Plymouth
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Plymouth clear sky chart from 2005-11-06 to 2025-10-04.

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	12% 8% 3 7% 5% 4 4 4 6% 8% 39%	126584
	Night	12% 9% 3 7% 5% 4 4 3 5% 7% 40%	123063
Transparency	Day	42% 5% 12% 26% 13% 1	130071
	Night	43% 4 12% 24% 15% 2	127650
Seeing	Day	16% 11% 31% 25% 12% 4	17220
	Night	17% 10% 30% 29% 12% 1	27011

By Month

Percentage Cloud Cover SE (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	9% 10% 4 12% 6% 5% 5% 4 6% 7% 33%	3617
	11th to 20th	Day	9% 10% 4 10% 5% 5% 5% 5% 6% 7% 34%	3620
	21st to end	Day	13% 13% 6% 10% 4 3 4 3 4 5% 34%	3982
February	1st to 10th	Day	12% 10% 5% 9% 6% 4 4 4 5% 8% 31%	1775
	11th to 20th	Day	11% 13% 4 7% 5% 4 5% 4 6% 6% 36%	1941
	21st to end	Day	11% 13% 4 8% 5% 5% 4 4 5% 7% 33%	1650
March	1st to 10th	Day	13% 7% 3 7% 4 4 4 3 6% 7% 43%	4480
	11th to 20th	Day	11% 7% 3 7% 4 4 4 4 5% 8% 42%	4680
	21st to end	Day	21% 12% 4 10% 5% 4 4 3 5% 5% 28%	5967
April	1st to 10th	Day	15% 12% 5% 6% 5% 5% 4 4 5% 6% 31%	2400
	11th to 20th	Day	17% 9% 3 7% 5% 5% 4 4 5% 6% 34%	2374
	21st to end	Day	17% 9% 3 8% 5% 4 5% 3 6% 6% 33%	2560
May	1st to 10th	Day	14% 9% 3 6% 4 4 4 3 5% 6% 42%	5903
	11th to 20th	Day	10% 7% 3 6% 5% 5% 5% 4 6% 8% 42%	6319
	21st to end	Day	14% 6% 2 6% 4 4 5% 4 6% 9% 40%	6935
June	1st to 10th	Day	10% 6% 2 6% 4 5% 4 4 6% 9% 44%	2800
	11th to 20th	Day	11% 6% 2 7% 5% 5% 5% 4 7% 8% 41%	2800
	21st to end	Day	8% 7% 2 6% 5% 4 4 4 7% 9% 44%	2800
July	1st to 10th	Day	12% 6% 2 7% 4 4 5% 4 7% 10% 39%	6360
	11th to 20th	Day	6% 4 2 5% 4 5% 5% 4 7% 10% 47%	6360
	21st to end	Day	7% 5% 2 6% 4 4 5% 4 6% 10% 46%	6924
August	1st to 10th	Day	9% 6% 2 6% 4 4 5% 4 6% 10% 43%	4563
	11th to 20th	Day	11% 5% 2 6% 4 4 4 4 7% 9% 45%	3989
	21st to end	Day	9% 6% 2 7% 5% 4 5% 4 6% 8% 43%	4312
September	1st to 10th	Day	10% 5% 2 7% 4 5% 4 4 6% 7% 45%	2400
	11th to 20th	Day	10% 5% 2 5% 4 4 4 3 6% 8% 48%	2275
	21st to end	Day	12% 8% 2 6% 4 5% 5% 4 7% 7% 42%	2200
October	1st to 10th	Day	14% 6% 3 5% 4 4 4 4 5% 6% 45%	2057
	11th to 20th	Day	20% 8% 4 7% 5% 6% 5% 4 6% 6% 30%	1853
	21st to end	Day	16% 10% 3 8% 5% 5% 5% 3 5% 6% 35%	1881
November	1st to 10th	Day	12% 8% 2 7% 5% 5% 4 3 6% 7% 41%	1762
	11th to 20th	Day	11% 11% 5% 9% 6% 5% 5% 4 6% 8% 31%	1800
	21st to end	Day	17% 11% 3 8% 5% 3 4 3 5% 6% 35%	1645
December	1st to 10th	Day	10% 10% 4 7% 5% 4 5% 4 8% 8% 35%	2610
	11th to 20th	Day	12% 15% 4 9% 5% 4 5% 4 6% 7% 29%	3008
	21st to end	Day	20% 19% 7% 11% 6% 3 4 3 4 4 20%	3982
January	1st to 10th	Night	10% 10% 5% 10% 6% 5% 4 3 4 5% 38%	6348
	11th to 20th	Night	11% 12% 4 11% 7% 5% 7% 5% 6% 6% 29%	6360
	21st to end	Night	11% 13% 5% 9% 4 4 4 3 5% 7% 35%	6996
February	1st to 10th	Night	12% 13% 4 8% 6% 4 5% 3 5% 5% 36%	2620
	11th to 20th	Night	10% 10% 3 8% 6% 4 5% 3 4 6% 39%	2343
