Turtle Rock Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Turtle Rock Observatory clear sky chart from 2005-11-06 to 2026-09-29.

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	13% 9% 3 8% 5% 5% 5% 4 6% 8% 34%	149042
	Night	14% 9% 4 8% 5% 4 4 3 5% 7% 36%	142377
Transparency	Day	46% 6% 12% 21% 13% 3	152727
	Night	47% 5% 11% 21% 14% 3	146764
Seeing	Day	20% 13% 22% 23% 16% 7%	17622
	Night	19% 10% 20% 31% 18% 2	27640

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	16% 17% 5% 10% 5% 3 3 3 4 5% 30%	5258
	11th to 20th	Day	11% 14% 9% 12% 6% 4 3 2 3 3 33%	5280
	21st to end	Day	14% 14% 5% 12% 4 4 3 3 3 4 35%	5780
February	1st to 10th	Day	17% 13% 4 9% 5% 4 5% 3 5% 5% 31%	1897
	11th to 20th	Day	22% 12% 4 10% 4 4 3 3 4 4 31%	2048
	21st to end	Day	21% 11% 4 8% 4 3 3 3 5% 5% 34%	1730
March	1st to 10th	Day	15% 14% 4 7% 4 4 4 3 4 5% 34%	4989
	11th to 20th	Day	11% 9% 3 7% 4 3 4 3 4 7% 44%	5090
	21st to end	Day	16% 9% 3 6% 4 4 4 3 5% 6% 39%	5586
April	1st to 10th	Day	14% 8% 3 7% 4 5% 5% 4 5% 8% 38%	2310
	11th to 20th	Day	14% 8% 2 6% 5% 4 4 4 5% 7% 42%	2542
	21st to end	Day	16% 8% 2 6% 6% 5% 4 4 5% 6% 38%	2730
May	1st to 10th	Day	15% 8% 4 9% 5% 5% 5% 4 6% 8% 30%	7616
	11th to 20th	Day	15% 7% 3 8% 5% 5% 6% 5% 6% 8% 33%	7626
	21st to end	Day	14% 8% 3 10% 6% 6% 6% 5% 8% 9% 24%	8391
June	1st to 10th	Day	11% 7% 3 7% 6% 6% 5% 5% 8% 9% 35%	2730
	11th to 20th	Day	9% 6% 2 8% 6% 6% 6% 6% 7% 9% 33%	2730
	21st to end	Day	9% 6% 2 8% 6% 5% 5% 5% 8% 9% 37%	2730
July	1st to 10th	Day	7% 5% 2 6% 5% 5% 6% 5% 9% 12% 37%	7680
	11th to 20th	Day	13% 7% 3 8% 5% 5% 6% 4 7% 10% 31%	7680
	21st to end	Day	16% 8% 3 9% 6% 6% 6% 5% 7% 9% 25%	8340
August	1st to 10th	Day	10% 7% 3 9% 7% 7% 7% 6% 9% 12% 23%	6014
	11th to 20th	Day	10% 6% 2 8% 6% 6% 6% 5% 9% 11% 31%	5951
	21st to end	Day	5% 4 1 4 3 4 4 4 6% 10% 54%	6289
September	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 5% 7% 8% 43%	2310
	11th to 20th	Day	12% 7% 3 7% 4 5% 5% 4 6% 7% 39%	2310
	21st to end	Day	15% 6% 3 7% 5% 5% 5% 4 6% 8% 37%	2288
October	1st to 10th	Day	13% 5% 2 5% 3 4 4 3 5% 6% 49%	2121
	11th to 20th	Day	14% 6% 2 5% 3 4 4 3 5% 5% 50%	2000
	21st to end	Day	17% 8% 2 5% 4 4 3 3 4 6% 44%	2200
November	1st to 10th	Day	11% 7% 2 6% 4 3 3 3 5% 7% 49%	1968
	11th to 20th	Day	11% 10% 2 6% 4 4 4 4 5% 7% 42%	1890
	21st to end	Day	14% 10% 3 7% 4 4 4 3 5% 6% 40%	1890
December	1st to 10th	Day	15% 13% 4 11% 5% 4 3 3 4 5% 34%	3020
	11th to 20th	Day	20% 14% 4 8% 3 4 4 4 6% 6% 27%	3463
	21st to end	Day	13% 12% 7% 8% 5% 5% 6% 5% 7% 6% 26%	4565
January	1st to 10th	Night	17% 15% 6% 12% 5% 3 3 2 4 5% 27%	7635
	11th to 20th	Night	16% 11% 4 10% 5% 4 3 2 3 4 37%	7680
	21st to end	Night	16% 11% 4 10% 5% 4 4 3 4 5% 35%	8401
February	1st to 10th	Night	16% 11% 4 8% 5% 3 4 3 5% 6% 34%	2719
	11th to 20th	Night	20% 12% 5% 8% 5% 4 4 3 4 4 30%	2458
