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 2024-01-13.

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% 7% 3 6% 6% 4 5% 5% 6% 7% 35%	74794
	Night	17% 7% 3 5% 5% 3 5% 5% 6% 7% 37%	74924
Transparency	Day	46% 6% 11% 22% 13% 2	74499
	Night	46% 6% 12% 22% 13% 2	74753
Seeing	Day	25% 17% 27% 19% 9% 3	13754
	Night	23% 15% 24% 28% 10% 1	22761

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	14% 8% 3 7% 6% 3 5% 6% 7% 9% 32%	1517
	11th to 20th	Day	17% 9% 4 6% 5% 4 6% 5% 6% 8% 31%	1456
	21st to end	Day	11% 9% 4 7% 7% 5% 4 5% 6% 8% 35%	1584
February	1st to 10th	Day	15% 10% 4 7% 8% 4 6% 5% 5% 8% 30%	1593
	11th to 20th	Day	12% 9% 3 8% 8% 3 6% 6% 6% 6% 34%	1748
	21st to end	Day	16% 9% 5% 7% 6% 3 5% 5% 5% 6% 31%	1480
March	1st to 10th	Day	13% 8% 3 7% 7% 4 6% 4 6% 7% 35%	1797
	11th to 20th	Day	16% 9% 4 6% 5% 3 5% 5% 6% 7% 34%	1877
	21st to end	Day	16% 9% 3 8% 6% 3 5% 4 4 6% 37%	2322
April	1st to 10th	Day	19% 9% 4 6% 5% 3 5% 4 4 7% 33%	2160
	11th to 20th	Day	17% 6% 2 5% 5% 4 5% 5% 5% 8% 37%	2155
	21st to end	Day	22% 8% 4 6% 6% 3 6% 5% 5% 6% 30%	2340
May	1st to 10th	Day	23% 8% 3 7% 6% 3 4 4 4 5% 32%	2340
	11th to 20th	Day	20% 6% 2 5% 6% 3 3 4 5% 7% 37%	2425
	21st to end	Day	16% 5% 3 5% 5% 3 5% 5% 5% 8% 37%	2725
June	1st to 10th	Day	20% 6% 2 6% 6% 4 6% 6% 7% 8% 28%	2520
	11th to 20th	Day	18% 5% 3 5% 6% 3 5% 5% 6% 8% 36%	2520
	21st to end	Day	19% 7% 3 6% 7% 4 6% 5% 6% 7% 30%	2520
July	1st to 10th	Day	16% 7% 3 4 5% 4 5% 6% 6% 8% 36%	2520
	11th to 20th	Day	13% 5% 2 6% 6% 5% 6% 5% 7% 9% 36%	2520
	21st to end	Day	14% 6% 2 5% 5% 4 6% 6% 7% 7% 37%	2744
August	1st to 10th	Day	13% 5% 2 5% 6% 4 6% 7% 8% 8% 36%	2424
	11th to 20th	Day	14% 5% 2 6% 7% 4 6% 6% 8% 8% 34%	2292
	21st to end	Day	13% 4 2 4 5% 3 6% 6% 6% 9% 43%	2376
September	1st to 10th	Day	12% 4 2 4 5% 3 5% 6% 6% 8% 45%	2160
	11th to 20th	Day	15% 4 2 4 4 3 4 4 4 5% 52%	1998
	21st to end	Day	16% 4 2 4 5% 3 5% 4 6% 7% 45%	1980
October	1st to 10th	Day	23% 7% 2 6% 5% 3 5% 4 5% 6% 34%	1980
	11th to 20th	Day	17% 6% 2 5% 6% 3 5% 5% 5% 7% 38%	1715
	21st to end	Day	26% 8% 2 5% 5% 4 5% 4 6% 5% 30%	1782
November	1st to 10th	Day	15% 6% 2 6% 5% 4 5% 4 5% 8% 41%	1671
	11th to 20th	Day	20% 5% 2 4 5% 4 4 5% 6% 7% 38%	1710
	21st to end	Day	21% 7% 2 4 5% 3 4 5% 6% 7% 35%	1645
December	1st to 10th	Day	21% 9% 3 8% 6% 4 5% 5% 6% 7% 26%	1520
