Clear Lake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Clear Lake clear sky chart from 2005-11-06 to 2026-07-16.

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	6% 4 1 3 2 2 3 2 4 6% 66%	146900
	Night	8% 5% 2 4 3 3 3 3 4 6% 61%	143229
Transparency	Day	22% 4 7% 19% 26% 22%	141784
	Night	26% 5% 7% 18% 23% 21%	139338
Seeing	Day	10% 10% 22% 28% 22% 8%	17471
	Night	10% 7% 16% 30% 29% 7%	26688

By Month

Percentage Cloud Cover SW (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% 2 6% 5% 4 5% 4 6% 7% 41%	4944
	11th to 20th	Day	12% 5% 2 4 3 3 3 3 4 5% 57%	5340
	21st to end	Day	14% 6% 3 6% 3 3 4 4 5% 7% 46%	5843
February	1st to 10th	Day	13% 9% 2 5% 3 4 4 3 6% 6% 44%	1890
	11th to 20th	Day	11% 9% 2 6% 4 4 5% 3 6% 8% 41%	1871
	21st to end	Day	14% 7% 2 5% 4 4 4 3 6% 8% 42%	1730
March	1st to 10th	Day	13% 12% 4 8% 4 4 4 3 4 5% 39%	5031
	11th to 20th	Day	9% 9% 2 5% 4 4 4 3 5% 7% 48%	5110
	21st to end	Day	11% 9% 3 7% 5% 5% 5% 4 7% 8% 37%	5643
April	1st to 10th	Day	9% 9% 2 6% 5% 4 4 3 5% 9% 44%	2520
	11th to 20th	Day	7% 6% 2 4 4 4 4 3 5% 7% 54%	2489
	21st to end	Day	4 4 1 4 4 3 3 4 6% 8% 59%	2520
May	1st to 10th	Day	3 3 1 3 2 2 2 2 4 7% 71%	7570
	11th to 20th	Day	1 1 1 1 1 2 2 3 4 83%	7820
	21st to end	Day	2 1 2 2 2 2 2 4 6% 77%	8992
June	1st to 10th	Day	4 4 1 3 3 3 3 3 5% 7% 65%	2940
	11th to 20th	Day	1 1 1 2 2 1 2 2 4 6% 79%	2940
	21st to end	Day	1 1 1 2 1 1 1 2 3 86%	2940
July	1st to 10th	Day	1 1 1 1 1 1 2 92%	6459
	11th to 20th	Day	1 1 1 1 1 1 2 3 90%	6065
	21st to end	Day	1 1 1 1 1 2 4 89%	6402
August	1st to 10th	Day	1 1 1 1 1 2 2 3 5% 84%	5777
	11th to 20th	Day	1 1 1 1 1 3 92%	5669
	21st to end	Day	1 1 1 1 1 1 2 4 86%	5984
September	1st to 10th	Day	1 1 1 2 2 1 2 4 85%	2400
	11th to 20th	Day	2 1 1 2 2 2 2 2 4 7% 74%	2390
	21st to end	Day	2 1 1 2 1 2 2 2 3 5% 79%	2106
October	1st to 10th	Day	3 2 1 2 1 2 2 2 3 5% 76%	3280
	11th to 20th	Day	5% 3 1 3 2 2 3 2 4 5% 71%	3261
	21st to end	Day	5% 2 1 3 3 4 5% 4 5% 7% 60%	3558
November	1st to 10th	Day	9% 4 1 5% 3 3 4 3 5% 7% 56%	2012
	11th to 20th	Day	9% 7% 2 6% 4 4 4 4 6% 7% 48%	1824