	21st to end	Night	20% 12% 3 10% 5% 4 3 3 4 6% 30%	2076
March	1st to 10th	Night	17% 13% 3 7% 4 3 3 3 3 5% 38%	5082
	11th to 20th	Night	12% 6% 3 6% 4 2 4 3 5% 7% 50%	5090
	21st to end	Night	11% 8% 3 7% 4 4 5% 4 6% 7% 40%	5588
April	1st to 10th	Night	13% 8% 3 8% 4 4 5% 4 6% 7% 39%	2310
	11th to 20th	Night	13% 8% 2 5% 4 4 5% 4 6% 6% 42%	2041
	21st to end	Night	13% 8% 2 6% 5% 5% 6% 4 5% 6% 41%	1890
May	1st to 10th	Night	16% 9% 3 8% 5% 5% 5% 4 6% 9% 30%	5237
	11th to 20th	Night	12% 9% 3 9% 5% 5% 5% 4 5% 9% 34%	5234
	21st to end	Night	19% 10% 4 9% 6% 5% 5% 3 6% 7% 25%	5761
June	1st to 10th	Night	11% 6% 2 7% 4 4 5% 4 7% 10% 39%	1890
	11th to 20th	Night	7% 5% 3 6% 6% 6% 5% 6% 6% 11% 39%	1890
	21st to end	Night	9% 6% 2 7% 5% 5% 4 4 7% 11% 39%	1890
July	1st to 10th	Night	11% 6% 3 7% 5% 5% 5% 5% 8% 10% 35%	5278
	11th to 20th	Night	15% 8% 3 8% 5% 5% 5% 4 6% 10% 31%	5277
	21st to end	Night	17% 10% 4 11% 6% 6% 5% 5% 6% 7% 23%	5736
August	1st to 10th	Night	11% 8% 4 9% 7% 6% 7% 5% 8% 11% 25%	4135
	11th to 20th	Night	11% 7% 3 7% 5% 5% 5% 5% 7% 10% 36%	4095
	21st to end	Night	6% 5% 2 5% 3 4 4 4 6% 9% 53%	4786
September	1st to 10th	Night	11% 6% 2 6% 4 5% 4 3 4 7% 46%	2310
	11th to 20th	Night	12% 6% 3 6% 4 4 4 3 5% 8% 45%	2310
	21st to end	Night	14% 7% 2 6% 4 4 4 3 5% 7% 44%	2289
October	1st to 10th	Night	14% 6% 1 5% 4 4 3 3 4 6% 50%	2277
	11th to 20th	Night	15% 7% 2 4 4 3 3 3 5% 5% 50%	2400
	21st to end	Night	15% 8% 2 5% 4 3 3 2 4 6% 46%	2640
November	1st to 10th	Night	12% 7% 3 5% 3 3 4 3 4 5% 50%	2547
	11th to 20th	Night	14% 9% 3 6% 4 3 4 3 4 6% 43%	2730
	21st to end	Night	13% 11% 2 6% 4 4 3 3 5% 5% 44%	2730
December	1st to 10th	Night	15% 12% 5% 10% 5% 4 4 3 4 5% 34%	4380
	11th to 20th	Night	18% 13% 6% 10% 5% 4 4 3 4 5% 28%	4952
	21st to end	Night	13% 14% 6% 9% 5% 5% 4 3 5% 8% 28%	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	58% 3 9% 16% 13% 1	5259
	11th to 20th	Day	57% 3 3 16% 17% 4	5280
	21st to end	Day	55% 2 7% 22% 13% 1	5780
February	1st to 10th	Day	55% 3 7% 19% 14% 1	1897
	11th to 20th	Day	58% 3 6% 18% 13% 2	2058
	21st to end	Day	54% 2 8% 16% 16% 4	1730
March	1st to 10th	Day	52% 2 7% 16% 16% 7%	4989
	11th to 20th	Day	41% 3 6% 21% 24% 5%	5090
	21st to end	Day	46% 3 7% 28% 15% 2	5586
April	1st to 10th	Day	45% 3 11% 24% 14% 3	2310
	11th to 20th	Day	43% 3 9% 23% 18% 4	2542
	21st to end	Day	48% 3 11% 25% 11% 2	2730
May	1st to 10th	Day	51% 5% 11% 21% 11%	7651
	11th to 20th	Day	47% 5% 16% 22% 9% 1	7630
	21st to end	Day	53% 7% 12% 17% 10% 2	8405
June	1st to 10th	Day	42% 8% 15% 21% 13% 1	2730
	11th to 20th	Day	43% 9% 15% 19% 12% 2	2730
	21st to end	Day	41% 7% 15% 21% 15% 1	2730
July	1st to 10th	Day	36% 11% 21% 20% 11% 1	7615
	11th to 20th	Day	45% 10% 18% 21% 6% 1	7611
	21st to end	Day	53% 9% 12% 15% 11%	8156
August	1st to 10th	Day	49% 12% 18% 15% 5%	6001
	11th to 20th	Day	40% 11% 20% 21% 6% 1	5990
