Portland(ME)
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Portland(ME) clear sky chart from 2005-11-06 to 2025-04-11.

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	17% 8% 2 6% 4 4 4 4 5% 7% 40%	113105
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 38%	115154
Transparency	Day	26% 43% 7% 13% 9% 2	116673
	Night	28% 40% 7% 13% 10% 2	119750
Seeing	Day	20% 10% 30% 27% 11% 2	15467
	Night	21% 10% 30% 27% 10% 1	24411

By Month

Percentage Cloud Cover NE (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	13% 8% 2 7% 6% 5% 5% 4 5% 7% 40%	3597
	11th to 20th	Day	14% 7% 3 5% 4 4 4 3 4 6% 46%	3570
	21st to end	Day	14% 9% 3 6% 3 3 3 3 4 5% 46%	3960
February	1st to 10th	Day	17% 8% 2 6% 4 4 5% 4 6% 6% 37%	1788
	11th to 20th	Day	13% 7% 2 7% 6% 4 5% 4 6% 7% 39%	1948
	21st to end	Day	15% 11% 3 8% 4 4 4 4 5% 5% 38%	1650
March	1st to 10th	Day	22% 10% 2 6% 4 4 4 4 4 5% 34%	4476
	11th to 20th	Day	18% 10% 2 6% 4 3 4 3 5% 7% 38%	4878
	21st to end	Day	28% 8% 2 5% 3 3 3 3 5% 6% 34%	5620
April	1st to 10th	Day	19% 9% 3 6% 4 4 4 4 4 6% 37%	2400
	11th to 20th	Day	17% 6% 2 5% 5% 5% 4 3 6% 7% 41%	2338
	21st to end	Day	21% 8% 2 6% 4 4 4 4 5% 6% 36%	2470
May	1st to 10th	Day	21% 9% 3 7% 5% 4 4 3 6% 7% 30%	4106
	11th to 20th	Day	19% 8% 3 8% 5% 4 4 4 5% 8% 33%	4133
	21st to end	Day	16% 6% 2 5% 4 4 5% 4 6% 9% 38%	4837
June	1st to 10th	Day	21% 7% 2 7% 5% 4 4 3 6% 7% 34%	2660
	11th to 20th	Day	15% 6% 2 5% 5% 4 4 4 5% 7% 42%	2660
	21st to end	Day	21% 6% 2 7% 5% 5% 5% 4 5% 7% 33%	2660
July	1st to 10th	Day	13% 9% 3 8% 5% 5% 5% 5% 6% 9% 33%	4440
	11th to 20th	Day	10% 6% 2 6% 4 4 5% 5% 8% 10% 40%	4440
	21st to end	Day	18% 7% 2 5% 3 4 4 4 6% 8% 39%	4814
August	1st to 10th	Day	14% 6% 3 7% 4 5% 5% 4 6% 8% 38%	4118
	11th to 20th	Day	16% 8% 3 7% 4 4 4 4 6% 8% 36%	4045
	21st to end	Day	8% 4 1 5% 5% 4 5% 4 7% 9% 48%	4314
September	1st to 10th	Day	12% 5% 1 5% 4 4 4 3 5% 7% 50%	2237
	11th to 20th	Day	12% 6% 2 4 3 3 3 2 4 5% 56%	2090
	21st to end	Day	16% 7% 2 4 3 4 4 4 5% 6% 46%	2090
October	1st to 10th	Day	18% 7% 2 6% 3 4 4 4 5% 5% 42%	2090
	11th to 20th	Day	17% 6% 2 6% 4 3 4 3 5% 7% 44%	1836
	21st to end	Day	23% 6% 2 7% 3 3 4 4 4 5% 41%	1881
November	1st to 10th	Day	16% 6% 2 4 3 3 3 2 4 7% 48%	1761
	11th to 20th	Day	19% 4 2 4 2 3 4 4 5% 6% 48%	1800
