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 2024-07-08.

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% 7% 2 6% 4 4 4 4 5% 7% 40%	89692
	Night	16% 7% 2 6% 4 4 4 4 5% 7% 42%	89632
Transparency	Day	42% 7% 12% 22% 14% 2	89414
	Night	41% 7% 12% 23% 15% 2	89463
Seeing	Day	22% 16% 29% 20% 10% 3	14484
	Night	20% 15% 30% 26% 9% 1	23824

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% 8% 2 6% 4 4 4 3 4 5% 45%	2517
	11th to 20th	Day	14% 10% 3 10% 5% 4 4 3 5% 6% 34%	2630
	21st to end	Day	19% 9% 3 8% 5% 4 4 4 4 5% 35%	3124
February	1st to 10th	Day	15% 8% 3 6% 5% 4 4 4 5% 7% 39%	1710
	11th to 20th	Day	15% 8% 3 8% 4 4 4 3 5% 6% 39%	1774
	21st to end	Day	19% 10% 3 7% 4 4 3 3 4 6% 37%	1570
March	1st to 10th	Day	26% 6% 2 6% 4 3 4 3 3 6% 36%	3175
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 4 6% 8% 44%	3480
	21st to end	Day	22% 7% 2 5% 3 3 3 2 4 5% 43%	4001
April	1st to 10th	Day	15% 7% 3 6% 4 4 4 4 5% 8% 40%	2280
	11th to 20th	Day	15% 7% 2 5% 5% 4 4 4 5% 7% 42%	2254
	21st to end	Day	16% 5% 2 7% 5% 4 4 4 6% 8% 39%	2280
May	1st to 10th	Day	27% 7% 3 6% 4 4 4 4 5% 7% 29%	4085
	11th to 20th	Day	25% 9% 3 7% 5% 4 4 4 5% 6% 28%	4398
	21st to end	Day	11% 6% 2 6% 4 4 4 4 5% 9% 43%	4837
June	1st to 10th	Day	13% 6% 2 5% 5% 5% 5% 4 7% 8% 39%	2660
	11th to 20th	Day	12% 6% 2 7% 5% 4 5% 5% 7% 9% 37%	2660
	21st to end	Day	10% 5% 2 6% 4 5% 6% 5% 6% 10% 41%	2660
July	1st to 10th	Day	9% 5% 2 6% 6% 5% 6% 5% 7% 10% 38%	2621
	11th to 20th	Day	7% 5% 2 5% 5% 4 5% 5% 9% 10% 42%	2520
	21st to end	Day	9% 7% 2 7% 5% 6% 6% 6% 6% 8% 37%	2668
August	1st to 10th	Day	9% 6% 2 7% 5% 5% 6% 5% 7% 8% 39%	2327
	11th to 20th	Day	11% 5% 3 7% 5% 5% 6% 4 6% 10% 39%	2157
	21st to end	Day	14% 5% 2 6% 5% 5% 5% 4 6% 8% 42%	2376
September	1st to 10th	Day	16% 6% 2 6% 4 4 5% 4 5% 7% 43%	2160
	11th to 20th	Day	13% 5% 2 5% 3 3 4 2 5% 8% 50%	2152
	21st to end	Day	18% 7% 2 7% 4 4 4 3 4 6% 40%	1946
October	1st to 10th	Day	18% 6% 2 5% 4 3 4 3 5% 7% 43%	1800
	11th to 20th	Day	16% 6% 2 5% 4 4 4 3 5% 8% 45%	1800
	21st to end	Day	20% 6% 2 5% 3 3 4 3 5% 6% 42%	1942
November	1st to 10th	Day	12% 8% 2 4 3 2 4 2 4 7% 51%	1671
	11th to 20th	Day	16% 5% 2 4 5% 3 4 3 6% 6% 46%	1710
	21st to end	Day	15% 6% 1 5% 4 5% 3 5% 5% 7% 44%	1549
December	1st to 10th	Day	17% 10% 2 7% 4 4 4 3 6% 6% 37%	1520
	11th to 20th	Day	19% 8% 2 5% 4 3 3 3 5% 7% 41%	1906
	21st to end	Day	16% 7% 3 8% 6% 5% 6% 4 6% 6% 34%	2772
January	1st to 10th	Night	15% 7% 2 6% 5% 4 4 3 4 7% 43%	4429
	11th to 20th	Night	18% 8% 2 6% 4 4 4 3 5% 6% 42%	4315
	21st to end	Night	22% 9% 3 8% 4 3 3 3 4 5% 37%	4532
February	1st to 10th	Night	16% 9% 2 6% 5% 4 3 4 4 6% 40%	2470
	11th to 20th	Night	16% 8% 3 7% 5% 4 4 3 5% 5% 40%	2291
	21st to end	Night	18% 7% 3 5% 4 4 4 4 5% 7% 40%	1884
March	1st to 10th	Night	21% 10% 2 6% 4 3 4 3 4 5% 37%	3661
