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 2025-03-12.

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	17% 7% 2 6% 4 4 4 4 5% 7% 39%	108707
	Night	17% 7% 2 6% 4 4 4 3 5% 6% 42%	110280
Transparency	Day	24% 44% 6% 14% 10% 2	112269
	Night	25% 42% 7% 14% 11% 2	114882
Seeing	Day	21% 16% 30% 21% 9% 3	15323
	Night	20% 13% 30% 27% 9% 1	24599

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	12% 7% 1 5% 4 4 5% 4 5% 6% 46%	3597
	11th to 20th	Day	14% 9% 3 6% 5% 5% 5% 3 4 5% 41%	3570
	21st to end	Day	18% 8% 3 7% 4 4 4 4 5% 4 39%	4282
February	1st to 10th	Day	17% 7% 3 7% 5% 5% 4 4 5% 7% 36%	1801
	11th to 20th	Day	16% 7% 2 6% 5% 4 4 3 4 5% 43%	1948
	21st to end	Day	19% 9% 2 5% 5% 3 4 4 4 6% 39%	1650
March	1st to 10th	Day	26% 7% 2 6% 4 3 4 3 5% 5% 36%	3216
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 4 7% 8% 43%	3348
	21st to end	Day	20% 7% 2 6% 3 4 3 3 4 5% 42%	4133
April	1st to 10th	Day	19% 8% 2 5% 4 4 4 4 5% 7% 38%	2280
	11th to 20th	Day	17% 7% 2 6% 4 4 4 3 5% 7% 41%	2254
	21st to end	Day	20% 6% 2 5% 4 4 4 4 6% 6% 39%	2308
May	1st to 10th	Day	28% 8% 3 5% 3 3 3 3 5% 8% 30%	4106
	11th to 20th	Day	23% 8% 3 8% 5% 4 4 4 5% 7% 30%	4390
	21st to end	Day	14% 5% 2 5% 4 4 4 4 5% 10% 43%	4837
June	1st to 10th	Day	17% 7% 2 5% 5% 4 5% 4 7% 9% 36%	2660
	11th to 20th	Day	16% 7% 2 6% 4 4 6% 4 7% 8% 35%	2660
	21st to end	Day	13% 6% 2 7% 4 5% 5% 4 7% 9% 38%	2660
July	1st to 10th	Day	11% 6% 2 8% 5% 6% 6% 5% 7% 9% 36%	4440
	11th to 20th	Day	11% 7% 2 6% 4 5% 6% 4 7% 10% 37%	4440
	21st to end	Day	15% 8% 3 9% 6% 5% 5% 5% 6% 7% 32%	4814
August	1st to 10th	Day	21% 6% 2 7% 5% 5% 5% 4 6% 7% 31%	4208
	11th to 20th	Day	14% 6% 2 6% 4 4 4 4 6% 9% 40%	3730
	21st to end	Day	12% 6% 2 6% 4 4 4 3 6% 9% 46%	4180
September	1st to 10th	Day	15% 5% 2 7% 5% 5% 5% 4 5% 6% 42%	2280
	11th to 20th	Day	13% 5% 2 5% 3 3 4 3 5% 7% 49%	2199
	21st to end	Day	18% 6% 3 7% 4 4 4 4 6% 6% 40%	2090
October	1st to 10th	Day	22% 6% 2 5% 3 4 4 3 5% 6% 41%	1981
	11th to 20th	Day	15% 5% 2 4 3 4 4 3 5% 7% 48%	1900
	21st to end	Day	21% 5% 2 4 4 4 4 3 5% 7% 41%	1933
November	1st to 10th	Day	14% 6% 3 4 3 3 3 3 5% 5% 52%	1761
	11th to 20th	Day	18% 6% 1 4 4 3 3 4 5% 6% 47%	1800
	21st to end	Day	16% 7% 2 4 4 4 4 4 6% 7% 42%	1705
December	1st to 10th	Day	16% 7% 3 5% 4 4 4 3 6% 7% 42%	2600
	11th to 20th	Day	23% 7% 1 4 3 3 3 3 4 5% 45%	2986
	21st to end	Day	18% 7% 3 7% 4 4 4 3 4 7% 39%	3960
January	1st to 10th	Night	14% 5% 2 5% 4 4 4 4 5% 6% 48%	6349
	11th to 20th	Night	13% 7% 3 7% 4 4 4 3 4 6% 46%	6315
	21st to end	Night	17% 9% 3 5% 4 4 4 3 5% 6% 40%	6656
February	1st to 10th	Night	17% 9% 2 6% 4 4 4 3 5% 6% 41%	2597
	11th to 20th	Night	17% 8% 2 6% 4 3 5% 4 5% 5% 41%	2338
	21st to end	Night	17% 8% 3 6% 4 4 4 3 4 5% 41%	1980
March	1st to 10th	Night	23% 9% 2 5% 4 3 4 3 4 6% 36%	3844
