Southern Maine Astronomers Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Southern Maine Astronomers Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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% 2 6% 4 4 4 3 5% 7% 38%	130895
	Night	17% 8% 3 7% 4 4 4 3 5% 6% 38%	131192
Transparency	Day	29% 37% 8% 14% 9% 2	134415
	Night	31% 36% 7% 14% 10% 2	135759
Seeing	Day	21% 14% 33% 22% 8% 2	16746
	Night	21% 12% 31% 26% 9% 1	25714

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% 7% 3 8% 6% 5% 5% 4 5% 6% 39%	4661
	11th to 20th	Day	14% 11% 4 5% 4 3 4 2 4 6% 43%	4650
	21st to end	Day	11% 10% 2 5% 3 3 4 3 5% 5% 49%	5124
February	1st to 10th	Day	16% 8% 3 7% 4 4 4 4 6% 5% 38%	1869
	11th to 20th	Day	12% 8% 3 6% 5% 5% 5% 4 6% 6% 39%	2048
	21st to end	Day	15% 11% 3 7% 5% 4 4 4 5% 4 39%	1730
March	1st to 10th	Day	23% 10% 2 7% 5% 4 4 3 4 4 34%	4576
	11th to 20th	Day	18% 11% 3 6% 4 3 3 3 5% 6% 38%	4984
	21st to end	Day	27% 9% 2 5% 3 3 3 3 5% 6% 34%	5762
April	1st to 10th	Day	20% 10% 3 6% 4 4 4 3 4 6% 36%	2520
	11th to 20th	Day	18% 7% 1 5% 4 5% 4 4 5% 6% 40%	2536
	21st to end	Day	22% 8% 2 6% 4 4 4 3 4 6% 36%	2600
May	1st to 10th	Day	33% 11% 3 7% 4 4 3 3 4 5% 23%	5886
	11th to 20th	Day	19% 9% 3 8% 5% 5% 4 4 5% 8% 30%	5952
	21st to end	Day	20% 6% 2 6% 4 4 5% 4 6% 8% 33%	6949
June	1st to 10th	Day	22% 8% 2 5% 4 5% 5% 3 6% 7% 33%	2800
	11th to 20th	Day	17% 7% 2 5% 5% 4 4 4 6% 7% 40%	2800
	21st to end	Day	22% 6% 2 6% 5% 4 4 4 6% 7% 34%	2800
July	1st to 10th	Day	17% 8% 3 6% 5% 5% 5% 4 6% 8% 33%	6360
	11th to 20th	Day	12% 7% 2 6% 5% 4 5% 5% 7% 9% 37%	6360
	21st to end	Day	16% 6% 2 5% 3 3 4 3 6% 8% 45%	6926
August	1st to 10th	Day	15% 6% 2 7% 4 4 5% 4 6% 9% 38%	4269
	11th to 20th	Day	15% 8% 3 8% 5% 4 4 4 5% 7% 35%	4175
	21st to end	Day	8% 4 2 6% 5% 5% 5% 4 6% 9% 46%	4451
September	1st to 10th	Day	12% 4 1 5% 3 4 4 3 6% 7% 50%	2360
	11th to 20th	Day	12% 5% 1 6% 3 3 3 3 3 5% 56%	2200
	21st to end	Day	16% 5% 1 4 3 4 4 3 5% 7% 47%	2200
October	1st to 10th	Day	18% 7% 3 6% 4 4 3 2 5% 6% 42%	2200
	11th to 20th	Day	17% 7% 2 6% 4 4 3 3 4 7% 43%	1928
	21st to end	Day	22% 7% 2 5% 3 3 4 4 4 5% 40%	1980
November	1st to 10th	Day	17% 7% 1 4 3 3 4 3 5% 6% 48%	1851
	11th to 20th	Day	19% 4 2 5% 3 3 4 3 6% 6% 46%	1890
	21st to end	Day	22% 7% 2 7% 3 3 3 3 3 6% 42%	1890
December	1st to 10th	Day	16% 10% 2 4 3 4 4 3 5% 7% 41%	2560
	11th to 20th	Day	19% 7% 2 4 3 3 3 3 4 6% 47%	3000
	21st to end	Day	21% 10% 2 6% 4 4 4 3 4 5% 35%	4048
January	1st to 10th	Night	12% 8% 3 7% 5% 4 4 3 5% 6% 43%	8234
	11th to 20th	Night	14% 11% 4 10% 5% 5% 4 3 5% 5% 34%	8235
	21st to end	Night	12% 10% 3 6% 4 4 4 3 5% 5% 43%	9057
February	1st to 10th	Night	15% 10% 2 7% 5% 5% 4 4 5% 6% 38%	2749
	11th to 20th	Night	14% 10% 2 6% 5% 4 4 4 5% 5% 41%	2458
	21st to end	Night	15% 9% 3 6% 5% 4 4 3 5% 7% 39%	2076
