Winter Bay Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Winter Bay Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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	17% 12% 4 10% 5% 5% 5% 4 5% 7% 29%	128266
	Night	18% 13% 4 11% 6% 5% 5% 3 5% 6% 26%	122269
Transparency	Day	54% 5% 13% 20% 7%	131794
	Night	57% 5% 12% 18% 7% 1	126832
Seeing	Day	24% 12% 30% 24% 9% 1	18976
	Night	25% 12% 30% 25% 7% 1	25991

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% 23% 9% 22% 5% 4 4 3 3 4 10%	3616
	11th to 20th	Day	12% 15% 10% 14% 8% 6% 5% 4 4 5% 17%	3591
	21st to end	Day	9% 10% 5% 13% 6% 7% 7% 6% 5% 7% 25%	4332
February	1st to 10th	Day	17% 13% 4 11% 6% 5% 6% 4 4 5% 25%	1800
	11th to 20th	Day	16% 15% 5% 9% 7% 5% 5% 4 4 5% 25%	1753
	21st to end	Day	11% 12% 4 10% 8% 6% 5% 4 5% 7% 30%	1535
March	1st to 10th	Day	18% 13% 5% 11% 7% 6% 4 3 4 5% 23%	4746
	11th to 20th	Day	12% 14% 6% 12% 7% 5% 5% 3 6% 7% 23%	4980
	21st to end	Day	21% 17% 5% 14% 6% 5% 4 3 4 5% 17%	5557
April	1st to 10th	Day	18% 13% 4 9% 5% 4 3 3 5% 6% 29%	2400
	11th to 20th	Day	20% 11% 3 7% 4 4 4 2 5% 8% 33%	2424
	21st to end	Day	20% 12% 3 8% 3 3 4 3 5% 7% 31%	2600
May	1st to 10th	Day	25% 13% 3 7% 5% 4 4 3 4 5% 28%	6286
	11th to 20th	Day	19% 9% 3 7% 5% 4 5% 3 4 6% 35%	6318
	21st to end	Day	17% 10% 3 7% 5% 4 4 4 5% 8% 33%	6948
June	1st to 10th	Day	23% 11% 3 8% 4 4 5% 4 5% 7% 25%	2800
	11th to 20th	Day	19% 9% 2 7% 4 4 4 3 6% 7% 35%	2990
	21st to end	Day	22% 10% 3 7% 5% 3 4 3 5% 7% 30%	3000
July	1st to 10th	Day	16% 9% 3 7% 5% 5% 5% 5% 7% 8% 30%	6804
	11th to 20th	Day	13% 6% 2 7% 4 4 4 4 6% 8% 42%	6360
	21st to end	Day	12% 7% 2 7% 5% 4 5% 4 7% 9% 39%	6927
August	1st to 10th	Day	16% 10% 3 8% 5% 4 5% 4 6% 7% 32%	4316
	11th to 20th	Day	14% 7% 2 7% 4 4 5% 4 7% 8% 38%	4175
	21st to end	Day	13% 6% 2 6% 4 4 4 4 6% 8% 43%	4458
September	1st to 10th	Day	16% 7% 2 5% 3 3 4 4 6% 8% 41%	2400
	11th to 20th	Day	18% 8% 3 7% 4 3 4 3 5% 6% 38%	2391
	21st to end	Day	18% 9% 3 6% 4 4 3 3 4 6% 39%	2093
October	1st to 10th	Day	19% 8% 3 7% 4 4 5% 5% 5% 8% 33%	2000
	11th to 20th	Day	20% 12% 3 9% 6% 5% 4 4 6% 7% 25%	2000
	21st to end	Day	20% 14% 4 9% 5% 4 5% 4 5% 6% 24%	2127
November	1st to 10th	Day	20% 18% 4 9% 6% 4 5% 3 5% 5% 22%	1695
	11th to 20th	Day	23% 21% 5% 10% 6% 5% 4 3 3 6% 15%	1656
	21st to end	Day	19% 18% 6% 11% 7% 4 5% 4 5% 7% 14%	1600
December	1st to 10th	Day	16% 18% 7% 17% 7% 5% 4 3 4 5% 15%	2610
	11th to 20th	Day	19% 17% 7% 15% 6% 7% 4 2 3 4 17%	2996
	21st to end	Day	18% 14% 9% 19% 6% 5% 5% 3 3 4 14%	3982
January	1st to 10th	Night	16% 19% 7% 20% 8% 4 3 3 3 3 14%	6349
	11th to 20th	Night	14% 17% 9% 15% 7% 5% 4 3 4 4 19%	6314
	21st to end	Night	9% 11% 6% 16% 9% 6% 6% 5% 5% 5% 22%	6628
February	1st to 10th	Night	17% 15% 3 10% 6% 4 4 4 5% 5% 27%	2600
	11th to 20th	Night	17% 15% 4 10% 7% 5% 5% 4 5% 5% 23%	2533