	11th to 20th	Day	22% 8% 2 5% 4 2 4 4 5% 6% 36%	1906
	21st to end	Day	24% 11% 3 5% 6% 4 5% 5% 5% 6% 27%	2772
January	1st to 10th	Night	14% 8% 3 6% 6% 4 4 5% 7% 8% 34%	2649
	11th to 20th	Night	16% 11% 3 6% 5% 3 6% 6% 6% 8% 30%	2549
	21st to end	Night	10% 8% 4 7% 7% 4 5% 5% 6% 7% 36%	2772
February	1st to 10th	Night	15% 9% 3 6% 7% 3 6% 5% 6% 7% 32%	2363
	11th to 20th	Night	12% 7% 4 6% 6% 3 5% 6% 7% 9% 35%	2098
	21st to end	Night	13% 9% 3 5% 6% 4 6% 5% 6% 9% 35%	1776
March	1st to 10th	Night	14% 9% 3 5% 7% 4 6% 4 6% 7% 36%	2150
	11th to 20th	Night	16% 8% 3 6% 5% 4 5% 6% 6% 7% 33%	2081
	21st to end	Night	16% 8% 3 5% 5% 2 5% 4 5% 7% 39%	2030
April	1st to 10th	Night	19% 8% 3 5% 6% 3 5% 4 5% 5% 37%	1800
	11th to 20th	Night	19% 7% 2 5% 5% 4 5% 4 6% 6% 37%	1753
	21st to end	Night	22% 8% 3 5% 5% 2 5% 3 5% 7% 36%	1620
May	1st to 10th	Night	21% 9% 3 7% 5% 3 5% 4 4 7% 32%	1620
	11th to 20th	Night	18% 7% 2 5% 6% 3 5% 6% 5% 6% 37%	1483
	21st to end	Night	17% 5% 3 5% 5% 4 5% 4 5% 8% 39%	1550
June	1st to 10th	Night	21% 7% 3 5% 5% 3 5% 4 6% 8% 33%	1440
	11th to 20th	Night	20% 6% 2 5% 5% 3 5% 4 5% 7% 38%	1440
	21st to end	Night	17% 6% 2 6% 5% 2 5% 6% 6% 8% 38%	1440
July	1st to 10th	Night	17% 5% 2 5% 4 2 4 6% 6% 8% 41%	1440
	11th to 20th	Night	15% 4 2 5% 4 3 5% 5% 6% 9% 42%	1440
	21st to end	Night	14% 4 1 5% 5% 3 5% 5% 6% 8% 44%	1574
August	1st to 10th	Night	15% 5% 2 4 4 2 4 5% 6% 8% 44%	1495
	11th to 20th	Night	15% 7% 2 6% 6% 3 4 4 5% 7% 41%	1604
	21st to end	Night	16% 4 2 4 4 3 3 5% 5% 8% 45%	1980
September	1st to 10th	Night	12% 4 2 3 5% 2 3 4 7% 8% 50%	1800
	11th to 20th	Night	15% 5% 2 5% 4 2 4 4 4 6% 50%	1962
	21st to end	Night	16% 6% 1 4 4 2 4 5% 6% 7% 45%	1980
October	1st to 10th	Night	21% 7% 2 5% 5% 4 4 5% 5% 6% 38%	1980
	11th to 20th	Night	17% 7% 2 6% 6% 3 4 6% 7% 6% 37%	2241
	21st to end	Night	25% 7% 2 5% 5% 3 4 5% 5% 7% 32%	2574
November	1st to 10th	Night	15% 6% 2 5% 5% 3 5% 6% 6% 7% 40%	2407
	11th to 20th	Night	17% 6% 3 6% 5% 3 5% 4 5% 8% 38%	2470
	21st to end	Night	16% 7% 2 5% 6% 4 4 5% 6% 7% 37%	2531
December	1st to 10th	Night	20% 9% 3 7% 7% 3 5% 5% 6% 7% 28%	2660
	11th to 20th	Night	20% 9% 3 5% 5% 3 4 5% 6% 7% 33%	3288
	21st to end	Night	23% 12% 4 7% 5% 3 5% 5% 4 5% 27%	4884

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	46% 6% 10% 25% 13%	1518
	11th to 20th	Day	50% 7% 12% 18% 10% 3	1456
