Twin Peeks Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Twin Peeks Observatory clear sky chart from 2005-11-06 to 2025-12-18.

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	13% 6% 2 6% 4 4 5% 4 6% 8% 43%	126454
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	124543
Transparency	Day	38% 6% 13% 22% 17% 4	130059
	Night	38% 6% 11% 21% 19% 5%	129006
Seeing	Day	18% 12% 26% 24% 14% 5%	16576
	Night	17% 10% 27% 30% 14% 2	26566

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% 5% 2 5% 3 3 3 2 4 5% 53%	4056
	11th to 20th	Day	13% 6% 2 5% 3 3 2 3 4 5% 55%	4060
	21st to end	Day	17% 7% 2 6% 4 3 3 2 4 5% 47%	4466
February	1st to 10th	Day	12% 7% 2 6% 4 4 6% 4 6% 8% 41%	1800
	11th to 20th	Day	21% 7% 1 5% 3 3 3 3 5% 6% 42%	1753
	21st to end	Day	19% 6% 2 6% 5% 4 4 4 5% 7% 39%	1756
March	1st to 10th	Day	17% 6% 2 6% 5% 4 5% 4 5% 8% 39%	4935
	11th to 20th	Day	11% 5% 1 5% 3 3 4 3 6% 7% 51%	4980
	21st to end	Day	17% 6% 2 6% 4 4 5% 4 5% 5% 42%	5465
April	1st to 10th	Day	11% 5% 2 5% 4 4 4 3 6% 7% 51%	2340
	11th to 20th	Day	12% 6% 2 5% 4 4 3 3 4 7% 48%	2374
	21st to end	Day	15% 6% 2 6% 5% 4 4 3 6% 7% 43%	2420
May	1st to 10th	Day	12% 6% 2 7% 5% 5% 6% 5% 8% 11% 33%	5862
	11th to 20th	Day	20% 8% 3 7% 5% 5% 5% 5% 6% 8% 30%	5861
	21st to end	Day	14% 5% 2 5% 4 4 5% 4 6% 10% 42%	6433
June	1st to 10th	Day	10% 4 2 5% 5% 5% 5% 5% 7% 11% 43%	2800
	11th to 20th	Day	10% 4 2 5% 5% 5% 4 5% 8% 11% 41%	2800
	21st to end	Day	7% 5% 2 5% 5% 5% 6% 4 7% 12% 43%	2705
July	1st to 10th	Day	7% 5% 2 7% 5% 5% 6% 5% 8% 11% 40%	5900
	11th to 20th	Day	9% 5% 2 7% 5% 6% 7% 6% 9% 14% 30%	5900
	21st to end	Day	9% 5% 2 6% 5% 6% 6% 5% 8% 10% 38%	6424
August	1st to 10th	Day	16% 7% 2 7% 5% 5% 6% 5% 7% 11% 30%	4234
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 5% 8% 10% 38%	4085
	21st to end	Day	8% 5% 2 5% 3 4 4 4 6% 9% 50%	4312
September	1st to 10th	Day	11% 6% 2 5% 4 4 5% 3 5% 8% 49%	2400
	11th to 20th	Day	14% 6% 2 6% 4 4 5% 4 5% 7% 44%	2285
	21st to end	Day	22% 7% 2 5% 4 4 4 4 5% 6% 36%	2147
October	1st to 10th	Day	14% 6% 2 5% 3 3 4 2 4 6% 52%	2000
	11th to 20th	Day	14% 5% 2 4 4 4 3 2 3 5% 54%	2000
	21st to end	Day	13% 5% 2 6% 3 3 3 2 4 6% 54%	2200
November	1st to 10th	Day	12% 5% 2 5% 3 4 3 3 4 6% 55%	2057
	11th to 20th	Day	14% 6% 2 5% 3 2 4 3 5% 6% 50%	1974
	21st to end	Day	14% 4 2 5% 3 3 3 3 4 7% 52%	1680
December	1st to 10th	Day	13% 6% 2 5% 3 4 4 3 5% 6% 50%	2680
	11th to 20th	Day	19% 6% 2 4 3 3 3 3 4 6% 47%	3046
	21st to end	Day	18% 6% 2 5% 3 3 5% 3 6% 7% 41%	4264
January	1st to 10th	Night	13% 6% 2 4 3 3 3 2 4 5% 55%	5889
	11th to 20th	Night	13% 6% 2 5% 3 2 3 2 4 4 57%	5900
	21st to end	Night	17% 6% 2 5% 3 3 3 2 4 5% 51%	6490
February	1st to 10th	Night	15% 6% 2 5% 4 4 4 3 6% 7% 44%	2600
	11th to 20th	Night	19% 7% 2 4 3 4 3 3 5% 6% 43%	2533
	21st to end	Night	14% 7% 2 5% 4 4 4 3 5% 6% 45%	1869
