Twin Pines Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Twin Pines Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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% 4 4 4 4 5% 7% 41%	126602
	Night	15% 8% 3 6% 4 4 4 3 5% 7% 43%	123875
Transparency	Day	40% 7% 11% 22% 17% 4	130281
	Night	39% 5% 10% 22% 19% 5%	128270
Seeing	Day	18% 11% 24% 24% 17% 6%	16645
	Night	18% 10% 24% 28% 17% 3	26361

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	15% 11% 6% 10% 4 3 3 3 4 5% 35%	3617
	11th to 20th	Day	17% 7% 3 10% 3 3 3 2 3 5% 45%	3775
	21st to end	Day	19% 8% 4 5% 3 3 3 3 5% 6% 41%	4817
February	1st to 10th	Day	19% 11% 4 7% 4 3 3 3 4 6% 37%	2000
	11th to 20th	Day	20% 9% 3 8% 5% 3 3 3 3 5% 37%	1948
	21st to end	Day	21% 10% 3 6% 3 4 3 3 4 5% 37%	1650
March	1st to 10th	Day	17% 13% 3 8% 4 4 4 2 4 5% 36%	4480
	11th to 20th	Day	14% 9% 2 5% 4 4 4 3 4 5% 47%	4666
	21st to end	Day	13% 7% 2 7% 4 4 5% 3 5% 7% 43%	5967
April	1st to 10th	Day	15% 6% 2 5% 5% 4 4 3 4 6% 46%	2400
	11th to 20th	Day	16% 7% 3 6% 4 3 3 3 5% 6% 44%	2374
	21st to end	Day	14% 6% 3 6% 5% 4 5% 3 6% 7% 41%	2400
May	1st to 10th	Day	20% 8% 2 6% 5% 5% 6% 5% 6% 9% 28%	5761
	11th to 20th	Day	16% 8% 4 9% 6% 5% 5% 4 6% 7% 30%	5861
	21st to end	Day	18% 7% 2 6% 4 4 4 3 6% 7% 37%	6433
June	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 4 7% 8% 41%	2727
	11th to 20th	Day	5% 4 2 6% 6% 5% 5% 5% 7% 9% 46%	2800
	21st to end	Day	6% 4 1 5% 5% 5% 7% 7% 8% 10% 42%	2800
July	1st to 10th	Day	8% 5% 2 6% 5% 6% 6% 6% 7% 11% 39%	6360
	11th to 20th	Day	8% 5% 2 6% 5% 5% 6% 6% 8% 12% 36%	6360
	21st to end	Day	12% 6% 3 7% 5% 5% 6% 5% 8% 10% 32%	6417
August	1st to 10th	Day	5% 2 1 5% 4 4 5% 4 6% 10% 54%	3905
	11th to 20th	Day	8% 4 2 6% 4 4 4 4 7% 11% 45%	3849
	21st to end	Day	6% 3 2 5% 3 4 4 4 6% 8% 55%	4312
September	1st to 10th	Day	11% 5% 2 6% 4 5% 5% 4 7% 7% 44%	2400
	11th to 20th	Day	13% 5% 2 6% 4 4 4 4 6% 7% 47%	2225
	21st to end	Day	11% 6% 2 6% 4 3 4 4 5% 6% 49%	2200
October	1st to 10th	Day	10% 5% 2 4 3 3 3 3 4 6% 58%	2200
	11th to 20th	Day	11% 5% 1 5% 3 3 3 3 4 6% 55%	2070
	21st to end	Day	19% 7% 2 5% 3 3 2 3 4 5% 49%	2072
November	1st to 10th	Day	14% 6% 2 5% 3 3 3 2 4 6% 54%	1852
	11th to 20th	Day	15% 7% 2 4 3 3 3 2 4 6% 51%	1863
	21st to end	Day	17% 8% 2 5% 4 3 3 3 5% 7% 44%	1800
December	1st to 10th	Day	17% 8% 2 5% 3 3 3 3 4 4 46%	2930
	11th to 20th	Day	22% 9% 4 7% 3 4 3 3 4 4 36%	3329
	21st to end	Day	18% 10% 3 7% 4 5% 5% 3 6% 6% 32%	3982
January	1st to 10th	Night	17% 14% 4 9% 3 3 3 3 4 5% 36%	6348
	11th to 20th	Night	18% 10% 4 8% 3 3 2 2 3 3 44%	6204
	21st to end	Night	21% 10% 3 6% 4 3 3 3 4 5% 39%	6141
February	1st to 10th	Night	22% 11% 4 7% 5% 3 3 3 4 5% 33%	2400
	11th to 20th	Night	23% 10% 4 6% 4 4 3 3 4 4 36%	2338
	21st to end	Night	22% 9% 2 6% 4 4 4 2 5% 5% 38%	1980
