Monsey Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Monsey Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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% 39%	138040
	Night	17% 7% 2 6% 4 4 4 3 5% 6% 42%	134640
Transparency	Day	28% 36% 7% 15% 11% 2	141556
	Night	28% 35% 7% 15% 12% 2	139213
Seeing	Day	21% 16% 30% 21% 9% 3	17117
	Night	20% 13% 30% 27% 9% 1	26228

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% 7% 2 6% 4 4 5% 4 6% 6% 45%	4661
	11th to 20th	Day	15% 8% 3 5% 4 4 4 3 4 5% 44%	4650
	21st to end	Day	16% 7% 3 6% 4 4 4 4 6% 6% 42%	5541
February	1st to 10th	Day	16% 7% 3 7% 5% 5% 4 4 5% 7% 37%	1891
	11th to 20th	Day	16% 7% 3 6% 5% 4 4 3 5% 6% 42%	2048
	21st to end	Day	18% 10% 2 5% 5% 3 4 4 4 6% 40%	1730
March	1st to 10th	Day	22% 6% 2 5% 4 3 4 3 5% 5% 41%	4576
	11th to 20th	Day	12% 6% 2 6% 5% 4 5% 4 6% 7% 43%	4873
	21st to end	Day	18% 7% 2 6% 4 4 4 3 5% 5% 41%	6054
April	1st to 10th	Day	20% 8% 2 5% 4 3 4 4 5% 6% 38%	2520
	11th to 20th	Day	17% 6% 2 6% 4 3 4 3 5% 8% 41%	2494
	21st to end	Day	19% 6% 2 6% 4 4 4 3 6% 6% 39%	2551
May	1st to 10th	Day	27% 7% 3 5% 3 3 4 3 5% 7% 33%	7639
	11th to 20th	Day	20% 7% 2 7% 4 4 4 3 4 6% 39%	8213
	21st to end	Day	23% 6% 2 5% 4 4 5% 4 6% 8% 32%	9060
June	1st to 10th	Day	16% 6% 2 5% 5% 4 5% 4 7% 9% 37%	2940
	11th to 20th	Day	16% 7% 2 6% 4 4 6% 4 7% 8% 36%	2940
	21st to end	Day	13% 6% 2 7% 4 5% 5% 4 7% 9% 38%	2940
July	1st to 10th	Day	12% 7% 2 7% 5% 5% 6% 5% 6% 9% 36%	6500
	11th to 20th	Day	11% 7% 2 7% 5% 6% 7% 5% 8% 11% 32%	6430
	21st to end	Day	13% 6% 2 7% 5% 4 4 4 6% 8% 39%	6926
August	1st to 10th	Day	21% 6% 2 7% 5% 5% 5% 4 6% 7% 32%	4341
	11th to 20th	Day	14% 6% 2 6% 4 4 4 4 7% 9% 40%	3850
	21st to end	Day	12% 5% 2 6% 4 4 4 4 6% 9% 47%	4312
September	1st to 10th	Day	15% 5% 2 7% 5% 5% 5% 4 5% 6% 43%	2400
	11th to 20th	Day	13% 5% 2 5% 3 3 4 3 5% 7% 49%	2315
	21st to end	Day	18% 6% 3 7% 4 5% 4 4 6% 6% 39%	2200
October	1st to 10th	Day	21% 6% 2 5% 3 4 3 3 5% 6% 43%	2085
	11th to 20th	Day	15% 5% 2 5% 3 4 4 3 5% 7% 47%	2000
	21st to end	Day	21% 5% 2 4 4 4 4 3 5% 6% 42%	2035
November	1st to 10th	Day	14% 6% 2 4 3 3 3 3 5% 6% 52%	1851
	11th to 20th	Day	17% 6% 1 4 4 3 3 4 5% 6% 46%	1890
	21st to end	Day	16% 7% 2 4 4 4 4 4 6% 7% 41%	1790
December	1st to 10th	Day	16% 7% 3 5% 4 4 4 3 6% 7% 42%	2680
	11th to 20th	Day	22% 7% 1 4 3 3 3 3 4 5% 45%	3066
	21st to end	Day	18% 7% 3 7% 4 5% 4 3 4 7% 38%	4048
January	1st to 10th	Night	13% 7% 3 6% 4 4 4 4 5% 6% 44%	8234
	11th to 20th	Night	15% 8% 3 6% 4 3 4 3 4 6% 44%	8235
	21st to end	Night	14% 9% 2 5% 4 3 4 3 5% 6% 45%	8622
February	1st to 10th	Night	16% 9% 2 6% 4 4 4 3 5% 6% 41%	2727
	11th to 20th	Night	16% 8% 2 6% 4 3 5% 4 5% 5% 40%	2458
	21st to end	Night	17% 8% 3 7% 4 4 4 3 4 5% 41%	2076
March	1st to 10th	Night	20% 8% 2 4 3 3 4 2 5% 7% 43%	5484