	21st to end	Night	12% 15% 4 9% 6% 5% 4 4 4 6% 30%	2095
March	1st to 10th	Night	21% 14% 5% 10% 6% 5% 6% 3 4 5% 22%	5090
	11th to 20th	Night	17% 15% 5% 11% 6% 4 4 3 5% 5% 24%	4979
	21st to end	Night	23% 16% 5% 11% 5% 5% 5% 3 4 4 20%	5361
April	1st to 10th	Night	22% 11% 3 9% 5% 5% 5% 4 5% 5% 26%	2000
	11th to 20th	Night	22% 12% 3 6% 4 5% 4 2 4 5% 32%	1923
	21st to end	Night	22% 15% 3 9% 5% 4 3 2 4 5% 29%	1800
May	1st to 10th	Night	29% 14% 3 7% 4 4 4 3 4 5% 22%	3670
	11th to 20th	Night	17% 10% 2 7% 4 5% 5% 4 5% 7% 33%	3596
	21st to end	Night	14% 11% 3 7% 5% 4 5% 4 5% 8% 33%	3948
June	1st to 10th	Night	26% 8% 4 7% 5% 4 4 3 4 6% 28%	1600
	11th to 20th	Night	17% 10% 2 5% 3 4 5% 4 4 7% 39%	1423
	21st to end	Night	23% 9% 4 8% 5% 4 5% 3 6% 6% 26%	1400
July	1st to 10th	Night	14% 9% 3 10% 6% 7% 5% 5% 7% 8% 26%	3160
	11th to 20th	Night	11% 6% 3 7% 5% 4 6% 4 7% 9% 38%	3574
	21st to end	Night	15% 7% 3 6% 4 4 4 4 6% 9% 38%	3948
August	1st to 10th	Night	18% 8% 4 7% 5% 4 5% 5% 6% 8% 31%	2833
	11th to 20th	Night	10% 7% 3 7% 5% 5% 5% 4 5% 8% 40%	2872
	21st to end	Night	13% 6% 2 6% 4 4 4 4 5% 7% 44%	3439
September	1st to 10th	Night	15% 7% 2 6% 4 4 5% 3 4 7% 43%	2000
	11th to 20th	Night	17% 6% 2 6% 4 5% 5% 3 5% 7% 41%	2005
	21st to end	Night	19% 6% 2 6% 4 3 4 4 5% 7% 40%	2306
October	1st to 10th	Night	18% 7% 3 8% 5% 3 4 3 5% 6% 37%	2400
	11th to 20th	Night	19% 11% 4 7% 5% 4 4 4 5% 6% 31%	2400
	21st to end	Night	24% 13% 4 8% 4 4 4 4 6% 6% 25%	2712
November	1st to 10th	Night	19% 17% 3 10% 6% 4 4 3 4 5% 24%	2823
	11th to 20th	Night	24% 17% 5% 10% 4 4 4 4 5% 6% 18%	2899
	21st to end	Night	26% 18% 5% 9% 5% 5% 4 3 3 5% 17%	2800
December	1st to 10th	Night	16% 14% 5% 14% 7% 6% 6% 4 4 5% 19%	4580
	11th to 20th	Night	19% 16% 6% 17% 6% 5% 4 2 3 3 20%	5213
	21st to end	Night	22% 15% 7% 19% 5% 4 3 2 4 4 16%	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	79% 3 10% 5% 3	3617
	11th to 20th	Day	64% 5% 12% 13% 6%	3620
	21st to end	Day	52% 4 12% 22% 9% 1	4332
February	1st to 10th	Day	59% 4 10% 17% 9% 1	1800
	11th to 20th	Day	60% 4 10% 17% 7% 1	1761
	21st to end	Day	53% 5% 12% 17% 11% 2	1535
March	1st to 10th	Day	63% 3 8% 18% 7% 1	4750
	11th to 20th	Day	60% 6% 15% 14% 5% 1	4980
	21st to end	Day	69% 4 8% 12% 5% 1	5557
April	1st to 10th	Day	57% 4 9% 17% 10% 3	2400
	11th to 20th	Day	51% 4 10% 22% 12% 1	2424
	21st to end	Day	54% 5% 12% 16% 11% 2	2600
May	1st to 10th	Day	60% 4 12% 16% 8%	6298
	11th to 20th	Day	51% 5% 14% 24% 5%	6322
	21st to end	Day	50% 7% 12% 22% 9%	6948
June	1st to 10th	Day	57% 5% 12% 20% 6%	2800
	11th to 20th	Day	48% 6% 12% 24% 9% 1	2990
	21st to end	Day	54% 5% 13% 18% 9% 1	3000
July	1st to 10th	Day	51% 6% 17% 19% 7%	6729
	11th to 20th	Day	41% 5% 18% 29% 7%	6284
	21st to end	Day	41% 8% 19% 26% 5%	6730
August	1st to 10th	Day	51% 6% 13% 24% 6%	4302
