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 2009-12-29.

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	18% 9% 5% 7% 6% 5% 5% 6% 26%	16452
	Night	18% 10% 6% 7% 6% 5% 5% 6% 26%	16482
Cloud Cover	Day	18% 10% 5% 7% 6% 5% 5% 7% 26%	16465
	Night	18% 10% 6% 7% 6% 5% 6% 26%	16499
Transparency	Day	15% 22% 11%	12102
	Night	16% 21% 10%	12143
Seeing	Day	10% 22% 23% 18% 7%	961
	Night	15% 26% 29% 10%	4604

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	17% 13% 11% 12% 10% 6% 12%	1027
	Night	24% 14% 8% 8% 9% 5% 13%	1672
February	Day	17% 11% 10% 16% 12% 11%	1021
	Night	13% 12% 10% 14% 8% 7% 5% 5% 12%	1386
March	Day	19% 13% 11% 7% 5% 5% 6% 22%	1331
	Night	18% 11% 7% 7% 5% 5% 26%	1347
April	Day	20% 10% 5% 6% 34%	1424
	Night	20% 9% 5% 5% 5% 32%	1183
May	Day	16% 6% 5% 5% 6% 5% 7% 36%	1711
	Night	14% 5% 5% 6% 7% 39%	999
June	Day	17% 7% 5% 5% 6% 6% 6% 8% 29%	1663
	Night	20% 6% 6% 5% 6% 8% 32%	956
July	Day	15% 5% 5% 6% 8% 7% 6% 8% 30%	1711
	Night	11% 5% 5% 5% 6% 7% 8% 37%	979
August	Day	16% 6% 5% 5% 5% 5% 5% 9% 33%	1547
	Night	14% 5% 6% 5% 7% 40%	1166
September	Day	21% 7% 5% 5% 35%	1368
	Night	17% 7% 5% 42%	1235
October	Day	25% 11% 5% 5% 5% 5% 26%	1212
	Night	24% 10% 5% 5% 5% 5% 28%	1482
November	Day	19% 14% 6% 8% 6% 5% 5% 6% 20%	1220
	Night	19% 14% 5% 7% 5% 5% 5% 7% 22%	1951
December	Day	19% 17% 9% 11% 8% 5% 11%	1217
	Night	20% 14% 9% 11% 8% 5% 5% 12%	2126

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% 12% 9% 12% 10% 5% 11%	1025
	Night	24% 14% 9% 9% 8% 6% 14%	1673
February	Day	17% 12% 9% 16% 12% 11%	1019
	Night	13% 12% 9% 14% 8% 7% 5% 13%	1377
March	Day	19% 13% 10% 8% 5% 22%	1354
	Night	18% 11% 8% 7% 5% 5% 26%	1369
April	Day	20% 10% 5% 5% 8% 33%	1423
	Night	20% 9% 5% 5% 5% 5% 33%	1183
May	Day	16% 6% 5% 6% 5% 7% 36%	1705
	Night	13% 5% 5% 6% 6% 8% 39%	993
June	Day	18% 7% 5% 5% 6% 5% 6% 8% 28%	1672
	Night	20% 7% 5% 5% 5% 8% 31%	958
July	Day	15% 5% 5% 6% 7% 6% 6% 9% 29%	1711
	Night	12% 5% 6% 5% 5% 6% 9% 37%	980
August	Day	16% 6% 6% 5% 5% 9% 33%	1542
	Night	14% 5% 5% 6% 5% 7% 40%	1168
September	Day	21% 7% 5% 5% 5% 6% 34%	1367
	Night	17% 6% 5% 43%	1236
October	Day	24% 11% 5% 6% 26%	1212
	Night	23% 10% 6% 6% 28%	1482
November	Day	18% 15% 6% 8% 5% 5% 5% 6% 20%	1218
	Night	19% 12% 6% 7% 6% 5% 7% 23%	1950
December	Day	19% 18% 10% 11% 7% 11%	1217
	Night	19% 14% 9% 11% 7% 6% 13%	2130

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	6% 17% 12%	611
	Night	5% 17% 12%	997
February	Day	5% 11% 15%	631
	Night	18% 14%	878
March	Day	14% 24% 10%	870
	Night	15% 22% 12%	926
April	Day	15% 23% 22%	1005
	Night	15% 21% 18%	834
May	Day	18% 24% 17%	1338
	Night	17% 23% 18%	820
June	Day	6% 20% 25% 9%	1252
	Night	6% 19% 31% 9%	722
July	Day	5% 19% 25% 8%	1118
	Night	5% 21% 25% 14%	650
August	Day	12% 26% 15%	1370
	Night	14% 28% 16%	1006
September	Day	10% 27% 17%	1119
	Night	11% 30% 17%	1043
October	Day	10% 20% 11%	1020
	Night	14% 19% 12%	1283
November	Day	5% 19% 15% 7%	918
	Night	15% 21% 6%	1480
December	Day	15% 12%	850
	Night	5% 16% 11%	1504

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	22% 35% 22%	390
February	Day	28% 18% 18% 9%	32
	Night	27% 33% 20%	365
March	Day	22% 27% 22% 8%	113
	Night	24% 27% 23%	446
April	Day	13% 23% 23% 16%	101
	Night	16% 29% 32% 7%	403
May	Day	10% 28% 21% 17% 7%	116
	Night	8% 27% 39% 12%	247
June	Day	20% 33% 23%	103
	Night	6% 17% 48% 15%	205
July	Day	7% 27% 26% 20% 8%	105
	Night	16% 48% 20%	213
August	Day	17% 18% 27% 15%	117
	Night	19% 38% 22%	391
September	Day	5% 16% 18% 18% 19%	104
	Night	7% 20% 32% 22%	427
October	Day	10% 15% 29% 15%	113
	Night	11% 24% 27% 11%	452
November	Day	12% 29% 24% 15%	57
	Night	16% 29% 24% 7%	523
December	Night	23% 32% 19%	542

Page generated at 2009-12-30 05:20:40. Version ab11