	11th to 20th	Night	12% 7% 2 6% 4 4 4 4 5% 7% 46%	3636
	21st to end	Night	18% 10% 3 6% 3 3 3 3 4 5% 42%	3498
April	1st to 10th	Night	18% 8% 3 6% 4 4 5% 4 5% 7% 38%	1900
	11th to 20th	Night	19% 6% 2 6% 4 3 4 3 5% 8% 40%	1875
	21st to end	Night	20% 7% 2 6% 3 3 3 2 5% 6% 42%	1870
May	1st to 10th	Night	27% 8% 3 7% 4 3 4 3 5% 6% 30%	2834
	11th to 20th	Night	24% 9% 3 7% 4 3 3 3 5% 5% 34%	2531
	21st to end	Night	15% 7% 2 6% 4 4 4 3 5% 8% 42%	2738
June	1st to 10th	Night	17% 6% 2 5% 3 3 6% 4 6% 8% 41%	1520
	11th to 20th	Night	14% 6% 2 5% 4 4 4 4 6% 10% 40%	1520
	21st to end	Night	12% 5% 1 6% 5% 4 4 4 7% 9% 44%	1520
July	1st to 10th	Night	11% 6% 2 6% 5% 5% 5% 4 7% 9% 40%	2520
	11th to 20th	Night	14% 6% 2 6% 4 4 4 5% 6% 10% 38%	2520
	21st to end	Night	18% 8% 3 6% 3 3 5% 4 7% 8% 37%	2739
August	1st to 10th	Night	22% 6% 3 7% 5% 5% 5% 4 4 8% 32%	2684
	11th to 20th	Night	15% 7% 2 7% 5% 4 5% 3 6% 6% 39%	3097
	21st to end	Night	11% 4 2 5% 4 3 4 3 5% 7% 52%	3476
September	1st to 10th	Night	15% 6% 3 5% 4 3 4 3 5% 6% 45%	1900
	11th to 20th	Night	14% 6% 2 3 4 3 3 3 5% 6% 53%	1981
	21st to end	Night	18% 6% 2 5% 4 3 4 3 5% 5% 45%	2090
October	1st to 10th	Night	21% 7% 2 6% 4 3 3 3 4 5% 42%	2194
	11th to 20th	Night	15% 5% 2 5% 3 3 3 3 5% 6% 49%	2280
	21st to end	Night	22% 6% 3 5% 2 3 3 3 4 5% 43%	2664
November	1st to 10th	Night	13% 5% 3 5% 3 3 3 3 5% 6% 51%	2537
	11th to 20th	Night	19% 7% 2 4 3 3 3 3 4 5% 47%	2600
	21st to end	Night	15% 5% 2 5% 4 4 5% 3 4 7% 46%	2693
December	1st to 10th	Night	14% 7% 2 6% 4 3 3 3 4 6% 48%	4580
	11th to 20th	Night	25% 7% 2 4 3 3 3 3 4 4 42%	5208
	21st to end	Night	19% 9% 3 6% 4 4 3 2 4 5% 40%	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	24% 28% 6% 24% 15% 3	3598
	11th to 20th	Day	32% 27% 6% 17% 12% 5%	3600
	21st to end	Day	36% 30% 7% 16% 11% 1	4282
February	1st to 10th	Day	18% 62% 6% 8% 4 1	1801
	11th to 20th	Day	17% 60% 2 10% 9% 1	1957
	21st to end	Day	17% 62% 5% 9% 6% 2	1650
March	1st to 10th	Day	35% 39% 7% 12% 5% 2	3219
	11th to 20th	Day	18% 42% 8% 22% 9% 1	3348
	21st to end	Day	28% 39% 4 8% 17% 4	4133
April	1st to 10th	Day	18% 64% 4 7% 5% 1	2280
	11th to 20th	Day	14% 65% 4 7% 9% 1	2254
	21st to end	Day	17% 65% 3 9% 6% 1	2308
May	1st to 10th	Day	34% 41% 9% 9% 6% 1	4120
	11th to 20th	Day	35% 42% 8% 12% 3	4394
	21st to end	Day	21% 41% 6% 19% 12% 1	4837
June	1st to 10th	Day	15% 65% 3 9% 7%	2660
	11th to 20th	Day	14% 65% 4 10% 7%	2660
	21st to end	Day	14% 64% 4 9% 9%	2660
July	1st to 10th	Day	28% 40% 7% 14% 11% 1	4370
	11th to 20th	Day	27% 39% 11% 14% 9% 1	4365
	21st to end	Day	34% 37% 6% 12% 10% 2	4661
August	1st to 10th	Day	35% 39% 8% 13% 5%	4194
	11th to 20th	Day	24% 39% 9% 22% 6%	3766
	21st to end	Day	19% 38% 7% 22% 13% 1	4180
September	1st to 10th	Day	16% 60% 5% 12% 8%	2280
