Above the Clouds Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Above the Clouds Observatory clear sky chart from 2005-11-06 to 2024-03-01.

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	15% 9% 4 7% 6% 4 5% 5% 6% 7% 33%	78314
	Night	14% 8% 4 7% 6% 4 5% 5% 6% 7% 33%	81091
Transparency	Day	50% 6% 12% 20% 10% 1	78019
	Night	49% 6% 13% 20% 10% 1	80920
Seeing	Day	23% 16% 29% 19% 9% 3	13827
	Night	20% 13% 27% 29% 10% 1	22944

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% 11% 5% 11% 7% 5% 5% 5% 6% 7% 28%	2517
	11th to 20th	Day	13% 11% 6% 11% 11% 5% 5% 5% 6% 5% 23%	2520
	21st to end	Day	13% 14% 8% 11% 7% 3 6% 5% 5% 6% 21%	2772
February	1st to 10th	Day	11% 10% 4 9% 7% 4 7% 5% 6% 7% 28%	1675
	11th to 20th	Day	9% 9% 6% 10% 10% 5% 6% 6% 5% 8% 27%	1848
	21st to end	Day	14% 11% 5% 8% 6% 4 6% 5% 5% 6% 30%	1570
March	1st to 10th	Day	11% 10% 4 8% 8% 4 6% 6% 6% 6% 30%	1797
	11th to 20th	Day	15% 11% 5% 8% 6% 4 4 5% 5% 5% 32%	1841
	21st to end	Day	15% 9% 4 8% 7% 3 5% 5% 4 6% 34%	2376
April	1st to 10th	Day	17% 10% 4 8% 6% 3 4 4 4 6% 33%	2160
	11th to 20th	Day	15% 9% 3 6% 6% 4 5% 5% 5% 7% 35%	2134
	21st to end	Day	21% 9% 4 8% 6% 4 4 4 5% 7% 28%	2322
May	1st to 10th	Day	20% 11% 4 7% 6% 4 5% 4 4 5% 31%	2348
	11th to 20th	Day	18% 6% 4 5% 6% 3 4 4 6% 6% 38%	2478
	21st to end	Day	14% 6% 3 6% 6% 3 5% 6% 6% 8% 38%	2725
June	1st to 10th	Day	19% 8% 3 6% 6% 4 5% 6% 6% 8% 29%	2520
	11th to 20th	Day	17% 6% 3 5% 5% 4 6% 5% 5% 9% 35%	2520
	21st to end	Day	17% 7% 4 6% 7% 4 6% 6% 6% 8% 29%	2520
July	1st to 10th	Day	16% 6% 3 5% 5% 3 6% 5% 6% 8% 38%	2520
	11th to 20th	Day	11% 5% 3 6% 7% 4 7% 6% 7% 10% 34%	2520
	21st to end	Day	14% 5% 2 5% 6% 4 6% 6% 7% 9% 36%	2744
August	1st to 10th	Day	11% 5% 2 6% 7% 5% 6% 6% 7% 9% 35%	2471
	11th to 20th	Day	13% 7% 3 6% 5% 3 6% 7% 6% 8% 35%	2237
	21st to end	Day	12% 6% 2 4 7% 4 4 6% 6% 9% 41%	2376
September	1st to 10th	Day	12% 5% 2 5% 5% 3 4 5% 6% 9% 44%	2160
	11th to 20th	Day	13% 6% 3 5% 5% 3 4 3 5% 6% 48%	2034
	21st to end	Day	15% 6% 2 5% 4 2 5% 4 6% 8% 42%	1980
October	1st to 10th	Day	20% 7% 2 7% 7% 3 5% 5% 4 6% 34%	1943
	11th to 20th	Day	13% 9% 3 8% 6% 4 5% 5% 5% 8% 36%	1738
	21st to end	Day	23% 12% 3 6% 5% 4 5% 5% 4 5% 27%	1782
November	1st to 10th	Day	11% 9% 4 5% 6% 4 5% 6% 6% 6% 38%	1671
	11th to 20th	Day	15% 9% 4 6% 6% 3 5% 6% 5% 7% 33%	1710
	21st to end	Day	16% 9% 5% 9% 6% 3 5% 5% 5% 8% 30%	1587
December	1st to 10th	Day	16% 13% 5% 9% 8% 5% 6% 4 7% 6% 22%	1520
	11th to 20th	Day	19% 11% 4 7% 6% 3 5% 5% 5% 6% 29%	1906
	21st to end	Day	20% 14% 4 9% 8% 4 5% 5% 5% 4 22%	2772
January	1st to 10th	Night	14% 9% 4 8% 6% 3 6% 6% 6% 8% 31%	4429
	11th to 20th	Night	10% 11% 6% 11% 10% 5% 7% 6% 6% 7% 23%	4440
	21st to end	Night	16% 13% 6% 11% 8% 5% 5% 5% 5% 5% 22%	4884
February	1st to 10th	Night	10% 11% 4 9% 8% 5% 6% 6% 7% 9% 26%	2501
	11th to 20th	Night	9% 7% 4 10% 8% 5% 6% 8% 7% 6% 29%	2218
