Whiskey Bay Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Whiskey Bay Observatory clear sky chart from 2005-11-06 to 2026-01-02.

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	13% 10% 3 8% 5% 4 4 4 5% 7% 35%	126998
	Night	15% 12% 4 9% 5% 5% 5% 4 5% 6% 31%	124532
Transparency	Day	29% 40% 8% 15% 7% 1	130471
	Night	33% 40% 7% 13% 6% 1	129146
Seeing	Day	17% 10% 29% 29% 13% 3	17007
	Night	20% 10% 29% 28% 12% 2	26488

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	12% 14% 5% 16% 7% 6% 5% 4 6% 6% 20%	3440
	11th to 20th	Day	11% 16% 5% 12% 10% 7% 7% 5% 7% 6% 16%	3589
	21st to end	Day	11% 18% 8% 15% 7% 6% 5% 4 4 6% 15%	3982
February	1st to 10th	Day	8% 14% 5% 12% 8% 6% 6% 4 6% 7% 24%	1759
	11th to 20th	Day	11% 12% 4 11% 7% 6% 5% 5% 6% 5% 29%	1833
	21st to end	Day	10% 13% 5% 11% 6% 6% 5% 4 5% 6% 28%	1650
March	1st to 10th	Day	13% 10% 4 9% 4 5% 4 4 6% 7% 35%	4478
	11th to 20th	Day	11% 12% 4 10% 7% 6% 5% 4 5% 7% 30%	4979
	21st to end	Day	16% 11% 5% 9% 6% 5% 4 4 5% 5% 30%	5479
April	1st to 10th	Day	16% 12% 4 7% 5% 4 5% 4 6% 6% 32%	2400
	11th to 20th	Day	17% 9% 3 7% 4 4 4 4 5% 5% 37%	2563
	21st to end	Day	17% 8% 3 8% 4 5% 5% 4 5% 8% 34%	2600
May	1st to 10th	Day	16% 7% 2 6% 5% 4 4 3 5% 6% 42%	5888
	11th to 20th	Day	13% 11% 3 6% 4 3 4 4 5% 6% 42%	5989
	21st to end	Day	12% 8% 3 7% 5% 4 4 3 5% 6% 43%	7406
June	1st to 10th	Day	13% 6% 2 6% 5% 4 5% 4 6% 8% 42%	3000
	11th to 20th	Day	12% 6% 2 6% 4 5% 4 5% 5% 8% 41%	3000
	21st to end	Day	14% 7% 3 5% 4 4 4 3 6% 8% 41%	3000
July	1st to 10th	Day	14% 6% 2 5% 3 4 4 4 5% 9% 44%	6819
	11th to 20th	Day	8% 4 2 5% 3 4 4 3 6% 9% 51%	6389
	21st to end	Day	9% 4 2 5% 3 3 4 3 6% 10% 50%	6926
August	1st to 10th	Day	7% 5% 2 6% 5% 4 5% 4 6% 10% 45%	4309
	11th to 20th	Day	12% 6% 2 5% 4 5% 4 4 6% 7% 45%	4174
	21st to end	Day	12% 6% 3 6% 4 3 3 3 5% 8% 47%	4675
September	1st to 10th	Day	13% 7% 2 6% 4 4 4 3 6% 7% 43%	2334
	11th to 20th	Day	13% 4 2 6% 4 4 4 3 6% 9% 44%	2200
	21st to end	Day	15% 8% 3 7% 5% 4 4 3 6% 8% 38%	2200
October	1st to 10th	Day	19% 7% 2 7% 5% 3 4 4 5% 7% 37%	2135
	11th to 20th	Day	20% 9% 4 8% 7% 5% 5% 4 6% 8% 22%	1945
	21st to end	Day	21% 10% 4 9% 6% 6% 6% 5% 5% 7% 23%	1980
November	1st to 10th	Day	19% 11% 4 9% 6% 4 6% 4 6% 8% 25%	1852
	11th to 20th	Day	14% 16% 5% 11% 7% 6% 5% 4 6% 6% 19%	1767
	21st to end	Day	20% 18% 6% 12% 6% 5% 5% 3 4 6% 15%	1664
December	1st to 10th	Day	14% 19% 5% 11% 7% 6% 5% 4 6% 5% 17%	2350
	11th to 20th	Day	16% 15% 6% 16% 8% 5% 4 4 5% 6% 16%	2691
	21st to end	Day	24% 22% 7% 13% 5% 4 3 3 4 4 12%	3553
January	1st to 10th	Night	12% 12% 5% 16% 6% 5% 5% 4 5% 6% 23%	6533
	11th to 20th	Night	12% 17% 6% 15% 8% 7% 6% 4 5% 6% 15%	6316
	21st to end	Night	10% 15% 7% 17% 8% 7% 5% 4 5% 6% 16%	6996
February	1st to 10th	Night	10% 16% 5% 11% 8% 6% 5% 4 6% 6% 22%	2640
