Bailyhill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bailyhill Observatory clear sky chart from 2005-11-06 to 2025-12-03.

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	17% 9% 3 9% 5% 5% 5% 4 5% 7% 31%	126915
	Night	17% 10% 4 9% 6% 5% 5% 4 5% 6% 30%	123787
Transparency	Day	48% 7% 13% 19% 11% 2	130409
	Night	48% 7% 14% 19% 11% 1	128380
Seeing	Day	25% 19% 28% 18% 8% 2	16203
	Night	23% 16% 26% 26% 9% 1	25102

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	11% 10% 6% 14% 8% 7% 6% 4 5% 5% 23%	3597
	11th to 20th	Day	11% 10% 5% 12% 10% 5% 5% 4 6% 6% 26%	3570
	21st to end	Day	11% 12% 8% 12% 5% 5% 4 4 4 4 31%	3960
February	1st to 10th	Day	14% 12% 4 10% 6% 6% 5% 4 5% 6% 26%	1766
	11th to 20th	Day	11% 11% 4 13% 7% 5% 5% 4 5% 6% 28%	1948
	21st to end	Day	15% 13% 4 10% 6% 5% 4 4 5% 5% 30%	1650
March	1st to 10th	Day	18% 14% 4 13% 5% 5% 4 3 3 4 26%	4476
	11th to 20th	Day	16% 10% 5% 8% 5% 5% 5% 4 5% 5% 31%	4702
	21st to end	Day	22% 11% 4 9% 5% 4 3 3 4 6% 29%	5873
April	1st to 10th	Day	20% 12% 4 8% 5% 4 4 3 4 6% 30%	2400
	11th to 20th	Day	16% 11% 3 7% 6% 5% 5% 3 5% 6% 34%	2388
	21st to end	Day	21% 10% 3 8% 6% 4 4 4 5% 6% 29%	2600
May	1st to 10th	Day	33% 12% 4 8% 4 4 3 3 4 5% 20%	5886
	11th to 20th	Day	19% 11% 3 9% 6% 5% 5% 4 6% 7% 25%	6224
	21st to end	Day	18% 8% 3 7% 5% 5% 5% 4 7% 7% 31%	6949
June	1st to 10th	Day	19% 9% 3 7% 6% 5% 6% 4 6% 7% 28%	2800
	11th to 20th	Day	17% 8% 3 6% 5% 5% 5% 4 7% 8% 34%	2800
	21st to end	Day	20% 7% 3 9% 6% 5% 5% 4 6% 7% 29%	2800
July	1st to 10th	Day	14% 7% 2 7% 6% 5% 6% 5% 8% 11% 31%	6360
	11th to 20th	Day	12% 6% 2 7% 5% 6% 6% 5% 7% 10% 34%	6360
	21st to end	Day	16% 6% 2 6% 4 4 5% 5% 6% 8% 39%	6926
August	1st to 10th	Day	14% 7% 3 8% 6% 6% 5% 5% 6% 7% 32%	4433
	11th to 20th	Day	16% 9% 3 8% 5% 5% 5% 5% 7% 8% 29%	4121
	21st to end	Day	10% 5% 3 7% 5% 5% 5% 5% 7% 9% 40%	4312
September	1st to 10th	Day	12% 5% 2 5% 5% 5% 5% 4 6% 7% 44%	2400
	11th to 20th	Day	13% 6% 2 5% 3 4 3 3 5% 6% 50%	2230
	21st to end	Day	16% 6% 2 6% 4 4 4 3 6% 7% 42%	2200
October	1st to 10th	Day	21% 9% 2 7% 4 4 4 3 4 6% 35%	2200
	11th to 20th	Day	16% 9% 3 8% 5% 4 5% 3 4 6% 37%	1900
	21st to end	Day	24% 12% 4 8% 4 4 5% 3 4 5% 28%	1980
November	1st to 10th	Day	12% 9% 4 9% 6% 4 5% 3 5% 7% 37%	1851
	11th to 20th	Day	16% 10% 4 8% 6% 4 5% 3 6% 6% 32%	1890
	21st to end	Day	20% 10% 3 9% 6% 4 5% 4 4 5% 28%	1801
December	1st to 10th	Day	15% 15% 4 15% 7% 6% 5% 3 5% 6% 19%	2616
	11th to 20th	Day	18% 13% 6% 10% 4 4 4 3 4 5% 29%	2986
	21st to end	Day	20% 13% 4 10% 5% 4 4 3 3 5% 28%	3960
January	1st to 10th	Night	11% 10% 6% 16% 10% 5% 4 4 5% 5% 24%	6349
	11th to 20th	Night	10% 14% 5% 14% 9% 6% 6% 4 6% 6% 20%	6315
	21st to end	Night	13% 11% 4 9% 7% 7% 6% 4 5% 6% 27%	6996
February	1st to 10th	Night	14% 11% 5% 9% 7% 5% 5% 5% 6% 6% 26%	2627
	11th to 20th	Night	11% 10% 4 10% 8% 7% 6% 4 6% 7% 26%	2338
