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 2026-08-15.

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% 39%	143428
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 38%	138448
Transparency	Day	30% 35% 8% 16% 10% 2	146950
	Night	31% 34% 7% 15% 11% 2	143015
Seeing	Day	20% 9% 29% 27% 12% 3	17249
	Night	21% 9% 30% 28% 11% 1	25971

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	12% 8% 3 7% 6% 5% 5% 4 5% 7% 39%	4661
	11th to 20th	Day	15% 10% 3 6% 4 3 3 3 4 6% 44%	4650
	21st to end	Day	11% 9% 2 5% 3 3 3 3 5% 6% 49%	5124
February	1st to 10th	Day	16% 8% 2 6% 4 4 5% 4 6% 6% 38%	1878
	11th to 20th	Day	13% 7% 3 7% 6% 4 4 4 6% 7% 39%	2048
	21st to end	Day	14% 11% 2 8% 4 4 4 4 5% 5% 39%	1730
March	1st to 10th	Day	22% 10% 2 6% 4 4 4 4 4 5% 34%	4576
	11th to 20th	Day	18% 10% 2 6% 4 3 4 3 5% 7% 38%	4986
	21st to end	Day	28% 8% 2 5% 2 3 3 3 5% 6% 34%	5744
April	1st to 10th	Day	19% 9% 3 6% 4 4 4 4 4 6% 37%	2520
	11th to 20th	Day	18% 6% 2 5% 4 5% 4 3 6% 7% 40%	2588
	21st to end	Day	21% 8% 2 6% 4 4 4 4 5% 6% 37%	2730
May	1st to 10th	Day	28% 9% 3 7% 4 4 4 3 5% 6% 27%	7639
	11th to 20th	Day	18% 9% 3 7% 5% 5% 5% 4 5% 8% 32%	7719
	21st to end	Day	20% 7% 3 7% 5% 5% 5% 4 5% 7% 33%	9060
June	1st to 10th	Day	20% 7% 2 7% 4 4 4 3 6% 8% 34%	2940
	11th to 20th	Day	15% 6% 2 5% 5% 5% 4 4 5% 7% 41%	2940
	21st to end	Day	20% 6% 2 7% 4 5% 5% 4 5% 7% 34%	2940
July	1st to 10th	Day	15% 8% 3 7% 5% 5% 5% 5% 6% 9% 33%	8280
	11th to 20th	Day	9% 7% 2 6% 4 4 4 4 7% 10% 44%	8280
	21st to end	Day	15% 7% 2 5% 3 3 3 4 5% 8% 44%	8993
August	1st to 10th	Day	14% 6% 3 7% 5% 5% 5% 4 6% 8% 39%	4379
	11th to 20th	Day	15% 8% 3 7% 4 4 4 4 6% 8% 37%	4227
	21st to end	Day	8% 4 1 5% 5% 4 5% 4 6% 9% 49%	4450
September	1st to 10th	Day	12% 5% 1 5% 4 4 4 3 5% 7% 50%	2355
	11th to 20th	Day	11% 6% 2 4 3 3 3 2 4 6% 56%	2200
	21st to end	Day	16% 7% 2 4 3 4 4 4 5% 6% 46%	2200
October	1st to 10th	Day	18% 7% 2 6% 3 3 4 3 5% 5% 44%	2200
	11th to 20th	Day	17% 6% 2 6% 4 3 4 3 5% 7% 43%	1932
	21st to end	Day	22% 6% 2 7% 3 3 4 4 4 5% 41%	1980
November	1st to 10th	Day	16% 6% 2 5% 3 3 3 2 4 7% 48%	1851
	11th to 20th	Day	19% 4 2 4 3 2 3 4 5% 7% 47%	1890
	21st to end	Day	22% 6% 2 6% 3 3 3 3 3 5% 43%	1890
December	1st to 10th	Day	15% 10% 2 5% 4 3 4 3 4 6% 44%	2734
	11th to 20th	Day	18% 7% 2 3 3 3 3 2 4 5% 49%	3066
	21st to end	Day	21% 10% 3 6% 4 4 4 4 5% 5% 35%	4048
January	1st to 10th	Night	13% 8% 3 7% 4 4 4 3 5% 6% 43%	8234
	11th to 20th	Night	15% 11% 4 10% 5% 4 4 4 5% 5% 34%	8235
	21st to end	Night	13% 9% 3 7% 4 4 4 3 5% 6% 42%	9057
February	1st to 10th	Night	16% 9% 2 6% 4 4 5% 4 5% 6% 37%	2737
	11th to 20th	Night	13% 10% 3 5% 5% 5% 4 4 5% 5% 40%	2458
	21st to end	Night	15% 9% 2 7% 5% 4 4 3 5% 5% 40%	2076
March	1st to 10th	Night	27% 9% 2 6% 3 4 4 3 4 5% 33%	5484