	11th to 20th	Night	14% 9% 3 5% 3 3 3 3 4 6% 45%	5288
	21st to end	Night	19% 9% 3 6% 4 3 4 3 5% 5% 40%	5119
April	1st to 10th	Night	20% 8% 3 6% 4 4 4 3 5% 7% 37%	2100
	11th to 20th	Night	19% 6% 2 6% 4 3 4 3 5% 8% 40%	2075
	21st to end	Night	19% 7% 2 6% 3 3 3 2 5% 6% 42%	2066
May	1st to 10th	Night	26% 8% 3 6% 4 3 4 3 5% 6% 31%	5259
	11th to 20th	Night	20% 8% 2 6% 4 4 4 3 6% 6% 37%	4702
	21st to end	Night	23% 8% 3 6% 4 4 4 3 4 6% 34%	5114
June	1st to 10th	Night	16% 6% 2 5% 3 3 5% 4 5% 8% 42%	1680
	11th to 20th	Night	15% 6% 3 5% 4 4 4 4 6% 10% 39%	1680
	21st to end	Night	12% 5% 1 7% 6% 4 4 3 6% 9% 43%	1680
July	1st to 10th	Night	13% 6% 2 7% 5% 5% 5% 4 6% 9% 39%	3680
	11th to 20th	Night	15% 8% 3 7% 5% 4 5% 4 6% 9% 34%	3640
	21st to end	Night	13% 6% 2 5% 3 3 4 3 6% 7% 45%	3926
August	1st to 10th	Night	22% 6% 2 7% 5% 5% 4 4 4 8% 33%	2770
	11th to 20th	Night	15% 7% 2 7% 5% 4 5% 4 6% 6% 40%	3197
	21st to end	Night	11% 4 2 5% 4 3 3 3 5% 7% 53%	3586
September	1st to 10th	Night	15% 6% 3 5% 4 3 4 3 5% 6% 45%	2000
	11th to 20th	Night	13% 6% 2 4 4 3 3 3 5% 6% 53%	2085
	21st to end	Night	18% 7% 2 5% 4 3 4 3 4 6% 45%	2200
October	1st to 10th	Night	20% 7% 2 6% 4 3 3 2 4 5% 43%	2310
	11th to 20th	Night	16% 5% 2 5% 3 3 3 3 5% 6% 49%	2400
	21st to end	Night	21% 6% 2 5% 2 3 3 3 4 6% 44%	2804
November	1st to 10th	Night	13% 5% 3 5% 3 3 3 3 6% 7% 51%	2667
	11th to 20th	Night	18% 7% 2 4 3 3 3 3 4 5% 47%	2730
	21st to end	Night	15% 5% 2 5% 4 4 4 3 4 7% 46%	2828
December	1st to 10th	Night	14% 7% 2 6% 3 3 3 3 4 6% 48%	4720
	11th to 20th	Night	25% 7% 2 4 3 3 3 3 4 4 42%	5348
	21st to end	Night	19% 9% 3 6% 4 4 4 3 5% 5% 39%	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% 23% 8% 23% 15% 2	4662
	11th to 20th	Day	33% 21% 7% 18% 15% 6%	4680
	21st to end	Day	33% 24% 9% 19% 14% 1	5541
February	1st to 10th	Day	19% 60% 6% 9% 5% 1	1891
	11th to 20th	Day	19% 57% 2 11% 9% 1	2057
	21st to end	Day	18% 59% 5% 11% 6% 1	1730
March	1st to 10th	Day	34% 29% 8% 12% 9% 8%	4579
	11th to 20th	Day	27% 30% 9% 23% 7% 4	4873
	21st to end	Day	33% 29% 7% 7% 19% 5%	6054
April	1st to 10th	Day	21% 59% 4 8% 6% 2	2520
	11th to 20th	Day	16% 59% 6% 8% 10% 1	2494
	21st to end	Day	19% 59% 3 11% 7% 1	2551
May	1st to 10th	Day	41% 25% 10% 13% 11% 1	7651
	11th to 20th	Day	37% 24% 8% 18% 12% 1	8217
	21st to end	Day	40% 24% 9% 17% 10%	9061
June	1st to 10th	Day	17% 60% 3 10% 10%	2940
	11th to 20th	Day	17% 59% 4 11% 8%	2940
	21st to end	Day	17% 58% 5% 10% 9% 1	2940
July	1st to 10th	Day	33% 30% 8% 16% 12% 1	6430
	11th to 20th	Day	33% 30% 13% 16% 7% 1	6355
	21st to end	Day	31% 27% 7% 17% 15% 2	6728
August	1st to 10th	Day	35% 38% 8% 14% 6% 1	4327
	11th to 20th	Day	24% 38% 9% 22% 6%	3886
	21st to end	Day	19% 37% 8% 22% 14% 1	4312
