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 2025-12-03.

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 7% 5% 5% 5% 4 6% 7% 34%	126905
	Night	17% 8% 3 7% 5% 4 4 4 6% 7% 34%	123796
Transparency	Day	47% 6% 11% 21% 13% 2	130399
	Night	47% 5% 12% 21% 13% 2	128376
Seeing	Day	25% 17% 27% 19% 9% 3	16223
	Night	23% 15% 24% 28% 10% 1	25107

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	13% 9% 4 11% 6% 5% 5% 5% 6% 6% 30%	3597
	11th to 20th	Day	12% 9% 5% 10% 6% 5% 5% 4 6% 6% 32%	3570
	21st to end	Day	13% 9% 3 9% 6% 5% 4 4 5% 5% 36%	3960
February	1st to 10th	Day	13% 10% 4 8% 7% 6% 5% 4 5% 7% 31%	1770
	11th to 20th	Day	11% 9% 3 10% 6% 5% 5% 5% 5% 7% 33%	1948
	21st to end	Day	16% 10% 4 8% 5% 5% 5% 4 5% 6% 32%	1650
March	1st to 10th	Day	19% 12% 2 11% 5% 5% 4 3 5% 5% 29%	4476
	11th to 20th	Day	17% 10% 4 8% 5% 5% 5% 4 5% 6% 33%	4737
	21st to end	Day	23% 9% 2 6% 4 4 4 3 5% 7% 32%	5832
April	1st to 10th	Day	19% 10% 3 7% 5% 4 4 3 5% 7% 32%	2400
	11th to 20th	Day	17% 7% 2 5% 5% 5% 5% 4 6% 8% 36%	2398
	21st to end	Day	21% 8% 3 7% 4 5% 5% 4 5% 6% 32%	2600
May	1st to 10th	Day	33% 10% 3 8% 4 4 4 3 4 5% 21%	5886
	11th to 20th	Day	20% 8% 3 8% 6% 6% 5% 4 6% 7% 27%	6195
	21st to end	Day	19% 6% 2 6% 4 5% 5% 4 7% 8% 32%	6949
June	1st to 10th	Day	20% 7% 2 7% 5% 6% 5% 5% 7% 8% 28%	2800
	11th to 20th	Day	17% 6% 2 6% 5% 5% 5% 4 7% 8% 36%	2800
	21st to end	Day	19% 7% 3 6% 6% 6% 5% 4 6% 8% 31%	2800
July	1st to 10th	Day	14% 6% 3 6% 5% 5% 6% 6% 7% 10% 33%	6360
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 4 8% 10% 37%	6360
	21st to end	Day	16% 5% 1 5% 4 4 4 4 6% 7% 42%	6926
August	1st to 10th	Day	14% 6% 2 6% 6% 6% 6% 5% 8% 8% 34%	4407
	11th to 20th	Day	18% 7% 2 6% 5% 4 6% 5% 7% 9% 31%	4148
	21st to end	Day	10% 4 2 5% 4 4 6% 5% 7% 10% 43%	4312
September	1st to 10th	Day	12% 4 2 5% 4 4 4 4 6% 8% 46%	2400
	11th to 20th	Day	14% 4 1 5% 3 4 4 4 4 5% 53%	2220
	21st to end	Day	16% 5% 2 5% 4 4 4 4 6% 7% 44%	2200
October	1st to 10th	Day	21% 7% 2 6% 4 5% 4 3 5% 6% 37%	2200
	11th to 20th	Day	17% 7% 2 6% 4 5% 5% 3 5% 7% 39%	1904
	21st to end	Day	24% 8% 2 6% 5% 5% 5% 3 5% 5% 31%	1980
November	1st to 10th	Day	14% 6% 2 7% 5% 5% 5% 3 5% 8% 40%	1851
	11th to 20th	Day	18% 6% 2 6% 5% 5% 5% 4 6% 7% 38%	1890
	21st to end	Day	20% 9% 1 6% 4 5% 4 3 6% 7% 33%	1817
December	1st to 10th	Day	16% 9% 4 11% 6% 6% 6% 4 6% 8% 24%	2616
	11th to 20th	Day	20% 9% 2 6% 4 4 4 3 7% 6% 35%	2986
	21st to end	Day	21% 11% 4 6% 5% 4 4 3 4 5% 32%	3960
January	1st to 10th	Night	12% 9% 4 13% 6% 5% 4 4 5% 6% 32%	6349
	11th to 20th	Night	11% 12% 4 12% 7% 5% 6% 5% 6% 7% 26%	6315
	21st to end	Night	12% 10% 4 9% 6% 5% 5% 4 7% 7% 33%	6996
February	1st to 10th	Night	14% 10% 3 7% 6% 5% 5% 4 6% 7% 32%	2625
	11th to 20th	Night	11% 8% 3 8% 5% 5% 5% 5% 7% 9% 34%	2338
	21st to end	Night	13% 9% 3 6% 6% 5% 5% 4 6% 9% 34%	1980
