Portland
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Portland clear sky chart from 2005-11-06 to 2024-11-06.

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	19% 8% 3 7% 5% 4 4 3 5% 6% 36%	105465
	Night	21% 9% 3 7% 4 4 4 3 4 5% 36%	102280
Transparency	Day	50% 4 12% 17% 12% 4	105208
	Night	52% 4 13% 18% 10% 3	102126
Seeing	Day	22% 9% 20% 27% 17% 5%	14630
	Night	25% 6% 18% 33% 16% 1	23554

By Month

Percentage Cloud Cover SW (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	37% 15% 4 10% 6% 5% 5% 3 4 4 7%	2539
	11th to 20th	Day	39% 14% 3 9% 5% 3 3 2 2 3 16%	2540
	21st to end	Day	34% 13% 4 11% 6% 4 3 3 4 4 14%	3047
February	1st to 10th	Day	31% 12% 4 9% 5% 4 5% 3 6% 6% 15%	1710
	11th to 20th	Day	29% 15% 5% 10% 4 5% 4 2 4 4 17%	1665
	21st to end	Day	31% 13% 4 9% 6% 4 3 3 3 5% 18%	1490
March	1st to 10th	Day	20% 13% 5% 11% 7% 6% 5% 4 6% 6% 16%	3283
	11th to 20th	Day	20% 9% 3 7% 5% 5% 4 3 4 5% 35%	3490
	21st to end	Day	25% 13% 4 11% 6% 5% 5% 4 5% 6% 16%	3912
April	1st to 10th	Day	28% 12% 3 9% 6% 5% 4 3 5% 6% 18%	2280
	11th to 20th	Day	24% 11% 4 9% 5% 4 4 3 5% 5% 25%	2249
	21st to end	Day	22% 10% 3 8% 6% 5% 6% 5% 5% 6% 23%	2460
May	1st to 10th	Day	21% 7% 3 7% 5% 5% 4 3 5% 5% 34%	4371
	11th to 20th	Day	12% 6% 2 6% 4 5% 5% 4 6% 7% 43%	4377
	21st to end	Day	24% 8% 3 8% 6% 6% 6% 4 5% 6% 24%	4813
June	1st to 10th	Day	22% 8% 3 7% 5% 4 5% 4 6% 6% 31%	2660
	11th to 20th	Day	20% 9% 3 8% 6% 5% 5% 5% 6% 6% 27%	2660
	21st to end	Day	9% 6% 2 6% 4 4 4 3 5% 7% 50%	2660
July	1st to 10th	Day	5% 3 1 3 3 2 3 2 4 5% 69%	4420
	11th to 20th	Day	4 3 1 2 2 2 2 2 3 4 74%	4420
	21st to end	Day	6% 3 1 3 2 2 3 2 3 6% 69%	4790
August	1st to 10th	Day	4 3 1 4 3 3 3 3 4 5% 66%	4137
	11th to 20th	Day	6% 4 2 5% 4 4 5% 4 6% 8% 52%	4026
	21st to end	Day	11% 6% 2 5% 4 3 4 3 4 7% 52%	4304
September	1st to 10th	Day	8% 4 2 4 3 3 3 3 4 7% 59%	2280
	11th to 20th	Day	13% 8% 3 6% 4 4 5% 3 6% 7% 42%	2280
	21st to end	Day	15% 6% 3 6% 4 4 4 3 4 6% 44%	2000
October	1st to 10th	Day	17% 6% 2 5% 4 4 4 3 5% 5% 44%	3180
	11th to 20th	Day	19% 8% 3 9% 5% 5% 5% 4 6% 7% 28%	3161
	21st to end	Day	27% 9% 4 9% 6% 5% 4 3 5% 7% 21%	3361
November	1st to 10th	Day	31% 12% 5% 10% 5% 5% 4 4 5% 5% 15%	1618
	11th to 20th	Day	31% 11% 5% 9% 5% 3 3 3 4 5% 19%	1520
