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 2024-01-13.

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% 5% 2 5% 5% 3 5% 5% 6% 8% 43%	74593
	Night	14% 6% 2 5% 5% 3 4 4 5% 7% 45%	75129
Transparency	Day	38% 7% 12% 22% 17% 4	74314
	Night	38% 6% 11% 22% 19% 5%	74927
Seeing	Day	18% 12% 26% 24% 15% 5%	13990
	Night	17% 10% 27% 30% 15% 2	24021

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% 8% 3 5% 4 2 4 4 5% 5% 46%	1706
	11th to 20th	Day	17% 6% 2 6% 4 3 3 4 5% 6% 43%	1644
	21st to end	Day	17% 6% 2 5% 4 2 4 3 4 5% 47%	1782
February	1st to 10th	Day	13% 6% 2 5% 5% 3 5% 5% 6% 9% 40%	1620
	11th to 20th	Day	20% 6% 2 4 4 2 4 4 4 6% 44%	1573
	21st to end	Day	20% 6% 2 6% 6% 4 4 5% 5% 6% 37%	1575
March	1st to 10th	Day	14% 5% 2 4 4 2 5% 4 4 6% 50%	1980
	11th to 20th	Day	17% 6% 2 5% 5% 3 6% 5% 6% 7% 40%	1980
	21st to end	Day	17% 8% 3 6% 5% 3 5% 4 4 6% 40%	2178
April	1st to 10th	Day	10% 5% 2 4 4 2 4 3 6% 8% 53%	2106
	11th to 20th	Day	12% 5% 3 4 5% 3 4 4 4 7% 48%	2134
	21st to end	Day	16% 5% 2 5% 5% 3 5% 4 6% 7% 42%	2178
May	1st to 10th	Day	14% 5% 2 5% 5% 4 5% 5% 7% 8% 40%	2340
	11th to 20th	Day	16% 6% 3 6% 5% 4 5% 5% 6% 9% 36%	2301
	21st to end	Day	12% 6% 3 5% 5% 3 6% 6% 6% 10% 38%	2531
June	1st to 10th	Day	11% 4 2 4 5% 3 5% 6% 6% 11% 43%	2520
	11th to 20th	Day	11% 3 3 5% 5% 4 6% 5% 7% 10% 42%	2520
	21st to end	Day	8% 4 2 5% 6% 3 6% 5% 7% 11% 42%	2435
July	1st to 10th	Day	9% 5% 2 6% 6% 4 6% 6% 7% 11% 38%	2340
	11th to 20th	Day	6% 4 2 4 5% 4 7% 7% 8% 13% 40%	2340
	21st to end	Day	8% 5% 2 6% 6% 5% 7% 6% 8% 10% 36%	2547
August	1st to 10th	Day	10% 6% 2 6% 6% 4 7% 6% 7% 10% 36%	2327
	11th to 20th	Day	12% 6% 3 6% 7% 4 6% 6% 7% 9% 34%	2259
	21st to end	Day	11% 6% 2 6% 5% 4 6% 6% 7% 9% 39%	2376
September	1st to 10th	Day	11% 6% 2 5% 4 2 5% 4 5% 7% 48%	2160
	11th to 20th	Day	13% 5% 2 5% 5% 3 5% 4 4 7% 45%	2057
	21st to end	Day	21% 7% 2 5% 5% 3 4 4 5% 6% 38%	1933
October	1st to 10th	Day	15% 5% 2 4 3 3 3 3 5% 6% 51%	1800
	11th to 20th	Day	15% 5% 2 4 4 3 3 3 3 5% 52%	1800
	21st to end	Day	13% 4 2 5% 4 2 3 3 4 6% 53%	1980
November	1st to 10th	Day	13% 5% 2 4 3 2 3 3 3 6% 55%	1857
	11th to 20th	Day	14% 5% 2 5% 3 2 4 4 5% 6% 50%	1786
	21st to end	Day	15% 4 2 4 4 2 3 4 4 8% 52%	1520
December	1st to 10th	Day	17% 7% 2 5% 5% 3 5% 6% 5% 6% 39%	1520
	11th to 20th	Day	16% 5% 2 4 3 2 3 3 3 5% 54%	1910
	21st to end	Day	18% 6% 2 4 4 3 5% 4 5% 8% 41%	2978
January	1st to 10th	Night	16% 6% 2 5% 4 3 4 4 5% 5% 47%	2459
	11th to 20th	Night	16% 6% 3 6% 5% 3 4 4 5% 6% 43%	2378
	21st to end	Night	16% 5% 2 4 4 3 4 3 4 5% 50%	2574
February	1st to 10th	Night	15% 6% 2 4 4 3 4 4 6% 8% 44%	2340
	11th to 20th	Night	19% 7% 3 4 4 3 3 3 5% 6% 43%	2273
	21st to end	Night	16% 6% 3 4 5% 2 4 4 5% 7% 44%	1676
March	1st to 10th	Night	13% 5% 2 4 4 3 4 3 5% 7% 50%	1980
