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 2024-12-24.

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 4 5% 7% 38%	101577
	Night	18% 8% 2 6% 4 4 4 3 5% 6% 38%	98496
Transparency	Day	25% 47% 7% 12% 8% 1	105115
	Night	25% 47% 6% 12% 8% 1	103047
Seeing	Day	21% 13% 32% 22% 9% 3	15103
	Night	21% 12% 30% 26% 9% 1	24002

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	15% 8% 2 6% 4 4 4 3 4 6% 43%	2517
	11th to 20th	Day	17% 7% 3 5% 4 4 4 3 4 6% 42%	2520
	21st to end	Day	17% 12% 3 7% 3 3 4 4 5% 5% 38%	2772
February	1st to 10th	Day	18% 9% 3 7% 4 4 4 4 6% 5% 36%	1691
	11th to 20th	Day	12% 9% 3 6% 6% 5% 4 4 6% 6% 38%	1848
	21st to end	Day	15% 11% 3 7% 5% 4 4 4 5% 4 38%	1570
March	1st to 10th	Day	26% 11% 3 7% 5% 3 4 3 4 4 30%	3116
	11th to 20th	Day	17% 8% 3 6% 4 4 4 3 6% 7% 41%	3405
	21st to end	Day	23% 9% 2 6% 3 3 3 3 6% 5% 36%	3932
April	1st to 10th	Day	20% 10% 3 6% 4 4 4 3 4 6% 36%	2280
	11th to 20th	Day	17% 6% 1 5% 5% 5% 4 4 6% 7% 40%	2338
	21st to end	Day	22% 9% 2 6% 4 4 4 3 4 6% 35%	2470
May	1st to 10th	Day	24% 12% 3 7% 5% 4 4 4 5% 6% 28%	4106
	11th to 20th	Day	19% 9% 3 8% 5% 4 4 4 5% 7% 33%	4144
	21st to end	Day	16% 6% 2 6% 4 4 5% 4 7% 9% 38%	4837
June	1st to 10th	Day	22% 8% 2 5% 5% 5% 5% 3 6% 7% 33%	2660
	11th to 20th	Day	17% 6% 2 5% 5% 4 4 3 6% 7% 40%	2660
	21st to end	Day	22% 7% 2 6% 5% 4 4 4 6% 7% 33%	2660
July	1st to 10th	Day	14% 7% 3 6% 5% 5% 5% 4 6% 9% 36%	4440
	11th to 20th	Day	12% 6% 3 6% 5% 5% 5% 5% 7% 10% 38%	4440
	21st to end	Day	20% 7% 2 5% 4 3 4 4 6% 7% 38%	4814
August	1st to 10th	Day	16% 6% 2 7% 4 4 5% 4 6% 9% 37%	4138
	11th to 20th	Day	15% 8% 3 8% 5% 4 4 4 6% 8% 35%	4045
	21st to end	Day	9% 4 2 6% 5% 5% 5% 4 6% 9% 46%	4315
September	1st to 10th	Day	12% 4 1 5% 4 4 4 3 6% 7% 50%	2242
	11th to 20th	Day	12% 5% 1 6% 3 3 2 3 3 5% 55%	2090
	21st to end	Day	16% 5% 1 4 3 4 4 3 5% 7% 46%	2090
October	1st to 10th	Day	19% 8% 3 6% 4 4 3 2 5% 6% 40%	2090
	11th to 20th	Day	17% 7% 2 6% 4 4 3 3 4 7% 43%	1832
	21st to end	Day	23% 7% 2 5% 3 3 4 4 4 5% 40%	1881
November	1st to 10th	Day	17% 6% 1 4 3 2 4 3 5% 6% 48%	1761
	11th to 20th	Day	19% 4 2 5% 3 3 4 3 6% 6% 46%	1800
	21st to end	Day	23% 7% 2 6% 3 3 3 3 3 6% 42%	1800
December	1st to 10th	Day	22% 9% 2 5% 4 4 3 3 4 6% 38%	1531
	11th to 20th	Day	22% 7% 2 4 3 3 3 3 4 5% 43%	1946
	21st to end	Day	25% 10% 2 6% 4 4 4 3 5% 6% 30%	2796
January	1st to 10th	Night	16% 9% 3 5% 4 3 3 3 4 6% 44%	4429
	11th to 20th	Night	14% 8% 3 9% 5% 4 4 3 5% 6% 39%	4440
	21st to end	Night	19% 12% 3 7% 4 4 4 3 5% 5% 34%	4884
February	1st to 10th	Night	16% 10% 2 7% 5% 5% 4 4 5% 6% 37%	2489
	11th to 20th	Night	14% 10% 2 7% 6% 5% 4 3 5% 4 41%	2218
	21st to end	Night	15% 9% 2 6% 5% 4 4 3 5% 7% 39%	1884
March	1st to 10th	Night	28% 11% 2 4 4 3 4 3 4 4 33%	3724