	21st to end	Day	23% 6% 2 6% 3 3 3 3 3 6% 43%	1800
December	1st to 10th	Day	15% 11% 2 5% 4 3 4 3 5% 6% 44%	2652
	11th to 20th	Day	19% 7% 2 3 3 3 3 2 4 5% 49%	2986
	21st to end	Day	21% 10% 3 6% 4 4 4 4 5% 5% 35%	3960
January	1st to 10th	Night	13% 8% 3 6% 4 4 4 3 5% 6% 44%	6349
	11th to 20th	Night	13% 9% 4 10% 5% 5% 4 4 5% 6% 36%	6315
	21st to end	Night	15% 10% 3 7% 4 4 4 3 5% 6% 41%	6996
February	1st to 10th	Night	17% 9% 3 6% 5% 4 5% 3 5% 6% 37%	2607
	11th to 20th	Night	14% 10% 3 6% 5% 5% 4 4 5% 5% 40%	2338
	21st to end	Night	15% 9% 2 7% 5% 4 4 3 6% 5% 40%	1980
March	1st to 10th	Night	27% 9% 2 6% 4 4 4 3 4 5% 33%	5364
	11th to 20th	Night	15% 10% 3 7% 5% 5% 5% 4 5% 6% 35%	5080
	21st to end	Night	25% 8% 3 6% 3 4 5% 3 5% 6% 32%	5295
April	1st to 10th	Night	22% 9% 3 5% 5% 4 4 3 4 5% 37%	2000
	11th to 20th	Night	19% 7% 2 6% 4 3 5% 4 5% 6% 39%	1797
	21st to end	Night	22% 8% 3 6% 4 4 4 3 5% 5% 37%	1710
May	1st to 10th	Night	23% 8% 3 7% 4 4 5% 4 6% 7% 28%	2834
	11th to 20th	Night	17% 8% 3 8% 5% 7% 7% 5% 6% 8% 26%	2789
	21st to end	Night	18% 9% 3 6% 4 5% 4 3 6% 7% 36%	2738
June	1st to 10th	Night	21% 8% 2 8% 5% 5% 4 4 5% 6% 32%	1520
	11th to 20th	Night	17% 7% 2 6% 4 3 4 3 6% 8% 39%	1520
	21st to end	Night	23% 7% 3 4 4 4 5% 3 6% 8% 34%	1520
July	1st to 10th	Night	13% 8% 3 9% 6% 4 6% 4 7% 9% 31%	2520
	11th to 20th	Night	12% 7% 3 7% 5% 4 4 5% 8% 9% 36%	2520
	21st to end	Night	15% 7% 2 6% 5% 5% 4 4 6% 8% 39%	2739
August	1st to 10th	Night	20% 7% 2 8% 5% 5% 4 4 6% 7% 32%	2780
	11th to 20th	Night	15% 9% 3 7% 4 5% 4 4 6% 8% 34%	2782
	21st to end	Night	10% 4 2 5% 4 4 5% 4 6% 9% 46%	3338
September	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 5% 6% 49%	1941
	11th to 20th	Night	15% 7% 2 5% 3 2 3 3 4 7% 50%	2090
	21st to end	Night	15% 6% 3 4 3 3 4 3 5% 7% 47%	2090
October	1st to 10th	Night	18% 7% 3 6% 4 5% 4 3 5% 6% 40%	2090
	11th to 20th	Night	16% 5% 3 5% 4 4 5% 4 6% 7% 43%	2341
	21st to end	Night	24% 6% 2 6% 3 3 4 3 5% 5% 38%	2717
November	1st to 10th	Night	16% 6% 2 5% 4 4 4 4 4 6% 46%	2537
	11th to 20th	Night	18% 6% 2 4 4 4 3 3 4 6% 46%	2600
	21st to end	Night	17% 7% 2 6% 4 4 3 3 5% 7% 43%	2600
December	1st to 10th	Night	17% 10% 3 7% 4 4 4 3 5% 5% 39%	4513
	11th to 20th	Night	23% 7% 2 5% 3 3 3 3 4 5% 42%	5208
	21st to end	Night	21% 11% 3 7% 4 3 3 3 4 5% 36%	6996