	21st to end	Night	11% 9% 4 8% 8% 4 5% 5% 5% 8% 32%	1884
March	1st to 10th	Night	11% 10% 3 8% 9% 5% 5% 6% 6% 6% 32%	2157
	11th to 20th	Night	14% 8% 4 7% 8% 5% 6% 5% 6% 7% 31%	2114
	21st to end	Night	14% 9% 4 7% 5% 3 4 5% 6% 5% 39%	1980
April	1st to 10th	Night	16% 8% 4 7% 6% 4 6% 6% 5% 6% 34%	1800
	11th to 20th	Night	15% 8% 4 7% 6% 4 5% 5% 6% 7% 35%	1775
	21st to end	Night	19% 8% 4 6% 5% 3 4 4 6% 7% 33%	1638
May	1st to 10th	Night	20% 10% 4 7% 5% 3 6% 4 6% 7% 30%	1620
	11th to 20th	Night	16% 7% 3 5% 6% 4 4 5% 5% 7% 38%	1423
	21st to end	Night	12% 6% 2 5% 6% 3 5% 6% 6% 8% 39%	1550
June	1st to 10th	Night	17% 6% 3 5% 5% 4 6% 5% 6% 8% 34%	1440
	11th to 20th	Night	18% 6% 3 4 5% 3 4 6% 4 8% 40%	1440
	21st to end	Night	16% 5% 2 6% 5% 4 6% 5% 6% 8% 38%	1440
July	1st to 10th	Night	14% 5% 2 4 6% 2 4 5% 7% 10% 42%	1440
	11th to 20th	Night	13% 4 2 4 4 3 4 5% 7% 10% 44%	1440
	21st to end	Night	10% 5% 2 5% 5% 3 6% 5% 7% 9% 44%	1574
August	1st to 10th	Night	11% 4 3 4 5% 3 5% 5% 6% 9% 44%	1448
	11th to 20th	Night	12% 5% 2 6% 6% 3 5% 4 6% 8% 43%	1660
	21st to end	Night	14% 5% 2 4 5% 3 3 5% 6% 8% 45%	1980
September	1st to 10th	Night	12% 4 2 4 5% 3 4 4 7% 9% 47%	1800
	11th to 20th	Night	12% 5% 2 5% 4 2 4 4 5% 5% 51%	1926
	21st to end	Night	12% 5% 2 4 5% 3 4 5% 6% 7% 46%	1980
October	1st to 10th	Night	18% 7% 3 7% 6% 2 4 4 5% 8% 35%	2016
	11th to 20th	Night	16% 7% 3 4 5% 4 6% 6% 6% 9% 34%	2222
	21st to end	Night	21% 8% 3 6% 5% 4 5% 5% 6% 7% 29%	2574
November	1st to 10th	Night	12% 8% 4 7% 7% 3 5% 5% 5% 6% 36%	2407
	11th to 20th	Night	15% 6% 4 5% 6% 4 5% 7% 6% 8% 35%	2470
	21st to end	Night	13% 9% 4 7% 7% 4 5% 5% 6% 7% 33%	2589
December	1st to 10th	Night	15% 11% 5% 10% 9% 4 7% 5% 6% 8% 21%	2660
	11th to 20th	Night	18% 9% 5% 8% 5% 4 5% 5% 5% 8% 28%	3288
	21st to end	Night	18% 12% 5% 12% 8% 4 5% 4 5% 5% 21%	4884

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% 7% 11% 23% 5%	2518
	11th to 20th	Day	62% 6% 10% 15% 5% 1	2520
	21st to end	Day	64% 5% 10% 14% 6% 1	2772
February	1st to 10th	Day	54% 6% 13% 17% 8% 1	1675
	11th to 20th	Day	55% 7% 11% 17% 9% 1	1857
	21st to end	Day	54% 5% 12% 15% 12% 2	1570
March	1st to 10th	Day	53% 7% 12% 16% 10% 3	1800
	11th to 20th	Day	54% 5% 11% 14% 14% 3	1841
	21st to end	Day	52% 5% 8% 17% 14% 4	2376
April	1st to 10th	Day	51% 5% 10% 20% 11% 3	2160
	11th to 20th	Day	47% 6% 11% 20% 15% 1	2134
	21st to end	Day	56% 5% 10% 16% 11% 2	2322
May	1st to 10th	Day	55% 4 7% 20% 12% 2	2348
	11th to 20th	Day	46% 5% 10% 21% 16% 2	2482
	21st to end	Day	43% 8% 16% 22% 10% 2	2725
June	1st to 10th	Day	52% 7% 15% 17% 9%	2520
	11th to 20th	Day	46% 6% 12% 21% 14% 1	2520
	21st to end	Day	52% 8% 12% 17% 9% 1	2520
July	1st to 10th	Day	42% 7% 13% 23% 15% 1	2450
	11th to 20th	Day	43% 7% 16% 25% 9%	2445
	21st to end	Day	41% 8% 15% 25% 10% 1	2591
August	1st to 10th	Day	43% 8% 16% 24% 10%	2457