	11th to 20th	Night	14% 8% 3 8% 6% 5% 5% 4 6% 7% 35%	3553
	21st to end	Night	24% 9% 2 6% 4 3 4 4 6% 5% 32%	3699
April	1st to 10th	Night	21% 10% 2 6% 4 4 3 3 4 5% 38%	1900
	11th to 20th	Night	19% 7% 3 5% 4 3 4 4 5% 6% 39%	1790
	21st to end	Night	22% 7% 2 6% 4 5% 4 3 5% 6% 36%	1710
May	1st to 10th	Night	24% 9% 3 6% 4 4 5% 4 6% 6% 28%	2834
	11th to 20th	Night	17% 8% 2 9% 5% 6% 7% 4 7% 8% 27%	2764
	21st to end	Night	17% 8% 3 7% 5% 4 4 3 5% 7% 38%	2738
June	1st to 10th	Night	22% 7% 3 8% 6% 4 3 3 5% 6% 33%	1520
	11th to 20th	Night	18% 6% 2 6% 4 4 4 3 6% 8% 40%	1520
	21st to end	Night	23% 6% 2 5% 5% 4 4 4 5% 8% 34%	1520
July	1st to 10th	Night	12% 7% 2 8% 6% 5% 6% 5% 7% 9% 34%	2520
	11th to 20th	Night	13% 7% 3 7% 5% 4 4 5% 7% 8% 38%	2520
	21st to end	Night	15% 7% 2 6% 4 5% 4 4 6% 7% 40%	2739
August	1st to 10th	Night	21% 6% 2 8% 5% 5% 5% 3 5% 7% 33%	2759
	11th to 20th	Night	15% 10% 3 7% 4 4 4 4 6% 7% 35%	2782
	21st to end	Night	10% 5% 2 5% 5% 4 4 3 6% 9% 47%	3338
September	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 5% 6% 49%	1938
	11th to 20th	Night	15% 6% 2 4 3 3 3 3 4 7% 51%	2090
	21st to end	Night	15% 6% 2 6% 4 3 3 2 5% 6% 48%	2090
October	1st to 10th	Night	18% 7% 3 6% 4 5% 4 3 5% 6% 39%	2090
	11th to 20th	Night	16% 6% 3 5% 4 4 4 4 4 6% 43%	2344
	21st to end	Night	24% 7% 2 6% 3 4 4 3 5% 4 38%	2717
November	1st to 10th	Night	16% 6% 2 5% 4 4 4 3 4 5% 46%	2537
	11th to 20th	Night	18% 6% 2 4 3 4 4 3 4 7% 46%	2600
	21st to end	Night	17% 7% 2 6% 4 4 4 3 5% 6% 43%	2600
December	1st to 10th	Night	21% 10% 3 6% 3 4 4 3 4 5% 36%	2840
	11th to 20th	Night	22% 8% 2 5% 4 3 4 3 4 5% 40%	3443
	21st to end	Night	24% 10% 3 7% 4 4 4 2 4 4 33%	4933

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	27% 38% 7% 20% 7% 1	2518
	11th to 20th	Day	27% 38% 7% 18% 8% 1	2520
	21st to end	Day	32% 41% 7% 14% 6%	2772
February	1st to 10th	Day	17% 65% 5% 7% 5% 1	1691
	11th to 20th	Day	18% 64% 3 7% 7% 2	1857
	21st to end	Day	16% 65% 4 7% 6% 2	1570
March	1st to 10th	Day	40% 41% 4 5% 9% 1	3119
	11th to 20th	Day	23% 43% 9% 10% 10% 4	3405
	21st to end	Day	33% 41% 2 7% 12% 4	3932
April	1st to 10th	Day	19% 65% 2 6% 6% 2	2280
	11th to 20th	Day	16% 66% 3 9% 5% 1	2338
	21st to end	Day	19% 65% 3 6% 6%	2470
May	1st to 10th	Day	35% 42% 10% 9% 3	4120
	11th to 20th	Day	32% 42% 13% 10% 3	4148
	21st to end	Day	24% 43% 11% 13% 8%	4837
June	1st to 10th	Day	16% 66% 5% 9% 5%	2660
	11th to 20th	Day	14% 65% 5% 9% 7% 1	2660
	21st to end	Day	17% 64% 4 10% 5%	2660
July	1st to 10th	Day	29% 39% 7% 17% 9%	4370
	11th to 20th	Day	24% 37% 12% 19% 8%	4365
	21st to end	Day	31% 36% 9% 14% 9% 1	4661
August	1st to 10th	Day	30% 38% 10% 17% 5%	4124
	11th to 20th	Day	32% 39% 11% 14% 3	4084
	21st to end	Day	18% 39% 13% 16% 14% 1	4315