	21st to end	Day	45% 6% 9% 23% 15% 2	1584
February	1st to 10th	Day	53% 5% 12% 19% 9% 2	1593
	11th to 20th	Day	48% 6% 11% 19% 12% 3	1757
	21st to end	Day	52% 5% 10% 18% 13% 2	1480
March	1st to 10th	Day	48% 5% 12% 19% 12% 4	1800
	11th to 20th	Day	47% 5% 11% 17% 16% 4	1877
	21st to end	Day	48% 5% 9% 18% 17% 4	2322
April	1st to 10th	Day	49% 5% 9% 20% 13% 4	2160
	11th to 20th	Day	43% 6% 12% 21% 16% 2	2155
	21st to end	Day	54% 5% 8% 18% 13% 3	2340
May	1st to 10th	Day	54% 5% 7% 21% 12% 2	2340
	11th to 20th	Day	45% 5% 9% 21% 17% 2	2429
	21st to end	Day	43% 7% 14% 22% 12% 2	2725
June	1st to 10th	Day	50% 7% 13% 20% 10% 1	2520
	11th to 20th	Day	44% 7% 10% 21% 15% 2	2520
	21st to end	Day	49% 8% 12% 18% 11% 1	2520
July	1st to 10th	Day	42% 6% 13% 22% 15% 1	2450
	11th to 20th	Day	44% 7% 13% 26% 9% 1	2445
	21st to end	Day	41% 6% 14% 25% 12% 1	2591
August	1st to 10th	Day	42% 8% 15% 23% 12%	2410
	11th to 20th	Day	43% 7% 14% 24% 11% 1	2292
	21st to end	Day	35% 7% 14% 25% 17% 1	2376
September	1st to 10th	Day	35% 7% 13% 30% 15% 1	2160
	11th to 20th	Day	35% 5% 8% 29% 21% 2	1998
	21st to end	Day	37% 6% 12% 26% 17% 2	1980
October	1st to 10th	Day	49% 6% 8% 22% 12% 2	1980
	11th to 20th	Day	42% 6% 14% 28% 9% 1	1715
	21st to end	Day	54% 6% 9% 18% 10% 2	1782
November	1st to 10th	Day	41% 5% 14% 24% 15% 1	1671
	11th to 20th	Day	44% 6% 11% 19% 17% 2	1710
	21st to end	Day	47% 5% 18% 19% 9% 1	1645
December	1st to 10th	Day	54% 7% 13% 16% 9% 1	1520
	11th to 20th	Day	49% 6% 9% 19% 17% 1	1906
	21st to end	Day	58% 5% 9% 18% 10%	2772
January	1st to 10th	Night	45% 5% 12% 22% 14% 1	2649
	11th to 20th	Night	52% 7% 13% 19% 9% 1	2549
	21st to end	Night	45% 7% 10% 21% 16% 2	2772
February	1st to 10th	Night	50% 6% 12% 19% 12% 2	2363
	11th to 20th	Night	44% 5% 10% 23% 15% 2	2100
	21st to end	Night	46% 5% 11% 22% 13% 3	1776
March	1st to 10th	Night	45% 6% 10% 22% 14% 3	2160
	11th to 20th	Night	48% 5% 10% 20% 13% 3	2082
	21st to end	Night	44% 5% 9% 18% 19% 5%	2030
April	1st to 10th	Night	49% 4 9% 20% 16% 2	1800
	11th to 20th	Night	46% 4 9% 24% 16% 1	1753
	21st to end	Night	49% 3 12% 19% 14% 3	1620
May	1st to 10th	Night	53% 4 8% 21% 13% 2	1620
	11th to 20th	Night	46% 6% 10% 21% 14% 3	1484
	21st to end	Night	45% 6% 15% 22% 11% 2	1551
June	1st to 10th	Night	48% 7% 13% 20% 11%	1440
	11th to 20th	Night	46% 5% 14% 20% 13% 2	1440
