Sourland Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sourland Mountain Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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	17% 7% 2 6% 4 4 4 4 5% 7% 40%	126973
	Night	17% 7% 2 6% 4 4 4 3 5% 6% 43%	123415
Transparency	Day	41% 5% 11% 22% 17% 3	130487
	Night	41% 5% 11% 23% 17% 3	127975
Seeing	Day	22% 14% 29% 22% 10% 3	16328
	Night	20% 12% 31% 28% 9% 1	25528

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% 6% 3 5% 3 3 3 3 5% 5% 50%	3597
	11th to 20th	Day	14% 8% 3 9% 4 4 4 3 5% 6% 41%	3570
	21st to end	Day	18% 8% 3 7% 5% 3 3 3 4 5% 41%	4464
February	1st to 10th	Day	17% 8% 3 6% 5% 3 5% 4 5% 7% 39%	1800
	11th to 20th	Day	16% 8% 3 7% 5% 4 3 3 5% 5% 40%	1923
	21st to end	Day	20% 8% 3 6% 4 3 4 4 4 4 39%	1650
March	1st to 10th	Day	22% 6% 2 4 4 3 3 3 4 5% 44%	4476
	11th to 20th	Day	12% 7% 2 6% 5% 5% 4 3 5% 7% 43%	4904
	21st to end	Day	18% 5% 2 6% 4 4 4 3 5% 6% 43%	5848
April	1st to 10th	Day	18% 7% 3 7% 4 4 4 4 5% 6% 39%	2400
	11th to 20th	Day	16% 6% 2 6% 4 4 4 3 6% 7% 40%	2374
	21st to end	Day	17% 6% 2 6% 4 4 4 3 5% 7% 40%	2400
May	1st to 10th	Day	27% 7% 2 6% 4 4 4 3 6% 8% 29%	5854
	11th to 20th	Day	25% 9% 3 6% 4 4 4 3 4 6% 34%	6318
	21st to end	Day	20% 7% 2 6% 4 4 4 4 6% 8% 35%	6949
June	1st to 10th	Day	14% 6% 2 6% 5% 4 5% 5% 7% 8% 38%	2800
	11th to 20th	Day	15% 7% 2 7% 5% 4 5% 4 7% 7% 37%	2800
	21st to end	Day	12% 6% 2 6% 5% 5% 4 4 6% 9% 41%	2800
July	1st to 10th	Day	10% 6% 2 7% 5% 6% 5% 5% 6% 10% 37%	6360
	11th to 20th	Day	11% 7% 3 7% 5% 5% 5% 6% 8% 10% 32%	6360
	21st to end	Day	12% 6% 2 6% 5% 5% 5% 4 6% 9% 40%	6926
August	1st to 10th	Day	20% 6% 2 6% 5% 4 5% 4 6% 8% 33%	4218
	11th to 20th	Day	13% 6% 2 5% 4 4 5% 4 6% 9% 42%	3850
	21st to end	Day	12% 5% 3 5% 3 3 4 3 6% 8% 49%	4312
September	1st to 10th	Day	14% 6% 3 5% 4 4 4 3 6% 7% 45%	2400
	11th to 20th	Day	13% 4 2 5% 3 3 3 3 5% 8% 50%	2346
	21st to end	Day	19% 6% 3 7% 5% 5% 4 3 4 6% 39%	2200
October	1st to 10th	Day	18% 6% 2 5% 3 4 4 3 5% 6% 44%	2035
	11th to 20th	Day	16% 5% 2 5% 4 3 4 3 5% 6% 48%	2000
	21st to end	Day	19% 6% 2 4 3 3 4 3 5% 6% 46%	2092
November	1st to 10th	Day	13% 6% 2 5% 3 3 3 2 4 6% 52%	1851
	11th to 20th	Day	16% 6% 1 5% 3 3 3 3 6% 6% 47%	1863
	21st to end	Day	16% 6% 2 5% 4 3 4 2 6% 7% 45%	1687
December	1st to 10th	Day	15% 8% 2 5% 3 4 4 3 5% 7% 44%	2600
	11th to 20th	Day	22% 8% 1 4 3 3 3 2 4 5% 43%	2986
	21st to end	Day	17% 6% 2 6% 5% 4 4 4 5% 8% 38%	3960
January	1st to 10th	Night	13% 6% 2 5% 4 3 4 3 5% 6% 51%	6349
	11th to 20th	Night	14% 8% 2 6% 4 4 4 3 4 5% 47%	6315
	21st to end	Night	18% 8% 2 5% 4 4 3 3 4 6% 42%	6490
February	1st to 10th	Night	17% 9% 2 5% 4 3 4 4 5% 6% 41%	2600
	11th to 20th	Night	18% 8% 3 6% 4 3 3 3 4 5% 41%	2363
	21st to end	Night	18% 8% 2 6% 3 3 4 3 5% 6% 42%	1980