March	1st to 10th	Night	16% 5% 2 6% 4 5% 4 3 5% 5% 45%	4918
	11th to 20th	Night	11% 4 2 4 3 3 3 3 5% 7% 55%	4980
	21st to end	Night	14% 6% 2 6% 4 4 4 3 4 7% 46%	5467
April	1st to 10th	Night	15% 7% 2 5% 4 4 4 3 5% 8% 44%	2059
	11th to 20th	Night	10% 5% 2 4 4 4 4 4 6% 8% 49%	1975
	21st to end	Night	13% 5% 2 5% 5% 4 4 4 5% 7% 46%	1996
May	1st to 10th	Night	14% 7% 3 8% 5% 4 4 4 7% 8% 35%	4033
	11th to 20th	Night	22% 8% 3 6% 4 4 4 3 5% 7% 32%	4032
	21st to end	Night	15% 5% 2 5% 3 4 4 4 6% 9% 42%	4404
June	1st to 10th	Night	10% 6% 1 6% 5% 4 5% 4 6% 11% 43%	1600
	11th to 20th	Night	11% 5% 2 5% 5% 5% 5% 4 6% 11% 42%	1600
	21st to end	Night	7% 5% 2 5% 5% 5% 5% 4 7% 10% 44%	1686
July	1st to 10th	Night	11% 5% 2 6% 5% 6% 6% 5% 8% 9% 36%	4058
	11th to 20th	Night	11% 8% 3 8% 5% 6% 7% 5% 8% 11% 27%	4057
	21st to end	Night	10% 7% 3 8% 6% 6% 6% 6% 8% 9% 32%	4427
August	1st to 10th	Night	17% 8% 3 7% 4 5% 5% 5% 7% 10% 29%	2915
	11th to 20th	Night	8% 5% 2 7% 4 5% 6% 5% 8% 10% 39%	2931
	21st to end	Night	8% 5% 2 6% 5% 5% 5% 4 7% 9% 44%	3586
September	1st to 10th	Night	10% 7% 2 6% 4 4 4 3 6% 7% 46%	2000
	11th to 20th	Night	13% 6% 2 6% 5% 4 4 3 5% 6% 45%	2110
	21st to end	Night	22% 8% 3 8% 4 3 4 3 5% 5% 36%	2251
October	1st to 10th	Night	15% 6% 2 5% 3 4 4 3 4 6% 48%	2400
	11th to 20th	Night	13% 6% 1 5% 3 3 4 3 5% 6% 50%	2400
	21st to end	Night	14% 4 2 4 3 2 3 3 4 5% 55%	2640
November	1st to 10th	Night	12% 6% 2 6% 4 2 3 3 4 5% 53%	2461
	11th to 20th	Night	13% 6% 2 4 3 3 4 3 5% 7% 50%	2638
	21st to end	Night	12% 4 2 4 4 3 4 4 5% 6% 51%	2940
December	1st to 10th	Night	11% 6% 2 4 4 4 4 3 5% 5% 53%	4720
	11th to 20th	Night	20% 7% 2 7% 4 4 3 3 4 5% 41%	5304
	21st to end	Night	17% 6% 2 5% 3 4 4 4 6% 6% 44%	6674

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	34% 3 6% 22% 27% 8%	4057
	11th to 20th	Day	33% 3 6% 20% 28% 11%	4060
	21st to end	Day	41% 3 7% 25% 20% 4	4466
February	1st to 10th	Day	40% 5% 12% 23% 16% 4	1800
	11th to 20th	Day	43% 4 8% 21% 19% 5%	1762
	21st to end	Day	45% 6% 11% 15% 17% 6%	1756
March	1st to 10th	Day	43% 4 8% 19% 16% 10%	4935
	11th to 20th	Day	31% 6% 9% 20% 24% 10%	4980
	21st to end	Day	43% 5% 9% 18% 18% 6%	5465
April	1st to 10th	Day	33% 6% 11% 23% 20% 8%	2340
	11th to 20th	Day	36% 5% 10% 23% 20% 6%	2374
	21st to end	Day	40% 6% 11% 23% 15% 4	2420
May	1st to 10th	Day	42% 9% 16% 22% 10% 1	5897
	11th to 20th	Day	50% 7% 11% 19% 11% 1	5865
	21st to end	Day	37% 7% 15% 20% 18% 3	6447
June	1st to 10th	Day	33% 9% 16% 26% 15% 1	2800
	11th to 20th	Day	35% 9% 17% 22% 15% 2	2800
	21st to end	Day	33% 9% 17% 24% 15% 2	2705
July	1st to 10th	Day	36% 10% 20% 19% 13% 3	5835
	11th to 20th	Day	40% 12% 24% 18% 5% 1	5831
	21st to end	Day	38% 10% 15% 23% 14%	6240
August	1st to 10th	Day	46% 8% 17% 23% 6%	4221
	11th to 20th	Day	36% 10% 21% 24% 9% 1	4124
