Ithaca
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ithaca clear sky chart from 2005-11-05 to 2008-11-10.

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	19% 9% 6% 6% 5% 6% 5% 6% 27%	12110
	Night	19% 10% 6% 6% 5% 5% 5% 6% 26%	11925
Cloud Cover	Day	19% 9% 6% 6% 5% 5% 5% 7% 26%	12129
	Night	19% 10% 6% 6% 5% 5% 5% 6% 26%	11948
Transparency	Day	5% 19% 24% 13%	7766
	Night	5% 19% 23% 12%	7588
Seeing	Day	11% 23% 24% 19% 7%	697
	Night	16% 28% 30% 10%	3183

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	19% 13% 12% 10% 9% 5% 13%	776
	Night	26% 14% 9% 6% 6% 6% 14%	1264
February	Day	18% 11% 10% 15% 11% 6% 9%	759
	Night	14% 13% 11% 13% 7% 8% 5% 6% 10%	1032
March	Day	21% 12% 13% 7% 6% 5% 5% 18%	992
	Night	17% 11% 8% 7% 6% 5% 25%	1005
April	Day	20% 8% 6% 6% 36%	1082
	Night	21% 8% 6% 5% 5% 5% 5% 31%	897
May	Day	16% 5% 5% 6% 5% 7% 36%	1291
	Night	12% 5% 6% 8% 41%	755
June	Day	16% 5% 6% 6% 6% 5% 8% 32%	1251
	Night	21% 6% 5% 5% 6% 8% 31%	718
July	Day	15% 5% 5% 8% 7% 6% 9% 30%	1297
	Night	11% 5% 5% 7% 8% 9% 35%	744
August	Day	17% 5% 5% 5% 10% 35%	1158
	Night	16% 5% 6% 6% 5% 7% 37%	874
September	Day	22% 6% 5% 5% 36%	1037
	Night	19% 7% 5% 41%	939
October	Day	24% 11% 5% 5% 5% 6% 28%	915
	Night	21% 8% 5% 5% 6% 31%	1119
November	Day	22% 12% 5% 6% 6% 5% 5% 5% 6% 19%	808
	Night	23% 13% 6% 5% 5% 5% 7% 21%	1281
December	Day	20% 18% 10% 9% 6% 6% 11%	744
	Night	20% 15% 10% 7% 6% 7% 5% 5% 5% 11%	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	21% 12% 10% 12% 8% 6% 12%	775
	Night	26% 13% 9% 6% 7% 6% 14%	1265
February	Day	18% 12% 9% 15% 11% 5% 5% 9%	757
	Night	15% 13% 11% 12% 8% 7% 5% 6% 11%	1023
March	Day	21% 13% 12% 7% 5% 5% 18%	1015
	Night	18% 11% 9% 7% 6% 5% 25%	1027
April	Day	21% 9% 6% 9% 35%	1081
	Night	22% 7% 5% 5% 5% 5% 32%	897
May	Day	16% 5% 5% 7% 5% 7% 36%	1290
	Night	12% 5% 6% 7% 9% 40%	755
June	Day	16% 5% 5% 5% 7% 5% 6% 8% 31%	1260
	Night	20% 6% 5% 5% 6% 8% 31%	720
July	Day	15% 5% 5% 8% 6% 7% 9% 29%	1297
	Night	11% 5% 6% 5% 6% 5% 6% 10% 35%	745
August	Day	17% 6% 5% 5% 10% 34%	1153
	Night	16% 5% 6% 6% 5% 6% 38%	876
September	Day	22% 6% 5% 6% 34%	1036
	Night	19% 7% 5% 41%	940
October	Day	23% 12% 5% 7% 27%	915
	Night	20% 10% 5% 5% 7% 31%	1119
November	Day	21% 13% 6% 7% 5% 5% 5% 6% 19%	806
	Night	23% 11% 6% 6% 5% 5% 6% 22%	1280
December	Day	20% 19% 11% 8% 6% 6% 10%	744
	Night	19% 15% 10% 8% 6% 7% 5% 5% 12%	1301

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	6% 23% 13% 6%	360
	Night	6% 25% 14% 5%	589
February	Day	7% 11% 15% 6%	369
	Night	21% 15% 7%	524
March	Day	5% 18% 25% 7%	539
	Night	5% 21% 27% 10%	584
April	Day	22% 23% 26% 5%	663
	Night	18% 20% 21% 6%	551
May	Day	23% 27% 18%	918
	Night	5% 22% 26% 20%	575
June	Day	7% 26% 29% 10%	840
	Night	8% 22% 33% 9%	484
July	Day	5% 26% 26% 9%	704
	Night	6% 27% 24% 13%	415
August	Day	12% 29% 17%	981
	Night	14% 28% 16%	714
September	Day	12% 29% 18%	788
	Night	12% 32% 14%	747
October	Day	8% 23% 15%	721
	Night	5% 14% 21% 16%	920
November	Day	7% 26% 18% 6%	506
	Night	5% 19% 22% 7%	810
December	Day	5% 22% 14%	377
	Night	8% 22% 14%	675

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	26% 37% 21%	270
February	Day	18% 27% 22% 13%	22
	Night	30% 34% 20%	253
March	Day	28% 25% 18% 10%	82
	Night	27% 30% 22% 5%	322
April	Day	16% 25% 23% 19% 6%	72
	Night	18% 27% 35% 8%	290
May	Day	12% 27% 25% 17% 9%	87
	Night	8% 27% 40% 15%	185
June	Day	24% 32% 28%	74
	Night	8% 17% 47% 17%	146
July	Day	6% 28% 28% 21% 10%	76
	Night	17% 50% 20%	154
August	Day	20% 18% 26% 13%	86
	Night	22% 36% 22%	285
September	Day	6% 15% 21% 18% 18%	76
	Night	9% 19% 32% 22%	314
October	Day	10% 15% 31% 18%	83
	Night	11% 25% 29% 14%	332
November	Day	17% 28% 23% 15%	39
	Night	18% 30% 23% 7%	327
December	Night	23% 38% 24%	305

Page generated at 2008-11-11 07:54:38. Version ab11