	11th to 20th	Night	15% 10% 3 7% 5% 5% 5% 4 5% 6% 35%	5192
	21st to end	Night	25% 8% 3 6% 3 4 5% 3 5% 6% 32%	5413
April	1st to 10th	Night	22% 9% 3 5% 4 4 3 3 4 5% 36%	2100
	11th to 20th	Night	20% 7% 2 6% 4 3 4 4 5% 6% 38%	1980
	21st to end	Night	21% 8% 3 6% 4 4 3 3 5% 5% 37%	1890
May	1st to 10th	Night	29% 8% 3 7% 4 4 3 3 4 6% 28%	5259
	11th to 20th	Night	19% 9% 2 7% 5% 5% 6% 4 5% 7% 31%	5183
	21st to end	Night	20% 9% 3 7% 5% 5% 4 3 6% 7% 30%	5114
June	1st to 10th	Night	20% 8% 2 8% 5% 5% 4 3 5% 6% 33%	1680
	11th to 20th	Night	17% 7% 3 6% 4 3 4 3 6% 8% 39%	1680
	21st to end	Night	22% 6% 3 5% 4 4 5% 3 6% 8% 35%	1680
July	1st to 10th	Night	14% 8% 3 8% 5% 4 6% 4 6% 10% 33%	4680
	11th to 20th	Night	12% 8% 2 7% 4 4 4 4 6% 8% 41%	4680
	21st to end	Night	14% 5% 2 4 4 4 4 4 5% 8% 46%	5084
August	1st to 10th	Night	19% 7% 3 9% 5% 5% 4 4 6% 7% 33%	2956
	11th to 20th	Night	14% 9% 3 7% 4 5% 4 4 6% 8% 35%	2909
	21st to end	Night	10% 4 2 5% 4 4 5% 4 6% 10% 46%	3444
September	1st to 10th	Night	12% 6% 2 5% 4 4 4 3 5% 6% 49%	2043
	11th to 20th	Night	14% 7% 2 4 3 2 3 3 4 7% 51%	2200
	21st to end	Night	15% 6% 3 4 3 3 4 4 5% 7% 47%	2200
October	1st to 10th	Night	17% 7% 3 6% 4 5% 4 3 5% 6% 42%	2200
	11th to 20th	Night	16% 5% 3 5% 4 4 5% 4 6% 7% 42%	2464
	21st to end	Night	23% 6% 2 6% 3 3 5% 3 5% 5% 38%	2860
November	1st to 10th	Night	17% 6% 2 4 4 4 4 3 4 6% 46%	2667
	11th to 20th	Night	17% 6% 2 4 4 4 3 3 4 6% 46%	2730
	21st to end	Night	16% 7% 2 6% 4 4 3 3 5% 7% 43%	2730
December	1st to 10th	Night	17% 10% 2 6% 4 4 4 3 5% 6% 39%	4651
	11th to 20th	Night	23% 7% 2 5% 3 3 3 3 4 5% 42%	5348
	21st to end	Night	20% 11% 3 7% 4 3 3 3 4 5% 36%	7150

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	37% 22% 9% 19% 13% 1	4662
	11th to 20th	Day	35% 22% 7% 21% 14% 3	4680
	21st to end	Day	30% 23% 7% 20% 18% 4	5124
February	1st to 10th	Day	18% 59% 6% 9% 6% 1	1878
	11th to 20th	Day	19% 58% 4 9% 8% 2	2057
	21st to end	Day	17% 59% 4 9% 8% 2	1730
March	1st to 10th	Day	40% 30% 5% 7% 11% 8%	4579
	11th to 20th	Day	34% 30% 13% 12% 8% 3	4986
	21st to end	Day	42% 28% 4 10% 12% 5%	5744
April	1st to 10th	Day	22% 59% 3 7% 7% 2	2520
	11th to 20th	Day	18% 60% 4 9% 6% 2	2588
	21st to end	Day	20% 60% 3 8% 8% 1	2730
May	1st to 10th	Day	47% 27% 9% 12% 5%	7651
	11th to 20th	Day	41% 25% 11% 16% 6% 1	7723
	21st to end	Day	40% 24% 10% 17% 8%	9061
June	1st to 10th	Day	17% 60% 5% 11% 6%	2940
	11th to 20th	Day	16% 59% 5% 10% 9% 1	2940
	21st to end	Day	19% 58% 5% 11% 7%	2940
July	1st to 10th	Day	40% 22% 11% 18% 9%	8210
	11th to 20th	Day	28% 22% 11% 25% 13% 1	8205
	21st to end	Day	32% 21% 10% 22% 14%	8795
August	1st to 10th	Day	30% 36% 10% 18% 6%	4365
	11th to 20th	Day	31% 37% 10% 17% 4	4266
	21st to end	Day	18% 37% 10% 19% 14% 1	4450
