Twin Lakes Preserve Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Twin Lakes Preserve Observatory clear sky chart from 2005-11-06 to 2025-11-03.

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	16% 8% 3 7% 5% 4 5% 4 6% 8% 36%	126958
	Night	16% 7% 3 7% 4 4 4 4 5% 7% 39%	123086
Transparency	Day	46% 4 11% 23% 15% 2	130447
	Night	44% 3 12% 23% 15% 2	127671
Seeing	Day	24% 17% 28% 19% 9% 3	16167
	Night	23% 15% 28% 24% 9% 1	25313

By Month

Percentage Cloud Cover NE (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	11% 9% 4 11% 5% 5% 5% 4 5% 7% 34%	3597
	11th to 20th	Day	13% 12% 4 12% 6% 5% 4 4 5% 5% 30%	3570
	21st to end	Day	16% 9% 4 7% 4 5% 5% 4 5% 5% 37%	4391
February	1st to 10th	Day	15% 10% 3 8% 7% 5% 5% 5% 6% 6% 32%	1800
	11th to 20th	Day	13% 10% 3 8% 6% 4 4 4 5% 7% 36%	1889
	21st to end	Day	18% 10% 2 7% 6% 4 5% 3 5% 6% 33%	1650
March	1st to 10th	Day	20% 8% 3 7% 5% 4 4 3 5% 4 37%	4476
	11th to 20th	Day	11% 8% 3 7% 5% 4 4 4 6% 7% 40%	4906
	21st to end	Day	17% 8% 2 6% 4 4 4 3 5% 7% 39%	5832
April	1st to 10th	Day	16% 11% 3 7% 5% 4 5% 4 5% 6% 35%	2400
	11th to 20th	Day	17% 9% 3 6% 4 4 4 3 5% 6% 38%	2374
	21st to end	Day	17% 6% 2 7% 5% 6% 4 4 5% 7% 37%	2400
May	1st to 10th	Day	29% 6% 2 6% 4 4 4 4 5% 7% 28%	6050
	11th to 20th	Day	22% 8% 3 6% 5% 4 5% 4 5% 6% 32%	6318
	21st to end	Day	20% 6% 2 5% 4 4 5% 4 7% 9% 32%	6949
June	1st to 10th	Day	15% 6% 2 7% 5% 5% 5% 4 6% 9% 35%	2800
	11th to 20th	Day	16% 8% 2 6% 5% 5% 5% 4 6% 8% 34%	2800
	21st to end	Day	13% 7% 2 7% 5% 4 5% 5% 6% 9% 37%	2800
July	1st to 10th	Day	9% 6% 2 8% 5% 6% 6% 4 7% 10% 36%	6360
	11th to 20th	Day	11% 6% 2 7% 5% 5% 6% 5% 8% 11% 33%	6360
	21st to end	Day	13% 7% 2 6% 4 4 4 4 6% 9% 40%	6926
August	1st to 10th	Day	21% 5% 2 7% 5% 5% 6% 5% 6% 7% 31%	4310
	11th to 20th	Day	13% 5% 2 7% 4 5% 5% 5% 6% 9% 39%	3883
	21st to end	Day	16% 5% 2 5% 3 3 4 4 6% 10% 43%	4312
September	1st to 10th	Day	15% 6% 2 6% 5% 4 5% 3 5% 8% 41%	2400
	11th to 20th	Day	13% 5% 1 5% 3 4 4 3 6% 8% 49%	2355
	21st to end	Day	18% 6% 3 7% 5% 4 4 4 5% 7% 38%	2200
October	1st to 10th	Day	18% 7% 2 5% 4 4 4 3 5% 6% 41%	2019
	11th to 20th	Day	14% 8% 2 6% 5% 4 5% 4 5% 7% 41%	2000
	21st to end	Day	21% 7% 2 6% 4 4 5% 4 5% 7% 36%	2076
November	1st to 10th	Day	13% 7% 2 5% 4 4 3 2 4 7% 49%	1779
	11th to 20th	Day	16% 8% 2 6% 4 3 4 4 5% 7% 40%	1800
