Bays Mountain Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bays Mountain Park clear sky chart from 2005-11-06 to 2025-11-03.

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% 8% 3 7% 5% 5% 5% 4 6% 8% 36%	126675
	Night	14% 8% 3 7% 5% 4 4 4 6% 7% 38%	123381
Transparency	Day	43% 7% 12% 21% 14% 3	130386
	Night	43% 7% 11% 21% 15% 3	127739
Seeing	Day	17% 9% 19% 23% 21% 10%	16340
	Night	17% 6% 20% 35% 19% 2	26365

By Month

Percentage Cloud Cover SE (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	15% 15% 4 10% 4 3 4 3 4 5% 32%	4056
	11th to 20th	Day	15% 12% 7% 10% 6% 4 3 2 3 4 33%	4060
	21st to end	Day	16% 12% 3 7% 5% 4 4 3 4 5% 37%	4466
February	1st to 10th	Day	18% 11% 4 7% 4 4 4 3 5% 6% 33%	1860
	11th to 20th	Day	22% 10% 4 9% 5% 5% 4 3 4 6% 30%	1948
	21st to end	Day	22% 9% 3 7% 4 4 4 3 4 6% 34%	1650
March	1st to 10th	Day	17% 12% 3 8% 4 4 4 3 4 6% 35%	4797
	11th to 20th	Day	12% 7% 2 6% 4 4 3 3 5% 6% 47%	4980
	21st to end	Day	15% 8% 3 7% 5% 4 4 3 5% 6% 41%	5465
April	1st to 10th	Day	13% 8% 2 7% 5% 3 4 4 7% 8% 40%	2200
	11th to 20th	Day	15% 7% 2 5% 5% 4 4 3 5% 7% 43%	2435
	21st to end	Day	14% 7% 2 7% 5% 5% 4 4 5% 8% 39%	2600
May	1st to 10th	Day	15% 9% 3 9% 6% 6% 6% 4 7% 8% 26%	5862
	11th to 20th	Day	16% 8% 3 9% 6% 6% 6% 5% 6% 7% 26%	5861
	21st to end	Day	15% 6% 2 7% 5% 5% 6% 5% 7% 9% 33%	6433
June	1st to 10th	Day	10% 6% 2 7% 5% 6% 6% 5% 7% 8% 38%	2600
	11th to 20th	Day	7% 5% 3 7% 7% 7% 6% 5% 8% 10% 36%	2600
	21st to end	Day	9% 7% 2 6% 5% 5% 6% 5% 8% 10% 37%	2600
July	1st to 10th	Day	6% 5% 2 6% 5% 5% 7% 6% 9% 11% 39%	5900
	11th to 20th	Day	9% 6% 3 7% 6% 6% 6% 5% 8% 12% 31%	5900
	21st to end	Day	13% 9% 3 10% 6% 7% 7% 5% 7% 9% 25%	6424
August	1st to 10th	Day	10% 7% 3 8% 5% 6% 6% 5% 8% 11% 30%	4234
	11th to 20th	Day	9% 5% 2 7% 5% 5% 5% 5% 9% 11% 36%	4174
	21st to end	Day	6% 3 1 5% 4 4 4 4 6% 11% 53%	4392
September	1st to 10th	Day	10% 6% 3 6% 5% 4 5% 4 7% 8% 43%	2200
	11th to 20th	Day	11% 5% 2 6% 5% 5% 5% 4 5% 8% 43%	2200
	21st to end	Day	15% 6% 2 5% 5% 5% 4 3 5% 7% 42%	2200
October	1st to 10th	Day	12% 6% 2 5% 4 4 4 3 3 7% 51%	2160
	11th to 20th	Day	13% 5% 1 5% 3 4 3 3 4 6% 52%	2000
	21st to end	Day	15% 8% 2 5% 4 3 3 3 4 5% 46%	2200
November	1st to 10th	Day	11% 5% 2 5% 4 3 3 4 6% 6% 52%	1849
	11th to 20th	Day	12% 7% 2 6% 5% 4 4 3 5% 7% 46%	1800
	21st to end	Day	14% 8% 3 6% 4 3 4 3 5% 6% 43%	1800
December	1st to 10th	Day	14% 10% 3 7% 4 4 3 3 4 5% 41%	2930
	11th to 20th	Day	18% 12% 4 9% 5% 4 5% 4 5% 6% 27%	3373
	21st to end	Day	13% 12% 4 7% 5% 5% 5% 4 7% 8% 28%	4466
January	1st to 10th	Night	15% 12% 5% 10% 5% 5% 4 3 4 5% 32%	5889
	11th to 20th	Night	20% 10% 3 9% 5% 3 3 3 4 4 38%	5900
	21st to end	Night	16% 10% 3 7% 5% 4 4 3 5% 7% 38%	6490
February	1st to 10th	Night	18% 9% 3 7% 5% 4 5% 4 5% 6% 36%	2535
	11th to 20th	Night	22% 9% 3 7% 5% 4 4 3 5% 5% 33%	2338
