North Falmouth
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the North Falmouth clear sky chart from 2005-11-06 to 2025-05-11.

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	18% 9% 3 6% 4 4 4 3 5% 7% 40%	113473
	Night	18% 9% 3 6% 4 4 4 3 5% 6% 39%	115468
Transparency	Day	44% 5% 12% 22% 14% 2	117083
	Night	46% 4 11% 21% 15% 2	120022
Seeing	Day	20% 11% 30% 25% 11% 3	15815
	Night	21% 12% 29% 27% 10% 1	24791

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% 9% 3 6% 3 3 4 4 4 6% 48%	3597
	11th to 20th	Day	16% 9% 2 5% 3 2 3 3 4 4 48%	3570
	21st to end	Day	18% 9% 3 8% 4 4 4 3 3 4 41%	4030
February	1st to 10th	Day	19% 10% 4 7% 4 4 4 4 5% 6% 35%	1904
	11th to 20th	Day	15% 8% 2 6% 4 3 4 4 4 7% 42%	1948
	21st to end	Day	18% 9% 3 5% 4 3 4 3 5% 6% 38%	1650
March	1st to 10th	Day	21% 13% 3 6% 3 3 3 3 4 5% 37%	4476
	11th to 20th	Day	14% 10% 4 6% 3 3 3 3 5% 7% 40%	4857
	21st to end	Day	25% 11% 3 7% 3 4 3 3 4 5% 32%	5643
April	1st to 10th	Day	21% 9% 2 5% 3 3 3 4 4 6% 39%	2400
	11th to 20th	Day	19% 8% 2 5% 4 3 4 4 5% 6% 40%	2409
	21st to end	Day	19% 8% 2 5% 4 4 3 4 5% 7% 38%	2600
May	1st to 10th	Day	27% 11% 4 7% 4 3 4 3 5% 6% 26%	4236
	11th to 20th	Day	23% 13% 3 6% 3 3 3 3 5% 7% 31%	4081
	21st to end	Day	16% 7% 2 6% 4 3 4 3 5% 8% 42%	4736
June	1st to 10th	Day	20% 7% 2 6% 4 3 4 4 6% 9% 35%	2660
	11th to 20th	Day	17% 7% 2 5% 5% 4 4 4 6% 8% 39%	2660
	21st to end	Day	15% 8% 3 5% 3 4 4 3 7% 9% 40%	2660
July	1st to 10th	Day	16% 9% 3 7% 4 4 4 4 5% 7% 36%	4440
	11th to 20th	Day	14% 7% 3 6% 4 4 5% 4 7% 9% 37%	4440
	21st to end	Day	18% 7% 3 7% 4 4 4 4 6% 8% 34%	4726
August	1st to 10th	Day	18% 8% 3 8% 5% 4 4 3 5% 8% 34%	4105
	11th to 20th	Day	12% 7% 3 6% 4 5% 5% 4 7% 8% 40%	4045
	21st to end	Day	8% 4 1 5% 4 3 4 4 7% 9% 52%	4180
September	1st to 10th	Day	14% 6% 2 6% 4 4 4 4 5% 6% 47%	2238
	11th to 20th	Day	14% 6% 2 5% 3 3 4 3 5% 7% 49%	2090
	21st to end	Day	20% 7% 2 6% 4 4 3 4 4 6% 41%	2090
October	1st to 10th	Day	21% 6% 2 5% 3 3 4 3 4 6% 42%	2090
	11th to 20th	Day	15% 6% 2 4 4 3 3 3 4 7% 48%	1893
	21st to end	Day	19% 6% 2 6% 3 3 3 3 5% 7% 43%	1881
November	1st to 10th	Day	17% 8% 2 6% 3 4 4 2 4 5% 45%	1761
	11th to 20th	Day	17% 7% 2 4 4 3 4 4 5% 6% 45%	1800
	21st to end	Day	18% 8% 2 6% 4 3 4 3 5% 6% 43%	1800
December	1st to 10th	Day	15% 9% 3 5% 3 3 3 2 4 6% 48%	2831
	11th to 20th	Day	18% 7% 3 6% 4 3 3 2 3 5% 45%	2986
	21st to end	Day	20% 12% 4 7% 4 5% 4 3 4 5% 33%	3960
January	1st to 10th	Night	14% 6% 2 4 3 3 3 3 4 6% 52%	6349
	11th to 20th	Night	15% 12% 3 8% 3 3 3 2 4 4 42%	6315
	21st to end	Night	17% 11% 4 8% 4 4 4 3 4 5% 36%	6923
February	1st to 10th	Night	17% 9% 3 8% 4 4 4 3 4 7% 37%	2495
	11th to 20th	Night	19% 9% 3 6% 4 4 4 3 4 6% 39%	2338
	21st to end	Night	16% 10% 3 7% 5% 3 3 3 4 5% 41%	1980
March	1st to 10th	Night	24% 11% 2 5% 3 3 3 3 4 5% 37%	5364