September	1st to 10th	Day	11% 61% 7% 13% 7% 1	2242
	11th to 20th	Day	16% 60% 4 11% 8% 1	2090
	21st to end	Day	14% 61% 3 14% 7% 1	2090
October	1st to 10th	Day	29% 38% 7% 15% 10% 1	3480
	11th to 20th	Day	17% 37% 5% 22% 18% 1	3022
	21st to end	Day	20% 37% 7% 18% 15% 3	3081
November	1st to 10th	Day	17% 56% 7% 12% 7% 1	1761
	11th to 20th	Day	16% 57% 3 9% 12% 3	1800
	21st to end	Day	19% 57% 6% 10% 7% 1	1800
December	1st to 10th	Day	20% 57% 5% 12% 6%	1531
	11th to 20th	Day	24% 47% 4 11% 8% 5%	1946
	21st to end	Day	36% 38% 7% 10% 7% 2	2796
January	1st to 10th	Night	26% 38% 9% 19% 6% 3	4429
	11th to 20th	Night	26% 38% 8% 15% 9% 3	4440
	21st to end	Night	37% 40% 6% 13% 5%	4884
February	1st to 10th	Night	17% 65% 5% 6% 5% 2	2489
	11th to 20th	Night	17% 64% 3 7% 9% 1	2220
	21st to end	Night	17% 65% 3 7% 6% 3	1884
March	1st to 10th	Night	36% 41% 6% 9% 6% 2	3734
	11th to 20th	Night	28% 42% 5% 12% 10% 2	3554
	21st to end	Night	35% 41% 3 7% 9% 4	3699
April	1st to 10th	Night	19% 65% 2 6% 7% 1	1900
	11th to 20th	Night	17% 66% 3 7% 7% 1	1790
	21st to end	Night	18% 65% 4 6% 5% 2	1710
May	1st to 10th	Night	35% 44% 11% 8% 3	2840
	11th to 20th	Night	34% 43% 11% 9% 3	2765
	21st to end	Night	27% 43% 10% 11% 8% 1	2739
June	1st to 10th	Night	18% 66% 4 8% 5%	1520
	11th to 20th	Night	14% 66% 6% 8% 6% 1	1520
	21st to end	Night	18% 64% 4 10% 4 1	1520
July	1st to 10th	Night	29% 39% 9% 12% 11%	2480
	11th to 20th	Night	29% 38% 7% 16% 10%	2472
	21st to end	Night	29% 37% 8% 17% 9%	2651
August	1st to 10th	Night	37% 38% 9% 11% 5%	2749
	11th to 20th	Night	32% 39% 10% 13% 5%	2809
	21st to end	Night	19% 39% 12% 17% 13%	3338
September	1st to 10th	Night	14% 61% 5% 13% 7% 1	1938
	11th to 20th	Night	17% 60% 4 11% 7% 1	2090
	21st to end	Night	14% 60% 4 15% 6% 1	2090
October	1st to 10th	Night	29% 38% 9% 14% 9% 1	3480
	11th to 20th	Night	16% 37% 5% 19% 21% 2	3893
	21st to end	Night	22% 37% 6% 19% 13% 2	4467
November	1st to 10th	Night	18% 56% 3 13% 7% 1	2537
	11th to 20th	Night	17% 57% 3 9% 12% 2	2600
	21st to end	Night	17% 57% 7% 10% 8% 1	2600
December	1st to 10th	Night	22% 57% 4 10% 7%	2840
	11th to 20th	Night	25% 46% 5% 10% 13% 1	3443
	21st to end	Night	36% 37% 6% 13% 8% 1	4933

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	28% 19% 26% 20% 6%	180
	11th to 20th	Day	26% 19% 29% 21% 4 1	180
	21st to end	Day	17% 19% 38% 19% 7%	198
February	1st to 10th	Day	27% 14% 38% 16% 4	341
	11th to 20th	Day	20% 23% 40% 15% 2	456
	21st to end	Day	25% 14% 38% 17% 5%	382
March	1st to 10th	Day	26% 14% 38% 17% 4	460
	11th to 20th	Day	21% 23% 37% 15% 3 1	460
	21st to end	Day	25% 20% 32% 14% 6% 2	506
April	1st to 10th	Day	28% 20% 30% 16% 5% 2	460
	11th to 20th	Day	22% 19% 34% 18% 6% 2	452
	21st to end	Day	29% 15% 32% 16% 4 2	460
May	1st to 10th	Day	27% 18% 32% 16% 5% 2	463
