Three Pines Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Three Pines Observatory clear sky chart from 2005-11-06 to 2026-03-18.

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	11% 8% 3 8% 5% 5% 5% 4 6% 7% 40%	131434
	Night	10% 8% 3 8% 5% 5% 5% 4 5% 6% 41%	130362
Transparency	Day	42% 6% 13% 24% 13% 1	135004
	Night	42% 5% 13% 25% 14% 2	134859
Seeing	Day	15% 11% 32% 27% 12% 2	17785
	Night	14% 11% 36% 29% 9% 1	27847

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	6% 13% 4 16% 11% 8% 6% 3 4 5% 22%	4181
	11th to 20th	Day	4 10% 5% 12% 7% 7% 8% 5% 9% 8% 24%	5058
	21st to end	Day	3 8% 8% 11% 6% 5% 6% 5% 6% 7% 36%	5812
February	1st to 10th	Day	8% 12% 4 12% 9% 6% 6% 5% 6% 6% 26%	1890
	11th to 20th	Day	8% 10% 6% 11% 5% 6% 6% 5% 6% 6% 31%	1844
	21st to end	Day	11% 11% 4 11% 6% 6% 4 4 4 5% 33%	1679
March	1st to 10th	Day	11% 8% 3 7% 4 4 4 3 5% 7% 43%	4931
	11th to 20th	Day	9% 9% 3 7% 5% 4 4 3 5% 8% 43%	5057
	21st to end	Day	22% 12% 4 12% 5% 4 3 3 4 5% 24%	5462
April	1st to 10th	Day	18% 9% 2 6% 4 4 4 4 6% 8% 36%	2303
	11th to 20th	Day	17% 11% 4 6% 4 4 4 3 5% 6% 35%	2542
	21st to end	Day	22% 9% 3 7% 3 4 4 4 4 5% 36%	2600
May	1st to 10th	Day	11% 5% 2 5% 3 4 4 3 5% 7% 49%	5862
	11th to 20th	Day	18% 7% 2 5% 3 3 3 3 4 6% 45%	6121
	21st to end	Day	17% 7% 2 6% 4 5% 5% 4 6% 7% 36%	6935
June	1st to 10th	Day	12% 5% 2 5% 4 4 5% 4 6% 7% 46%	2890
	11th to 20th	Day	15% 6% 2 6% 4 4 5% 4 5% 7% 41%	3000
	21st to end	Day	9% 5% 2 6% 4 5% 5% 4 7% 9% 45%	3000
July	1st to 10th	Day	8% 4 2 5% 4 5% 5% 4 7% 11% 44%	6800
	11th to 20th	Day	8% 4 1 5% 3 3 3 3 5% 8% 56%	6538
	21st to end	Day	8% 5% 2 5% 4 4 5% 4 7% 10% 46%	6422
August	1st to 10th	Day	14% 7% 2 6% 4 3 4 4 6% 7% 42%	4234
	11th to 20th	Day	12% 6% 3 6% 5% 5% 5% 4 7% 7% 40%	4171
	21st to end	Day	8% 5% 2 5% 3 4 5% 4 6% 7% 51%	4652
September	1st to 10th	Day	11% 4 2 5% 4 3 4 3 4 7% 52%	2385
	11th to 20th	Day	9% 5% 2 5% 4 4 3 3 7% 7% 50%	2200
	21st to end	Day	12% 6% 2 5% 3 3 3 3 4 6% 53%	2200
October	1st to 10th	Day	14% 7% 2 5% 5% 4 3 3 5% 6% 47%	2020
	11th to 20th	Day	14% 9% 2 5% 3 3 4 4 5% 6% 46%	2000
	21st to end	Day	16% 10% 3 6% 4 3 3 3 4 5% 43%	2143
November	1st to 10th	Day	13% 9% 3 7% 5% 3 4 4 4 6% 42%	1806
	11th to 20th	Day	9% 11% 4 13% 5% 5% 4 3 5% 6% 37%	1680
	21st to end	Day	10% 12% 3 10% 5% 4 4 3 6% 8% 34%	1680
December	1st to 10th	Day	5% 9% 4 12% 9% 7% 5% 3 6% 6% 32%	2690
	11th to 20th	Day	9% 12% 7% 12% 7% 7% 6% 4 5% 7% 24%	3071
	21st to end	Day	14% 11% 5% 18% 6% 4 4 3 5% 6% 23%	3575
January	1st to 10th	Night	6% 10% 5% 15% 10% 8% 7% 4 6% 5% 24%	8739
	11th to 20th	Night	3 9% 4 12% 8% 8% 11% 6% 8% 7% 23%	7927
	21st to end	Night	4 8% 6% 10% 5% 5% 6% 4 6% 6% 39%	8402
February	1st to 10th	Night	8% 10% 5% 10% 7% 7% 6% 4 6% 6% 32%	2730
	11th to 20th	Night	7% 9% 4 9% 6% 5% 6% 4 6% 6% 38%	2663
