Big "A" Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Big "A" Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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% 6% 2 6% 5% 5% 5% 4 6% 8% 41%	126350
	Night	13% 6% 2 6% 4 4 4 4 6% 7% 44%	123377
Transparency	Day	39% 7% 11% 19% 18% 5%	130149
	Night	37% 6% 10% 20% 20% 7%	127646
Seeing	Day	16% 10% 23% 23% 18% 9%	16089
	Night	15% 9% 23% 28% 20% 4	26056

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	17% 9% 3 5% 3 3 3 3 4 4 47%	4056
	11th to 20th	Day	12% 7% 3 4 3 2 3 2 4 6% 54%	4060
	21st to end	Day	22% 10% 3 5% 3 3 3 2 4 5% 39%	4770
February	1st to 10th	Day	15% 8% 3 7% 5% 4 5% 4 4 6% 40%	2000
	11th to 20th	Day	21% 7% 2 5% 4 3 3 3 3 5% 41%	1948
	21st to end	Day	19% 7% 3 6% 4 4 5% 3 5% 6% 39%	1650
March	1st to 10th	Day	21% 9% 3 8% 4 4 4 3 4 4 35%	4560
	11th to 20th	Day	10% 7% 2 6% 3 4 4 4 5% 6% 49%	4980
	21st to end	Day	16% 6% 2 7% 4 4 4 4 5% 5% 44%	5465
April	1st to 10th	Day	12% 6% 2 6% 5% 4 5% 4 5% 6% 45%	2305
	11th to 20th	Day	11% 6% 2 6% 5% 4 4 3 5% 6% 47%	2374
	21st to end	Day	10% 5% 2 4 5% 4 4 4 6% 7% 49%	2515
May	1st to 10th	Day	13% 7% 3 7% 4 5% 5% 5% 6% 9% 37%	5862
	11th to 20th	Day	17% 8% 2 7% 5% 6% 5% 5% 7% 8% 30%	5861
	21st to end	Day	11% 6% 2 7% 5% 4 5% 4 6% 8% 41%	6433
June	1st to 10th	Day	10% 6% 3 9% 7% 6% 6% 5% 7% 9% 33%	2600
	11th to 20th	Day	5% 4 2 6% 6% 6% 7% 6% 8% 11% 41%	2600
	21st to end	Day	6% 4 2 7% 6% 8% 7% 6% 8% 9% 37%	2600
July	1st to 10th	Day	7% 6% 3 8% 6% 7% 7% 6% 8% 10% 31%	5900
	11th to 20th	Day	6% 5% 2 7% 6% 6% 7% 6% 8% 14% 33%	5900
	21st to end	Day	8% 8% 4 8% 7% 8% 8% 7% 9% 11% 22%	6424
August	1st to 10th	Day	7% 4 2 6% 5% 5% 6% 5% 9% 12% 37%	4178
	11th to 20th	Day	7% 5% 2 5% 5% 5% 6% 4 6% 9% 46%	3849
	21st to end	Day	8% 4 2 5% 4 4 5% 5% 7% 8% 47%	4312
September	1st to 10th	Day	10% 7% 2 7% 5% 5% 6% 5% 6% 8% 40%	2235
	11th to 20th	Day	16% 5% 2 6% 5% 5% 4 4 7% 7% 40%	2200
	21st to end	Day	13% 6% 2 6% 4 4 5% 3 5% 6% 48%	2200
October	1st to 10th	Day	13% 4 2 4 3 4 3 2 5% 5% 54%	2200
	11th to 20th	Day	11% 3 2 4 3 3 3 3 4 7% 58%	2109
	21st to end	Day	14% 4 1 5% 4 3 3 2 4 5% 55%	2073
November	1st to 10th	Day	13% 7% 2 5% 3 3 3 3 3 6% 52%	1762
	11th to 20th	Day	15% 5% 1 3 4 3 3 3 5% 6% 52%	1800
	21st to end	Day	13% 7% 2 5% 3 4 4 3 5% 7% 47%	1800
December	1st to 10th	Day	17% 6% 2 5% 3 4 3 3 5% 6% 47%	2930
	11th to 20th	Day	17% 9% 3 6% 4 4 4 3 4 5% 42%	3373
	21st to end	Day	22% 6% 2 5% 3 3 4 3 4 6% 41%	4466
January	1st to 10th	Night	15% 7% 3 6% 3 3 3 3 4 5% 47%	5889
	11th to 20th	Night	13% 8% 3 6% 3 3 3 3 5% 5% 47%	5900
	21st to end	Night	19% 9% 2 5% 3 3 4 3 4 4 44%	6170
February	1st to 10th	Night	15% 8% 3 6% 3 4 3 4 6% 6% 43%	2400
	11th to 20th	Night	21% 8% 2 5% 4 3 4 3 4 5% 42%	2338
	21st to end	Night	18% 8% 2 6% 4 4 4 3 6% 6% 41%	1980
March	1st to 10th	Night	21% 9% 3 6% 4 4 4 3 5% 6% 34%	5292
