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 2025-11-03.

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% 8% 3 7% 5% 4 4 3 5% 6% 37%	136058
	Night	21% 9% 3 7% 4 4 4 3 4 5% 36%	131314
Transparency	Day	49% 4 12% 19% 12% 4	130815
	Night	51% 4 12% 18% 11% 3	127555
Seeing	Day	21% 9% 20% 27% 18% 5%	15883
	Night	25% 6% 18% 33% 16% 1	24780

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	38% 15% 4 10% 5% 4 4 3 4 4 10%	3639
	11th to 20th	Day	28% 12% 3 10% 6% 5% 4 3 4 5% 20%	3640
	21st to end	Day	26% 9% 3 8% 5% 4 4 3 4 4 30%	4369
February	1st to 10th	Day	29% 13% 5% 10% 5% 4 5% 4 5% 6% 15%	1800
	11th to 20th	Day	30% 15% 5% 10% 4 5% 4 2 4 4 17%	1755
	21st to end	Day	31% 13% 4 9% 6% 4 3 2 4 5% 19%	1566
March	1st to 10th	Day	18% 11% 5% 10% 6% 6% 6% 5% 6% 7% 19%	4728
	11th to 20th	Day	25% 13% 4 8% 5% 5% 4 3 4 4 25%	5000
	21st to end	Day	25% 12% 4 11% 6% 6% 6% 4 6% 7% 13%	5612
April	1st to 10th	Day	28% 12% 4 9% 6% 6% 4 3 5% 6% 17%	2400
	11th to 20th	Day	23% 11% 4 9% 5% 4 4 3 4 5% 27%	2369
	21st to end	Day	21% 9% 3 8% 6% 5% 6% 5% 5% 6% 24%	2590
May	1st to 10th	Day	16% 7% 3 6% 5% 4 4 3 5% 6% 42%	6245
	11th to 20th	Day	16% 9% 3 9% 6% 6% 5% 4 5% 6% 31%	6277
	21st to end	Day	18% 7% 3 7% 5% 6% 5% 4 6% 8% 31%	6902
June	1st to 10th	Day	22% 7% 3 6% 5% 4 4 4 6% 6% 33%	2800
	11th to 20th	Day	19% 9% 3 8% 6% 5% 5% 5% 6% 6% 28%	2800
	21st to end	Day	9% 7% 2 6% 4 4 4 3 5% 7% 49%	2800
July	1st to 10th	Day	4 3 1 3 2 2 3 2 3 5% 71%	6319
	11th to 20th	Day	3 2 1 2 2 1 2 2 3 4 79%	6320
	21st to end	Day	5% 2 1 3 2 2 2 2 3 6% 72%	6880
August	1st to 10th	Day	4 4 1 4 3 3 3 3 5% 6% 63%	5849
	11th to 20th	Day	7% 4 2 5% 3 4 5% 4 6% 8% 52%	5796
	21st to end	Day	9% 6% 2 5% 4 4 4 4 5% 7% 51%	6160
September	1st to 10th	Day	7% 4 2 4 3 3 4 3 4 7% 57%	2400
	11th to 20th	Day	13% 8% 3 6% 4 4 5% 3 5% 6% 43%	2400
	21st to end	Day	15% 6% 3 6% 4 4 4 3 4 6% 44%	2105
October	1st to 10th	Day	17% 7% 2 5% 4 4 4 3 5% 5% 44%	3280
	11th to 20th	Day	19% 7% 3 9% 6% 5% 5% 4 6% 7% 28%	3261
	21st to end	Day	27% 9% 4 9% 6% 5% 5% 3 5% 7% 21%	3468
November	1st to 10th	Day	31% 12% 5% 10% 5% 5% 4 4 5% 5% 15%	1676
	11th to 20th	Day	31% 12% 5% 9% 5% 3 4 3 4 5% 19%	1600
	21st to end	Day	29% 12% 4 8% 6% 4 4 4 4 5% 21%	1600
December	1st to 10th	Day	23% 10% 3 7% 5% 4 3 3 4 6% 32%	2620
	11th to 20th	Day	33% 14% 5% 10% 6% 5% 4 3 5% 5% 12%	3028
	21st to end	Day	34% 15% 4 12% 6% 6% 5% 3 4 4 8%	4004
January	1st to 10th	Night	36% 13% 4 9% 5% 5% 4 3 4 4 13%	6306
	11th to 20th	Night	28% 11% 3 7% 5% 4 5% 3 4 5% 23%	6320
	21st to end	Night	27% 10% 3 8% 4 3 3 2 4 5% 32%	6600
February	1st to 10th	Night	27% 13% 4 10% 6% 5% 4 4 4 5% 18%	2600
	11th to 20th	Night	31% 11% 3 7% 5% 5% 4 3 4 4 24%	2532
	21st to end	Night	29% 13% 4 8% 5% 4 5% 3 4 5% 21%	2063