	11th to 20th	Night	18% 5% 3 6% 4 3 5% 5% 5% 6% 40%	1980
	21st to end	Night	16% 7% 3 5% 5% 3 4 4 5% 7% 42%	2178
April	1st to 10th	Night	14% 5% 3 5% 4 3 4 4 5% 8% 45%	1854
	11th to 20th	Night	11% 5% 3 4 4 3 4 4 5% 8% 50%	1775
	21st to end	Night	15% 5% 2 4 5% 3 4 5% 5% 6% 45%	1797
May	1st to 10th	Night	13% 6% 3 5% 4 2 4 5% 7% 7% 44%	1620
	11th to 20th	Night	17% 6% 4 5% 5% 3 4 5% 5% 8% 38%	1592
	21st to end	Night	15% 4 2 4 4 3 5% 5% 5% 10% 43%	1740
June	1st to 10th	Night	10% 5% 1 5% 5% 3 5% 4 6% 12% 43%	1440
	11th to 20th	Night	11% 4 3 4 5% 3 5% 5% 5% 11% 43%	1440
	21st to end	Night	7% 4 3 4 5% 5% 5% 5% 7% 10% 44%	1516
July	1st to 10th	Night	9% 5% 3 5% 6% 4 5% 5% 7% 10% 43%	1620
	11th to 20th	Night	4 3 2 4 5% 4 6% 7% 7% 13% 44%	1620
	21st to end	Night	7% 4 2 4 6% 4 5% 7% 8% 11% 40%	1771
August	1st to 10th	Night	11% 6% 3 5% 5% 5% 6% 6% 7% 10% 37%	1611
	11th to 20th	Night	12% 6% 3 6% 5% 4 5% 6% 6% 8% 38%	1624
	21st to end	Night	9% 5% 2 5% 6% 4 7% 6% 7% 10% 38%	1980
September	1st to 10th	Night	11% 7% 2 6% 5% 3 6% 4 6% 7% 44%	1800
	11th to 20th	Night	12% 6% 3 5% 6% 3 4 4 5% 7% 46%	1898
	21st to end	Night	21% 8% 3 6% 5% 2 4 4 5% 5% 38%	2025
October	1st to 10th	Night	16% 6% 2 4 4 3 3 4 4 6% 48%	2160
	11th to 20th	Night	14% 5% 2 5% 3 2 4 4 5% 6% 49%	2160
	21st to end	Night	14% 4 2 4 4 2 3 4 5% 5% 55%	2376
November	1st to 10th	Night	12% 6% 3 5% 4 2 3 3 3 5% 53%	2221
	11th to 20th	Night	13% 5% 2 4 3 2 4 4 5% 7% 51%	2386
	21st to end	Night	13% 3 2 3 4 2 4 5% 5% 7% 51%	2660
December	1st to 10th	Night	17% 8% 2 5% 4 3 4 4 5% 5% 42%	2660
	11th to 20th	Night	17% 5% 2 5% 5% 2 3 3 3 6% 49%	3285
	21st to end	Night	17% 6% 2 5% 4 3 5% 5% 7% 7% 40%	4660

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	41% 5% 9% 22% 19% 5%	1707
	11th to 20th	Day	41% 5% 9% 22% 19% 3	1644
	21st to end	Day	40% 3 7% 24% 20% 5%	1782
February	1st to 10th	Day	40% 6% 12% 23% 15% 4	1620
	11th to 20th	Day	42% 4 8% 21% 20% 5%	1582
	21st to end	Day	47% 6% 11% 14% 16% 6%	1575
March	1st to 10th	Day	35% 5% 8% 23% 22% 6%	1980
	11th to 20th	Day	42% 7% 10% 17% 17% 6%	1980
	21st to end	Day	47% 5% 10% 20% 15% 4	2178
April	1st to 10th	Day	30% 6% 11% 24% 20% 8%	2106
	11th to 20th	Day	36% 5% 10% 22% 20% 6%	2134
	21st to end	Day	41% 6% 11% 22% 15% 5%	2178
May	1st to 10th	Day	39% 8% 16% 19% 14% 3	2340
	11th to 20th	Day	43% 9% 13% 22% 10% 2	2305
	21st to end	Day	41% 8% 15% 23% 12%	2531
June	1st to 10th	Day	34% 9% 16% 26% 15% 1	2520
	11th to 20th	Day	36% 9% 17% 21% 15% 2	2520
	21st to end	Day	35% 10% 17% 23% 14% 2	2435
July	1st to 10th	Day	38% 10% 15% 20% 15% 3	2275
	11th to 20th	Day	32% 11% 20% 23% 13% 2	2271
	21st to end	Day	38% 10% 14% 24% 13%	2405
August	1st to 10th	Day	40% 10% 16% 25% 9% 1	2314
	11th to 20th	Day	42% 11% 16% 20% 10% 1	2259
	21st to end	Day	38% 8% 18% 21% 13% 3	2376
