 12% 28% 27% 11% 2	379
November	1st to 10th	Day	20% 8% 32% 28% 9% 2	330
	11th to 20th	Day	17% 15% 38% 25% 4 1	330
	21st to end	Day	21% 18% 35% 16% 9% 2	330
December	1st to 10th	Day	26% 22% 32% 15% 5%	330
	11th to 20th	Day	23% 22% 32% 14% 8%	330
	21st to end	Day	27% 19% 34% 15% 4 1	363
January	1st to 10th	Night	24% 18% 33% 20% 5%	840
	11th to 20th	Night	26% 20% 30% 20% 4	840
	21st to end	Night	21% 16% 38% 23% 2	924
February	1st to 10th	Night	24% 15% 33% 24% 3	840
	11th to 20th	Night	28% 18% 32% 19% 2	823
	21st to end	Night	28% 20% 30% 20% 2	692
March	1st to 10th	Night	22% 18% 31% 25% 4	840
	11th to 20th	Night	20% 16% 34% 23% 6% 1	840
	21st to end	Night	19% 15% 33% 24% 8%	851
April	1st to 10th	Night	15% 15% 33% 27% 10%	630
	11th to 20th	Night	17% 9% 33% 29% 10% 1	623
	21st to end	Night	16% 9% 30% 32% 12% 1	630
May	1st to 10th	Night	14% 7% 32% 29% 17% 2	630
	11th to 20th	Night	15% 5% 24% 32% 19% 4	621
	21st to end	Night	10% 4 19% 35% 27% 5%	683
June	1st to 10th	Night	8% 4 18% 35% 30% 4	630
	11th to 20th	Night	6% 2 11% 38% 35% 9%	630
	21st to end	Night	6% 1 12% 35% 37% 8%	630
July	1st to 10th	Night	8% 1 10% 27% 41% 13%	615
	11th to 20th	Night	8% 3 8% 25% 44% 12%	612
	21st to end	Night	7% 3 8% 31% 40% 11%	690
August	1st to 10th	Night	6% 3 7% 26% 49% 9%	627
	11th to 20th	Night	8% 2 9% 32% 42% 7%	606
	21st to end	Night	9% 2 9% 28% 41% 10%	660
September	1st to 10th	Night	10% 3 13% 31% 37% 6%	640
	11th to 20th	Night	10% 3 16% 28% 37% 6%	800
	21st to end	Night	11% 3 17% 33% 34% 3	800
October	1st to 10th	Night	11% 6% 18% 30% 28% 6%	800
	11th to 20th	Night	15% 8% 23% 36% 17% 1	800
	21st to end	Night	19% 8% 24% 31% 16% 2	880
November	1st to 10th	Night	17% 9% 21% 33% 17% 2	820
	11th to 20th	Night	16% 12% 31% 29% 11% 1	840
	21st to end	Night	21% 13% 27% 28% 11% 1	840
December	1st to 10th	Night	26% 15% 28% 26% 5%	840
	11th to 20th	Night	25% 11% 29% 26% 7% 1	840
	21st to end	Night	25% 18% 28% 25% 4	924

Page generated at 2026-08-12 02:37:13. Version ac07

Copyright © 2026 Attilla Danko.