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 2025-03-12.

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	7% 4 1 4 3 2 3 3 4 6% 64%	112917
	Night	8% 5% 2 4 3 3 3 3 4 6% 60%	114869
Transparency	Day	24% 4 7% 19% 24% 23%	112675
	Night	27% 5% 8% 17% 22% 21%	114701
Seeing	Day	10% 10% 21% 28% 22% 8%	15660
	Night	11% 7% 16% 30% 29% 7%	25025

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	10% 7% 2 6% 6% 4 5% 4 6% 7% 44%	3805
	11th to 20th	Day	15% 7% 3 5% 3 3 2 2 3 4 52%	4100
	21st to end	Day	18% 6% 2 5% 3 2 3 3 4 6% 48%	4510
February	1st to 10th	Day	14% 9% 2 5% 3 4 4 3 6% 6% 44%	1800
	11th to 20th	Day	11% 9% 2 6% 4 4 5% 3 6% 8% 42%	1780
	21st to end	Day	14% 6% 2 5% 4 4 4 3 6% 8% 44%	1650
March	1st to 10th	Day	16% 14% 4 9% 5% 4 4 3 4 5% 31%	3534
	11th to 20th	Day	9% 7% 2 5% 3 3 3 2 4 7% 55%	3503
	21st to end	Day	14% 9% 3 7% 5% 4 5% 4 6% 6% 35%	3847
April	1st to 10th	Day	10% 9% 2 6% 5% 4 4 3 5% 9% 43%	2280
	11th to 20th	Day	7% 6% 2 4 4 4 4 3 5% 7% 54%	2249
	21st to end	Day	4 4 2 4 4 3 3 4 6% 8% 59%	2280
May	1st to 10th	Day	4 3 1 2 2 2 2 2 4 6% 72%	4079
	11th to 20th	Day	3 2 1 2 1 2 2 2 4 4 78%	4192
	21st to end	Day	3 1 1 2 2 2 2 2 4 6% 75%	4813
June	1st to 10th	Day	4 4 2 3 3 3 3 3 5% 8% 63%	2660
	11th to 20th	Day	1 1 1 2 2 2 2 2 4 6% 78%	2660
	21st to end	Day	2 1 1 2 1 2 1 2 3 85%	2660
July	1st to 10th	Day	1 1 1 1 1 1 3 92%	4420
	11th to 20th	Day	1 1 1 1 2 1 2 3 87%	4187
	21st to end	Day	1 1 1 1 1 2 3 91%	4455
August	1st to 10th	Day	1 1 1 1 1 2 3 4 6% 79%	4091
	11th to 20th	Day	1 1 1 1 1 2 3 90%	3938
	21st to end	Day	1 1 1 1 1 1 1 2 4 88%	4180
September	1st to 10th	Day	1 1 2 2 1 2 4 86%	2280
	11th to 20th	Day	2 1 1 2 2 2 2 2 4 7% 74%	2271
	21st to end	Day	2 1 1 2 1 2 2 2 3 5% 80%	2001
October	1st to 10th	Day	3 2 1 2 1 2 2 2 3 5% 77%	3180
	11th to 20th	Day	5% 2 1 3 2 2 3 2 4 6% 72%	3161
	21st to end	Day	5% 2 1 3 3 4 5% 4 5% 7% 60%	3448
November	1st to 10th	Day	9% 4 1 5% 3 3 4 3 5% 7% 56%	1914
	11th to 20th	Day	9% 6% 2 6% 4 4 4 3 5% 7% 49%	1737
	21st to end	Day	10% 6% 2 4 4 3 3 4 6% 7% 52%	1600
December	1st to 10th	Day	8% 5% 2 4 3 3 3 4 5% 8% 55%	2620
	11th to 20th	Day	23% 11% 3 6% 4 4 5% 4 6% 7% 27%	3028
	21st to end	Day	19% 10% 3 7% 4 4 5% 5% 7% 8% 27%	4004
January	1st to 10th	Night	9% 7% 2 6% 5% 4 5% 4 7% 9% 42%	6131
	11th to 20th	Night	13% 8% 2 6% 4 4 4 2 3 4 51%	5880
	21st to end	Night	16% 7% 2 6% 3 3 4 3 4 5% 46%	6468
February	1st to 10th	Night	13% 7% 2 6% 4 4 4 4 5% 5% 48%	2600
	11th to 20th	Night	10% 7% 3 6% 4 4 4 4 7% 7% 44%	2506
	21st to end	Night	10% 7% 2 5% 4 4 4 3 5% 7% 51%	1980
March	1st to 10th	Night	14% 14% 3 7% 5% 5% 4 3 5% 7% 32%	3557
	11th to 20th	Night	10% 6% 2 4 3 2 2 2 4 5% 59%	3505
	21st to end	Night	11% 14% 4 8% 5% 4 4 4 5% 6% 36%	3804