March	1st to 10th	Night	21% 11% 3 7% 5% 4 5% 3 6% 7% 28%	5364
	11th to 20th	Night	15% 10% 3 9% 6% 5% 5% 3 5% 6% 32%	5221
	21st to end	Night	23% 11% 3 6% 4 3 4 3 5% 7% 31%	5082
April	1st to 10th	Night	19% 9% 2 6% 5% 4 4 3 5% 5% 35%	2000
	11th to 20th	Night	19% 9% 2 6% 4 5% 4 3 6% 6% 36%	1950
	21st to end	Night	20% 8% 3 5% 4 4 4 3 5% 7% 37%	1800
May	1st to 10th	Night	36% 9% 2 7% 4 4 4 3 4 6% 20%	4054
	11th to 20th	Night	18% 11% 3 8% 5% 5% 5% 4 5% 6% 28%	3741
	21st to end	Night	20% 7% 2 7% 5% 5% 4 4 6% 7% 33%	3926
June	1st to 10th	Night	21% 7% 3 6% 4 4 4 4 6% 8% 33%	1600
	11th to 20th	Night	19% 6% 2 6% 5% 4 5% 4 5% 7% 38%	1600
	21st to end	Night	18% 6% 2 6% 5% 4 4 5% 6% 7% 37%	1600
July	1st to 10th	Night	14% 5% 2 6% 4 4 5% 5% 7% 10% 39%	3600
	11th to 20th	Night	16% 6% 2 6% 4 4 5% 4 6% 8% 39%	3600
	21st to end	Night	12% 4 1 4 4 4 4 4 6% 8% 49%	3926
August	1st to 10th	Night	21% 7% 3 7% 4 4 4 4 5% 6% 36%	2709
	11th to 20th	Night	17% 8% 2 6% 4 4 4 3 5% 8% 38%	2895
	21st to end	Night	10% 4 1 4 4 5% 4 4 7% 10% 47%	3586
September	1st to 10th	Night	12% 5% 2 4 4 3 3 3 6% 7% 51%	2000
	11th to 20th	Night	14% 5% 1 5% 3 3 4 3 5% 7% 51%	2180
	21st to end	Night	16% 6% 2 5% 4 4 4 4 6% 7% 43%	2200
October	1st to 10th	Night	20% 7% 2 6% 4 4 4 4 5% 6% 39%	2200
	11th to 20th	Night	16% 7% 2 6% 5% 4 4 4 6% 7% 38%	2491
	21st to end	Night	24% 7% 2 6% 5% 4 4 4 6% 7% 33%	2860
November	1st to 10th	Night	15% 6% 2 6% 5% 4 4 5% 6% 6% 41%	2667
	11th to 20th	Night	16% 7% 2 6% 5% 5% 4 3 5% 8% 38%	2730
	21st to end	Night	16% 7% 2 6% 5% 5% 4 4 7% 7% 36%	2798
December	1st to 10th	Night	17% 10% 3 9% 7% 5% 5% 4 6% 7% 27%	4609
	11th to 20th	Night	23% 8% 3 5% 4 4 4 4 6% 6% 33%	5208
	21st to end	Night	20% 11% 3 9% 4 4 4 3 4 5% 32%	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	52% 5% 9% 23% 11%	3598
	11th to 20th	Day	48% 8% 10% 20% 11% 3	3600
	21st to end	Day	46% 6% 8% 20% 17% 3	3960
February	1st to 10th	Day	51% 5% 11% 20% 10% 3	1770
	11th to 20th	Day	48% 6% 11% 20% 12% 3	1957
	21st to end	Day	51% 5% 11% 18% 13% 2	1650
March	1st to 10th	Day	57% 4 9% 13% 10% 7%	4479
	11th to 20th	Day	50% 5% 13% 17% 11% 3	4737
	21st to end	Day	52% 4 5% 17% 18% 4	5832
April	1st to 10th	Day	50% 5% 8% 19% 14% 4	2400
	11th to 20th	Day	45% 6% 11% 20% 16% 2	2398
	21st to end	Day	52% 5% 8% 19% 14% 3	2600
May	1st to 10th	Day	65% 6% 8% 15% 6% 1	5897
	11th to 20th	Day	53% 7% 14% 16% 9% 1	6199
	21st to end	Day	46% 8% 13% 21% 11% 1	6949
June	1st to 10th	Day	49% 7% 12% 21% 11%	2800
	11th to 20th	Day	44% 7% 12% 20% 15% 2	2800
	21st to end	Day	49% 8% 11% 19% 12% 1	2800
July	1st to 10th	Day	45% 6% 15% 22% 12%	6290
	11th to 20th	Day	41% 7% 15% 24% 13% 1	6285
	21st to end	Day	39% 5% 12% 29% 15% 1	6728
August	1st to 10th	Day	45% 7% 15% 23% 11%	4393
	11th to 20th	Day	46% 8% 14% 24% 8%	4187
	21st to end	Day	33% 6% 16% 27% 17% 1	4312
