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 2024-11-10.

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% 2 6% 4 4 4 4 5% 7% 42%	101004
	Night	14% 7% 2 6% 4 4 4 3 5% 7% 44%	98137
Transparency	Day	39% 6% 10% 23% 18% 4	104723
	Night	38% 6% 10% 23% 19% 5%	102558
Seeing	Day	18% 11% 24% 24% 17% 6%	15267
	Night	18% 10% 24% 28% 17% 3	25012

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	13% 14% 6% 9% 6% 4 4 3 4 5% 33%	2527
	11th to 20th	Day	22% 8% 3 8% 3 4 3 3 3 5% 38%	2629
	21st to end	Day	23% 11% 4 5% 3 3 3 3 4 5% 36%	3360
February	1st to 10th	Day	18% 11% 4 7% 4 3 3 3 4 6% 37%	1900
	11th to 20th	Day	20% 10% 3 8% 4 3 3 3 3 5% 38%	1848
	21st to end	Day	22% 10% 3 6% 3 3 3 3 4 5% 36%	1570
March	1st to 10th	Day	20% 13% 3 8% 4 5% 4 3 4 5% 33%	3120
	11th to 20th	Day	14% 7% 2 5% 3 4 4 3 4 6% 47%	3264
	21st to end	Day	13% 7% 2 7% 5% 4 5% 4 6% 6% 41%	4165
April	1st to 10th	Day	15% 6% 2 5% 5% 4 4 4 4 6% 46%	2280
	11th to 20th	Day	17% 8% 3 6% 4 3 3 3 5% 6% 43%	2254
	21st to end	Day	14% 6% 3 6% 5% 4 5% 3 6% 7% 42%	2280
May	1st to 10th	Day	18% 7% 2 5% 5% 4 5% 4 5% 9% 35%	4009
	11th to 20th	Day	17% 7% 3 6% 4 4 4 4 6% 8% 37%	4081
	21st to end	Day	12% 7% 3 7% 5% 4 5% 4 7% 9% 39%	4476
June	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 4 7% 9% 41%	2590
	11th to 20th	Day	5% 4 2 5% 6% 4 5% 5% 7% 9% 48%	2660
	21st to end	Day	6% 4 1 5% 5% 5% 7% 6% 7% 10% 43%	2660
July	1st to 10th	Day	8% 5% 2 6% 6% 5% 5% 6% 7% 11% 39%	4440
	11th to 20th	Day	9% 6% 2 6% 5% 5% 5% 5% 7% 11% 38%	4440
	21st to end	Day	17% 9% 3 9% 6% 5% 5% 4 6% 8% 28%	4459
August	1st to 10th	Day	5% 2 1 5% 4 4 5% 4 6% 10% 54%	3785
	11th to 20th	Day	8% 4 2 6% 4 4 4 4 7% 11% 44%	3729
	21st to end	Day	6% 3 2 5% 3 4 4 4 6% 8% 55%	4180
September	1st to 10th	Day	11% 5% 2 6% 4 5% 5% 4 7% 7% 43%	2280
	11th to 20th	Day	13% 5% 2 6% 4 4 4 3 6% 7% 45%	2114
	21st to end	Day	11% 6% 2 6% 4 3 4 4 5% 7% 49%	2090
October	1st to 10th	Day	11% 5% 2 3 3 3 3 3 4 6% 57%	2090
	11th to 20th	Day	12% 5% 1 5% 3 3 3 3 4 6% 53%	1967
	21st to end	Day	18% 7% 1 5% 3 3 2 3 4 5% 49%	1969
November	1st to 10th	Day	14% 5% 1 5% 3 2 3 2 4 5% 56%	1753
	11th to 20th	Day	15% 7% 2 4 3 3 3 2 4 6% 50%	1710
	21st to end	Day	17% 8% 2 5% 4 3 3 3 5% 7% 44%	1710
December	1st to 10th	Day	22% 9% 3 5% 3 4 3 3 5% 5% 37%	1710
	11th to 20th	Day	17% 10% 3 7% 3 4 4 3 4 5% 40%	2122
	21st to end	Day	15% 12% 4 7% 4 5% 5% 3 5% 6% 33%	2783
January	1st to 10th	Night	19% 13% 3 5% 3 3 3 2 4 6% 39%	4428
	11th to 20th	Night	19% 11% 4 9% 4 4 3 2 3 4 37%	4340
	21st to end	Night	25% 11% 4 5% 3 2 3 3 4 4 36%	4295
February	1st to 10th	Night	21% 11% 4 7% 5% 3 3 3 4 5% 34%	2280
	11th to 20th	Night	22% 10% 4 6% 4 4 3 3 3 4 38%	2218
	21st to end	Night	23% 10% 2 6% 4 4 4 2 5% 5% 36%	1884
March	1st to 10th	Night	23% 10% 3 6% 4 4 3 3 4 5% 35%	3733
