WWYS Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the WWYS Observatory clear sky chart from 2005-11-06 to 2026-04-02.

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% 7% 2 6% 4 4 4 4 6% 8% 40%	131463
	Night	15% 7% 2 6% 4 4 4 3 5% 7% 42%	130356
Transparency	Day	41% 7% 12% 23% 15% 3	135007
	Night	40% 6% 12% 23% 17% 3	134898
Seeing	Day	23% 16% 28% 20% 10% 3	16686
	Night	20% 15% 30% 26% 9% 1	26118

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	13% 7% 2 7% 5% 5% 5% 3 5% 6% 44%	4661
	11th to 20th	Day	15% 8% 3 8% 5% 4 4 3 4 6% 41%	4871
	21st to end	Day	14% 6% 3 7% 5% 5% 5% 4 6% 7% 38%	5780
February	1st to 10th	Day	14% 8% 3 6% 5% 4 5% 4 5% 7% 39%	1890
	11th to 20th	Day	16% 9% 3 8% 4 4 4 3 5% 6% 38%	1966
	21st to end	Day	18% 10% 3 7% 4 4 3 3 4 6% 38%	1730
March	1st to 10th	Day	21% 5% 2 5% 4 3 4 3 4 6% 44%	4665
	11th to 20th	Day	13% 6% 2 6% 5% 5% 4 4 5% 7% 43%	5090
	21st to end	Day	18% 6% 2 6% 4 4 4 3 5% 6% 43%	5862
April	1st to 10th	Day	16% 7% 3 6% 4 4 4 4 5% 8% 39%	2412
	11th to 20th	Day	15% 7% 2 6% 5% 4 4 4 5% 7% 41%	2374
	21st to end	Day	16% 5% 2 6% 5% 4 4 4 6% 8% 40%	2400
May	1st to 10th	Day	25% 7% 3 6% 4 4 4 4 6% 8% 29%	5851
	11th to 20th	Day	25% 8% 3 6% 4 3 3 3 5% 7% 31%	6318
	21st to end	Day	18% 6% 2 6% 4 5% 4 4 6% 9% 36%	6949
June	1st to 10th	Day	13% 6% 2 5% 5% 5% 5% 4 7% 8% 39%	2800
	11th to 20th	Day	14% 6% 2 7% 5% 4 5% 5% 7% 9% 37%	2800
	21st to end	Day	10% 5% 2 6% 4 5% 6% 4 6% 10% 42%	2800
July	1st to 10th	Day	8% 5% 2 7% 5% 5% 6% 5% 7% 11% 39%	6360
	11th to 20th	Day	10% 7% 3 8% 5% 5% 6% 5% 8% 11% 32%	6360
	21st to end	Day	10% 7% 3 7% 4 5% 5% 4 6% 9% 40%	6727
August	1st to 10th	Day	19% 6% 2 6% 5% 5% 5% 4 6% 6% 35%	4234
	11th to 20th	Day	13% 5% 3 6% 4 4 5% 4 6% 9% 41%	3896
	21st to end	Day	12% 5% 2 5% 4 4 4 3 5% 8% 49%	4312
September	1st to 10th	Day	15% 6% 2 6% 4 4 4 4 5% 7% 45%	2400
	11th to 20th	Day	13% 5% 2 4 3 3 4 2 5% 8% 50%	2390
	21st to end	Day	19% 7% 3 8% 4 4 4 3 4 6% 38%	2162
October	1st to 10th	Day	17% 6% 2 5% 4 3 4 3 5% 7% 45%	2000
	11th to 20th	Day	16% 6% 2 4 4 4 4 3 5% 8% 47%	2000
	21st to end	Day	18% 6% 2 5% 3 3 3 3 5% 6% 45%	2157
November	1st to 10th	Day	12% 7% 2 4 3 3 4 3 4 7% 52%	1851
	11th to 20th	Day	15% 5% 2 4 5% 4 4 3 6% 6% 46%	1890
	21st to end	Day	15% 7% 2 5% 4 4 3 5% 5% 7% 43%	1711
December	1st to 10th	Day	13% 9% 2 6% 4 4 4 4 6% 6% 42%	2680
	11th to 20th	Day	20% 9% 2 5% 3 3 3 3 5% 6% 42%	3066
	21st to end	Day	16% 6% 2 6% 5% 5% 5% 4 5% 7% 38%	4048
January	1st to 10th	Night	11% 7% 3 7% 4 4 4 3 5% 7% 45%	8234
	11th to 20th	Night	15% 8% 2 5% 4 3 3 3 5% 6% 46%	7999
	21st to end	Night	15% 8% 2 6% 3 3 3 3 4 6% 46%	8401
February	1st to 10th	Night	16% 9% 2 6% 5% 4 3 4 5% 6% 40%	2730
	11th to 20th	Night	17% 8% 3 7% 5% 4 4 3 5% 5% 39%	2538
	21st to end	Night	17% 8% 3 5% 4 4 4 4 5% 7% 40%	2076