March	1st to 10th	Night	19% 7% 2 5% 3 3 3 3 5% 6% 44%	5364
	11th to 20th	Night	13% 9% 2 5% 3 3 3 3 4 5% 49%	5053
	21st to end	Night	17% 7% 2 7% 4 4 4 3 4 5% 42%	5082
April	1st to 10th	Night	18% 7% 3 7% 4 4 4 4 5% 6% 39%	2000
	11th to 20th	Night	17% 7% 2 7% 5% 4 4 4 4 6% 40%	1975
	21st to end	Night	19% 6% 2 4 3 3 4 3 5% 7% 45%	2000
May	1st to 10th	Night	25% 9% 3 7% 4 4 4 4 6% 8% 27%	4128
	11th to 20th	Night	23% 10% 3 7% 4 3 3 3 4 6% 34%	3575
	21st to end	Night	21% 7% 3 5% 4 4 3 4 5% 9% 35%	3926
June	1st to 10th	Night	16% 6% 3 5% 4 4 4 4 5% 7% 42%	1600
	11th to 20th	Night	13% 6% 2 6% 5% 5% 5% 4 5% 8% 41%	1600
	21st to end	Night	11% 4 2 6% 5% 5% 5% 4 7% 9% 43%	1600
July	1st to 10th	Night	9% 6% 3 8% 6% 5% 5% 5% 7% 9% 37%	3600
	11th to 20th	Night	15% 8% 3 8% 5% 5% 5% 4 7% 9% 32%	3600
	21st to end	Night	12% 6% 2 6% 4 3 5% 4 6% 8% 44%	3926
August	1st to 10th	Night	23% 6% 2 6% 4 4 4 4 6% 8% 32%	2901
	11th to 20th	Night	14% 7% 2 7% 4 4 5% 4 6% 7% 40%	3191
	21st to end	Night	11% 5% 2 5% 4 4 3 3 6% 7% 50%	3586
September	1st to 10th	Night	15% 6% 2 6% 4 3 4 4 5% 7% 45%	2000
	11th to 20th	Night	12% 6% 2 4 3 3 4 4 4 6% 52%	2052
	21st to end	Night	19% 6% 2 6% 3 4 3 3 4 8% 40%	2200
October	1st to 10th	Night	19% 6% 2 6% 3 3 4 3 5% 6% 45%	2360
	11th to 20th	Night	16% 4 2 5% 3 3 4 3 5% 6% 50%	2400
	21st to end	Night	18% 6% 2 5% 3 3 3 2 5% 5% 47%	2746
November	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 5% 6% 53%	2667
	11th to 20th	Night	18% 6% 2 5% 3 3 3 3 5% 4 49%	2692
	21st to end	Night	15% 6% 2 5% 4 4 4 3 6% 6% 45%	2710
December	1st to 10th	Night	14% 8% 2 6% 3 4 4 3 5% 5% 47%	4580
	11th to 20th	Night	23% 7% 2 5% 3 3 3 3 4 5% 42%	5208
	21st to end	Night	20% 8% 3 6% 4 3 3 3 4 5% 41%	6996

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	35% 3 9% 28% 22% 3	3598
	11th to 20th	Day	44% 4 9% 22% 16% 6%	3600
	21st to end	Day	47% 4 10% 21% 16% 1	4464
February	1st to 10th	Day	44% 5% 13% 24% 12% 2	1800
	11th to 20th	Day	46% 4 9% 22% 16% 3	1932
	21st to end	Day	46% 5% 10% 22% 13% 4	1650
March	1st to 10th	Day	42% 3 9% 23% 15% 7%	4479
	11th to 20th	Day	39% 4 12% 27% 14% 5%	4904
	21st to end	Day	43% 4 10% 15% 23% 6%	5848
April	1st to 10th	Day	45% 5% 10% 20% 16% 5%	2400
	11th to 20th	Day	40% 6% 12% 20% 18% 3	2374
	21st to end	Day	41% 5% 10% 20% 19% 5%	2400
May	1st to 10th	Day	52% 6% 13% 17% 10% 1	5865
	11th to 20th	Day	52% 5% 9% 21% 13% 1	6322
	21st to end	Day	46% 6% 10% 23% 14% 2	6949
June	1st to 10th	Day	40% 8% 13% 22% 16% 1	2800
	11th to 20th	Day	43% 8% 13% 22% 12% 2	2800
	21st to end	Day	38% 8% 14% 22% 17% 1	2800
July	1st to 10th	Day	42% 9% 13% 20% 15% 2	6290
	11th to 20th	Day	44% 8% 17% 19% 11% 1	6285
	21st to end	Day	39% 6% 12% 20% 22% 1	6728
August	1st to 10th	Day	48% 7% 12% 22% 10% 1	4204
	11th to 20th	Day	38% 6% 13% 28% 14% 1	3886
	21st to end	Day	32% 6% 10% 27% 23% 2	4312