March	1st to 10th	Night	18% 13% 4 9% 6% 5% 6% 4 6% 6% 22%	5164
	11th to 20th	Night	24% 12% 3 8% 5% 4 4 3 4 5% 28%	5000
	21st to end	Night	27% 12% 4 11% 6% 6% 5% 4 6% 6% 14%	5363
April	1st to 10th	Night	25% 13% 3 9% 5% 5% 4 3 5% 5% 22%	2000
	11th to 20th	Night	20% 11% 3 7% 5% 4 4 3 5% 6% 34%	1979
	21st to end	Night	25% 9% 2 6% 5% 4 4 3 5% 7% 29%	1806
May	1st to 10th	Night	17% 7% 2 5% 3 3 3 3 4 6% 49%	3716
	11th to 20th	Night	14% 7% 3 6% 4 4 4 3 6% 6% 43%	3616
	21st to end	Night	14% 8% 2 7% 5% 5% 5% 4 6% 9% 36%	3972
June	1st to 10th	Night	18% 8% 3 6% 5% 5% 4 2 3 6% 40%	1600
	11th to 20th	Night	15% 9% 2 9% 4 4 4 3 5% 5% 39%	1600
	21st to end	Night	8% 5% 2 5% 3 3 4 3 5% 5% 58%	1600
July	1st to 10th	Night	3 2 1 2 2 2 2 3 3 4 77%	3640
	11th to 20th	Night	3 2 1 3 2 1 1 1 2 3 82%	3640
	21st to end	Night	4 2 1 2 2 2 2 2 4 5% 75%	3970
August	1st to 10th	Night	5% 3 1 3 3 3 3 3 4 6% 66%	3943
	11th to 20th	Night	12% 5% 2 5% 3 3 3 3 4 6% 54%	4052
	21st to end	Night	8% 5% 2 5% 4 3 4 3 4 5% 57%	4831
September	1st to 10th	Night	7% 3 2 4 2 3 3 3 4 6% 64%	2000
	11th to 20th	Night	13% 6% 2 7% 4 4 4 2 5% 5% 49%	2000
	21st to end	Night	11% 5% 2 5% 4 4 4 3 5% 7% 51%	2292
October	1st to 10th	Night	13% 5% 2 5% 4 4 4 3 5% 7% 48%	3920
	11th to 20th	Night	18% 6% 2 7% 4 5% 5% 4 5% 8% 37%	3897
	21st to end	Night	25% 10% 3 8% 4 5% 4 4 6% 5% 26%	4338
November	1st to 10th	Night	30% 9% 3 8% 5% 4 4 3 6% 6% 21%	2666
	11th to 20th	Night	35% 10% 3 8% 4 5% 4 3 4 4 20%	2800
	21st to end	Night	28% 8% 4 8% 5% 5% 4 3 4 5% 26%	2800
December	1st to 10th	Night	22% 8% 2 6% 4 3 4 4 5% 7% 35%	4560
	11th to 20th	Night	37% 13% 4 9% 5% 4 4 3 3 4 13%	5176
	21st to end	Night	34% 13% 5% 10% 6% 5% 4 3 4 4 13%	6952

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	80% 2 7% 6% 4	3639
	11th to 20th	Day	68% 3 6% 15% 7% 1	3640
	21st to end	Day	59% 3 7% 13% 15% 3	4369
February	1st to 10th	Day	70% 5% 9% 11% 5%	1800
	11th to 20th	Day	72% 4 8% 10% 4 1	1764
	21st to end	Day	70% 4 10% 9% 6% 2	1566
March	1st to 10th	Day	63% 4 14% 14% 5%	4728
	11th to 20th	Day	63% 4 7% 19% 7%	5000
	21st to end	Day	70% 5% 14% 8% 3	5612
April	1st to 10th	Day	68% 4 13% 13% 3	2400
	11th to 20th	Day	60% 5% 9% 15% 9% 2	2372
	21st to end	Day	59% 5% 13% 13% 8% 1	2590
May	1st to 10th	Day	44% 3 11% 25% 17%	6245
	11th to 20th	Day	52% 6% 15% 20% 7% 1	6280
	21st to end	Day	52% 6% 21% 17% 4	6904
June	1st to 10th	Day	51% 5% 17% 19% 8% 1	2800
	11th to 20th	Day	54% 6% 18% 15% 6%	2800
	21st to end	Day	35% 5% 18% 27% 10% 4	2800
July	1st to 10th	Day	18% 2 11% 28% 19% 21%	6251
	11th to 20th	Day	13% 2 7% 22% 40% 17%	6245
	21st to end	Day	17% 3 17% 35% 25% 4	6681
August	1st to 10th	Day	21% 3 9% 28% 26% 12%	4253
	11th to 20th	Day	30% 6% 18% 29% 13% 5%	4195
	21st to end	Day	33% 3 12% 24% 20% 7%	4439
September	1st to 10th	Day	28% 4 14% 26% 18% 11%	2400