September	1st to 10th	Day	13% 58% 6% 15% 8% 1	2355
	11th to 20th	Day	16% 57% 4 13% 10% 1	2200
	21st to end	Day	16% 58% 3 15% 8% 1	2200
October	1st to 10th	Day	29% 36% 6% 18% 10% 1	3590
	11th to 20th	Day	18% 36% 5% 22% 18% 1	3122
	21st to end	Day	20% 36% 6% 20% 15% 2	3180
November	1st to 10th	Day	19% 53% 7% 12% 8% 1	1851
	11th to 20th	Day	18% 54% 4 10% 12% 3	1890
	21st to end	Day	20% 54% 7% 10% 8% 1	1890
December	1st to 10th	Day	25% 35% 8% 24% 8%	2734
	11th to 20th	Day	27% 30% 9% 13% 13% 8%	3066
	21st to end	Day	38% 28% 4 15% 12% 3	4048
January	1st to 10th	Night	34% 21% 9% 24% 12% 2	8234
	11th to 20th	Night	42% 21% 8% 15% 11% 3	8280
	21st to end	Night	35% 22% 4 17% 16% 5%	9057
February	1st to 10th	Night	20% 59% 5% 8% 7% 2	2737
	11th to 20th	Night	18% 58% 4 10% 10% 1	2460
	21st to end	Night	20% 59% 3 10% 6% 2	2076
March	1st to 10th	Night	41% 28% 6% 8% 11% 6%	5494
	11th to 20th	Night	35% 29% 12% 12% 9% 2	5193
	21st to end	Night	42% 28% 3 8% 15% 4	5413
April	1st to 10th	Night	23% 59% 2 7% 8% 2	2100
	11th to 20th	Night	20% 60% 3 7% 8% 1	1980
	21st to end	Night	22% 59% 4 6% 8% 2	1890
May	1st to 10th	Night	47% 24% 10% 9% 8% 1	5265
	11th to 20th	Night	41% 25% 9% 19% 5%	5184
	21st to end	Night	42% 24% 9% 17% 7% 1	5115
June	1st to 10th	Night	19% 60% 3 11% 6%	1680
	11th to 20th	Night	16% 60% 7% 10% 6% 1	1680
	21st to end	Night	20% 58% 5% 12% 5% 1	1680
July	1st to 10th	Night	38% 21% 12% 19% 11%	4640
	11th to 20th	Night	32% 21% 9% 22% 15%	4632
	21st to end	Night	30% 20% 11% 26% 13%	4967
August	1st to 10th	Night	37% 36% 9% 11% 6% 1	2946
	11th to 20th	Night	32% 37% 9% 16% 6%	2936
	21st to end	Night	18% 37% 11% 18% 16%	3444
September	1st to 10th	Night	15% 57% 5% 14% 8% 1	2043
	11th to 20th	Night	18% 57% 4 12% 9% 1	2200
	21st to end	Night	16% 57% 4 15% 6% 1	2200
October	1st to 10th	Night	29% 36% 7% 16% 10% 1	3590
	11th to 20th	Night	16% 35% 6% 20% 21% 2	4013
	21st to end	Night	22% 36% 6% 21% 14% 2	4610
November	1st to 10th	Night	20% 53% 3 14% 8% 2	2667
	11th to 20th	Night	19% 54% 4 10% 12% 2	2730
	21st to end	Night	19% 55% 7% 11% 8% 1	2730
December	1st to 10th	Night	30% 35% 5% 22% 8%	4651
	11th to 20th	Night	32% 30% 8% 11% 15% 3	5348
	21st to end	Night	38% 27% 6% 14% 14% 1	7150

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	22% 19% 31% 24% 4	220
	11th to 20th	Day	24% 16% 30% 23% 6% 1	220
	21st to end	Day	17% 16% 42% 19% 7%	242
February	1st to 10th	Day	23% 14% 40% 19% 4	408
	11th to 20th	Day	19% 18% 39% 21% 4	536
	21st to end	Day	22% 14% 35% 24% 5%	446
March	1st to 10th	Day	24% 14% 35% 23% 3	540
	11th to 20th	Day	20% 17% 35% 21% 6% 1	540
	21st to end	Day	25% 14% 36% 18% 6% 2	594
April	1st to 10th	Day	25% 16% 31% 22% 5% 1	540
	11th to 20th	Day	22% 11% 34% 24% 8% 1	532
	21st to end	Day	27% 10% 30% 22% 10% 1	540
May	1st to 10th	Day	28% 8% 31% 23% 7% 1	543