	21st to end	Night	16% 13% 3 6% 4 4 3 3 4 7% 37%	1980
March	1st to 10th	Night	15% 6% 2 5% 4 4 4 4 5% 6% 45%	5373
	11th to 20th	Night	10% 7% 2 6% 5% 5% 5% 4 5% 9% 43%	5322
	21st to end	Night	18% 10% 4 11% 5% 4 4 3 5% 5% 32%	4962
April	1st to 10th	Night	18% 9% 3 5% 5% 4 3 3 6% 5% 37%	2000
	11th to 20th	Night	20% 8% 3 6% 4 4 4 3 5% 4 39%	1975
	21st to end	Night	16% 9% 3 6% 4 4 4 3 6% 6% 39%	1840
May	1st to 10th	Night	14% 6% 2 7% 5% 4 4 3 4 8% 41%	3990
	11th to 20th	Night	9% 7% 4 7% 5% 5% 5% 5% 6% 7% 41%	3596
	21st to end	Night	18% 8% 2 6% 4 4 4 3 5% 6% 40%	3942
June	1st to 10th	Night	11% 5% 2 4 4 4 5% 4 7% 8% 46%	1600
	11th to 20th	Night	11% 6% 2 6% 4 4 4 4 6% 8% 44%	1600
	21st to end	Night	8% 6% 3 6% 4 3 5% 4 6% 8% 48%	1600
July	1st to 10th	Night	8% 6% 2 6% 4 4 4 4 6% 9% 44%	3618
	11th to 20th	Night	6% 5% 1 5% 3 4 5% 3 6% 8% 54%	3617
	21st to end	Night	5% 4 2 5% 5% 4 4 4 5% 8% 53%	3947
August	1st to 10th	Night	11% 4 2 5% 5% 4 4 3 6% 9% 48%	2597
	11th to 20th	Night	9% 5% 2 6% 4 4 4 4 7% 7% 47%	3055
	21st to end	Night	8% 5% 2 5% 4 4 4 3 6% 7% 49%	3586
September	1st to 10th	Night	8% 5% 2 6% 3 4 4 3 6% 6% 53%	2000
	11th to 20th	Night	10% 3 2 6% 3 3 4 3 5% 7% 54%	2105
	21st to end	Night	11% 7% 2 6% 4 4 3 3 5% 7% 48%	2200
October	1st to 10th	Night	12% 6% 2 6% 4 4 3 3 4 6% 49%	2186
	11th to 20th	Night	16% 7% 2 6% 4 4 5% 4 5% 7% 41%	2327
	21st to end	Night	16% 9% 3 7% 3 4 4 4 6% 8% 36%	2717
November	1st to 10th	Night	12% 7% 2 6% 4 4 4 3 5% 7% 46%	2536
	11th to 20th	Night	14% 9% 3 8% 5% 5% 5% 4 6% 7% 34%	2600
	21st to end	Night	15% 10% 3 7% 5% 5% 4 4 5% 6% 36%	2750
December	1st to 10th	Night	10% 9% 3 5% 4 4 4 4 6% 8% 42%	4580
	11th to 20th	Night	14% 15% 6% 10% 5% 5% 4 3 4 5% 29%	5199
	21st to end	Night	20% 16% 5% 10% 6% 4 4 3 4 5% 22%	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	51% 4 15% 22% 8%	3618
	11th to 20th	Day	48% 4 10% 20% 16% 2	3620
	21st to end	Day	52% 3 9% 21% 14%	3982
February	1st to 10th	Day	50% 5% 11% 22% 11% 1	1775
	11th to 20th	Day	47% 5% 11% 23% 12% 2	1951
	21st to end	Day	51% 3 11% 22% 11% 2	1650
March	1st to 10th	Day	40% 4 11% 23% 15% 5%	4480
	11th to 20th	Day	40% 4 14% 24% 16% 2	4680
	21st to end	Day	58% 5% 10% 16% 11% 1	5969
April	1st to 10th	Day	53% 5% 11% 20% 11%	2400
	11th to 20th	Day	50% 4 9% 21% 13% 2	2374
	21st to end	Day	51% 4 12% 19% 12% 2	2560
May	1st to 10th	Day	41% 5% 11% 25% 17% 1	5938
	11th to 20th	Day	38% 7% 11% 28% 14% 1	6323
	21st to end	Day	41% 5% 10% 28% 13% 3	6949
June	1st to 10th	Day	36% 5% 13% 29% 16% 2	2800
	11th to 20th	Day	40% 5% 14% 25% 15% 1	2800
	21st to end	Day	35% 6% 15% 27% 15% 2	2800
July	1st to 10th	Day	39% 6% 13% 32% 10% 1	6291
	11th to 20th	Day	29% 8% 14% 34% 14% 1	6285
	21st to end	Day	32% 8% 14% 26% 18% 2	6728
August	1st to 10th	Day	35% 6% 15% 30% 12% 1	4548
