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 2024-11-11.

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% 6% 2 6% 4 4 5% 4 6% 8% 42%	100878
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 44%	98199
Transparency	Day	38% 7% 13% 22% 17% 4	104527
	Night	38% 6% 11% 22% 19% 5%	102688
Seeing	Day	18% 12% 26% 25% 15% 5%	15160
	Night	17% 10% 27% 30% 14% 2	25095

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	18% 5% 2 5% 3 3 3 2 5% 5% 50%	2836
	11th to 20th	Day	14% 7% 2 6% 3 3 3 3 5% 6% 47%	2840
	21st to end	Day	21% 9% 3 6% 4 4 3 2 4 5% 40%	3124
February	1st to 10th	Day	12% 7% 2 6% 4 5% 5% 4 6% 8% 41%	1710
	11th to 20th	Day	20% 7% 1 4 3 3 3 3 5% 6% 43%	1663
	21st to end	Day	20% 6% 2 7% 5% 4 4 4 5% 7% 37%	1670
March	1st to 10th	Day	20% 7% 2 6% 4 4 5% 4 5% 7% 37%	3435
	11th to 20th	Day	10% 5% 2 5% 3 4 5% 3 5% 6% 52%	3480
	21st to end	Day	20% 7% 2 6% 5% 4 5% 4 5% 5% 39%	3815
April	1st to 10th	Day	10% 5% 1 5% 4 4 4 3 6% 7% 52%	2223
	11th to 20th	Day	12% 6% 2 5% 4 4 3 3 4 7% 48%	2254
	21st to end	Day	16% 6% 2 5% 5% 4 4 3 6% 7% 43%	2299
May	1st to 10th	Day	15% 7% 3 7% 5% 5% 6% 5% 7% 8% 32%	4082
	11th to 20th	Day	19% 7% 3 7% 4 4 4 4 6% 9% 33%	4081
	21st to end	Day	8% 4 2 5% 4 4 5% 4 7% 11% 45%	4476
June	1st to 10th	Day	10% 4 2 4 5% 5% 5% 5% 7% 11% 43%	2660
	11th to 20th	Day	10% 4 2 5% 5% 6% 4 5% 8% 11% 41%	2660
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 4 7% 12% 42%	2570
July	1st to 10th	Day	6% 4 2 6% 5% 5% 6% 5% 9% 12% 39%	4120
	11th to 20th	Day	11% 5% 2 6% 4 5% 6% 5% 8% 13% 35%	4120
	21st to end	Day	13% 7% 2 7% 6% 7% 6% 5% 8% 9% 30%	4466
August	1st to 10th	Day	15% 7% 2 7% 5% 5% 6% 5% 7% 11% 30%	4104
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 5% 8% 10% 39%	3958
	21st to end	Day	8% 5% 2 5% 3 4 4 4 6% 9% 50%	4180
September	1st to 10th	Day	11% 6% 2 5% 4 4 5% 3 5% 7% 48%	2280
	11th to 20th	Day	13% 6% 2 6% 4 4 5% 3 4 7% 44%	2171
	21st to end	Day	22% 7% 3 5% 4 4 4 3 5% 6% 37%	2040
October	1st to 10th	Day	15% 6% 2 5% 3 3 3 2 4 6% 51%	1900
	11th to 20th	Day	14% 5% 2 4 4 4 3 2 3 5% 54%	1900
	21st to end	Day	12% 4 2 6% 3 3 3 2 4 6% 54%	2090
November	1st to 10th	Day	13% 5% 2 5% 3 3 3 3 4 6% 55%	1957
	11th to 20th	Day	14% 6% 2 5% 3 2 4 3 5% 6% 49%	1786
	21st to end	Day	15% 4 1 4 3 3 3 3 4 7% 52%	1520
December	1st to 10th	Day	17% 7% 2 6% 4 5% 5% 4 6% 6% 39%	1520
	11th to 20th	Day	16% 6% 1 4 3 3 3 2 3 5% 54%	1910
	21st to end	Day	18% 6% 2 5% 3 4 5% 3 6% 7% 41%	2978
January	1st to 10th	Night	14% 7% 2 5% 4 3 4 3 5% 5% 48%	4109
	11th to 20th	Night	16% 7% 3 6% 3 3 3 2 4 5% 48%	4120
	21st to end	Night	22% 7% 2 6% 4 3 4 2 4 4 43%	4532
