Mount Cuba Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mount Cuba Astronomical Observatory clear sky chart from 2005-11-06 to 2026-03-18.

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% 6% 2 6% 4 4 5% 4 6% 8% 41%	131265
	Night	16% 7% 2 6% 4 4 4 3 5% 7% 43%	130223
Transparency	Day	25% 39% 8% 15% 11% 2	134823
	Night	26% 37% 7% 15% 12% 3	134751
Seeing	Day	20% 16% 28% 22% 10% 3	16677
	Night	19% 14% 30% 26% 10% 1	26115

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	13% 5% 2 6% 5% 5% 5% 4 5% 5% 46%	4661
	11th to 20th	Day	15% 7% 2 7% 4 4 3 3 4 5% 46%	4906
	21st to end	Day	16% 6% 2 5% 4 5% 5% 4 6% 7% 42%	5780
February	1st to 10th	Day	14% 7% 3 6% 4 4 5% 3 6% 7% 39%	1890
	11th to 20th	Day	17% 8% 3 6% 4 5% 4 3 5% 6% 39%	1981
	21st to end	Day	17% 11% 3 6% 4 3 3 2 5% 6% 39%	1730
March	1st to 10th	Day	20% 5% 2 4 3 4 4 3 4 6% 46%	4659
	11th to 20th	Day	13% 7% 2 5% 4 3 5% 3 5% 7% 46%	5057
	21st to end	Day	18% 6% 2 5% 4 3 4 3 5% 6% 43%	5740
April	1st to 10th	Day	16% 7% 2 6% 5% 4 4 4 5% 6% 41%	2400
	11th to 20th	Day	14% 6% 2 5% 5% 4 4 4 5% 7% 43%	2374
	21st to end	Day	17% 5% 2 6% 4 4 5% 4 6% 7% 40%	2400
May	1st to 10th	Day	26% 7% 2 5% 3 4 4 4 6% 8% 31%	5781
	11th to 20th	Day	25% 7% 2 6% 4 4 4 3 6% 7% 31%	6318
	21st to end	Day	18% 6% 2 6% 4 4 5% 4 6% 9% 37%	6949
June	1st to 10th	Day	11% 6% 3 5% 4 5% 5% 5% 7% 9% 40%	2800
	11th to 20th	Day	13% 7% 2 6% 5% 4 5% 4 7% 9% 37%	2800
	21st to end	Day	9% 5% 2 6% 4 4 5% 4 6% 10% 43%	2800
July	1st to 10th	Day	9% 5% 2 6% 5% 5% 6% 5% 7% 11% 39%	6360
	11th to 20th	Day	11% 7% 3 7% 5% 6% 6% 5% 8% 11% 31%	6360
	21st to end	Day	10% 6% 2 6% 5% 5% 5% 5% 7% 9% 39%	6703
August	1st to 10th	Day	18% 6% 2 7% 5% 5% 5% 5% 7% 8% 33%	4234
	11th to 20th	Day	12% 5% 2 6% 4 4 5% 4 7% 9% 41%	3881
	21st to end	Day	10% 5% 2 5% 4 4 5% 4 6% 8% 47%	4312
September	1st to 10th	Day	15% 5% 3 5% 4 3 5% 4 5% 7% 45%	2400
	11th to 20th	Day	12% 5% 1 4 4 4 4 3 5% 8% 49%	2385
	21st to end	Day	18% 8% 3 7% 4 4 4 4 5% 6% 37%	2171
October	1st to 10th	Day	16% 6% 2 5% 4 4 4 3 5% 6% 45%	2000
	11th to 20th	Day	16% 5% 2 5% 3 3 3 3 5% 7% 48%	2000
	21st to end	Day	17% 5% 2 5% 3 4 4 3 5% 6% 47%	2160
November	1st to 10th	Day	12% 6% 2 5% 2 3 3 3 4 7% 53%	1851
	11th to 20th	Day	15% 5% 2 4 3 3 4 4 6% 7% 47%	1890
	21st to end	Day	15% 7% 2 5% 4 3 4 4 5% 5% 46%	1738
December	1st to 10th	Day	14% 7% 3 5% 4 4 4 4 5% 6% 46%	2680
	11th to 20th	Day	20% 9% 2 4 3 3 3 3 5% 6% 43%	3066
	21st to end	Day	16% 6% 2 6% 4 4 5% 4 6% 7% 40%	4048
January	1st to 10th	Night	13% 7% 2 6% 4 4 4 3 5% 6% 45%	8234
	11th to 20th	Night	16% 8% 2 5% 3 2 3 2 4 6% 49%	7963
	21st to end	Night	15% 7% 2 5% 4 4 3 3 4 6% 46%	8401
February	1st to 10th	Night	16% 9% 2 6% 4 4 4 3 5% 6% 41%	2730
	11th to 20th	Night	19% 7% 2 5% 4 4 4 4 4 5% 41%	2523
	21st to end	Night	17% 7% 3 6% 4 4 4 4 5% 6% 41%	2076