	11th to 20th	Day	42% 6% 15% 26% 10% 1	4214
	21st to end	Day	38% 5% 15% 29% 12%	4458
September	1st to 10th	Day	41% 6% 15% 28% 11%	2400
	11th to 20th	Day	46% 5% 13% 24% 11%	2391
	21st to end	Day	45% 5% 11% 26% 13% 1	2093
October	1st to 10th	Day	50% 6% 13% 24% 7%	3260
	11th to 20th	Day	50% 5% 15% 24% 6%	3240
	21st to end	Day	52% 5% 16% 20% 6%	3421
November	1st to 10th	Day	63% 4 11% 17% 5%	1695
	11th to 20th	Day	68% 5% 12% 12% 3	1656
	21st to end	Day	67% 5% 15% 12% 2	1600
December	1st to 10th	Day	67% 5% 10% 16% 1	2610
	11th to 20th	Day	69% 3 12% 9% 7%	2995
	21st to end	Day	73% 4 11% 10% 2	3982
January	1st to 10th	Night	68% 5% 13% 12% 2	6349
	11th to 20th	Night	65% 5% 11% 14% 5%	6360
	21st to end	Night	55% 5% 11% 17% 10% 1	6628
February	1st to 10th	Night	58% 4 10% 15% 11% 2	2600
	11th to 20th	Night	61% 3 10% 16% 9% 1	2536
	21st to end	Night	55% 4 9% 20% 10% 2	2095
March	1st to 10th	Night	64% 3 5% 17% 10% 2	5099
	11th to 20th	Night	63% 4 11% 14% 7% 1	4980
	21st to end	Night	68% 3 6% 15% 7% 1	5361
April	1st to 10th	Night	59% 4 10% 14% 11% 1	2000
	11th to 20th	Night	55% 3 8% 21% 11% 2	1923
	21st to end	Night	58% 4 8% 16% 13% 2	1800
May	1st to 10th	Night	65% 4 8% 13% 10% 1	3675
	11th to 20th	Night	50% 5% 14% 22% 8% 1	3597
	21st to end	Night	49% 6% 17% 23% 5%	3949
June	1st to 10th	Night	58% 5% 11% 19% 7%	1600
	11th to 20th	Night	44% 6% 13% 23% 13%	1423
	21st to end	Night	57% 7% 13% 14% 9% 1	1400
July	1st to 10th	Night	53% 7% 15% 18% 6% 1	3125
	11th to 20th	Night	41% 7% 18% 25% 8%	3527
	21st to end	Night	42% 7% 20% 23% 8%	3831
August	1st to 10th	Night	51% 7% 15% 22% 5%	2822
	11th to 20th	Night	41% 7% 19% 23% 9% 1	2899
	21st to end	Night	38% 5% 18% 28% 11%	3438
September	1st to 10th	Night	41% 5% 12% 31% 11%	2000
	11th to 20th	Night	43% 4 13% 28% 12%	2005
	21st to end	Night	43% 5% 12% 29% 11%	2306
October	1st to 10th	Night	48% 5% 16% 22% 8%	3920
	11th to 20th	Night	44% 4 17% 25% 10%	3899
	21st to end	Night	53% 5% 13% 20% 9%	4372
November	1st to 10th	Night	61% 4 13% 15% 6%	2823
	11th to 20th	Night	63% 5% 13% 13% 5%	2899
	21st to end	Night	67% 5% 14% 11% 2	2800
December	1st to 10th	Night	59% 5% 14% 20% 2	4580
	11th to 20th	Night	67% 4 8% 15% 5% 1	5215
	21st to end	Night	71% 3 12% 11% 3	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	27% 20% 35% 13% 4	400
	11th to 20th	Day	26% 20% 28% 20% 6% 1	400
	21st to end	Day	16% 17% 39% 24% 3	440
February	1st to 10th	Day	28% 17% 34% 15% 5% 1	400
	11th to 20th	Day	23% 24% 32% 16% 5%	396
	21st to end	Day	17% 17% 33% 26% 6% 1	331
March	1st to 10th	Day	23% 16% 35% 22% 4	459
	11th to 20th	Day	18% 18% 34% 23% 6% 1	500
	21st to end	Day	28% 15% 33% 16% 7% 1	550
April	1st to 10th	Day	27% 17% 36% 15% 4	500
	11th to 20th	Day	24% 16% 34% 18% 7% 1	492
	21st to end	Day	26% 13% 29% 23% 7% 1	500
