Portland
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Portland 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	23% 7% 5% 6% 5% 6% 29%	12248
	Night	24% 8% 5% 5% 33%	12180
Cloud Cover	Day	24% 8% 5% 5% 5% 30%	12255
	Night	26% 7% 5% 34%	12176
Transparency	Day	6% 19% 20% 12%	7982
	Night	6% 20% 22% 12%	8191
Seeing	Day	13% 28% 31% 13%	726
	Night	8% 19% 38% 18%	3261

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	37% 11% 7% 5% 18%	764
	Night	33% 9% 7% 6% 5% 19%	1274
February	Day	29% 9% 8% 8% 7% 6% 15%	738
	Night	32% 10% 5% 5% 23%	1040
March	Day	33% 11% 7% 9% 7% 7% 5% 5%	1008
	Night	34% 9% 7% 8% 5% 5% 5% 11%	1032
April	Day	25% 9% 7% 6% 6% 6% 7% 16%	1104
	Night	22% 13% 5% 7% 6% 5% 23%	868
May	Day	22% 7% 5% 6% 6% 6% 8% 27%	1296
	Night	20% 5% 5% 6% 6% 35%	749
June	Day	18% 6% 6% 5% 6% 5% 7% 31%	1260
	Night	16% 6% 5% 5% 5% 5% 41%	720
July	Day	11% 6% 59%	1301
	Night	5% 68%	739
August	Day	10% 5% 5% 7% 50%	1184
	Night	7% 70%	852
September	Day	7% 5% 5% 6% 55%	1033
	Night	8% 5% 61%	944
October	Day	24% 8% 5% 8% 6% 5% 26%	912
	Night	19% 7% 5% 7% 39%	1121
November	Day	34% 10% 5% 6% 5% 5% 16%	904
	Night	33% 8% 5% 5% 27%	1540
December	Day	40% 12% 6% 5% 5% 11%	744
	Night	37% 11% 5% 5% 5% 5% 5% 15%	1301

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	41% 9% 6% 6% 19%	765
	Night	36% 9% 5% 5% 5% 20%	1271
February	Day	32% 9% 7% 7% 6% 6% 16%	740
	Night	35% 8% 5% 5% 23%	1039
March	Day	36% 10% 5% 8% 8% 7% 5% 6%	1010
	Night	35% 11% 6% 7% 5% 5% 12%	1030
April	Day	26% 11% 5% 6% 5% 6% 6% 17%	1105
	Night	25% 11% 5% 5% 5% 5% 5% 23%	870
May	Day	23% 7% 5% 5% 5% 5% 5% 7% 27%	1296
	Night	22% 5% 5% 5% 8% 35%	749
June	Day	20% 5% 6% 5% 5% 6% 32%	1260
	Night	18% 5% 5% 5% 6% 41%	720
July	Day	11% 6% 59%	1301
	Night	5% 5% 68%	741
August	Day	10% 6% 5% 7% 50%	1185
	Night	7% 71%	852
September	Day	9% 5% 6% 55%	1031
	Night	9% 6% 61%	944
October	Day	27% 9% 5% 6% 28%	912
	Night	20% 6% 5% 7% 38%	1120
November	Day	36% 10% 5% 5% 5% 16%	906
	Night	35% 8% 5% 27%	1540
December	Day	42% 13% 5% 5% 13%	744
	Night	41% 10% 5% 5% 5% 15%	1300

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	16% 21% 7%	369
	Night	8% 20% 16% 10%	666
February	Day	12% 13% 17% 10%	400
	Night	8% 20% 17% 13%	599
March	Day	10% 23%	458
	Night	13% 23% 9%	512
April	Day	13% 21% 13%	594
	Night	12% 21% 20% 5%	486
May	Day	7% 24% 20% 12%	845
	Night	6% 33% 18% 8% 5%	532
June	Day	8% 31% 22% 6%	786
	Night	5% 31% 26% 10% 5%	476
July	Day	16% 29% 28% 9%	771
	Night	21% 28% 23% 13%	482
August	Day	15% 28% 19%	1096
	Night	18% 40% 18%	814
September	Day	16% 25% 17% 13%	935
	Night	17% 27% 18% 13%	870
October	Day	12% 14% 12%	756
	Night	8% 14% 18% 14% 6%	987
November	Day	6% 17% 13% 6%	583
	Night	17% 22% 11%	1048
December	Day	8% 17% 16%	389
	Night	9% 19% 14%	719

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	Day	5% 31% 21% 21% 5%	19
	Night	12% 26% 30% 11%	302
February	Day	13% 28% 25% 15%	66
	Night	10% 23% 36% 12%	249
March	Day	28% 27% 20% 5%	77
	Night	14% 19% 38% 10%	292
April	Day	25% 26% 28%	78
	Night	11% 22% 37% 15%	286
May	Day	18% 31% 27% 9%	83
	Night	8% 23% 42% 15%	170
June	Day	13% 27% 32% 11%	79
	Night	5% 18% 53% 12%	155
July	Day	6% 28% 50% 8%	73
	Night	14% 65% 16%	155
August	Day	32% 36% 20%	88
	Night	10% 46% 29%	281
September	Day	6% 28% 33% 22%	87
	Night	11% 41% 33%	344
October	Day	7% 22% 27% 17%	76
	Night	15% 32% 25%	354
November	Night	10% 21% 28% 16%	376
December	Night	12% 24% 28% 7%	297

Page generated at 2008-11-27 10:47:48. Version ab11