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 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	16% 6% 3 5% 5% 3 5% 5% 5% 7% 40%	74744
	Night	16% 6% 3 5% 5% 3 4 4 5% 7% 43%	74970
Transparency	Day	42% 6% 12% 22% 16% 3	74449
	Night	41% 6% 11% 22% 17% 3	74799
Seeing	Day	21% 14% 29% 22% 11% 4	13856
	Night	19% 11% 31% 28% 10% 1	23192

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 6% 5% 3 5% 4 6% 6% 42%	1517
	11th to 20th	Day	16% 7% 3 6% 6% 4 5% 5% 5% 7% 37%	1454
	21st to end	Day	15% 7% 3 5% 6% 3 5% 5% 4 7% 41%	1780
February	1st to 10th	Day	18% 7% 4 6% 5% 2 4 5% 5% 7% 38%	1620
	11th to 20th	Day	16% 7% 3 7% 6% 3 4 3 5% 5% 40%	1726
	21st to end	Day	21% 8% 4 5% 5% 3 4 4 4 5% 38%	1480
March	1st to 10th	Day	15% 7% 2 5% 5% 3 4 4 4 7% 44%	1797
	11th to 20th	Day	16% 7% 2 6% 7% 3 4 4 4 7% 39%	1946
	21st to end	Day	21% 6% 3 5% 4 3 5% 4 4 5% 40%	2324
April	1st to 10th	Day	17% 7% 3 6% 5% 3 4 4 5% 6% 41%	2160
	11th to 20th	Day	16% 5% 2 5% 5% 3 4 4 6% 8% 41%	2134
	21st to end	Day	18% 5% 3 5% 5% 3 5% 4 5% 7% 39%	2160
May	1st to 10th	Day	21% 7% 3 5% 5% 3 4 4 5% 7% 35%	2322
	11th to 20th	Day	21% 6% 2 4 5% 3 5% 6% 4 7% 37%	2478
	21st to end	Day	15% 7% 2 5% 6% 3 5% 4 5% 8% 40%	2725
June	1st to 10th	Day	14% 6% 2 5% 5% 3 5% 6% 7% 9% 38%	2520
	11th to 20th	Day	15% 7% 2 6% 6% 3 5% 6% 6% 8% 36%	2520
	21st to end	Day	13% 5% 2 5% 6% 4 4 6% 6% 9% 41%	2520
July	1st to 10th	Day	12% 5% 2 5% 7% 4 6% 6% 6% 9% 37%	2520
	11th to 20th	Day	9% 5% 2 4 6% 3 6% 6% 7% 9% 42%	2520
	21st to end	Day	12% 6% 3 6% 6% 4 6% 5% 7% 8% 38%	2744
August	1st to 10th	Day	10% 6% 2 6% 6% 4 6% 7% 7% 8% 39%	2325
	11th to 20th	Day	12% 5% 2 5% 6% 4 6% 5% 7% 9% 39%	2126
	21st to end	Day	15% 6% 3 5% 5% 3 5% 5% 7% 8% 40%	2376
September	1st to 10th	Day	14% 6% 3 5% 4 3 4 4 6% 8% 43%	2160
	11th to 20th	Day	14% 4 2 4 4 3 3 4 4 8% 50%	2112
	21st to end	Day	18% 5% 2 5% 6% 3 5% 4 4 6% 41%	1980
October	1st to 10th	Day	20% 6% 2 4 4 3 4 4 5% 6% 41%	1832
	11th to 20th	Day	16% 5% 2 4 5% 3 4 5% 5% 6% 46%	1800
	21st to end	Day	20% 6% 2 4 4 2 4 4 4 7% 43%	1884
November	1st to 10th	Day	14% 6% 2 4 4 2 4 2 3 6% 52%	1671
	11th to 20th	Day	17% 5% 2 4 4 2 3 4 5% 5% 47%	1710
	21st to end	Day	15% 5% 2 4 4 3 4 3 6% 8% 45%	1603
December	1st to 10th	Day	18% 8% 3 5% 4 3 4 4 4 7% 39%	1520
	11th to 20th	Day	20% 6% 2 4 4 2 4 4 4 6% 42%	1906
	21st to end	Day	18% 7% 3 6% 6% 4 5% 6% 5% 7% 33%	2772
January	1st to 10th	Night	15% 6% 3 4 4 2 4 4 4 7% 46%	2649
	11th to 20th	Night	18% 7% 2 6% 5% 3 4 4 5% 7% 40%	2546
	21st to end	Night	14% 8% 3 5% 4 3 4 4 5% 7% 45%	2574
February	1st to 10th	Night	18% 8% 3 5% 5% 2 3 5% 5% 6% 41%	2340
	11th to 20th	Night	18% 7% 4 5% 5% 3 4 4 4 5% 41%	2120
	21st to end	Night	19% 7% 2 5% 4 2 4 3 4 6% 43%	1776
March	1st to 10th	Night	16% 5% 2 4 4 3 4 4 6% 7% 46%	2150