	11th to 20th	Night	14% 7% 2 6% 4 4 4 3 4 6% 47%	3718
	21st to end	Night	16% 8% 2 7% 5% 4 4 4 4 6% 40%	3471
April	1st to 10th	Night	12% 6% 3 6% 4 4 4 3 5% 7% 47%	1900
	11th to 20th	Night	16% 6% 2 5% 3 3 4 3 5% 6% 47%	1875
	21st to end	Night	13% 8% 2 5% 4 4 5% 3 5% 7% 44%	1900
May	1st to 10th	Night	16% 7% 3 6% 4 4 4 3 4 6% 41%	2882
	11th to 20th	Night	13% 8% 3 7% 4 4 4 3 5% 8% 42%	2812
	21st to end	Night	9% 5% 2 6% 4 4 5% 4 7% 10% 43%	3079
June	1st to 10th	Night	11% 6% 2 5% 4 4 6% 4 7% 8% 43%	1585
	11th to 20th	Night	4 3 1 4 4 4 4 4 8% 10% 56%	1520
	21st to end	Night	4 2 1 4 3 4 6% 5% 6% 13% 52%	1520
July	1st to 10th	Night	8% 4 2 6% 6% 5% 6% 5% 7% 10% 41%	2530
	11th to 20th	Night	6% 4 2 5% 4 4 4 4 7% 11% 49%	2530
	21st to end	Night	11% 8% 3 8% 5% 5% 6% 4 7% 10% 33%	3075
August	1st to 10th	Night	4 3 1 3 3 3 4 4 5% 7% 63%	3144
	11th to 20th	Night	4 3 1 5% 4 4 5% 5% 8% 10% 52%	3100
	21st to end	Night	5% 3 1 4 3 3 4 4 6% 8% 58%	3476
September	1st to 10th	Night	10% 5% 2 5% 4 3 4 4 6% 7% 50%	1900
	11th to 20th	Night	11% 5% 2 6% 4 3 3 3 5% 7% 51%	2046
	21st to end	Night	10% 4 2 5% 3 3 4 4 5% 5% 54%	2090
October	1st to 10th	Night	9% 3 1 4 3 3 2 2 4 6% 62%	2090
	11th to 20th	Night	15% 4 1 5% 3 3 3 3 4 6% 53%	2209
	21st to end	Night	17% 6% 2 5% 3 3 3 3 4 6% 50%	2627
November	1st to 10th	Night	14% 5% 2 5% 3 3 3 3 5% 5% 53%	2530
	11th to 20th	Night	12% 5% 2 4 3 3 3 3 5% 8% 50%	2470
	21st to end	Night	17% 8% 2 5% 4 3 3 3 4 5% 46%	2470
December	1st to 10th	Night	20% 11% 3 7% 4 3 3 3 4 5% 37%	2470
	11th to 20th	Night	17% 9% 2 5% 3 3 3 3 4 5% 46%	3056
	21st to end	Night	18% 13% 4 7% 5% 5% 5% 4 5% 6% 30%	4884

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 7% 21% 14% 2	2528
	11th to 20th	Day	49% 5% 8% 19% 19% 1	2629
	21st to end	Day	50% 4 7% 23% 15% 1	3360
February	1st to 10th	Day	49% 4 9% 20% 16% 3	1900
	11th to 20th	Day	50% 3 6% 20% 16% 4	1858
	21st to end	Day	51% 3 8% 16% 15% 6%	1570
March	1st to 10th	Day	54% 5% 5% 14% 14% 8%	3120
	11th to 20th	Day	38% 6% 9% 24% 17% 6%	3264
	21st to end	Day	41% 6% 10% 23% 18% 1	4165
April	1st to 10th	Day	39% 5% 9% 22% 21% 4	2280
	11th to 20th	Day	41% 5% 8% 20% 21% 6%	2254
	21st to end	Day	40% 6% 11% 24% 17% 2	2280
May	1st to 10th	Day	44% 9% 11% 22% 13% 1	4046
	11th to 20th	Day	44% 8% 12% 22% 13% 1	4085
	21st to end	Day	40% 8% 10% 22% 18% 1	4489
June	1st to 10th	Day	38% 8% 12% 21% 17% 3	2590
	11th to 20th	Day	29% 9% 16% 25% 18% 3	2660
	21st to end	Day	32% 11% 14% 23% 15% 4	2660
July	1st to 10th	Day	36% 9% 18% 27% 8% 2	4371
	11th to 20th	Day	37% 8% 16% 29% 8% 2	4365
	21st to end	Day	53% 9% 11% 20% 7% 1	4317
August	1st to 10th	Day	24% 7% 16% 30% 21% 2	3773
	11th to 20th	Day	32% 8% 14% 27% 17% 2	3765
	21st to end	Day	26% 7% 11% 30% 25% 2	4179
September	1st to 10th	Day	37% 9% 11% 22% 16% 5%	2280