	11th to 20th	Night	16% 13% 5% 7% 4 4 4 3 4 4 36%	5101
	21st to end	Night	23% 12% 3 7% 5% 4 4 3 4 4 32%	5286
April	1st to 10th	Night	20% 9% 3 7% 5% 4 4 3 4 5% 36%	2000
	11th to 20th	Night	20% 9% 3 6% 4 4 3 3 6% 6% 37%	1938
	21st to end	Night	20% 8% 3 7% 4 5% 4 3 5% 5% 37%	1800
May	1st to 10th	Night	25% 12% 5% 8% 5% 3 3 3 5% 5% 25%	2924
	11th to 20th	Night	25% 12% 3 7% 5% 4 4 3 6% 6% 25%	2820
	21st to end	Night	16% 9% 4 7% 5% 4 5% 4 6% 7% 32%	2866
June	1st to 10th	Night	18% 10% 4 6% 4 4 5% 4 6% 8% 33%	1520
	11th to 20th	Night	19% 7% 2 7% 3 3 5% 3 5% 7% 39%	1520
	21st to end	Night	20% 9% 3 6% 4 3 4 2 7% 10% 33%	1520
July	1st to 10th	Night	21% 14% 3 6% 4 3 4 3 5% 7% 30%	2520
	11th to 20th	Night	17% 9% 3 7% 4 4 5% 4 6% 9% 33%	2520
	21st to end	Night	18% 10% 3 8% 5% 5% 4 4 6% 6% 30%	2793
August	1st to 10th	Night	22% 9% 3 7% 4 4 5% 4 6% 7% 28%	2825
	11th to 20th	Night	10% 8% 2 8% 6% 4 6% 4 7% 7% 37%	2782
	21st to end	Night	9% 5% 2 5% 4 3 4 4 6% 8% 52%	3472
September	1st to 10th	Night	14% 6% 2 6% 4 4 4 3 5% 6% 45%	1941
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 5% 6% 50%	2090
	21st to end	Night	20% 7% 2 6% 4 4 4 3 5% 4 40%	2090
October	1st to 10th	Night	20% 8% 3 7% 4 4 4 3 4 5% 38%	2090
	11th to 20th	Night	16% 5% 2 5% 4 3 3 3 5% 7% 47%	2284
	21st to end	Night	19% 9% 3 6% 4 4 4 3 4 6% 39%	2717
November	1st to 10th	Night	16% 6% 2 6% 3 4 4 4 4 5% 45%	2537
	11th to 20th	Night	15% 9% 2 6% 3 4 4 3 4 6% 44%	2600
	21st to end	Night	16% 6% 2 6% 3 3 3 3 5% 7% 45%	2600
December	1st to 10th	Night	18% 8% 2 6% 4 3 3 3 3 4 47%	4344
	11th to 20th	Night	20% 10% 3 6% 4 3 3 3 4 5% 41%	5208
	21st to end	Night	18% 10% 5% 7% 4 4 4 2 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	38% 3 7% 25% 23% 3	3598
	11th to 20th	Day	40% 3 11% 23% 16% 7%	3600
	21st to end	Day	49% 3 9% 20% 17% 2	4030
February	1st to 10th	Day	50% 4 13% 18% 12% 2	1904
	11th to 20th	Day	43% 4 11% 24% 15% 2	1957
	21st to end	Day	47% 3 12% 20% 15% 3	1650
March	1st to 10th	Day	50% 3 12% 13% 11% 10%	4479
	11th to 20th	Day	42% 4 16% 26% 10% 2	4857
	21st to end	Day	57% 3 6% 14% 14% 6%	5643
April	1st to 10th	Day	46% 5% 9% 21% 17% 3	2400
	11th to 20th	Day	44% 5% 9% 24% 17% 2	2409
	21st to end	Day	45% 5% 9% 22% 16% 4	2600
May	1st to 10th	Day	60% 5% 12% 14% 8% 1	4250
	11th to 20th	Day	54% 5% 14% 16% 10% 1	4085
	21st to end	Day	42% 5% 12% 22% 18% 1	4736
June	1st to 10th	Day	46% 6% 13% 23% 11%	2660
	11th to 20th	Day	42% 6% 15% 23% 13% 1	2660
	21st to end	Day	41% 6% 17% 24% 12% 1	2660
July	1st to 10th	Day	47% 7% 12% 20% 13% 1	4370
	11th to 20th	Day	41% 6% 17% 23% 12% 1	4365
	21st to end	Day	47% 6% 13% 20% 12% 1	4575
August	1st to 10th	Day	49% 5% 17% 21% 7%	4092
	11th to 20th	Day	41% 6% 16% 29% 8% 1	4084
	21st to end	Day	28% 6% 16% 30% 19% 1	4180
