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 2024-01-12.

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 5% 6% 3 5% 5% 6% 7% 41%	74853
	Night	13% 6% 2 5% 5% 3 4 4 6% 8% 45%	74873
Transparency	Day	40% 7% 11% 19% 18% 5%	74576
	Night	37% 6% 11% 20% 20% 6%	74669
Seeing	Day	16% 10% 22% 23% 19% 10%	13691
	Night	15% 9% 23% 28% 20% 4	23775

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	18% 8% 2 5% 4 2 4 4 4 5% 43%	1706
	11th to 20th	Day	16% 8% 3 5% 4 2 3 4 4 6% 45%	1629
	21st to end	Day	17% 7% 3 6% 5% 2 4 4 4 5% 44%	1904
February	1st to 10th	Day	16% 8% 3 6% 6% 3 4 5% 4 6% 39%	1800
	11th to 20th	Day	20% 6% 3 5% 4 3 4 4 3 5% 42%	1748
	21st to end	Day	21% 7% 3 5% 5% 2 4 4 5% 6% 38%	1480
March	1st to 10th	Day	14% 6% 2 6% 5% 3 3 4 4 5% 47%	1824
	11th to 20th	Day	18% 7% 3 6% 4 3 4 5% 4 6% 38%	1980
	21st to end	Day	16% 6% 3 5% 5% 3 5% 5% 6% 7% 39%	2178
April	1st to 10th	Day	12% 6% 2 5% 6% 3 5% 4 4 6% 46%	2074
	11th to 20th	Day	12% 6% 2 5% 6% 3 4 4 5% 6% 46%	2134
	21st to end	Day	10% 4 2 4 5% 3 4 5% 5% 7% 52%	2263
May	1st to 10th	Day	9% 5% 3 5% 4 3 4 5% 5% 8% 49%	2340
	11th to 20th	Day	10% 6% 2 5% 7% 4 6% 5% 7% 8% 39%	2301
	21st to end	Day	9% 5% 2 5% 6% 4 5% 7% 6% 8% 42%	2531
June	1st to 10th	Day	10% 6% 3 7% 8% 4 6% 7% 7% 10% 33%	2340
	11th to 20th	Day	5% 3 2 5% 6% 4 7% 8% 7% 11% 41%	2340
	21st to end	Day	6% 4 2 6% 7% 6% 8% 8% 8% 10% 35%	2340
July	1st to 10th	Day	9% 5% 3 7% 8% 5% 8% 9% 8% 11% 26%	2340
	11th to 20th	Day	8% 5% 3 6% 7% 4 8% 7% 9% 11% 29%	2340
	21st to end	Day	6% 5% 3 6% 7% 5% 11% 9% 9% 11% 29%	2547
August	1st to 10th	Day	8% 4 2 6% 8% 5% 9% 8% 9% 12% 29%	2301
	11th to 20th	Day	11% 6% 2 6% 7% 5% 8% 7% 6% 9% 33%	2125
	21st to end	Day	14% 6% 3 6% 8% 4 6% 7% 8% 9% 29%	2376
September	1st to 10th	Day	10% 7% 2 6% 6% 4 6% 6% 6% 8% 39%	2012
	11th to 20th	Day	16% 5% 2 5% 5% 3 5% 4 7% 7% 40%	1980
	21st to end	Day	14% 6% 2 5% 5% 3 4 4 4 5% 49%	1980
October	1st to 10th	Day	13% 4 2 3 4 2 4 3 4 5% 54%	1980
	11th to 20th	Day	12% 3 2 4 4 2 4 3 4 7% 55%	1917
	21st to end	Day	14% 4 1 4 4 3 2 3 5% 5% 54%	1964
November	1st to 10th	Day	13% 5% 2 4 4 2 3 4 3 6% 55%	1672
	11th to 20th	Day	14% 4 2 3 4 2 4 4 5% 5% 53%	1710
	21st to end	Day	14% 6% 2 4 4 3 5% 4 5% 7% 46%	1710
December	1st to 10th	Day	18% 7% 2 6% 4 3 5% 4 5% 6% 39%	1710
	11th to 20th	Day	17% 7% 3 6% 5% 3 4 4 4 5% 43%	2153
	21st to end	Day	21% 7% 2 5% 4 3 4 4 5% 6% 38%	3124
January	1st to 10th	Night	18% 7% 3 5% 4 2 4 3 4 6% 44%	2459
	11th to 20th	Night	17% 8% 3 6% 4 3 4 4 5% 6% 41%	2360
	21st to end	Night	17% 5% 3 5% 4 3 4 3 4 5% 46%	2450
February	1st to 10th	Night	15% 7% 3 5% 4 2 4 4 5% 7% 42%	2160
	11th to 20th	Night	20% 7% 2 5% 4 3 4 3 3 5% 44%	2098
	21st to end	Night	20% 8% 2 5% 5% 3 3 4 6% 6% 39%	1776
March	1st to 10th	Night	14% 6% 3 4 4 3 4 3 5% 7% 48%	2135