	21st to end	Night	21% 9% 3 6% 4 4 4 4 6% 6% 33%	1980
March	1st to 10th	Night	18% 10% 3 8% 4 4 3 3 4 5% 37%	5055
	11th to 20th	Night	11% 6% 2 4 4 3 3 3 4 8% 52%	4980
	21st to end	Night	11% 6% 2 6% 5% 5% 5% 4 6% 8% 41%	5467
April	1st to 10th	Night	12% 7% 2 7% 4 5% 4 4 6% 7% 42%	2200
	11th to 20th	Night	13% 6% 2 7% 4 4 4 3 6% 7% 44%	1930
	21st to end	Night	12% 7% 3 7% 6% 5% 4 4 5% 6% 40%	1800
May	1st to 10th	Night	18% 9% 3 8% 5% 5% 5% 4 6% 9% 28%	4033
	11th to 20th	Night	16% 8% 4 10% 6% 6% 5% 4 6% 7% 28%	4032
	21st to end	Night	15% 7% 3 8% 6% 5% 4 4 6% 7% 35%	4421
June	1st to 10th	Night	10% 6% 3 7% 5% 5% 4 5% 8% 10% 37%	1800
	11th to 20th	Night	6% 5% 2 7% 5% 5% 6% 5% 7% 10% 41%	1800
	21st to end	Night	9% 6% 2 7% 5% 5% 5% 4 8% 10% 39%	1800
July	1st to 10th	Night	9% 6% 2 7% 5% 6% 6% 6% 9% 12% 34%	4058
	11th to 20th	Night	10% 7% 3 8% 5% 5% 5% 5% 7% 10% 33%	4057
	21st to end	Night	11% 9% 4 11% 7% 8% 7% 5% 8% 9% 22%	4427
August	1st to 10th	Night	11% 5% 3 8% 5% 5% 6% 5% 8% 10% 34%	2915
	11th to 20th	Night	10% 5% 2 8% 5% 5% 6% 5% 6% 9% 40%	2875
	21st to end	Night	6% 3 2 5% 4 3 4 3 7% 10% 53%	3467
September	1st to 10th	Night	12% 7% 3 7% 5% 5% 3 5% 5% 6% 43%	2200
	11th to 20th	Night	14% 6% 2 6% 5% 4 4 4 6% 7% 42%	2200
	21st to end	Night	14% 6% 3 6% 5% 4 3 3 5% 6% 45%	2200
October	1st to 10th	Night	13% 5% 2 6% 4 3 4 3 4 6% 50%	2240
	11th to 20th	Night	15% 5% 2 4 3 3 3 2 5% 5% 52%	2400
	21st to end	Night	15% 6% 2 5% 4 3 4 3 4 6% 49%	2640
November	1st to 10th	Night	13% 6% 2 5% 3 3 3 4 5% 5% 51%	2490
	11th to 20th	Night	13% 10% 2 5% 4 4 3 3 5% 7% 45%	2600
	21st to end	Night	15% 9% 2 5% 4 3 3 3 6% 6% 44%	2600
December	1st to 10th	Night	16% 9% 3 8% 5% 4 4 3 4 6% 36%	4250
	11th to 20th	Night	19% 11% 5% 10% 5% 5% 4 3 4 5% 30%	4822
	21st to end	Night	15% 13% 3 7% 5% 4 4 4 6% 8% 31%	6490

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% 5% 7% 23% 11% 1	4057
	11th to 20th	Day	55% 6% 5% 16% 17% 2	4060
	21st to end	Day	49% 5% 8% 23% 13% 1	4466
February	1st to 10th	Day	52% 5% 9% 19% 14% 2	1860
	11th to 20th	Day	57% 4 8% 17% 13% 2	1958
	21st to end	Day	51% 4 8% 16% 16% 5%	1650
March	1st to 10th	Day	49% 5% 7% 16% 17% 7%	4797
	11th to 20th	Day	37% 5% 6% 19% 27% 6%	4980
	21st to end	Day	43% 5% 7% 30% 14% 2	5465
April	1st to 10th	Day	39% 7% 12% 23% 15% 3	2200
	11th to 20th	Day	39% 5% 10% 21% 19% 5%	2435
	21st to end	Day	43% 7% 12% 24% 12% 2	2600
May	1st to 10th	Day	50% 9% 13% 18% 9% 1	5897
	11th to 20th	Day	52% 10% 13% 19% 6% 1	5865
	21st to end	Day	42% 9% 14% 19% 12% 3	6447
June	1st to 10th	Day	39% 9% 16% 21% 13% 1	2600
	11th to 20th	Day	39% 11% 17% 18% 14% 2	2600
	21st to end	Day	38% 10% 15% 21% 15% 2	2600
July	1st to 10th	Day	33% 12% 20% 20% 13% 1	5835
	11th to 20th	Day	42% 12% 20% 19% 5% 1	5831
	21st to end	Day	52% 10% 14% 16% 8%	6240
August	1st to 10th	Day	43% 10% 18% 22% 8%	4221