	11th to 20th	Night	11% 5% 1 4 3 3 4 4 5% 7% 52%	4980
	21st to end	Night	14% 6% 2 6% 4 4 4 3 5% 6% 46%	5467
April	1st to 10th	Night	11% 5% 1 5% 5% 4 4 4 6% 8% 47%	2110
	11th to 20th	Night	14% 5% 1 5% 3 4 3 2 4 8% 51%	1975
	21st to end	Night	9% 5% 2 5% 3 4 5% 4 5% 7% 52%	1884
May	1st to 10th	Night	10% 6% 3 7% 5% 5% 6% 5% 7% 9% 38%	4033
	11th to 20th	Night	18% 7% 3 7% 5% 5% 6% 4 6% 8% 32%	4032
	21st to end	Night	12% 5% 2 5% 3 4 4 4 8% 11% 42%	4421
June	1st to 10th	Night	6% 6% 3 7% 5% 6% 5% 5% 8% 12% 38%	1800
	11th to 20th	Night	3 4 2 6% 5% 5% 5% 5% 8% 12% 45%	1800
	21st to end	Night	6% 4 2 6% 5% 5% 5% 4 7% 12% 44%	1800
July	1st to 10th	Night	9% 7% 3 8% 7% 6% 7% 4 8% 11% 33%	4058
	11th to 20th	Night	5% 4 2 6% 5% 6% 6% 6% 9% 12% 37%	4057
	21st to end	Night	9% 9% 3 9% 6% 6% 7% 6% 9% 10% 28%	4427
August	1st to 10th	Night	7% 4 2 5% 5% 5% 6% 5% 8% 12% 40%	2967
	11th to 20th	Night	7% 5% 2 6% 4 5% 4 4 6% 9% 48%	3200
	21st to end	Night	9% 5% 2 5% 4 3 4 3 6% 9% 51%	3586
September	1st to 10th	Night	9% 5% 2 6% 5% 5% 6% 5% 6% 9% 42%	2145
	11th to 20th	Night	12% 6% 2 6% 4 4 3 4 5% 8% 46%	2200
	21st to end	Night	11% 5% 2 5% 4 4 5% 3 5% 7% 48%	2200
October	1st to 10th	Night	10% 6% 2 4 2 3 4 3 5% 6% 56%	2200
	11th to 20th	Night	11% 5% 2 4 4 2 2 3 3 4 60%	2247
	21st to end	Night	12% 5% 2 4 3 3 3 2 4 5% 57%	2521
November	1st to 10th	Night	14% 5% 2 4 3 3 3 3 4 5% 56%	2536
	11th to 20th	Night	13% 6% 2 4 3 3 3 2 4 6% 54%	2600
	21st to end	Night	14% 6% 2 5% 4 3 4 3 5% 6% 48%	2600
December	1st to 10th	Night	16% 7% 2 5% 3 3 2 2 4 5% 50%	4250
	11th to 20th	Night	18% 6% 2 5% 3 3 3 3 5% 6% 46%	4822
	21st to end	Night	21% 7% 2 5% 4 4 4 3 5% 6% 40%	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	41% 2 5% 22% 23% 6%	4057
	11th to 20th	Day	34% 2 5% 22% 30% 7%	4060
	21st to end	Day	50% 2 5% 15% 21% 7%	4770
February	1st to 10th	Day	45% 5% 8% 17% 21% 5%	2000
	11th to 20th	Day	46% 3 5% 18% 21% 7%	1958
	21st to end	Day	46% 4 7% 14% 20% 9%	1650
March	1st to 10th	Day	52% 3 8% 10% 20% 7%	4560
	11th to 20th	Day	35% 4 6% 14% 24% 16%	4980
	21st to end	Day	42% 4 7% 20% 21% 6%	5465
April	1st to 10th	Day	40% 5% 8% 21% 19% 7%	2305
	11th to 20th	Day	37% 5% 7% 19% 24% 9%	2374
	21st to end	Day	33% 6% 9% 25% 24% 3	2515
May	1st to 10th	Day	41% 9% 13% 20% 16% 2	5897
	11th to 20th	Day	49% 9% 15% 18% 9% 1	5865
	21st to end	Day	39% 7% 10% 23% 18% 2	6447
June	1st to 10th	Day	44% 10% 14% 19% 12% 2	2600
	11th to 20th	Day	33% 10% 19% 23% 14% 2	2600
	21st to end	Day	37% 12% 17% 20% 12% 2	2600
July	1st to 10th	Day	42% 12% 18% 21% 6% 1	5835
	11th to 20th	Day	36% 15% 27% 17% 4 1	5831
	21st to end	Day	49% 14% 17% 17% 3	6240
August	1st to 10th	Day	35% 9% 14% 30% 11%	4165
	11th to 20th	Day	32% 9% 16% 28% 13% 2	3885
	21st to end	Day	31% 8% 15% 23% 20% 3	4311
September	1st to 10th	Day	40% 8% 13% 18% 17% 5%	2235