	21st to end	Night	9% 9% 4 8% 6% 6% 4 5% 6% 6% 38%	2144
March	1st to 10th	Night	8% 9% 3 6% 4 3 3 3 5% 6% 49%	5148
	11th to 20th	Night	10% 6% 3 6% 5% 4 4 3 5% 9% 44%	5057
	21st to end	Night	21% 12% 4 9% 4 3 3 3 4 5% 31%	5467
April	1st to 10th	Night	15% 10% 2 6% 4 4 4 4 4 6% 40%	2096
	11th to 20th	Night	15% 9% 3 6% 4 4 4 4 5% 6% 40%	1807
	21st to end	Night	19% 8% 2 6% 5% 3 4 3 4 6% 39%	1800
May	1st to 10th	Night	13% 6% 1 4 2 2 3 2 5% 5% 57%	4053
	11th to 20th	Night	14% 7% 2 5% 3 4 3 3 4 6% 48%	3790
	21st to end	Night	18% 7% 2 6% 3 4 4 3 4 6% 42%	3942
June	1st to 10th	Night	8% 5% 1 5% 3 4 3 4 6% 8% 53%	1505
	11th to 20th	Night	10% 4 2 4 5% 4 4 3 6% 9% 49%	1400
	21st to end	Night	9% 4 1 4 3 3 4 3 6% 7% 56%	1400
July	1st to 10th	Night	8% 4 2 5% 3 4 4 4 7% 9% 50%	3178
	11th to 20th	Night	6% 4 2 5% 3 4 4 3 5% 7% 56%	3433
	21st to end	Night	8% 5% 2 4 3 3 4 4 6% 9% 50%	4449
August	1st to 10th	Night	12% 5% 2 5% 3 4 4 3 5% 7% 49%	2926
	11th to 20th	Night	11% 5% 2 5% 3 4 3 3 5% 7% 52%	2888
	21st to end	Night	7% 4 2 4 3 4 4 3 5% 7% 57%	3256
September	1st to 10th	Night	8% 4 2 4 3 3 3 3 6% 7% 58%	2012
	11th to 20th	Night	8% 4 2 4 4 3 4 3 5% 7% 55%	2200
	21st to end	Night	11% 4 2 4 3 2 4 3 4 7% 57%	2200
October	1st to 10th	Night	15% 5% 2 4 2 3 3 3 4 6% 52%	2360
	11th to 20th	Night	14% 8% 2 5% 3 3 3 3 4 6% 48%	2400
	21st to end	Night	15% 8% 3 6% 4 3 4 3 4 6% 43%	2695
November	1st to 10th	Night	10% 8% 2 7% 5% 4 4 3 5% 6% 46%	2711
	11th to 20th	Night	10% 10% 3 10% 6% 4 4 3 4 5% 41%	2940
	21st to end	Night	9% 9% 3 9% 5% 4 4 3 5% 6% 41%	2940
December	1st to 10th	Night	5% 8% 4 13% 8% 7% 4 4 5% 6% 35%	4720
	11th to 20th	Night	10% 10% 4 12% 8% 6% 5% 4 5% 6% 29%	5339
	21st to end	Night	15% 12% 5% 15% 6% 5% 4 4 5% 6% 23%	7645

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	64% 5% 11% 12% 7% 1	4182
	11th to 20th	Day	54% 4 10% 20% 12%	5058
	21st to end	Day	47% 6% 15% 22% 10%	5812
February	1st to 10th	Day	57% 6% 14% 15% 8%	1890
	11th to 20th	Day	52% 6% 11% 20% 10% 2	1853
	21st to end	Day	52% 6% 12% 19% 10% 2	1679
March	1st to 10th	Day	39% 6% 13% 27% 13% 1	4931
	11th to 20th	Day	40% 4 11% 29% 14% 2	5057
	21st to end	Day	61% 5% 10% 17% 6%	5464
April	1st to 10th	Day	46% 7% 13% 22% 11% 1	2303
	11th to 20th	Day	50% 4 12% 20% 12% 1	2542
	21st to end	Day	49% 6% 12% 23% 9%	2600
May	1st to 10th	Day	34% 6% 11% 27% 22% 1	5900
	11th to 20th	Day	41% 5% 9% 21% 15% 9%	6125
	21st to end	Day	45% 7% 11% 26% 11% 1	6949
June	1st to 10th	Day	36% 7% 15% 28% 13% 1	2890
	11th to 20th	Day	42% 7% 13% 22% 14% 2	3000
	21st to end	Day	34% 8% 16% 26% 15% 2	3000
July	1st to 10th	Day	31% 9% 21% 28% 11% 1	6726
	11th to 20th	Day	27% 6% 14% 35% 17% 2	6457
	21st to end	Day	31% 8% 17% 26% 16% 2	6240
August	1st to 10th	Day	38% 8% 11% 28% 13% 2	4220
	11th to 20th	Day	40% 8% 15% 29% 7% 1	4210
	21st to end	Day	30% 7% 18% 27% 15% 2	4651