	21st to end	Day	17% 8% 3 6% 5% 6% 5% 3 6% 6% 36%	1630
December	1st to 10th	Day	13% 10% 3 7% 4 5% 4 4 6% 7% 36%	2600
	11th to 20th	Day	20% 10% 3 6% 4 3 3 3 5% 6% 37%	2986
	21st to end	Day	16% 11% 3 10% 5% 4 5% 3 6% 7% 30%	3960
January	1st to 10th	Night	12% 7% 3 9% 6% 6% 5% 4 6% 8% 35%	6349
	11th to 20th	Night	11% 9% 3 11% 6% 5% 5% 4 6% 6% 33%	6315
	21st to end	Night	15% 9% 3 6% 5% 5% 5% 4 6% 7% 35%	6564
February	1st to 10th	Night	15% 9% 3 6% 5% 4 4 4 6% 7% 36%	2600
	11th to 20th	Night	14% 8% 2 8% 6% 4 5% 5% 7% 7% 35%	2395
	21st to end	Night	18% 8% 3 6% 4 5% 4 4 5% 6% 36%	1980
March	1st to 10th	Night	18% 9% 3 6% 4 3 3 3 5% 6% 39%	5364
	11th to 20th	Night	13% 7% 3 6% 4 4 4 3 6% 7% 43%	5036
	21st to end	Night	15% 8% 3 7% 5% 5% 4 4 5% 6% 39%	5100
April	1st to 10th	Night	16% 9% 3 8% 6% 4 4 3 6% 6% 33%	2000
	11th to 20th	Night	19% 7% 2 6% 4 4 5% 4 5% 7% 37%	1975
	21st to end	Night	19% 6% 2 6% 5% 4 4 3 5% 6% 41%	2000
May	1st to 10th	Night	32% 7% 2 5% 3 3 4 3 5% 6% 28%	3931
	11th to 20th	Night	23% 9% 3 7% 3 4 4 3 4 6% 34%	3575
	21st to end	Night	21% 7% 2 6% 4 4 5% 4 6% 7% 34%	3926
June	1st to 10th	Night	16% 7% 2 5% 4 5% 4 4 5% 8% 41%	1600
	11th to 20th	Night	14% 6% 2 6% 4 5% 4 4 5% 9% 41%	1600
	21st to end	Night	11% 4 2 5% 3 4 5% 5% 7% 9% 45%	1600
July	1st to 10th	Night	10% 6% 2 8% 4 5% 5% 3 6% 9% 41%	3600
	11th to 20th	Night	12% 7% 2 7% 5% 5% 5% 5% 7% 9% 36%	3600
	21st to end	Night	13% 5% 2 4 3 3 4 4 5% 7% 49%	3926
August	1st to 10th	Night	20% 5% 2 6% 4 4 4 4 7% 8% 35%	2836
	11th to 20th	Night	15% 6% 2 7% 5% 4 4 3 5% 6% 43%	3131
	21st to end	Night	12% 5% 2 5% 4 3 4 3 5% 7% 51%	3586
September	1st to 10th	Night	14% 6% 2 5% 3 3 4 3 6% 8% 46%	2000
	11th to 20th	Night	14% 4 2 6% 3 3 3 2 5% 6% 53%	2045
	21st to end	Night	17% 6% 2 5% 4 4 4 4 5% 6% 42%	2200
October	1st to 10th	Night	20% 6% 2 5% 3 4 3 3 4 6% 44%	2377
	11th to 20th	Night	18% 6% 2 5% 3 3 3 3 5% 6% 46%	2400
	21st to end	Night	20% 7% 2 5% 3 3 3 3 5% 8% 40%	2762
November	1st to 10th	Night	12% 6% 2 5% 3 4 4 3 5% 6% 48%	2564
	11th to 20th	Night	17% 7% 2 5% 4 3 4 3 5% 6% 43%	2600
	21st to end	Night	14% 7% 2 6% 5% 4 4 3 6% 7% 40%	2765
December	1st to 10th	Night	14% 8% 3 7% 4 4 4 3 6% 8% 39%	4580
	11th to 20th	Night	23% 8% 2 6% 3 4 4 3 4 5% 37%	5208
	21st to end	Night	19% 10% 4 8% 5% 4 5% 3 5% 6% 31%	6996

