Jarnac Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Jarnac Observatory clear sky chart from 2005-11-05 to 2008-11-27.

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	6% 63%	12280
	Night	7% 65%	12151
Transparency	Day	6% 13% 30% 34%	10509
	Night	7% 15% 31% 33%	10550
Seeing	Day	12% 30% 37% 13%	672
	Night	14% 38% 35% 7%	3899

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	5% 5% 7% 61%	835
	Night	6% 6% 5% 58%	1203
February	Day	5% 7% 64%	802
	Night	6% 63%	977
March	Day	6% 64%	1020
	Night	6% 68%	1020
April	Day	5% 75%	1042
	Night	75%	934
May	Day	79%	1206
	Night	5% 80%	839
June	Day	5% 5% 71%	1170
	Night	6% 73%	810
July	Day	10% 5% 5% 6% 8% 6% 7% 8% 33%	1208
	Night	5% 7% 8% 12% 14% 35%	833
August	Day	7% 5% 8% 9% 7% 8% 9% 32%	1160
	Night	6% 5% 5% 8% 12% 42%	875
September	Day	5% 7% 65%	985
	Night	7% 69%	990
October	Day	6% 78%	935
	Night	79%	1098
November	Day	7% 71%	1080
	Night	7% 69%	1365
December	Day	6% 7% 68%	837
	Night	6% 68%	1207

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	7% 24% 29% 25%	715
	Night	5% 10% 20% 25% 26%	1028
February	Day	7% 17% 38% 27%	688
	Night	5% 18% 38% 27%	844
March	Day	6% 9% 37% 38%	843
	Night	14% 36% 36%	851
April	Day	7% 29% 54%	939
	Night	8% 31% 51%	807
May	Day	10% 31% 47%	1125
	Night	12% 32% 43%	793
June	Day	7% 22% 59%	1010
	Night	21% 64%	695
July	Day	12% 12% 21% 17% 8%	707
	Night	13% 23% 16% 18% 8%	507
August	Day	15% 14% 19% 15%	911
	Night	14% 18% 20% 17% 6%	738
September	Day	8% 15% 37% 23%	910
	Night	8% 21% 37% 20%	909
October	Day	9% 26% 52%	870
	Night	5% 6% 26% 54%	1022
November	Day	5% 10% 37% 33%	1039
	Night	15% 32% 33%	1291
December	Day	5% 20% 41% 23%	752
	Night	5% 20% 43% 21%	1065

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	24% 42% 21%	364
February	Day	18% 43% 20% 9%	53
	Night	23% 38% 28%	317
March	Day	29% 39% 20% 6%	87
	Night	5% 20% 45% 26%	364
April	Day	33% 40% 18% 6%	87
	Night	26% 46% 20%	298
May	Day	8% 45% 38%	91
	Night	14% 41% 36%	279
June	Day	5% 24% 58% 10%	89
	Night	9% 32% 49% 5%	253
July	Day	21% 47% 18%	76
	Night	24% 57% 12%	240
August	Day	9% 44% 32%	88
	Night	15% 55% 22%	267
September	Day	26% 43% 23%	86
	Night	11% 33% 42% 11%	347
October	Day	20% 73%	15
	Night	10% 42% 35% 8%	364
November	Night	10% 36% 39% 9%	442
December	Night	11% 46% 28% 6%	364

Page generated at 2008-11-27 11:47:39. Version ab11