	11th to 20th	Day	38% 7% 10% 21% 22% 3	2114
	21st to end	Day	33% 7% 9% 23% 22% 6%	2090
October	1st to 10th	Day	20% 4 7% 15% 44% 12%	3480
	11th to 20th	Day	22% 3 7% 24% 28% 16%	3246
	21st to end	Day	30% 3 5% 24% 31% 7%	3220
November	1st to 10th	Day	32% 4 7% 23% 28% 5%	1753
	11th to 20th	Day	36% 4 7% 21% 25% 7%	1710
	21st to end	Day	41% 4 7% 23% 20% 5%	1710
December	1st to 10th	Day	48% 6% 6% 20% 16% 3	1710
	11th to 20th	Day	46% 5% 8% 21% 17% 2	2119
	21st to end	Day	49% 7% 10% 22% 11% 2	2783
January	1st to 10th	Night	48% 3 7% 24% 17% 1	4428
	11th to 20th	Night	52% 4 6% 17% 16% 4	4340
	21st to end	Night	51% 4 7% 22% 13% 2	4295
February	1st to 10th	Night	54% 3 9% 16% 16% 2	2280
	11th to 20th	Night	51% 4 6% 18% 16% 5%	2219
	21st to end	Night	51% 4 8% 17% 16% 5%	1884
March	1st to 10th	Night	51% 5% 7% 12% 16% 8%	3733
	11th to 20th	Night	38% 5% 9% 22% 16% 10%	3718
	21st to end	Night	44% 6% 9% 20% 20% 1	3471
April	1st to 10th	Night	37% 5% 11% 24% 20% 4	1900
	11th to 20th	Night	37% 5% 7% 19% 23% 8%	1875
	21st to end	Night	39% 5% 10% 25% 18% 3	1900
May	1st to 10th	Night	43% 8% 11% 23% 14% 2	2910
	11th to 20th	Night	39% 8% 16% 20% 16% 1	2813
	21st to end	Night	32% 11% 14% 26% 15% 2	3087
June	1st to 10th	Night	36% 7% 12% 24% 19% 2	1585
	11th to 20th	Night	22% 7% 16% 33% 20% 4	1520
	21st to end	Night	23% 9% 20% 26% 18% 5%	1520
July	1st to 10th	Night	34% 9% 18% 28% 9% 2	2490
	11th to 20th	Night	26% 8% 18% 38% 7% 3	2482
	21st to end	Night	47% 10% 13% 22% 9% 1	2977
August	1st to 10th	Night	18% 7% 12% 38% 24% 1	3132
	11th to 20th	Night	24% 8% 21% 28% 17% 2	3130
	21st to end	Night	22% 6% 13% 30% 25% 3	3476
September	1st to 10th	Night	32% 6% 12% 26% 18% 5%	1900
	11th to 20th	Night	33% 6% 12% 19% 27% 3	2046
	21st to end	Night	31% 6% 8% 24% 24% 7%	2090
October	1st to 10th	Night	18% 4 6% 18% 41% 13%	3480
	11th to 20th	Night	20% 3 4 20% 33% 19%	3669
	21st to end	Night	26% 3 6% 26% 27% 12%	4326
November	1st to 10th	Night	34% 5% 7% 21% 26% 7%	2530
	11th to 20th	Night	33% 5% 7% 21% 26% 8%	2470
	21st to end	Night	40% 4 7% 22% 22% 5%	2470
December	1st to 10th	Night	50% 4 8% 19% 16% 3	2470
	11th to 20th	Night	41% 3 8% 23% 22% 4	3058
	21st to end	Night	52% 7% 9% 18% 10% 3	4884

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	27% 22% 37% 11% 3	270
	11th to 20th	Day	33% 24% 31% 8% 3 1	270
	21st to end	Day	26% 27% 36% 9% 2	297
February	1st to 10th	Day	29% 24% 30% 14% 2 1	270
	11th to 20th	Day	37% 26% 28% 7% 1	270
	21st to end	Day	40% 24% 28% 7% 1	225
March	1st to 10th	Day	31% 21% 33% 12% 3	270
	11th to 20th	Day	27% 24% 31% 14% 3 1	312
	21st to end	Day	28% 17% 33% 17% 4	506
April	1st to 10th	Day	27% 16% 31% 18% 7% 1	460
	11th to 20th	Day	24% 17% 32% 20% 6% 1	452
	21st to end	Day	22% 15% 32% 21% 8% 2	460
May	1st to 10th	Day	23% 13% 29% 23% 10% 3	463