	21st to end	Day	30% 11% 4 8% 6% 4 4 3 4 5% 22%	1520
December	1st to 10th	Day	29% 14% 3 9% 5% 3 3 3 3 5% 23%	1520
	11th to 20th	Day	31% 14% 4 7% 6% 4 4 3 5% 6% 16%	1928
	21st to end	Day	31% 15% 4 12% 7% 7% 5% 3 4 4 9%	2794
January	1st to 10th	Night	35% 15% 3 9% 6% 5% 5% 3 4 4 11%	4406
	11th to 20th	Night	38% 14% 3 8% 5% 3 3 2 2 4 17%	4420
	21st to end	Night	36% 14% 4 9% 5% 4 3 2 4 4 15%	4614
February	1st to 10th	Night	28% 13% 4 9% 5% 4 4 4 4 5% 19%	2470
	11th to 20th	Night	31% 12% 3 7% 5% 5% 4 3 4 4 24%	2402
	21st to end	Night	29% 13% 4 8% 5% 4 5% 3 4 5% 20%	1963
March	1st to 10th	Night	19% 15% 4 9% 7% 5% 5% 4 6% 6% 20%	3589
	11th to 20th	Night	18% 8% 2 6% 5% 4 5% 3 4 5% 39%	3490
	21st to end	Night	25% 13% 4 11% 7% 5% 5% 4 5% 5% 16%	3741
April	1st to 10th	Night	26% 13% 4 9% 5% 5% 4 3 5% 5% 22%	1900
	11th to 20th	Night	20% 11% 3 7% 5% 4 4 3 5% 6% 32%	1879
	21st to end	Night	26% 9% 3 6% 5% 4 4 3 5% 7% 28%	1716
May	1st to 10th	Night	21% 9% 3 6% 4 3 3 3 4 4 41%	2588
	11th to 20th	Night	11% 5% 1 3 4 3 3 3 5% 6% 55%	2516
	21st to end	Night	16% 7% 2 8% 5% 5% 6% 4 6% 7% 33%	2762
June	1st to 10th	Night	19% 8% 3 6% 5% 5% 3 2 3 6% 38%	1520
	11th to 20th	Night	16% 9% 3 9% 4 4 4 3 5% 4 38%	1520
	21st to end	Night	7% 4 2 5% 3 3 4 3 4 5% 59%	1520
July	1st to 10th	Night	4 2 1 2 2 2 2 1 2 3 79%	2540
	11th to 20th	Night	4 3 1 4 2 2 2 1 2 4 77%	2540
	21st to end	Night	4 2 1 2 2 2 2 2 3 4 78%	2761
August	1st to 10th	Night	3 2 1 2 2 2 2 2 4 5% 76%	2784
	11th to 20th	Night	7% 4 2 5% 4 4 4 3 4 6% 57%	2812
	21st to end	Night	10% 5% 2 4 3 2 3 2 3 5% 61%	3370
September	1st to 10th	Night	6% 3 1 4 2 3 3 3 4 5% 66%	1900
	11th to 20th	Night	13% 6% 2 7% 4 4 4 2 5% 5% 49%	1900
	21st to end	Night	11% 5% 2 5% 4 4 4 3 5% 6% 51%	2177
October	1st to 10th	Night	13% 5% 2 5% 4 4 4 3 5% 8% 48%	3800
	11th to 20th	Night	18% 6% 2 7% 4 5% 5% 4 5% 8% 37%	3777
	21st to end	Night	25% 10% 3 8% 4 5% 4 4 6% 5% 26%	4203
November	1st to 10th	Night	31% 9% 3 8% 5% 4 4 3 5% 6% 21%	2582
	11th to 20th	Night	35% 9% 3 8% 4 5% 4 3 4 4 21%	2660
	21st to end	Night	28% 8% 3 8% 4 5% 4 3 4 5% 27%	2660
December	1st to 10th	Night	31% 10% 3 6% 4 4 4 3 5% 6% 24%	2660
	11th to 20th	Night	31% 12% 4 9% 5% 4 4 3 4 6% 20%	3276
	21st to end	Night	30% 13% 4 10% 5% 5% 5% 3 4 4 15%	4862