May	1st to 10th	Day	34% 11% 28% 20% 6% 2	670
	11th to 20th	Day	23% 9% 28% 26% 15% 1	724
	21st to end	Day	22% 10% 32% 26% 9% 1	790
June	1st to 10th	Day	30% 7% 28% 24% 9% 1	730
	11th to 20th	Day	21% 6% 30% 31% 10% 1	730
	21st to end	Day	26% 7% 25% 29% 11% 2	730
July	1st to 10th	Day	20% 4 30% 32% 12% 2	720
	11th to 20th	Day	15% 4 30% 33% 17% 1	718
	21st to end	Day	19% 5% 25% 35% 14% 2	795
August	1st to 10th	Day	14% 3 24% 40% 15% 4	581
	11th to 20th	Day	19% 5% 23% 33% 17% 4	519
	21st to end	Day	18% 7% 25% 32% 14% 4	583
September	1st to 10th	Day	16% 6% 32% 32% 11% 3	530
	11th to 20th	Day	23% 11% 27% 27% 10% 3	530
	21st to end	Day	21% 10% 31% 25% 11% 2	470
October	1st to 10th	Day	21% 12% 25% 27% 13% 3	420
	11th to 20th	Day	24% 14% 30% 20% 11% 2	420
	21st to end	Day	26% 17% 32% 18% 6% 1	462
November	1st to 10th	Day	29% 18% 32% 15% 5% 1	425
	11th to 20th	Day	38% 19% 25% 14% 4 1	421
	21st to end	Day	33% 19% 30% 16% 3	400
December	1st to 10th	Day	32% 16% 32% 17% 3	400
	11th to 20th	Day	32% 24% 31% 12% 2	400
	21st to end	Day	25% 23% 32% 17% 2 1	440
January	1st to 10th	Night	28% 16% 34% 18% 4	900
	11th to 20th	Night	26% 16% 32% 21% 5% 1	900
	21st to end	Night	20% 22% 31% 22% 5% 1	910
February	1st to 10th	Night	27% 14% 33% 21% 5%	800
	11th to 20th	Night	25% 17% 37% 17% 3	783
	21st to end	Night	20% 13% 35% 22% 9% 1	660
March	1st to 10th	Night	24% 13% 38% 22% 3	800
	11th to 20th	Night	21% 14% 32% 25% 8% 1	800
	21st to end	Night	29% 14% 27% 21% 8% 1	839
April	1st to 10th	Night	29% 13% 30% 23% 6%	600
	11th to 20th	Night	27% 11% 31% 21% 8% 1	594
	21st to end	Night	29% 9% 30% 23% 8% 1	600
May	1st to 10th	Night	32% 8% 25% 24% 10% 2	600
	11th to 20th	Night	23% 5% 28% 33% 10% 2	592
	21st to end	Night	21% 7% 37% 28% 6% 1	649
June	1st to 10th	Night	29% 7% 25% 31% 8% 1	600
	11th to 20th	Night	22% 5% 31% 34% 9%	423
	21st to end	Night	24% 5% 31% 33% 7% 1	400
July	1st to 10th	Night	19% 6% 27% 35% 13%	390
	11th to 20th	Night	17% 2 30% 39% 12%	563
	21st to end	Night	20% 5% 27% 36% 12% 1	656
August	1st to 10th	Night	17% 4 25% 39% 15% 1	598
	11th to 20th	Night	17% 5% 23% 37% 16% 2	592
	21st to end	Night	15% 5% 24% 37% 18% 1	660
September	1st to 10th	Night	17% 5% 34% 34% 11% 1	600
	11th to 20th	Night	20% 8% 23% 38% 11% 1	605
	21st to end	Night	20% 10% 31% 28% 9% 1	800
October	1st to 10th	Night	23% 9% 27% 29% 11%	800
	11th to 20th	Night	26% 11% 28% 23% 10% 2	800
	21st to end	Night	32% 15% 29% 20% 4	921
November	1st to 10th	Night	31% 16% 32% 16% 4 1	930
	11th to 20th	Night	33% 17% 28% 17% 4 1	936
	21st to end	Night	35% 18% 27% 17% 2	900
December	1st to 10th	Night	31% 17% 31% 17% 4	900
	11th to 20th	Night	29% 23% 32% 14% 2	900
	21st to end	Night	32% 17% 31% 17% 3 1	990

Page generated at 2025-11-15 01:56:02. Version ac07

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