	21st to end	Day	9% 6% 2 4 4 3 3 4 5% 7% 53%	1680
December	1st to 10th	Day	8% 5% 2 4 3 3 3 4 5% 8% 56%	2700
	11th to 20th	Day	22% 11% 3 6% 4 4 5% 5% 6% 7% 27%	3108
	21st to end	Day	19% 10% 3 7% 4 4 5% 5% 7% 8% 27%	4092
January	1st to 10th	Night	13% 10% 3 6% 4 4 4 4 6% 8% 38%	7945
	11th to 20th	Night	10% 6% 2 5% 3 3 3 2 4 5% 57%	7650
	21st to end	Night	14% 7% 3 7% 4 4 5% 4 5% 6% 43%	8371
February	1st to 10th	Night	12% 7% 1 6% 4 4 4 4 5% 5% 48%	2730
	11th to 20th	Night	11% 7% 3 6% 4 4 4 4 7% 7% 43%	2634
	21st to end	Night	10% 7% 2 5% 4 4 4 3 5% 7% 49%	2076
March	1st to 10th	Night	12% 12% 3 7% 4 5% 4 3 5% 7% 38%	5080
	11th to 20th	Night	11% 7% 3 5% 3 3 3 3 5% 6% 51%	5110
	21st to end	Night	11% 12% 3 7% 5% 4 4 4 5% 7% 37%	5572
April	1st to 10th	Night	7% 7% 2 5% 5% 3 4 4 5% 8% 50%	2100
	11th to 20th	Night	7% 4 1 5% 4 3 4 3 6% 8% 54%	2079
	21st to end	Night	4 3 1 3 3 3 4 4 5% 8% 62%	2100
May	1st to 10th	Night	4 3 1 3 3 3 3 2 3 4 71%	5363
	11th to 20th	Night	1 1 1 2 2 1 2 2 3 6% 80%	5043
	21st to end	Night	2 1 1 2 1 1 1 2 3 5% 81%	5181
June	1st to 10th	Night	4 3 1 3 2 3 3 3 5% 8% 66%	1680
	11th to 20th	Night	1 1 1 1 1 2 2 3 4 84%	1680
	21st to end	Night	1 2 2 2 2 2 2 2 3 82%	1680
July	1st to 10th	Night	1 1 1 1 2 93%	3720
	11th to 20th	Night	1 1 1 1 1 1 1 2 91%	4003
	21st to end	Night	1 1 1 1 2 3 5% 86%	4472
August	1st to 10th	Night	2 1 1 1 1 1 2 2 3 86%	4035
	11th to 20th	Night	1 1 1 1 1 2 3 91%	4185
	21st to end	Night	1 1 1 1 1 1 1 2 3 86%	5015
September	1st to 10th	Night	1 1 1 1 1 1 2 5% 87%	2000
	11th to 20th	Night	2 2 1 3 1 2 3 2 4 5% 74%	2005
	21st to end	Night	1 1 1 2 1 2 2 1 3 7% 79%	2292
October	1st to 10th	Night	2 2 1 3 2 2 2 2 2 4 78%	3920
	11th to 20th	Night	3 2 1 4 2 3 3 2 4 6% 71%	3897
	21st to end	Night	5% 3 1 3 2 3 4 4 5% 8% 61%	4253
November	1st to 10th	Night	7% 5% 1 5% 4 4 5% 4 6% 8% 53%	2503
	11th to 20th	Night	9% 6% 1 5% 4 4 4 4 7% 8% 47%	2793
	21st to end	Night	7% 5% 2 5% 4 4 3 3 4 7% 54%	2940
December	1st to 10th	Night	9% 4 2 4 3 3 3 3 5% 7% 56%	4700
	11th to 20th	Night	22% 9% 3 7% 5% 4 5% 4 6% 8% 28%	5316
	21st to end	Night	20% 14% 4 7% 4 4 4 3 4 5% 30%	7106