	21st to end	Night	12% 11% 3 11% 7% 5% 6% 4 6% 7% 28%	1980
March	1st to 10th	Night	19% 13% 3 9% 6% 5% 5% 4 5% 6% 24%	5364
	11th to 20th	Night	15% 11% 4 10% 6% 6% 4 3 4 6% 30%	5256
	21st to end	Night	23% 13% 4 7% 4 4 4 3 5% 6% 28%	5056
April	1st to 10th	Night	20% 10% 3 8% 6% 5% 4 3 4 5% 32%	2000
	11th to 20th	Night	19% 11% 3 8% 4 3 4 3 5% 6% 33%	1960
	21st to end	Night	19% 9% 4 8% 5% 4 3 3 4 6% 34%	1800
May	1st to 10th	Night	35% 10% 2 7% 4 4 4 3 4 6% 19%	4054
	11th to 20th	Night	19% 13% 4 8% 6% 5% 5% 4 5% 5% 27%	3698
	21st to end	Night	19% 9% 3 8% 5% 5% 5% 4 5% 7% 30%	3926
June	1st to 10th	Night	21% 8% 3 8% 5% 5% 4 4 6% 7% 31%	1600
	11th to 20th	Night	18% 9% 3 7% 4 4 4 4 6% 6% 37%	1600
	21st to end	Night	16% 8% 2 7% 5% 5% 4 4 7% 7% 35%	1600
July	1st to 10th	Night	13% 6% 2 7% 5% 5% 6% 5% 7% 10% 36%	3600
	11th to 20th	Night	16% 7% 3 8% 5% 4 5% 4 5% 7% 37%	3600
	21st to end	Night	12% 5% 2 6% 6% 4 4 3 5% 7% 44%	3926
August	1st to 10th	Night	20% 9% 3 8% 4 4 4 3 5% 6% 34%	2684
	11th to 20th	Night	16% 9% 3 7% 5% 4 4 4 6% 7% 35%	2922
	21st to end	Night	9% 5% 2 6% 6% 4 5% 4 7% 10% 43%	3586
September	1st to 10th	Night	13% 6% 2 6% 4 3 3 3 5% 7% 48%	2000
	11th to 20th	Night	14% 6% 2 5% 4 3 3 3 4 7% 49%	2170
	21st to end	Night	17% 7% 2 5% 4 4 4 3 5% 6% 43%	2200
October	1st to 10th	Night	20% 7% 3 7% 4 4 4 3 6% 6% 36%	2200
	11th to 20th	Night	16% 11% 3 7% 5% 4 5% 4 5% 6% 35%	2496
	21st to end	Night	23% 11% 2 7% 5% 5% 4 3 5% 6% 28%	2860
November	1st to 10th	Night	14% 9% 4 9% 5% 4 5% 3 5% 5% 36%	2667
	11th to 20th	Night	16% 9% 4 9% 6% 5% 4 4 5% 7% 32%	2730
	21st to end	Night	16% 11% 3 9% 6% 5% 6% 4 6% 6% 29%	2814
December	1st to 10th	Night	15% 14% 5% 12% 7% 7% 6% 4 5% 6% 19%	4609
	11th to 20th	Night	23% 12% 4 9% 5% 5% 4 3 5% 5% 27%	5208
	21st to end	Night	20% 13% 5% 11% 5% 4 4 4 5% 4 25%	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	52% 9% 14% 18% 7%	3598
	11th to 20th	Day	52% 8% 12% 16% 9% 3	3600
	21st to end	Day	50% 7% 10% 16% 14% 2	3960
February	1st to 10th	Day	54% 6% 13% 17% 8% 2	1766
	11th to 20th	Day	52% 6% 13% 19% 9% 2	1957
	21st to end	Day	52% 7% 11% 16% 12% 1	1650
March	1st to 10th	Day	59% 5% 10% 11% 9% 6%	4479
	11th to 20th	Day	48% 9% 15% 18% 8% 3	4702
	21st to end	Day	53% 5% 8% 16% 16% 3	5873
April	1st to 10th	Day	53% 5% 10% 18% 12% 3	2400
	11th to 20th	Day	47% 7% 11% 18% 14% 2	2388
	21st to end	Day	53% 6% 9% 17% 12% 2	2600
May	1st to 10th	Day	65% 7% 8% 14% 6% 1	5897
	11th to 20th	Day	54% 9% 14% 14% 8% 1	6228
	21st to end	Day	46% 9% 14% 21% 9% 1	6949
June	1st to 10th	Day	51% 8% 13% 18% 10%	2800
	11th to 20th	Day	45% 8% 13% 19% 13% 2	2800
	21st to end	Day	51% 8% 12% 18% 10% 1	2800
July	1st to 10th	Day	44% 8% 16% 21% 12%	6290
	11th to 20th	Day	41% 8% 16% 22% 12% 1	6285
	21st to end	Day	39% 7% 13% 29% 12% 1	6728
August	1st to 10th	Day	45% 8% 16% 20% 10%	4419