	11th to 20th	Night	10% 13% 4 11% 8% 5% 5% 4 6% 6% 26%	2448
	21st to end	Night	12% 14% 5% 10% 7% 6% 5% 5% 5% 6% 26%	1980
March	1st to 10th	Night	16% 10% 3 7% 5% 4 4 3 5% 7% 36%	5362
	11th to 20th	Night	12% 11% 4 8% 5% 5% 5% 4 6% 8% 33%	4979
	21st to end	Night	19% 10% 5% 9% 4 4 4 3 4 5% 32%	5467
April	1st to 10th	Night	18% 10% 4 8% 5% 5% 5% 4 5% 6% 31%	2015
	11th to 20th	Night	19% 8% 4 6% 4 3 4 3 5% 6% 37%	1785
	21st to end	Night	19% 9% 3 7% 5% 4 4 4 5% 6% 35%	1800
May	1st to 10th	Night	16% 7% 2 7% 5% 5% 4 4 4 6% 39%	4053
	11th to 20th	Night	15% 10% 3 9% 5% 4 4 3 4 5% 38%	3909
	21st to end	Night	15% 9% 3 8% 4 4 3 3 5% 6% 38%	3478
June	1st to 10th	Night	13% 6% 2 6% 4 4 5% 4 7% 9% 40%	1400
	11th to 20th	Night	10% 7% 3 6% 5% 4 6% 4 7% 9% 41%	1400
	21st to end	Night	13% 8% 2 7% 4 4 4 3 6% 8% 41%	1400
July	1st to 10th	Night	13% 6% 2 5% 4 4 5% 3 6% 9% 43%	3160
	11th to 20th	Night	8% 4 2 6% 4 4 4 3 6% 8% 51%	3574
	21st to end	Night	8% 5% 2 5% 3 3 4 3 5% 10% 53%	3948
August	1st to 10th	Night	8% 6% 3 7% 4 4 5% 4 6% 8% 45%	2840
	11th to 20th	Night	12% 5% 2 6% 4 4 4 3 5% 10% 45%	2875
	21st to end	Night	10% 5% 3 6% 4 4 4 3 5% 6% 52%	3223
September	1st to 10th	Night	12% 6% 2 5% 5% 5% 4 4 5% 6% 45%	2044
	11th to 20th	Night	14% 6% 2 5% 3 3 4 3 6% 8% 48%	2200
	21st to end	Night	17% 9% 3 7% 4 4 4 5% 5% 5% 37%	2200
October	1st to 10th	Night	17% 8% 2 6% 5% 3 4 2 6% 7% 39%	2265
	11th to 20th	Night	20% 11% 3 8% 5% 5% 5% 3 5% 7% 28%	2450
	21st to end	Night	26% 12% 3 7% 5% 5% 4 3 5% 6% 25%	2860
November	1st to 10th	Night	17% 12% 3 8% 6% 4 5% 3 6% 7% 29%	2666
	11th to 20th	Night	18% 16% 4 10% 7% 5% 5% 4 5% 6% 20%	2851
	21st to end	Night	19% 19% 6% 11% 6% 4 4 3 4 5% 18%	2952
December	1st to 10th	Night	13% 17% 5% 12% 6% 6% 6% 4 6% 6% 19%	5060
	11th to 20th	Night	15% 17% 6% 13% 6% 5% 5% 3 5% 5% 20%	5736
	21st to end	Night	25% 23% 7% 11% 4 4 3 2 3 4 13%	7667

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	46% 34% 5% 8% 4 3	3441
	11th to 20th	Day	48% 32% 7% 9% 5%	3620
	21st to end	Day	52% 33% 3 6% 5%	3982
February	1st to 10th	Day	22% 64% 4 8% 2	1759
	11th to 20th	Day	22% 62% 4 7% 5%	1842
	21st to end	Day	22% 63% 4 7% 4	1650
March	1st to 10th	Day	33% 34% 10% 10% 11% 2	4480
	11th to 20th	Day	40% 31% 10% 12% 6% 1	4979
	21st to end	Day	40% 33% 8% 11% 8% 1	5479
April	1st to 10th	Day	21% 63% 4 8% 4 1	2400
	11th to 20th	Day	18% 63% 3 8% 6% 1	2563
	21st to end	Day	18% 62% 5% 9% 6% 1	2600
May	1st to 10th	Day	29% 33% 6% 17% 13% 2	5897
	11th to 20th	Day	30% 33% 6% 16% 13% 1	5993
	21st to end	Day	30% 31% 9% 14% 11% 4	7406
June	1st to 10th	Day	15% 63% 5% 10% 6% 2	3000
	11th to 20th	Day	18% 63% 5% 9% 5% 1	3000
	21st to end	Day	16% 61% 5% 11% 6% 1	3000
July	1st to 10th	Day	30% 28% 11% 21% 10%	6746
	11th to 20th	Day	21% 28% 14% 31% 5%	6313
	21st to end	Day	21% 27% 10% 28% 12%	6728
