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Ladd Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ladd Observatory 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	23% 6% 7% 35%	8739
	Night	24% 5% 5% 6% 35%	8435
Transparency	Day	17% 15% 41% 21%	4607
	Night	17% 16% 40% 21%	4245
Seeing	Day	15% 33% 28% 16% 6%	550
	Night	18% 35% 32% 12%	2244

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	24% 5% 5% 38%	490
	Night	26% 5% 5% 39%	863
February	Day	16% 5% 5% 6% 46%	561
	Night	19% 5% 47%	693
March	Day	27% 5% 37%	667
	Night	21% 7% 5% 6% 35%	697
April	Day	26% 6% 5% 40%	740
	Night	31% 5% 5% 35%	580
May	Day	26% 5% 6% 5% 8% 29%	825
	Night	26% 7% 5% 7% 8% 24%	530
June	Day	17% 5% 5% 5% 8% 6% 9% 29%	840
	Night	19% 7% 7% 5% 5% 6% 33%	476
July	Day	20% 5% 10% 6% 5% 9% 27%	866
	Night	22% 5% 5% 5% 7% 5% 6% 9% 23%	496
August	Day	26% 5% 7% 7% 28%	1169
	Night	26% 5% 6% 5% 6% 6% 8% 24%	870
September	Day	21% 5% 6% 43%	1001
	Night	21% 5% 5% 6% 41%	944
October	Day	28% 6% 7% 32%	612
	Night	36% 6% 6% 29%	749
November	Day	16% 6% 6% 6% 41%	475
	Night	21% 5% 5% 6% 40%	692
December	Day	24% 6% 5% 35%	493
	Night	22% 6% 5% 5% 5% 39%	845

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	21% 14% 40% 19%	247
	Night	17% 14% 44% 20%	399
February	Day	17% 12% 41% 25%	335
	Night	14% 12% 49% 18% 5%	400
March	Day	24% 16% 46% 10%	329
	Night	22% 19% 40% 15%	346
April	Day	15% 10% 36% 31% 5%	385
	Night	12% 16% 40% 28%	264
May	Day	27% 20% 35% 14%	378
	Night	30% 18% 26% 23%	204
June	Day	16% 12% 47% 22%	406
	Night	14% 14% 37% 30%	222
July	Day	21% 15% 35% 22%	409
	Night	33% 9% 40% 13%	211
August	Day	19% 23% 34% 16% 5%	520
	Night	24% 20% 34% 15%	338
September	Day	6% 11% 40% 36%	582
	Night	6% 15% 47% 25%	518
October	Day	14% 13% 53% 18%	441
	Night	16% 16% 46% 20%	504
November	Day	16% 18% 37% 19% 8%	312
	Night	15% 25% 28% 27%	410
December	Day	20% 20% 49% 7%	263
	Night	15% 17% 40% 22%	429

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	34% 45% 18%	179
February	Day	9% 70% 18%	44
	Night	17% 53% 20% 7%	181
March	Day	21% 41% 28% 8%	46
	Night	23% 39% 32%	186
April	Day	22% 36% 25% 11%	44
	Night	15% 46% 33%	159
May	Day	11% 32% 37% 16%	43
	Night	14% 40% 37% 7%	129
June	Day	9% 26% 50% 13%	53
	Night	11% 32% 50%	104
July	Day	6% 25% 27% 31% 10%	48
	Night	14% 29% 34% 20%	107
August	Day	14% 20% 28% 25% 10%	74
	Night	14% 26% 33% 23%	258
September	Day	14% 23% 28% 18% 15%	71
	Night	11% 20% 42% 24%	302
October	Day	21% 37% 11% 17% 11%	69
	Night	16% 26% 29% 22%	260
November	Day	20% 32% 33% 13%	53
	Night	14% 44% 32% 7%	188
December	Day	20% 60% 20%	5
	Night	29% 42% 24%	191

Page generated at 2008-08-20 18:20:59. Version ab11