	11th to 20th	Day	37% 9% 18% 23% 11% 1	4213
	21st to end	Day	24% 7% 18% 34% 16% 1	4392
September	1st to 10th	Day	36% 8% 14% 20% 15% 5%	2200
	11th to 20th	Day	38% 8% 11% 24% 17% 2	2200
	21st to end	Day	40% 7% 13% 20% 17% 3	2200
October	1st to 10th	Day	28% 4 8% 21% 33% 6%	3524
	11th to 20th	Day	25% 4 10% 24% 22% 15%	3240
	21st to end	Day	30% 4 7% 30% 19% 10%	3538
November	1st to 10th	Day	33% 4 9% 21% 28% 5%	1849
	11th to 20th	Day	38% 6% 7% 23% 21% 5%	1800
	21st to end	Day	41% 5% 8% 23% 21% 3	1800
December	1st to 10th	Day	45% 4 8% 19% 22% 1	2930
	11th to 20th	Day	55% 5% 10% 17% 11% 1	3370
	21st to end	Day	50% 8% 13% 20% 8%	4466
January	1st to 10th	Night	53% 5% 8% 18% 15% 1	5889
	11th to 20th	Night	50% 5% 5% 17% 19% 4	5900
	21st to end	Night	45% 5% 8% 23% 16% 2	6490
February	1st to 10th	Night	49% 5% 10% 20% 16% 1	2535
	11th to 20th	Night	52% 4 9% 19% 12% 4	2339
	21st to end	Night	51% 5% 8% 19% 13% 4	1980
March	1st to 10th	Night	47% 5% 7% 21% 16% 5%	5055
	11th to 20th	Night	32% 4 8% 23% 25% 8%	4980
	21st to end	Night	39% 6% 11% 27% 15% 2	5467
April	1st to 10th	Night	39% 7% 12% 22% 16% 3	2200
	11th to 20th	Night	39% 6% 10% 21% 18% 6%	1930
	21st to end	Night	43% 7% 11% 25% 13% 2	1800
May	1st to 10th	Night	50% 10% 13% 18% 8% 1	4060
	11th to 20th	Night	52% 9% 13% 20% 5%	4033
	21st to end	Night	43% 11% 14% 21% 10% 2	4429
June	1st to 10th	Night	39% 10% 17% 19% 14% 2	1800
	11th to 20th	Night	34% 12% 18% 22% 12% 2	1800
	21st to end	Night	36% 13% 14% 21% 13% 3	1800
July	1st to 10th	Night	39% 11% 19% 21% 9% 1	4015
	11th to 20th	Night	42% 12% 21% 18% 5% 2	4006
	21st to end	Night	54% 12% 12% 14% 8%	4297
August	1st to 10th	Night	42% 10% 19% 23% 6% 1	2904
	11th to 20th	Night	39% 9% 17% 24% 10% 1	2902
	21st to end	Night	25% 7% 17% 37% 12% 1	3467
September	1st to 10th	Night	39% 8% 13% 19% 17% 4	2200
	11th to 20th	Night	38% 9% 13% 21% 17% 1	2200
	21st to end	Night	39% 7% 11% 22% 17% 3	2200
October	1st to 10th	Night	34% 5% 9% 18% 27% 6%	3656
	11th to 20th	Night	26% 5% 6% 24% 26% 12%	3899
	21st to end	Night	26% 4 6% 29% 27% 9%	4252
November	1st to 10th	Night	35% 5% 8% 21% 24% 6%	2490
	11th to 20th	Night	39% 5% 6% 21% 24% 6%	2600
	21st to end	Night	41% 3 8% 26% 20% 3	2600
December	1st to 10th	Night	49% 6% 7% 20% 17% 1	4250
	11th to 20th	Night	57% 4 8% 18% 11% 1	4824
	21st to end	Night	49% 7% 14% 18% 11%	6490

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	24% 21% 31% 19% 4 1	300
	11th to 20th	Day	30% 19% 26% 18% 6%	300
	21st to end	Day	23% 22% 29% 20% 6%	330
February	1st to 10th	Day	24% 17% 27% 23% 8% 1	300
	11th to 20th	Day	34% 24% 29% 10% 3	300
	21st to end	Day	30% 19% 31% 16% 4	249
March	1st to 10th	Day	23% 12% 28% 26% 9% 1	435
	11th to 20th	Day	23% 16% 27% 27% 6%	500
	21st to end	Day	22% 15% 27% 26% 9% 1	550
April	1st to 10th	Day	20% 16% 23% 25% 13% 3	500
	11th to 20th	Day	16% 16% 23% 26% 16% 3	492