March	1st to 10th	Night	25% 10% 3 6% 4 4 4 3 4 4 33%	5484
	11th to 20th	Night	15% 9% 3 7% 5% 5% 5% 3 5% 6% 35%	5194
	21st to end	Night	25% 9% 2 6% 4 3 4 4 6% 5% 33%	5410
April	1st to 10th	Night	21% 10% 2 6% 4 4 4 3 4 6% 36%	2100
	11th to 20th	Night	20% 7% 3 6% 4 3 4 4 5% 6% 38%	1945
	21st to end	Night	22% 7% 3 7% 4 5% 4 3 4 6% 36%	1800
May	1st to 10th	Night	33% 9% 3 7% 4 4 4 3 5% 5% 23%	4054
	11th to 20th	Night	16% 10% 3 9% 5% 6% 6% 4 6% 7% 29%	3970
	21st to end	Night	19% 9% 3 8% 6% 5% 5% 3 5% 7% 31%	3926
June	1st to 10th	Night	21% 7% 3 8% 6% 5% 3 3 5% 6% 33%	1600
	11th to 20th	Night	18% 6% 2 6% 4 4 4 3 6% 7% 40%	1600
	21st to end	Night	22% 6% 2 5% 5% 4 4 4 5% 8% 35%	1600
July	1st to 10th	Night	13% 7% 2 7% 5% 4 5% 4 7% 8% 36%	3600
	11th to 20th	Night	15% 9% 3 7% 5% 4 4 5% 6% 7% 36%	3600
	21st to end	Night	11% 5% 2 5% 4 4 4 4 6% 7% 47%	3926
August	1st to 10th	Night	21% 6% 2 8% 5% 5% 5% 3 5% 7% 33%	2847
	11th to 20th	Night	15% 9% 3 7% 4 4 4 4 6% 7% 36%	2872
	21st to end	Night	10% 5% 2 5% 5% 4 4 3 6% 9% 47%	3444
September	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 5% 6% 50%	2040
	11th to 20th	Night	15% 5% 2 4 3 3 3 3 4 7% 51%	2200
	21st to end	Night	15% 6% 2 5% 4 3 3 3 5% 6% 48%	2200
October	1st to 10th	Night	17% 7% 3 6% 4 5% 4 3 5% 5% 41%	2200
	11th to 20th	Night	17% 6% 3 5% 4 4 4 4 4 6% 43%	2468
	21st to end	Night	23% 7% 2 6% 3 4 4 3 5% 5% 38%	2860
November	1st to 10th	Night	16% 6% 2 5% 4 4 4 3 4 5% 46%	2667
	11th to 20th	Night	17% 6% 2 5% 3 4 4 3 4 6% 46%	2730
	21st to end	Night	17% 7% 2 6% 4 4 4 3 5% 6% 42%	2730
December	1st to 10th	Night	17% 11% 3 7% 3 4 4 3 5% 6% 39%	4802
	11th to 20th	Night	23% 7% 2 5% 4 3 3 2 4 4 42%	5364
	21st to end	Night	21% 11% 3 7% 4 3 3 2 4 4 37%	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	36% 22% 9% 19% 13% 1	4662
	11th to 20th	Day	34% 21% 8% 21% 13% 2	4680
	21st to end	Day	29% 23% 7% 19% 18% 4	5124
February	1st to 10th	Day	19% 59% 5% 9% 7% 1	1869
	11th to 20th	Day	19% 58% 5% 9% 8% 2	2057
	21st to end	Day	18% 59% 4 9% 8% 2	1730
March	1st to 10th	Day	41% 28% 4 7% 11% 8%	4579
	11th to 20th	Day	34% 30% 12% 12% 9% 3	4984
	21st to end	Day	40% 29% 3 11% 13% 4	5762
April	1st to 10th	Day	22% 59% 3 6% 7% 2	2520
	11th to 20th	Day	19% 61% 4 9% 6% 2	2536
	21st to end	Day	20% 62% 3 7% 7%	2600
May	1st to 10th	Day	49% 31% 9% 9% 2	5897
	11th to 20th	Day	41% 30% 11% 12% 4 1	5956
	21st to end	Day	36% 31% 12% 15% 7%	6949
June	1st to 10th	Day	17% 63% 5% 9% 6%	2800
	11th to 20th	Day	15% 62% 5% 9% 8% 1	2800
	21st to end	Day	17% 61% 4 11% 6% 1	2800
July	1st to 10th	Day	38% 28% 9% 16% 9%	6290
	11th to 20th	Day	32% 28% 11% 18% 10% 1	6285
	21st to end	Day	28% 26% 10% 22% 12%	6728
August	1st to 10th	Day	29% 37% 10% 17% 5%	4255
	11th to 20th	Day	32% 38% 11% 15% 4	4214