September	1st to 10th	Day	32% 6% 13% 32% 16% 1	2385
	11th to 20th	Day	33% 7% 17% 30% 13% 1	2200
	21st to end	Day	33% 5% 10% 30% 20% 2	2200
October	1st to 10th	Day	28% 4 8% 29% 29% 1	3293
	11th to 20th	Day	38% 5% 11% 23% 21% 2	3240
	21st to end	Day	42% 4 14% 26% 14% 1	3448
November	1st to 10th	Day	43% 5% 12% 23% 15% 3	1806
	11th to 20th	Day	49% 5% 13% 22% 11% 1	1680
	21st to end	Day	48% 6% 14% 20% 10% 1	1680
December	1st to 10th	Day	51% 6% 15% 21% 7% 1	2690
	11th to 20th	Day	59% 6% 15% 15% 5%	3068
	21st to end	Day	60% 5% 12% 15% 7% 1	3575
January	1st to 10th	Night	60% 6% 9% 18% 7% 1	8740
	11th to 20th	Night	55% 5% 13% 16% 11% 1	7927
	21st to end	Night	44% 5% 13% 25% 13%	8402
February	1st to 10th	Night	51% 6% 12% 20% 10% 2	2730
	11th to 20th	Night	45% 6% 12% 23% 12% 3	2664
	21st to end	Night	45% 6% 13% 20% 12% 4	2144
March	1st to 10th	Night	36% 4 9% 34% 15% 2	5148
	11th to 20th	Night	37% 4 12% 32% 14% 1	5057
	21st to end	Night	56% 5% 12% 19% 7% 1	5467
April	1st to 10th	Night	44% 5% 13% 25% 12% 1	2096
	11th to 20th	Night	45% 4 11% 23% 15% 2	1807
	21st to end	Night	46% 6% 11% 23% 12% 2	1800
May	1st to 10th	Night	30% 5% 12% 29% 21% 3	4080
	11th to 20th	Night	37% 5% 8% 20% 21% 9%	3791
	21st to end	Night	43% 5% 10% 28% 13% 1	3950
June	1st to 10th	Night	27% 8% 16% 29% 17% 4	1505
	11th to 20th	Night	33% 6% 16% 23% 19% 2	1400
	21st to end	Night	26% 6% 17% 30% 19% 2	1400
July	1st to 10th	Night	28% 7% 18% 30% 15% 1	3145
	11th to 20th	Night	26% 6% 14% 36% 16% 2	3394
	21st to end	Night	29% 8% 18% 29% 14% 2	4320
August	1st to 10th	Night	34% 6% 15% 26% 16% 3	2916
	11th to 20th	Night	31% 7% 16% 33% 12% 1	2915
	21st to end	Night	27% 6% 18% 29% 18% 3	3256
September	1st to 10th	Night	25% 6% 14% 33% 20% 2	2012
	11th to 20th	Night	28% 7% 14% 31% 18% 2	2200
	21st to end	Night	28% 6% 10% 31% 23% 2	2200
October	1st to 10th	Night	22% 3 7% 28% 37% 4	3854
	11th to 20th	Night	29% 4 13% 28% 23% 3	3899
	21st to end	Night	39% 4 12% 29% 14% 1	4343
November	1st to 10th	Night	40% 5% 13% 22% 18% 3	2711
	11th to 20th	Night	44% 5% 12% 23% 14% 2	2940
	21st to end	Night	43% 5% 16% 23% 11% 1	2940
December	1st to 10th	Night	49% 4 16% 22% 8%	4720
	11th to 20th	Night	55% 6% 16% 19% 4	5341
	21st to end	Night	60% 6% 11% 17% 5%	7645

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	15% 12% 37% 27% 7% 2	330
	11th to 20th	Day	14% 17% 42% 21% 5% 1	330
	21st to end	Day	13% 15% 42% 22% 8% 1	363
February	1st to 10th	Day	17% 18% 45% 17% 3	330
	11th to 20th	Day	12% 17% 41% 22% 7% 1	330
	21st to end	Day	18% 18% 43% 19% 2	273
March	1st to 10th	Day	18% 13% 43% 23% 3	330
	11th to 20th	Day	22% 11% 38% 22% 6% 1	321
	21st to end	Day	25% 15% 29% 21% 8% 2	330
April	1st to 10th	Day	28% 14% 31% 24% 3	403
	11th to 20th	Day	27% 16% 29% 21% 6% 1	660
	21st to end	Day	23% 18% 29% 19% 10% 1	700
