The Rolnick Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Rolnick 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	22% 5% 5% 7% 36%	8736
	Night	24% 5% 6% 36%	8438
Transparency	Day	18% 16% 39% 23%	4577
	Night	17% 17% 39% 22%	4394
Seeing	Day	15% 32% 29% 16% 5%	545
	Night	17% 37% 30% 12%	2308

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	25% 5% 7% 38%	499
	Night	27% 7% 5% 38%	854
February	Day	16% 6% 5% 6% 45%	555
	Night	16% 6% 7% 45%	699
March	Day	29% 5% 5% 33%	672
	Night	26% 6% 5% 7% 31%	692
April	Day	22% 5% 42%	728
	Night	28% 5% 5% 36%	592
May	Day	17% 5% 6% 5% 5% 9% 36%	837
	Night	27% 5% 8% 29%	518
June	Day	15% 5% 6% 6% 6% 8% 36%	840
	Night	17% 5% 5% 8% 8% 33%	476
July	Day	18% 5% 5% 7% 6% 6% 9% 29%	866
	Night	20% 5% 5% 5% 6% 8% 31%	496
August	Day	24% 5% 6% 5% 9% 28%	1158
	Night	25% 5% 5% 5% 5% 31%	881
September	Day	25% 6% 5% 39%	1014
	Night	25% 6% 5% 42%	931
October	Day	25% 5% 6% 37%	611
	Night	30% 7% 35%	751
November	Day	20% 8% 5% 5% 8% 36%	470
	Night	23% 5% 8% 37%	696
December	Day	21% 5% 5% 5% 37%	486
	Night	21% 5% 5% 41%	852

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	23% 18% 38% 14% 5%	240
	Night	19% 15% 46% 18%	385
February	Day	18% 8% 41% 28%	312
	Night	17% 8% 46% 20% 6%	407
March	Day	17% 18% 46% 17%	291
	Night	22% 18% 41% 16%	329
April	Day	16% 13% 33% 30% 6%	409
	Night	18% 13% 35% 27% 5%	276
May	Day	27% 18% 41% 12%	417
	Night	24% 20% 34% 18%	225
June	Day	18% 9% 50% 20%	448
	Night	9% 16% 41% 29%	224
July	Day	19% 18% 37% 18% 5%	377
	Night	28% 14% 33% 20%	242
August	Day	21% 24% 30% 22%	526
	Night	18% 20% 40% 14% 6%	372
September	Day	7% 12% 30% 46%	542
	Night	7% 15% 35% 37%	503
October	Day	19% 14% 46% 19%	461
	Night	16% 21% 38% 22%	559
November	Day	18% 22% 39% 15%	309
	Night	18% 24% 32% 23%	434
December	Day	26% 16% 41% 13%	245
	Night	19% 16% 40% 20%	438

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	44% 36% 14%	184
February	Day	10% 75% 13%	37
	Night	13% 60% 17% 8%	191
March	Day	23% 36% 34%	46
	Night	21% 50% 27%	190
April	Day	18% 43% 20% 13%	44
	Night	18% 44% 26% 10%	176
May	Day	15% 39% 31% 9%	51
	Night	12% 36% 37% 12%	124
June	Day	7% 40% 28% 23%	52
	Night	5% 33% 55%	106
July	Day	10% 28% 33% 17% 8%	56
	Night	30% 42% 20%	101
August	Day	12% 15% 33% 27% 11%	72
	Night	10% 23% 34% 26%	267
September	Day	11% 24% 28% 24% 11%	70
	Night	7% 27% 42% 21%	288
October	Day	19% 30% 28% 12% 8%	71
	Night	16% 33% 27% 16% 5%	283
November	Day	23% 13% 39% 21%	46
	Night	23% 33% 31% 10%	200
December	Night	28% 41% 24% 5%	198

Page generated at 2008-10-12 20:06:18. Version ab11