March	1st to 10th	Night	19% 12% 2 6% 4 4 4 3 5% 6% 36%	5373
	11th to 20th	Night	13% 6% 2 5% 4 3 4 4 5% 6% 49%	5330
	21st to end	Night	16% 7% 2 7% 4 4 4 3 5% 7% 42%	4967
April	1st to 10th	Night	13% 6% 3 6% 4 4 4 3 5% 7% 46%	2000
	11th to 20th	Night	16% 6% 2 5% 3 3 4 3 5% 6% 47%	1975
	21st to end	Night	13% 8% 2 5% 4 5% 5% 3 6% 7% 43%	2000
May	1st to 10th	Night	17% 9% 4 7% 5% 5% 5% 4 4 6% 35%	4130
	11th to 20th	Night	15% 8% 4 9% 5% 5% 5% 4 5% 7% 33%	4032
	21st to end	Night	15% 7% 3 6% 4 4 5% 4 6% 8% 37%	4421
June	1st to 10th	Night	10% 6% 2 5% 4 4 5% 4 7% 8% 44%	1668
	11th to 20th	Night	4 3 1 4 4 4 5% 4 7% 9% 55%	1600
	21st to end	Night	4 2 2 4 3 4 6% 4 6% 13% 52%	1600
July	1st to 10th	Night	9% 5% 2 6% 5% 5% 6% 5% 7% 9% 42%	3618
	11th to 20th	Night	5% 4 1 5% 4 4 5% 5% 8% 14% 43%	3617
	21st to end	Night	8% 6% 3 7% 5% 5% 6% 4 8% 14% 35%	4404
August	1st to 10th	Night	4 3 1 3 3 3 4 4 5% 7% 63%	3244
	11th to 20th	Night	4 3 1 5% 4 4 5% 5% 8% 10% 52%	3200
	21st to end	Night	5% 3 1 4 4 3 4 4 6% 8% 58%	3586
September	1st to 10th	Night	10% 5% 2 5% 4 3 4 4 6% 7% 51%	2000
	11th to 20th	Night	10% 5% 2 6% 3 3 3 3 5% 7% 52%	2154
	21st to end	Night	11% 4 2 5% 4 3 4 4 5% 5% 53%	2200
October	1st to 10th	Night	9% 4 1 4 3 2 2 2 4 6% 63%	2200
	11th to 20th	Night	14% 4 1 5% 3 3 3 3 4 6% 54%	2325
	21st to end	Night	17% 6% 2 5% 3 3 3 3 4 5% 49%	2766
November	1st to 10th	Night	14% 5% 2 5% 3 3 3 3 5% 5% 54%	2666
	11th to 20th	Night	12% 5% 2 4 3 3 3 3 5% 7% 51%	2692
	21st to end	Night	16% 8% 2 5% 4 3 3 3 4 5% 47%	2600
December	1st to 10th	Night	18% 10% 2 6% 3 3 3 3 3 5% 43%	4250
	11th to 20th	Night	23% 9% 3 5% 3 2 2 2 4 5% 42%	4850
	21st to end	Night	20% 11% 5% 8% 5% 5% 4 3 5% 5% 29%	6996

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% 4 10% 20% 14% 1	3618
	11th to 20th	Day	44% 4 6% 24% 23% 1	3775
	21st to end	Day	43% 4 7% 21% 20% 5%	4817
February	1st to 10th	Day	50% 4 9% 20% 15% 2	2000
	11th to 20th	Day	52% 3 7% 20% 15% 4	1958
	21st to end	Day	49% 3 7% 17% 16% 7%	1650
March	1st to 10th	Day	52% 4 4 15% 16% 9%	4480
	11th to 20th	Day	39% 5% 7% 21% 19% 8%	4666
	21st to end	Day	40% 5% 9% 25% 19% 1	5969
April	1st to 10th	Day	39% 5% 9% 22% 21% 4	2400
	11th to 20th	Day	41% 5% 8% 21% 20% 6%	2374
	21st to end	Day	41% 6% 11% 24% 16% 2	2400
May	1st to 10th	Day	50% 10% 12% 19% 9% 1	5797
	11th to 20th	Day	50% 9% 13% 18% 10%	5865
	21st to end	Day	44% 7% 8% 22% 16% 2	6447
June	1st to 10th	Day	38% 8% 12% 21% 18% 2	2727
	11th to 20th	Day	31% 9% 16% 24% 17% 3	2800
	21st to end	Day	32% 12% 14% 23% 15% 4	2800
July	1st to 10th	Day	35% 10% 16% 26% 11% 1	6291
	11th to 20th	Day	36% 11% 20% 26% 6% 1	6285
	21st to end	Day	43% 11% 21% 20% 6%	6233
August	1st to 10th	Day	24% 7% 16% 30% 20% 2	3893
	11th to 20th	Day	32% 8% 14% 27% 17% 2	3885
	21st to end	Day	26% 7% 10% 30% 25% 2	4311