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	34% 27% 6% 19% 13% 1	3598
	11th to 20th	Day	29% 28% 8% 22% 11% 2	3600
	21st to end	Day	30% 29% 5% 15% 16% 4	3960
February	1st to 10th	Day	18% 62% 6% 7% 6% 2	1788
	11th to 20th	Day	19% 61% 4 7% 7% 2	1957
	21st to end	Day	17% 62% 3 8% 8% 2	1650
March	1st to 10th	Day	40% 30% 5% 7% 11% 8%	4479
	11th to 20th	Day	34% 30% 13% 12% 8% 3	4878
	21st to end	Day	41% 28% 4 10% 12% 5%	5620
April	1st to 10th	Day	21% 62% 2 7% 6% 2	2400
	11th to 20th	Day	15% 66% 4 8% 5% 1	2338
	21st to end	Day	18% 66% 3 6% 7%	2470
May	1st to 10th	Day	33% 46% 9% 9% 3	4120
	11th to 20th	Day	31% 44% 11% 11% 3	4137
	21st to end	Day	23% 43% 11% 14% 9%	4837
June	1st to 10th	Day	15% 67% 5% 9% 5%	2660
	11th to 20th	Day	14% 65% 4 9% 8% 1	2660
	21st to end	Day	16% 64% 4 9% 6%	2660
July	1st to 10th	Day	33% 37% 6% 14% 9%	4370
	11th to 20th	Day	23% 37% 12% 19% 8%	4365
	21st to end	Day	31% 36% 9% 14% 9% 1	4661
August	1st to 10th	Day	30% 38% 9% 17% 5%	4104
	11th to 20th	Day	32% 38% 10% 16% 4	4084
	21st to end	Day	18% 38% 10% 19% 14% 1	4314
September	1st to 10th	Day	12% 61% 6% 14% 7% 1	2237
	11th to 20th	Day	16% 60% 4 12% 8% 1	2090
	21st to end	Day	15% 60% 3 14% 7% 1	2090
October	1st to 10th	Day	30% 37% 6% 17% 10% 1	3480
	11th to 20th	Day	17% 37% 5% 22% 18% 1	3026
	21st to end	Day	20% 37% 6% 19% 15% 2	3081
November	1st to 10th	Day	17% 56% 7% 12% 7% 1	1761
	11th to 20th	Day	17% 56% 4 9% 12% 3	1800
	21st to end	Day	19% 57% 7% 9% 8% 1	1800
December	1st to 10th	Day	25% 36% 9% 23% 7%	2652
	11th to 20th	Day	27% 31% 9% 12% 13% 8%	2986
	21st to end	Day	38% 28% 4 14% 12% 3	3960
January	1st to 10th	Night	30% 26% 8% 24% 10% 2	6349
	11th to 20th	Night	37% 27% 9% 16% 9% 2	6360
	21st to end	Night	35% 28% 5% 14% 14% 5%	6996
February	1st to 10th	Night	19% 62% 5% 6% 6% 2	2607
	11th to 20th	Night	17% 60% 4 8% 9% 1	2340
	21st to end	Night	19% 62% 3 9% 6% 2	1980
March	1st to 10th	Night	41% 29% 6% 7% 11% 6%	5374
	11th to 20th	Night	35% 30% 12% 12% 9% 2	5081
	21st to end	Night	42% 28% 3 8% 15% 4	5295
April	1st to 10th	Night	21% 62% 2 6% 7% 2	2000
	11th to 20th	Night	17% 66% 3 7% 7% 1	1797
	21st to end	Night	18% 65% 4 5% 6% 2	1710
May	1st to 10th	Night	35% 44% 11% 8% 3	2840
	11th to 20th	Night	33% 44% 11% 9% 3	2790
	21st to end	Night	28% 43% 11% 10% 8% 1	2739
June	1st to 10th	Night	17% 67% 3 8% 5%	1520
	11th to 20th	Night	14% 66% 6% 9% 5% 1	1520
	21st to end	Night	17% 64% 4 10% 4 1	1520
July	1st to 10th	Night	30% 37% 9% 14% 10%	2480
	11th to 20th	Night	29% 37% 8% 15% 11%	2472
	21st to end	Night	30% 36% 7% 16% 10%	2651
August	1st to 10th	Night	37% 38% 8% 10% 6%	2770
	11th to 20th	Night	32% 39% 9% 14% 6%	2809
	21st to end	Night	18% 38% 12% 17% 16%	3338
September	1st to 10th	Night	14% 60% 5% 13% 7% 1	1941
	11th to 20th	Night	17% 60% 4 11% 7% 1	2090
	21st to end	Night	15% 60% 4 15% 6% 1	2090
October	1st to 10th	Night	30% 37% 7% 16% 9% 1	3480
	11th to 20th	Night	15% 36% 6% 20% 21% 2	3890
	21st to end	Night	21% 37% 6% 20% 14% 2	4467
November	1st to 10th	Night	19% 56% 3 14% 7% 2	2537
	11th to 20th	Night	17% 56% 4 9% 12% 2	2600
	21st to end	Night	18% 58% 7% 10% 8% 1	2600
December	1st to 10th	Night	29% 36% 5% 22% 8%	4513
	11th to 20th	Night	32% 31% 8% 11% 15% 3	5208
	21st to end	Night	38% 28% 6% 14% 13% 1	6996

