West Lake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the West Lake clear sky chart from 2005-11-06 to 2025-09-19.

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	15% 9% 3 8% 5% 4 4 4 5% 7% 36%	126721
	Night	17% 10% 4 9% 5% 4 4 4 5% 6% 32%	122323
Transparency	Day	46% 7% 14% 22% 11% 1	130263
	Night	49% 7% 12% 19% 11% 1	126867
Seeing	Day	18% 10% 27% 27% 14% 4	15617
	Night	20% 10% 29% 28% 12% 2	24802

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	14% 14% 8% 10% 5% 4 4 3 5% 6% 26%	3597
	11th to 20th	Day	12% 19% 8% 19% 9% 5% 4 3 4 4 14%	3584
	21st to end	Day	14% 17% 6% 11% 7% 5% 5% 4 5% 6% 19%	4466
February	1st to 10th	Day	14% 16% 5% 13% 6% 5% 5% 4 5% 5% 23%	1800
	11th to 20th	Day	15% 11% 4 12% 7% 5% 4 4 5% 5% 29%	1753
	21st to end	Day	20% 12% 3 9% 5% 4 4 4 5% 6% 27%	1608
March	1st to 10th	Day	19% 10% 4 9% 4 4 3 3 4 5% 34%	4720
	11th to 20th	Day	11% 11% 3 9% 5% 5% 5% 4 5% 7% 35%	4980
	21st to end	Day	19% 9% 3 7% 4 4 4 3 4 4 39%	5602
April	1st to 10th	Day	19% 9% 3 7% 5% 5% 3 4 5% 6% 34%	2400
	11th to 20th	Day	19% 9% 3 6% 4 4 4 3 4 5% 40%	2374
	21st to end	Day	19% 8% 3 7% 5% 5% 4 3 4 7% 34%	2536
May	1st to 10th	Day	22% 9% 3 8% 5% 5% 4 3 5% 6% 30%	6197
	11th to 20th	Day	14% 9% 3 9% 4 5% 4 3 5% 7% 36%	6318
	21st to end	Day	18% 7% 2 6% 5% 4 4 3 5% 8% 37%	6949
June	1st to 10th	Day	17% 6% 3 6% 5% 4 5% 4 7% 8% 36%	2800
	11th to 20th	Day	12% 5% 2 5% 5% 4 5% 4 6% 9% 42%	2800
	21st to end	Day	15% 6% 2 6% 4 3 4 3 6% 8% 43%	2800
July	1st to 10th	Day	10% 6% 2 5% 4 4 5% 4 6% 9% 46%	6360
	11th to 20th	Day	9% 5% 2 5% 4 4 5% 4 6% 9% 48%	6360
	21st to end	Day	8% 4 2 6% 3 4 5% 4 7% 9% 48%	6890
August	1st to 10th	Day	16% 6% 2 6% 3 4 4 3 6% 7% 43%	4234
	11th to 20th	Day	13% 5% 2 6% 4 4 4 4 6% 10% 42%	4174
	21st to end	Day	12% 5% 2 5% 3 4 4 4 6% 9% 46%	4323
September	1st to 10th	Day	11% 5% 2 6% 4 4 4 4 5% 7% 48%	2400
	11th to 20th	Day	9% 5% 1 5% 4 3 4 3 6% 8% 51%	2376
	21st to end	Day	15% 6% 2 6% 5% 4 5% 5% 5% 7% 40%	1995
October	1st to 10th	Day	17% 7% 3 6% 6% 5% 4 4 5% 7% 36%	1900
	11th to 20th	Day	20% 6% 3 6% 5% 4 4 4 6% 8% 33%	1900
	21st to end	Day	25% 11% 2 6% 5% 5% 5% 3 4 5% 27%	2043
November	1st to 10th	Day	15% 10% 2 6% 5% 5% 4 4 6% 7% 36%	1736
	11th to 20th	Day	15% 12% 3 9% 6% 5% 5% 4 6% 7% 29%	1600
	21st to end	Day	21% 14% 4 8% 6% 5% 4 4 5% 6% 25%	1600
December	1st to 10th	Day	21% 17% 4 8% 5% 5% 5% 4 5% 6% 20%	2600
	11th to 20th	Day	18% 16% 5% 11% 5% 4 3 3 4 6% 25%	2986
	21st to end	Day	26% 12% 4 9% 5% 5% 4 4 4 5% 23%	3960
January	1st to 10th	Night	16% 16% 5% 11% 6% 5% 4 3 4 6% 24%	6349
	11th to 20th	Night	13% 15% 8% 22% 8% 6% 5% 3 4 4 12%	6297
	21st to end	Night	17% 16% 8% 13% 7% 5% 4 3 4 4 18%	6490
February	1st to 10th	Night	16% 17% 5% 11% 5% 4 5% 4 5% 5% 23%	2600
	11th to 20th	Night	16% 14% 5% 10% 5% 4 3 4 6% 5% 28%	2533
