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 2026-09-29.

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% 3 6% 4 4 4 3 5% 7% 39%	148694
	Night	18% 9% 3 6% 4 4 4 3 5% 6% 38%	142727
Transparency	Day	44% 5% 13% 23% 13% 2	152258
	Night	46% 4 11% 21% 15% 2	147252
Seeing	Day	20% 12% 30% 25% 11% 3	17671
	Night	22% 12% 29% 27% 10% 1	26455

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	11% 10% 3 6% 4 3 4 4 4 6% 45%	4661
	11th to 20th	Day	17% 9% 3 5% 3 2 3 2 4 4 47%	4650
	21st to end	Day	15% 9% 3 6% 3 3 3 3 3 4 47%	5222
February	1st to 10th	Day	19% 10% 4 7% 4 3 4 4 5% 6% 35%	1999
	11th to 20th	Day	15% 8% 2 6% 4 3 4 4 4 7% 42%	2048
	21st to end	Day	18% 10% 3 5% 4 3 4 3 5% 6% 38%	1730
March	1st to 10th	Day	21% 13% 3 6% 3 3 3 3 4 5% 37%	4576
	11th to 20th	Day	15% 10% 4 6% 3 3 3 3 6% 7% 40%	4964
	21st to end	Day	25% 11% 3 7% 3 4 4 3 4 5% 32%	5768
April	1st to 10th	Day	22% 9% 2 5% 3 3 3 4 4 6% 39%	2520
	11th to 20th	Day	19% 8% 2 5% 4 3 4 4 5% 6% 40%	2531
	21st to end	Day	19% 8% 2 5% 4 4 3 4 5% 7% 38%	2730
May	1st to 10th	Day	24% 12% 4 7% 4 4 4 3 5% 7% 26%	7639
	11th to 20th	Day	19% 11% 3 6% 4 3 4 3 5% 7% 35%	7625
	21st to end	Day	23% 10% 3 7% 4 4 4 3 5% 7% 31%	8875
June	1st to 10th	Day	19% 7% 2 6% 4 3 4 4 6% 9% 36%	2940
	11th to 20th	Day	16% 7% 2 6% 5% 4 4 4 6% 8% 38%	2940
	21st to end	Day	15% 7% 2 5% 3 4 4 3 7% 9% 41%	2940
July	1st to 10th	Day	20% 9% 3 6% 4 4 4 4 5% 9% 34%	8280
	11th to 20th	Day	11% 7% 3 7% 4 4 5% 4 6% 10% 40%	8280
	21st to end	Day	16% 6% 2 6% 4 4 4 3 5% 8% 41%	8821
August	1st to 10th	Day	15% 7% 3 7% 5% 5% 5% 4 6% 9% 36%	6015
	11th to 20th	Day	12% 6% 3 6% 4 4 4 4 6% 8% 42%	5953
	21st to end	Day	11% 4 1 5% 3 4 4 4 7% 9% 47%	6080
September	1st to 10th	Day	14% 6% 2 5% 4 4 4 4 5% 6% 46%	2474
	11th to 20th	Day	14% 5% 2 5% 3 3 3 3 5% 7% 49%	2310
	21st to end	Day	21% 7% 2 6% 4 4 3 3 5% 6% 40%	2288
October	1st to 10th	Day	20% 6% 2 5% 3 3 4 3 4 6% 44%	2200
	11th to 20th	Day	16% 6% 2 4 4 3 3 3 4 7% 47%	1992
	21st to end	Day	18% 6% 2 6% 3 3 3 3 5% 7% 44%	1980
November	1st to 10th	Day	17% 8% 2 6% 3 4 4 2 4 5% 45%	1851
	11th to 20th	Day	16% 7% 2 4 4 3 4 4 5% 6% 45%	1890
	21st to end	Day	17% 8% 2 6% 4 3 4 3 5% 6% 43%	1890
December	1st to 10th	Day	15% 9% 3 5% 3 3 3 2 4 6% 48%	2918
	11th to 20th	Day	18% 7% 3 6% 4 3 3 2 3 5% 45%	3066
	21st to end	Day	20% 12% 3 7% 4 4 4 3 4 5% 33%	4048
January	1st to 10th	Night	15% 7% 2 5% 3 3 3 3 3 5% 50%	8234
	11th to 20th	Night	17% 11% 3 7% 3 3 3 3 4 5% 40%	8235
	21st to end	Night	15% 10% 3 8% 5% 5% 4 3 4 5% 38%	8956
February	1st to 10th	Night	17% 8% 3 8% 4 4 4 3 4 7% 37%	2620
	11th to 20th	Night	19% 9% 3 6% 4 3 3 3 4 6% 39%	2458
	21st to end	Night	17% 10% 3 7% 4 3 3 3 4 5% 41%	2076
March	1st to 10th	Night	25% 11% 2 5% 3 3 3 3 4 5% 37%	5484