	11th to 20th	Night	18% 7% 2 5% 5% 3 4 4 4 6% 43%	2011
	21st to end	Night	18% 6% 3 5% 5% 3 4 5% 4 5% 43%	2030
April	1st to 10th	Night	17% 6% 3 6% 4 3 4 5% 6% 7% 40%	1800
	11th to 20th	Night	16% 7% 2 6% 6% 3 4 4 4 7% 41%	1775
	21st to end	Night	21% 5% 2 4 4 2 4 4 5% 6% 44%	1800
May	1st to 10th	Night	21% 9% 3 6% 5% 3 4 4 4 7% 35%	1652
	11th to 20th	Night	19% 7% 3 6% 5% 3 4 4 5% 7% 39%	1415
	21st to end	Night	15% 6% 2 5% 5% 3 4 5% 6% 8% 41%	1550
June	1st to 10th	Night	16% 5% 4 4 4 3 4 5% 5% 6% 43%	1440
	11th to 20th	Night	13% 4 3 5% 5% 3 5% 5% 5% 8% 41%	1440
	21st to end	Night	11% 4 2 4 5% 4 4 6% 6% 9% 45%	1440
July	1st to 10th	Night	11% 5% 2 5% 7% 3 5% 6% 7% 9% 39%	1440
	11th to 20th	Night	10% 4 1 5% 7% 4 5% 5% 7% 10% 41%	1440
	21st to end	Night	10% 4 2 5% 4 3 5% 7% 6% 10% 43%	1574
August	1st to 10th	Night	10% 5% 3 5% 5% 3 6% 7% 6% 9% 41%	1597
	11th to 20th	Night	12% 6% 2 7% 5% 4 6% 5% 6% 8% 38%	1768
	21st to end	Night	14% 5% 2 5% 5% 3 4 4 7% 9% 41%	1980
September	1st to 10th	Night	15% 6% 2 5% 5% 2 4 5% 5% 7% 43%	1800
	11th to 20th	Night	13% 5% 2 3 4 2 4 4 3 6% 53%	1846
	21st to end	Night	17% 6% 2 5% 4 3 4 4 4 7% 43%	1980
October	1st to 10th	Night	20% 6% 2 5% 4 2 4 4 4 6% 43%	2124
	11th to 20th	Night	16% 4 2 4 4 3 4 4 5% 6% 49%	2160
	21st to end	Night	20% 7% 2 4 4 2 4 3 4 5% 45%	2470
November	1st to 10th	Night	12% 5% 2 5% 4 2 3 3 4 6% 53%	2407
	11th to 20th	Night	19% 6% 2 5% 3 2 3 4 5% 4 48%	2470
	21st to end	Night	15% 6% 3 4 4 3 4 4 6% 7% 46%	2574
December	1st to 10th	Night	21% 7% 2 4 5% 3 4 4 5% 6% 40%	2660
	11th to 20th	Night	18% 5% 2 4 5% 3 4 4 5% 6% 42%	3288
	21st to end	Night	19% 8% 3 6% 5% 3 4 4 4 5% 38%	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	42% 4 12% 24% 14% 4	1518
	11th to 20th	Day	45% 7% 12% 21% 12% 3	1454
	21st to end	Day	43% 5% 10% 24% 15% 3	1780
February	1st to 10th	Day	46% 5% 13% 23% 11% 2	1620
	11th to 20th	Day	46% 4 9% 23% 15% 3	1735
	21st to end	Day	48% 5% 9% 21% 13% 4	1480
March	1st to 10th	Day	41% 4 12% 25% 14% 3	1800
	11th to 20th	Day	45% 4 10% 21% 16% 4	1946
	21st to end	Day	47% 4 8% 19% 17% 5%	2324
April	1st to 10th	Day	44% 5% 10% 20% 16% 5%	2160
	11th to 20th	Day	39% 6% 12% 22% 18% 4	2134
	21st to end	Day	43% 6% 10% 19% 18% 5%	2160
May	1st to 10th	Day	48% 6% 9% 16% 17% 3	2322
	11th to 20th	Day	46% 6% 11% 21% 15% 2	2482
	21st to end	Day	42% 7% 14% 21% 14% 2	2725
June	1st to 10th	Day	40% 9% 13% 22% 15% 1	2520
	11th to 20th	Day	44% 8% 13% 22% 12% 2	2520
	21st to end	Day	38% 8% 14% 23% 16% 1	2520
July	1st to 10th	Day	41% 8% 12% 21% 15% 2	2450
	11th to 20th	Day	35% 8% 13% 25% 17% 2	2445
	21st to end	Day	41% 7% 12% 20% 18% 2	2591
August	1st to 10th	Day	39% 8% 13% 22% 16% 1	2311
	11th to 20th	Day	40% 8% 15% 21% 15% 2	2126
	21st to end	Day	39% 8% 11% 20% 18% 4	2376
September	1st to 10th	Day	40% 6% 12% 20% 20% 3	2160