	11th to 20th	Day	41% 6% 14% 19% 15% 7%	2400
	21st to end	Day	40% 5% 14% 19% 16% 5%	2105
October	1st to 10th	Day	42% 4 7% 25% 20% 2	3280
	11th to 20th	Day	53% 6% 14% 15% 11% 1	3261
	21st to end	Day	62% 4 7% 17% 9% 1	3468
November	1st to 10th	Day	72% 4 10% 11% 4	1676
	11th to 20th	Day	69% 4 11% 9% 6% 2	1600
	21st to end	Day	66% 5% 6% 11% 10% 1	1600
December	1st to 10th	Day	54% 4 9% 17% 15% 1	2620
	11th to 20th	Day	75% 4 10% 10% 1	3028
	21st to end	Day	81% 4 7% 6% 2	4004
January	1st to 10th	Night	77% 3 8% 7% 4 1	6306
	11th to 20th	Night	64% 3 8% 16% 7% 1	6320
	21st to end	Night	58% 2 7% 13% 15% 5%	6600
February	1st to 10th	Night	67% 6% 11% 11% 5%	2600
	11th to 20th	Night	65% 4 12% 11% 6% 2	2534
	21st to end	Night	67% 3 11% 11% 5% 2	2063
March	1st to 10th	Night	62% 4 12% 16% 6%	5164
	11th to 20th	Night	59% 4 11% 18% 7% 1	5000
	21st to end	Night	71% 4 13% 10% 1	5363
April	1st to 10th	Night	65% 4 15% 12% 4	2000
	11th to 20th	Night	52% 5% 14% 17% 8% 3	1979
	21st to end	Night	56% 5% 14% 17% 6% 2	1806
May	1st to 10th	Night	39% 2 11% 32% 15% 1	3713
	11th to 20th	Night	41% 5% 19% 29% 6% 1	3616
	21st to end	Night	46% 6% 26% 18% 4	3972
June	1st to 10th	Night	48% 3 16% 26% 7% 1	1600
	11th to 20th	Night	47% 5% 18% 24% 6%	1600
	21st to end	Night	29% 4 19% 32% 12% 4	1600
July	1st to 10th	Night	13% 2 12% 30% 23% 20%	3600
	11th to 20th	Night	12% 2 8% 21% 42% 15%	3592
	21st to end	Night	13% 4 19% 33% 27% 4	3857
August	1st to 10th	Night	14% 1 11% 36% 30% 8%	2864
	11th to 20th	Night	28% 5% 23% 28% 10% 5%	2929
	21st to end	Night	29% 3 14% 28% 20% 7%	3476
September	1st to 10th	Night	23% 5% 15% 29% 18% 11%	2000
	11th to 20th	Night	38% 4 13% 23% 13% 9%	2000
	21st to end	Night	34% 3 18% 22% 17% 6%	2292
October	1st to 10th	Night	36% 4 10% 29% 19% 2	3920
	11th to 20th	Night	46% 5% 17% 19% 11% 2	3897
	21st to end	Night	59% 4 9% 17% 10% 1	4338
November	1st to 10th	Night	64% 5% 9% 15% 6%	2666
	11th to 20th	Night	68% 4 10% 10% 6% 2	2800
	21st to end	Night	62% 5% 10% 10% 12% 2	2800
December	1st to 10th	Night	49% 5% 9% 19% 18%	4560
	11th to 20th	Night	75% 4 9% 8% 3	5176
	21st to end	Night	76% 4 10% 8% 2	6952

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% 12% 18% 13% 10% 2	200
	11th to 20th	Day	38% 8% 25% 18% 8% 4	200
	21st to end	Day	35% 10% 18% 21% 13% 4	377
February	1st to 10th	Day	36% 13% 17% 21% 11% 3	400
	11th to 20th	Day	32% 13% 27% 18% 7% 2	395
	21st to end	Day	38% 15% 17% 21% 8% 1	372
March	1st to 10th	Day	31% 14% 22% 23% 9% 2	500
	11th to 20th	Day	30% 11% 24% 18% 13% 4	500
	21st to end	Day	31% 16% 23% 19% 10% 1	550
April	1st to 10th	Day	28% 15% 21% 23% 9% 4	500
	11th to 20th	Day	26% 14% 27% 21% 9% 3	492
	21st to end	Day	22% 12% 27% 23% 14% 3	500
May	1st to 10th	Day	17% 14% 21% 28% 15% 4	503