September	1st to 10th	Day	17% 57% 5% 13% 8% 1	2400
	11th to 20th	Day	16% 57% 4 10% 12% 1	2315
	21st to end	Day	22% 57% 5% 9% 6% 1	2200
October	1st to 10th	Day	22% 38% 5% 11% 19% 5%	3397
	11th to 20th	Day	14% 36% 5% 17% 24% 4	3240
	21st to end	Day	13% 36% 10% 18% 20% 3	3260
November	1st to 10th	Day	18% 53% 6% 11% 11% 1	1851
	11th to 20th	Day	16% 54% 3 12% 13% 2	1890
	21st to end	Day	18% 54% 7% 15% 5% 1	1790
December	1st to 10th	Day	26% 36% 10% 20% 7% 1	2680
	11th to 20th	Day	29% 30% 9% 17% 12% 3	3066
	21st to end	Day	35% 27% 4 17% 13% 2	4048
January	1st to 10th	Night	32% 22% 9% 20% 14% 3	8234
	11th to 20th	Night	32% 22% 9% 16% 15% 6%	8280
	21st to end	Night	33% 23% 7% 17% 16% 4	8622
February	1st to 10th	Night	18% 59% 5% 12% 5% 1	2727
	11th to 20th	Night	19% 58% 3 9% 9% 2	2460
	21st to end	Night	19% 59% 3 9% 7% 2	2076
March	1st to 10th	Night	33% 30% 6% 14% 10% 7%	5494
	11th to 20th	Night	27% 29% 12% 18% 10% 3	5289
	21st to end	Night	36% 29% 4 9% 18% 4	5119
April	1st to 10th	Night	21% 59% 4 9% 5% 2	2100
	11th to 20th	Night	16% 60% 5% 9% 9% 1	2075
	21st to end	Night	19% 59% 3 9% 8% 1	2066
May	1st to 10th	Night	43% 25% 8% 14% 9%	5265
	11th to 20th	Night	38% 25% 11% 17% 9% 1	4703
	21st to end	Night	43% 23% 8% 17% 7% 1	5115
June	1st to 10th	Night	16% 59% 2 11% 11% 1	1680
	11th to 20th	Night	17% 59% 6% 11% 6% 1	1680
	21st to end	Night	16% 58% 5% 12% 8% 1	1680
July	1st to 10th	Night	32% 30% 10% 15% 12% 2	3640
	11th to 20th	Night	37% 28% 11% 16% 8%	3592
	21st to end	Night	28% 27% 9% 19% 16% 1	3809
August	1st to 10th	Night	38% 38% 8% 10% 5% 1	2760
	11th to 20th	Night	27% 38% 8% 20% 6%	3227
	21st to end	Night	15% 38% 7% 26% 13% 1	3586
September	1st to 10th	Night	15% 57% 4 14% 8% 1	2000
	11th to 20th	Night	14% 58% 4 10% 11% 2	2085
	21st to end	Night	21% 56% 4 9% 8% 1	2200
October	1st to 10th	Night	24% 37% 7% 16% 15% 2	3778
	11th to 20th	Night	12% 36% 5% 20% 24% 3	3899
	21st to end	Night	16% 36% 8% 23% 16% 1	4529
November	1st to 10th	Night	17% 54% 6% 12% 10% 1	2667
	11th to 20th	Night	17% 54% 3 11% 13% 3	2730
	21st to end	Night	17% 54% 6% 14% 8% 1	2828
December	1st to 10th	Night	25% 35% 8% 23% 8% 1	4720
	11th to 20th	Night	33% 30% 7% 14% 12% 3	5348
	21st to end	Night	36% 26% 8% 12% 13% 4	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% 32% 13% 3	220
	11th to 20th	Day	31% 26% 31% 10% 1	220
	21st to end	Day	19% 29% 37% 10% 4 2	319
February	1st to 10th	Day	26% 23% 36% 11% 3	331
	11th to 20th	Day	26% 24% 35% 15% 1	536
	21st to end	Day	27% 18% 35% 16% 3	446
March	1st to 10th	Day	26% 25% 29% 18% 3	540
	11th to 20th	Day	22% 24% 33% 16% 4 1	540
	21st to end	Day	28% 23% 29% 15% 3 1	594
April	1st to 10th	Day	28% 25% 26% 13% 6% 1	540
	11th to 20th	Day	24% 23% 31% 14% 7% 2	532
	21st to end	Day	24% 21% 34% 16% 4 1	540
May	1st to 10th	Day	30% 19% 31% 13% 6% 2	543