	11th to 20th	Day	47% 9% 15% 23% 6%	4160
	21st to end	Day	35% 8% 18% 25% 14% 1	4312
September	1st to 10th	Day	35% 7% 12% 30% 15% 1	2400
	11th to 20th	Day	34% 6% 10% 30% 19% 2	2230
	21st to end	Day	39% 6% 14% 26% 14% 2	2200
October	1st to 10th	Day	45% 7% 10% 22% 15% 1	3590
	11th to 20th	Day	43% 5% 13% 23% 14% 2	3064
	21st to end	Day	41% 4 11% 27% 14% 2	3180
November	1st to 10th	Day	44% 6% 14% 20% 14% 1	1851
	11th to 20th	Day	47% 6% 14% 17% 14% 2	1890
	21st to end	Day	53% 6% 19% 17% 5%	1801
December	1st to 10th	Day	61% 6% 15% 14% 3	2616
	11th to 20th	Day	52% 7% 10% 16% 14% 1	2986
	21st to end	Day	56% 5% 8% 14% 12% 4	3960
January	1st to 10th	Night	54% 7% 14% 19% 6%	6349
	11th to 20th	Night	56% 8% 14% 13% 7% 1	6360
	21st to end	Night	49% 8% 12% 15% 13% 3	6996
February	1st to 10th	Night	51% 8% 14% 17% 9% 1	2627
	11th to 20th	Night	49% 7% 15% 19% 10% 1	2340
	21st to end	Night	47% 7% 15% 20% 8% 2	1980
March	1st to 10th	Night	54% 6% 12% 14% 12% 3	5374
	11th to 20th	Night	51% 8% 16% 15% 10% 2	5257
	21st to end	Night	52% 5% 8% 16% 17% 2	5056
April	1st to 10th	Night	52% 5% 11% 18% 12% 2	2000
	11th to 20th	Night	49% 6% 10% 22% 13% 1	1960
	21st to end	Night	48% 7% 12% 18% 14% 3	1800
May	1st to 10th	Night	63% 8% 9% 12% 7% 1	4060
	11th to 20th	Night	55% 9% 11% 17% 7% 1	3699
	21st to end	Night	49% 8% 16% 16% 9% 1	3927
June	1st to 10th	Night	50% 8% 13% 19% 11%	1600
	11th to 20th	Night	46% 8% 15% 19% 11% 2	1600
	21st to end	Night	45% 9% 14% 21% 10% 1	1600
July	1st to 10th	Night	39% 8% 20% 23% 10%	3560
	11th to 20th	Night	43% 8% 14% 22% 13%	3552
	21st to end	Night	34% 7% 15% 32% 11% 1	3809
August	1st to 10th	Night	49% 8% 13% 21% 9% 1	2674
	11th to 20th	Night	45% 7% 17% 20% 11%	2949
	21st to end	Night	30% 8% 21% 27% 13%	3586
September	1st to 10th	Night	34% 6% 16% 29% 15%	2000
	11th to 20th	Night	34% 7% 12% 29% 17% 1	2170
	21st to end	Night	39% 6% 13% 25% 15% 1	2200
October	1st to 10th	Night	43% 7% 15% 24% 10% 1	3590
	11th to 20th	Night	38% 7% 14% 23% 16% 3	4071
	21st to end	Night	43% 6% 12% 27% 11% 1	4610
November	1st to 10th	Night	44% 6% 14% 22% 11% 2	2667
	11th to 20th	Night	45% 6% 15% 21% 12% 1	2730
	21st to end	Night	48% 8% 17% 20% 7%	2814
December	1st to 10th	Night	55% 9% 15% 18% 3	4609
	11th to 20th	Night	54% 7% 12% 13% 14% 1	5208
	21st to end	Night	57% 5% 11% 15% 9% 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	31% 23% 30% 15% 3 1	200
	11th to 20th	Day	32% 27% 28% 10% 3 1	200
	21st to end	Day	23% 28% 31% 13% 4	220
February	1st to 10th	Day	34% 29% 27% 8% 2	311
	11th to 20th	Day	32% 27% 27% 11% 2	496
	21st to end	Day	31% 26% 27% 14% 3	414
March	1st to 10th	Day	29% 22% 32% 13% 4	500
	11th to 20th	Day	33% 26% 24% 11% 5% 1	500
	21st to end	Day	33% 23% 23% 13% 5% 2	550
April	1st to 10th	Day	38% 21% 21% 14% 4 2	500
	11th to 20th	Day	34% 24% 22% 13% 5% 1	492
	21st to end	Day	31% 25% 26% 12% 4 2	500