September	1st to 10th	Day	36% 7% 14% 19% 20% 4	2160
	11th to 20th	Day	39% 6% 10% 23% 19% 3	2057
	21st to end	Day	47% 5% 10% 17% 18% 3	1933
October	1st to 10th	Day	35% 4 10% 26% 21% 4	1800
	11th to 20th	Day	35% 4 8% 26% 22% 5%	1800
	21st to end	Day	34% 4 8% 25% 21% 8%	1980
November	1st to 10th	Day	32% 4 8% 25% 26% 6%	1857
	11th to 20th	Day	35% 4 10% 21% 21% 8%	1786
	21st to end	Day	32% 4 11% 23% 25% 5%	1520
December	1st to 10th	Day	43% 6% 10% 20% 17% 4	1520
	11th to 20th	Day	34% 3 8% 26% 21% 8%	1906
	21st to end	Day	41% 5% 11% 25% 16% 2	2978
January	1st to 10th	Night	39% 4 9% 20% 20% 7%	2459
	11th to 20th	Night	42% 6% 9% 20% 19% 4	2378
	21st to end	Night	38% 4 8% 24% 22% 5%	2574
February	1st to 10th	Night	38% 5% 11% 24% 18% 4	2340
	11th to 20th	Night	43% 4 8% 19% 19% 7%	2275
	21st to end	Night	40% 5% 9% 22% 18% 6%	1676
March	1st to 10th	Night	34% 5% 9% 23% 23% 6%	1980
	11th to 20th	Night	44% 5% 8% 19% 19% 5%	1980
	21st to end	Night	42% 6% 9% 20% 18% 5%	2178
April	1st to 10th	Night	38% 6% 11% 18% 22% 6%	1854
	11th to 20th	Night	33% 6% 10% 23% 22% 7%	1775
	21st to end	Night	38% 7% 11% 22% 17% 5%	1797
May	1st to 10th	Night	37% 7% 15% 22% 15% 4	1620
	11th to 20th	Night	44% 9% 11% 21% 12% 3	1593
	21st to end	Night	36% 9% 17% 23% 14% 1	1741
June	1st to 10th	Night	33% 9% 16% 26% 15% 1	1440
	11th to 20th	Night	35% 10% 19% 20% 15% 2	1440
	21st to end	Night	34% 9% 16% 20% 17% 4	1516
July	1st to 10th	Night	36% 10% 17% 18% 15% 4	1575
	11th to 20th	Night	28% 11% 24% 22% 12% 2	1566
	21st to end	Night	34% 10% 20% 24% 12%	1672
August	1st to 10th	Night	40% 10% 17% 20% 11% 1	1600
	11th to 20th	Night	40% 10% 16% 20% 12% 1	1624
	21st to end	Night	38% 10% 17% 22% 10% 3	1980
September	1st to 10th	Night	38% 8% 13% 19% 17% 6%	1800
	11th to 20th	Night	38% 7% 11% 20% 22% 2	1898
	21st to end	Night	48% 5% 9% 18% 17% 4	2025
October	1st to 10th	Night	38% 5% 10% 24% 20% 3	2160
	11th to 20th	Night	35% 6% 8% 24% 23% 5%	2160
	21st to end	Night	32% 3 8% 23% 27% 7%	2376
November	1st to 10th	Night	35% 4 8% 20% 25% 7%	2221
	11th to 20th	Night	34% 5% 9% 19% 24% 9%	2386
	21st to end	Night	33% 5% 8% 25% 23% 7%	2660
December	1st to 10th	Night	44% 5% 7% 23% 17% 4	2660
	11th to 20th	Night	39% 4 8% 21% 22% 7%	3288
	21st to end	Night	40% 6% 11% 23% 17% 2	4660

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% 26% 29% 13% 4 1	270
	11th to 20th	Day	27% 24% 36% 11% 1	248
	21st to end	Day	26% 22% 39% 11% 2	264
February	1st to 10th	Day	23% 25% 38% 11% 3	240
	11th to 20th	Day	38% 28% 26% 7% 2	240
	21st to end	Day	33% 18% 33% 13% 3	338
March	1st to 10th	Day	23% 20% 34% 18% 5%	420
	11th to 20th	Day	23% 25% 30% 18% 4	420
	21st to end	Day	23% 18% 37% 16% 5% 1	462
April	1st to 10th	Day	21% 21% 29% 22% 6% 1	420
	11th to 20th	Day	21% 14% 30% 23% 9% 4	412
	21st to end	Day	20% 13% 35% 21% 8% 1	420