	21st to end	Night	17% 12% 4 8% 5% 5% 5% 4 6% 6% 28%	2021
March	1st to 10th	Night	18% 10% 4 8% 6% 5% 4 4 4 5% 31%	5116
	11th to 20th	Night	14% 10% 3 8% 5% 4 5% 4 5% 7% 36%	4979
	21st to end	Night	21% 8% 3 7% 4 3 3 2 4 6% 39%	5330
April	1st to 10th	Night	20% 10% 2 7% 4 4 5% 4 5% 6% 33%	2000
	11th to 20th	Night	19% 9% 2 7% 5% 4 4 3 5% 5% 37%	1975
	21st to end	Night	19% 8% 2 8% 5% 3 4 3 5% 7% 37%	1880
May	1st to 10th	Night	28% 10% 3 8% 4 3 4 3 5% 5% 27%	3742
	11th to 20th	Night	15% 7% 2 7% 5% 5% 4 4 6% 7% 38%	3575
	21st to end	Night	19% 7% 3 7% 5% 4 5% 4 5% 7% 35%	3926
June	1st to 10th	Night	15% 7% 2 7% 5% 5% 4 5% 6% 8% 36%	1600
	11th to 20th	Night	10% 5% 3 6% 5% 4 5% 4 6% 9% 42%	1600
	21st to end	Night	11% 7% 3 8% 5% 4 5% 4 5% 7% 41%	1600
July	1st to 10th	Night	9% 5% 2 7% 5% 4 5% 5% 7% 9% 41%	3600
	11th to 20th	Night	10% 5% 2 5% 4 4 5% 4 6% 9% 46%	3600
	21st to end	Night	9% 5% 2 6% 4 3 4 3 6% 9% 48%	3959
August	1st to 10th	Night	10% 5% 2 6% 4 5% 5% 4 5% 8% 46%	2915
	11th to 20th	Night	12% 5% 2 6% 4 4 4 4 5% 8% 46%	2875
	21st to end	Night	8% 4 1 5% 4 4 5% 4 6% 10% 48%	3540
September	1st to 10th	Night	12% 5% 2 5% 5% 3 4 3 6% 8% 47%	2000
	11th to 20th	Night	12% 4 2 4 3 3 3 4 5% 7% 52%	1981
	21st to end	Night	17% 6% 3 6% 4 5% 4 3 6% 7% 40%	2180
October	1st to 10th	Night	17% 7% 2 8% 5% 5% 4 4 6% 7% 36%	2280
	11th to 20th	Night	20% 8% 4 7% 5% 4 4 4 4 6% 33%	2280
	21st to end	Night	22% 8% 3 9% 5% 4 4 4 5% 6% 29%	2555
November	1st to 10th	Night	18% 8% 3 6% 4 4 4 4 6% 6% 38%	2561
	11th to 20th	Night	17% 10% 3 7% 5% 4 4 4 6% 7% 32%	2800
	21st to end	Night	23% 12% 3 7% 5% 5% 4 4 5% 7% 26%	2800
December	1st to 10th	Night	20% 19% 4 9% 5% 4 4 4 5% 6% 20%	4580
	11th to 20th	Night	21% 15% 5% 10% 5% 4 4 4 4 6% 22%	5208
	21st to end	Night	28% 14% 4 9% 4 4 4 3 5% 5% 21%	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	54% 9% 11% 16% 9% 1	3598
	11th to 20th	Day	69% 10% 7% 9% 5% 1	3614
	21st to end	Day	64% 7% 11% 10% 7% 1	4466
February	1st to 10th	Day	62% 8% 11% 14% 4 1	1800
	11th to 20th	Day	57% 6% 12% 15% 9% 1	1762
	21st to end	Day	57% 7% 12% 16% 8% 1	1608
March	1st to 10th	Day	51% 7% 11% 15% 15% 1	4723
	11th to 20th	Day	47% 6% 21% 14% 10% 2	4980
	21st to end	Day	47% 6% 8% 21% 17% 2	5602
April	1st to 10th	Day	50% 6% 10% 19% 11% 3	2400
	11th to 20th	Day	46% 5% 10% 22% 15% 2	2374
	21st to end	Day	50% 7% 11% 18% 12% 3	2536
May	1st to 10th	Day	54% 7% 10% 17% 10% 2	6207
	11th to 20th	Day	46% 9% 12% 18% 14% 1	6322
	21st to end	Day	45% 7% 14% 22% 11% 2	6949
June	1st to 10th	Day	43% 8% 14% 20% 14% 1	2800
	11th to 20th	Day	37% 7% 15% 26% 13% 2	2800
	21st to end	Day	39% 7% 16% 23% 14% 1	2800
July	1st to 10th	Day	35% 5% 15% 30% 14% 1	6290
	11th to 20th	Day	32% 5% 16% 31% 15% 1	6285
	21st to end	Day	32% 7% 15% 35% 11%	6692
August	1st to 10th	Day	41% 6% 16% 24% 13% 1	4221
	11th to 20th	Day	37% 7% 19% 28% 9%	4213