	21st to end	Day	18% 38% 13% 16% 14% 1	4451
September	1st to 10th	Day	12% 58% 7% 14% 8% 1	2360
	11th to 20th	Day	16% 57% 4 13% 9% 1	2200
	21st to end	Day	15% 58% 3 15% 8% 1	2200
October	1st to 10th	Day	28% 36% 7% 16% 10% 1	3590
	11th to 20th	Day	18% 36% 6% 22% 17% 1	3118
	21st to end	Day	21% 36% 8% 19% 14% 3	3180
November	1st to 10th	Day	19% 53% 7% 12% 8% 1	1851
	11th to 20th	Day	18% 54% 4 9% 12% 3	1890
	21st to end	Day	21% 54% 6% 10% 7% 1	1890
December	1st to 10th	Day	25% 35% 10% 23% 8%	2560
	11th to 20th	Day	26% 31% 9% 14% 12% 9%	3000
	21st to end	Day	38% 27% 6% 13% 12% 4	4048
January	1st to 10th	Night	34% 21% 9% 23% 11% 2	8234
	11th to 20th	Night	42% 22% 8% 15% 11% 3	8280
	21st to end	Night	35% 22% 4 18% 15% 5%	9057
February	1st to 10th	Night	19% 59% 5% 8% 6% 2	2749
	11th to 20th	Night	18% 58% 4 9% 10% 2	2460
	21st to end	Night	19% 59% 3 9% 7% 2	2076
March	1st to 10th	Night	42% 28% 5% 8% 10% 6%	5494
	11th to 20th	Night	35% 29% 12% 13% 8% 2	5195
	21st to end	Night	40% 29% 3 9% 15% 4	5410
April	1st to 10th	Night	23% 59% 3 7% 8% 1	2100
	11th to 20th	Night	21% 60% 4 7% 7% 1	1945
	21st to end	Night	20% 62% 4 6% 6% 2	1800
May	1st to 10th	Night	49% 31% 8% 7% 3	4060
	11th to 20th	Night	41% 30% 9% 14% 5%	3971
	21st to end	Night	40% 32% 9% 13% 6% 1	3927
June	1st to 10th	Night	19% 63% 4 9% 6%	1600
	11th to 20th	Night	15% 63% 6% 10% 6% 1	1600
	21st to end	Night	18% 61% 5% 11% 5% 1	1600
July	1st to 10th	Night	34% 28% 11% 18% 10%	3560
	11th to 20th	Night	36% 28% 8% 15% 12%	3552
	21st to end	Night	25% 27% 13% 25% 10%	3809
August	1st to 10th	Night	37% 37% 9% 11% 6%	2837
	11th to 20th	Night	32% 38% 10% 14% 5%	2899
	21st to end	Night	19% 38% 12% 18% 13%	3444
September	1st to 10th	Night	15% 58% 5% 14% 8% 1	2040
	11th to 20th	Night	17% 57% 4 12% 9% 1	2200
	21st to end	Night	15% 58% 4 15% 7% 1	2200
October	1st to 10th	Night	28% 37% 9% 15% 10% 1	3590
	11th to 20th	Night	17% 36% 6% 20% 20% 2	4017
	21st to end	Night	23% 36% 6% 20% 13% 2	4610
November	1st to 10th	Night	20% 54% 4 13% 8% 1	2667
	11th to 20th	Night	18% 54% 4 10% 12% 2	2730
	21st to end	Night	19% 55% 8% 10% 8% 1	2730
December	1st to 10th	Night	30% 35% 5% 21% 8%	4802
	11th to 20th	Night	33% 30% 7% 11% 15% 3	5364
	21st to end	Night	39% 26% 6% 13% 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	25% 23% 28% 20% 5%	220
	11th to 20th	Day	25% 19% 31% 20% 4 1	220
	21st to end	Day	15% 21% 42% 16% 6%	242
February	1st to 10th	Day	24% 16% 39% 18% 4	399
	11th to 20th	Day	20% 21% 39% 17% 2	536
	21st to end	Day	23% 15% 41% 17% 5%	446
March	1st to 10th	Day	26% 16% 39% 16% 3	540
	11th to 20th	Day	22% 23% 37% 15% 3 1	540
	21st to end	Day	26% 22% 31% 13% 5% 2	594
April	1st to 10th	Day	28% 20% 32% 15% 4 1	540
	11th to 20th	Day	24% 19% 34% 17% 5% 1	516
	21st to end	Day	29% 15% 32% 17% 4 2	500