	11th to 20th	Day	43% 8% 18% 22% 9%	2237
	21st to end	Day	38% 7% 14% 26% 13% 1	2376
September	1st to 10th	Day	36% 6% 14% 30% 13% 1	2160
	11th to 20th	Day	38% 4 10% 29% 17% 2	2034
	21st to end	Day	38% 6% 15% 26% 12% 2	1980
October	1st to 10th	Day	51% 6% 10% 20% 12% 2	1943
	11th to 20th	Day	46% 6% 14% 26% 7% 1	1738
	21st to end	Day	58% 6% 11% 16% 8% 1	1782
November	1st to 10th	Day	44% 7% 14% 22% 13%	1671
	11th to 20th	Day	50% 7% 12% 17% 12% 2	1710
	21st to end	Day	53% 5% 19% 17% 6% 1	1587
December	1st to 10th	Day	62% 6% 13% 14% 5% 1	1520
	11th to 20th	Day	57% 5% 10% 17% 9% 1	1906
	21st to end	Day	65% 6% 10% 12% 8%	2772
January	1st to 10th	Night	50% 7% 19% 16% 7%	4429
	11th to 20th	Night	60% 8% 13% 13% 6%	4440
	21st to end	Night	64% 6% 10% 13% 7% 1	4884
February	1st to 10th	Night	53% 6% 15% 17% 8% 1	2501
	11th to 20th	Night	51% 7% 11% 20% 10% 1	2220
	21st to end	Night	50% 6% 14% 20% 8% 3	1884
March	1st to 10th	Night	51% 6% 11% 20% 11% 2	2167
	11th to 20th	Night	52% 6% 11% 17% 11% 2	2115
	21st to end	Night	46% 5% 11% 19% 16% 4	1980
April	1st to 10th	Night	50% 5% 10% 21% 12% 2	1800
	11th to 20th	Night	47% 5% 11% 23% 13% 1	1775
	21st to end	Night	51% 6% 10% 18% 11% 3	1638
May	1st to 10th	Night	53% 4 9% 18% 15% 2	1620
	11th to 20th	Night	45% 6% 11% 22% 14% 2	1424
	21st to end	Night	42% 8% 17% 21% 12% 1	1551
June	1st to 10th	Night	46% 8% 15% 18% 12%	1440
	11th to 20th	Night	44% 7% 15% 20% 13% 2	1440
	21st to end	Night	44% 8% 14% 24% 10%	1440
July	1st to 10th	Night	37% 7% 20% 22% 14% 1	1400
	11th to 20th	Night	34% 8% 21% 26% 11%	1392
	21st to end	Night	34% 8% 16% 29% 11% 1	1486
August	1st to 10th	Night	35% 7% 18% 27% 11%	1438
	11th to 20th	Night	39% 6% 17% 27% 11%	1660
	21st to end	Night	36% 7% 16% 28% 13%	1980
September	1st to 10th	Night	33% 7% 18% 30% 12%	1800
	11th to 20th	Night	34% 6% 13% 30% 16% 1	1926
	21st to end	Night	37% 6% 15% 26% 15% 2	1980
October	1st to 10th	Night	47% 6% 13% 22% 11% 2	2016
	11th to 20th	Night	47% 7% 16% 23% 7%	2222
	21st to end	Night	53% 7% 13% 18% 8% 1	2574
November	1st to 10th	Night	48% 6% 13% 21% 11% 2	2407
	11th to 20th	Night	45% 8% 13% 21% 11% 2	2470
	21st to end	Night	50% 7% 16% 20% 7%	2589
December	1st to 10th	Night	61% 7% 12% 15% 4	2660
	11th to 20th	Night	55% 7% 13% 15% 10% 1	3288
	21st to end	Night	64% 6% 10% 16% 4	4884

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	25% 21% 28% 18% 7% 1	180
	11th to 20th	Day	31% 22% 29% 12% 6% 1	180
	21st to end	Day	20% 25% 32% 15% 7% 2	198
February	1st to 10th	Day	32% 27% 29% 9% 3	260
	11th to 20th	Day	27% 24% 31% 15% 2	456
	21st to end	Day	28% 21% 29% 18% 4	382
March	1st to 10th	Day	26% 16% 35% 19% 3	420
	11th to 20th	Day	31% 21% 29% 13% 6%	420
	21st to end	Day	30% 21% 26% 13% 6% 4	462
April	1st to 10th	Day	34% 15% 27% 16% 5% 2	420
	11th to 20th	Day	34% 19% 24% 13% 8% 2	412
