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 2009-12-29.

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%	16517
	Night	6% 66%	16445
Transparency	Day	6% 13% 29% 32%	14745
	Night	7% 15% 29% 32%	14849
Seeing	Day	12% 27% 39% 14%	906
	Night	13% 39% 35% 6%	5309

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	6% 5% 6% 63%	1105
	Night	5% 5% 5% 62%	1592
February	Day	7% 66%	1080
	Night	6% 63%	1315
March	Day	6% 64%	1361
	Night	6% 66%	1361
April	Day	5% 74%	1369
	Night	75%	1234
May	Day	75%	1597
	Night	6% 77%	1101
June	Day	5% 6% 66%	1550
	Night	6% 70%	1080
July	Day	8% 5% 5% 6% 7% 6% 8% 9% 35%	1596
	Night	5% 7% 8% 11% 13% 39%	1094
August	Day	5% 7% 8% 7% 7% 8% 40%	1550
	Night	5% 5% 7% 10% 50%	1167
September	Day	5% 6% 64%	1303
	Night	8% 69%	1299
October	Day	5% 80%	1240
	Night	79%	1453
November	Day	7% 70%	1397
	Night	7% 68%	1773
December	Day	6% 5% 6% 62%	1369
	Night	6% 65%	1976

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	5% 21% 27% 30%	985
	Night	8% 17% 24% 31%	1418
February	Day	5% 15% 37% 30%	966
	Night	17% 35% 29%	1182
March	Day	5% 7% 34% 38%	1175
	Night	5% 12% 33% 36%	1192
April	Day	7% 30% 51%	1265
	Night	8% 33% 48%	1106
May	Day	10% 30% 44%	1523
	Night	10% 30% 43%	1060
June	Day	8% 26% 47%	1390
	Night	7% 24% 52%	965
July	Day	10% 12% 20% 18% 8%	1095
	Night	12% 23% 18% 18% 9%	768
August	Day	12% 12% 17% 19% 10%	1301
	Night	11% 14% 20% 20% 13%	1030
September	Day	8% 16% 35% 22%	1228
	Night	8% 24% 34% 19%	1218
October	Day	9% 29% 49%	1177
	Night	9% 26% 50%	1377
November	Day	5% 11% 34% 34%	1356
	Night	14% 30% 34%	1699
December	Day	8% 19% 36% 17%	1284
	Night	6% 20% 37% 19%	1834

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% 43% 24%	484
February	Day	21% 39% 19% 8%	73
	Night	22% 44% 24%	429
March	Day	27% 36% 19% 9%	118
	Night	5% 20% 43% 26%	488
April	Day	31% 36% 22% 6%	114
	Night	25% 47% 19%	396
May	Day	8% 38% 44% 5%	122
	Night	12% 41% 38%	366
June	Day	5% 25% 56% 8%	119
	Night	9% 36% 44%	341
July	Day	16% 49% 21%	106
	Night	21% 60% 12%	327
August	Day	8% 47% 33%	119
	Night	16% 58% 19%	361
September	Day	23% 48% 20%	115
	Night	10% 34% 42% 11%	459
October	Day	10% 30% 55%	20
	Night	11% 43% 33% 7%	484
November	Night	9% 38% 39% 8%	574
December	Night	11% 47% 26% 6%	600

Page generated at 2009-12-30 03:20:43. Version ab11