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	42% 8% 11% 21% 12% 7%	4945
	11th to 20th	Day	32% 3 5% 8% 24% 27%	5340
	21st to end	Day	39% 4 7% 16% 14% 20%	5843
February	1st to 10th	Day	41% 5% 7% 18% 19% 10%	1890
	11th to 20th	Day	42% 6% 9% 19% 18% 7%	1880
	21st to end	Day	40% 6% 10% 21% 16% 7%	1730
March	1st to 10th	Day	48% 5% 6% 18% 18% 4	5031
	11th to 20th	Day	35% 7% 11% 25% 13% 9%	5110
	21st to end	Day	43% 9% 12% 22% 11% 2	5643
April	1st to 10th	Day	39% 6% 11% 23% 17% 4	2520
	11th to 20th	Day	31% 6% 13% 22% 21% 8%	2492
	21st to end	Day	23% 7% 13% 30% 21% 6%	2520
May	1st to 10th	Day	15% 4 9% 26% 31% 15%	7570
	11th to 20th	Day	8% 2 5% 27% 41% 16%	7823
	21st to end	Day	10% 3 7% 33% 42% 5%	8994
June	1st to 10th	Day	19% 4 11% 28% 29% 9%	2940
	11th to 20th	Day	9% 3 8% 22% 36% 22%	2940
	21st to end	Day	8% 2 3 16% 37% 34%	2940
July	1st to 10th	Day	4 1 1 7% 23% 64%	6391
	11th to 20th	Day	4 1 2 5% 16% 73%	5993
	21st to end	Day	4 1 5% 18% 27% 45%	6217
August	1st to 10th	Day	8% 2 3 11% 27% 50%	4206
	11th to 20th	Day	3 1 2 12% 43% 39%	4104
	21st to end	Day	6% 2 3 12% 38% 38%	4311
September	1st to 10th	Day	6% 2 3 11% 30% 49%	2400
	11th to 20th	Day	13% 4 5% 25% 37% 16%	2390
	21st to end	Day	10% 3 5% 20% 34% 28%	2106
October	1st to 10th	Day	13% 2 4 13% 25% 43%	3280
	11th to 20th	Day	17% 4 5% 25% 31% 19%	3261
	21st to end	Day	22% 6% 9% 20% 30% 13%	3558
November	1st to 10th	Day	28% 7% 7% 26% 20% 13%	2012
	11th to 20th	Day	36% 6% 11% 20% 19% 9%	1824
	21st to end	Day	31% 6% 9% 18% 20% 16%	1680
December	1st to 10th	Day	27% 5% 7% 20% 30% 12%	2700
	11th to 20th	Day	54% 6% 7% 15% 15% 4	3108
	21st to end	Day	52% 8% 10% 16% 10% 3	4092
January	1st to 10th	Night	43% 8% 15% 17% 13% 4	7945
	11th to 20th	Night	32% 3 5% 12% 23% 25%	7650
	21st to end	Night	41% 5% 6% 13% 17% 18%	8371
February	1st to 10th	Night	38% 6% 7% 17% 20% 13%	2730
	11th to 20th	Night	39% 6% 9% 20% 19% 7%	2636
	21st to end	Night	34% 6% 13% 23% 19% 6%	2076
March	1st to 10th	Night	45% 8% 9% 17% 16% 5%	5080
	11th to 20th	Night	33% 8% 8% 24% 19% 8%	5110
	21st to end	Night	43% 10% 12% 19% 13% 2	5572
April	1st to 10th	Night	34% 7% 13% 23% 18% 5%	2100
	11th to 20th	Night	27% 7% 13% 25% 19% 9%	2079
	21st to end	Night	22% 6% 12% 31% 25% 5%	2100
May	1st to 10th	Night	19% 4 5% 22% 33% 17%	5361
	11th to 20th	Night	10% 3 5% 24% 38% 19%	5043
	21st to end	Night	9% 3 8% 32% 42% 7%	5182
June	1st to 10th	Night	19% 5% 10% 23% 34% 10%	1680
	11th to 20th	Night	8% 3 8% 20% 36% 25%	1680
	21st to end	Night	10% 3 3 13% 34% 37%	1680
July	1st to 10th	Night	3 1 1 6% 25% 64%	3680
	11th to 20th	Night	4 1 2 4 13% 76%	3953
	21st to end	Night	3 2 5% 16% 26% 48%	4345
August	1st to 10th	Night	9% 2 2 8% 26% 54%	2929
	11th to 20th	Night	3 1 3 7% 41% 44%	3023
	21st to end	Night	6% 2 3 11% 36% 41%	3608
September	1st to 10th	Night	4 1 3 11% 28% 53%	2000
	11th to 20th	Night	14% 3 5% 19% 36% 23%	2005
	21st to end	Night	9% 4 4 21% 29% 33%	2292
October	1st to 10th	Night	13% 2 4 12% 22% 47%	3920
	11th to 20th	Night	15% 5% 8% 25% 28% 19%	3897
	21st to end	Night	21% 6% 7% 22% 25% 19%	4253
November	1st to 10th	Night	29% 7% 10% 21% 18% 13%	2503
	11th to 20th	Night	32% 6% 15% 19% 19% 9%	2793
	21st to end	Night	31% 5% 8% 18% 23% 15%	2940
December	1st to 10th	Night	28% 5% 5% 22% 26% 15%	4700
	11th to 20th	Night	51% 7% 7% 14% 15% 4	5316
	21st to end	Night	55% 6% 10% 16% 10% 3	7106