	11th to 20th	Night	15% 6% 2 5% 4 2 5% 5% 6% 7% 43%	1980
	21st to end	Night	13% 5% 3 5% 4 3 5% 4 5% 6% 46%	2178
April	1st to 10th	Night	11% 5% 2 4 5% 3 5% 4 6% 8% 48%	1900
	11th to 20th	Night	15% 5% 2 4 3 3 3 3 4 8% 50%	1775
	21st to end	Night	9% 4 2 4 2 2 5% 4 5% 7% 54%	1696
May	1st to 10th	Night	8% 4 2 3 4 2 5% 5% 6% 8% 52%	1620
	11th to 20th	Night	9% 5% 3 5% 6% 3 7% 5% 7% 8% 42%	1592
	21st to end	Night	10% 5% 3 4 5% 3 4 5% 6% 9% 46%	1744
June	1st to 10th	Night	5% 5% 3 6% 6% 4 6% 6% 8% 13% 38%	1620
	11th to 20th	Night	3 3 3 5% 5% 4 6% 5% 8% 12% 48%	1620
	21st to end	Night	6% 4 2 5% 5% 4 6% 5% 7% 12% 44%	1620
July	1st to 10th	Night	8% 6% 3 6% 7% 3 7% 6% 7% 12% 36%	1620
	11th to 20th	Night	6% 3 3 5% 6% 4 8% 7% 10% 12% 36%	1620
	21st to end	Night	5% 5% 2 6% 6% 4 7% 8% 9% 12% 36%	1771
August	1st to 10th	Night	6% 4 3 5% 6% 5% 6% 8% 8% 12% 37%	1633
	11th to 20th	Night	10% 5% 3 6% 6% 5% 5% 5% 8% 10% 38%	1776
	21st to end	Night	13% 6% 3 5% 6% 3 4 5% 8% 10% 36%	1980
September	1st to 10th	Night	9% 5% 2 5% 6% 3 6% 6% 6% 9% 43%	1930
	11th to 20th	Night	12% 6% 2 5% 6% 3 4 4 5% 8% 47%	1980
	21st to end	Night	11% 5% 2 4 4 3 4 5% 5% 7% 50%	1980
October	1st to 10th	Night	10% 5% 2 3 3 2 3 3 4 7% 57%	1980
	11th to 20th	Night	12% 5% 2 3 5% 2 2 4 4 5% 56%	2042
	21st to end	Night	12% 5% 2 4 4 2 3 3 4 5% 56%	2388
November	1st to 10th	Night	13% 4 2 3 3 2 3 3 4 5% 58%	2406
	11th to 20th	Night	12% 5% 2 4 4 2 3 3 4 6% 54%	2470
	21st to end	Night	15% 5% 2 4 5% 3 4 4 5% 6% 47%	2470
December	1st to 10th	Night	17% 7% 2 6% 4 3 3 4 5% 7% 41%	2470
	11th to 20th	Night	18% 6% 2 5% 4 2 2 3 4 6% 48%	3042
	21st to end	Night	20% 8% 3 6% 5% 3 5% 4 6% 7% 33%	4532

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	43% 4 8% 18% 22% 5%	1707
	11th to 20th	Day	42% 3 8% 19% 22% 5%	1629
	21st to end	Day	44% 3 6% 19% 22% 7%	1904
February	1st to 10th	Day	46% 5% 8% 17% 20% 4	1800
	11th to 20th	Day	46% 3 5% 19% 21% 6%	1758
	21st to end	Day	48% 4 7% 13% 20% 8%	1480
March	1st to 10th	Day	40% 4 8% 16% 22% 10%	1824
	11th to 20th	Day	47% 6% 10% 14% 17% 6%	1980
	21st to end	Day	43% 6% 9% 21% 17% 4	2178
April	1st to 10th	Day	40% 5% 9% 21% 19% 7%	2074
	11th to 20th	Day	38% 5% 7% 19% 23% 8%	2134
	21st to end	Day	31% 6% 9% 25% 25% 3	2263
May	1st to 10th	Day	33% 6% 12% 22% 24% 4	2340
	11th to 20th	Day	40% 9% 15% 19% 14% 2	2305
	21st to end	Day	37% 8% 11% 25% 18% 2	2531
June	1st to 10th	Day	45% 10% 13% 19% 12% 1	2340
	11th to 20th	Day	33% 10% 19% 23% 13% 2	2340
	21st to end	Day	38% 13% 17% 20% 11% 2	2340
July	1st to 10th	Day	45% 12% 17% 16% 8% 1	2275
	11th to 20th	Day	41% 13% 20% 15% 9% 2	2271
	21st to end	Day	40% 15% 18% 19% 7%	2405
August	1st to 10th	Day	41% 14% 19% 21% 5%	2288
	11th to 20th	Day	44% 11% 17% 17% 9% 2	2125
	21st to end	Day	47% 10% 15% 15% 10% 2	2376
September	1st to 10th	Day	41% 8% 14% 17% 16% 4	2012