September	1st to 10th	Day	36% 9% 12% 22% 16% 5%	2400
	11th to 20th	Day	36% 7% 10% 22% 22% 3	2225
	21st to end	Day	34% 7% 9% 22% 23% 6%	2200
October	1st to 10th	Day	20% 3 6% 15% 43% 12%	3590
	11th to 20th	Day	21% 3 7% 24% 28% 16%	3349
	21st to end	Day	31% 3 5% 24% 31% 7%	3323
November	1st to 10th	Day	34% 4 7% 23% 28% 5%	1852
	11th to 20th	Day	36% 4 7% 22% 25% 6%	1863
	21st to end	Day	41% 4 7% 23% 20% 5%	1800
December	1st to 10th	Day	41% 5% 6% 28% 19% 2	2930
	11th to 20th	Day	52% 4 7% 20% 13% 3	3326
	21st to end	Day	50% 6% 9% 24% 9% 1	3982
January	1st to 10th	Night	51% 3 9% 20% 16% 1	6348
	11th to 20th	Night	47% 3 6% 18% 22% 4	6204
	21st to end	Night	49% 4 5% 21% 17% 5%	6141
February	1st to 10th	Night	54% 3 10% 16% 16% 2	2400
	11th to 20th	Night	53% 4 6% 18% 15% 4	2339
	21st to end	Night	49% 4 7% 17% 18% 5%	1980
March	1st to 10th	Night	50% 4 6% 15% 18% 8%	5373
	11th to 20th	Night	36% 5% 7% 20% 18% 14%	5330
	21st to end	Night	42% 5% 8% 23% 18% 3	4967
April	1st to 10th	Night	38% 4 11% 24% 19% 4	2000
	11th to 20th	Night	37% 5% 8% 20% 23% 8%	1975
	21st to end	Night	40% 6% 11% 24% 17% 2	2000
May	1st to 10th	Night	49% 8% 12% 20% 10% 1	4158
	11th to 20th	Night	49% 8% 14% 17% 11% 1	4033
	21st to end	Night	42% 9% 12% 23% 13% 1	4429
June	1st to 10th	Night	35% 7% 12% 24% 19% 2	1668
	11th to 20th	Night	23% 7% 16% 32% 19% 4	1600
	21st to end	Night	23% 9% 20% 26% 18% 5%	1600
July	1st to 10th	Night	34% 9% 16% 27% 12% 1	3580
	11th to 20th	Night	27% 11% 25% 30% 5% 2	3572
	21st to end	Night	38% 11% 23% 22% 6%	4275
August	1st to 10th	Night	18% 7% 12% 38% 24% 1	3232
	11th to 20th	Night	24% 8% 21% 29% 17% 2	3230
	21st to end	Night	22% 6% 13% 30% 25% 3	3586
September	1st to 10th	Night	31% 6% 12% 27% 19% 5%	2000
	11th to 20th	Night	32% 6% 12% 20% 27% 3	2154
	21st to end	Night	32% 6% 8% 23% 25% 7%	2200
October	1st to 10th	Night	18% 4 6% 18% 41% 13%	3590
	11th to 20th	Night	20% 3 4 21% 33% 19%	3785
	21st to end	Night	27% 3 6% 26% 27% 11%	4465
November	1st to 10th	Night	33% 5% 7% 22% 26% 7%	2666
	11th to 20th	Night	32% 5% 7% 22% 27% 8%	2692
	21st to end	Night	40% 4 7% 23% 21% 5%	2600
December	1st to 10th	Night	45% 4 7% 19% 23% 2	4250
	11th to 20th	Night	47% 2 6% 22% 20% 3	4852
	21st to end	Night	56% 6% 8% 20% 8% 2	6996

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% 22% 36% 11% 3	300
	11th to 20th	Day	31% 25% 33% 8% 3 1	300
	21st to end	Day	25% 28% 37% 8% 2	330
February	1st to 10th	Day	28% 26% 30% 13% 2 1	300
	11th to 20th	Day	37% 27% 27% 6% 1	300
	21st to end	Day	37% 24% 30% 8% 1	249
March	1st to 10th	Day	31% 23% 31% 12% 3	300
	11th to 20th	Day	28% 24% 31% 13% 2 1	344
	21st to end	Day	28% 18% 33% 16% 4	550
April	1st to 10th	Day	28% 17% 30% 18% 7% 1	500
	11th to 20th	Day	23% 17% 32% 20% 7% 1	492
	21st to end	Day	22% 15% 32% 21% 8% 2	500
May	1st to 10th	Day	24% 13% 30% 22% 9% 2	503