May	1st to 10th	Day	34% 22% 25% 12% 5% 2	503
	11th to 20th	Day	23% 20% 33% 17% 6% 2	527
	21st to end	Day	22% 17% 32% 21% 7% 1	573
June	1st to 10th	Day	22% 15% 36% 17% 9% 1	530
	11th to 20th	Day	20% 19% 31% 18% 10% 2	530
	21st to end	Day	21% 18% 31% 19% 9% 2	530
July	1st to 10th	Day	18% 13% 35% 21% 11% 3	525
	11th to 20th	Day	16% 10% 33% 29% 10% 2	524
	21st to end	Day	16% 12% 36% 26% 8% 2	576
August	1st to 10th	Day	12% 14% 31% 25% 15% 3	526
	11th to 20th	Day	18% 13% 27% 23% 14% 6%	519
	21st to end	Day	13% 13% 31% 25% 13% 5%	583
September	1st to 10th	Day	17% 12% 28% 22% 16% 5%	530
	11th to 20th	Day	15% 14% 27% 21% 16% 8%	530
	21st to end	Day	18% 17% 27% 22% 11% 6%	530
October	1st to 10th	Day	25% 15% 22% 21% 12% 4	530
	11th to 20th	Day	26% 19% 22% 22% 9% 2	472
	21st to end	Day	36% 16% 23% 18% 6%	462
November	1st to 10th	Day	27% 19% 22% 21% 10% 1	425
	11th to 20th	Day	29% 22% 26% 17% 6% 1	430
	21st to end	Day	36% 21% 24% 15% 4	341
December	1st to 10th	Day	30% 24% 28% 15% 3	204
	11th to 20th	Day	38% 20% 31% 8% 3	200
	21st to end	Day	31% 20% 28% 17% 4	220
January	1st to 10th	Night	25% 23% 28% 22% 3	800
	11th to 20th	Night	28% 20% 26% 23% 3 1	800
	21st to end	Night	18% 24% 34% 20% 4	880
February	1st to 10th	Night	24% 21% 34% 20% 2	800
	11th to 20th	Night	25% 24% 31% 18% 2	783
	21st to end	Night	25% 20% 29% 23% 3	660
March	1st to 10th	Night	27% 18% 32% 20% 3	800
	11th to 20th	Night	26% 22% 25% 22% 5%	800
	21st to end	Night	29% 18% 26% 21% 6% 1	880
April	1st to 10th	Night	28% 20% 26% 22% 4	800
	11th to 20th	Night	23% 23% 26% 22% 6%	776
	21st to end	Night	27% 20% 28% 21% 5%	600
May	1st to 10th	Night	30% 14% 26% 24% 7%	600
	11th to 20th	Night	18% 16% 29% 30% 6%	452
	21st to end	Night	22% 9% 24% 35% 10% 1	433
June	1st to 10th	Night	23% 10% 24% 33% 10% 1	400
	11th to 20th	Night	20% 10% 26% 32% 12%	400
	21st to end	Night	18% 8% 25% 39% 10% 1	400
July	1st to 10th	Night	15% 5% 22% 42% 15%	390
	11th to 20th	Night	17% 5% 20% 37% 19% 2	388
	21st to end	Night	15% 5% 22% 41% 16% 1	438
August	1st to 10th	Night	16% 3 18% 43% 19%	457
	11th to 20th	Night	16% 7% 25% 31% 19% 2	616
	21st to end	Night	16% 8% 21% 31% 23% 2	880
September	1st to 10th	Night	16% 9% 23% 32% 19% 2	800
	11th to 20th	Night	15% 10% 21% 28% 22% 4	800
	21st to end	Night	19% 11% 21% 30% 16% 2	800
October	1st to 10th	Night	24% 11% 20% 27% 16% 1	800
	11th to 20th	Night	23% 15% 24% 27% 11% 1	800
	21st to end	Night	29% 16% 26% 24% 6%	880
November	1st to 10th	Night	22% 16% 26% 27% 8% 1	820
	11th to 20th	Night	23% 21% 26% 22% 7% 1	840
	21st to end	Night	24% 21% 26% 23% 5% 1	840
December	1st to 10th	Night	25% 21% 31% 19% 4	809
	11th to 20th	Night	30% 19% 27% 20% 3	800
	21st to end	Night	28% 22% 28% 20% 3	880

Page generated at 2025-11-30 01:00:55. Version ac07

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