	11th to 20th	Day	40% 9% 10% 20% 20% 2	2200
	21st to end	Day	38% 4 12% 19% 21% 6%	2200
October	1st to 10th	Day	26% 4 6% 13% 37% 14%	3590
	11th to 20th	Day	19% 2 5% 17% 30% 27%	3427
	21st to end	Day	25% 3 7% 18% 35% 12%	3399
November	1st to 10th	Day	35% 4 7% 14% 29% 10%	1762
	11th to 20th	Day	33% 4 6% 17% 29% 11%	1800
	21st to end	Day	38% 5% 7% 17% 26% 7%	1800
December	1st to 10th	Day	40% 3 5% 18% 26% 7%	2930
	11th to 20th	Day	46% 3 6% 17% 21% 6%	3370
	21st to end	Day	44% 3 8% 23% 20% 2	4466
January	1st to 10th	Night	40% 3 5% 20% 27% 5%	5889
	11th to 20th	Night	38% 3 5% 20% 26% 8%	5900
	21st to end	Night	45% 3 4 15% 28% 6%	6170
February	1st to 10th	Night	41% 5% 8% 19% 20% 8%	2400
	11th to 20th	Night	45% 4 6% 17% 20% 8%	2339
	21st to end	Night	44% 5% 7% 15% 19% 11%	1980
March	1st to 10th	Night	50% 3 8% 13% 18% 8%	5292
	11th to 20th	Night	31% 4 9% 14% 25% 18%	4980
	21st to end	Night	39% 3 6% 21% 21% 9%	5467
April	1st to 10th	Night	35% 5% 10% 19% 22% 8%	2110
	11th to 20th	Night	34% 4 6% 22% 24% 10%	1975
	21st to end	Night	31% 6% 11% 26% 21% 5%	1884
May	1st to 10th	Night	39% 10% 14% 21% 14% 2	4060
	11th to 20th	Night	48% 8% 14% 21% 8% 1	4033
	21st to end	Night	33% 8% 14% 24% 18% 2	4429
June	1st to 10th	Night	35% 12% 16% 21% 15% 1	1800
	11th to 20th	Night	28% 10% 22% 25% 13% 2	1800
	21st to end	Night	32% 10% 19% 20% 16% 4	1800
July	1st to 10th	Night	42% 11% 18% 22% 6% 1	4015
	11th to 20th	Night	34% 15% 28% 17% 5% 1	4006
	21st to end	Night	46% 13% 21% 16% 4 1	4297
August	1st to 10th	Night	33% 9% 15% 32% 10% 1	2956
	11th to 20th	Night	31% 8% 15% 30% 15% 2	3230
	21st to end	Night	30% 8% 14% 20% 25% 3	3586
September	1st to 10th	Night	35% 10% 15% 18% 17% 5%	2145
	11th to 20th	Night	38% 7% 10% 23% 19% 3	2200
	21st to end	Night	34% 6% 12% 20% 20% 8%	2200
October	1st to 10th	Night	26% 4 8% 14% 34% 15%	3590
	11th to 20th	Night	18% 2 4 17% 31% 29%	3668
	21st to end	Night	22% 3 4 22% 31% 18%	4145
November	1st to 10th	Night	32% 3 9% 16% 27% 13%	2536
	11th to 20th	Night	33% 4 5% 17% 28% 12%	2600
	21st to end	Night	37% 4 7% 18% 26% 8%	2600
December	1st to 10th	Night	37% 4 5% 19% 27% 7%	4250
	11th to 20th	Night	41% 4 10% 15% 25% 6%	4824
	21st to end	Night	45% 3 6% 25% 19% 2	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% 26% 36% 13% 1	300
	11th to 20th	Day	25% 22% 33% 12% 5% 2	300
	21st to end	Day	25% 25% 35% 11% 3 1	330
February	1st to 10th	Day	22% 22% 35% 17% 4	300
	11th to 20th	Day	35% 20% 33% 11% 1	300
	21st to end	Day	29% 19% 40% 10% 3	249
March	1st to 10th	Day	27% 19% 34% 15% 3 1	328
	11th to 20th	Day	23% 19% 32% 19% 7% 1	500
	21st to end	Day	22% 18% 33% 21% 6% 1	550
April	1st to 10th	Day	21% 18% 30% 19% 11% 1	500
	11th to 20th	Day	13% 16% 34% 22% 13% 2	492
	21st to end	Day	15% 13% 26% 26% 17% 3	500
May	1st to 10th	Day	14% 12% 28% 28% 15% 3	503
