Nanjemoy Creek Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Nanjemoy Creek 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	20% 6% 5% 7% 36%	8721
	Night	19% 6% 5% 6% 36%	8450
Transparency	Day	17% 15% 39% 25%	4659
	Night	16% 13% 40% 26%	4551
Seeing	Day	7% 30% 29% 25% 7%	527
	Night	12% 31% 34% 19%	2396

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	22% 6% 6% 7% 38%	539
	Night	19% 5% 5% 41%	814
February	Day	20% 5% 6% 9% 40%	537
	Night	20% 6% 40%	717
March	Day	17% 5% 6% 6% 6% 7% 5% 6% 32%	682
	Night	21% 7% 5% 6% 37%	681
April	Day	22% 6% 7% 40%	720
	Night	24% 5% 5% 5% 36%	600
May	Day	16% 5% 5% 5% 8% 38%	827
	Night	15% 5% 8% 7% 7% 32%	526
June	Day	15% 5% 5% 8% 5% 8% 10% 29%	840
	Night	14% 5% 6% 8% 6% 9% 10% 27%	476
July	Day	19% 5% 5% 5% 5% 9% 5% 6% 8% 25%	832
	Night	18% 6% 5% 9% 6% 7% 8% 25%	530
August	Day	21% 5% 5% 6% 8% 32%	1156
	Night	18% 5% 5% 7% 5% 10% 27%	882
September	Day	22% 7% 40%	1018
	Night	16% 5% 5% 6% 40%	927
October	Day	25% 7% 6% 38%	618
	Night	26% 5% 38%	744
November	Day	21% 6% 8% 39%	466
	Night	24% 5% 7% 38%	701
December	Day	19% 6% 45%	486
	Night	16% 6% 5% 6% 43%	852

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	28% 13% 42% 13%	263
	Night	21% 12% 46% 18%	411
February	Day	15% 14% 37% 31%	311
	Night	11% 10% 44% 26% 6%	368
March	Day	18% 23% 35% 20%	306
	Night	12% 10% 42% 33%	331
April	Day	14% 5% 39% 32% 7%	385
	Night	7% 8% 44% 33% 5%	299
May	Day	18% 15% 44% 19%	419
	Night	32% 10% 38% 15%	255
June	Day	27% 15% 40% 15%	408
	Night	25% 11% 48% 12%	224
July	Day	23% 13% 44% 14%	343
	Night	19% 17% 40% 19%	246
August	Day	17% 20% 38% 21%	557
	Night	21% 25% 38% 13%	402
September	Day	10% 14% 30% 36% 7%	585
	Night	11% 11% 31% 37% 7%	513
October	Day	11% 12% 42% 31%	461
	Night	17% 9% 35% 35%	568
November	Day	11% 22% 37% 28%	336
	Night	11% 15% 42% 28%	442
December	Day	17% 10% 42% 26%	285
	Night	14% 16% 37% 28%	492

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% 45% 25%	199
February	Day	31% 36% 31%	19
	Night	15% 46% 35%	180
March	Day	15% 32% 39% 13%	53
	Night	20% 43% 31% 5%	195
April	Day	8% 45% 33% 6% 6%	48
	Night	19% 37% 33% 9%	183
May	Day	7% 32% 33% 25%	56
	Night	7% 33% 41% 17%	137
June	Day	33% 30% 22% 13%	53
	Night	26% 53% 17%	103
July	Day	12% 27% 50% 8%	48
	Night	15% 44% 31% 5%	135
August	Day	27% 21% 35% 13%	73
	Night	20% 30% 40%	285
September	Day	5% 24% 24% 30% 15%	73
	Night	24% 35% 32%	302
October	Day	36% 24% 27% 7%	66
	Night	14% 27% 29% 23%	282
November	Day	15% 26% 34% 23%	38
	Night	15% 26% 33% 24%	194
December	Night	21% 41% 30% 5%	201

Page generated at 2008-05-11 04:45:53. Version ab11