April	1st to 10th	Night	7% 8% 2 5% 5% 4 3 3 5% 8% 50%	1900
	11th to 20th	Night	7% 4 1 5% 3 3 4 3 6% 8% 55%	1879
	21st to end	Night	4 2 1 3 3 3 3 3 5% 8% 64%	1900
May	1st to 10th	Night	4 4 1 3 2 2 2 2 3 4 73%	2874
	11th to 20th	Night	1 1 1 2 2 1 2 2 3 5% 81%	2693
	21st to end	Night	2 1 2 1 1 2 2 3 5% 81%	2762
June	1st to 10th	Night	5% 3 1 3 2 3 3 3 5% 8% 63%	1520
	11th to 20th	Night	1 2 1 1 1 2 2 3 4 83%	1520
	21st to end	Night	1 2 1 2 2 1 2 2 3 3 81%	1520
July	1st to 10th	Night	1 1 1 2 93%	2540
	11th to 20th	Night	1 1 1 1 1 2 1 2 2 88%	2763
	21st to end	Night	1 1 1 1 2 2 4 89%	3109
August	1st to 10th	Night	2 1 1 2 1 1 2 2 3 4 81%	2848
	11th to 20th	Night	1 1 1 1 1 2 2 91%	2903
	21st to end	Night	1 1 1 1 1 1 1 2 3 87%	3498
September	1st to 10th	Night	1 1 1 1 1 2 5% 88%	1900
	11th to 20th	Night	2 2 1 3 1 2 3 2 4 6% 75%	1905
	21st to end	Night	1 1 1 1 1 2 2 1 4 7% 79%	2177
October	1st to 10th	Night	2 2 1 2 2 2 2 2 2 4 79%	3800
	11th to 20th	Night	3 2 1 3 2 3 3 3 4 6% 71%	3777
	21st to end	Night	5% 3 1 3 2 3 4 4 5% 8% 61%	4121
November	1st to 10th	Night	7% 5% 1 5% 4 4 5% 4 6% 8% 52%	2381
	11th to 20th	Night	9% 5% 1 4 4 4 4 4 7% 8% 49%	2660
	21st to end	Night	8% 5% 3 5% 4 4 3 3 4 7% 54%	2800
December	1st to 10th	Night	9% 5% 2 4 3 3 3 3 5% 7% 56%	4560
	11th to 20th	Night	22% 9% 2 6% 5% 4 5% 4 6% 8% 27%	5176
	21st to end	Night	20% 14% 4 7% 4 4 4 3 4 5% 30%	6952

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% 7% 12% 20% 12% 8%	3805
	11th to 20th	Day	38% 4 6% 10% 25% 17%	4100
	21st to end	Day	40% 3 6% 13% 14% 24%	4510
February	1st to 10th	Day	42% 5% 7% 18% 19% 10%	1800
	11th to 20th	Day	41% 5% 8% 19% 19% 8%	1789
	21st to end	Day	39% 6% 9% 22% 16% 7%	1650
March	1st to 10th	Day	57% 6% 7% 15% 12% 3	3534
	11th to 20th	Day	31% 5% 9% 28% 17% 11%	3503
	21st to end	Day	48% 7% 12% 20% 11% 2	3847
April	1st to 10th	Day	41% 6% 11% 23% 15% 4	2280
	11th to 20th	Day	31% 6% 13% 23% 21% 7%	2252
	21st to end	Day	22% 7% 14% 29% 22% 6%	2280
May	1st to 10th	Day	16% 5% 12% 30% 22% 16%	4079
	11th to 20th	Day	12% 3 5% 23% 42% 15%	4195
	21st to end	Day	12% 4 10% 43% 27% 5%	4814
June	1st to 10th	Day	21% 4 12% 29% 26% 8%	2660
	11th to 20th	Day	9% 3 9% 23% 36% 20%	2660
	21st to end	Day	8% 2 3 16% 37% 34%	2660
July	1st to 10th	Day	3 1 2 7% 18% 69%	4351
	11th to 20th	Day	5% 2 3 7% 20% 64%	4115
	21st to end	Day	4 1 1 7% 25% 62%	4315
August	1st to 10th	Day	8% 2 3 11% 25% 51%	4076
	11th to 20th	Day	3 1 2 12% 43% 39%	3977
	21st to end	Day	5% 2 4 12% 39% 38%	4179
September	1st to 10th	Day	5% 2 2 9% 30% 51%	2280
	11th to 20th	Day	12% 4 5% 25% 37% 16%	2271
	21st to end	Day	10% 3 5% 20% 33% 28%	2001
October	1st to 10th	Day	13% 2 3 12% 25% 44%	3180
	11th to 20th	Day	16% 4 5% 25% 31% 19%	3161