	21st to end	Day	19% 10% 24% 25% 17% 5%	500
May	1st to 10th	Day	17% 14% 25% 22% 16% 6%	503
	11th to 20th	Day	18% 7% 17% 28% 21% 9%	527
	21st to end	Day	15% 6% 17% 19% 23% 18%	573
June	1st to 10th	Day	13% 4 16% 25% 28% 14%	530
	11th to 20th	Day	9% 3 15% 25% 31% 17%	530
	21st to end	Day	12% 2 18% 22% 29% 17%	530
July	1st to 10th	Day	11% 3 11% 20% 28% 26%	525
	11th to 20th	Day	13% 1 9% 21% 33% 23%	524
	21st to end	Day	16% 2 8% 24% 30% 20%	576
August	1st to 10th	Day	16% 3 13% 21% 28% 19%	526
	11th to 20th	Day	14% 3 9% 24% 30% 19%	519
	21st to end	Day	11% 2 8% 18% 36% 24%	583
September	1st to 10th	Day	11% 3 10% 22% 35% 19%	530
	11th to 20th	Day	12% 3 12% 18% 32% 22%	530
	21st to end	Day	19% 4 17% 23% 27% 9%	530
October	1st to 10th	Day	15% 4 14% 27% 29% 12%	530
	11th to 20th	Day	13% 9% 18% 29% 25% 6%	530
	21st to end	Day	20% 8% 20% 27% 20% 5%	583
November	1st to 10th	Day	14% 8% 24% 29% 20% 4	375
	11th to 20th	Day	16% 15% 25% 27% 14% 2	300
	21st to end	Day	17% 14% 26% 23% 18% 2	300
December	1st to 10th	Day	26% 15% 27% 25% 7% 1	300
	11th to 20th	Day	23% 15% 23% 27% 10% 2	300
	21st to end	Day	21% 15% 30% 23% 9% 2	330
January	1st to 10th	Night	22% 13% 27% 34% 4	800
	11th to 20th	Night	28% 11% 26% 32% 4	800
	21st to end	Night	19% 10% 36% 34% 2	880
February	1st to 10th	Night	22% 10% 28% 37% 3	800
	11th to 20th	Night	26% 12% 34% 26% 2	783
	21st to end	Night	25% 12% 33% 27% 2	660
March	1st to 10th	Night	20% 10% 29% 37% 5%	800
	11th to 20th	Night	20% 11% 31% 31% 8%	800
	21st to end	Night	17% 8% 30% 36% 8%	880
April	1st to 10th	Night	16% 11% 27% 36% 10%	800
	11th to 20th	Night	15% 8% 26% 38% 13% 1	662
	21st to end	Night	15% 6% 24% 43% 12% 1	600
May	1st to 10th	Night	17% 6% 21% 40% 15% 2	600
	11th to 20th	Night	17% 5% 17% 37% 23% 1	591
	21st to end	Night	11% 3 16% 32% 36% 4	650
June	1st to 10th	Night	11% 2 12% 39% 33% 4	600
	11th to 20th	Night	8% 1 10% 36% 40% 5%	600
	21st to end	Night	12% 1 9% 35% 39% 4	600
July	1st to 10th	Night	12% 2 6% 28% 46% 6%	585
	11th to 20th	Night	13% 1 5% 25% 47% 9%	582
	21st to end	Night	11% 1 7% 33% 41% 7%	657
August	1st to 10th	Night	13% 1 6% 30% 45% 4	597
	11th to 20th	Night	14% 1 6% 33% 39% 7%	594
	21st to end	Night	10% 1 7% 30% 43% 8%	695
September	1st to 10th	Night	14% 2 9% 29% 40% 6%	800
	11th to 20th	Night	16% 2 8% 30% 38% 6%	800
	21st to end	Night	17% 2 15% 37% 28% 2	800
October	1st to 10th	Night	15% 3 13% 34% 31% 4	800
	11th to 20th	Night	16% 5% 19% 41% 18% 1	800
	21st to end	Night	18% 6% 19% 40% 16% 1	880
November	1st to 10th	Night	15% 5% 17% 44% 17% 2	789
	11th to 20th	Night	18% 8% 22% 42% 10% 1	800
	21st to end	Night	19% 7% 22% 41% 10% 1	800
December	1st to 10th	Night	25% 8% 27% 34% 6%	800
	11th to 20th	Night	22% 8% 25% 39% 7%	800
	21st to end	Night	24% 10% 24% 36% 6%	880

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