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 2024-11-01.

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	14% 9% 3 8% 5% 5% 5% 4 6% 7% 32%	101360
	Night	14% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	97595
Transparency	Day	49% 7% 13% 20% 10% 1	101118
	Night	49% 7% 14% 19% 10% 1	97457
Seeing	Day	23% 16% 29% 19% 9% 3	14792
	Night	20% 13% 27% 29% 10% 1	23716

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	10% 11% 4 12% 6% 6% 5% 4 6% 6% 28%	2517
	11th to 20th	Day	13% 11% 6% 13% 9% 6% 5% 4 6% 5% 23%	2520
	21st to end	Day	13% 15% 8% 13% 6% 5% 5% 4 5% 6% 21%	2772
February	1st to 10th	Day	11% 11% 3 10% 6% 7% 5% 4 6% 7% 28%	1675
	11th to 20th	Day	9% 10% 5% 12% 8% 7% 6% 5% 6% 7% 27%	1848
	21st to end	Day	14% 12% 3 9% 6% 5% 5% 4 5% 6% 30%	1570
March	1st to 10th	Day	19% 16% 5% 10% 6% 4 5% 3 4 4 24%	3116
	11th to 20th	Day	12% 12% 5% 11% 6% 6% 5% 4 6% 6% 27%	3243
	21st to end	Day	16% 11% 4 11% 5% 5% 4 4 4 5% 32%	4166
April	1st to 10th	Day	17% 11% 3 9% 5% 4 4 3 4 6% 33%	2280
	11th to 20th	Day	15% 10% 2 7% 5% 6% 5% 4 5% 7% 34%	2254
	21st to end	Day	20% 9% 3 9% 5% 5% 4 4 5% 6% 29%	2451
May	1st to 10th	Day	21% 11% 4 9% 6% 5% 5% 4 5% 5% 27%	4128
	11th to 20th	Day	16% 8% 3 8% 6% 5% 6% 4 7% 7% 30%	4398
	21st to end	Day	12% 5% 2 6% 4 5% 5% 5% 8% 9% 40%	4837
June	1st to 10th	Day	18% 9% 3 7% 5% 5% 5% 5% 6% 7% 29%	2660
	11th to 20th	Day	16% 7% 2 6% 5% 5% 6% 4 6% 8% 35%	2660
	21st to end	Day	17% 8% 3 7% 6% 6% 5% 4 6% 8% 28%	2660
July	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 5% 8% 10% 37%	4440
	11th to 20th	Day	9% 5% 2 7% 5% 6% 7% 5% 9% 11% 34%	4440
	21st to end	Day	13% 7% 3 7% 5% 5% 5% 4 7% 7% 37%	4814
August	1st to 10th	Day	14% 6% 2 7% 6% 6% 6% 5% 8% 9% 32%	4361
	11th to 20th	Day	14% 8% 3 7% 5% 5% 6% 4 7% 8% 33%	3922
	21st to end	Day	10% 6% 2 6% 5% 5% 5% 5% 7% 11% 40%	4180
September	1st to 10th	Day	11% 5% 2 6% 4 3 5% 4 7% 9% 45%	2280
	11th to 20th	Day	12% 6% 2 6% 4 4 4 3 5% 6% 48%	2147
	21st to end	Day	15% 7% 2 6% 4 4 4 3 7% 8% 41%	2090
October	1st to 10th	Day	19% 7% 2 8% 6% 5% 5% 4 5% 6% 34%	2050
	11th to 20th	Day	13% 10% 3 8% 5% 5% 4 3 5% 7% 37%	1834
	21st to end	Day	22% 11% 3 7% 5% 5% 4 5% 4 5% 29%	1881
November	1st to 10th	Day	11% 9% 3 6% 5% 5% 5% 5% 7% 6% 38%	1671
	11th to 20th	Day	15% 11% 4 7% 5% 4 5% 5% 5% 6% 33%	1710
	21st to end	Day	15% 10% 4 9% 5% 4 4 4 6% 7% 29%	1587