	11th to 20th	Day	40% 9% 10% 18% 21% 2	1980
	21st to end	Day	38% 4 9% 20% 23% 6%	1980
October	1st to 10th	Day	33% 5% 9% 17% 28% 8%	1980
	11th to 20th	Day	30% 4 8% 24% 26% 8%	1917
	21st to end	Day	33% 4 6% 16% 31% 10%	1964
November	1st to 10th	Day	33% 4 7% 15% 31% 11%	1672
	11th to 20th	Day	34% 4 6% 17% 28% 11%	1710
	21st to end	Day	38% 5% 7% 17% 25% 7%	1710
December	1st to 10th	Day	46% 5% 8% 18% 17% 7%	1710
	11th to 20th	Day	45% 2 5% 15% 24% 7%	2150
	21st to end	Day	46% 4 8% 18% 21% 3	3124
January	1st to 10th	Night	43% 3 7% 18% 23% 5%	2459
	11th to 20th	Night	44% 4 8% 19% 20% 5%	2360
	21st to end	Night	41% 4 4 20% 25% 6%	2450
February	1st to 10th	Night	41% 5% 8% 19% 19% 8%	2160
	11th to 20th	Night	44% 4 6% 17% 21% 8%	2099
	21st to end	Night	46% 5% 8% 15% 18% 9%	1776
March	1st to 10th	Night	38% 3 8% 19% 21% 12%	2135
	11th to 20th	Night	39% 6% 12% 17% 19% 6%	1980
	21st to end	Night	39% 4 9% 23% 20% 5%	2178
April	1st to 10th	Night	35% 5% 10% 20% 23% 8%	1900
	11th to 20th	Night	34% 4 6% 22% 23% 10%	1775
	21st to end	Night	29% 5% 10% 27% 22% 6%	1696
May	1st to 10th	Night	30% 7% 12% 24% 23% 4	1620
	11th to 20th	Night	37% 9% 16% 22% 15% 2	1593
	21st to end	Night	33% 9% 11% 25% 19% 3	1745
June	1st to 10th	Night	34% 12% 17% 22% 14% 1	1620
	11th to 20th	Night	26% 9% 23% 27% 13% 2	1620
	21st to end	Night	33% 10% 19% 20% 15% 3	1620
July	1st to 10th	Night	40% 11% 18% 20% 10% 2	1575
	11th to 20th	Night	34% 15% 23% 16% 10% 2	1566
	21st to end	Night	34% 13% 23% 20% 8% 1	1672
August	1st to 10th	Night	36% 12% 18% 26% 6% 1	1622
	11th to 20th	Night	38% 11% 18% 18% 12% 2	1776
	21st to end	Night	42% 10% 17% 18% 11% 3	1980
September	1st to 10th	Night	35% 10% 15% 18% 17% 5%	1930
	11th to 20th	Night	37% 7% 10% 22% 20% 4	1980
	21st to end	Night	33% 6% 11% 20% 21% 9%	1980
October	1st to 10th	Night	28% 6% 9% 21% 27% 9%	1980
	11th to 20th	Night	31% 4 6% 21% 28% 10%	2042
	21st to end	Night	32% 3 5% 17% 30% 13%	2388
November	1st to 10th	Night	31% 3 8% 17% 28% 13%	2406
	11th to 20th	Night	33% 4 5% 18% 27% 13%	2470
	21st to end	Night	37% 4 7% 17% 26% 8%	2470
December	1st to 10th	Night	43% 5% 8% 19% 19% 6%	2470
	11th to 20th	Night	40% 3 7% 15% 29% 6%	3044
	21st to end	Night	49% 4 7% 19% 18% 3	4532

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	23% 25% 36% 14% 1	270
	11th to 20th	Day	25% 20% 34% 14% 6% 2	243
	21st to end	Day	23% 28% 36% 9% 3 1	264
February	1st to 10th	Day	24% 25% 33% 16% 3	240
	11th to 20th	Day	33% 23% 35% 10% 1	240
	21st to end	Day	30% 19% 40% 11% 1	198
March	1st to 10th	Day	21% 19% 39% 17% 3 1	264
	11th to 20th	Day	24% 20% 31% 17% 8% 1	420
	21st to end	Day	22% 17% 33% 21% 7% 1	462
April	1st to 10th	Day	19% 18% 29% 20% 13% 1	420
	11th to 20th	Day	14% 17% 31% 20% 15% 2	412
	21st to end	Day	14% 15% 26% 24% 17% 3	420