	11th to 20th	Night	11% 6% 2 6% 5% 4 4 3 5% 6% 48%	3479
	21st to end	Night	16% 9% 3 7% 4 3 3 2 4 5% 46%	3629
April	1st to 10th	Night	16% 6% 2 7% 4 4 4 4 5% 7% 40%	1900
	11th to 20th	Night	14% 7% 2 7% 4 4 4 3 5% 7% 42%	1875
	21st to end	Night	18% 6% 2 4 3 4 4 4 4 7% 43%	1900
May	1st to 10th	Night	24% 8% 3 6% 4 4 5% 4 6% 6% 30%	2884
	11th to 20th	Night	25% 10% 3 7% 4 3 4 5% 5% 6% 28%	2495
	21st to end	Night	13% 6% 2 6% 4 4 3 4 7% 10% 42%	2738
June	1st to 10th	Night	14% 5% 2 6% 5% 4 4 4 5% 8% 42%	1520
	11th to 20th	Night	11% 5% 3 6% 5% 5% 5% 4 7% 10% 40%	1520
	21st to end	Night	10% 5% 1 6% 5% 3 5% 4 7% 10% 43%	1520
July	1st to 10th	Night	9% 6% 3 7% 6% 5% 5% 5% 7% 9% 39%	1504
	11th to 20th	Night	7% 4 1 5% 5% 4 5% 6% 7% 10% 45%	1440
	21st to end	Night	10% 5% 2 6% 4 4 5% 5% 8% 10% 41%	1646
August	1st to 10th	Night	9% 5% 2 5% 4 5% 6% 5% 6% 9% 43%	1611
	11th to 20th	Night	11% 7% 3 7% 5% 5% 4 5% 6% 7% 39%	1724
	21st to end	Night	13% 5% 2 6% 4 5% 4 4 6% 8% 43%	1980
September	1st to 10th	Night	15% 6% 3 6% 4 4 3 3 5% 6% 46%	1800
	11th to 20th	Night	13% 6% 2 4 2 3 3 4 5% 6% 52%	1807
	21st to end	Night	17% 6% 2 5% 4 4 4 4 4 7% 43%	2011
October	1st to 10th	Night	21% 7% 2 5% 4 4 3 2 3 6% 43%	2160
	11th to 20th	Night	16% 6% 2 4 4 3 4 2 5% 6% 48%	2160
	21st to end	Night	19% 6% 2 4 3 3 3 3 4 6% 47%	2412
November	1st to 10th	Night	11% 6% 1 5% 3 3 3 3 5% 6% 52%	2407
	11th to 20th	Night	16% 7% 3 4 3 3 4 2 5% 5% 47%	2470
	21st to end	Night	13% 5% 2 5% 4 5% 4 4 5% 7% 46%	2626
December	1st to 10th	Night	19% 6% 3 6% 4 4 5% 3 5% 7% 38%	2660
	11th to 20th	Night	18% 6% 2 6% 4 4 5% 3 5% 6% 41%	3288
	21st to end	Night	17% 7% 3 7% 5% 4 4 4 6% 7% 36%	4884

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% 6% 14% 25% 12% 3	2518
	11th to 20th	Day	49% 6% 12% 19% 12% 2	2630
	21st to end	Day	50% 7% 12% 21% 9% 1	3124
February	1st to 10th	Day	44% 7% 14% 23% 11% 2	1710
	11th to 20th	Day	45% 6% 9% 25% 13% 2	1783
	21st to end	Day	49% 4 10% 22% 12% 3	1570
March	1st to 10th	Day	48% 6% 9% 26% 9% 2	3178
	11th to 20th	Day	34% 8% 13% 27% 15% 3	3480
	21st to end	Day	44% 5% 9% 19% 19% 3	4001
April	1st to 10th	Day	42% 7% 10% 21% 15% 5%	2280
	11th to 20th	Day	40% 7% 11% 22% 16% 4	2254
	21st to end	Day	41% 8% 11% 19% 17% 4	2280
May	1st to 10th	Day	53% 8% 10% 18% 9% 2	4097
	11th to 20th	Day	54% 8% 12% 17% 8% 1	4402
	21st to end	Day	34% 9% 14% 25% 16% 2	4837
June	1st to 10th	Day	38% 8% 15% 23% 15% 1	2660
	11th to 20th	Day	40% 10% 15% 23% 10% 3	2660
	21st to end	Day	36% 9% 14% 25% 14% 1	2660
July	1st to 10th	Day	40% 10% 13% 21% 14% 3	2551
	11th to 20th	Day	32% 9% 17% 25% 14% 2	2445
	21st to end	Day	41% 10% 12% 21% 15% 2	2519
August	1st to 10th	Day	39% 10% 14% 22% 15% 1	2314
	11th to 20th	Day	38% 9% 15% 21% 15% 1	2157
	21st to end	Day	38% 9% 12% 21% 16% 3	2376