	11th to 20th	Day	34% 4 11% 24% 24% 4	2112
	21st to end	Day	44% 5% 9% 18% 21% 3	1980
October	1st to 10th	Day	42% 7% 11% 22% 18% 2	1832
	11th to 20th	Day	38% 5% 10% 24% 21% 2	1800
	21st to end	Day	42% 5% 9% 23% 18% 2	1884
November	1st to 10th	Day	35% 3 13% 25% 21% 3	1671
	11th to 20th	Day	38% 4 10% 23% 22% 3	1710
	21st to end	Day	37% 4 13% 30% 13% 3	1603
December	1st to 10th	Day	44% 5% 15% 22% 14% 1	1520
	11th to 20th	Day	44% 4 13% 25% 12% 2	1906
	21st to end	Day	47% 6% 13% 23% 11% 1	2772
January	1st to 10th	Night	40% 5% 10% 25% 16% 4	2649
	11th to 20th	Night	44% 6% 14% 23% 11% 2	2546
	21st to end	Night	39% 5% 10% 25% 18% 3	2574
February	1st to 10th	Night	44% 5% 12% 24% 12% 2	2340
	11th to 20th	Night	45% 4 9% 19% 18% 4	2122
	21st to end	Night	43% 5% 7% 23% 19% 4	1776
March	1st to 10th	Night	38% 4 12% 24% 17% 4	2160
	11th to 20th	Night	44% 5% 9% 21% 18% 4	2012
	21st to end	Night	44% 4 10% 17% 19% 6%	2030
April	1st to 10th	Night	42% 6% 11% 20% 17% 4	1800
	11th to 20th	Night	43% 5% 10% 19% 20% 3	1775
	21st to end	Night	40% 6% 10% 20% 19% 5%	1800
May	1st to 10th	Night	49% 5% 10% 16% 17% 2	1652
	11th to 20th	Night	46% 6% 12% 16% 18% 3	1416
	21st to end	Night	42% 8% 15% 19% 13% 2	1551
June	1st to 10th	Night	40% 8% 11% 21% 19% 1	1440
	11th to 20th	Night	41% 7% 13% 25% 13% 1	1440
	21st to end	Night	34% 8% 15% 26% 14% 2	1440
July	1st to 10th	Night	39% 11% 15% 19% 15% 2	1400
	11th to 20th	Night	36% 8% 13% 26% 14% 2	1392
	21st to end	Night	33% 10% 15% 23% 16% 3	1486
August	1st to 10th	Night	36% 10% 16% 22% 14% 2	1587
	11th to 20th	Night	42% 9% 13% 22% 13% 2	1768
	21st to end	Night	40% 7% 13% 19% 20% 2	1980
September	1st to 10th	Night	41% 6% 11% 20% 19% 2	1800
	11th to 20th	Night	33% 5% 10% 22% 26% 4	1846
	21st to end	Night	41% 5% 7% 23% 23% 2	1980
October	1st to 10th	Night	42% 5% 11% 23% 16% 2	2124
	11th to 20th	Night	35% 5% 11% 29% 17% 2	2160
	21st to end	Night	42% 4 9% 25% 18% 2	2470
November	1st to 10th	Night	34% 4 11% 25% 22% 4	2407
	11th to 20th	Night	40% 4 9% 23% 20% 5%	2470
	21st to end	Night	38% 4 13% 27% 16% 2	2574
December	1st to 10th	Night	45% 4 12% 23% 14% 2	2660
	11th to 20th	Night	42% 6% 13% 20% 17% 4	3288
	21st to end	Night	47% 5% 12% 25% 11% 1	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	26% 24% 27% 19% 2 1	180
	11th to 20th	Day	32% 28% 27% 10% 2	164
	21st to end	Day	23% 25% 34% 13% 4	264
February	1st to 10th	Day	33% 25% 30% 8% 3	240
	11th to 20th	Day	29% 21% 36% 12% 2	394
	21st to end	Day	31% 21% 30% 15% 3	346
March	1st to 10th	Day	27% 21% 30% 19% 3	420
	11th to 20th	Day	24% 25% 33% 13% 4 1	420
	21st to end	Day	27% 22% 29% 15% 5% 1	462
April	1st to 10th	Day	26% 21% 32% 12% 6% 1	420
	11th to 20th	Day	22% 20% 31% 15% 10% 2	412
	21st to end	Day	29% 20% 30% 14% 4 2	420
May	1st to 10th	Day	30% 18% 29% 14% 7% 2	423