May	1st to 10th	Day	14% 12% 28% 27% 16% 3	423
	11th to 20th	Day	13% 6% 23% 30% 18% 11%	447
	21st to end	Day	12% 6% 17% 29% 24% 13%	485
June	1st to 10th	Day	14% 3 15% 28% 28% 13%	450
	11th to 20th	Day	7% 3 14% 24% 28% 25%	450
	21st to end	Day	10% 2 15% 28% 27% 18%	450
July	1st to 10th	Day	9% 2 8% 24% 33% 22%	445
	11th to 20th	Day	7% 3 8% 20% 34% 28%	444
	21st to end	Day	7% 1 9% 22% 38% 23%	488
August	1st to 10th	Day	11% 2 12% 23% 29% 23%	446
	11th to 20th	Day	10% 1 8% 27% 32% 22%	439
	21st to end	Day	17% 2 8% 27% 27% 18%	495
September	1st to 10th	Day	10% 4 14% 25% 28% 19%	450
	11th to 20th	Day	18% 3 13% 24% 31% 12%	450
	21st to end	Day	14% 3 17% 25% 28% 12%	450
October	1st to 10th	Day	16% 6% 20% 30% 19% 9%	450
	11th to 20th	Day	14% 8% 24% 28% 20% 6%	450
	21st to end	Day	18% 11% 23% 26% 19% 3	479
November	1st to 10th	Day	23% 10% 26% 26% 13% 3	270
	11th to 20th	Day	17% 17% 37% 21% 6% 1	270
	21st to end	Day	10% 18% 41% 22% 8% 1	270
December	1st to 10th	Day	24% 21% 31% 19% 4	270
	11th to 20th	Day	23% 20% 32% 19% 5% 1	270
	21st to end	Day	30% 15% 35% 13% 5% 1	297
January	1st to 10th	Night	23% 19% 31% 23% 4 1	760
	11th to 20th	Night	21% 17% 30% 25% 7%	727
	21st to end	Night	20% 18% 38% 22% 3	792
February	1st to 10th	Night	20% 14% 35% 26% 5%	720
	11th to 20th	Night	23% 17% 35% 22% 2	703
	21st to end	Night	22% 17% 36% 21% 4	592
March	1st to 10th	Night	17% 17% 35% 24% 6%	720
	11th to 20th	Night	18% 13% 33% 25% 10% 1	720
	21st to end	Night	16% 15% 31% 29% 10% 1	792
April	1st to 10th	Night	14% 13% 32% 27% 13% 1	640
	11th to 20th	Night	18% 11% 29% 25% 14% 2	533
	21st to end	Night	12% 8% 25% 36% 17% 2	540
May	1st to 10th	Night	11% 8% 27% 32% 22% 1	540
	11th to 20th	Night	11% 6% 20% 33% 25% 5%	531
	21st to end	Night	13% 2 13% 31% 35% 7%	584
June	1st to 10th	Night	7% 2 13% 38% 33% 7%	540
	11th to 20th	Night	2 3 14% 29% 35% 18%	540
	21st to end	Night	6% 1 10% 34% 41% 8%	540
July	1st to 10th	Night	8% 3 8% 24% 40% 16%	525
	11th to 20th	Night	7% 1 5% 24% 47% 17%	522
	21st to end	Night	8% 1 8% 27% 43% 13%	591
August	1st to 10th	Night	9% 7% 27% 44% 13%	537
	11th to 20th	Night	13% 2 7% 27% 42% 9%	534
	21st to end	Night	15% 2 6% 27% 38% 12%	594
September	1st to 10th	Night	10% 2 8% 25% 43% 11%	670
	11th to 20th	Night	15% 3 11% 27% 38% 6%	720
	21st to end	Night	13% 2 16% 34% 33% 3	720
October	1st to 10th	Night	12% 5% 21% 32% 23% 7%	720
	11th to 20th	Night	14% 6% 26% 31% 21% 3	720
	21st to end	Night	15% 9% 22% 33% 18% 3	792
November	1st to 10th	Night	15% 8% 24% 31% 20% 2	740
	11th to 20th	Night	16% 12% 29% 29% 13% 1	760
	21st to end	Night	18% 11% 30% 29% 12% 1	760
December	1st to 10th	Night	20% 14% 30% 26% 10% 1	760
	11th to 20th	Night	23% 10% 32% 25% 9% 1	760
	21st to end	Night	28% 14% 28% 25% 6%	836

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