	11th to 20th	Night	16% 13% 5% 7% 4 4 4 3 4 4 36%	5214
	21st to end	Night	23% 12% 3 7% 5% 4 4 3 4 4 31%	5403
April	1st to 10th	Night	20% 9% 3 7% 5% 4 3 3 4 5% 36%	2100
	11th to 20th	Night	20% 9% 3 6% 4 4 3 3 6% 6% 37%	2036
	21st to end	Night	20% 8% 3 7% 4 5% 4 3 5% 5% 37%	1890
May	1st to 10th	Night	27% 10% 4 7% 4 3 3 3 5% 6% 29%	5259
	11th to 20th	Night	20% 12% 4 7% 4 4 4 3 6% 6% 31%	5235
	21st to end	Night	22% 10% 4 8% 6% 5% 5% 4 5% 6% 24%	5354
June	1st to 10th	Night	18% 9% 3 6% 4 4 5% 4 5% 8% 34%	1680
	11th to 20th	Night	19% 7% 3 7% 3 3 4 3 5% 7% 39%	1680
	21st to end	Night	19% 9% 3 6% 4 3 5% 2 7% 9% 34%	1680
July	1st to 10th	Night	22% 13% 3 7% 4 3 4 3 5% 7% 29%	4680
	11th to 20th	Night	16% 9% 3 5% 3 4 4 4 6% 9% 37%	4680
	21st to end	Night	16% 8% 3 7% 4 4 4 4 5% 7% 38%	5194
August	1st to 10th	Night	18% 8% 3 7% 4 5% 5% 4 7% 8% 31%	4135
	11th to 20th	Night	11% 8% 2 7% 5% 4 6% 4 7% 8% 39%	4092
	21st to end	Night	12% 5% 2 6% 4 3 4 4 7% 8% 47%	5041
September	1st to 10th	Night	13% 6% 2 6% 4 4 4 3 6% 6% 44%	2145
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 5% 6% 50%	2310
	21st to end	Night	21% 8% 2 6% 4 4 4 3 5% 4 39%	2289
October	1st to 10th	Night	19% 7% 2 7% 4 4 4 3 4 5% 40%	2200
	11th to 20th	Night	17% 6% 2 5% 4 3 3 3 5% 7% 46%	2405
	21st to end	Night	18% 9% 3 5% 4 4 4 3 4 6% 40%	2860
November	1st to 10th	Night	16% 6% 2 6% 3 4 4 4 4 5% 45%	2667
	11th to 20th	Night	15% 8% 2 6% 4 4 4 3 5% 6% 44%	2730
	21st to end	Night	15% 6% 2 6% 3 3 3 3 5% 7% 45%	2730
December	1st to 10th	Night	18% 8% 2 6% 4 3 3 3 3 4 46%	4477
	11th to 20th	Night	20% 10% 3 6% 4 3 3 3 4 5% 42%	5348
	21st to end	Night	18% 10% 5% 7% 4 4 4 2 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	40% 3 11% 23% 20% 3	4662
	11th to 20th	Day	42% 3 9% 22% 17% 7%	4680
	21st to end	Day	42% 2 10% 23% 20% 2	5222
February	1st to 10th	Day	50% 4 12% 18% 13% 2	1999
	11th to 20th	Day	43% 5% 11% 25% 15% 2	2057
	21st to end	Day	47% 3 11% 21% 15% 3	1730
March	1st to 10th	Day	51% 3 12% 14% 11% 10%	4579
	11th to 20th	Day	42% 4 16% 26% 10% 1	4964
	21st to end	Day	57% 3 6% 14% 14% 6%	5768
April	1st to 10th	Day	46% 5% 9% 20% 17% 3	2520
	11th to 20th	Day	44% 5% 9% 23% 17% 2	2531
	21st to end	Day	45% 5% 9% 22% 16% 4	2730
May	1st to 10th	Day	58% 5% 12% 16% 7% 1	7651
	11th to 20th	Day	49% 5% 14% 21% 10% 1	7629
	21st to end	Day	53% 5% 10% 20% 11% 1	8876
June	1st to 10th	Day	45% 6% 13% 23% 12% 1	2940
	11th to 20th	Day	43% 6% 14% 23% 13% 1	2940
	21st to end	Day	40% 6% 17% 24% 13% 1	2940
July	1st to 10th	Day	48% 6% 14% 20% 10% 1	8210
	11th to 20th	Day	38% 6% 17% 27% 12%	8205
	21st to end	Day	41% 5% 12% 26% 15% 1	8625
August	1st to 10th	Day	45% 6% 20% 21% 8%	6002
	11th to 20th	Day	38% 6% 19% 30% 7% 1	5992
	21st to end	Day	32% 6% 15% 29% 16% 1	6080
September	1st to 10th	Day	39% 5% 10% 27% 18% 1	2474