	21st to end	Day	23% 6% 9% 20% 30% 13%	3448
November	1st to 10th	Day	28% 7% 7% 26% 19% 13%	1914
	11th to 20th	Day	35% 5% 11% 21% 19% 9%	1737
	21st to end	Day	32% 6% 10% 18% 18% 16%	1600
December	1st to 10th	Day	28% 5% 6% 20% 30% 12%	2620
	11th to 20th	Day	55% 6% 7% 15% 15% 3	3028
	21st to end	Day	52% 8% 10% 17% 10% 3	4004
January	1st to 10th	Night	38% 7% 19% 17% 14% 5%	6131
	11th to 20th	Night	39% 4 6% 14% 23% 14%	5880
	21st to end	Night	41% 5% 6% 9% 18% 22%	6468
February	1st to 10th	Night	39% 6% 7% 17% 20% 12%	2600
	11th to 20th	Night	38% 6% 8% 20% 19% 8%	2508
	21st to end	Night	32% 6% 12% 24% 20% 6%	1980
March	1st to 10th	Night	52% 8% 11% 12% 14% 3	3557
	11th to 20th	Night	29% 4 7% 27% 22% 11%	3505
	21st to end	Night	46% 9% 11% 20% 10% 3	3804
April	1st to 10th	Night	35% 7% 14% 22% 17% 5%	1900
	11th to 20th	Night	27% 7% 13% 25% 19% 8%	1879
	21st to end	Night	20% 6% 12% 31% 25% 6%	1900
May	1st to 10th	Night	16% 5% 5% 30% 28% 15%	2874
	11th to 20th	Night	10% 3 5% 18% 38% 25%	2693
	21st to end	Night	8% 4 10% 39% 32% 7%	2762
June	1st to 10th	Night	20% 5% 10% 24% 31% 10%	1520
	11th to 20th	Night	8% 3 9% 21% 37% 22%	1520
	21st to end	Night	10% 3 3 12% 33% 38%	1520
July	1st to 10th	Night	3 1 1 6% 17% 72%	2500
	11th to 20th	Night	5% 1 3 6% 17% 68%	2713
	21st to end	Night	3 1 1 6% 23% 64%	3011
August	1st to 10th	Night	9% 2 2 7% 25% 55%	2839
	11th to 20th	Night	4 1 2 7% 42% 44%	2930
	21st to end	Night	7% 2 3 11% 36% 41%	3498
September	1st to 10th	Night	3 1 3 10% 27% 55%	1900
	11th to 20th	Night	13% 3 5% 19% 37% 24%	1905
	21st to end	Night	8% 4 4 21% 28% 34%	2177
October	1st to 10th	Night	13% 2 4 11% 22% 48%	3800
	11th to 20th	Night	15% 5% 8% 24% 28% 20%	3777
	21st to end	Night	21% 6% 7% 22% 25% 20%	4121
November	1st to 10th	Night	30% 7% 10% 21% 18% 14%	2381
	11th to 20th	Night	31% 6% 15% 19% 20% 10%	2660
	21st to end	Night	33% 5% 8% 18% 22% 15%	2800
December	1st to 10th	Night	28% 5% 5% 21% 26% 15%	4560
	11th to 20th	Night	52% 7% 7% 14% 16% 4	5176
	21st to end	Night	55% 6% 10% 16% 10% 3	6952

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% 12% 24% 18% 14% 2	275
	11th to 20th	Day	23% 13% 22% 25% 14% 4	400
	21st to end	Day	15% 15% 22% 26% 16% 6%	440
February	1st to 10th	Day	23% 13% 23% 22% 17% 3	400
	11th to 20th	Day	22% 20% 23% 20% 15% 1	408
	21st to end	Day	18% 14% 23% 29% 14% 2	414
March	1st to 10th	Day	24% 19% 26% 20% 9% 2	500
	11th to 20th	Day	22% 14% 25% 22% 14% 3	465
	21st to end	Day	18% 17% 33% 22% 8% 1	506
April	1st to 10th	Day	17% 16% 27% 23% 12% 5%	460
	11th to 20th	Day	15% 20% 25% 21% 15% 4	452
	21st to end	Day	7% 18% 31% 22% 17% 5%	460
May	1st to 10th	Day	10% 16% 29% 25% 16% 4	463
	11th to 20th	Day	8% 13% 29% 24% 16% 9%	488
	21st to end	Day	6% 11% 21% 31% 24% 7%	529