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	83% 2 8% 4 2 1	2539
	11th to 20th	Day	76% 2 6% 10% 4 2	2540
	21st to end	Day	76% 3 9% 8% 4	3047
February	1st to 10th	Day	70% 5% 10% 11% 5%	1710
	11th to 20th	Day	72% 4 8% 11% 4 1	1674
	21st to end	Day	70% 4 10% 9% 6% 2	1490
March	1st to 10th	Day	67% 4 14% 12% 3	3283
	11th to 20th	Day	52% 4 8% 26% 9%	3490
	21st to end	Day	69% 3 15% 9% 4	3912
April	1st to 10th	Day	68% 4 13% 12% 3	2280
	11th to 20th	Day	63% 5% 9% 13% 8% 2	2252
	21st to end	Day	61% 5% 13% 12% 8% 1	2460
May	1st to 10th	Day	52% 3 13% 20% 12%	4371
	11th to 20th	Day	39% 5% 18% 26% 11% 1	4380
	21st to end	Day	61% 6% 18% 13% 2	4814
June	1st to 10th	Day	53% 5% 17% 17% 7%	2660
	11th to 20th	Day	56% 6% 18% 14% 6%	2660
	21st to end	Day	35% 4 18% 28% 10% 4	2660
July	1st to 10th	Day	20% 3 9% 18% 22% 29%	4351
	11th to 20th	Day	16% 3 6% 19% 38% 18%	4345
	21st to end	Day	20% 3 11% 27% 34% 6%	4639
August	1st to 10th	Day	21% 3 9% 28% 27% 12%	4121
	11th to 20th	Day	29% 6% 18% 29% 13% 4	4065
	21st to end	Day	33% 3 12% 25% 20% 7%	4303
September	1st to 10th	Day	26% 4 13% 27% 19% 12%	2280
	11th to 20th	Day	42% 5% 14% 18% 14% 7%	2280
	21st to end	Day	41% 6% 15% 19% 15% 5%	2000
October	1st to 10th	Day	41% 4 7% 25% 21% 3	3180
	11th to 20th	Day	54% 6% 14% 15% 10% 1	3161
	21st to end	Day	62% 4 7% 17% 9% 1	3361
November	1st to 10th	Day	72% 4 9% 11% 4	1618
	11th to 20th	Day	68% 4 11% 9% 6% 2	1520
	21st to end	Day	66% 5% 6% 12% 11% 1	1520
December	1st to 10th	Day	66% 4 9% 12% 8% 2	1520
	11th to 20th	Day	70% 4 12% 12% 2	1928
	21st to end	Day	81% 3 7% 6% 3	2794
January	1st to 10th	Night	78% 3 9% 6% 3 1	4406
	11th to 20th	Night	74% 2 6% 11% 4 1	4420
	21st to end	Night	75% 2 8% 9% 5% 1	4614
February	1st to 10th	Night	67% 6% 11% 11% 5%	2470
	11th to 20th	Night	65% 4 12% 12% 6% 2	2404
	21st to end	Night	68% 3 11% 10% 6% 2	1963
March	1st to 10th	Night	64% 3 13% 16% 3	3589
	11th to 20th	Night	47% 4 14% 24% 10% 1	3490
	21st to end	Night	71% 3 14% 11% 1	3741
April	1st to 10th	Night	65% 4 15% 12% 4	1900
	11th to 20th	Night	54% 6% 15% 16% 7% 3	1879
	21st to end	Night	58% 5% 13% 16% 6% 2	1716
May	1st to 10th	Night	49% 3 13% 24% 9% 1	2588
	11th to 20th	Night	30% 4 20% 36% 8% 2	2516
	21st to end	Night	50% 6% 27% 13% 3	2762
June	1st to 10th	Night	50% 3 16% 25% 6% 1	1520
	11th to 20th	Night	48% 5% 19% 23% 6%	1520
	21st to end	Night	29% 4 19% 33% 12% 5%	1520
July	1st to 10th	Night	14% 2 8% 23% 25% 28%	2500
	11th to 20th	Night	15% 2 7% 21% 37% 17%	2492
	21st to end	Night	12% 3 11% 31% 37% 6%	2674
August	1st to 10th	Night	13% 1 10% 36% 31% 8%	2776
	11th to 20th	Night	28% 5% 23% 29% 10% 4	2839
	21st to end	Night	28% 3 14% 28% 20% 7%	3370
September	1st to 10th	Night	22% 4 13% 29% 19% 12%	1900
	11th to 20th	Night	39% 4 13% 24% 11% 9%	1900
	21st to end	Night	35% 4 17% 22% 16% 6%	2177
October	1st to 10th	Night	36% 4 10% 29% 19% 2	3800
	11th to 20th	Night	46% 5% 17% 19% 11% 2	3777
	21st to end	Night	59% 4 9% 17% 10% 2	4203
November	1st to 10th	Night	64% 5% 9% 15% 6%	2582
	11th to 20th	Night	67% 4 10% 10% 7% 2	2660
	21st to end	Night	60% 4 10% 11% 13% 2	2660
December	1st to 10th	Night	62% 5% 12% 12% 10% 1	2660
	11th to 20th	Night	68% 4 12% 11% 5%	3276
	21st to end	Night	74% 3 12% 9% 2	4862