September	1st to 10th	Day	34% 6% 12% 31% 16% 1	2400
	11th to 20th	Day	33% 5% 9% 30% 21% 2	2220
	21st to end	Day	38% 6% 12% 26% 17% 2	2200
October	1st to 10th	Day	45% 5% 9% 24% 16% 1	3590
	11th to 20th	Day	41% 4 13% 25% 15% 2	3068
	21st to end	Day	39% 4 10% 28% 16% 3	3180
November	1st to 10th	Day	41% 5% 14% 24% 15% 1	1851
	11th to 20th	Day	43% 6% 11% 19% 18% 3	1890
	21st to end	Day	48% 5% 17% 20% 9% 1	1817
December	1st to 10th	Day	54% 7% 13% 19% 6% 1	2616
	11th to 20th	Day	46% 5% 10% 18% 20% 2	2986
	21st to end	Day	54% 4 7% 17% 14% 4	3960
January	1st to 10th	Night	52% 5% 11% 21% 11% 1	6349
	11th to 20th	Night	53% 9% 11% 15% 10% 2	6360
	21st to end	Night	47% 5% 10% 17% 17% 4	6996
February	1st to 10th	Night	49% 5% 12% 20% 12% 2	2625
	11th to 20th	Night	44% 6% 11% 23% 14% 2	2340
	21st to end	Night	46% 5% 11% 22% 12% 3	1980
March	1st to 10th	Night	54% 4 10% 14% 14% 4	5374
	11th to 20th	Night	51% 5% 15% 16% 11% 2	5222
	21st to end	Night	52% 4 5% 16% 19% 4	5082
April	1st to 10th	Night	50% 4 10% 19% 15% 2	2000
	11th to 20th	Night	48% 4 8% 23% 15% 1	1950
	21st to end	Night	47% 4 12% 19% 16% 3	1800
May	1st to 10th	Night	65% 5% 8% 14% 7% 1	4060
	11th to 20th	Night	55% 7% 11% 19% 8% 1	3742
	21st to end	Night	50% 5% 16% 19% 9% 1	3927
June	1st to 10th	Night	48% 6% 12% 21% 12%	1600
	11th to 20th	Night	45% 6% 15% 20% 12% 2	1600
	21st to end	Night	44% 7% 15% 22% 11% 1	1600
July	1st to 10th	Night	39% 7% 18% 25% 11%	3560
	11th to 20th	Night	44% 7% 13% 22% 15%	3552
	21st to end	Night	33% 6% 14% 35% 12% 1	3809
August	1st to 10th	Night	50% 6% 12% 22% 9% 1	2699
	11th to 20th	Night	44% 7% 17% 21% 11%	2922
	21st to end	Night	31% 6% 18% 29% 15%	3586
September	1st to 10th	Night	33% 5% 15% 30% 18%	2000
	11th to 20th	Night	33% 5% 11% 29% 20% 1	2180
	21st to end	Night	39% 5% 11% 28% 16% 1	2200
October	1st to 10th	Night	43% 6% 14% 24% 12% 1	3590
	11th to 20th	Night	37% 4 12% 26% 18% 2	4053
	21st to end	Night	40% 4 11% 30% 13% 2	4610
November	1st to 10th	Night	41% 6% 11% 25% 15% 2	2667
	11th to 20th	Night	43% 5% 11% 23% 16% 2	2730
	21st to end	Night	45% 6% 17% 22% 9% 1	2798
December	1st to 10th	Night	53% 7% 12% 22% 6%	4609
	11th to 20th	Night	50% 5% 11% 15% 18% 2	5208
	21st to end	Night	53% 5% 8% 18% 13% 3	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	31% 23% 30% 14% 3 1	200
	11th to 20th	Day	31% 27% 28% 11% 4 2	200
	21st to end	Day	23% 27% 28% 17% 5%	220
February	1st to 10th	Day	31% 26% 28% 12% 3	320
	11th to 20th	Day	29% 27% 29% 12% 3	496
	21st to end	Day	31% 24% 25% 17% 3	414
March	1st to 10th	Day	30% 22% 28% 16% 4	500
	11th to 20th	Day	32% 23% 26% 13% 4 1	500
	21st to end	Day	32% 22% 24% 14% 6% 2	550
April	1st to 10th	Day	34% 21% 24% 13% 6% 2	500
	11th to 20th	Day	32% 25% 22% 14% 5% 1	492
	21st to end	Day	30% 22% 30% 11% 5% 2	500