September	1st to 10th	Day	38% 5% 10% 25% 19% 1	2238
	11th to 20th	Day	35% 4 11% 27% 20% 3	2090
	21st to end	Day	44% 5% 8% 23% 18% 2	2090
October	1st to 10th	Day	38% 4 11% 27% 16% 3	3480
	11th to 20th	Day	31% 3 9% 30% 25% 2	3122
	21st to end	Day	30% 3 15% 33% 17% 2	3081
November	1st to 10th	Day	43% 5% 13% 21% 16% 2	1761
	11th to 20th	Day	39% 5% 12% 22% 19% 3	1800
	21st to end	Day	42% 5% 12% 27% 12% 2	1800
December	1st to 10th	Day	40% 3 15% 28% 14% 1	2831
	11th to 20th	Day	43% 3 15% 18% 16% 5%	2986
	21st to end	Day	52% 6% 10% 20% 8% 3	3960
January	1st to 10th	Night	35% 3 9% 34% 17% 2	6349
	11th to 20th	Night	47% 3 10% 21% 15% 4	6360
	21st to end	Night	51% 3 9% 19% 14% 4	6923
February	1st to 10th	Night	49% 4 12% 22% 11% 1	2495
	11th to 20th	Night	47% 4 8% 23% 14% 3	2340
	21st to end	Night	46% 3 10% 24% 14% 3	1980
March	1st to 10th	Night	51% 3 9% 16% 13% 6%	5374
	11th to 20th	Night	51% 4 13% 18% 11% 2	5102
	21st to end	Night	57% 3 5% 11% 19% 5%	5286
April	1st to 10th	Night	51% 4 7% 18% 16% 4	2000
	11th to 20th	Night	48% 4 9% 20% 17% 3	1938
	21st to end	Night	48% 4 9% 20% 15% 3	1800
May	1st to 10th	Night	62% 4 12% 14% 7% 1	2930
	11th to 20th	Night	60% 4 12% 14% 7% 2	2821
	21st to end	Night	52% 6% 11% 19% 12% 1	2867
June	1st to 10th	Night	49% 6% 12% 21% 12% 1	1520
	11th to 20th	Night	44% 5% 11% 25% 14% 1	1520
	21st to end	Night	47% 5% 14% 22% 11% 1	1520
July	1st to 10th	Night	54% 5% 9% 18% 14%	2480
	11th to 20th	Night	47% 7% 16% 21% 10% 1	2472
	21st to end	Night	52% 6% 13% 17% 10% 1	2704
August	1st to 10th	Night	53% 8% 13% 18% 9% 1	2813
	11th to 20th	Night	42% 7% 16% 27% 8%	2809
	21st to end	Night	29% 5% 16% 28% 21% 1	3472
September	1st to 10th	Night	38% 6% 11% 25% 19% 1	1941
	11th to 20th	Night	35% 4 11% 25% 22% 2	2090
	21st to end	Night	46% 6% 6% 21% 18% 3	2090
October	1st to 10th	Night	42% 5% 9% 28% 14% 2	3480
	11th to 20th	Night	27% 3 9% 26% 32% 3	3794
	21st to end	Night	34% 4 11% 28% 21% 2	4467
November	1st to 10th	Night	42% 5% 11% 23% 16% 3	2537
	11th to 20th	Night	42% 4 11% 22% 18% 4	2600
	21st to end	Night	38% 5% 15% 28% 12% 2	2600
December	1st to 10th	Night	43% 3 12% 26% 15% 1	4344
	11th to 20th	Night	47% 3 14% 16% 17% 2	5208
	21st to end	Night	49% 6% 12% 19% 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	25% 19% 35% 17% 6%	200
	11th to 20th	Day	22% 22% 35% 17% 5%	200
	21st to end	Day	19% 16% 42% 16% 5% 2	251
February	1st to 10th	Day	26% 17% 35% 19% 3	453
	11th to 20th	Day	26% 20% 36% 16% 2	496
	21st to end	Day	21% 22% 36% 16% 4 1	414
March	1st to 10th	Day	26% 15% 35% 19% 5%	500
	11th to 20th	Day	19% 18% 39% 22% 3	500
	21st to end	Day	25% 16% 33% 16% 7% 2	550
April	1st to 10th	Day	28% 13% 33% 19% 6% 1	500
	11th to 20th	Day	23% 16% 34% 22% 5% 1	492
	21st to end	Day	22% 12% 33% 22% 8% 2	500
May	1st to 10th	Day	29% 10% 32% 22% 7% 1	503