	11th to 20th	Day	35% 4 10% 27% 21% 3	2310
	21st to end	Day	45% 5% 8% 22% 17% 2	2288
October	1st to 10th	Day	38% 4 11% 28% 16% 3	3590
	11th to 20th	Day	32% 3 9% 29% 24% 2	3221
	21st to end	Day	30% 3 15% 33% 17% 2	3180
November	1st to 10th	Day	43% 5% 13% 21% 16% 2	1851
	11th to 20th	Day	39% 5% 11% 23% 19% 3	1890
	21st to end	Day	42% 5% 12% 26% 12% 2	1890
December	1st to 10th	Day	40% 3 14% 27% 15% 1	2918
	11th to 20th	Day	43% 3 15% 18% 17% 5%	3066
	21st to end	Day	52% 6% 10% 19% 9% 3	4048
January	1st to 10th	Night	38% 2 10% 31% 17% 2	8234
	11th to 20th	Night	49% 3 9% 20% 15% 4	8280
	21st to end	Night	49% 3 8% 19% 17% 4	8956
February	1st to 10th	Night	48% 4 12% 22% 13% 1	2620
	11th to 20th	Night	47% 4 8% 24% 14% 3	2460
	21st to end	Night	45% 3 10% 25% 14% 3	2076
March	1st to 10th	Night	51% 3 9% 17% 14% 6%	5494
	11th to 20th	Night	50% 4 13% 18% 11% 2	5215
	21st to end	Night	58% 3 5% 11% 18% 5%	5403
April	1st to 10th	Night	50% 4 8% 18% 16% 4	2100
	11th to 20th	Night	47% 4 10% 20% 16% 3	2036
	21st to end	Night	49% 4 9% 19% 15% 3	1890
May	1st to 10th	Night	57% 4 13% 14% 11% 1	5265
	11th to 20th	Night	54% 4 14% 18% 9% 2	5236
	21st to end	Night	60% 5% 10% 17% 8%	5355
June	1st to 10th	Night	48% 5% 12% 22% 12% 1	1680
	11th to 20th	Night	45% 5% 11% 24% 14% 1	1680
	21st to end	Night	47% 5% 13% 23% 11% 1	1680
July	1st to 10th	Night	56% 4 10% 17% 12%	4640
	11th to 20th	Night	44% 5% 14% 23% 13% 1	4632
	21st to end	Night	45% 5% 12% 23% 14% 1	5076
August	1st to 10th	Night	49% 8% 16% 16% 11% 1	4123
	11th to 20th	Night	40% 7% 17% 29% 7%	4119
	21st to end	Night	35% 5% 16% 28% 16% 1	5041
September	1st to 10th	Night	38% 5% 11% 26% 18% 1	2145
	11th to 20th	Night	35% 4 11% 25% 22% 3	2310
	21st to end	Night	47% 5% 7% 20% 18% 3	2289
October	1st to 10th	Night	41% 5% 9% 28% 14% 2	3590
	11th to 20th	Night	28% 3 9% 26% 31% 3	3915
	21st to end	Night	34% 4 10% 28% 21% 2	4610
November	1st to 10th	Night	41% 5% 11% 24% 16% 3	2667
	11th to 20th	Night	41% 4 10% 23% 18% 4	2730
	21st to end	Night	39% 5% 15% 28% 12% 2	2730
December	1st to 10th	Night	43% 3 12% 26% 15% 1	4477
	11th to 20th	Night	47% 3 14% 16% 17% 2	5348
	21st to end	Night	49% 6% 12% 19% 13% 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	24% 18% 35% 18% 5%	220
	11th to 20th	Day	22% 25% 33% 15% 5%	220
	21st to end	Day	19% 18% 41% 16% 4 2	275
February	1st to 10th	Day	24% 17% 36% 20% 3	488
	11th to 20th	Day	25% 19% 37% 16% 3	536
	21st to end	Day	21% 21% 37% 16% 4 1	446
March	1st to 10th	Day	27% 15% 34% 19% 5%	540
	11th to 20th	Day	20% 17% 38% 21% 3	540
	21st to end	Day	25% 18% 33% 15% 6% 2	594
April	1st to 10th	Day	28% 14% 32% 18% 6% 1	540
	11th to 20th	Day	23% 15% 35% 21% 5% 1	532
	21st to end	Day	22% 13% 34% 22% 7% 2	540
May	1st to 10th	Day	29% 11% 32% 21% 6% 1	543