	21st to end	Day	34% 6% 16% 35% 9% 1	4323
September	1st to 10th	Day	34% 7% 16% 29% 12% 1	2400
	11th to 20th	Day	31% 6% 16% 31% 15% 1	2376
	21st to end	Day	41% 8% 15% 27% 9%	1995
October	1st to 10th	Day	38% 6% 14% 30% 12%	3160
	11th to 20th	Day	40% 6% 19% 25% 10%	3140
	21st to end	Day	48% 6% 14% 23% 8%	3345
November	1st to 10th	Day	46% 8% 17% 19% 10% 1	1736
	11th to 20th	Day	51% 9% 15% 15% 9% 1	1600
	21st to end	Day	59% 8% 13% 14% 6%	1600
December	1st to 10th	Day	63% 6% 19% 9% 3	2600
	11th to 20th	Day	61% 7% 12% 14% 6%	2986
	21st to end	Day	61% 7% 11% 15% 5%	3960
January	1st to 10th	Night	60% 8% 11% 15% 6%	6349
	11th to 20th	Night	68% 12% 8% 8% 3	6342
	21st to end	Night	66% 8% 8% 10% 8%	6490
February	1st to 10th	Night	61% 7% 13% 16% 3 1	2600
	11th to 20th	Night	56% 6% 11% 17% 8% 1	2535
	21st to end	Night	52% 8% 14% 18% 7%	2021
March	1st to 10th	Night	53% 6% 8% 16% 12% 4	5126
	11th to 20th	Night	46% 8% 17% 18% 9% 2	4980
	21st to end	Night	45% 5% 9% 22% 17% 2	5330
April	1st to 10th	Night	50% 6% 9% 20% 12% 2	2000
	11th to 20th	Night	48% 7% 10% 18% 17% 1	1975
	21st to end	Night	46% 6% 11% 17% 16% 3	1880
May	1st to 10th	Night	57% 6% 9% 13% 11% 3	3748
	11th to 20th	Night	42% 8% 16% 19% 14% 1	3576
	21st to end	Night	45% 8% 12% 19% 15% 1	3927
June	1st to 10th	Night	42% 8% 13% 22% 13% 2	1600
	11th to 20th	Night	37% 7% 13% 21% 20% 3	1600
	21st to end	Night	41% 8% 13% 22% 15% 2	1600
July	1st to 10th	Night	38% 6% 15% 21% 19% 1	3560
	11th to 20th	Night	34% 6% 13% 31% 15% 1	3552
	21st to end	Night	31% 6% 14% 32% 16% 1	3842
August	1st to 10th	Night	36% 7% 14% 28% 14% 2	2904
	11th to 20th	Night	35% 6% 18% 27% 12% 1	2902
	21st to end	Night	29% 8% 15% 36% 11% 1	3540
September	1st to 10th	Night	34% 7% 16% 28% 12% 1	2000
	11th to 20th	Night	30% 7% 13% 32% 17% 1	1981
	21st to end	Night	41% 8% 12% 26% 13% 1	2180
October	1st to 10th	Night	37% 7% 11% 27% 19% 1	3800
	11th to 20th	Night	42% 6% 16% 28% 9%	3779
	21st to end	Night	46% 7% 13% 25% 9%	4203
November	1st to 10th	Night	45% 7% 15% 20% 11% 2	2561
	11th to 20th	Night	48% 9% 14% 18% 9% 1	2800
	21st to end	Night	56% 10% 15% 14% 4	2800
December	1st to 10th	Night	63% 7% 13% 15% 2	4580
	11th to 20th	Night	59% 8% 12% 13% 7%	5208
	21st to end	Night	64% 7% 10% 11% 8%	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	21% 16% 36% 18% 9% 2	200
	11th to 20th	Day	25% 20% 29% 19% 5%	201
	21st to end	Day	18% 18% 32% 24% 5% 2	330
February	1st to 10th	Day	25% 20% 35% 16% 4	300
	11th to 20th	Day	22% 16% 37% 22% 3	300
	21st to end	Day	28% 17% 32% 19% 3	372
March	1st to 10th	Day	21% 14% 34% 26% 4 1	500
	11th to 20th	Day	19% 17% 34% 24% 6%	500
	21st to end	Day	28% 11% 32% 19% 10% 1	550
April	1st to 10th	Day	22% 14% 31% 25% 8% 1	500
	11th to 20th	Day	24% 12% 33% 18% 10% 3	492
	21st to end	Day	23% 10% 32% 25% 7% 2	500