	21st to end	Day	30% 6% 19% 29% 16% 1	4312
September	1st to 10th	Day	35% 7% 14% 18% 22% 4	2400
	11th to 20th	Day	40% 6% 10% 23% 19% 3	2285
	21st to end	Day	49% 5% 11% 16% 17% 3	2147
October	1st to 10th	Day	33% 4 8% 19% 30% 6%	3260
	11th to 20th	Day	24% 3 5% 24% 29% 15%	3240
	21st to end	Day	23% 3 11% 25% 25% 12%	3538
November	1st to 10th	Day	32% 3 9% 25% 25% 6%	2057
	11th to 20th	Day	34% 4 10% 23% 21% 7%	1974
	21st to end	Day	33% 4 10% 24% 25% 4	1680
December	1st to 10th	Day	35% 4 8% 25% 21% 7%	2680
	11th to 20th	Day	40% 3 11% 21% 19% 5%	3042
	21st to end	Day	41% 5% 13% 24% 16% 1	4264
January	1st to 10th	Night	32% 3 6% 20% 27% 12%	5889
	11th to 20th	Night	32% 4 6% 18% 32% 8%	5900
	21st to end	Night	38% 3 6% 27% 20% 6%	6490
February	1st to 10th	Night	39% 5% 11% 24% 19% 3	2600
	11th to 20th	Night	42% 4 9% 19% 20% 7%	2535
	21st to end	Night	39% 5% 9% 22% 18% 7%	1869
March	1st to 10th	Night	40% 4 7% 19% 20% 9%	4918
	11th to 20th	Night	29% 5% 7% 17% 30% 12%	4980
	21st to end	Night	39% 4 8% 22% 22% 6%	5467
April	1st to 10th	Night	39% 6% 11% 18% 21% 6%	2059
	11th to 20th	Night	33% 6% 11% 23% 21% 6%	1975
	21st to end	Night	37% 7% 12% 22% 17% 4	1996
May	1st to 10th	Night	45% 7% 16% 19% 11% 2	4060
	11th to 20th	Night	50% 7% 11% 19% 12% 2	4033
	21st to end	Night	37% 7% 16% 24% 14% 2	4411
June	1st to 10th	Night	34% 9% 16% 24% 16% 1	1600
	11th to 20th	Night	35% 9% 19% 20% 15% 2	1600
	21st to end	Night	33% 9% 16% 21% 17% 3	1686
July	1st to 10th	Night	41% 11% 17% 17% 12% 2	4015
	11th to 20th	Night	49% 12% 19% 14% 5% 1	4006
	21st to end	Night	45% 9% 14% 20% 11%	4297
August	1st to 10th	Night	48% 8% 17% 20% 6%	2904
	11th to 20th	Night	36% 10% 18% 25% 10% 1	2958
	21st to end	Night	35% 7% 14% 31% 12% 2	3586
September	1st to 10th	Night	37% 7% 13% 19% 18% 6%	2000
	11th to 20th	Night	39% 6% 11% 20% 21% 2	2110
	21st to end	Night	50% 5% 9% 16% 16% 3	2251
October	1st to 10th	Night	39% 4 10% 18% 24% 5%	3920
	11th to 20th	Night	25% 4 7% 22% 29% 13%	3899
	21st to end	Night	22% 3 6% 29% 28% 13%	4252
November	1st to 10th	Night	35% 4 8% 21% 25% 7%	2461
	11th to 20th	Night	34% 4 9% 19% 24% 9%	2638
	21st to end	Night	33% 5% 8% 24% 24% 6%	2940
December	1st to 10th	Night	33% 4 6% 28% 24% 5%	4720
	11th to 20th	Night	46% 4 8% 18% 16% 7%	5307
	21st to end	Night	38% 5% 12% 26% 17% 2	6674

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	30% 27% 27% 12% 4 1	300
	11th to 20th	Day	29% 27% 33% 10% 1	300
	21st to end	Day	25% 22% 41% 11% 2	330
February	1st to 10th	Day	20% 27% 38% 12% 3	300
	11th to 20th	Day	40% 27% 23% 7% 3	300
	21st to end	Day	31% 18% 33% 15% 3	405
March	1st to 10th	Day	24% 19% 34% 18% 5% 1	500
	11th to 20th	Day	23% 24% 30% 19% 4	500
	21st to end	Day	23% 19% 38% 15% 4 1	550
April	1st to 10th	Day	21% 23% 30% 19% 5% 1	500
	11th to 20th	Day	21% 15% 32% 22% 8% 3	492
	21st to end	Day	18% 13% 36% 23% 8% 1	500