May	1st to 10th	Day	20% 17% 31% 23% 7% 2	423
	11th to 20th	Day	21% 9% 29% 23% 14% 3	447
	21st to end	Day	19% 5% 22% 27% 21% 6%	485
June	1st to 10th	Day	12% 6% 24% 30% 23% 5%	450
	11th to 20th	Day	12% 7% 24% 29% 19% 8%	450
	21st to end	Day	10% 6% 21% 34% 22% 7%	450
July	1st to 10th	Day	11% 4 16% 32% 27% 10%	445
	11th to 20th	Day	5% 2 15% 36% 31% 11%	444
	21st to end	Day	9% 3 19% 35% 23% 10%	488
August	1st to 10th	Day	13% 4 20% 28% 25% 11%	446
	11th to 20th	Day	13% 3 13% 32% 28% 12%	439
	21st to end	Day	11% 3 17% 28% 25% 17%	495
September	1st to 10th	Day	15% 5% 15% 24% 26% 16%	450
	11th to 20th	Day	14% 6% 14% 32% 22% 12%	450
	21st to end	Day	28% 6% 22% 27% 12% 4	450
October	1st to 10th	Day	19% 9% 22% 26% 18% 6%	450
	11th to 20th	Day	14% 13% 24% 29% 18% 3	450
	21st to end	Day	16% 11% 31% 29% 9% 3	495
November	1st to 10th	Day	17% 10% 30% 26% 12% 4	455
	11th to 20th	Day	19% 16% 29% 22% 13% 1	376
	21st to end	Day	19% 22% 28% 21% 8% 2	180
December	1st to 10th	Day	20% 24% 35% 13% 6% 1	180
	11th to 20th	Day	27% 14% 31% 23% 3 2	180
	21st to end	Day	25% 19% 32% 17% 6% 1	258
January	1st to 10th	Night	21% 17% 36% 24% 3	760
	11th to 20th	Night	20% 17% 34% 26% 2	732
	21st to end	Night	19% 17% 39% 24% 2	792
February	1st to 10th	Night	19% 16% 38% 23% 3	720
	11th to 20th	Night	25% 19% 38% 17% 1	703
	21st to end	Night	22% 17% 35% 23% 3	592
March	1st to 10th	Night	17% 18% 35% 26% 3	720
	11th to 20th	Night	23% 14% 34% 22% 7% 1	720
	21st to end	Night	19% 14% 38% 24% 5%	792
April	1st to 10th	Night	17% 18% 36% 23% 5%	720
	11th to 20th	Night	14% 14% 34% 28% 10%	711
	21st to end	Night	18% 8% 34% 32% 7%	717
May	1st to 10th	Night	15% 12% 33% 28% 11% 1	540
	11th to 20th	Night	18% 7% 26% 33% 15% 1	531
	21st to end	Night	16% 2 16% 37% 27% 2	580
June	1st to 10th	Night	10% 2 19% 41% 27% 1	360
	11th to 20th	Night	12% 3 16% 41% 24% 4	360
	21st to end	Night	7% 1 17% 48% 24% 3	436
July	1st to 10th	Night	12% 1 12% 37% 33% 5%	525
	11th to 20th	Night	5% 1 12% 37% 40% 5%	522
	21st to end	Night	9% 2 13% 41% 32% 3	591
August	1st to 10th	Night	13% 2 13% 37% 31% 5%	537
	11th to 20th	Night	13% 3 12% 36% 33% 3	560
	21st to end	Night	12% 2 13% 35% 32% 6%	792
September	1st to 10th	Night	16% 1 11% 34% 31% 7%	720
	11th to 20th	Night	14% 4 14% 34% 29% 5%	720
	21st to end	Night	25% 5% 17% 32% 19% 2	720
October	1st to 10th	Night	19% 4 20% 27% 24% 6%	720
	11th to 20th	Night	16% 8% 25% 33% 16% 1	720
	21st to end	Night	17% 9% 27% 32% 13% 1	792
November	1st to 10th	Night	16% 8% 29% 32% 13% 2	740
	11th to 20th	Night	17% 12% 29% 29% 12%	760
	21st to end	Night	14% 12% 31% 30% 12% 1	760
December	1st to 10th	Night	22% 13% 33% 25% 6% 1	760
	11th to 20th	Night	18% 11% 35% 30% 5%	760
	21st to end	Night	22% 14% 32% 29% 4	836

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