September	1st to 10th	Day	38% 6% 11% 20% 21% 3	2400
	11th to 20th	Day	33% 4 11% 25% 24% 3	2346
	21st to end	Day	47% 5% 9% 17% 19% 3	2200
October	1st to 10th	Day	36% 5% 10% 18% 26% 5%	3308
	11th to 20th	Day	26% 3 11% 25% 29% 6%	3240
	21st to end	Day	26% 4 13% 25% 28% 5%	3356
November	1st to 10th	Day	34% 3 13% 25% 22% 3	1851
	11th to 20th	Day	37% 4 10% 24% 22% 3	1863
	21st to end	Day	37% 4 14% 30% 13% 3	1687
December	1st to 10th	Day	39% 3 15% 28% 13% 1	2600
	11th to 20th	Day	45% 3 11% 27% 12% 2	2986
	21st to end	Day	43% 5% 12% 22% 17% 2	3960
January	1st to 10th	Night	35% 3 9% 26% 22% 5%	6349
	11th to 20th	Night	39% 3 13% 22% 15% 7%	6360
	21st to end	Night	44% 3 10% 22% 18% 4	6490
February	1st to 10th	Night	44% 5% 13% 25% 12% 2	2600
	11th to 20th	Night	45% 4 8% 20% 19% 4	2365
	21st to end	Night	43% 4 7% 22% 19% 4	1980
March	1st to 10th	Night	43% 3 9% 23% 13% 9%	5374
	11th to 20th	Night	38% 4 14% 25% 14% 4	5054
	21st to end	Night	44% 4 7% 16% 24% 5%	5082
April	1st to 10th	Night	44% 5% 10% 19% 17% 4	2000
	11th to 20th	Night	44% 5% 10% 18% 20% 3	1975
	21st to end	Night	39% 6% 9% 21% 20% 5%	2000
May	1st to 10th	Night	54% 6% 11% 16% 10% 2	4134
	11th to 20th	Night	52% 6% 11% 16% 13% 2	3576
	21st to end	Night	46% 6% 13% 21% 12% 2	3927
June	1st to 10th	Night	40% 7% 11% 21% 20% 2	1600
	11th to 20th	Night	41% 7% 13% 26% 12% 1	1600
	21st to end	Night	36% 8% 15% 25% 14% 2	1600
July	1st to 10th	Night	42% 8% 16% 18% 14% 2	3560
	11th to 20th	Night	46% 7% 16% 20% 10% 1	3552
	21st to end	Night	36% 7% 14% 22% 20% 1	3809
August	1st to 10th	Night	48% 8% 17% 17% 9% 1	2891
	11th to 20th	Night	42% 7% 11% 27% 12% 1	3221
	21st to end	Night	32% 5% 10% 30% 22% 1	3586
September	1st to 10th	Night	40% 6% 10% 21% 21% 2	2000
	11th to 20th	Night	33% 5% 10% 23% 25% 4	2052
	21st to end	Night	43% 5% 8% 21% 21% 2	2200
October	1st to 10th	Night	38% 5% 10% 23% 22% 2	3854
	11th to 20th	Night	25% 4 9% 29% 29% 4	3899
	21st to end	Night	28% 4 11% 31% 24% 3	4432
November	1st to 10th	Night	33% 4 11% 25% 22% 5%	2667
	11th to 20th	Night	39% 4 8% 23% 21% 5%	2692
	21st to end	Night	39% 4 13% 26% 16% 2	2710
December	1st to 10th	Night	39% 3 12% 31% 13% 2	4580
	11th to 20th	Night	45% 4 10% 24% 14% 4	5208
	21st to end	Night	46% 4 10% 20% 17% 3	6996

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	29% 26% 26% 18% 2 1	200
	11th to 20th	Day	36% 25% 30% 8% 2	200
	21st to end	Day	24% 25% 32% 13% 4 1	330
February	1st to 10th	Day	31% 24% 33% 9% 3	300
	11th to 20th	Day	33% 20% 34% 11% 1	471
	21st to end	Day	31% 21% 29% 15% 3	414
March	1st to 10th	Day	30% 21% 29% 17% 3	500
	11th to 20th	Day	24% 26% 31% 13% 4 1	500
	21st to end	Day	28% 23% 29% 15% 5% 1	550
April	1st to 10th	Day	29% 21% 32% 11% 6% 1	500
	11th to 20th	Day	25% 22% 31% 13% 8% 2	492
	21st to end	Day	28% 19% 30% 16% 5% 2	500