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	24% 21% 30% 23% 3 1	200
	11th to 20th	Day	24% 17% 30% 22% 7% 1	200
	21st to end	Day	17% 17% 41% 18% 7%	220
February	1st to 10th	Day	24% 15% 41% 17% 2	378
	11th to 20th	Day	20% 19% 40% 18% 3	496
	21st to end	Day	23% 15% 35% 22% 5%	414
March	1st to 10th	Day	24% 15% 36% 22% 3	500
	11th to 20th	Day	20% 18% 36% 20% 6% 1	500
	21st to end	Day	25% 15% 35% 17% 7% 2	550
April	1st to 10th	Day	26% 17% 31% 19% 5% 2	500
	11th to 20th	Day	21% 13% 36% 21% 8% 1	452
	21st to end	Day	27% 12% 33% 20% 7% 1	460
May	1st to 10th	Day	26% 10% 35% 22% 6% 2	463
	11th to 20th	Day	18% 9% 36% 28% 9% 1	487
	21st to end	Day	20% 8% 33% 28% 11% 1	529
June	1st to 10th	Day	22% 5% 31% 31% 11% 2	490
	11th to 20th	Day	16% 7% 34% 28% 12% 4	490
	21st to end	Day	21% 4 27% 34% 11% 2	490
July	1st to 10th	Day	17% 4 23% 35% 19% 3	485
	11th to 20th	Day	15% 4 19% 40% 18% 5%	484
	21st to end	Day	13% 3 24% 36% 21% 3	532
August	1st to 10th	Day	13% 2 24% 37% 21% 2	486
	11th to 20th	Day	16% 3 19% 35% 21% 6%	479
	21st to end	Day	13% 2 22% 38% 19% 6%	539
September	1st to 10th	Day	15% 3 24% 33% 20% 4	490
	11th to 20th	Day	16% 7% 27% 23% 18% 9%	490
	21st to end	Day	18% 8% 23% 31% 14% 6%	490
October	1st to 10th	Day	22% 10% 21% 27% 17% 4	490
	11th to 20th	Day	18% 11% 29% 27% 13% 3	400
	21st to end	Day	27% 8% 30% 26% 8%	429
November	1st to 10th	Day	17% 12% 32% 27% 10% 2	395
	11th to 20th	Day	22% 15% 33% 26% 5% 1	400
	21st to end	Day	26% 15% 29% 23% 7% 1	400
December	1st to 10th	Day	21% 15% 37% 18% 8%	239
	11th to 20th	Day	26% 19% 32% 20% 4 1	200
	21st to end	Day	24% 20% 30% 15% 10% 1	220
January	1st to 10th	Night	21% 17% 39% 20% 4	800
	11th to 20th	Night	26% 16% 35% 19% 4	800
	21st to end	Night	18% 18% 39% 21% 4	880
February	1st to 10th	Night	25% 16% 36% 21% 3	800
	11th to 20th	Night	20% 18% 39% 20% 3	783
	21st to end	Night	20% 15% 35% 24% 6%	660
March	1st to 10th	Night	25% 13% 37% 23% 2	800
	11th to 20th	Night	23% 12% 35% 21% 8% 1	800
	21st to end	Night	25% 12% 34% 23% 6%	880
April	1st to 10th	Night	28% 10% 32% 26% 4	800
	11th to 20th	Night	21% 9% 33% 28% 8% 1	669
	21st to end	Night	24% 6% 31% 30% 8% 1	570
May	1st to 10th	Night	25% 5% 30% 28% 11% 1	570
	11th to 20th	Night	21% 5% 31% 30% 11% 1	551
	21st to end	Night	27% 3 23% 32% 14% 1	411
June	1st to 10th	Night	24% 5% 22% 34% 13% 2	380
	11th to 20th	Night	22% 4 19% 34% 19% 2	380
	21st to end	Night	24% 2 17% 41% 15% 1	380
July	1st to 10th	Night	19% 2 14% 42% 21% 2	370
	11th to 20th	Night	15% 2 13% 44% 23% 2	368
	21st to end	Night	15% 1 15% 46% 21% 2	416
August	1st to 10th	Night	15% 2 16% 44% 22% 2	535
	11th to 20th	Night	14% 4 13% 43% 24% 3	562
	21st to end	Night	15% 2 18% 37% 24% 3	750
September	1st to 10th	Night	13% 2 22% 38% 22% 3	760
	11th to 20th	Night	18% 4 24% 27% 21% 7%	760
	21st to end	Night	19% 4 23% 31% 20% 3	760
October	1st to 10th	Night	22% 7% 21% 28% 18% 3	760
	11th to 20th	Night	18% 8% 29% 30% 13% 2	760
	21st to end	Night	26% 9% 29% 27% 8%	836
November	1st to 10th	Night	19% 9% 34% 30% 7% 1	780
	11th to 20th	Night	21% 13% 36% 22% 7% 1	800
	21st to end	Night	21% 15% 36% 23% 6%	800
December	1st to 10th	Night	25% 11% 41% 16% 6% 1	800
	11th to 20th	Night	25% 16% 34% 21% 3	800
	21st to end	Night	26% 13% 36% 22% 4	880

Page generated at 2025-04-08 09:17:30. Version ac07

Copyright © 2025 Attilla Danko.