	21st to end	Day	25% 6% 21% 31% 15% 1	6289
September	1st to 10th	Day	37% 7% 13% 23% 15% 5%	2310
	11th to 20th	Day	42% 7% 9% 24% 16% 1	2310
	21st to end	Day	45% 5% 11% 20% 16% 3	2288
October	1st to 10th	Day	30% 4 7% 21% 31% 7%	3459
	11th to 20th	Day	29% 2 9% 26% 18% 16%	3240
	21st to end	Day	31% 2 5% 32% 15% 14%	3538
November	1st to 10th	Day	37% 3 8% 21% 26% 5%	1968
	11th to 20th	Day	41% 3 7% 23% 21% 5%	1890
	21st to end	Day	46% 3 8% 22% 18% 3	1890
December	1st to 10th	Day	52% 3 8% 17% 17% 2	3020
	11th to 20th	Day	56% 4 8% 16% 13% 2	3460
	21st to end	Day	56% 5% 10% 19% 9% 1	4565
January	1st to 10th	Night	60% 3 10% 17% 9% 1	7636
	11th to 20th	Night	51% 3 6% 17% 21% 3	7680
	21st to end	Night	53% 2 7% 22% 14% 2	8401
February	1st to 10th	Night	51% 3 10% 20% 14% 1	2719
	11th to 20th	Night	59% 3 6% 19% 10% 3	2459
	21st to end	Night	56% 3 7% 19% 11% 4	2076
March	1st to 10th	Night	50% 2 6% 22% 15% 4	5082
	11th to 20th	Night	36% 2 8% 23% 25% 6%	5090
	21st to end	Night	43% 4 10% 27% 15% 1	5588
April	1st to 10th	Night	43% 5% 11% 23% 15% 3	2310
	11th to 20th	Night	41% 5% 10% 23% 17% 5%	2041
	21st to end	Night	45% 4 11% 27% 12% 1	1890
May	1st to 10th	Night	51% 6% 13% 20% 10% 1	5264
	11th to 20th	Night	47% 6% 14% 24% 10%	5235
	21st to end	Night	58% 5% 11% 17% 8% 1	5771
June	1st to 10th	Night	38% 8% 18% 20% 15% 1	1890
	11th to 20th	Night	37% 8% 20% 22% 11% 1	1890
	21st to end	Night	38% 10% 15% 23% 12% 3	1890
July	1st to 10th	Night	42% 10% 17% 24% 8% 1	5235
	11th to 20th	Night	47% 10% 18% 18% 5% 1	5226
	21st to end	Night	57% 10% 12% 13% 9%	5606
August	1st to 10th	Night	51% 12% 16% 16% 4	4124
	11th to 20th	Night	40% 11% 21% 20% 7%	4122
	21st to end	Night	27% 7% 20% 35% 11% 1	4786
September	1st to 10th	Night	39% 6% 14% 21% 17% 4	2310
	11th to 20th	Night	37% 6% 13% 24% 18% 1	2310
	21st to end	Night	41% 5% 10% 24% 18% 2	2289
October	1st to 10th	Night	35% 5% 8% 19% 28% 5%	3719
	11th to 20th	Night	31% 2 8% 23% 23% 12%	3899
	21st to end	Night	28% 3 5% 30% 23% 11%	4252
November	1st to 10th	Night	37% 3 9% 22% 23% 5%	2547
	11th to 20th	Night	43% 3 7% 20% 22% 6%	2730
	21st to end	Night	43% 3 7% 24% 19% 3	2730
December	1st to 10th	Night	52% 4 7% 21% 13% 1	4380
	11th to 20th	Night	59% 5% 8% 16% 11% 2	4954
	21st to end	Night	56% 4 12% 17% 11% 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	31% 29% 26% 11% 2 1	330
	11th to 20th	Day	37% 26% 26% 9% 2	330
	21st to end	Day	33% 28% 25% 12% 2	363
February	1st to 10th	Day	29% 20% 34% 12% 4	330
	11th to 20th	Day	44% 24% 23% 8% 1	330
	21st to end	Day	38% 23% 27% 10% 2	273
March	1st to 10th	Day	26% 20% 27% 20% 5% 2	508
	11th to 20th	Day	27% 24% 26% 19% 3 1	540
	21st to end	Day	29% 19% 26% 20% 6% 1	594
April	1st to 10th	Day	26% 17% 24% 21% 10% 2	540
	11th to 20th	Day	20% 19% 29% 19% 11% 2	532
	21st to end	Day	24% 15% 24% 22% 12% 4	540