June	1st to 10th	Day	6% 13% 23% 32% 18% 8%	490
	11th to 20th	Day	5% 11% 30% 30% 21% 4	490
	21st to end	Day	1 6% 24% 33% 28% 9%	490
July	1st to 10th	Day	2 2 16% 41% 30% 8%	486
	11th to 20th	Day	1 1 15% 38% 33% 12%	484
	21st to end	Day	1 9% 42% 32% 16%	531
August	1st to 10th	Day	2 1 17% 36% 32% 12%	487
	11th to 20th	Day	2 3 17% 35% 29% 13%	477
	21st to end	Day	1 4 13% 33% 35% 15%	539
September	1st to 10th	Day	2 16% 37% 37% 8%	490
	11th to 20th	Day	4 6% 18% 34% 29% 9%	490
	21st to end	Day	2 7% 19% 34% 27% 11%	490
October	1st to 10th	Day	6% 7% 22% 27% 26% 11%	490
	11th to 20th	Day	6% 10% 17% 26% 28% 13%	490
	21st to end	Day	10% 11% 18% 29% 24% 7%	539
November	1st to 10th	Day	11% 10% 22% 21% 27% 9%	450
	11th to 20th	Day	12% 9% 17% 25% 22% 15%	267
	21st to end	Day	14% 10% 22% 25% 21% 10%	200
December	1st to 10th	Day	20% 10% 19% 22% 18% 13%	200
	11th to 20th	Day	28% 14% 17% 20% 14% 10%	200
	21st to end	Day	21% 16% 20% 26% 13% 4	220
January	1st to 10th	Night	20% 9% 16% 29% 21% 5%	800
	11th to 20th	Night	20% 8% 16% 33% 19% 5%	800
	21st to end	Night	17% 7% 17% 31% 22% 5%	880
February	1st to 10th	Night	19% 8% 15% 33% 21% 3	800
	11th to 20th	Night	17% 10% 19% 29% 21% 4	784
	21st to end	Night	15% 12% 20% 33% 18% 2	660
March	1st to 10th	Night	21% 12% 17% 29% 18% 3	800
	11th to 20th	Night	17% 9% 17% 30% 22% 4	766
	21st to end	Night	16% 14% 25% 28% 15% 3	836
April	1st to 10th	Night	12% 12% 25% 28% 20% 4	760
	11th to 20th	Night	10% 13% 22% 30% 20% 6%	751
	21st to end	Night	6% 12% 26% 27% 23% 6%	760
May	1st to 10th	Night	7% 8% 25% 29% 24% 6%	584
	11th to 20th	Night	4 4 20% 32% 33% 6%	486
	21st to end	Night	4 7% 21% 30% 33% 5%	411
June	1st to 10th	Night	6% 9% 19% 32% 31% 4	380
	11th to 20th	Night	3 6% 21% 34% 35% 1	380
	21st to end	Night	2 3 10% 41% 40% 4	380
July	1st to 10th	Night	1 1 13% 34% 46% 5%	370
	11th to 20th	Night	1 8% 27% 50% 13%	498
	21st to end	Night	6% 25% 52% 17%	624
August	1st to 10th	Night	2 1 6% 29% 46% 17%	567
	11th to 20th	Night	1 11% 30% 43% 15%	616
	21st to end	Night	1 2 7% 27% 52% 12%	836
September	1st to 10th	Night	1 2 8% 31% 48% 11%	760
	11th to 20th	Night	3 3 12% 33% 36% 14%	760
	21st to end	Night	2 5% 14% 29% 37% 14%	760
October	1st to 10th	Night	5% 4 15% 29% 35% 12%	760
	11th to 20th	Night	5% 5% 13% 26% 41% 9%	760
	21st to end	Night	9% 5% 13% 30% 30% 13%	836
November	1st to 10th	Night	10% 9% 14% 26% 32% 8%	780
	11th to 20th	Night	12% 6% 15% 33% 28% 6%	800
	21st to end	Night	10% 10% 17% 33% 23% 7%	800
December	1st to 10th	Night	20% 8% 14% 30% 22% 8%	800
	11th to 20th	Night	24% 10% 13% 28% 20% 4	800
	21st to end	Night	22% 10% 19% 31% 16% 2	880

Page generated at 2025-03-09 05:08:46. Version ac07

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