February	1st to 10th	Night	14% 6% 2 5% 3 4 4 3 6% 7% 45%	2470
	11th to 20th	Night	19% 7% 2 5% 3 4 3 3 5% 6% 44%	2403
	21st to end	Night	15% 7% 2 5% 4 4 4 3 5% 7% 44%	1779
March	1st to 10th	Night	20% 7% 3 7% 5% 5% 4 3 5% 5% 36%	3418
	11th to 20th	Night	12% 4 2 5% 4 3 4 3 5% 5% 53%	3480
	21st to end	Night	16% 6% 2 5% 3 3 3 3 4 7% 47%	3817
April	1st to 10th	Night	14% 7% 2 5% 4 4 4 3 5% 8% 45%	1956
	11th to 20th	Night	11% 5% 2 4 4 4 4 4 5% 8% 49%	1875
	21st to end	Night	14% 6% 2 4 5% 4 4 4 5% 6% 45%	1896
May	1st to 10th	Night	16% 8% 4 7% 4 4 4 4 6% 7% 37%	2813
	11th to 20th	Night	24% 7% 3 5% 4 4 4 3 5% 6% 36%	2812
	21st to end	Night	10% 4 2 5% 3 4 4 4 7% 11% 46%	3062
June	1st to 10th	Night	10% 5% 1 6% 5% 4 5% 4 7% 12% 43%	1520
	11th to 20th	Night	10% 5% 2 5% 5% 5% 5% 4 6% 11% 43%	1520
	21st to end	Night	7% 5% 2 5% 5% 5% 5% 4 7% 10% 44%	1601
July	1st to 10th	Night	7% 5% 3 6% 5% 7% 6% 4 8% 10% 40%	2840
	11th to 20th	Night	9% 6% 2 7% 5% 6% 6% 5% 8% 11% 35%	2840
	21st to end	Night	13% 9% 4 8% 6% 6% 6% 5% 7% 8% 29%	3087
August	1st to 10th	Night	17% 8% 3 7% 4 5% 5% 5% 7% 10% 29%	2825
	11th to 20th	Night	7% 5% 2 7% 4 5% 6% 5% 8% 10% 40%	2839
	21st to end	Night	9% 5% 2 6% 5% 5% 5% 4 7% 9% 44%	3476
September	1st to 10th	Night	11% 7% 2 7% 4 4 4 3 6% 7% 45%	1900
	11th to 20th	Night	13% 7% 2 6% 5% 4 3 3 5% 6% 45%	2004
	21st to end	Night	22% 8% 3 7% 3 3 4 3 5% 5% 37%	2138
October	1st to 10th	Night	16% 7% 2 5% 3 4 4 3 4 6% 47%	2280
	11th to 20th	Night	13% 6% 1 5% 3 3 4 3 5% 6% 50%	2280
	21st to end	Night	13% 4 2 4 3 2 3 3 5% 5% 56%	2508
November	1st to 10th	Night	12% 7% 2 6% 4 3 3 3 4 5% 52%	2341
	11th to 20th	Night	13% 6% 2 4 3 3 4 3 5% 7% 50%	2393
	21st to end	Night	13% 4 2 4 4 3 4 4 6% 7% 50%	2660
December	1st to 10th	Night	17% 8% 2 5% 4 4 4 4 5% 5% 42%	2660
	11th to 20th	Night	17% 6% 2 6% 4 3 2 3 4 6% 48%	3285
	21st to end	Night	17% 7% 2 5% 4 4 4 4 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	36% 3 8% 28% 19% 5%	2837
	11th to 20th	Day	38% 4 8% 20% 25% 6%	2840
	21st to end	Day	48% 3 6% 22% 17% 3	3124
February	1st to 10th	Day	39% 6% 12% 24% 16% 4	1710
	11th to 20th	Day	41% 4 8% 22% 20% 5%	1672
	21st to end	Day	47% 6% 11% 14% 17% 5%	1670
March	1st to 10th	Day	46% 5% 9% 20% 13% 7%	3435
	11th to 20th	Day	30% 7% 10% 22% 22% 10%	3480
	21st to end	Day	46% 6% 10% 18% 17% 3	3815
April	1st to 10th	Day	31% 6% 11% 24% 20% 8%	2223
	11th to 20th	Day	36% 5% 10% 23% 20% 6%	2254
	21st to end	Day	40% 6% 11% 23% 15% 5%	2299
May	1st to 10th	Day	46% 9% 14% 20% 11% 2	4120
	11th to 20th	Day	46% 8% 11% 23% 10% 1	4085
	21st to end	Day	31% 9% 17% 21% 20% 2	4489
June	1st to 10th	Day	33% 9% 16% 26% 14% 1	2660