May	1st to 10th	Day	23% 17% 28% 19% 8% 4	543
	11th to 20th	Day	19% 11% 24% 25% 15% 7%	567
	21st to end	Day	18% 9% 22% 22% 18% 12%	617
June	1st to 10th	Day	12% 7% 22% 29% 21% 9%	570
	11th to 20th	Day	10% 7% 22% 27% 24% 10%	570
	21st to end	Day	12% 8% 23% 23% 25% 9%	570
July	1st to 10th	Day	13% 5% 12% 26% 26% 17%	565
	11th to 20th	Day	13% 4 13% 32% 25% 14%	564
	21st to end	Day	16% 6% 13% 30% 25% 11%	620
August	1st to 10th	Day	14% 6% 16% 26% 24% 14%	566
	11th to 20th	Day	14% 4 14% 29% 24% 14%	559
	21st to end	Day	11% 2 12% 28% 29% 18%	627
September	1st to 10th	Day	11% 3 15% 27% 29% 15%	570
	11th to 20th	Day	14% 5% 10% 25% 31% 16%	570
	21st to end	Day	19% 9% 20% 27% 19% 6%	562
October	1st to 10th	Day	16% 5% 18% 28% 25% 8%	530
	11th to 20th	Day	16% 14% 20% 27% 19% 4	530
	21st to end	Day	21% 13% 22% 26% 14% 4	583
November	1st to 10th	Day	17% 14% 25% 28% 13% 3	446
	11th to 20th	Day	20% 21% 29% 19% 8% 1	330
	21st to end	Day	21% 23% 33% 18% 5% 1	330
December	1st to 10th	Day	30% 19% 32% 15% 2	330
	11th to 20th	Day	28% 19% 27% 19% 7%	330
	21st to end	Day	30% 18% 29% 16% 6% 1	363
January	1st to 10th	Night	25% 19% 29% 24% 3	840
	11th to 20th	Night	28% 17% 29% 23% 3	840
	21st to end	Night	22% 19% 33% 24% 2	924
February	1st to 10th	Night	24% 16% 29% 30% 2	840
	11th to 20th	Night	30% 21% 30% 19% 1	823
	21st to end	Night	28% 19% 29% 23% 1	692
March	1st to 10th	Night	23% 14% 30% 28% 5%	840
	11th to 20th	Night	24% 16% 26% 27% 7%	840
	21st to end	Night	18% 14% 32% 29% 7%	924
April	1st to 10th	Night	19% 15% 27% 30% 9%	840
	11th to 20th	Night	18% 13% 24% 33% 11% 1	752
	21st to end	Night	16% 7% 24% 40% 12% 2	630
May	1st to 10th	Night	17% 10% 22% 35% 14% 2	630
	11th to 20th	Night	16% 7% 17% 37% 21% 2	621
	21st to end	Night	14% 5% 13% 33% 31% 5%	683
June	1st to 10th	Night	11% 4 10% 40% 30% 5%	630
	11th to 20th	Night	8% 4 13% 36% 34% 5%	630
	21st to end	Night	12% 2 9% 37% 37% 3	630
July	1st to 10th	Night	14% 3 7% 31% 41% 4	615
	11th to 20th	Night	14% 2 4 31% 42% 7%	612
	21st to end	Night	12% 3 7% 35% 33% 9%	690
August	1st to 10th	Night	13% 3 6% 32% 42% 4	627
	11th to 20th	Night	14% 2 6% 34% 37% 7%	624
	21st to end	Night	11% 3 5% 32% 41% 7%	766
September	1st to 10th	Night	13% 3 8% 34% 37% 5%	840
	11th to 20th	Night	15% 4 9% 30% 37% 6%	840
	21st to end	Night	15% 5% 15% 35% 28% 2	833
October	1st to 10th	Night	18% 4 14% 31% 29% 4	800
	11th to 20th	Night	18% 7% 20% 36% 18% 1	800
	21st to end	Night	20% 11% 18% 35% 14% 1	880
November	1st to 10th	Night	17% 8% 18% 40% 15% 2	820
	11th to 20th	Night	19% 12% 23% 35% 10% 1	840
	21st to end	Night	20% 11% 27% 33% 9% 1	840
December	1st to 10th	Night	27% 13% 26% 28% 5%	840
	11th to 20th	Night	25% 14% 25% 30% 5% 1	840
	21st to end	Night	26% 13% 24% 32% 5%	924

Page generated at 2026-09-26 04:18:38. Version ac07

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