	11th to 20th	Day	16% 11% 28% 25% 14% 5%	527
	21st to end	Day	14% 8% 26% 27% 15% 10%	573
June	1st to 10th	Day	12% 6% 24% 26% 21% 10%	657
	11th to 20th	Day	5% 5% 21% 34% 24% 11%	730
	21st to end	Day	7% 3 23% 34% 22% 11%	730
July	1st to 10th	Day	9% 3 17% 32% 28% 11%	721
	11th to 20th	Day	9% 4 13% 32% 33% 10%	718
	21st to end	Day	8% 3 14% 28% 33% 14%	629
August	1st to 10th	Day	10% 3 14% 31% 32% 11%	526
	11th to 20th	Day	11% 3 15% 31% 30% 9%	519
	21st to end	Day	11% 3 12% 25% 34% 16%	583
September	1st to 10th	Day	11% 3 16% 32% 27% 11%	530
	11th to 20th	Day	11% 3 13% 28% 31% 14%	530
	21st to end	Day	15% 7% 19% 27% 23% 8%	530
October	1st to 10th	Day	12% 5% 22% 31% 23% 7%	530
	11th to 20th	Day	12% 14% 32% 28% 12% 2	400
	21st to end	Day	25% 15% 32% 21% 6% 1	363
November	1st to 10th	Day	21% 11% 31% 27% 8% 2	330
	11th to 20th	Day	17% 20% 36% 24% 4	321
	21st to end	Day	26% 16% 29% 16% 11% 2	300
December	1st to 10th	Day	31% 19% 29% 17% 3	300
	11th to 20th	Day	29% 18% 28% 17% 7%	300
	21st to end	Day	30% 21% 36% 11% 2	330
January	1st to 10th	Night	27% 18% 31% 22% 3	1000
	11th to 20th	Night	29% 20% 30% 18% 3	1000
	21st to end	Night	23% 17% 39% 20% 1	946
February	1st to 10th	Night	30% 14% 33% 20% 3	800
	11th to 20th	Night	30% 19% 30% 20% 2	783
	21st to end	Night	29% 20% 32% 17% 2	660
March	1st to 10th	Night	24% 16% 31% 26% 4	800
	11th to 20th	Night	22% 15% 39% 20% 4 1	800
	21st to end	Night	20% 14% 35% 23% 7%	665
April	1st to 10th	Night	19% 15% 34% 25% 7% 1	600
	11th to 20th	Night	20% 12% 31% 28% 9% 1	593
	21st to end	Night	17% 12% 33% 28% 10% 1	600
May	1st to 10th	Night	16% 8% 32% 29% 14% 2	600
	11th to 20th	Night	13% 7% 25% 35% 17% 3	591
	21st to end	Night	10% 4 23% 32% 25% 6%	650
June	1st to 10th	Night	12% 3 16% 35% 29% 4	600
	11th to 20th	Night	5% 3 13% 39% 37% 4	600
	21st to end	Night	4 1 15% 37% 35% 8%	600
July	1st to 10th	Night	8% 3 10% 30% 38% 10%	585
	11th to 20th	Night	7% 2 10% 29% 41% 11%	582
	21st to end	Night	6% 1 9% 36% 39% 8%	657
August	1st to 10th	Night	6% 1 6% 31% 46% 9%	597
	11th to 20th	Night	6% 1 9% 43% 37% 4	594
	21st to end	Night	9% 2 9% 32% 39% 9%	660
September	1st to 10th	Night	10% 2 12% 35% 37% 5%	600
	11th to 20th	Night	12% 2 13% 31% 37% 5%	754
	21st to end	Night	13% 2 18% 32% 31% 5%	800
October	1st to 10th	Night	10% 6% 16% 34% 30% 5%	800
	11th to 20th	Night	17% 7% 22% 34% 18% 2	800
	21st to end	Night	21% 8% 26% 29% 15% 1	880
November	1st to 10th	Night	16% 8% 24% 35% 15% 2	820
	11th to 20th	Night	14% 12% 32% 31% 9% 1	829
	21st to end	Night	23% 11% 30% 27% 10% 1	800
December	1st to 10th	Night	27% 14% 28% 24% 7%	800
	11th to 20th	Night	25% 12% 30% 26% 6%	815
	21st to end	Night	29% 15% 28% 24% 4	1100

Page generated at 2025-11-14 23:55:59. Version ac07

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