May	1st to 10th	Day	33% 21% 26% 13% 5% 3	503
	11th to 20th	Day	23% 18% 33% 17% 7% 2	527
	21st to end	Day	23% 14% 31% 22% 9% 2	573
June	1st to 10th	Day	22% 13% 36% 18% 11% 1	530
	11th to 20th	Day	21% 15% 31% 20% 10% 2	530
	21st to end	Day	22% 15% 30% 20% 11% 2	530
July	1st to 10th	Day	19% 11% 30% 24% 13% 3	525
	11th to 20th	Day	15% 8% 31% 31% 13% 3	524
	21st to end	Day	16% 11% 34% 26% 9% 3	576
August	1st to 10th	Day	13% 12% 28% 27% 17% 3	526
	11th to 20th	Day	18% 11% 26% 23% 14% 8%	519
	21st to end	Day	13% 12% 29% 25% 15% 6%	583
September	1st to 10th	Day	15% 12% 27% 23% 16% 7%	530
	11th to 20th	Day	16% 10% 25% 22% 16% 10%	530
	21st to end	Day	17% 16% 25% 25% 11% 7%	530
October	1st to 10th	Day	27% 14% 19% 21% 14% 5%	530
	11th to 20th	Day	26% 16% 23% 21% 11% 2	467
	21st to end	Day	36% 15% 22% 19% 8%	462
November	1st to 10th	Day	25% 18% 24% 22% 10% 1	425
	11th to 20th	Day	32% 17% 24% 20% 7% 1	430
	21st to end	Day	35% 20% 24% 15% 6%	357
December	1st to 10th	Day	28% 24% 25% 18% 5%	204
	11th to 20th	Day	37% 20% 28% 12% 4	200
	21st to end	Day	30% 18% 27% 19% 6%	220
January	1st to 10th	Night	23% 22% 27% 25% 3	800
	11th to 20th	Night	27% 19% 24% 27% 3	800
	21st to end	Night	19% 24% 30% 23% 4	880
February	1st to 10th	Night	23% 21% 32% 22% 2	800
	11th to 20th	Night	23% 24% 31% 19% 3	783
	21st to end	Night	22% 21% 28% 26% 4	660
March	1st to 10th	Night	28% 16% 29% 24% 3	800
	11th to 20th	Night	24% 21% 24% 24% 6%	800
	21st to end	Night	28% 18% 24% 23% 7% 1	880
April	1st to 10th	Night	28% 17% 28% 23% 4	800
	11th to 20th	Night	25% 18% 26% 25% 6%	766
	21st to end	Night	27% 17% 28% 23% 6%	600
May	1st to 10th	Night	30% 12% 24% 27% 7% 1	600
	11th to 20th	Night	20% 13% 26% 32% 10%	469
	21st to end	Night	21% 7% 20% 37% 13% 1	433
June	1st to 10th	Night	22% 9% 22% 35% 12% 1	400
	11th to 20th	Night	20% 9% 24% 32% 15% 1	400
	21st to end	Night	21% 5% 23% 38% 14% 1	400
July	1st to 10th	Night	14% 6% 19% 43% 17% 1	390
	11th to 20th	Night	15% 4 19% 38% 22% 2	388
	21st to end	Night	16% 3 19% 42% 19% 1	438
August	1st to 10th	Night	17% 3 16% 42% 20% 1	469
	11th to 20th	Night	17% 7% 25% 28% 20% 2	602
	21st to end	Night	16% 6% 18% 33% 24% 3	880
September	1st to 10th	Night	14% 9% 22% 34% 20% 2	800
	11th to 20th	Night	16% 9% 17% 32% 21% 5%	800
	21st to end	Night	19% 9% 20% 33% 17% 2	800
October	1st to 10th	Night	24% 12% 17% 29% 17% 2	800
	11th to 20th	Night	22% 16% 22% 26% 13% 1	800
	21st to end	Night	29% 15% 21% 27% 8%	880
November	1st to 10th	Night	21% 15% 24% 30% 9% 1	820
	11th to 20th	Night	24% 18% 25% 24% 8% 1	840
	21st to end	Night	25% 19% 23% 28% 5% 1	840
December	1st to 10th	Night	27% 19% 27% 22% 5%	809
	11th to 20th	Night	28% 19% 26% 22% 4 1	800
	21st to end	Night	29% 21% 23% 24% 3	880

Page generated at 2025-11-30 01:00:52. Version ac07

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