	11th to 20th	Day	34% 6% 13% 35% 11% 1	4028
	21st to end	Day	38% 6% 14% 25% 16% 1	4311
September	1st to 10th	Day	37% 6% 12% 29% 15% 1	2400
	11th to 20th	Day	34% 5% 14% 33% 13% 1	2275
	21st to end	Day	39% 5% 12% 32% 11% 1	2200
October	1st to 10th	Day	25% 3 8% 38% 24% 2	3395
	11th to 20th	Day	41% 5% 9% 25% 19%	3056
	21st to end	Day	42% 5% 12% 27% 13% 1	3081
November	1st to 10th	Day	42% 4 13% 26% 14% 2	1762
	11th to 20th	Day	51% 4 14% 21% 9% 1	1800
	21st to end	Day	51% 3 14% 23% 7% 1	1645
December	1st to 10th	Day	45% 6% 19% 24% 5%	2610
	11th to 20th	Day	52% 5% 13% 19% 10% 2	3005
	21st to end	Day	68% 4 6% 14% 8%	3982
January	1st to 10th	Night	49% 3 9% 24% 14% 1	6348
	11th to 20th	Night	55% 4 9% 13% 17% 2	6360
	21st to end	Night	49% 4 14% 20% 13%	6996
February	1st to 10th	Night	51% 4 9% 21% 13% 2	2620
	11th to 20th	Night	47% 4 9% 25% 14% 2	2344
	21st to end	Night	49% 3 12% 20% 13% 2	1980
March	1st to 10th	Night	40% 3 14% 26% 13% 4	5373
	11th to 20th	Night	37% 5% 16% 23% 15% 3	5322
	21st to end	Night	56% 4 9% 18% 13% 1	4962
April	1st to 10th	Night	48% 5% 8% 20% 18% 1	2000
	11th to 20th	Night	47% 4 8% 21% 17% 3	1975
	21st to end	Night	45% 4 11% 24% 15% 2	1840
May	1st to 10th	Night	42% 4 11% 20% 20% 2	4017
	11th to 20th	Night	39% 6% 13% 23% 16% 2	3597
	21st to end	Night	46% 5% 8% 24% 12% 5%	3950
June	1st to 10th	Night	34% 6% 13% 30% 17% 1	1600
	11th to 20th	Night	37% 5% 11% 26% 19% 2	1600
	21st to end	Night	34% 6% 16% 25% 17% 3	1600
July	1st to 10th	Night	35% 6% 14% 32% 10% 4	3580
	11th to 20th	Night	27% 6% 15% 32% 18% 2	3572
	21st to end	Night	28% 6% 15% 26% 21% 3	3832
August	1st to 10th	Night	32% 6% 14% 28% 18% 2	2588
	11th to 20th	Night	33% 6% 13% 30% 17%	3082
	21st to end	Night	32% 6% 13% 25% 22% 3	3586
September	1st to 10th	Night	31% 4 12% 28% 23% 2	2000
	11th to 20th	Night	31% 4 11% 36% 17% 1	2105
	21st to end	Night	36% 4 10% 32% 17% 1	2200
October	1st to 10th	Night	27% 3 7% 40% 22% 1	3628
	11th to 20th	Night	32% 5% 13% 29% 19% 3	3863
	21st to end	Night	41% 4 11% 26% 17%	4467
November	1st to 10th	Night	38% 4 12% 27% 17% 2	2536
	11th to 20th	Night	49% 4 13% 21% 11% 2	2600
	21st to end	Night	48% 5% 15% 24% 8% 1	2750
December	1st to 10th	Night	39% 5% 20% 26% 6% 2	4580
	11th to 20th	Night	58% 4 10% 14% 13% 1	5201
	21st to end	Night	64% 5% 8% 15% 8%	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	17% 14% 37% 24% 5% 2	300
	11th to 20th	Day	17% 18% 36% 22% 6% 1	300
	21st to end	Day	21% 13% 35% 25% 5% 2	330
February	1st to 10th	Day	15% 20% 42% 17% 5% 1	300
	11th to 20th	Day	21% 24% 34% 16% 5%	300
	21st to end	Day	22% 23% 38% 14% 2	249
March	1st to 10th	Day	18% 21% 43% 15% 3	300
	11th to 20th	Day	23% 19% 30% 20% 5% 2	330
	21st to end	Day	27% 12% 31% 22% 7% 1	550
April	1st to 10th	Day	25% 11% 37% 19% 8%	500
	11th to 20th	Day	23% 21% 28% 18% 9% 2	492