March	1st to 10th	Night	18% 7% 2 5% 4 3 3 2 4 6% 45%	5395
	11th to 20th	Night	11% 8% 3 5% 4 4 3 3 4 5% 51%	5057
	21st to end	Night	15% 7% 3 6% 4 3 4 3 5% 6% 44%	5192
April	1st to 10th	Night	16% 8% 3 6% 5% 4 3 3 5% 7% 40%	2000
	11th to 20th	Night	15% 6% 2 6% 5% 4 4 3 5% 8% 42%	1975
	21st to end	Night	18% 5% 2 5% 4 4 4 3 5% 7% 44%	2000
May	1st to 10th	Night	24% 8% 3 8% 4 4 4 4 6% 7% 28%	4187
	11th to 20th	Night	23% 8% 3 7% 4 4 3 4 6% 8% 31%	3575
	21st to end	Night	21% 7% 2 6% 3 3 4 3 6% 8% 38%	3926
June	1st to 10th	Night	15% 5% 2 6% 5% 4 3 4 5% 8% 43%	1600
	11th to 20th	Night	11% 7% 2 6% 6% 4 5% 4 6% 9% 39%	1600
	21st to end	Night	10% 5% 2 6% 5% 4 4 5% 8% 11% 41%	1600
July	1st to 10th	Night	8% 6% 3 8% 6% 5% 6% 4 7% 9% 37%	3600
	11th to 20th	Night	14% 9% 3 8% 5% 5% 5% 5% 8% 10% 30%	3600
	21st to end	Night	12% 6% 2 6% 4 4 4 4 6% 9% 43%	4145
August	1st to 10th	Night	20% 6% 2 6% 5% 4 5% 4 5% 9% 34%	2915
	11th to 20th	Night	14% 6% 2 7% 5% 4 4 3 6% 7% 42%	3134
	21st to end	Night	11% 5% 2 5% 4 4 5% 3 5% 7% 50%	3586
September	1st to 10th	Night	15% 6% 2 6% 4 3 4 3 4 7% 46%	2000
	11th to 20th	Night	12% 6% 2 4 3 2 3 3 5% 7% 52%	2013
	21st to end	Night	20% 7% 2 6% 4 4 4 3 6% 7% 38%	2225
October	1st to 10th	Night	20% 6% 2 6% 4 4 3 3 4 6% 44%	2400
	11th to 20th	Night	17% 4 2 4 3 3 3 3 4 7% 50%	2400
	21st to end	Night	16% 5% 2 5% 3 3 3 3 4 6% 50%	2678
November	1st to 10th	Night	10% 6% 2 5% 3 3 3 4 5% 5% 53%	2667
	11th to 20th	Night	16% 7% 3 5% 3 3 3 3 4 5% 48%	2730
	21st to end	Night	13% 5% 2 5% 4 4 5% 4 6% 6% 46%	2878
December	1st to 10th	Night	13% 7% 2 6% 4 3 4 3 5% 7% 47%	4720
	11th to 20th	Night	22% 6% 2 5% 3 3 4 3 5% 7% 41%	5348
	21st to end	Night	18% 7% 2 6% 3 4 4 4 6% 6% 40%	7150

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	29% 24% 7% 20% 17% 2	4662
	11th to 20th	Day	32% 22% 4 19% 17% 6%	4938
	21st to end	Day	30% 25% 10% 23% 11% 1	5780
February	1st to 10th	Day	18% 60% 7% 8% 5% 1	1890
	11th to 20th	Day	20% 58% 3 9% 8% 1	1990
	21st to end	Day	19% 59% 4 11% 5% 2	1730
March	1st to 10th	Day	29% 29% 6% 16% 14% 6%	4662
	11th to 20th	Day	24% 31% 7% 22% 12% 4	5057
	21st to end	Day	28% 30% 7% 12% 18% 5%	5740
April	1st to 10th	Day	19% 63% 4 7% 6% 2	2400
	11th to 20th	Day	16% 63% 5% 9% 6% 1	2374
	21st to end	Day	16% 63% 4 8% 8% 1	2400
May	1st to 10th	Day	38% 32% 11% 13% 5% 1	5790
	11th to 20th	Day	38% 32% 7% 16% 8%	6322
	21st to end	Day	31% 32% 9% 16% 11% 1	6949
June	1st to 10th	Day	16% 63% 3 9% 7% 1	2800
	11th to 20th	Day	16% 63% 3 10% 7% 1	2800
	21st to end	Day	13% 62% 5% 10% 9% 1	2800
July	1st to 10th	Day	26% 33% 14% 13% 12% 2	6290
	11th to 20th	Day	34% 32% 15% 14% 5%	6285
	21st to end	Day	28% 29% 7% 18% 18% 1	6510
August	1st to 10th	Day	31% 39% 10% 15% 5%	4221
	11th to 20th	Day	22% 39% 9% 22% 7%	3918