May	1st to 10th	Day	18% 13% 33% 23% 11% 2	703
	11th to 20th	Day	22% 10% 30% 23% 12% 3	725
	21st to end	Day	18% 8% 30% 30% 12% 2	789
June	1st to 10th	Day	10% 9% 26% 32% 18% 4	730
	11th to 20th	Day	17% 10% 34% 27% 10% 3	730
	21st to end	Day	9% 9% 34% 31% 15% 2	730
July	1st to 10th	Day	8% 4 23% 37% 22% 6%	721
	11th to 20th	Day	6% 7% 33% 32% 17% 5%	718
	21st to end	Day	7% 3 26% 38% 21% 4	794
August	1st to 10th	Day	12% 4 22% 37% 20% 4	726
	11th to 20th	Day	14% 4 25% 32% 21% 5%	715
	21st to end	Day	7% 6% 31% 34% 20% 3	793
September	1st to 10th	Day	12% 6% 33% 29% 16% 4	515
	11th to 20th	Day	11% 5% 34% 32% 14% 4	330
	21st to end	Day	15% 15% 30% 22% 14% 5%	330
October	1st to 10th	Day	15% 14% 29% 25% 13% 3	330
	11th to 20th	Day	18% 20% 35% 19% 7% 1	330
	21st to end	Day	22% 19% 33% 20% 6%	363
November	1st to 10th	Day	22% 18% 34% 19% 6% 1	330
	11th to 20th	Day	16% 19% 45% 16% 4	330
	21st to end	Day	19% 18% 36% 21% 5% 1	330
December	1st to 10th	Day	12% 16% 39% 25% 6%	330
	11th to 20th	Day	17% 20% 40% 18% 5%	330
	21st to end	Day	22% 18% 36% 20% 2 1	363
January	1st to 10th	Night	12% 13% 44% 24% 6% 1	1050
	11th to 20th	Night	12% 16% 42% 24% 5% 1	1050
	21st to end	Night	15% 15% 40% 24% 5%	1155
February	1st to 10th	Night	13% 13% 45% 24% 5% 1	1050
	11th to 20th	Night	12% 16% 41% 24% 6%	1029
	21st to end	Night	13% 14% 41% 24% 7% 1	760
March	1st to 10th	Night	13% 14% 40% 27% 6% 1	688
	11th to 20th	Night	19% 14% 38% 24% 5%	621
	21st to end	Night	23% 11% 30% 27% 8% 1	660
April	1st to 10th	Night	20% 13% 38% 24% 5%	600
	11th to 20th	Night	22% 14% 30% 27% 7% 1	593
	21st to end	Night	22% 11% 34% 24% 9% 1	600
May	1st to 10th	Night	18% 10% 35% 28% 9%	600
	11th to 20th	Night	17% 6% 36% 30% 9% 1	591
	21st to end	Night	16% 6% 35% 32% 11% 1	650
June	1st to 10th	Night	10% 7% 26% 41% 16%	600
	11th to 20th	Night	13% 5% 37% 37% 9%	600
	21st to end	Night	10% 5% 27% 46% 13%	600
July	1st to 10th	Night	6% 4 24% 43% 20% 2	585
	11th to 20th	Night	4 3 28% 45% 18% 1	582
	21st to end	Night	5% 3 26% 45% 19% 2	657
August	1st to 10th	Night	9% 3 24% 45% 18% 1	597
	11th to 20th	Night	10% 4 24% 41% 19% 2	594
	21st to end	Night	6% 3 28% 40% 21% 2	660
September	1st to 10th	Night	9% 4 28% 40% 17% 2	600
	11th to 20th	Night	9% 6% 33% 36% 14% 2	600
	21st to end	Night	11% 10% 33% 33% 13% 1	600
October	1st to 10th	Night	17% 10% 33% 30% 9% 1	760
	11th to 20th	Night	17% 15% 37% 24% 7% 1	800
	21st to end	Night	20% 13% 34% 26% 6%	935
November	1st to 10th	Night	15% 13% 39% 27% 7%	1025
	11th to 20th	Night	15% 15% 41% 22% 6%	1050
	21st to end	Night	14% 14% 42% 25% 5%	1050
December	1st to 10th	Night	13% 13% 43% 24% 5%	1050
	11th to 20th	Night	19% 12% 38% 25% 5%	1050
	21st to end	Night	21% 14% 40% 21% 4 1	1155

Page generated at 2026-03-15 01:23:57. Version ac07

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