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	46% 10% 18% 14% 11% 2	180
	11th to 20th	Day	41% 8% 22% 17% 8% 3	180
	21st to end	Day	37% 10% 18% 20% 12% 4	347
February	1st to 10th	Day	37% 12% 16% 22% 11% 3	370
	11th to 20th	Day	32% 13% 25% 20% 7% 2	365
	21st to end	Day	38% 15% 17% 21% 8% 1	344
March	1st to 10th	Day	32% 14% 21% 22% 9% 2	460
	11th to 20th	Day	28% 11% 25% 19% 13% 4	460
	21st to end	Day	30% 17% 22% 19% 10% 2	506
April	1st to 10th	Day	29% 15% 20% 22% 9% 4	460
	11th to 20th	Day	28% 15% 27% 20% 8% 4	452
	21st to end	Day	22% 12% 28% 24% 11% 2	460
May	1st to 10th	Day	18% 14% 22% 27% 14% 5%	463
	11th to 20th	Day	27% 9% 17% 27% 16% 4	488
	21st to end	Day	20% 8% 20% 27% 20% 5%	529
June	1st to 10th	Day	23% 8% 19% 29% 18% 3	490
	11th to 20th	Day	20% 9% 26% 30% 13% 3	490
	21st to end	Day	7% 7% 20% 38% 22% 5%	490
July	1st to 10th	Day	9% 4 18% 38% 26% 6%	486
	11th to 20th	Day	5% 5% 21% 39% 24% 6%	484
	21st to end	Day	6% 2 15% 40% 30% 7%	531
August	1st to 10th	Day	5% 2 13% 41% 29% 9%	487
	11th to 20th	Day	4 2 14% 35% 32% 12%	477
	21st to end	Day	9% 5% 22% 33% 24% 8%	539
September	1st to 10th	Day	9% 6% 19% 32% 28% 7%	490
	11th to 20th	Day	15% 6% 15% 31% 26% 7%	490
	21st to end	Day	13% 8% 22% 31% 22% 3	490
October	1st to 10th	Day	20% 8% 19% 26% 20% 7%	490
	11th to 20th	Day	23% 8% 18% 26% 20% 5%	490
	21st to end	Day	31% 10% 23% 19% 12% 5%	488
November	1st to 10th	Day	29% 13% 25% 19% 10% 3	236
	11th to 20th	Day	29% 13% 25% 17% 12% 4	180
	21st to end	Day	31% 10% 20% 23% 14% 2	180
December	1st to 10th	Day	36% 9% 23% 18% 11% 3	180
	11th to 20th	Day	43% 11% 20% 18% 6% 1	180
	21st to end	Day	41% 14% 13% 21% 11% 1	198
January	1st to 10th	Night	41% 9% 19% 21% 9% 1	760
	11th to 20th	Night	38% 9% 16% 24% 11% 1	760
	21st to end	Night	34% 7% 16% 30% 12% 2	836
February	1st to 10th	Night	33% 9% 17% 28% 13% 1	760
	11th to 20th	Night	36% 8% 19% 23% 13% 1	744
	21st to end	Night	36% 7% 20% 28% 9%	628
March	1st to 10th	Night	31% 9% 18% 28% 13% 1	760
	11th to 20th	Night	29% 7% 20% 31% 12% 1	760
	21st to end	Night	30% 10% 20% 28% 12%	836
April	1st to 10th	Night	31% 8% 18% 27% 15% 1	760
	11th to 20th	Night	25% 8% 22% 30% 13% 2	751
	21st to end	Night	28% 8% 19% 30% 14% 1	576
May	1st to 10th	Night	18% 5% 27% 37% 13% 1	415
	11th to 20th	Night	21% 4 18% 37% 19% 1	374
	21st to end	Night	18% 6% 20% 36% 18%	411
June	1st to 10th	Night	21% 3 22% 38% 16%	380
	11th to 20th	Night	17% 8% 20% 44% 10% 1	380
	21st to end	Night	8% 2 27% 49% 14%	380
July	1st to 10th	Night	6% 2 17% 55% 19% 1	370
	11th to 20th	Night	7% 2 24% 49% 18%	368
	21st to end	Night	3 21% 57% 19%	416
August	1st to 10th	Night	4 1 13% 46% 32% 3	529
	11th to 20th	Night	5% 2 9% 42% 38% 4	564
	21st to end	Night	8% 3 17% 45% 25% 2	760
September	1st to 10th	Night	9% 2 18% 42% 26% 2	760
	11th to 20th	Night	14% 4 11% 37% 30% 3	760
	21st to end	Night	13% 3 15% 43% 23% 3	760
October	1st to 10th	Night	19% 4 17% 34% 23% 4	760
	11th to 20th	Night	22% 5% 13% 34% 24% 2	760
	21st to end	Night	28% 7% 19% 30% 14% 2	836
November	1st to 10th	Night	33% 7% 19% 29% 12% 1	764
	11th to 20th	Night	36% 9% 19% 25% 11% 1	760
	21st to end	Night	29% 7% 19% 31% 13%	760
December	1st to 10th	Night	32% 10% 19% 26% 11% 1	760
	11th to 20th	Night	41% 9% 17% 24% 9% 1	760
	21st to end	Night	42% 9% 19% 22% 8% 1	836

Page generated at 2024-11-03 10:19:01. Version ac07

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