	11th to 20th	Day	35% 9% 17% 22% 15% 2	2660
	21st to end	Day	34% 9% 17% 23% 14% 2	2570
July	1st to 10th	Day	34% 12% 22% 18% 10% 5%	4055
	11th to 20th	Day	38% 9% 22% 22% 8% 1	4051
	21st to end	Day	47% 10% 12% 19% 12%	4324
August	1st to 10th	Day	46% 8% 17% 22% 6%	4091
	11th to 20th	Day	36% 10% 21% 25% 9% 1	3997
	21st to end	Day	29% 6% 19% 29% 16% 1	4180
September	1st to 10th	Day	35% 7% 13% 18% 21% 4	2280
	11th to 20th	Day	40% 6% 10% 23% 19% 3	2171
	21st to end	Day	48% 5% 10% 17% 18% 3	2040
October	1st to 10th	Day	34% 4 8% 19% 29% 6%	3160
	11th to 20th	Day	24% 3 5% 24% 30% 15%	3140
	21st to end	Day	23% 3 11% 26% 25% 12%	3428
November	1st to 10th	Day	32% 3 8% 25% 26% 6%	1957
	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	37% 4 9% 22% 23% 5%	4109
	11th to 20th	Night	39% 4 8% 19% 25% 5%	4120
	21st to end	Night	45% 4 6% 22% 17% 6%	4532
February	1st to 10th	Night	38% 5% 10% 25% 19% 3	2470
	11th to 20th	Night	42% 4 8% 19% 20% 7%	2405
	21st to end	Night	40% 5% 9% 22% 18% 6%	1779
March	1st to 10th	Night	48% 5% 8% 18% 16% 4	3418
	11th to 20th	Night	32% 6% 6% 18% 27% 11%	3480
	21st to end	Night	39% 4 9% 23% 20% 5%	3817
April	1st to 10th	Night	38% 6% 11% 17% 22% 6%	1956
	11th to 20th	Night	33% 6% 11% 23% 22% 6%	1875
	21st to end	Night	37% 7% 12% 22% 17% 5%	1896
May	1st to 10th	Night	45% 7% 15% 19% 12% 2	2840
	11th to 20th	Night	47% 8% 9% 22% 12% 2	2813
	21st to end	Night	29% 9% 18% 24% 17% 3	3069
June	1st to 10th	Night	33% 10% 16% 25% 15% 1	1520
	11th to 20th	Night	34% 10% 19% 20% 15% 2	1520
	21st to end	Night	33% 9% 16% 21% 17% 4	1601
July	1st to 10th	Night	38% 11% 19% 17% 13% 3	2795
	11th to 20th	Night	41% 10% 22% 19% 7% 1	2786
	21st to end	Night	51% 9% 14% 19% 9%	2988
August	1st to 10th	Night	48% 8% 17% 19% 7%	2814
	11th to 20th	Night	36% 10% 18% 26% 10% 1	2866
	21st to end	Night	35% 7% 13% 31% 11% 2	3476
September	1st to 10th	Night	38% 7% 13% 18% 17% 6%	1900
	11th to 20th	Night	39% 6% 11% 19% 22% 2	2004
	21st to end	Night	49% 5% 8% 17% 16% 4	2138
October	1st to 10th	Night	40% 4 10% 18% 23% 5%	3800
	11th to 20th	Night	25% 4 7% 22% 29% 14%	3779
	21st to end	Night	21% 3 6% 29% 27% 13%	4120
November	1st to 10th	Night	35% 4 8% 20% 25% 7%	2341
	11th to 20th	Night	34% 5% 9% 19% 24% 9%	2393
	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	30% 25% 33% 10% 1	270
	21st to end	Day	25% 21% 40% 12% 2	297
February	1st to 10th	Day	21% 24% 38% 14% 3	270
	11th to 20th	Day	37% 28% 24% 7% 3	270
	21st to end	Day	33% 18% 32% 14% 3	373
March	1st to 10th	Day	24% 18% 34% 18% 5% 1	460
	11th to 20th	Day	22% 25% 31% 18% 4	460
	21st to end	Day	23% 18% 38% 16% 5% 1	506