May	1st to 10th	Day	23% 6% 27% 28% 12% 3	503
	11th to 20th	Day	16% 7% 25% 32% 16% 4	527
	21st to end	Day	15% 4 24% 33% 18% 7%	573
June	1st to 10th	Day	18% 6% 25% 34% 15% 2	530
	11th to 20th	Day	11% 5% 30% 32% 18% 4	530
	21st to end	Day	17% 5% 25% 32% 18% 3	530
July	1st to 10th	Day	8% 5% 21% 33% 28% 5%	525
	11th to 20th	Day	14% 4 21% 30% 25% 6%	524
	21st to end	Day	9% 4 24% 40% 19% 4	576
August	1st to 10th	Day	13% 3 17% 37% 23% 8%	526
	11th to 20th	Day	11% 3 19% 34% 25% 8%	519
	21st to end	Day	11% 5% 24% 32% 19% 8%	583
September	1st to 10th	Day	11% 7% 22% 31% 22% 8%	530
	11th to 20th	Day	8% 4 17% 29% 25% 16%	522
	21st to end	Day	16% 8% 22% 29% 21% 5%	490
October	1st to 10th	Day	21% 9% 26% 21% 18% 6%	490
	11th to 20th	Day	18% 15% 31% 22% 12% 2	490
	21st to end	Day	28% 12% 27% 24% 8%	514
November	1st to 10th	Day	22% 16% 25% 28% 9% 1	370
	11th to 20th	Day	18% 21% 35% 19% 7% 1	200
	21st to end	Day	24% 15% 41% 15% 5% 1	200
December	1st to 10th	Day	24% 21% 30% 20% 7% 1	200
	11th to 20th	Day	26% 27% 29% 14% 4 1	200
	21st to end	Day	32% 15% 30% 18% 4	220
January	1st to 10th	Night	27% 17% 33% 18% 5%	800
	11th to 20th	Night	27% 15% 32% 20% 6%	800
	21st to end	Night	22% 12% 36% 25% 5%	880
February	1st to 10th	Night	22% 16% 38% 21% 3	800
	11th to 20th	Night	23% 13% 39% 20% 4	783
	21st to end	Night	22% 13% 36% 23% 5%	660
March	1st to 10th	Night	20% 11% 35% 29% 4	800
	11th to 20th	Night	22% 13% 32% 27% 5% 1	800
	21st to end	Night	22% 8% 35% 25% 9% 1	880
April	1st to 10th	Night	24% 9% 34% 28% 5%	800
	11th to 20th	Night	21% 10% 33% 25% 10% 1	791
	21st to end	Night	21% 9% 27% 28% 14% 1	680
May	1st to 10th	Night	25% 5% 22% 31% 15% 2	470
	11th to 20th	Night	15% 3 25% 41% 14% 2	394
	21st to end	Night	11% 3 26% 37% 19% 3	433
June	1st to 10th	Night	15% 1 25% 40% 17% 2	400
	11th to 20th	Night	11% 3 24% 39% 21% 2	400
	21st to end	Night	14% 5% 24% 41% 16% 1	400
July	1st to 10th	Night	7% 4 18% 43% 26% 2	390
	11th to 20th	Night	11% 3 18% 38% 26% 4	388
	21st to end	Night	11% 2 17% 48% 19% 3	446
August	1st to 10th	Night	10% 2 14% 44% 26% 5%	597
	11th to 20th	Night	9% 4 16% 34% 32% 5%	594
	21st to end	Night	9% 4 21% 37% 26% 2	847
September	1st to 10th	Night	12% 3 25% 32% 23% 4	800
	11th to 20th	Night	12% 5% 16% 31% 27% 9%	793
	21st to end	Night	18% 7% 20% 30% 21% 5%	760
October	1st to 10th	Night	18% 10% 28% 23% 19% 3	760
	11th to 20th	Night	21% 12% 29% 27% 9% 2	760
	21st to end	Night	25% 13% 31% 25% 6%	836
November	1st to 10th	Night	21% 12% 28% 27% 11% 1	780
	11th to 20th	Night	21% 17% 33% 22% 7% 1	800
	21st to end	Night	27% 15% 30% 22% 6%	800
December	1st to 10th	Night	29% 17% 31% 19% 4	800
	11th to 20th	Night	25% 17% 37% 18% 3 1	800
	21st to end	Night	31% 14% 34% 17% 4	880

Page generated at 2025-09-16 01:49:11. Version ac07

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