Preston
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Preston clear sky chart from 2005-11-05 to 2008-11-20.

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	18% 5% 5% 7% 37%	12205
	Night	19% 6% 7% 37%	12044
Cloud Cover	Day	18% 5% 5% 5% 7% 37%	12220
	Night	18% 6% 5% 7% 37%	12071
Transparency	Day	9% 18% 25% 17%	8673
	Night	8% 18% 26% 17%	8699
Seeing	Day	12% 33% 27% 11%	806
	Night	15% 39% 26% 6%	3432

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	20% 6% 5% 5% 7% 35%	740
	Night	19% 6% 5% 5% 8% 36%	1300
February	Day	18% 6% 5% 5% 5% 5% 7% 35%	799
	Night	18% 6% 5% 8% 38%	990
March	Day	15% 6% 5% 5% 43%	978
	Night	16% 5% 5% 5% 7% 43%	1018
April	Day	22% 7% 39%	1102
	Night	21% 8% 5% 6% 37%	875
May	Day	22% 5% 8% 35%	1249
	Night	20% 7% 8% 36%	800
June	Day	19% 5% 5% 6% 7% 9% 29%	1251
	Night	22% 5% 5% 5% 5% 9% 28%	718
July	Day	16% 5% 6% 6% 6% 5% 9% 32%	1301
	Night	15% 6% 6% 6% 7% 5% 8% 9% 25%	740
August	Day	12% 5% 6% 6% 5% 5% 8% 38%	1160
	Night	12% 6% 5% 5% 6% 5% 5% 8% 37%	873
September	Day	16% 5% 51%	1021
	Night	18% 6% 6% 45%	956
October	Day	15% 5% 5% 5% 6% 43%	915
	Night	16% 6% 7% 42%	1119
November	Day	23% 6% 5% 36%	939
	Night	26% 8% 5% 34%	1364
December	Day	20% 6% 5% 5% 6% 38%	750
	Night	18% 5% 7% 39%	1291

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	21% 6% 6% 6% 35%	739
	Night	20% 5% 8% 36%	1301
February	Day	17% 6% 5% 5% 6% 5% 7% 35%	796
	Night	17% 7% 5% 5% 5% 6% 39%	982
March	Day	14% 5% 6% 5% 5% 43%	999
	Night	16% 5% 5% 5% 43%	1042
April	Day	21% 5% 9% 38%	1101
	Night	21% 8% 5% 5% 37%	875
May	Day	21% 5% 5% 5% 8% 35%	1248
	Night	19% 7% 5% 8% 36%	800
June	Day	18% 6% 5% 6% 5% 7% 10% 29%	1260
	Night	21% 6% 5% 5% 6% 5% 10% 28%	720
July	Day	15% 5% 5% 7% 5% 6% 9% 31%	1301
	Night	13% 6% 7% 7% 8% 7% 7% 8% 25%	741
August	Day	11% 5% 6% 7% 5% 5% 5% 6% 38%	1154
	Night	11% 5% 6% 5% 7% 6% 5% 8% 36%	876
September	Day	15% 5% 49%	1020
	Night	18% 5% 5% 45%	957
October	Day	15% 5% 5% 6% 43%	915
	Night	15% 5% 5% 7% 41%	1119
November	Day	22% 6% 5% 36%	937
	Night	26% 7% 5% 34%	1363
December	Day	20% 6% 5% 6% 38%	750
	Night	18% 6% 5% 7% 38%	1295

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	10% 19% 30% 15%	494
	Night	9% 20% 29% 15%	882
February	Day	8% 23% 23% 15%	559
	Night	8% 19% 27% 16%	715
March	Day	7% 18% 30% 20%	713
	Night	7% 17% 32% 19%	761
April	Day	10% 17% 25% 16% 9%	753
	Night	8% 12% 25% 22% 5%	596
May	Day	11% 18% 27% 15%	865
	Night	10% 20% 22% 15%	560
June	Day	18% 27% 25% 9%	798
	Night	16% 29% 23% 9%	425
July	Day	12% 20% 29% 12%	672
	Night	13% 20% 24% 8%	379
August	Day	7% 13% 22% 19%	968
	Night	9% 13% 27% 15%	736
September	Day	6% 9% 23% 31%	863
	Night	5% 9% 22% 30%	794
October	Day	6% 15% 25% 17%	794
	Night	6% 17% 23% 17%	977
November	Day	9% 18% 23% 14%	674
	Night	5% 18% 27% 13%	949
December	Day	9% 27% 24% 13%	520
	Night	9% 25% 24% 14%	925

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	25% 44% 21%	320
February	Day	21% 43% 21%	64
	Night	27% 44% 16%	298
March	Day	20% 39% 25%	88
	Night	21% 46% 19%	342
April	Day	18% 37% 17% 11%	79
	Night	16% 40% 26%	291
May	Day	10% 37% 28% 12%	82
	Night	13% 38% 31% 6%	214
June	Day	8% 26% 43% 10%	73
	Night	7% 38% 36% 10%	144
July	Day	7% 33% 26% 19%	71
	Night	34% 44% 10%	155
August	Day	5% 22% 35% 12% 11%	87
	Night	5% 29% 38% 14%	293
September	Day	9% 28% 22% 17% 7%	85
	Night	28% 37% 15%	327
October	Day	7% 28% 27% 17%	84
	Night	11% 37% 25% 9%	349
November	Day	12% 37% 26% 7%	88
	Night	15% 35% 24% 7%	364
December	Day	60% 40%	5
	Night	19% 50% 16%	335

Page generated at 2008-11-21 00:15:50. Version ab11