	11th to 20th	Day	22% 10% 32% 26% 8% 2	567
	21st to end	Day	23% 6% 27% 26% 15% 3	617
June	1st to 10th	Day	20% 4 28% 32% 14% 2	570
	11th to 20th	Day	16% 9% 30% 31% 11% 3	570
	21st to end	Day	15% 8% 25% 35% 15% 2	570
July	1st to 10th	Day	18% 4 20% 33% 22% 2	565
	11th to 20th	Day	13% 5% 23% 32% 23% 4	564
	21st to end	Day	14% 5% 27% 35% 17% 3	620
August	1st to 10th	Day	13% 3 25% 39% 17% 3	566
	11th to 20th	Day	14% 6% 19% 37% 18% 5%	559
	21st to end	Day	13% 5% 26% 29% 19% 7%	627
September	1st to 10th	Day	17% 6% 27% 27% 17% 7%	570
	11th to 20th	Day	16% 7% 27% 25% 16% 9%	570
	21st to end	Day	25% 8% 27% 23% 12% 4	562
October	1st to 10th	Day	23% 8% 24% 26% 13% 7%	530
	11th to 20th	Day	20% 11% 29% 24% 13% 4	454
	21st to end	Day	22% 11% 34% 20% 10% 3	462
November	1st to 10th	Day	19% 14% 31% 24% 10% 2	425
	11th to 20th	Day	19% 21% 33% 21% 6%	430
	21st to end	Day	21% 20% 31% 19% 8% 1	430
December	1st to 10th	Day	21% 17% 37% 20% 5%	367
	11th to 20th	Day	27% 22% 34% 13% 4	220
	21st to end	Day	24% 20% 33% 16% 5% 2	242
January	1st to 10th	Night	22% 16% 40% 18% 4	840
	11th to 20th	Night	27% 19% 34% 17% 3	840
	21st to end	Night	22% 20% 34% 20% 3 1	924
February	1st to 10th	Night	21% 15% 36% 25% 2	840
	11th to 20th	Night	25% 16% 40% 17% 1	823
	21st to end	Night	22% 17% 37% 20% 4	692
March	1st to 10th	Night	24% 14% 34% 23% 5%	840
	11th to 20th	Night	22% 14% 36% 22% 5% 1	840
	21st to end	Night	24% 14% 32% 22% 7%	924
April	1st to 10th	Night	25% 13% 31% 27% 4	840
	11th to 20th	Night	23% 12% 32% 28% 5%	792
	21st to end	Night	22% 9% 33% 27% 9%	630
May	1st to 10th	Night	27% 9% 28% 26% 8% 1	630
	11th to 20th	Night	23% 8% 34% 28% 5% 1	622
	21st to end	Night	23% 5% 21% 38% 12% 1	539
June	1st to 10th	Night	21% 6% 24% 33% 16%	420
	11th to 20th	Night	22% 5% 19% 41% 11% 1	420
	21st to end	Night	21% 6% 18% 40% 15%	420
July	1st to 10th	Night	19% 6% 17% 40% 18%	410
	11th to 20th	Night	17% 5% 18% 36% 22% 2	408
	21st to end	Night	17% 4 17% 43% 18% 1	494
August	1st to 10th	Night	12% 3 22% 42% 20% 1	628
	11th to 20th	Night	15% 3 18% 41% 22% 1	622
	21st to end	Night	16% 4 17% 36% 23% 3	920
September	1st to 10th	Night	16% 3 20% 38% 20% 2	840
	11th to 20th	Night	16% 5% 21% 30% 22% 6%	840
	21st to end	Night	24% 6% 22% 29% 16% 3	833
October	1st to 10th	Night	24% 8% 22% 26% 16% 4	800
	11th to 20th	Night	20% 11% 27% 26% 14% 2	800
	21st to end	Night	23% 12% 32% 24% 9% 1	880
November	1st to 10th	Night	20% 11% 30% 27% 12% 1	820
	11th to 20th	Night	20% 19% 33% 21% 7%	840
	21st to end	Night	18% 20% 30% 24% 7% 1	840
December	1st to 10th	Night	25% 17% 33% 20% 4 1	840
	11th to 20th	Night	25% 19% 33% 19% 4	840
	21st to end	Night	22% 19% 35% 20% 3	924

Page generated at 2026-09-26 04:18:38. Version ac07

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