May	1st to 10th	Day	29% 17% 31% 16% 5% 2	503
	11th to 20th	Day	20% 14% 38% 20% 5% 2	527
	21st to end	Day	21% 13% 33% 25% 6% 2	573
June	1st to 10th	Day	21% 12% 35% 25% 6% 2	530
	11th to 20th	Day	14% 14% 37% 23% 10% 2	530
	21st to end	Day	22% 8% 35% 24% 10% 2	530
July	1st to 10th	Day	19% 6% 32% 29% 11% 3	525
	11th to 20th	Day	14% 6% 32% 32% 11% 4	524
	21st to end	Day	14% 5% 34% 29% 14% 3	576
August	1st to 10th	Day	12% 6% 27% 34% 16% 4	526
	11th to 20th	Day	16% 5% 27% 31% 14% 7%	519
	21st to end	Day	12% 6% 31% 30% 15% 7%	583
September	1st to 10th	Day	14% 8% 30% 28% 15% 4	530
	11th to 20th	Day	14% 8% 32% 23% 15% 8%	530
	21st to end	Day	17% 10% 32% 23% 13% 6%	530
October	1st to 10th	Day	20% 13% 21% 28% 13% 4	530
	11th to 20th	Day	21% 13% 30% 23% 11% 3	431
	21st to end	Day	28% 13% 27% 23% 8%	462
November	1st to 10th	Day	20% 18% 28% 24% 8% 2	425
	11th to 20th	Day	23% 17% 33% 22% 5% 1	430
	21st to end	Day	26% 17% 29% 21% 6% 1	430
December	1st to 10th	Day	23% 16% 34% 18% 9%	247
	11th to 20th	Day	22% 23% 35% 17% 3	220
	21st to end	Day	25% 22% 28% 18% 8%	242
January	1st to 10th	Night	21% 19% 36% 20% 4	840
	11th to 20th	Night	25% 19% 34% 18% 4	840
	21st to end	Night	19% 20% 39% 19% 3	924
February	1st to 10th	Night	22% 17% 36% 21% 3	840
	11th to 20th	Night	20% 19% 39% 19% 2	823
	21st to end	Night	20% 16% 35% 24% 4	692
March	1st to 10th	Night	24% 17% 38% 19% 2	840
	11th to 20th	Night	23% 14% 35% 21% 7%	840
	21st to end	Night	25% 14% 33% 23% 4 1	924
April	1st to 10th	Night	28% 13% 30% 24% 4	840
	11th to 20th	Night	23% 12% 35% 23% 8%	725
	21st to end	Night	26% 11% 29% 29% 5% 1	600
May	1st to 10th	Night	27% 7% 30% 28% 8% 1	600
	11th to 20th	Night	20% 11% 30% 29% 9% 1	568
	21st to end	Night	24% 9% 23% 30% 13% 1	433
June	1st to 10th	Night	23% 9% 22% 33% 12% 2	400
	11th to 20th	Night	20% 4 25% 35% 17% 1	400
	21st to end	Night	25% 2 21% 38% 14% 1	400
July	1st to 10th	Night	18% 4 17% 44% 17% 1	390
	11th to 20th	Night	18% 3 18% 38% 21% 2	388
	21st to end	Night	15% 3 17% 45% 19% 1	438
August	1st to 10th	Night	14% 3 19% 44% 18% 2	555
	11th to 20th	Night	17% 4 19% 36% 21% 2	592
	21st to end	Night	15% 4 21% 38% 20% 3	790
September	1st to 10th	Night	15% 3 28% 34% 19% 2	800
	11th to 20th	Night	16% 6% 26% 28% 20% 4	800
	21st to end	Night	18% 7% 25% 32% 17% 2	800
October	1st to 10th	Night	21% 9% 23% 28% 17% 2	800
	11th to 20th	Night	19% 10% 32% 26% 12% 2	800
	21st to end	Night	26% 11% 30% 26% 6%	880
November	1st to 10th	Night	21% 12% 34% 26% 6% 2	820
	11th to 20th	Night	22% 13% 34% 22% 7% 1	840
	21st to end	Night	21% 18% 34% 22% 6%	840
December	1st to 10th	Night	25% 15% 39% 16% 5%	880
	11th to 20th	Night	24% 20% 34% 19% 3	848
	21st to end	Night	24% 16% 37% 21% 3	924

Page generated at 2026-04-14 02:44:30. Version ac07

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