	11th to 20th	Day	14% 7% 24% 29% 16% 10%	527
	21st to end	Day	13% 8% 20% 28% 20% 11%	573
June	1st to 10th	Day	14% 4 16% 28% 27% 11%	530
	11th to 20th	Day	6% 3 15% 26% 27% 22%	530
	21st to end	Day	9% 2 13% 28% 28% 19%	530
July	1st to 10th	Day	9% 2 8% 26% 33% 22%	525
	11th to 20th	Day	6% 3 7% 19% 37% 28%	524
	21st to end	Day	7% 2 9% 23% 39% 21%	576
August	1st to 10th	Day	13% 3 13% 24% 28% 20%	526
	11th to 20th	Day	10% 2 10% 25% 32% 21%	519
	21st to end	Day	15% 2 10% 28% 27% 19%	583
September	1st to 10th	Day	10% 4 16% 28% 25% 16%	530
	11th to 20th	Day	17% 3 14% 24% 31% 12%	530
	21st to end	Day	14% 4 18% 26% 27% 11%	530
October	1st to 10th	Day	16% 6% 21% 32% 17% 8%	530
	11th to 20th	Day	12% 8% 24% 30% 22% 5%	522
	21st to end	Day	16% 10% 23% 27% 20% 3	522
November	1st to 10th	Day	24% 10% 25% 25% 12% 3	300
	11th to 20th	Day	17% 16% 38% 21% 7% 1	300
	21st to end	Day	10% 19% 39% 23% 8% 1	300
December	1st to 10th	Day	25% 21% 32% 18% 4	300
	11th to 20th	Day	23% 19% 32% 20% 5% 1	300
	21st to end	Day	31% 15% 33% 15% 5% 1	330
January	1st to 10th	Night	23% 20% 30% 22% 4 1	800
	11th to 20th	Night	21% 18% 31% 24% 6%	800
	21st to end	Night	20% 17% 38% 22% 3	880
February	1st to 10th	Night	19% 14% 35% 27% 5%	800
	11th to 20th	Night	25% 19% 33% 22% 3	783
	21st to end	Night	22% 18% 36% 22% 3	660
March	1st to 10th	Night	19% 17% 34% 25% 6%	800
	11th to 20th	Night	18% 14% 33% 25% 9% 1	800
	21st to end	Night	15% 16% 31% 28% 9% 1	880
April	1st to 10th	Night	14% 15% 32% 26% 12% 1	710
	11th to 20th	Night	17% 12% 29% 27% 13% 2	593
	21st to end	Night	12% 9% 25% 37% 17% 2	600
May	1st to 10th	Night	11% 7% 26% 34% 21% 1	600
	11th to 20th	Night	12% 6% 21% 33% 23% 5%	591
	21st to end	Night	13% 2 14% 32% 33% 6%	650
June	1st to 10th	Night	7% 3 14% 38% 32% 6%	600
	11th to 20th	Night	3 3 15% 28% 35% 17%	600
	21st to end	Night	6% 1 10% 33% 43% 8%	600
July	1st to 10th	Night	9% 4 8% 24% 40% 15%	585
	11th to 20th	Night	7% 1 5% 23% 46% 18%	582
	21st to end	Night	8% 1 8% 26% 44% 13%	657
August	1st to 10th	Night	10% 1 7% 28% 42% 12%	597
	11th to 20th	Night	12% 2 8% 27% 41% 9%	594
	21st to end	Night	14% 2 7% 28% 37% 12%	660
September	1st to 10th	Night	10% 2 10% 27% 40% 10%	745
	11th to 20th	Night	15% 3 11% 27% 38% 6%	800
	21st to end	Night	14% 2 16% 33% 32% 3	800
October	1st to 10th	Night	12% 5% 21% 33% 22% 6%	800
	11th to 20th	Night	12% 6% 25% 33% 21% 3	793
	21st to end	Night	14% 8% 22% 34% 19% 3	836
November	1st to 10th	Night	16% 8% 23% 31% 20% 2	780
	11th to 20th	Night	16% 12% 29% 29% 13% 1	800
	21st to end	Night	17% 12% 30% 29% 12% 1	800
December	1st to 10th	Night	20% 14% 30% 26% 10% 1	800
	11th to 20th	Night	23% 10% 31% 26% 9% 1	800
	21st to end	Night	28% 13% 27% 25% 6%	880

Page generated at 2025-10-16 01:50:23. Version ac07

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