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	29% 14% 24% 17% 14% 2	299
	11th to 20th	Day	21% 12% 20% 26% 16% 5%	430
	21st to end	Day	14% 15% 21% 28% 16% 6%	473
February	1st to 10th	Day	21% 13% 23% 23% 17% 3	430
	11th to 20th	Day	23% 21% 23% 20% 14% 1	439
	21st to end	Day	19% 14% 24% 29% 13% 2	446
March	1st to 10th	Day	23% 18% 27% 21% 9% 2	540
	11th to 20th	Day	21% 15% 24% 21% 16% 3	540
	21st to end	Day	17% 17% 33% 22% 9% 2	594
April	1st to 10th	Day	16% 15% 28% 23% 14% 5%	540
	11th to 20th	Day	15% 18% 26% 21% 16% 5%	532
	21st to end	Day	7% 17% 30% 23% 17% 6%	540
May	1st to 10th	Day	9% 15% 29% 26% 16% 6%	543
	11th to 20th	Day	9% 14% 29% 23% 16% 8%	568
	21st to end	Day	6% 11% 22% 30% 24% 7%	617
June	1st to 10th	Day	5% 12% 23% 32% 19% 8%	570
	11th to 20th	Day	5% 10% 30% 29% 22% 4	570
	21st to end	Day	1 7% 24% 32% 27% 9%	570
July	1st to 10th	Day	2 2 18% 40% 29% 9%	566
	11th to 20th	Day	1 1 16% 37% 33% 13%	545
	21st to end	Day	1 9% 42% 31% 17%	575
August	1st to 10th	Day	2 1 17% 35% 32% 12%	527
	11th to 20th	Day	2 3 17% 36% 29% 14%	517
	21st to end	Day	1 4 13% 33% 35% 15%	583
September	1st to 10th	Day	1 3 16% 37% 35% 8%	530
	11th to 20th	Day	4 6% 18% 33% 29% 10%	530
	21st to end	Day	2 7% 20% 34% 26% 10%	530
October	1st to 10th	Day	6% 8% 23% 26% 26% 12%	530
	11th to 20th	Day	6% 12% 17% 26% 27% 13%	530
	21st to end	Day	10% 10% 17% 30% 25% 8%	583
November	1st to 10th	Day	11% 10% 22% 21% 26% 10%	488
	11th to 20th	Day	12% 10% 17% 27% 21% 13%	294
	21st to end	Day	12% 9% 21% 26% 21% 10%	220
December	1st to 10th	Day	18% 9% 19% 23% 19% 12%	220
	11th to 20th	Day	27% 13% 16% 20% 14% 10%	220
	21st to end	Day	22% 16% 20% 25% 13% 4	242
January	1st to 10th	Night	21% 9% 15% 28% 21% 5%	840
	11th to 20th	Night	19% 8% 15% 32% 22% 5%	840
	21st to end	Night	16% 7% 17% 33% 22% 5%	924
February	1st to 10th	Night	18% 8% 15% 34% 21% 3	840
	11th to 20th	Night	17% 11% 19% 28% 20% 4	824
	21st to end	Night	16% 12% 20% 32% 17% 2	692
March	1st to 10th	Night	20% 11% 18% 29% 18% 3	840
	11th to 20th	Night	17% 10% 17% 30% 22% 3	840
	21st to end	Night	15% 14% 25% 28% 15% 3	924
April	1st to 10th	Night	11% 12% 25% 28% 20% 4	840
	11th to 20th	Night	10% 13% 22% 30% 20% 5%	831
	21st to end	Night	7% 11% 27% 28% 22% 5%	840
May	1st to 10th	Night	6% 9% 24% 30% 24% 6%	646
	11th to 20th	Night	5% 7% 21% 31% 31% 6%	538
	21st to end	Night	4 9% 20% 31% 32% 4	455
June	1st to 10th	Night	5% 8% 21% 33% 30% 3	420
	11th to 20th	Night	3 5% 23% 34% 33% 1	420
	21st to end	Night	2 3 11% 40% 39% 4	420
July	1st to 10th	Night	1 1 14% 36% 44% 5%	410
	11th to 20th	Night	1 9% 27% 50% 12%	537
	21st to end	Night	6% 25% 51% 17%	657
August	1st to 10th	Night	2 1 7% 28% 45% 17%	597
	11th to 20th	Night	1 12% 31% 42% 14%	649
	21st to end	Night	1 2 7% 27% 52% 11%	880
September	1st to 10th	Night	1 2 9% 31% 47% 10%	800
	11th to 20th	Night	3 3 12% 32% 36% 14%	800
	21st to end	Night	2 5% 15% 29% 36% 14%	800
October	1st to 10th	Night	5% 4 16% 29% 35% 11%	800
	11th to 20th	Night	5% 6% 14% 26% 40% 9%	800
	21st to end	Night	9% 6% 12% 30% 30% 13%	880
November	1st to 10th	Night	10% 9% 14% 26% 32% 8%	820
	11th to 20th	Night	12% 7% 16% 32% 28% 5%	840
	21st to end	Night	10% 10% 16% 34% 24% 7%	840
December	1st to 10th	Night	19% 7% 14% 31% 22% 7%	840
	11th to 20th	Night	24% 10% 13% 28% 21% 5%	840
	21st to end	Night	23% 10% 19% 30% 16% 2	924

Page generated at 2026-07-13 00:14:51. Version ac07

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