September	1st to 10th	Day	41% 8% 12% 19% 18% 3	2160
	11th to 20th	Day	33% 6% 11% 25% 22% 4	2152
	21st to end	Day	46% 5% 8% 18% 20% 2	1946
October	1st to 10th	Day	41% 6% 13% 22% 16% 1	1800
	11th to 20th	Day	38% 6% 11% 23% 19% 2	1800
	21st to end	Day	42% 5% 10% 24% 17% 2	1942
November	1st to 10th	Day	34% 4 15% 25% 18% 4	1671
	11th to 20th	Day	37% 6% 11% 24% 20% 2	1710
	21st to end	Day	38% 6% 14% 30% 10% 2	1549
December	1st to 10th	Day	47% 6% 16% 19% 12% 1	1520
	11th to 20th	Day	42% 6% 15% 27% 9% 1	1906
	21st to end	Day	47% 8% 14% 21% 9% 1	2772
January	1st to 10th	Night	41% 5% 13% 26% 12% 3	4429
	11th to 20th	Night	43% 5% 13% 24% 12% 2	4315
	21st to end	Night	50% 4 9% 26% 10% 1	4532
February	1st to 10th	Night	45% 5% 13% 25% 11% 2	2470
	11th to 20th	Night	45% 4 10% 20% 17% 3	2293
	21st to end	Night	44% 5% 10% 22% 15% 4	1884
March	1st to 10th	Night	48% 6% 10% 23% 12% 2	3671
	11th to 20th	Night	33% 8% 13% 29% 15% 2	3480
	21st to end	Night	42% 4 8% 18% 23% 5%	3629
April	1st to 10th	Night	42% 6% 11% 20% 16% 4	1900
	11th to 20th	Night	41% 7% 10% 21% 19% 3	1875
	21st to end	Night	40% 6% 10% 22% 17% 5%	1900
May	1st to 10th	Night	52% 8% 9% 18% 11% 2	2891
	11th to 20th	Night	54% 9% 13% 12% 10% 2	2496
	21st to end	Night	36% 10% 14% 22% 15% 3	2739
June	1st to 10th	Night	39% 7% 12% 23% 18% 1	1520
	11th to 20th	Night	37% 9% 15% 23% 14% 1	1520
	21st to end	Night	34% 9% 15% 27% 13% 2	1520
July	1st to 10th	Night	39% 12% 15% 17% 14% 3	1464
	11th to 20th	Night	31% 11% 17% 24% 15% 3	1392
	21st to end	Night	33% 12% 15% 23% 15% 2	1554
August	1st to 10th	Night	35% 10% 17% 22% 15% 1	1600
	11th to 20th	Night	41% 8% 16% 18% 15% 2	1724
	21st to end	Night	37% 8% 16% 19% 18% 2	1980
September	1st to 10th	Night	39% 7% 10% 21% 20% 3	1800
	11th to 20th	Night	32% 6% 11% 21% 26% 4	1807
	21st to end	Night	40% 7% 9% 21% 22% 2	2011
October	1st to 10th	Night	43% 6% 11% 23% 15% 2	2160
	11th to 20th	Night	36% 6% 12% 27% 17% 2	2160
	21st to end	Night	39% 5% 10% 26% 18% 2	2412
November	1st to 10th	Night	32% 6% 12% 26% 19% 5%	2407
	11th to 20th	Night	39% 5% 9% 22% 19% 4	2470
	21st to end	Night	37% 6% 12% 28% 15% 2	2626
December	1st to 10th	Night	47% 6% 12% 21% 13% 1	2660
	11th to 20th	Night	42% 6% 12% 22% 15% 3	3288
	21st to end	Night	44% 8% 14% 23% 10% 1	4884

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	34% 23% 27% 13% 3	180
	11th to 20th	Day	37% 27% 28% 8%	213
	21st to end	Day	29% 29% 26% 13% 3	297
February	1st to 10th	Day	28% 28% 32% 9% 2 1	270
	11th to 20th	Day	32% 27% 32% 7% 1	382
	21st to end	Day	35% 22% 28% 12% 2	382
March	1st to 10th	Day	33% 19% 28% 15% 3	460
	11th to 20th	Day	30% 26% 28% 12% 3 1	460
	21st to end	Day	30% 25% 30% 10% 4 1	506
April	1st to 10th	Day	27% 25% 30% 13% 4 1	460
	11th to 20th	Day	24% 24% 27% 17% 6% 2	452
	21st to end	Day	28% 17% 35% 14% 3 2	460