	21st to end	Day	23% 13% 31% 21% 11% 1	500
May	1st to 10th	Day	20% 10% 39% 20% 9% 2	518
	11th to 20th	Day	17% 12% 31% 26% 10% 3	725
	21st to end	Day	16% 8% 33% 25% 13% 5%	789
June	1st to 10th	Day	12% 9% 34% 31% 11% 3	730
	11th to 20th	Day	15% 8% 33% 28% 13% 3	730
	21st to end	Day	12% 8% 32% 32% 11% 4	730
July	1st to 10th	Day	9% 4 25% 33% 20% 8%	721
	11th to 20th	Day	8% 6% 29% 31% 18% 7%	718
	21st to end	Day	6% 3 28% 36% 18% 8%	794
August	1st to 10th	Day	8% 4 26% 34% 19% 9%	726
	11th to 20th	Day	13% 3 20% 28% 25% 10%	519
	21st to end	Day	12% 7% 22% 28% 23% 8%	583
September	1st to 10th	Day	14% 6% 25% 28% 18% 9%	530
	11th to 20th	Day	9% 3 21% 35% 22% 10%	530
	21st to end	Day	18% 9% 25% 25% 18% 5%	530
October	1st to 10th	Day	19% 11% 26% 26% 14% 4	436
	11th to 20th	Day	22% 17% 31% 23% 7%	300
	21st to end	Day	22% 13% 40% 18% 6% 1	330
November	1st to 10th	Day	16% 18% 32% 24% 9% 2	300
	11th to 20th	Day	17% 20% 36% 20% 7%	300
	21st to end	Day	24% 12% 40% 18% 6% 1	300
December	1st to 10th	Day	21% 19% 35% 20% 4	300
	11th to 20th	Day	23% 20% 35% 16% 5% 1	300
	21st to end	Day	27% 19% 34% 15% 4 1	330
January	1st to 10th	Night	17% 15% 38% 24% 5%	1000
	11th to 20th	Night	20% 15% 37% 22% 5% 1	1000
	21st to end	Night	19% 10% 37% 26% 6%	1100
February	1st to 10th	Night	19% 17% 35% 24% 4 1	1000
	11th to 20th	Night	18% 18% 33% 24% 6%	788
	21st to end	Night	22% 15% 40% 20% 4	660
March	1st to 10th	Night	19% 12% 34% 25% 9% 1	800
	11th to 20th	Night	17% 14% 37% 27% 6%	790
	21st to end	Night	21% 8% 27% 33% 9% 1	660
April	1st to 10th	Night	21% 11% 37% 24% 6% 1	600
	11th to 20th	Night	24% 9% 33% 25% 8% 1	593
	21st to end	Night	21% 9% 29% 31% 10% 1	600
May	1st to 10th	Night	18% 6% 32% 34% 9% 1	600
	11th to 20th	Night	18% 7% 30% 31% 13% 1	591
	21st to end	Night	16% 4 22% 42% 15% 2	650
June	1st to 10th	Night	13% 5% 26% 39% 17% 1	600
	11th to 20th	Night	13% 4 29% 36% 16% 2	600
	21st to end	Night	9% 5% 26% 41% 19% 1	600
July	1st to 10th	Night	9% 1 17% 44% 27% 3	585
	11th to 20th	Night	7% 3 22% 40% 26% 3	582
	21st to end	Night	6% 2 21% 46% 24% 2	657
August	1st to 10th	Night	9% 1 15% 44% 29% 3	597
	11th to 20th	Night	9% 3 16% 38% 29% 6%	594
	21st to end	Night	9% 2 20% 39% 26% 4	660
September	1st to 10th	Night	10% 3 23% 33% 24% 7%	600
	11th to 20th	Night	12% 4 17% 35% 26% 6%	705
	21st to end	Night	14% 8% 24% 29% 22% 4	800
October	1st to 10th	Night	15% 8% 27% 30% 18% 2	773
	11th to 20th	Night	20% 12% 31% 27% 9% 1	760
	21st to end	Night	22% 11% 31% 26% 9% 1	836
November	1st to 10th	Night	16% 10% 37% 26% 9% 1	780
	11th to 20th	Night	21% 15% 35% 23% 6% 1	800
	21st to end	Night	22% 14% 35% 22% 7%	950
December	1st to 10th	Night	20% 15% 35% 24% 5%	1000
	11th to 20th	Night	23% 14% 35% 21% 6% 1	1000
	21st to end	Night	27% 14% 33% 21% 4	1100

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