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Hampden-Sydney College Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hampden-Sydney College 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	21% 6% 5% 7% 35%	8700
	Night	19% 5% 5% 7% 37%	8471
Transparency	Day	17% 16% 35% 27%	4464
	Night	15% 13% 38% 28%	4629
Seeing	Day	11% 23% 29% 25% 11%	525
	Night	14% 27% 34% 21%	2540

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	18% 5% 6% 42%	551
	Night	17% 7% 44%	802
February	Day	23% 5% 5% 39%	540
	Night	19% 7% 5% 7% 39%	713
March	Day	20% 6% 6% 7% 38%	682
	Night	18% 6% 5% 5% 6% 36%	682
April	Day	20% 6% 44%	716
	Night	23% 6% 7% 6% 5% 36%	604
May	Day	17% 6% 5% 5% 8% 37%	809
	Night	18% 5% 5% 6% 6% 7% 10% 31%	545
June	Day	21% 5% 7% 5% 5% 10% 5% 8% 20%	838
	Night	18% 5% 5% 6% 5% 7% 11% 28%	478
July	Day	17% 5% 7% 5% 6% 7% 5% 7% 10% 22%	804
	Night	19% 5% 5% 5% 6% 6% 6% 6% 7% 28%	558
August	Day	21% 5% 7% 5% 7% 11% 26%	1164
	Night	16% 6% 6% 8% 5% 7% 10% 26%	875
September	Day	25% 5% 7% 39%	1009
	Night	20% 5% 5% 7% 42%	936
October	Day	25% 7% 5% 5% 6% 34%	618
	Night	24% 5% 6% 6% 6% 34%	744
November	Day	22% 5% 6% 41%	474
	Night	21% 6% 5% 5% 39%	691
December	Day	18% 5% 5% 48%	495
	Night	16% 49%	843

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	16% 14% 44% 22%	268
	Night	21% 12% 42% 21%	435
February	Day	12% 11% 29% 42%	265
	Night	11% 12% 42% 27% 6%	362
March	Day	17% 21% 36% 24%	341
	Night	12% 10% 38% 35%	326
April	Day	12% 11% 36% 33% 6%	400
	Night	11% 6% 36% 37% 7%	293
May	Day	31% 15% 37% 13%	403
	Night	36% 10% 32% 17%	273
June	Day	37% 15% 32% 13%	301
	Night	33% 18% 30% 14%	225
July	Day	25% 19% 37% 17%	308
	Night	20% 17% 41% 20%	245
August	Day	18% 27% 35% 17%	544
	Night	20% 27% 36% 15%	421
September	Day	8% 9% 32% 41% 8%	545
	Night	10% 12% 33% 34% 8%	547
October	Day	14% 14% 38% 27% 5%	472
	Night	8% 12% 47% 26%	582
November	Day	11% 18% 30% 37%	332
	Night	8% 10% 38% 35% 6%	437
December	Day	7% 11% 34% 42%	285
	Night	10% 11% 31% 42%	483

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% 49% 25%	206
February	Day	30% 15% 53%	13
	Night	22% 39% 34%	187
March	Day	14% 40% 36% 8%	49
	Night	24% 44% 27%	205
April	Day	14% 40% 26% 10% 8%	49
	Night	18% 37% 36% 7%	194
May	Day	8% 35% 28% 26%	56
	Night	9% 23% 46% 20%	158
June	Day	13% 11% 37% 29% 7%	51
	Night	5% 16% 46% 28%	107
July	Day	5% 9% 31% 31% 21%	51
	Night	8% 6% 36% 45%	163
August	Day	7% 16% 15% 28% 32%	78
	Night	8% 10% 25% 48% 6%	293
September	Day	11% 16% 26% 31% 13%	72
	Night	8% 19% 36% 33%	308
October	Day	12% 20% 33% 27% 5%	72
	Night	12% 24% 39% 23%	295
November	Day	5% 26% 32% 35%	34
	Night	14% 25% 34% 25%	214
December	Night	18% 41% 30% 8%	210

Page generated at 2008-09-30 05:41:12. Version ab11