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	47% 3 8% 28% 12% 2	3598
	11th to 20th	Day	53% 3 9% 20% 11% 4	3600
	21st to end	Day	48% 3 9% 24% 15% 1	4391
February	1st to 10th	Day	51% 4 11% 20% 14% 1	1800
	11th to 20th	Day	48% 3 8% 22% 17% 2	1898
	21st to end	Day	50% 2 9% 23% 13% 3	1650
March	1st to 10th	Day	49% 2 9% 20% 13% 7%	4479
	11th to 20th	Day	44% 2 12% 29% 9% 4	4906
	21st to end	Day	46% 3 9% 12% 24% 6%	5832
April	1st to 10th	Day	49% 3 8% 21% 15% 4	2400
	11th to 20th	Day	47% 2 10% 21% 17% 3	2374
	21st to end	Day	47% 3 8% 23% 15% 4	2400
May	1st to 10th	Day	56% 4 13% 16% 10% 2	6061
	11th to 20th	Day	54% 3 8% 21% 12% 1	6322
	21st to end	Day	49% 4 10% 22% 13% 2	6949
June	1st to 10th	Day	46% 5% 11% 22% 15% 1	2800
	11th to 20th	Day	46% 5% 12% 24% 11% 2	2800
	21st to end	Day	43% 4 12% 26% 14% 1	2800
July	1st to 10th	Day	44% 7% 12% 22% 14% 1	6290
	11th to 20th	Day	42% 7% 16% 23% 10%	6285
	21st to end	Day	41% 5% 11% 23% 20% 1	6728
August	1st to 10th	Day	51% 5% 13% 21% 10%	4296
	11th to 20th	Day	40% 5% 14% 28% 11%	3920
	21st to end	Day	36% 4 11% 33% 15% 2	4312
September	1st to 10th	Day	40% 5% 11% 24% 18% 2	2400
	11th to 20th	Day	36% 2 9% 26% 25% 3	2355
	21st to end	Day	46% 4 9% 19% 19% 2	2200
October	1st to 10th	Day	40% 4 7% 21% 23% 4	3279
	11th to 20th	Day	33% 3 8% 29% 23% 3	3240
	21st to end	Day	31% 3 16% 27% 21% 3	3327
November	1st to 10th	Day	36% 3 13% 26% 20% 3	1779
	11th to 20th	Day	41% 2 10% 26% 18% 3	1800
	21st to end	Day	45% 5% 13% 26% 10% 2	1630
December	1st to 10th	Day	46% 4 15% 25% 9% 1	2600
	11th to 20th	Day	48% 2 14% 22% 12% 2	2986
	21st to end	Day	55% 3 9% 19% 14% 1	3960
January	1st to 10th	Night	46% 3 9% 25% 15% 2	6349
	11th to 20th	Night	48% 4 13% 19% 12% 4	6360
	21st to end	Night	47% 2 10% 20% 17% 4	6564
February	1st to 10th	Night	46% 3 14% 24% 12% 1	2600
	11th to 20th	Night	46% 3 9% 22% 17% 4	2397
	21st to end	Night	47% 3 7% 24% 16% 3	1980
March	1st to 10th	Night	47% 2 11% 21% 14% 5%	5374
	11th to 20th	Night	41% 2 16% 25% 12% 4	5037
	21st to end	Night	47% 3 7% 18% 22% 4	5100
April	1st to 10th	Night	51% 3 9% 20% 14% 3	2000
	11th to 20th	Night	47% 4 10% 20% 19% 1	1975
	21st to end	Night	44% 3 9% 23% 17% 4	2000
May	1st to 10th	Night	57% 3 12% 17% 10% 1	3937
	11th to 20th	Night	54% 3 12% 17% 13% 1	3576
	21st to end	Night	48% 4 13% 23% 11% 1	3927
June	1st to 10th	Night	42% 4 10% 24% 19% 1	1600
	11th to 20th	Night	41% 5% 15% 27% 11% 1	1600
	21st to end	Night	35% 4 16% 29% 14% 1	1600
July	1st to 10th	Night	41% 7% 15% 20% 16% 2	3560
	11th to 20th	Night	44% 8% 14% 22% 11% 1	3552
	21st to end	Night	35% 6% 12% 27% 19% 1	3809
August	1st to 10th	Night	48% 5% 16% 20% 9% 1	2826
	11th to 20th	Night	42% 5% 14% 28% 10%	3160
	21st to end	Night	33% 4 11% 34% 17% 1	3586
September	1st to 10th	Night	37% 4 12% 27% 19% 1	2000
	11th to 20th	Night	34% 3 9% 26% 25% 2	2045
	21st to end	Night	42% 3 9% 24% 21% 1	2200
October	1st to 10th	Night	41% 4 9% 24% 20% 1	3884
	11th to 20th	Night	31% 2 10% 28% 25% 3	3899
	21st to end	Night	31% 3 15% 30% 19% 1	4461
November	1st to 10th	Night	35% 4 13% 24% 22% 2	2564
	11th to 20th	Night	42% 2 10% 23% 20% 3	2600
	21st to end	Night	41% 2 14% 29% 12% 1	2765
December	1st to 10th	Night	43% 3 12% 32% 8% 1	4580
	11th to 20th	Night	50% 2 12% 20% 14% 2	5208
	21st to end	Night	54% 3 10% 19% 12% 2	6996