December	1st to 10th	Day	15% 14% 4 11% 7% 6% 5% 4 7% 6% 22%	1520
	11th to 20th	Day	19% 12% 4 9% 5% 4 4 4 5% 6% 29%	1906
	21st to end	Day	20% 14% 4 10% 7% 5% 6% 4 5% 4 22%	2772
January	1st to 10th	Night	14% 9% 3 9% 5% 5% 6% 5% 6% 7% 30%	4429
	11th to 20th	Night	10% 11% 6% 13% 7% 7% 6% 5% 6% 6% 23%	4440
	21st to end	Night	16% 14% 6% 13% 6% 6% 5% 4 5% 4 22%	4884
February	1st to 10th	Night	10% 12% 4 10% 7% 6% 5% 6% 7% 8% 26%	2501
	11th to 20th	Night	9% 8% 4 11% 7% 6% 6% 6% 7% 6% 29%	2218
	21st to end	Night	11% 10% 3 10% 7% 5% 5% 4 5% 8% 32%	1884
March	1st to 10th	Night	20% 13% 3 7% 6% 4 5% 4 5% 5% 27%	3724
	11th to 20th	Night	11% 10% 4 10% 6% 6% 6% 5% 5% 6% 31%	3698
	21st to end	Night	15% 12% 4 10% 5% 4 4 3 4 5% 34%	3466
April	1st to 10th	Night	16% 9% 3 8% 5% 5% 5% 4 5% 6% 33%	1900
	11th to 20th	Night	15% 9% 3 8% 5% 5% 5% 3 6% 7% 35%	1875
	21st to end	Night	18% 9% 4 6% 4 4 4 3 6% 7% 34%	1729
May	1st to 10th	Night	23% 12% 3 7% 4 4 5% 3 6% 7% 25%	2834
	11th to 20th	Night	15% 10% 3 8% 7% 5% 5% 4 5% 6% 30%	2503
	21st to end	Night	12% 5% 2 6% 5% 5% 6% 5% 7% 9% 40%	2738
June	1st to 10th	Night	17% 6% 2 6% 4 5% 6% 4 6% 9% 34%	1520
	11th to 20th	Night	17% 7% 3 4 5% 3 3 5% 5% 8% 40%	1520
	21st to end	Night	16% 6% 1 7% 5% 5% 5% 4 6% 8% 36%	1520
July	1st to 10th	Night	10% 7% 2 6% 5% 5% 5% 5% 7% 9% 38%	2520
	11th to 20th	Night	13% 5% 2 7% 4 4 4 5% 7% 8% 42%	2520
	21st to end	Night	11% 7% 2 7% 5% 5% 5% 4 7% 7% 40%	2739
August	1st to 10th	Night	16% 9% 3 8% 4 5% 5% 3 5% 7% 35%	2536
	11th to 20th	Night	14% 6% 3 7% 5% 4 4 4 6% 8% 39%	2902
	21st to end	Night	10% 5% 1 5% 4 3 5% 4 8% 10% 46%	3476
September	1st to 10th	Night	11% 5% 2 5% 4 4 4 3 6% 8% 48%	1900
	11th to 20th	Night	12% 5% 2 6% 3 3 4 3 6% 5% 51%	2033
	21st to end	Night	12% 6% 2 5% 4 4 4 4 7% 7% 45%	2090
October	1st to 10th	Night	17% 7% 3 9% 5% 4 4 3 5% 8% 35%	2128
	11th to 20th	Night	15% 8% 3 5% 4 5% 5% 5% 6% 9% 35%	2346
	21st to end	Night	20% 8% 3 7% 5% 5% 5% 4 6% 7% 30%	2717
November	1st to 10th	Night	12% 9% 4 8% 6% 5% 5% 4 4 6% 36%	2414
	11th to 20th	Night	15% 7% 3 6% 5% 5% 5% 5% 6% 8% 35%	2470
	21st to end	Night	13% 10% 4 8% 6% 5% 5% 4 6% 7% 33%	2589
December	1st to 10th	Night	15% 12% 4 11% 8% 6% 6% 4 6% 8% 20%	2660
	11th to 20th	Night	18% 9% 5% 9% 5% 5% 5% 4 5% 8% 28%	3288
	21st to end	Night	18% 13% 5% 13% 6% 5% 5% 3 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	63% 5% 9% 11% 10% 2	3119