	11th to 20th	Day	20% 14% 38% 21% 5% 2	487
	21st to end	Day	20% 13% 33% 25% 7% 2	529
June	1st to 10th	Day	21% 12% 34% 25% 6% 2	490
	11th to 20th	Day	13% 14% 37% 24% 10% 2	490
	21st to end	Day	23% 9% 34% 24% 9% 1	490
July	1st to 10th	Day	18% 6% 32% 28% 12% 4	485
	11th to 20th	Day	13% 5% 33% 33% 11% 4	484
	21st to end	Day	14% 5% 33% 30% 14% 4	532
August	1st to 10th	Day	13% 7% 28% 33% 16% 3	486
	11th to 20th	Day	17% 6% 27% 30% 14% 6%	479
	21st to end	Day	12% 6% 30% 29% 15% 7%	539
September	1st to 10th	Day	14% 8% 31% 28% 15% 4	490
	11th to 20th	Day	15% 9% 32% 22% 14% 8%	490
	21st to end	Day	17% 11% 30% 24% 12% 6%	490
October	1st to 10th	Day	22% 13% 21% 28% 13% 4	490
	11th to 20th	Day	21% 13% 30% 23% 11% 4	400
	21st to end	Day	29% 14% 27% 22% 9%	429
November	1st to 10th	Day	19% 17% 29% 24% 9% 2	395
	11th to 20th	Day	24% 17% 33% 22% 4 1	400
	21st to end	Day	27% 15% 29% 21% 7% 1	400
December	1st to 10th	Day	23% 15% 35% 18% 9%	226
	11th to 20th	Day	23% 23% 34% 18% 4	200
	21st to end	Day	26% 21% 30% 17% 7%	204
January	1st to 10th	Night	22% 18% 36% 20% 4	760
	11th to 20th	Night	25% 19% 34% 18% 4	760
	21st to end	Night	20% 19% 39% 20% 3	836
February	1st to 10th	Night	23% 16% 37% 21% 3	760
	11th to 20th	Night	20% 19% 39% 19% 2	743
	21st to end	Night	21% 17% 34% 24% 4	628
March	1st to 10th	Night	24% 16% 38% 20% 2	760
	11th to 20th	Night	22% 14% 35% 21% 8%	760
	21st to end	Night	25% 14% 33% 23% 4 1	836
April	1st to 10th	Night	28% 13% 30% 25% 4	760
	11th to 20th	Night	21% 12% 34% 24% 8%	666
	21st to end	Night	26% 11% 28% 29% 5% 1	570
May	1st to 10th	Night	25% 8% 30% 28% 8% 1	570
	11th to 20th	Night	20% 11% 29% 30% 9% 1	539
	21st to end	Night	25% 9% 23% 30% 13% 1	411
June	1st to 10th	Night	22% 10% 22% 33% 12% 2	380
	11th to 20th	Night	20% 3 24% 34% 17% 1	380
	21st to end	Night	26% 2 21% 38% 14% 1	380
July	1st to 10th	Night	18% 4 17% 43% 18% 1	370
	11th to 20th	Night	17% 3 18% 38% 21% 2	368
	21st to end	Night	16% 3 16% 44% 20% 1	416
August	1st to 10th	Night	15% 3 19% 44% 18% 2	527
	11th to 20th	Night	17% 4 20% 36% 21% 2	562
	21st to end	Night	15% 4 20% 38% 21% 3	750
September	1st to 10th	Night	14% 3 28% 34% 19% 2	760
	11th to 20th	Night	17% 7% 26% 27% 19% 4	760
	21st to end	Night	18% 7% 25% 31% 17% 3	760
October	1st to 10th	Night	22% 8% 23% 28% 17% 2	760
	11th to 20th	Night	19% 10% 32% 26% 11% 2	760
	21st to end	Night	27% 12% 30% 26% 6%	836
November	1st to 10th	Night	20% 11% 34% 27% 6% 2	780
	11th to 20th	Night	23% 13% 34% 22% 7% 1	800
	21st to end	Night	21% 18% 33% 22% 6%	800
December	1st to 10th	Night	25% 14% 39% 17% 5%	836
	11th to 20th	Night	24% 19% 34% 19% 3	807
	21st to end	Night	25% 16% 37% 20% 3	851

Page generated at 2024-12-21 04:13:51. Version ac07

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