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New York City
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the New York City clear sky chart from 2005-11-05 to 2008-11-27.

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% 5% 7% 39%	12263
	Night	17% 5% 5% 7% 39%	12129
Cloud Cover	Day	15% 5% 5% 8% 40%	12278
	Night	18% 5% 5% 7% 39%	12156
Transparency	Day	6% 18% 28% 19%	8941
	Night	6% 18% 26% 20%	8865
Seeing	Day	5% 26% 29% 21% 6%	789
	Night	8% 34% 33% 11%	3478

By Month

Percentage Cloud Cover
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month		Occurrence of each Forecast	Data Points
January	Day	18% 5% 6% 5% 5% 7% 36%	762
	Night	22% 7% 5% 7% 37%	1277
February	Day	14% 5% 5% 5% 7% 42%	785
	Night	15% 7% 5% 5% 6% 39%	1006
March	Day	13% 5% 5% 6% 6% 6% 40%	986
	Night	16% 6% 6% 6% 8% 39%	1010
April	Day	18% 5% 5% 5% 6% 39%	1082
	Night	19% 7% 5% 9% 35%	895
May	Day	14% 7% 6% 9% 38%	1267
	Night	16% 5% 5% 8% 41%	782
June	Day	14% 5% 6% 6% 6% 10% 32%	1251
	Night	13% 6% 7% 7% 9% 34%	718
July	Day	13% 5% 5% 8% 5% 7% 8% 35%	1301
	Night	11% 5% 7% 6% 7% 11% 34%	740
August	Day	12% 5% 5% 5% 6% 5% 6% 7% 40%	1145
	Night	14% 5% 9% 39%	888
September	Day	15% 6% 51%	1035
	Night	15% 7% 51%	942
October	Day	14% 5% 7% 46%	913
	Night	16% 5% 6% 46%	1120
November	Day	21% 5% 5% 5% 40%	992
	Night	24% 7% 5% 34%	1454
December	Day	15% 6% 6% 6% 5% 6% 37%	744
	Night	18% 5% 5% 8% 38%	1297

Percentage Cloud Cover
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month		Occurrence of each Forecast	Data Points
January	Day	18% 6% 6% 5% 8% 36%	761
	Night	23% 5% 6% 38%	1278
February	Day	14% 5% 5% 5% 7% 42%	782
	Night	16% 5% 5% 5% 5% 7% 39%	998
March	Day	14% 5% 7% 7% 40%	1007
	Night	16% 6% 5% 5% 9% 38%	1034
April	Day	19% 5% 5% 5% 6% 39%	1081
	Night	20% 7% 5% 8% 36%	895
May	Day	14% 6% 5% 6% 9% 38%	1266
	Night	17% 5% 5% 7% 42%	782
June	Day	14% 5% 5% 5% 5% 6% 11% 33%	1260
	Night	16% 5% 7% 5% 6% 10% 34%	720
July	Day	13% 5% 7% 5% 6% 9% 36%	1301
	Night	11% 5% 6% 7% 6% 6% 12% 33%	741
August	Day	12% 5% 6% 5% 6% 8% 40%	1139
	Night	14% 6% 5% 5% 8% 41%	891
September	Day	15% 5% 5% 52%	1034
	Night	15% 7% 51%	943
October	Day	13% 5% 8% 46%	913
	Night	16% 5% 5% 5% 47%	1120
November	Day	21% 6% 5% 5% 5% 41%	990
	Night	24% 7% 6% 34%	1453
December	Day	15% 6% 5% 5% 5% 7% 37%	744
	Night	18% 5% 7% 38%	1301

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	29% 26% 13%	519
	Night	6% 19% 35% 15%	847
February	Day	5% 20% 30% 16%	580
	Night	6% 20% 27% 19%	715
March	Day	5% 18% 32% 21%	710
	Night	7% 20% 29% 18%	735
April	Day	16% 24% 24% 6%	744
	Night	18% 20% 26%	620
May	Day	7% 18% 30% 20%	923
	Night	6% 17% 22% 25%	573
June	Day	15% 26% 32% 8%	837
	Night	12% 28% 24% 17%	457
July	Day	10% 17% 32% 15%	693
	Night	11% 21% 24% 19%	419
August	Day	5% 14% 24% 23%	957
	Night	11% 26% 23%	742
September	Day	5% 12% 28% 29%	880
	Night	10% 30% 31%	796
October	Day	13% 29% 20%	808
	Night	15% 26% 21%	971
November	Day	20% 24% 17%	757
	Night	15% 26% 15%	1050
December	Day	7% 28% 23% 14%	533
	Night	5% 29% 24% 13%	940

Seeing (when cloud cover <= 80%)
Color Meanings: Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month		Occurrence of each Forecast	Data Points
January	Night	12% 47% 28%	319
February	Day	9% 48% 30%	52
	Night	19% 43% 26%	296
March	Day	8% 46% 26% 7%	89
	Night	12% 47% 24%	343
April	Day	5% 32% 28% 19%	78
	Night	6% 37% 37% 7%	310
May	Day	29% 28% 24% 8%	82
	Night	6% 32% 40% 9%	207
June	Day	8% 10% 38% 28% 8%	75
	Night	9% 16% 45% 24%	153
July	Day	12% 21% 43% 10%	73
	Night	18% 50% 23%	155
August	Day	6% 8% 27% 27% 15%	83
	Night	15% 42% 24%	298
September	Day	12% 26% 31% 13%	80
	Night	14% 41% 24%	328
October	Day	6% 27% 34% 16%	86
	Night	6% 27% 38% 13%	344
November	Day	6% 36% 27% 14%	91
	Night	8% 37% 26% 11%	388
December	Night	10% 52% 23%	337

Page generated at 2008-11-27 13:47:41. Version ab11