May	1st to 10th	Day	30% 18% 29% 14% 7% 2	503
	11th to 20th	Day	24% 15% 32% 19% 9% 2	527
	21st to end	Day	21% 10% 25% 24% 16% 3	573
June	1st to 10th	Day	19% 10% 29% 25% 13% 3	530
	11th to 20th	Day	20% 12% 28% 27% 11% 3	530
	21st to end	Day	16% 10% 31% 29% 12% 3	530
July	1st to 10th	Day	13% 5% 23% 32% 22% 6%	525
	11th to 20th	Day	9% 6% 24% 33% 22% 6%	524
	21st to end	Day	13% 6% 32% 31% 14% 4	576
August	1st to 10th	Day	11% 6% 26% 33% 17% 7%	526
	11th to 20th	Day	14% 5% 21% 33% 21% 7%	519
	21st to end	Day	13% 6% 28% 29% 14% 9%	583
September	1st to 10th	Day	17% 9% 25% 25% 16% 8%	530
	11th to 20th	Day	16% 6% 24% 30% 15% 9%	530
	21st to end	Day	22% 12% 25% 24% 11% 5%	530
October	1st to 10th	Day	23% 7% 27% 22% 14% 8%	530
	11th to 20th	Day	20% 15% 28% 23% 11% 3	530
	21st to end	Day	21% 16% 29% 21% 10% 2	522
November	1st to 10th	Day	21% 12% 32% 23% 11% 3	425
	11th to 20th	Day	26% 16% 32% 19% 7%	421
	21st to end	Day	23% 18% 34% 17% 7% 1	287
December	1st to 10th	Day	27% 25% 31% 10% 8%	200
	11th to 20th	Day	40% 17% 26% 12% 6%	200
	21st to end	Day	28% 17% 30% 16% 7% 1	220
January	1st to 10th	Night	21% 19% 37% 20% 3	800
	11th to 20th	Night	24% 15% 35% 22% 3	800
	21st to end	Night	20% 18% 38% 22% 2	880
February	1st to 10th	Night	23% 15% 36% 24% 2	800
	11th to 20th	Night	27% 20% 36% 17% 1	783
	21st to end	Night	24% 18% 36% 20% 2	660
March	1st to 10th	Night	23% 20% 32% 22% 3	800
	11th to 20th	Night	24% 17% 35% 20% 4	800
	21st to end	Night	21% 14% 40% 20% 5%	880
April	1st to 10th	Night	22% 18% 37% 18% 5%	800
	11th to 20th	Night	21% 14% 37% 24% 5%	791
	21st to end	Night	21% 13% 35% 26% 5%	800
May	1st to 10th	Night	24% 15% 31% 24% 6%	635
	11th to 20th	Night	21% 10% 38% 25% 7%	394
	21st to end	Night	17% 7% 25% 38% 13% 1	433
June	1st to 10th	Night	17% 6% 29% 35% 14% 1	400
	11th to 20th	Night	14% 4 31% 40% 12%	400
	21st to end	Night	12% 4 24% 47% 12% 1	400
July	1st to 10th	Night	11% 3 25% 41% 18% 2	390
	11th to 20th	Night	12% 3 22% 42% 19% 1	388
	21st to end	Night	9% 5% 26% 43% 18%	438
August	1st to 10th	Night	14% 2 16% 46% 22% 2	583
	11th to 20th	Night	14% 3 19% 36% 26% 3	784
	21st to end	Night	15% 4 19% 39% 22% 1	880
September	1st to 10th	Night	16% 4 26% 34% 18% 3	800
	11th to 20th	Night	14% 4 23% 35% 21% 4	800
	21st to end	Night	22% 5% 22% 33% 17% 1	800
October	1st to 10th	Night	21% 7% 25% 26% 18% 3	800
	11th to 20th	Night	17% 11% 28% 32% 12%	800
	21st to end	Night	21% 12% 32% 26% 8%	880
November	1st to 10th	Night	16% 9% 34% 30% 11% 1	820
	11th to 20th	Night	21% 17% 29% 26% 7%	829
	21st to end	Night	20% 14% 34% 26% 7%	800
December	1st to 10th	Night	26% 15% 31% 22% 6% 1	800
	11th to 20th	Night	22% 14% 35% 25% 5%	800
	21st to end	Night	23% 15% 32% 28% 3	880

Page generated at 2025-11-15 00:55:57. Version ac07

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