	11th to 20th	Day	55% 6% 12% 14% 10% 2	3243
	21st to end	Day	55% 4 6% 14% 18% 3	4166
April	1st to 10th	Day	52% 5% 10% 20% 11% 3	2280
	11th to 20th	Day	48% 6% 11% 19% 15% 1	2254
	21st to end	Day	55% 5% 10% 16% 11% 2	2451
May	1st to 10th	Day	58% 5% 10% 18% 7% 1	4142
	11th to 20th	Day	50% 7% 17% 16% 9% 1	4402
	21st to end	Day	37% 8% 15% 27% 12% 1	4837
June	1st to 10th	Day	51% 7% 15% 17% 9%	2660
	11th to 20th	Day	45% 7% 12% 20% 14% 2	2660
	21st to end	Day	52% 8% 12% 18% 9% 1	2660
July	1st to 10th	Day	39% 7% 16% 24% 13%	4370
	11th to 20th	Day	38% 9% 20% 25% 8%	4365
	21st to end	Day	43% 8% 13% 24% 12%	4661
August	1st to 10th	Day	45% 9% 16% 20% 10%	4347
	11th to 20th	Day	46% 8% 17% 24% 5%	3961
	21st to end	Day	35% 9% 17% 25% 13% 1	4180
September	1st to 10th	Day	34% 6% 14% 31% 13% 1	2280
	11th to 20th	Day	37% 4 11% 29% 17% 1	2147
	21st to end	Day	39% 6% 15% 26% 12% 2	2090
October	1st to 10th	Day	49% 6% 10% 20% 13% 2	2050
	11th to 20th	Day	46% 6% 14% 26% 8% 1	1834
	21st to end	Day	56% 6% 11% 17% 9% 1	1881
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	56% 5% 11% 17% 9% 1	3734
	11th to 20th	Night	53% 5% 13% 16% 11% 1	3699
	21st to end	Night	53% 4 7% 15% 18% 3	3466
April	1st to 10th	Night	51% 4 10% 21% 12% 2	1900
	11th to 20th	Night	48% 5% 11% 23% 12% 1	1875
	21st to end	Night	50% 6% 10% 19% 12% 3	1729
May	1st to 10th	Night	57% 5% 11% 18% 8% 1	2840
	11th to 20th	Night	54% 6% 14% 17% 8% 1	2504
	21st to end	Night	39% 8% 19% 20% 13% 2	2739
June	1st to 10th	Night	45% 8% 16% 19% 11%	1520
	11th to 20th	Night	43% 7% 16% 20% 12% 2	1520
	21st to end	Night	45% 8% 14% 23% 10% 1	1520
July	1st to 10th	Night	41% 7% 19% 20% 13%	2480
	11th to 20th	Night	37% 10% 17% 26% 11%	2472
	21st to end	Night	40% 8% 12% 29% 10% 1	2651
August	1st to 10th	Night	48% 7% 14% 21% 9%	2526
	11th to 20th	Night	41% 8% 19% 22% 9%	2929
	21st to end	Night	29% 9% 22% 27% 12% 1	3476
September	1st to 10th	Night	33% 7% 17% 29% 13%	1900
	11th to 20th	Night	33% 6% 14% 30% 16% 1	2033
	21st to end	Night	37% 6% 15% 25% 15% 2	2090
October	1st to 10th	Night	47% 6% 13% 22% 10% 2	2128
	11th to 20th	Night	46% 6% 15% 24% 8% 1	2346
	21st to end	Night	52% 7% 13% 20% 8% 1	2717
November	1st to 10th	Night	48% 6% 13% 21% 11% 2	2414
	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	28% 16% 33% 18% 3	460