August	1st to 10th	Day	20% 39% 9% 24% 8%	4294
	11th to 20th	Day	26% 38% 9% 20% 7%	4213
	21st to end	Day	25% 37% 13% 21% 4	4674
September	1st to 10th	Day	18% 59% 7% 11% 4	2334
	11th to 20th	Day	14% 58% 7% 16% 5%	2200
	21st to end	Day	18% 59% 7% 11% 4	2200
October	1st to 10th	Day	19% 37% 9% 22% 13%	3473
	11th to 20th	Day	28% 38% 7% 23% 3	3148
	21st to end	Day	37% 38% 7% 13% 4	3180
November	1st to 10th	Day	27% 55% 7% 9% 3	1852
	11th to 20th	Day	29% 57% 6% 5% 2	1767
	21st to end	Day	34% 56% 4 4 2	1664
December	1st to 10th	Day	44% 37% 10% 6% 2	2350
	11th to 20th	Day	46% 35% 7% 8% 4	2691
	21st to end	Day	57% 29% 4 6% 3	3553
January	1st to 10th	Night	45% 31% 7% 8% 6% 2	6533
	11th to 20th	Night	50% 32% 7% 8% 3	6360
	21st to end	Night	48% 34% 6% 8% 3 1	6996
February	1st to 10th	Night	21% 64% 5% 7% 3	2640
	11th to 20th	Night	20% 63% 4 10% 3	2450
	21st to end	Night	21% 63% 4 7% 5%	1980
March	1st to 10th	Night	34% 31% 11% 11% 10% 2	5373
	11th to 20th	Night	35% 31% 14% 12% 6% 1	4980
	21st to end	Night	39% 32% 7% 12% 9% 1	5467
April	1st to 10th	Night	22% 62% 3 8% 5%	2015
	11th to 20th	Night	19% 62% 3 8% 6% 1	1785
	21st to end	Night	21% 62% 3 7% 6% 1	1800
May	1st to 10th	Night	32% 32% 6% 16% 11% 3	4060
	11th to 20th	Night	35% 31% 4 19% 11% 1	3910
	21st to end	Night	36% 31% 6% 14% 7% 5%	3479
June	1st to 10th	Night	15% 62% 5% 10% 8% 1	1400
	11th to 20th	Night	18% 63% 4 8% 5% 2	1400
	21st to end	Night	17% 61% 4 10% 7% 1	1400
July	1st to 10th	Night	31% 28% 10% 20% 10% 1	3125
	11th to 20th	Night	23% 28% 12% 27% 11%	3527
	21st to end	Night	19% 27% 11% 29% 13% 1	3832
August	1st to 10th	Night	24% 38% 8% 20% 9%	2831
	11th to 20th	Night	25% 39% 9% 19% 7% 1	2902
	21st to end	Night	22% 37% 8% 24% 9%	3223
September	1st to 10th	Night	16% 59% 8% 11% 6%	2044
	11th to 20th	Night	13% 58% 7% 16% 6%	2200
	21st to end	Night	19% 58% 7% 11% 5%	2200
October	1st to 10th	Night	25% 37% 9% 21% 7%	3707
	11th to 20th	Night	23% 37% 14% 18% 8%	3986
	21st to end	Night	34% 38% 11% 14% 2	4610
November	1st to 10th	Night	27% 54% 6% 8% 4	2666
	11th to 20th	Night	27% 56% 6% 7% 3	2851
	21st to end	Night	33% 56% 3 5% 2	2952
December	1st to 10th	Night	41% 39% 10% 9% 1	5060
	11th to 20th	Night	44% 36% 6% 7% 6% 1	5735
	21st to end	Night	59% 27% 5% 6% 3	7667

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	19% 15% 32% 23% 10% 1	262
	11th to 20th	Day	22% 10% 36% 26% 7% 1	300
	21st to end	Day	21% 12% 31% 27% 9% 1	330
February	1st to 10th	Day	16% 20% 39% 17% 6% 1	300
	11th to 20th	Day	16% 14% 44% 23% 2 1	300
	21st to end	Day	17% 21% 39% 20% 3	249
March	1st to 10th	Day	17% 16% 35% 25% 6% 1	300
	11th to 20th	Day	17% 14% 34% 27% 7% 1	499
	21st to end	Day	23% 12% 31% 25% 9% 1	550
April	1st to 10th	Day	21% 12% 31% 26% 9% 1	500
	11th to 20th	Day	22% 15% 29% 22% 10% 2	492
	21st to end	Day	21% 12% 34% 21% 11% 2	500