	11th to 20th	Day	16% 60% 3 9% 11% 1	2199
	21st to end	Day	20% 59% 5% 9% 6% 1	2090
October	1st to 10th	Day	22% 39% 5% 10% 18% 5%	3293
	11th to 20th	Day	13% 37% 5% 17% 25% 4	3140
	21st to end	Day	13% 37% 10% 17% 20% 3	3158
November	1st to 10th	Day	17% 55% 6% 10% 10% 1	1761
	11th to 20th	Day	14% 56% 3 12% 13% 2	1800
	21st to end	Day	16% 57% 7% 14% 5% 2	1705
December	1st to 10th	Day	26% 37% 10% 19% 7% 1	2600
	11th to 20th	Day	29% 31% 9% 16% 12% 2	2986
	21st to end	Day	35% 28% 5% 17% 14% 3	3960
January	1st to 10th	Night	26% 27% 6% 20% 17% 3	6349
	11th to 20th	Night	27% 27% 11% 16% 14% 5%	6360
	21st to end	Night	34% 29% 7% 13% 12% 5%	6656
February	1st to 10th	Night	17% 61% 5% 11% 4 1	2597
	11th to 20th	Night	17% 60% 2 8% 10% 2	2340
	21st to end	Night	18% 61% 3 8% 8% 2	1980
March	1st to 10th	Night	35% 40% 4 11% 7% 2	3854
	11th to 20th	Night	18% 41% 11% 17% 12% 1	3637
	21st to end	Night	29% 40% 4 7% 15% 4	3498
April	1st to 10th	Night	17% 65% 3 8% 5% 1	1900
	11th to 20th	Night	13% 66% 4 9% 8%	1875
	21st to end	Night	17% 64% 2 8% 7% 1	1870
May	1st to 10th	Night	36% 43% 6% 11% 4	2840
	11th to 20th	Night	36% 41% 10% 9% 4	2532
	21st to end	Night	25% 41% 9% 17% 7% 2	2739
June	1st to 10th	Night	15% 64% 2 10% 9% 1	1520
	11th to 20th	Night	13% 65% 5% 11% 5%	1520
	21st to end	Night	13% 63% 4 11% 8% 1	1520
July	1st to 10th	Night	25% 41% 9% 13% 10% 1	2480
	11th to 20th	Night	27% 38% 10% 16% 8%	2472
	21st to end	Night	33% 36% 9% 9% 13% 1	2651
August	1st to 10th	Night	38% 40% 9% 9% 4 1	2674
	11th to 20th	Night	27% 39% 8% 20% 6%	3127
	21st to end	Night	15% 39% 7% 26% 12%	3476
September	1st to 10th	Night	14% 60% 4 13% 8% 1	1900
	11th to 20th	Night	13% 60% 4 10% 10% 2	1981
	21st to end	Night	20% 59% 4 8% 8% 1	2090
October	1st to 10th	Night	24% 39% 7% 15% 14% 1	3662
	11th to 20th	Night	11% 37% 4 20% 24% 3	3779
	21st to end	Night	16% 37% 7% 22% 16% 1	4389
November	1st to 10th	Night	16% 56% 5% 11% 10% 1	2537
	11th to 20th	Night	16% 56% 3 10% 12% 3	2600
	21st to end	Night	15% 57% 6% 14% 7% 1	2693
December	1st to 10th	Night	25% 36% 8% 23% 7% 1	4580
	11th to 20th	Night	33% 31% 6% 14% 12% 3	5208
	21st to end	Night	36% 27% 8% 12% 13% 4	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	27% 27% 30% 14% 3	200
	11th to 20th	Day	30% 25% 33% 11% 2 1	200
	21st to end	Day	20% 27% 37% 10% 4 2	290
February	1st to 10th	Day	28% 24% 35% 11% 2	301
	11th to 20th	Day	27% 25% 33% 14% 1	496
	21st to end	Day	27% 19% 34% 16% 3	414
March	1st to 10th	Day	24% 26% 28% 19% 3	500
	11th to 20th	Day	21% 25% 32% 17% 4 1	464
	21st to end	Day	29% 22% 29% 16% 3 1	506
April	1st to 10th	Day	27% 25% 26% 14% 7% 1	460
	11th to 20th	Day	24% 22% 30% 15% 7% 2	452
	21st to end	Day	25% 21% 34% 15% 4 2	460
May	1st to 10th	Day	30% 18% 30% 13% 6% 2	463