	11th to 20th	Day	16% 11% 27% 26% 15% 6%	487
	21st to end	Day	12% 7% 25% 28% 16% 11%	529
June	1st to 10th	Day	12% 6% 22% 26% 22% 11%	610
	11th to 20th	Day	5% 4 20% 35% 24% 12%	680
	21st to end	Day	8% 3 24% 35% 20% 10%	680
July	1st to 10th	Day	10% 2 17% 31% 29% 11%	671
	11th to 20th	Day	9% 4 13% 31% 32% 11%	668
	21st to end	Day	9% 3 15% 29% 31% 14%	582
August	1st to 10th	Day	10% 3 13% 30% 32% 11%	486
	11th to 20th	Day	11% 4 16% 33% 29% 8%	479
	21st to end	Day	11% 2 12% 23% 35% 17%	539
September	1st to 10th	Day	11% 3 15% 32% 27% 12%	490
	11th to 20th	Day	12% 3 13% 28% 31% 13%	490
	21st to end	Day	16% 7% 18% 27% 23% 8%	490
October	1st to 10th	Day	12% 4 22% 30% 24% 8%	490
	11th to 20th	Day	13% 14% 34% 26% 11% 2	367
	21st to end	Day	25% 14% 31% 24% 7% 1	330
November	1st to 10th	Day	20% 9% 33% 27% 9% 2	297
	11th to 20th	Day	17% 19% 37% 23% 3	270
	21st to end	Day	25% 16% 29% 16% 12% 3	270
December	1st to 10th	Day	33% 18% 27% 18% 4	270
	11th to 20th	Day	27% 19% 28% 19% 7%	270
	21st to end	Day	29% 22% 36% 10% 2	297
January	1st to 10th	Night	27% 18% 30% 22% 3	950
	11th to 20th	Night	29% 20% 30% 17% 3	950
	21st to end	Night	23% 17% 39% 20% 1	899
February	1st to 10th	Night	30% 14% 33% 21% 3	760
	11th to 20th	Night	29% 18% 31% 20% 2	743
	21st to end	Night	31% 21% 31% 16% 1	628
March	1st to 10th	Night	24% 16% 30% 26% 4	760
	11th to 20th	Night	22% 15% 39% 20% 4 1	760
	21st to end	Night	20% 14% 35% 23% 7%	632
April	1st to 10th	Night	17% 15% 35% 26% 7% 1	570
	11th to 20th	Night	20% 11% 30% 28% 9% 1	563
	21st to end	Night	17% 12% 33% 27% 10% 1	570
May	1st to 10th	Night	16% 7% 32% 29% 14% 2	570
	11th to 20th	Night	13% 6% 24% 36% 17% 3	561
	21st to end	Night	9% 3 23% 34% 26% 6%	617
June	1st to 10th	Night	13% 3 15% 35% 30% 4	570
	11th to 20th	Night	4 2 12% 40% 37% 4	570
	21st to end	Night	4 1 15% 38% 34% 7%	570
July	1st to 10th	Night	8% 3 10% 31% 37% 11%	555
	11th to 20th	Night	8% 2 11% 28% 41% 11%	552
	21st to end	Night	7% 1 9% 36% 38% 8%	624
August	1st to 10th	Night	7% 1 7% 31% 46% 9%	567
	11th to 20th	Night	6% 1 9% 44% 36% 3	564
	21st to end	Night	9% 2 9% 31% 40% 9%	627
September	1st to 10th	Night	10% 2 11% 34% 38% 5%	570
	11th to 20th	Night	13% 2 13% 31% 36% 5%	716
	21st to end	Night	12% 2 18% 31% 32% 5%	760
October	1st to 10th	Night	10% 6% 16% 33% 30% 5%	760
	11th to 20th	Night	18% 8% 22% 32% 18% 2	760
	21st to end	Night	20% 8% 25% 30% 15% 1	836
November	1st to 10th	Night	16% 8% 24% 35% 15% 2	779
	11th to 20th	Night	14% 12% 32% 31% 10% 1	760
	21st to end	Night	23% 10% 30% 27% 10% 1	760
December	1st to 10th	Night	28% 13% 28% 24% 7%	760
	11th to 20th	Night	24% 12% 30% 27% 7% 1	774
	21st to end	Night	29% 15% 28% 23% 4	1045

Page generated at 2024-11-07 05:55:50. Version ac07

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