	21st to end	Day	31% 20% 30% 11% 7% 2	420
May	1st to 10th	Day	32% 20% 25% 14% 6% 2	423
	11th to 20th	Day	19% 20% 34% 19% 6% 2	447
	21st to end	Day	18% 17% 33% 20% 9% 2	485
June	1st to 10th	Day	24% 14% 38% 16% 7% 1	450
	11th to 20th	Day	20% 16% 31% 20% 11% 2	450
	21st to end	Day	20% 12% 35% 22% 9% 3	450
July	1st to 10th	Day	18% 11% 29% 27% 11% 4	445
	11th to 20th	Day	14% 9% 34% 29% 11% 4	444
	21st to end	Day	14% 8% 38% 25% 12% 3	488
August	1st to 10th	Day	11% 11% 32% 23% 19% 5%	446
	11th to 20th	Day	17% 9% 26% 25% 14% 9%	439
	21st to end	Day	13% 13% 26% 26% 14% 7%	495
September	1st to 10th	Day	15% 12% 26% 24% 15% 9%	450
	11th to 20th	Day	16% 12% 26% 20% 15% 10%	450
	21st to end	Day	19% 11% 29% 22% 12% 7%	450
October	1st to 10th	Day	24% 12% 24% 19% 15% 5%	450
	11th to 20th	Day	21% 16% 26% 24% 11% 2	419
	21st to end	Day	34% 15% 22% 20% 8% 1	396
November	1st to 10th	Day	23% 16% 24% 22% 14% 2	365
	11th to 20th	Day	27% 20% 24% 20% 8% 1	370
	21st to end	Day	31% 16% 27% 17% 8% 1	247
December	1st to 10th	Day	27% 20% 31% 15% 6% 1	180
	11th to 20th	Day	30% 23% 29% 16% 2	180
	21st to end	Day	34% 18% 26% 18% 4	198
January	1st to 10th	Night	20% 17% 35% 24% 4	760
	11th to 20th	Night	23% 18% 29% 26% 4	760
	21st to end	Night	16% 17% 33% 29% 4	836
February	1st to 10th	Night	18% 18% 35% 26% 3	760
	11th to 20th	Night	19% 19% 35% 23% 4	743
	21st to end	Night	22% 17% 31% 26% 4	628
March	1st to 10th	Night	22% 14% 33% 27% 3	723
	11th to 20th	Night	24% 18% 28% 24% 6%	720
	21st to end	Night	25% 16% 27% 24% 8% 1	792
April	1st to 10th	Night	23% 18% 29% 24% 6%	720
	11th to 20th	Night	21% 20% 28% 23% 8%	711
	21st to end	Night	28% 15% 27% 23% 7%	558
May	1st to 10th	Night	26% 13% 27% 25% 9%	540
	11th to 20th	Night	17% 13% 28% 35% 6% 1	362
	21st to end	Night	16% 10% 27% 33% 14%	389
June	1st to 10th	Night	20% 9% 26% 33% 12% 1	360
	11th to 20th	Night	21% 7% 23% 35% 14%	360
	21st to end	Night	17% 4 25% 39% 14% 1	360
July	1st to 10th	Night	16% 3 19% 43% 16% 2	350
	11th to 20th	Night	13% 1 25% 41% 18% 2	348
	21st to end	Night	11% 4 21% 45% 17% 1	394
August	1st to 10th	Night	13% 3 17% 41% 25%	374
	11th to 20th	Night	15% 6% 20% 36% 20% 3	596
	21st to end	Night	14% 6% 17% 36% 24% 2	792
September	1st to 10th	Night	14% 5% 25% 33% 21% 2	720
	11th to 20th	Night	16% 8% 20% 30% 22% 4	720
	21st to end	Night	15% 8% 26% 32% 17% 2	720
October	1st to 10th	Night	24% 12% 17% 30% 15% 2	720
	11th to 20th	Night	21% 12% 22% 32% 13% 1	720
	21st to end	Night	27% 14% 25% 27% 7%	792
November	1st to 10th	Night	20% 16% 25% 30% 9%	740
	11th to 20th	Night	19% 17% 26% 27% 9% 1	760
	21st to end	Night	21% 17% 26% 29% 7%	760
December	1st to 10th	Night	24% 18% 31% 22% 4	760
	11th to 20th	Night	24% 17% 26% 28% 4	760
	21st to end	Night	27% 15% 31% 24% 3	836

Page generated at 2024-02-27 06:30:18. Version ac07

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