May	1st to 10th	Day	33% 18% 28% 14% 5% 2	463
	11th to 20th	Day	24% 16% 32% 19% 7% 2	487
	21st to end	Day	17% 12% 29% 24% 17% 2	529
June	1st to 10th	Day	17% 13% 31% 25% 11% 2	490
	11th to 20th	Day	17% 10% 32% 25% 14% 2	490
	21st to end	Day	13% 11% 33% 26% 13% 3	490
July	1st to 10th	Day	12% 5% 24% 34% 20% 6%	474
	11th to 20th	Day	8% 5% 27% 35% 22% 4	444
	21st to end	Day	11% 8% 30% 31% 17% 3	488
August	1st to 10th	Day	10% 7% 28% 28% 21% 7%	446
	11th to 20th	Day	13% 5% 22% 34% 19% 7%	439
	21st to end	Day	12% 8% 25% 29% 17% 9%	495
September	1st to 10th	Day	20% 9% 21% 26% 16% 8%	450
	11th to 20th	Day	16% 6% 28% 24% 16% 10%	450
	21st to end	Day	21% 15% 25% 22% 13% 4	450
October	1st to 10th	Day	24% 10% 26% 22% 14% 5%	450
	11th to 20th	Day	19% 18% 26% 24% 12% 1	450
	21st to end	Day	22% 20% 32% 15% 8% 2	475
November	1st to 10th	Day	21% 15% 30% 22% 10% 3	365
	11th to 20th	Day	22% 22% 32% 19% 5%	370
	21st to end	Day	22% 22% 32% 21% 3	209
December	1st to 10th	Day	30% 26% 29% 9% 4 1	180
	11th to 20th	Day	36% 23% 26% 12% 3	180
	21st to end	Day	28% 25% 27% 14% 6% 1	198
January	1st to 10th	Night	21% 24% 36% 15% 3	760
	11th to 20th	Night	27% 20% 29% 21% 3	760
	21st to end	Night	19% 22% 39% 18% 2	836
February	1st to 10th	Night	23% 21% 35% 21% 1	760
	11th to 20th	Night	27% 26% 33% 12% 1	743
	21st to end	Night	27% 22% 35% 14% 3	628
March	1st to 10th	Night	23% 23% 31% 20% 3	760
	11th to 20th	Night	24% 24% 30% 17% 5%	760
	21st to end	Night	21% 18% 39% 17% 5%	836
April	1st to 10th	Night	21% 24% 35% 16% 4	760
	11th to 20th	Night	19% 17% 36% 22% 6%	751
	21st to end	Night	22% 16% 32% 24% 6%	760
May	1st to 10th	Night	24% 13% 32% 25% 6%	608
	11th to 20th	Night	19% 9% 34% 30% 7%	374
	21st to end	Night	14% 9% 26% 38% 11% 1	411
June	1st to 10th	Night	14% 7% 27% 41% 12%	380
	11th to 20th	Night	10% 8% 29% 42% 11%	380
	21st to end	Night	11% 4 29% 44% 13% 1	380
July	1st to 10th	Night	10% 5% 22% 41% 20% 1	366
	11th to 20th	Night	7% 2 23% 40% 25% 2	348
	21st to end	Night	10% 4 24% 39% 21% 2	466
August	1st to 10th	Night	10% 2 19% 42% 24% 2	537
	11th to 20th	Night	15% 4 19% 36% 25% 2	660
	21st to end	Night	12% 5% 19% 39% 22% 2	792
September	1st to 10th	Night	17% 4 28% 30% 18% 3	720
	11th to 20th	Night	14% 5% 20% 35% 22% 3	720
	21st to end	Night	19% 7% 23% 31% 19% 1	720
October	1st to 10th	Night	23% 9% 25% 24% 16% 3	720
	11th to 20th	Night	19% 11% 31% 28% 11%	720
	21st to end	Night	25% 14% 29% 23% 8%	792
November	1st to 10th	Night	16% 13% 31% 27% 11% 1	740
	11th to 20th	Night	22% 18% 27% 26% 7% 1	760
	21st to end	Night	18% 18% 35% 22% 7%	760
December	1st to 10th	Night	26% 19% 31% 19% 5% 1	760
	11th to 20th	Night	21% 19% 35% 21% 4	760
	21st to end	Night	22% 20% 32% 23% 2	836

Page generated at 2024-07-05 11:05:39. Version ac07

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