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 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	23% 8% 6% 5% 6% 30%	16454
	Night	24% 7% 5% 5% 34%	16504
Cloud Cover	Day	24% 8% 5% 5% 5% 30%	16461
	Night	25% 7% 5% 34%	16500
Transparency	Day	5% 15% 20% 11%	12187
	Night	6% 17% 21% 11%	12520
Seeing	Day	13% 26% 30% 13%	998
	Night	7% 18% 36% 17%	4597

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	35% 10% 7% 6% 20%	1012
	Night	33% 8% 6% 5% 23%	1684
February	Day	28% 10% 8% 9% 6% 6% 15%	994
	Night	28% 10% 5% 5% 6% 25%	1400
March	Day	35% 11% 5% 8% 6% 5% 5% 7%	1347
	Night	32% 10% 6% 8% 6% 5% 13%	1375
April	Day	25% 10% 6% 7% 6% 5% 7% 17%	1451
	Night	24% 12% 5% 5% 5% 6% 26%	1148
May	Day	20% 6% 5% 5% 5% 8% 31%	1716
	Night	19% 5% 5% 5% 6% 38%	983
June	Day	20% 7% 6% 6% 6% 5% 6% 30%	1669
	Night	16% 6% 5% 5% 5% 40%	960
July	Day	10% 6% 59%	1719
	Night	5% 69%	971
August	Day	11% 6% 7% 48%	1582
	Night	6% 5% 68%	1136
September	Day	9% 6% 53%	1366
	Night	9% 5% 61%	1237
October	Day	25% 8% 5% 8% 7% 5% 24%	1209
	Night	20% 7% 5% 5% 6% 37%	1484
November	Day	34% 10% 5% 5% 5% 5% 6% 14%	1173
	Night	33% 8% 5% 5% 25%	1997
December	Day	37% 12% 5% 5% 16%	1216
	Night	35% 10% 5% 5% 21%	2129

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	38% 9% 6% 5% 21%	1013
	Night	35% 8% 5% 24%	1681
February	Day	30% 10% 7% 7% 6% 5% 16%	996
	Night	30% 8% 5% 6% 25%	1399
March	Day	37% 11% 5% 7% 7% 5% 8%	1349
	Night	32% 12% 5% 7% 6% 5% 14%	1373
April	Day	26% 10% 5% 6% 5% 6% 6% 18%	1452
	Night	26% 11% 5% 5% 6% 26%	1150
May	Day	21% 6% 5% 5% 5% 7% 32%	1716
	Night	20% 5% 7% 38%	983
June	Day	20% 6% 6% 6% 5% 5% 6% 31%	1669
	Night	17% 6% 5% 5% 6% 40%	960
July	Day	10% 6% 59%	1719
	Night	5% 5% 69%	973
August	Day	11% 6% 7% 48%	1583
	Night	6% 5% 68%	1136
September	Day	10% 6% 53%	1364
	Night	9% 5% 61%	1237
October	Day	27% 9% 5% 6% 7% 25%	1209
	Night	21% 6% 5% 7% 36%	1483
November	Day	36% 9% 5% 6% 5% 5% 5% 14%	1175
	Night	35% 8% 6% 24%	1997
December	Day	39% 12% 17%	1216
	Night	37% 10% 5% 22%	2128

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	11% 15% 8%	617
	Night	7% 14% 15% 11%	1077
February	Day	9% 10% 16% 6%	656
	Night	6% 19% 19% 9%	959
March	Day	6% 18% 7%	788
	Night	10% 19% 10%	855
April	Day	9% 17% 14% 6%	939
	Night	9% 17% 20% 8%	766
May	Day	6% 20% 26% 9%	1273
	Night	5% 27% 23% 8%	770
June	Day	6% 25% 19% 7%	1195
	Night	6% 24% 25% 9%	716
July	Day	13% 28% 27% 10%	1189
	Night	17% 30% 23% 15%	714
August	Day	14% 28% 18%	1494
	Night	18% 42% 16%	1098
September	Day	16% 25% 16% 10%	1268
	Night	15% 29% 16% 12%	1163
October	Day	11% 14% 8%	1055
	Night	7% 15% 19% 11%	1350
November	Day	6% 15% 12% 5%	852
	Night	5% 15% 18% 8%	1505
December	Day	5% 9% 16% 6%	861
	Night	6% 13% 13% 10%	1547

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	7% 25% 22% 22%	27
	Night	9% 22% 30% 12%	422
February	Day	12% 23% 28% 15%	94
	Night	8% 22% 35% 14%	361
March	Day	25% 21% 19% 5%	108
	Night	13% 18% 33% 11%	416
April	Day	21% 28% 31%	105
	Night	10% 19% 35% 15%	392
May	Day	21% 28% 28% 7%	114
	Night	6% 24% 41% 14%	230
June	Day	11% 29% 30% 11%	109
	Night	5% 17% 50% 14%	213
July	Day	31% 43% 13%	103
	Night	18% 58% 17%	213
August	Day	6% 29% 33% 21%	119
	Night	10% 45% 31%	379
September	Day	6% 25% 33% 23%	116
	Night	12% 41% 32%	456
October	Day	8% 24% 28% 15%	103
	Night	15% 34% 21%	474
November	Night	9% 20% 28% 16%	508
December	Night	9% 21% 27% 9%	533

Page generated at 2009-12-30 02:20:39. Version ab11