March	1st to 10th	Night	18% 8% 2 6% 4 3 4 3 4 6% 43%	5391
	11th to 20th	Night	11% 6% 2 6% 4 4 4 3 5% 6% 49%	5089
	21st to end	Night	15% 8% 3 6% 4 4 4 3 4 6% 43%	5310
April	1st to 10th	Night	17% 6% 2 7% 4 4 4 4 5% 6% 39%	2011
	11th to 20th	Night	15% 7% 2 7% 4 4 4 3 5% 8% 41%	1975
	21st to end	Night	18% 6% 2 4 3 4 4 4 4 7% 44%	2000
May	1st to 10th	Night	24% 8% 3 7% 4 4 5% 4 6% 7% 28%	4118
	11th to 20th	Night	24% 9% 3 6% 4 3 3 4 5% 7% 31%	3575
	21st to end	Night	21% 6% 2 6% 3 4 3 3 6% 8% 37%	3926
June	1st to 10th	Night	14% 5% 2 6% 5% 4 4 4 5% 8% 41%	1600
	11th to 20th	Night	13% 5% 3 6% 5% 5% 5% 4 7% 10% 39%	1600
	21st to end	Night	11% 5% 1 6% 4 3 5% 4 7% 10% 43%	1600
July	1st to 10th	Night	8% 6% 3 7% 6% 6% 6% 4 7% 10% 38%	3600
	11th to 20th	Night	12% 8% 3 8% 5% 5% 6% 5% 7% 9% 30%	3600
	21st to end	Night	11% 6% 2 6% 4 4 5% 4 6% 9% 44%	4121
August	1st to 10th	Night	20% 5% 2 5% 4 5% 5% 4 5% 8% 36%	2915
	11th to 20th	Night	13% 7% 3 7% 4 4 4 5% 6% 7% 40%	3118
	21st to end	Night	11% 5% 2 5% 4 4 4 4 6% 7% 48%	3586
September	1st to 10th	Night	14% 6% 3 6% 3 3 3 3 5% 6% 47%	2000
	11th to 20th	Night	12% 6% 2 4 2 3 3 4 5% 6% 52%	2009
	21st to end	Night	19% 7% 2 5% 4 4 4 4 4 7% 39%	2235
October	1st to 10th	Night	20% 6% 2 5% 4 4 3 2 3 6% 45%	2400
	11th to 20th	Night	16% 6% 2 4 3 3 3 2 5% 6% 50%	2400
	21st to end	Night	17% 6% 2 4 3 3 3 3 4 6% 49%	2681
November	1st to 10th	Night	11% 5% 1 5% 3 3 3 3 6% 6% 52%	2667
	11th to 20th	Night	15% 8% 2 5% 3 3 4 3 5% 5% 47%	2730
	21st to end	Night	13% 5% 2 5% 5% 5% 4 4 5% 7% 45%	2903
December	1st to 10th	Night	12% 7% 3 6% 4 4 5% 3 5% 7% 45%	4720
	11th to 20th	Night	21% 7% 2 5% 4 3 4 3 4 6% 41%	5348
	21st to end	Night	18% 7% 3 7% 4 4 4 4 5% 6% 39%	7150

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	40% 5% 11% 26% 17% 2	4662
	11th to 20th	Day	43% 5% 9% 21% 16% 6%	4902
	21st to end	Day	43% 6% 12% 23% 14% 1	5780
February	1st to 10th	Day	43% 7% 13% 24% 11% 2	1890
	11th to 20th	Day	46% 6% 10% 24% 13% 2	1975
	21st to end	Day	48% 4 10% 23% 12% 3	1730
March	1st to 10th	Day	41% 5% 9% 24% 16% 6%	4668
	11th to 20th	Day	38% 7% 12% 26% 12% 5%	5090
	21st to end	Day	41% 6% 11% 17% 21% 5%	5862
April	1st to 10th	Day	43% 7% 10% 21% 14% 5%	2412
	11th to 20th	Day	41% 7% 11% 22% 16% 4	2374
	21st to end	Day	40% 8% 11% 20% 17% 4	2400
May	1st to 10th	Day	50% 8% 12% 18% 9% 2	5860
	11th to 20th	Day	51% 6% 9% 22% 11% 1	6322
	21st to end	Day	43% 7% 12% 23% 14% 1	6949
June	1st to 10th	Day	39% 8% 14% 23% 15% 1	2800
	11th to 20th	Day	40% 9% 15% 23% 10% 3	2800
	21st to end	Day	36% 9% 14% 25% 15% 1	2800
July	1st to 10th	Day	37% 11% 14% 21% 15% 2	6290
	11th to 20th	Day	41% 10% 18% 20% 9% 1	6285
	21st to end	Day	39% 8% 10% 20% 22% 1	6533
August	1st to 10th	Day	46% 8% 12% 23% 10%	4221
	11th to 20th	Day	37% 8% 12% 29% 13% 1	3933
	21st to end	Day	33% 7% 11% 29% 19% 2	4312