	11th to 20th	Day	32% 21% 29% 13% 6%	460
	21st to end	Day	31% 23% 25% 13% 6% 3	506
April	1st to 10th	Day	35% 15% 27% 16% 5% 2	460
	11th to 20th	Day	36% 19% 24% 12% 8% 2	452
	21st to end	Day	30% 20% 30% 11% 7% 2	460
May	1st to 10th	Day	32% 19% 26% 14% 6% 2	463
	11th to 20th	Day	19% 19% 32% 21% 6% 2	487
	21st to end	Day	18% 17% 34% 20% 9% 2	529
June	1st to 10th	Day	23% 14% 38% 17% 7% 1	490
	11th to 20th	Day	19% 16% 31% 21% 11% 2	490
	21st to end	Day	22% 14% 34% 20% 8% 2	490
July	1st to 10th	Day	16% 11% 31% 26% 11% 4	485
	11th to 20th	Day	13% 10% 35% 29% 10% 4	484
	21st to end	Day	14% 8% 38% 25% 11% 3	532
August	1st to 10th	Day	12% 11% 33% 22% 18% 5%	486
	11th to 20th	Day	18% 10% 25% 25% 14% 8%	479
	21st to end	Day	13% 12% 27% 27% 14% 7%	539
September	1st to 10th	Day	15% 12% 27% 23% 14% 9%	490
	11th to 20th	Day	15% 12% 25% 20% 17% 12%	490
	21st to end	Day	19% 11% 28% 22% 13% 7%	490
October	1st to 10th	Day	23% 13% 25% 19% 14% 6%	490
	11th to 20th	Day	22% 16% 25% 23% 12% 2	455
	21st to end	Day	33% 15% 23% 20% 8% 1	429
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	23% 14% 34% 27% 3	760
	11th to 20th	Night	23% 19% 28% 24% 6%	760
	21st to end	Night	26% 16% 26% 24% 8%	836
April	1st to 10th	Night	23% 18% 29% 24% 5%	760
	11th to 20th	Night	22% 20% 28% 22% 8%	751
	21st to end	Night	28% 15% 26% 24% 7%	589
May	1st to 10th	Night	26% 13% 28% 25% 9%	570
	11th to 20th	Night	16% 13% 28% 35% 6% 1	382
	21st to end	Night	16% 9% 27% 33% 13%	411
June	1st to 10th	Night	19% 9% 27% 33% 12% 1	380
	11th to 20th	Night	20% 7% 23% 36% 14%	380
	21st to end	Night	17% 6% 24% 38% 13% 1	380
July	1st to 10th	Night	16% 3 20% 43% 16% 2	370
	11th to 20th	Night	13% 2 25% 40% 18% 2	368
	21st to end	Night	12% 5% 21% 45% 17% 1	416
August	1st to 10th	Night	14% 4 17% 40% 25%	395
	11th to 20th	Night	15% 7% 20% 35% 20% 3	630
	21st to end	Night	14% 6% 17% 37% 24% 2	836
September	1st to 10th	Night	14% 6% 25% 33% 20% 2	760
	11th to 20th	Night	15% 7% 19% 30% 24% 4	760
	21st to end	Night	15% 8% 26% 32% 18% 2	760
October	1st to 10th	Night	23% 12% 17% 30% 16% 2	760
	11th to 20th	Night	21% 12% 22% 31% 13% 1	760
	21st to end	Night	26% 14% 26% 26% 8%	836
November	1st to 10th	Night	20% 16% 25% 30% 9%	743
	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 2024-10-29 11:20:52. Version ac07

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