May	1st to 10th	Day	19% 9% 31% 27% 12% 1	503
	11th to 20th	Day	16% 5% 34% 29% 12% 3	577
	21st to end	Day	13% 7% 29% 35% 14% 3	789
June	1st to 10th	Day	12% 7% 32% 35% 13% 2	730
	11th to 20th	Day	14% 7% 31% 35% 11% 2	730
	21st to end	Day	18% 8% 24% 35% 12% 2	730
July	1st to 10th	Day	10% 3 25% 40% 18% 3	721
	11th to 20th	Day	11% 5% 26% 36% 19% 3	718
	21st to end	Day	10% 5% 23% 37% 22% 4	794
August	1st to 10th	Day	12% 3 18% 37% 26% 4	576
	11th to 20th	Day	13% 4 17% 32% 25% 9%	519
	21st to end	Day	11% 7% 24% 32% 22% 4	583
September	1st to 10th	Day	15% 6% 24% 28% 21% 6%	530
	11th to 20th	Day	13% 5% 17% 34% 24% 8%	530
	21st to end	Day	18% 9% 20% 32% 17% 4	530
October	1st to 10th	Day	20% 12% 27% 21% 16% 3	530
	11th to 20th	Day	20% 16% 30% 28% 6% 1	475
	21st to end	Day	25% 14% 34% 22% 6% 1	363
November	1st to 10th	Day	20% 16% 33% 23% 7% 1	330
	11th to 20th	Day	23% 17% 38% 18% 5%	265
	21st to end	Day	25% 22% 36% 15% 2	220
December	1st to 10th	Day	24% 17% 32% 20% 7%	220
	11th to 20th	Day	26% 21% 35% 14% 5%	220
	21st to end	Day	29% 19% 35% 14% 3	242
January	1st to 10th	Night	21% 13% 34% 25% 7%	1006
	11th to 20th	Night	23% 13% 32% 24% 8% 1	1000
	21st to end	Night	21% 9% 33% 29% 7% 1	1100
February	1st to 10th	Night	15% 13% 40% 24% 8%	840
	11th to 20th	Night	16% 13% 36% 26% 8% 1	783
	21st to end	Night	18% 12% 36% 28% 6%	660
March	1st to 10th	Night	17% 10% 36% 28% 8% 1	800
	11th to 20th	Night	18% 12% 35% 28% 7% 1	800
	21st to end	Night	20% 8% 32% 30% 9% 1	880
April	1st to 10th	Night	22% 10% 31% 31% 6% 1	615
	11th to 20th	Night	22% 9% 26% 30% 12% 1	593
	21st to end	Night	23% 6% 30% 27% 12% 1	600
May	1st to 10th	Night	13% 7% 28% 38% 12% 2	600
	11th to 20th	Night	18% 6% 25% 33% 17% 1	591
	21st to end	Night	13% 6% 28% 35% 18% 1	452
June	1st to 10th	Night	14% 5% 26% 39% 16%	400
	11th to 20th	Night	11% 5% 26% 38% 19% 2	400
	21st to end	Night	15% 5% 23% 38% 19% 2	400
July	1st to 10th	Night	12% 1 19% 48% 19% 1	390
	11th to 20th	Night	8% 2 19% 40% 26% 5%	563
	21st to end	Night	11% 3 19% 37% 26% 3	657
August	1st to 10th	Night	11% 3 18% 40% 25% 3	597
	11th to 20th	Night	11% 3 18% 32% 30% 6%	594
	21st to end	Night	13% 4 19% 37% 25% 2	660
September	1st to 10th	Night	13% 6% 22% 30% 23% 6%	660
	11th to 20th	Night	14% 4 22% 31% 22% 6%	800
	21st to end	Night	20% 5% 23% 27% 21% 5%	800
October	1st to 10th	Night	20% 9% 26% 28% 15% 2	800
	11th to 20th	Night	24% 14% 28% 26% 7% 1	800
	21st to end	Night	32% 10% 28% 23% 7% 1	880
November	1st to 10th	Night	23% 14% 31% 24% 8%	820
	11th to 20th	Night	23% 18% 31% 21% 6% 1	840
	21st to end	Night	31% 18% 29% 18% 4	852
December	1st to 10th	Night	23% 17% 34% 20% 6% 1	1050
	11th to 20th	Night	26% 15% 37% 15% 6% 1	1050
	21st to end	Night	33% 13% 30% 18% 6%	1155

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