September	1st to 10th	Day	39% 7% 12% 20% 20% 3	2400
	11th to 20th	Day	32% 5% 11% 26% 22% 3	2390
	21st to end	Day	48% 5% 9% 17% 19% 2	2162
October	1st to 10th	Day	36% 5% 9% 18% 27% 5%	3260
	11th to 20th	Day	26% 4 12% 27% 24% 6%	3240
	21st to end	Day	25% 4 13% 26% 28% 4	3459
November	1st to 10th	Day	33% 4 15% 24% 20% 4	1851
	11th to 20th	Day	37% 6% 11% 24% 19% 3	1890
	21st to end	Day	39% 6% 14% 29% 10% 2	1711
December	1st to 10th	Day	42% 5% 16% 25% 11% 2	2680
	11th to 20th	Day	43% 5% 13% 29% 9% 1	3066
	21st to end	Day	43% 7% 13% 21% 14% 2	4048
January	1st to 10th	Night	39% 5% 14% 24% 15% 4	8234
	11th to 20th	Night	39% 4 12% 21% 18% 6%	8043
	21st to end	Night	40% 4 9% 26% 19% 2	8401
February	1st to 10th	Night	44% 5% 13% 25% 11% 2	2730
	11th to 20th	Night	46% 5% 10% 20% 16% 3	2540
	21st to end	Night	44% 5% 10% 22% 15% 4	2076
March	1st to 10th	Night	42% 5% 10% 23% 12% 8%	5401
	11th to 20th	Night	34% 6% 14% 27% 13% 5%	5090
	21st to end	Night	42% 5% 8% 17% 25% 4	5310
April	1st to 10th	Night	43% 6% 11% 20% 16% 4	2011
	11th to 20th	Night	41% 6% 10% 20% 19% 3	1975
	21st to end	Night	40% 6% 10% 22% 18% 5%	2000
May	1st to 10th	Night	53% 8% 11% 15% 11% 2	4125
	11th to 20th	Night	51% 8% 11% 17% 12% 1	3576
	21st to end	Night	44% 8% 11% 22% 14% 2	3927
June	1st to 10th	Night	40% 7% 12% 22% 19% 1	1600
	11th to 20th	Night	38% 9% 15% 23% 14% 1	1600
	21st to end	Night	34% 9% 15% 27% 14% 2	1600
July	1st to 10th	Night	40% 10% 15% 19% 14% 2	3560
	11th to 20th	Night	46% 11% 15% 18% 9% 1	3552
	21st to end	Night	35% 9% 11% 23% 21% 1	4000
August	1st to 10th	Night	45% 8% 19% 18% 9% 1	2904
	11th to 20th	Night	41% 7% 13% 23% 15% 1	3147
	21st to end	Night	32% 7% 12% 28% 19% 1	3586
September	1st to 10th	Night	38% 7% 10% 21% 21% 3	2000
	11th to 20th	Night	32% 6% 10% 22% 26% 4	2009
	21st to end	Night	43% 6% 9% 20% 20% 2	2235
October	1st to 10th	Night	42% 5% 9% 21% 21% 2	3920
	11th to 20th	Night	25% 4 10% 29% 27% 4	3899
	21st to end	Night	26% 4 12% 32% 24% 4	4329
November	1st to 10th	Night	31% 6% 12% 25% 20% 5%	2667
	11th to 20th	Night	39% 5% 9% 22% 20% 5%	2730
	21st to end	Night	38% 6% 12% 27% 16% 1	2903
December	1st to 10th	Night	39% 5% 13% 29% 12% 1	4720
	11th to 20th	Night	44% 4 11% 25% 13% 3	5348
	21st to end	Night	45% 7% 11% 20% 17% 1	7150

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% 26% 25% 11% 2	220
	11th to 20th	Day	41% 25% 26% 7%	261
	21st to end	Day	31% 31% 24% 12% 2	363
February	1st to 10th	Day	28% 27% 34% 8% 2 1	330
	11th to 20th	Day	36% 24% 31% 7% 1	454
	21st to end	Day	32% 24% 30% 12% 2	446
March	1st to 10th	Day	34% 20% 29% 14% 3	540
	11th to 20th	Day	31% 24% 29% 13% 3 1	540
	21st to end	Day	30% 25% 31% 10% 4 1	594
April	1st to 10th	Day	29% 25% 29% 12% 3 1	504
	11th to 20th	Day	26% 25% 26% 15% 5% 2	492
	21st to end	Day	27% 17% 35% 15% 4 1	500