	11th to 20th	Day	23% 15% 33% 19% 9% 2	447
	21st to end	Day	20% 9% 23% 26% 18% 3	485
June	1st to 10th	Day	17% 10% 30% 26% 14% 3	450
	11th to 20th	Day	19% 12% 28% 26% 11% 3	450
	21st to end	Day	16% 10% 30% 29% 12% 3	450
July	1st to 10th	Day	13% 5% 23% 30% 23% 7%	445
	11th to 20th	Day	9% 5% 23% 34% 23% 7%	444
	21st to end	Day	14% 6% 31% 30% 16% 4	488
August	1st to 10th	Day	10% 5% 25% 35% 18% 8%	446
	11th to 20th	Day	14% 3 21% 34% 21% 7%	439
	21st to end	Day	14% 5% 27% 29% 14% 11%	495
September	1st to 10th	Day	18% 7% 24% 25% 17% 9%	450
	11th to 20th	Day	17% 6% 26% 27% 15% 10%	450
	21st to end	Day	21% 13% 24% 24% 12% 6%	450
October	1st to 10th	Day	25% 7% 27% 21% 14% 7%	450
	11th to 20th	Day	19% 13% 29% 25% 12% 3	450
	21st to end	Day	22% 16% 29% 20% 9% 3	446
November	1st to 10th	Day	21% 10% 30% 24% 12% 3	365
	11th to 20th	Day	25% 15% 31% 20% 7% 1	370
	21st to end	Day	20% 17% 37% 19% 7% 1	263
December	1st to 10th	Day	26% 27% 29% 10% 8%	180
	11th to 20th	Day	39% 16% 27% 12% 6%	180
	21st to end	Day	30% 17% 29% 16% 7% 1	198
January	1st to 10th	Night	20% 18% 38% 21% 3	760
	11th to 20th	Night	23% 13% 37% 23% 4	728
	21st to end	Night	19% 18% 38% 23% 2	792
February	1st to 10th	Night	23% 15% 36% 24% 2	720
	11th to 20th	Night	25% 20% 36% 18% 1	703
	21st to end	Night	25% 18% 36% 19% 2	592
March	1st to 10th	Night	22% 18% 34% 23% 3	720
	11th to 20th	Night	24% 17% 35% 19% 4	720
	21st to end	Night	21% 13% 40% 21% 5%	792
April	1st to 10th	Night	21% 18% 38% 18% 5%	720
	11th to 20th	Night	19% 13% 38% 25% 6%	711
	21st to end	Night	22% 12% 35% 26% 5%	720
May	1st to 10th	Night	24% 15% 30% 24% 6%	572
	11th to 20th	Night	20% 8% 39% 26% 7%	354
	21st to end	Night	16% 6% 24% 40% 14% 1	389
June	1st to 10th	Night	17% 5% 28% 35% 14% 1	360
	11th to 20th	Night	13% 4 30% 41% 13%	360
	21st to end	Night	11% 3 25% 48% 13% 1	360
July	1st to 10th	Night	11% 3 25% 41% 18% 2	350
	11th to 20th	Night	10% 3 22% 43% 20% 1	348
	21st to end	Night	9% 4 25% 44% 17% 1	394
August	1st to 10th	Night	12% 2 16% 47% 21% 2	523
	11th to 20th	Night	14% 2 19% 36% 27% 3	704
	21st to end	Night	15% 4 17% 40% 22% 2	792
September	1st to 10th	Night	17% 3 25% 35% 18% 3	720
	11th to 20th	Night	15% 4 23% 34% 20% 4	720
	21st to end	Night	21% 5% 22% 33% 18% 1	720
October	1st to 10th	Night	22% 7% 25% 25% 18% 3	720
	11th to 20th	Night	17% 10% 28% 33% 13%	720
	21st to end	Night	22% 11% 32% 26% 8% 1	792
November	1st to 10th	Night	15% 9% 34% 30% 11% 1	740
	11th to 20th	Night	20% 15% 29% 27% 8%	760
	21st to end	Night	18% 13% 34% 27% 7%	760
December	1st to 10th	Night	26% 15% 31% 21% 6% 1	760
	11th to 20th	Night	20% 14% 35% 25% 5%	760
	21st to end	Night	22% 15% 33% 28% 3	836

Page generated at 2024-01-09 16:24:21. Version ac07

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