April	1st to 10th	Day	21% 22% 30% 20% 6% 1	460
	11th to 20th	Day	21% 15% 31% 22% 9% 3	452
	21st to end	Day	20% 13% 36% 22% 8% 1	460
May	1st to 10th	Day	20% 16% 31% 22% 8% 2	463
	11th to 20th	Day	21% 9% 29% 23% 15% 3	487
	21st to end	Day	17% 5% 24% 28% 20% 6%	529
June	1st to 10th	Day	11% 7% 25% 29% 23% 5%	490
	11th to 20th	Day	11% 6% 24% 31% 20% 8%	490
	21st to end	Day	10% 6% 21% 35% 22% 7%	490
July	1st to 10th	Day	10% 4 17% 32% 27% 10%	485
	11th to 20th	Day	6% 2 15% 36% 31% 11%	484
	21st to end	Day	10% 3 20% 35% 22% 9%	532
August	1st to 10th	Day	14% 4 20% 28% 24% 10%	486
	11th to 20th	Day	12% 3 15% 33% 27% 11%	479
	21st to end	Day	10% 2 16% 28% 26% 17%	539
September	1st to 10th	Day	14% 5% 16% 25% 25% 16%	490
	11th to 20th	Day	15% 6% 15% 31% 22% 11%	490
	21st to end	Day	30% 7% 22% 26% 11% 4	490
October	1st to 10th	Day	18% 9% 22% 27% 17% 6%	490
	11th to 20th	Day	13% 13% 23% 28% 19% 3	490
	21st to end	Day	15% 10% 31% 29% 11% 4	539
November	1st to 10th	Day	17% 9% 31% 26% 12% 4	495
	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	19% 19% 34% 25% 2	760
	21st to end	Night	19% 17% 38% 24% 2	836
February	1st to 10th	Night	19% 16% 38% 24% 3	760
	11th to 20th	Night	25% 19% 38% 17% 1	743
	21st to end	Night	22% 17% 34% 24% 4	628
March	1st to 10th	Night	18% 18% 34% 26% 4	760
	11th to 20th	Night	22% 15% 35% 21% 7% 1	760
	21st to end	Night	19% 14% 38% 24% 4	836
April	1st to 10th	Night	17% 19% 36% 23% 5%	760
	11th to 20th	Night	14% 13% 35% 28% 10%	751
	21st to end	Night	17% 9% 35% 32% 7%	756
May	1st to 10th	Night	15% 12% 33% 28% 11% 1	570
	11th to 20th	Night	19% 7% 25% 33% 15% 1	561
	21st to end	Night	16% 2 16% 37% 26% 2	612
June	1st to 10th	Night	9% 2 20% 40% 27% 1	380
	11th to 20th	Night	12% 3 16% 42% 24% 3	380
	21st to end	Night	7% 2 17% 48% 24% 2	461
July	1st to 10th	Night	11% 1 13% 38% 33% 5%	555
	11th to 20th	Night	6% 1 12% 38% 39% 5%	552
	21st to end	Night	9% 2 14% 41% 31% 3	624
August	1st to 10th	Night	13% 2 14% 36% 30% 4	567
	11th to 20th	Night	12% 3 12% 37% 32% 3	592
	21st to end	Night	11% 2 12% 35% 33% 6%	836
September	1st to 10th	Night	16% 2 12% 35% 30% 6%	760
	11th to 20th	Night	16% 4 14% 33% 29% 5%	760
	21st to end	Night	26% 4 18% 32% 19% 2	760
October	1st to 10th	Night	19% 4 20% 26% 24% 6%	760
	11th to 20th	Night	15% 9% 25% 34% 17% 1	760
	21st to end	Night	16% 9% 27% 32% 14% 1	836
November	1st to 10th	Night	15% 8% 29% 33% 13% 2	780
	11th to 20th	Night	18% 12% 29% 29% 12%	763
	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

Page generated at 2024-11-08 06:44:46. Version ac07

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