	11th to 20th	Day	22% 8% 30% 29% 9% 2	487
	21st to end	Day	21% 5% 25% 28% 17% 3	529
June	1st to 10th	Day	20% 3 27% 32% 16% 2	490
	11th to 20th	Day	15% 7% 30% 33% 13% 3	490
	21st to end	Day	15% 7% 24% 35% 16% 3	490
July	1st to 10th	Day	17% 4 19% 33% 24% 3	485
	11th to 20th	Day	14% 5% 20% 32% 25% 5%	484
	21st to end	Day	13% 5% 25% 36% 18% 3	532
August	1st to 10th	Day	13% 4 24% 38% 19% 3	486
	11th to 20th	Day	15% 6% 17% 37% 20% 6%	479
	21st to end	Day	13% 4 23% 31% 21% 8%	539
September	1st to 10th	Day	16% 6% 23% 28% 19% 8%	490
	11th to 20th	Day	16% 7% 26% 25% 16% 10%	490
	21st to end	Day	23% 8% 28% 24% 13% 4	490
October	1st to 10th	Day	24% 8% 24% 26% 13% 6%	490
	11th to 20th	Day	17% 11% 30% 25% 13% 4	421
	21st to end	Day	22% 10% 34% 20% 10% 3	429
November	1st to 10th	Day	17% 13% 31% 25% 11% 3	395
	11th to 20th	Day	19% 20% 33% 22% 6%	400
	21st to end	Day	21% 20% 31% 19% 9% 2	400
December	1st to 10th	Day	21% 16% 37% 20% 6%	340
	11th to 20th	Day	27% 21% 34% 15% 4	200
	21st to end	Day	22% 19% 35% 18% 5% 2	220
January	1st to 10th	Night	23% 16% 40% 18% 4	800
	11th to 20th	Night	26% 18% 35% 18% 3	800
	21st to end	Night	22% 18% 35% 21% 4 1	880
February	1st to 10th	Night	22% 16% 35% 25% 2	800
	11th to 20th	Night	26% 17% 40% 15% 1	783
	21st to end	Night	22% 18% 37% 19% 4	660
March	1st to 10th	Night	24% 15% 34% 22% 5%	800
	11th to 20th	Night	22% 13% 37% 23% 5% 1	800
	21st to end	Night	24% 13% 33% 22% 7%	880
April	1st to 10th	Night	25% 11% 32% 27% 4	800
	11th to 20th	Night	24% 11% 32% 28% 5%	754
	21st to end	Night	22% 9% 33% 26% 9% 1	600
May	1st to 10th	Night	27% 9% 27% 27% 9% 1	600
	11th to 20th	Night	25% 8% 33% 28% 6% 1	565
	21st to end	Night	22% 4 20% 40% 13% 1	487
June	1st to 10th	Night	22% 6% 23% 32% 17% 1	380
	11th to 20th	Night	22% 5% 16% 44% 12% 1	380
	21st to end	Night	22% 6% 17% 39% 15%	380
July	1st to 10th	Night	18% 6% 16% 40% 19% 1	370
	11th to 20th	Night	18% 4 16% 37% 23% 2	368
	21st to end	Night	17% 4 16% 42% 19% 1	446
August	1st to 10th	Night	12% 3 22% 42% 20% 1	568
	11th to 20th	Night	15% 3 17% 41% 23% 1	562
	21st to end	Night	15% 4 15% 38% 25% 4	832
September	1st to 10th	Night	17% 3 21% 36% 21% 2	760
	11th to 20th	Night	17% 4 22% 29% 22% 6%	760
	21st to end	Night	22% 6% 22% 30% 17% 3	760
October	1st to 10th	Night	25% 8% 22% 26% 16% 3	760
	11th to 20th	Night	19% 12% 28% 26% 14% 2	760
	21st to end	Night	23% 12% 32% 23% 9% 1	836
November	1st to 10th	Night	19% 10% 30% 27% 12% 1	780
	11th to 20th	Night	20% 19% 32% 21% 7% 1	800
	21st to end	Night	18% 20% 30% 23% 7% 1	800
December	1st to 10th	Night	25% 17% 33% 20% 5% 1	800
	11th to 20th	Night	25% 19% 33% 19% 4	800
	21st to end	Night	21% 19% 36% 21% 3	880

Page generated at 2025-05-08 09:34:01. Version ac07

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