May	1st to 10th	Day	19% 16% 33% 22% 7% 2	503
	11th to 20th	Day	22% 10% 29% 22% 14% 3	527
	21st to end	Day	20% 7% 23% 26% 19% 5%	573
June	1st to 10th	Day	10% 7% 27% 29% 22% 5%	530
	11th to 20th	Day	12% 6% 23% 32% 20% 7%	530
	21st to end	Day	10% 5% 19% 35% 24% 7%	530
July	1st to 10th	Day	10% 4 16% 33% 27% 10%	525
	11th to 20th	Day	6% 2 14% 35% 30% 12%	524
	21st to end	Day	9% 3 19% 34% 24% 10%	576
August	1st to 10th	Day	15% 4 19% 28% 24% 10%	526
	11th to 20th	Day	11% 3 15% 33% 27% 10%	519
	21st to end	Day	9% 3 18% 29% 25% 16%	583
September	1st to 10th	Day	14% 5% 18% 25% 24% 15%	530
	11th to 20th	Day	15% 5% 15% 31% 23% 11%	530
	21st to end	Day	29% 7% 22% 26% 11% 4	530
October	1st to 10th	Day	17% 9% 22% 28% 17% 7%	530
	11th to 20th	Day	14% 14% 24% 28% 17% 3	530
	21st to end	Day	17% 11% 30% 28% 10% 4	583
November	1st to 10th	Day	16% 10% 33% 26% 11% 4	535
	11th to 20th	Day	19% 17% 31% 21% 11% 1	444
	21st to end	Day	23% 22% 26% 18% 9% 1	220
December	1st to 10th	Day	21% 22% 38% 12% 5% 1	220
	11th to 20th	Day	27% 14% 33% 22% 3 1	214
	21st to end	Day	25% 18% 32% 17% 6% 1	287
January	1st to 10th	Night	21% 18% 37% 23% 3	800
	11th to 20th	Night	19% 20% 35% 25% 2	800
	21st to end	Night	19% 16% 39% 24% 2	880
February	1st to 10th	Night	19% 17% 38% 23% 3	800
	11th to 20th	Night	26% 19% 38% 16% 1	783
	21st to end	Night	21% 17% 34% 24% 3	660
March	1st to 10th	Night	18% 19% 35% 25% 3	800
	11th to 20th	Night	22% 14% 35% 22% 6% 1	800
	21st to end	Night	18% 15% 39% 24% 4	880
April	1st to 10th	Night	18% 19% 36% 22% 5%	800
	11th to 20th	Night	14% 14% 35% 27% 9%	791
	21st to end	Night	16% 9% 36% 33% 7%	796
May	1st to 10th	Night	15% 12% 34% 28% 11% 1	600
	11th to 20th	Night	19% 7% 26% 33% 14% 1	591
	21st to end	Night	16% 3 17% 36% 25% 2	645
June	1st to 10th	Night	10% 3 21% 41% 26% 1	400
	11th to 20th	Night	11% 3 16% 43% 24% 3	400
	21st to end	Night	7% 2 16% 47% 25% 2	486
July	1st to 10th	Night	12% 1 12% 38% 32% 4	585
	11th to 20th	Night	6% 1 12% 38% 38% 5%	582
	21st to end	Night	9% 2 13% 39% 32% 4	657
August	1st to 10th	Night	14% 2 14% 37% 29% 4	597
	11th to 20th	Night	13% 3 12% 36% 33% 3	624
	21st to end	Night	11% 2 13% 37% 32% 5%	880
September	1st to 10th	Night	15% 2 12% 36% 29% 6%	800
	11th to 20th	Night	16% 4 13% 33% 29% 5%	800
	21st to end	Night	27% 5% 17% 32% 18% 2	800
October	1st to 10th	Night	19% 5% 20% 26% 25% 6%	800
	11th to 20th	Night	15% 9% 26% 34% 16% 1	800
	21st to end	Night	16% 10% 27% 32% 14% 1	880
November	1st to 10th	Night	15% 9% 29% 33% 13% 2	820
	11th to 20th	Night	17% 13% 30% 29% 11%	840
	21st to end	Night	14% 13% 32% 30% 11% 1	840
December	1st to 10th	Night	21% 13% 35% 25% 5% 1	840
	11th to 20th	Night	19% 12% 35% 29% 5%	829
	21st to end	Night	21% 14% 31% 30% 4	880

Page generated at 2025-12-15 01:13:50. Version ac07

Copyright © 2025 Attilla Danko.