	11th to 20th	Day	21% 15% 34% 21% 8% 1	487
	21st to end	Day	19% 12% 25% 30% 12% 3	529
June	1st to 10th	Day	18% 12% 32% 24% 12% 2	490
	11th to 20th	Day	17% 15% 29% 27% 11% 1	490
	21st to end	Day	13% 13% 30% 26% 15% 2	490
July	1st to 10th	Day	15% 7% 27% 29% 16% 5%	485
	11th to 20th	Day	12% 5% 27% 35% 17% 4	484
	21st to end	Day	14% 8% 29% 33% 15% 1	532
August	1st to 10th	Day	15% 6% 26% 33% 14% 7%	486
	11th to 20th	Day	14% 6% 25% 33% 17% 5%	479
	21st to end	Day	13% 11% 26% 26% 18% 7%	539
September	1st to 10th	Day	17% 11% 27% 21% 18% 6%	490
	11th to 20th	Day	14% 8% 29% 23% 14% 12%	490
	21st to end	Day	22% 10% 28% 24% 10% 6%	490
October	1st to 10th	Day	26% 8% 29% 19% 13% 5%	490
	11th to 20th	Day	16% 18% 29% 23% 11% 2	490
	21st to end	Day	26% 14% 29% 18% 11% 1	456
November	1st to 10th	Day	16% 18% 31% 23% 9% 2	395
	11th to 20th	Day	25% 19% 32% 20% 5% 1	400
	21st to end	Day	21% 21% 32% 21% 4 2	305
December	1st to 10th	Day	26% 24% 31% 15% 5%	200
	11th to 20th	Day	32% 22% 33% 11% 3	200
	21st to end	Day	25% 17% 37% 15% 5%	220
January	1st to 10th	Night	19% 19% 39% 20% 3	800
	11th to 20th	Night	24% 17% 31% 25% 3	800
	21st to end	Night	20% 18% 36% 24% 3	880
February	1st to 10th	Night	22% 17% 35% 25% 2	800
	11th to 20th	Night	23% 21% 34% 20% 1	783
	21st to end	Night	23% 17% 36% 20% 4	660
March	1st to 10th	Night	23% 19% 35% 19% 4	800
	11th to 20th	Night	23% 17% 34% 22% 4	767
	21st to end	Night	22% 19% 32% 21% 5%	836
April	1st to 10th	Night	21% 21% 34% 21% 4	760
	11th to 20th	Night	22% 15% 34% 23% 6%	751
	21st to end	Night	22% 17% 31% 25% 5%	730
May	1st to 10th	Night	26% 15% 30% 22% 6%	570
	11th to 20th	Night	24% 11% 31% 28% 6%	397
	21st to end	Night	19% 9% 20% 41% 9% 2	411
June	1st to 10th	Night	19% 5% 28% 37% 11% 1	380
	11th to 20th	Night	15% 6% 27% 38% 14% 1	380
	21st to end	Night	13% 3 27% 43% 13%	380
July	1st to 10th	Night	14% 6% 20% 42% 18% 1	370
	11th to 20th	Night	12% 3 24% 43% 17% 1	368
	21st to end	Night	13% 4 20% 42% 19% 1	416
August	1st to 10th	Night	14% 1 22% 42% 20% 2	478
	11th to 20th	Night	14% 5% 19% 36% 24% 2	750
	21st to end	Night	16% 4 19% 37% 22% 2	836
September	1st to 10th	Night	17% 5% 25% 34% 16% 2	760
	11th to 20th	Night	17% 5% 22% 32% 21% 4	760
	21st to end	Night	21% 4 25% 33% 15% 2	760
October	1st to 10th	Night	24% 9% 28% 22% 15% 2	760
	11th to 20th	Night	18% 11% 30% 29% 11% 1	760
	21st to end	Night	25% 10% 32% 23% 9%	836
November	1st to 10th	Night	14% 12% 33% 28% 11% 1	780
	11th to 20th	Night	23% 13% 32% 24% 8% 1	800
	21st to end	Night	20% 16% 31% 26% 7%	800
December	1st to 10th	Night	25% 15% 32% 21% 5% 1	800
	11th to 20th	Night	24% 16% 34% 23% 4	800
	21st to end	Night	24% 15% 35% 24% 3	880

Page generated at 2025-03-09 08:08:47. Version ac07

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