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Ann Arbor
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ann Arbor clear sky chart from 2003-08-01 to 2005-11-07.

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	21% 6% 6% 5% 6% 7% 29%	8712
	Night	22% 6% 6% 5% 6% 7% 30%	8457
Transparency	Day	21% 17% 39% 19%	3957
	Night	20% 18% 41% 18%	3978
Seeing	Day	8% 26% 27% 24% 13%	571
	Night	12% 31% 32% 20%	2503

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	30% 10% 9% 6% 5% 6% 15%	506
	Night	32% 9% 8% 6% 5% 7% 17%	847
February	Day	25% 14% 10% 6% 7% 5% 17%	547
	Night	22% 11% 9% 7% 5% 6% 5% 22%	707
March	Day	33% 11% 6% 7% 6% 5% 15%	666
	Night	26% 10% 10% 5% 7% 7% 18%	698
April	Day	22% 5% 5% 6% 6% 5% 32%	750
	Night	24% 5% 5% 6% 36%	571
May	Day	19% 6% 5% 5% 9% 5% 7% 27%	802
	Night	19% 5% 5% 5% 5% 6% 9% 32%	550
June	Day	18% 5% 5% 5% 5% 5% 11% 34%	840
	Night	14% 6% 5% 10% 46%	476
July	Day	17% 5% 5% 8% 7% 5% 9% 31%	850
	Night	22% 5% 5% 11% 32%	512
August	Day	14% 5% 5% 8% 5% 7% 35%	1200
	Night	13% 7% 7% 10% 39%	842
September	Day	11% 5% 5% 8% 48%	982
	Night	13% 7% 54%	960
October	Day	23% 6% 6% 10% 31%	616
	Night	18% 6% 5% 5% 6% 5% 8% 33%	746
November	Day	34% 6% 5% 5% 6% 5% 24%	472
	Night	34% 8% 6% 5% 5% 6% 20%	694
December	Day	34% 7% 7% 5% 5% 19%	481
	Night	29% 8% 6% 6% 5% 5% 7% 19%	854

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	45% 21% 30%	128
	Night	33% 18% 42% 5%	197
February	Day	39% 22% 27% 10%	149
	Night	20% 30% 39% 9%	223
March	Day	36% 26% 27% 9%	152
	Night	33% 19% 40% 6%	228
April	Day	24% 15% 40% 19%	330
	Night	20% 9% 40% 26%	276
May	Day	25% 22% 42% 9%	331
	Night	20% 23% 46% 9%	272
June	Day	19% 14% 45% 19%	439
	Night	20% 14% 48% 16%	293
July	Day	19% 17% 39% 17% 5%	402
	Night	19% 15% 39% 21%	253
August	Day	21% 18% 37% 21%	588
	Night	17% 22% 40% 19%	475
September	Day	11% 12% 42% 31%	652
	Night	10% 14% 36% 33%	669
October	Day	17% 14% 43% 22%	433
	Night	21% 14% 44% 17%	550
November	Day	21% 28% 42% 7%	206
	Night	22% 27% 38% 10%	271
December	Day	25% 25% 27% 21%	147
	Night	31% 19% 37% 11%	271

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	21% 48% 23% 5%	202
February	Night	13% 37% 34% 13%	180
March	Day	14% 38% 29% 17%	34
	Night	20% 45% 29%	197
April	Day	18% 32% 36% 8%	49
	Night	18% 38% 32% 9%	153
May	Day	5% 35% 26% 26% 5%	56
	Night	7% 39% 34% 17%	166
June	Day	13% 27% 28% 22% 7%	104
	Night	26% 41% 26%	165
July	Day	25% 35% 17% 17%	96
	Night	8% 21% 35% 28% 6%	160
August	Day	17% 22% 28% 27%	88
	Night	14% 40% 33% 6%	262
September	Day	20% 15% 39% 20%	83
	Night	15% 31% 43%	313
October	Day	11% 24% 26% 31% 6%	61
	Night	11% 26% 33% 26%	325
November	Night	19% 43% 27% 8%	174
December	Night	19% 46% 26% 7%	206

Page generated at 2008-06-05 23:07:24. Version ab11