	11th to 20th	Day	20% 16% 34% 21% 7% 1	567
	21st to end	Day	22% 13% 25% 27% 10% 2	617
June	1st to 10th	Day	17% 13% 29% 25% 13% 2	570
	11th to 20th	Day	19% 16% 29% 25% 10% 1	570
	21st to end	Day	13% 13% 31% 26% 15% 2	570
July	1st to 10th	Day	15% 7% 26% 32% 16% 4	565
	11th to 20th	Day	11% 7% 26% 33% 18% 5%	544
	21st to end	Day	13% 8% 28% 34% 14% 1	576
August	1st to 10th	Day	15% 5% 26% 32% 15% 7%	526
	11th to 20th	Day	14% 5% 24% 34% 17% 6%	519
	21st to end	Day	12% 10% 28% 26% 17% 7%	583
September	1st to 10th	Day	17% 11% 28% 22% 17% 6%	530
	11th to 20th	Day	14% 8% 28% 24% 15% 12%	530
	21st to end	Day	21% 9% 28% 25% 11% 5%	530
October	1st to 10th	Day	24% 8% 29% 19% 13% 6%	530
	11th to 20th	Day	18% 18% 28% 23% 10% 2	530
	21st to end	Day	25% 15% 29% 18% 10% 1	492
November	1st to 10th	Day	16% 19% 31% 23% 9% 2	425
	11th to 20th	Day	25% 20% 30% 19% 5% 1	430
	21st to end	Day	22% 22% 31% 20% 4 2	330
December	1st to 10th	Day	25% 25% 32% 14% 5%	220
	11th to 20th	Day	32% 22% 34% 10% 2	220
	21st to end	Day	27% 17% 37% 14% 5%	242
January	1st to 10th	Night	19% 18% 39% 20% 3	840
	11th to 20th	Night	25% 18% 31% 24% 3	840
	21st to end	Night	19% 19% 37% 23% 2	924
February	1st to 10th	Night	22% 17% 34% 25% 2	840
	11th to 20th	Night	23% 21% 33% 21% 1	823
	21st to end	Night	23% 17% 36% 20% 3	692
March	1st to 10th	Night	23% 18% 35% 20% 4	840
	11th to 20th	Night	24% 17% 33% 22% 4	840
	21st to end	Night	23% 19% 33% 20% 4	924
April	1st to 10th	Night	23% 21% 32% 20% 4	840
	11th to 20th	Night	23% 15% 34% 22% 5%	831
	21st to end	Night	21% 16% 32% 25% 5%	806
May	1st to 10th	Night	27% 14% 30% 23% 6%	630
	11th to 20th	Night	23% 12% 30% 29% 7%	439
	21st to end	Night	21% 9% 21% 38% 8% 2	455
June	1st to 10th	Night	18% 5% 27% 38% 11%	420
	11th to 20th	Night	16% 6% 28% 37% 13%	420
	21st to end	Night	13% 4 25% 44% 14%	420
July	1st to 10th	Night	14% 6% 18% 43% 18%	410
	11th to 20th	Night	12% 3 23% 43% 17% 1	398
	21st to end	Night	13% 4 20% 43% 18% 1	438
August	1st to 10th	Night	14% 1 20% 42% 20% 2	504
	11th to 20th	Night	13% 5% 19% 37% 25% 2	790
	21st to end	Night	16% 4 21% 36% 21% 2	880
September	1st to 10th	Night	17% 5% 25% 35% 16% 3	800
	11th to 20th	Night	17% 5% 21% 33% 21% 4	800
	21st to end	Night	21% 4 24% 33% 16% 2	800
October	1st to 10th	Night	23% 9% 28% 22% 16% 3	800
	11th to 20th	Night	19% 12% 30% 29% 11% 1	800
	21st to end	Night	25% 11% 33% 23% 9%	880
November	1st to 10th	Night	15% 12% 33% 28% 10% 1	820
	11th to 20th	Night	23% 15% 31% 23% 8% 1	840
	21st to end	Night	20% 17% 30% 26% 7%	840
December	1st to 10th	Night	25% 15% 33% 21% 5% 1	840
	11th to 20th	Night	24% 16% 33% 23% 4	840
	21st to end	Night	24% 16% 34% 24% 2	924

Page generated at 2026-07-13 03:14:50. Version ac07

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