	21st to end	Day	17% 38% 8% 21% 15% 1	4312
September	1st to 10th	Day	15% 59% 6% 10% 9% 1	2400
	11th to 20th	Day	15% 58% 4 12% 8% 3	2385
	21st to end	Day	24% 58% 4 9% 6%	2171
October	1st to 10th	Day	20% 37% 5% 11% 22% 4	3260
	11th to 20th	Day	11% 37% 8% 15% 22% 7%	3240
	21st to end	Day	9% 35% 8% 20% 23% 4	3474
November	1st to 10th	Day	16% 53% 8% 11% 8% 3	1851
	11th to 20th	Day	15% 55% 4 12% 13% 2	1890
	21st to end	Day	17% 55% 6% 16% 6% 1	1738
December	1st to 10th	Day	26% 36% 11% 16% 8% 3	2680
	11th to 20th	Day	31% 32% 7% 22% 7% 2	3066
	21st to end	Day	28% 30% 9% 17% 14% 2	4048
January	1st to 10th	Night	31% 22% 10% 19% 14% 4	8234
	11th to 20th	Night	29% 21% 6% 19% 17% 8%	8006
	21st to end	Night	32% 23% 6% 20% 16% 3	8401
February	1st to 10th	Night	18% 60% 6% 10% 4 2	2730
	11th to 20th	Night	19% 58% 3 8% 10% 2	2525
	21st to end	Night	17% 59% 5% 9% 6% 3	2076
March	1st to 10th	Night	31% 29% 6% 17% 7% 9%	5405
	11th to 20th	Night	21% 31% 11% 21% 12% 4	5058
	21st to end	Night	30% 31% 5% 11% 20% 3	5192
April	1st to 10th	Night	19% 62% 2 8% 8% 1	2000
	11th to 20th	Night	15% 63% 5% 8% 7% 1	1975
	21st to end	Night	16% 62% 5% 7% 9% 1	2000
May	1st to 10th	Night	40% 33% 8% 11% 6% 2	4194
	11th to 20th	Night	39% 32% 8% 14% 6%	3576
	21st to end	Night	34% 31% 8% 15% 10% 2	3927
June	1st to 10th	Night	16% 63% 3 9% 10%	1600
	11th to 20th	Night	16% 62% 4 11% 6%	1600
	21st to end	Night	14% 62% 6% 10% 7% 2	1600
July	1st to 10th	Night	31% 31% 13% 14% 9% 1	3560
	11th to 20th	Night	36% 30% 16% 13% 4	3552
	21st to end	Night	27% 29% 9% 18% 16%	4023
August	1st to 10th	Night	34% 40% 12% 10% 4	2904
	11th to 20th	Night	24% 38% 8% 20% 9%	3163
	21st to end	Night	18% 37% 6% 23% 15% 1	3586
September	1st to 10th	Night	16% 58% 7% 10% 8% 1	2000
	11th to 20th	Night	15% 57% 4 11% 10% 2	2013
	21st to end	Night	23% 58% 4 8% 6%	2225
October	1st to 10th	Night	26% 38% 7% 12% 15% 3	3920
	11th to 20th	Night	11% 36% 7% 17% 24% 4	3899
	21st to end	Night	11% 36% 8% 23% 18% 5%	4314
November	1st to 10th	Night	16% 54% 6% 12% 8% 4	2667
	11th to 20th	Night	17% 54% 3 10% 13% 3	2730
	21st to end	Night	15% 55% 6% 15% 9% 1	2878
December	1st to 10th	Night	24% 35% 9% 22% 9% 2	4720
	11th to 20th	Night	30% 32% 8% 15% 11% 4	5348
	21st to end	Night	32% 30% 9% 13% 15% 2	7150

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	26% 26% 33% 12% 2	220
	11th to 20th	Day	30% 33% 27% 9% 1	267
	21st to end	Day	23% 34% 28% 12% 4	363
February	1st to 10th	Day	25% 30% 32% 9% 3 1	330
	11th to 20th	Day	33% 25% 32% 9% 1	469
	21st to end	Day	27% 26% 31% 13% 2	446
March	1st to 10th	Day	26% 26% 30% 14% 4	540
	11th to 20th	Day	28% 24% 30% 13% 4 1	528
	21st to end	Day	26% 26% 31% 13% 4 1	550
April	1st to 10th	Day	26% 26% 33% 10% 4 1	500
	11th to 20th	Day	25% 21% 29% 17% 5% 2	492
	21st to end	Day	23% 21% 35% 15% 5% 1	500