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	35% 25% 27% 12% 2 1	200
	11th to 20th	Day	44% 22% 26% 7% 1	200
	21st to end	Day	28% 31% 30% 7% 4 1	313
February	1st to 10th	Day	34% 30% 29% 7% 1	300
	11th to 20th	Day	39% 27% 26% 7% 1	437
	21st to end	Day	33% 25% 29% 12% 2	414
March	1st to 10th	Day	31% 23% 29% 14% 3	500
	11th to 20th	Day	31% 26% 28% 12% 3 1	500
	21st to end	Day	31% 26% 28% 9% 3 2	550
April	1st to 10th	Day	34% 26% 23% 12% 5%	500
	11th to 20th	Day	32% 23% 24% 12% 8% 1	492
	21st to end	Day	29% 20% 31% 13% 4 2	500
May	1st to 10th	Day	35% 18% 24% 13% 7% 2	503
	11th to 20th	Day	23% 20% 28% 17% 9% 2	527
	21st to end	Day	21% 12% 28% 23% 12% 3	573
June	1st to 10th	Day	18% 15% 30% 24% 10% 2	530
	11th to 20th	Day	23% 14% 24% 25% 13% 2	530
	21st to end	Day	17% 13% 33% 26% 10% 2	530
July	1st to 10th	Day	13% 7% 32% 28% 14% 6%	525
	11th to 20th	Day	12% 7% 27% 30% 20% 3	524
	21st to end	Day	14% 11% 33% 28% 11% 4	576
August	1st to 10th	Day	14% 8% 25% 29% 18% 6%	526
	11th to 20th	Day	16% 7% 25% 27% 19% 6%	519
	21st to end	Day	13% 12% 30% 24% 14% 8%	583
September	1st to 10th	Day	21% 10% 25% 22% 15% 7%	530
	11th to 20th	Day	14% 8% 25% 25% 16% 12%	530
	21st to end	Day	22% 11% 28% 23% 12% 5%	530
October	1st to 10th	Day	26% 11% 23% 20% 16% 5%	530
	11th to 20th	Day	22% 18% 28% 21% 9% 2	530
	21st to end	Day	29% 15% 29% 19% 8%	514
November	1st to 10th	Day	21% 18% 25% 24% 9% 2	401
	11th to 20th	Day	27% 22% 29% 19% 4 1	400
	21st to end	Day	30% 22% 30% 14% 3	230
December	1st to 10th	Day	32% 24% 30% 10% 5% 1	200
	11th to 20th	Day	40% 22% 27% 10% 2	200
	21st to end	Day	30% 22% 27% 15% 6%	220
January	1st to 10th	Night	24% 26% 34% 14% 2	800
	11th to 20th	Night	30% 18% 30% 20% 3	800
	21st to end	Night	22% 23% 36% 16% 2	880
February	1st to 10th	Night	26% 21% 33% 18% 2	800
	11th to 20th	Night	27% 27% 34% 12% 1	783
	21st to end	Night	30% 22% 30% 15% 3	660
March	1st to 10th	Night	27% 21% 34% 15% 3	800
	11th to 20th	Night	26% 22% 29% 19% 4	800
	21st to end	Night	23% 23% 33% 16% 4	880
April	1st to 10th	Night	25% 22% 32% 18% 3	800
	11th to 20th	Night	25% 20% 30% 20% 6%	791
	21st to end	Night	25% 17% 30% 22% 6%	800
May	1st to 10th	Night	31% 13% 27% 23% 6% 1	555
	11th to 20th	Night	23% 10% 33% 24% 9% 1	394
	21st to end	Night	18% 7% 24% 37% 13%	433
June	1st to 10th	Night	18% 7% 24% 38% 14%	400
	11th to 20th	Night	18% 7% 28% 35% 13%	400
	21st to end	Night	12% 5% 26% 44% 13% 1	400
July	1st to 10th	Night	14% 5% 20% 41% 19% 2	390
	11th to 20th	Night	13% 4 22% 41% 18% 2	388
	21st to end	Night	14% 5% 21% 41% 18% 1	438
August	1st to 10th	Night	13% 4 19% 40% 22% 2	544
	11th to 20th	Night	15% 4 18% 36% 25% 2	748
	21st to end	Night	15% 5% 21% 34% 23% 2	880
September	1st to 10th	Night	17% 5% 26% 30% 18% 2	800
	11th to 20th	Night	16% 7% 22% 30% 21% 5%	800
	21st to end	Night	19% 9% 23% 30% 17% 2	800
October	1st to 10th	Night	25% 9% 22% 25% 18% 2	800
	11th to 20th	Night	23% 14% 27% 28% 8%	800
	21st to end	Night	26% 15% 31% 21% 7%	880
November	1st to 10th	Night	19% 17% 28% 26% 8% 1	789
	11th to 20th	Night	26% 19% 26% 22% 7% 1	800
	21st to end	Night	22% 20% 31% 20% 6% 1	800
December	1st to 10th	Night	27% 20% 30% 19% 4	800
	11th to 20th	Night	27% 21% 29% 20% 3	800
	21st to end	Night	27% 21% 31% 18% 3	880

Page generated at 2025-10-31 02:02:40. Version ac07

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