May	1st to 10th	Day	33% 18% 28% 14% 5% 1	503
	11th to 20th	Day	25% 16% 33% 18% 7% 2	527
	21st to end	Day	20% 12% 28% 23% 16% 2	573
June	1st to 10th	Day	17% 13% 31% 25% 12% 2	530
	11th to 20th	Day	19% 9% 32% 24% 13% 2	530
	21st to end	Day	13% 11% 32% 26% 14% 3	530
July	1st to 10th	Day	11% 5% 25% 34% 19% 6%	525
	11th to 20th	Day	8% 5% 27% 34% 22% 5%	524
	21st to end	Day	10% 7% 29% 33% 16% 3	576
August	1st to 10th	Day	12% 7% 27% 28% 20% 7%	526
	11th to 20th	Day	13% 6% 21% 35% 18% 7%	519
	21st to end	Day	12% 8% 25% 28% 18% 8%	583
September	1st to 10th	Day	19% 10% 22% 26% 16% 7%	530
	11th to 20th	Day	16% 6% 26% 26% 17% 10%	530
	21st to end	Day	23% 14% 24% 23% 12% 4	530
October	1st to 10th	Day	22% 10% 26% 22% 14% 6%	530
	11th to 20th	Day	21% 17% 26% 22% 11% 2	530
	21st to end	Day	21% 20% 32% 16% 9% 2	558
November	1st to 10th	Day	21% 16% 30% 21% 9% 3	425
	11th to 20th	Day	23% 22% 31% 19% 5%	430
	21st to end	Day	28% 22% 29% 18% 2	251
December	1st to 10th	Day	30% 26% 30% 10% 4	220
	11th to 20th	Day	36% 23% 27% 11% 3	220
	21st to end	Day	28% 26% 26% 13% 7%	242
January	1st to 10th	Night	21% 25% 36% 15% 3	840
	11th to 20th	Night	28% 21% 29% 20% 3	840
	21st to end	Night	19% 23% 39% 17% 2	924
February	1st to 10th	Night	23% 20% 35% 20% 1	840
	11th to 20th	Night	29% 25% 32% 13% 1	823
	21st to end	Night	26% 22% 35% 15% 2	692
March	1st to 10th	Night	24% 22% 30% 20% 3	840
	11th to 20th	Night	24% 23% 30% 17% 5%	840
	21st to end	Night	21% 18% 39% 17% 4	924
April	1st to 10th	Night	22% 24% 34% 17% 3	805
	11th to 20th	Night	20% 17% 35% 22% 5%	791
	21st to end	Night	21% 16% 32% 25% 6%	800
May	1st to 10th	Night	24% 13% 33% 24% 6%	639
	11th to 20th	Night	19% 10% 34% 30% 7%	394
	21st to end	Night	16% 10% 26% 37% 11% 1	433
June	1st to 10th	Night	15% 6% 27% 41% 12%	400
	11th to 20th	Night	12% 8% 28% 41% 11%	400
	21st to end	Night	11% 4 29% 43% 13% 1	400
July	1st to 10th	Night	10% 5% 20% 43% 21% 1	390
	11th to 20th	Night	8% 2 23% 41% 24% 2	388
	21st to end	Night	10% 3 23% 40% 22% 2	519
August	1st to 10th	Night	12% 2 18% 42% 24% 2	597
	11th to 20th	Night	14% 4 19% 37% 24% 2	735
	21st to end	Night	12% 5% 19% 39% 22% 2	880
September	1st to 10th	Night	16% 4 28% 31% 19% 3	800
	11th to 20th	Night	14% 5% 19% 37% 22% 3	800
	21st to end	Night	21% 7% 22% 31% 18% 1	800
October	1st to 10th	Night	22% 9% 25% 25% 17% 3	800
	11th to 20th	Night	20% 12% 30% 28% 11%	800
	21st to end	Night	23% 14% 30% 25% 9%	880
November	1st to 10th	Night	16% 14% 31% 27% 11% 1	820
	11th to 20th	Night	23% 19% 26% 25% 6% 1	840
	21st to end	Night	20% 18% 34% 22% 6%	840
December	1st to 10th	Night	24% 20% 33% 18% 4 1	840
	11th to 20th	Night	22% 19% 34% 21% 4	840
	21st to end	Night	22% 21% 31% 23% 3	924

Page generated at 2026-03-30 02:18:46. Version ac07

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