	11th to 20th	Day	26% 10% 19% 26% 16% 3	528
	21st to end	Day	19% 8% 21% 26% 20% 5%	573
June	1st to 10th	Day	22% 8% 20% 28% 19% 3	530
	11th to 20th	Day	19% 9% 26% 31% 13% 3	530
	21st to end	Day	8% 7% 19% 38% 22% 5%	530
July	1st to 10th	Day	9% 4 17% 39% 26% 6%	526
	11th to 20th	Day	5% 5% 21% 39% 25% 5%	524
	21st to end	Day	6% 2 14% 38% 33% 7%	575
August	1st to 10th	Day	6% 2 13% 40% 29% 9%	527
	11th to 20th	Day	4 3 14% 35% 32% 12%	517
	21st to end	Day	8% 5% 21% 33% 25% 8%	583
September	1st to 10th	Day	9% 5% 18% 33% 28% 7%	530
	11th to 20th	Day	15% 6% 16% 31% 25% 7%	530
	21st to end	Day	14% 7% 22% 31% 22% 3	530
October	1st to 10th	Day	20% 8% 20% 26% 20% 6%	530
	11th to 20th	Day	24% 9% 18% 26% 19% 4	530
	21st to end	Day	31% 10% 23% 19% 12% 4	529
November	1st to 10th	Day	29% 13% 25% 20% 10% 3	252
	11th to 20th	Day	33% 13% 23% 17% 12% 4	200
	21st to end	Day	30% 13% 19% 22% 16% 2	200
December	1st to 10th	Day	34% 8% 26% 17% 13% 4	200
	11th to 20th	Day	43% 12% 23% 17% 6% 1	200
	21st to end	Day	41% 15% 14% 20% 10%	220
January	1st to 10th	Night	41% 9% 20% 20% 8% 1	800
	11th to 20th	Night	37% 8% 17% 25% 11% 1	800
	21st to end	Night	32% 6% 16% 30% 13% 2	880
February	1st to 10th	Night	32% 9% 18% 27% 13% 1	800
	11th to 20th	Night	36% 8% 19% 23% 13% 1	784
	21st to end	Night	36% 7% 20% 29% 8%	660
March	1st to 10th	Night	30% 9% 19% 29% 13% 1	800
	11th to 20th	Night	30% 8% 19% 31% 11% 1	800
	21st to end	Night	31% 10% 19% 28% 12%	880
April	1st to 10th	Night	31% 8% 18% 28% 15% 1	800
	11th to 20th	Night	24% 8% 23% 31% 13% 2	791
	21st to end	Night	27% 8% 19% 30% 15% 1	606
May	1st to 10th	Night	18% 5% 27% 37% 13% 1	437
	11th to 20th	Night	21% 4 19% 37% 18% 1	394
	21st to end	Night	18% 6% 21% 37% 18%	433
June	1st to 10th	Night	20% 4 22% 39% 15%	400
	11th to 20th	Night	17% 8% 21% 44% 10% 1	400
	21st to end	Night	9% 3 26% 49% 14%	400
July	1st to 10th	Night	6% 3 17% 55% 18% 1	390
	11th to 20th	Night	6% 2 24% 50% 17%	388
	21st to end	Night	3 1 21% 56% 19%	438
August	1st to 10th	Night	5% 1 13% 46% 31% 3	557
	11th to 20th	Night	6% 2 10% 42% 37% 4	594
	21st to end	Night	8% 3 17% 45% 25% 2	800
September	1st to 10th	Night	9% 2 18% 42% 26% 2	800
	11th to 20th	Night	14% 4 12% 37% 29% 4	800
	21st to end	Night	13% 4 15% 44% 23% 2	800
October	1st to 10th	Night	19% 4 17% 34% 22% 3	800
	11th to 20th	Night	21% 6% 14% 34% 23% 2	800
	21st to end	Night	28% 7% 19% 31% 14% 2	880
November	1st to 10th	Night	33% 7% 18% 30% 12% 1	788
	11th to 20th	Night	38% 9% 19% 24% 10% 1	800
	21st to end	Night	29% 7% 19% 30% 14% 1	800
December	1st to 10th	Night	31% 10% 19% 27% 12% 1	800
	11th to 20th	Night	41% 10% 17% 23% 9% 1	800
	21st to end	Night	43% 9% 19% 22% 7% 1	880

Page generated at 2025-10-30 23:02:37. Version ac07

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