May	1st to 10th	Day	30% 19% 28% 15% 7% 1	503
	11th to 20th	Day	22% 16% 32% 22% 8% 1	527
	21st to end	Day	20% 11% 25% 26% 14% 3	573
June	1st to 10th	Day	14% 14% 31% 24% 15% 3	530
	11th to 20th	Day	18% 10% 31% 26% 13% 3	530
	21st to end	Day	12% 8% 31% 31% 13% 4	530
July	1st to 10th	Day	13% 6% 22% 34% 19% 6%	525
	11th to 20th	Day	6% 3 25% 35% 25% 5%	524
	21st to end	Day	11% 6% 26% 35% 18% 4	576
August	1st to 10th	Day	12% 5% 24% 33% 20% 6%	526
	11th to 20th	Day	13% 4 20% 36% 18% 8%	519
	21st to end	Day	11% 7% 26% 30% 17% 9%	583
September	1st to 10th	Day	17% 11% 20% 25% 18% 8%	530
	11th to 20th	Day	14% 5% 25% 29% 18% 8%	530
	21st to end	Day	25% 12% 22% 28% 10% 3	530
October	1st to 10th	Day	20% 9% 27% 24% 14% 7%	530
	11th to 20th	Day	18% 16% 29% 22% 12% 2	530
	21st to end	Day	20% 16% 32% 20% 9% 2	561
November	1st to 10th	Day	16% 19% 32% 21% 10% 2	425
	11th to 20th	Day	20% 23% 31% 21% 5%	430
	21st to end	Day	23% 22% 31% 21% 4	278
December	1st to 10th	Day	27% 22% 35% 12% 4	220
	11th to 20th	Day	32% 23% 31% 10% 4	220
	21st to end	Day	26% 25% 27% 16% 6%	242
January	1st to 10th	Night	20% 23% 37% 17% 3	840
	11th to 20th	Night	26% 20% 31% 20% 3	840
	21st to end	Night	18% 23% 39% 19% 1	924
February	1st to 10th	Night	20% 21% 36% 21% 1	840
	11th to 20th	Night	27% 25% 32% 14% 1	823
	21st to end	Night	23% 21% 37% 17% 2	692
March	1st to 10th	Night	21% 22% 34% 20% 4	840
	11th to 20th	Night	22% 20% 35% 17% 5% 1	829
	21st to end	Night	20% 17% 40% 18% 5%	880
April	1st to 10th	Night	21% 22% 35% 18% 4	800
	11th to 20th	Night	19% 16% 34% 26% 5%	791
	21st to end	Night	22% 15% 33% 24% 7%	800
May	1st to 10th	Night	21% 13% 34% 26% 5%	669
	11th to 20th	Night	20% 12% 32% 29% 6%	394
	21st to end	Night	18% 7% 22% 37% 15% 1	433
June	1st to 10th	Night	17% 5% 27% 38% 15%	400
	11th to 20th	Night	13% 6% 28% 42% 11%	400
	21st to end	Night	10% 4 26% 45% 15%	400
July	1st to 10th	Night	12% 5% 22% 39% 21% 2	390
	11th to 20th	Night	8% 4 17% 43% 27% 1	388
	21st to end	Night	11% 4 19% 42% 22% 2	530
August	1st to 10th	Night	12% 2 18% 41% 25% 2	597
	11th to 20th	Night	15% 4 17% 37% 25% 2	751
	21st to end	Night	12% 4 21% 38% 22% 3	880
September	1st to 10th	Night	17% 5% 24% 34% 18% 2	800
	11th to 20th	Night	16% 4 18% 37% 22% 4	800
	21st to end	Night	23% 6% 24% 29% 17% 1	800
October	1st to 10th	Night	21% 8% 25% 26% 17% 4	800
	11th to 20th	Night	20% 11% 27% 30% 11% 1	800
	21st to end	Night	19% 13% 32% 25% 10% 1	880
November	1st to 10th	Night	15% 13% 35% 27% 10% 1	820
	11th to 20th	Night	22% 17% 30% 23% 7% 1	840
	21st to end	Night	17% 20% 31% 25% 7%	840
December	1st to 10th	Night	23% 19% 35% 17% 6% 1	840
	11th to 20th	Night	20% 19% 36% 21% 4	840
	21st to end	Night	22% 19% 32% 25% 2	924

Page generated at 2026-03-15 02:23:50. Version ac07

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