	11th to 20th	Day	19% 8% 32% 30% 10% 1	567
	21st to end	Day	20% 7% 30% 30% 11% 1	617
June	1st to 10th	Day	20% 4 26% 32% 14% 3	570
	11th to 20th	Day	16% 7% 31% 29% 14% 4	570
	21st to end	Day	20% 4 24% 35% 15% 3	570
July	1st to 10th	Day	16% 3 20% 36% 22% 2	565
	11th to 20th	Day	15% 4 17% 40% 19% 5%	564
	21st to end	Day	13% 3 21% 35% 22% 5%	620
August	1st to 10th	Day	12% 2 21% 35% 25% 5%	566
	11th to 20th	Day	15% 3 17% 35% 22% 7%	535
	21st to end	Day	12% 2 21% 39% 20% 6%	583
September	1st to 10th	Day	15% 3 23% 34% 21% 5%	530
	11th to 20th	Day	15% 7% 25% 24% 20% 10%	530
	21st to end	Day	18% 7% 22% 32% 15% 6%	530
October	1st to 10th	Day	21% 9% 21% 26% 18% 5%	530
	11th to 20th	Day	19% 10% 28% 27% 13% 3	431
	21st to end	Day	26% 8% 29% 27% 10% 1	462
November	1st to 10th	Day	17% 12% 32% 27% 10% 2	425
	11th to 20th	Day	21% 14% 33% 26% 6% 1	430
	21st to end	Day	25% 14% 30% 23% 6% 1	430
December	1st to 10th	Day	21% 14% 38% 19% 8%	261
	11th to 20th	Day	25% 18% 34% 20% 3	220
	21st to end	Day	23% 20% 31% 16% 9% 1	242
January	1st to 10th	Night	21% 16% 39% 20% 4	840
	11th to 20th	Night	26% 17% 35% 18% 4	840
	21st to end	Night	18% 18% 39% 21% 4	924
February	1st to 10th	Night	24% 15% 36% 21% 3	840
	11th to 20th	Night	20% 17% 39% 21% 3	823
	21st to end	Night	20% 15% 36% 23% 6%	692
March	1st to 10th	Night	25% 13% 37% 23% 2	840
	11th to 20th	Night	23% 12% 35% 22% 8%	840
	21st to end	Night	25% 12% 34% 23% 5%	924
April	1st to 10th	Night	28% 10% 32% 26% 4	840
	11th to 20th	Night	23% 9% 33% 28% 8% 1	736
	21st to end	Night	24% 6% 30% 30% 8% 1	630
May	1st to 10th	Night	26% 4 29% 29% 11% 1	630
	11th to 20th	Night	21% 4 30% 32% 12% 1	609
	21st to end	Night	27% 3 21% 34% 14% 1	455
June	1st to 10th	Night	23% 5% 21% 35% 16% 2	420
	11th to 20th	Night	21% 4 20% 35% 18% 2	420
	21st to end	Night	23% 2 15% 41% 17% 1	420
July	1st to 10th	Night	19% 2 13% 42% 22% 2	410
	11th to 20th	Night	15% 1 12% 45% 24% 3	408
	21st to end	Night	15% 1 14% 45% 22% 3	460
August	1st to 10th	Night	14% 2 15% 42% 25% 3	591
	11th to 20th	Night	13% 3 13% 43% 24% 3	605
	21st to end	Night	15% 2 18% 37% 25% 3	790
September	1st to 10th	Night	13% 2 22% 38% 23% 3	800
	11th to 20th	Night	17% 4 23% 28% 22% 7%	800
	21st to end	Night	19% 4 22% 32% 20% 3	800
October	1st to 10th	Night	21% 7% 22% 28% 20% 3	800
	11th to 20th	Night	18% 8% 29% 30% 13% 2	800
	21st to end	Night	26% 8% 29% 28% 8%	880
November	1st to 10th	Night	19% 9% 34% 29% 7% 1	820
	11th to 20th	Night	21% 13% 36% 22% 8% 1	840
	21st to end	Night	20% 15% 36% 23% 6%	840
December	1st to 10th	Night	25% 11% 41% 17% 5%	840
	11th to 20th	Night	25% 17% 34% 21% 3	840
	21st to end	Night	26% 13% 36% 22% 3	924

Page generated at 2026-08-12 03:37:15. Version ac07

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