	21st to end	Night	44% 7% 15% 22% 11% 1	1440
July	1st to 10th	Night	38% 8% 17% 23% 14% 1	1400
	11th to 20th	Night	38% 5% 19% 25% 13%	1392
	21st to end	Night	36% 6% 15% 29% 12% 1	1486
August	1st to 10th	Night	37% 7% 15% 28% 13% 1	1485
	11th to 20th	Night	42% 6% 16% 23% 12%	1604
	21st to end	Night	36% 6% 13% 28% 16%	1980
September	1st to 10th	Night	33% 5% 16% 30% 16%	1800
	11th to 20th	Night	35% 5% 10% 29% 19% 2	1962
	21st to end	Night	37% 5% 11% 29% 15% 1	1980
October	1st to 10th	Night	47% 5% 11% 23% 11% 2	1980
	11th to 20th	Night	44% 6% 14% 27% 9% 1	2241
	21st to end	Night	51% 6% 11% 20% 10% 2	2574
November	1st to 10th	Night	41% 6% 12% 24% 15% 2	2407
	11th to 20th	Night	43% 5% 11% 24% 15% 2	2470
	21st to end	Night	44% 6% 17% 23% 9% 1	2531
December	1st to 10th	Night	54% 7% 10% 21% 8%	2660
	11th to 20th	Night	48% 6% 12% 17% 15% 1	3288
	21st to end	Night	57% 6% 9% 21% 7% 1	4884

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	29% 19% 33% 16% 3 1	180
	11th to 20th	Day	30% 26% 29% 11% 2 2	164
	21st to end	Day	20% 28% 31% 16% 6%	176
February	1st to 10th	Day	33% 26% 29% 10% 2	263
	11th to 20th	Day	27% 27% 29% 13% 3	416
	21st to end	Day	32% 25% 24% 16% 3	346
March	1st to 10th	Day	29% 20% 30% 16% 4	420
	11th to 20th	Day	33% 24% 25% 13% 4 1	420
	21st to end	Day	29% 21% 25% 16% 6% 2	462
April	1st to 10th	Day	33% 20% 24% 14% 7% 2	420
	11th to 20th	Day	28% 25% 23% 16% 6% 2	412
	21st to end	Day	31% 20% 29% 11% 5% 3	420
May	1st to 10th	Day	31% 22% 26% 13% 5% 3	423
	11th to 20th	Day	23% 19% 33% 16% 7% 2	447
	21st to end	Day	23% 13% 30% 22% 9% 2	485
June	1st to 10th	Day	22% 14% 36% 17% 11% 1	450
	11th to 20th	Day	20% 15% 32% 20% 10% 3	450
	21st to end	Day	20% 14% 30% 21% 13% 2	450
July	1st to 10th	Day	20% 10% 29% 23% 14% 3	445
	11th to 20th	Day	16% 7% 31% 30% 13% 3	444
	21st to end	Day	15% 10% 35% 26% 10% 3	488
August	1st to 10th	Day	13% 12% 28% 27% 16% 4	446
	11th to 20th	Day	18% 11% 27% 23% 14% 7%	439
	21st to end	Day	15% 13% 27% 25% 15% 6%	495
September	1st to 10th	Day	16% 12% 26% 23% 16% 6%	450
	11th to 20th	Day	18% 12% 26% 21% 14% 9%	450
	21st to end	Day	16% 17% 25% 25% 10% 7%	450
October	1st to 10th	Day	30% 14% 18% 22% 12% 4	450
	11th to 20th	Day	25% 17% 23% 23% 11% 2	399
	21st to end	Day	37% 16% 20% 19% 8%	396
November	1st to 10th	Day	24% 17% 24% 23% 11% 1	365
	11th to 20th	Day	32% 16% 24% 20% 7% 1	370
	21st to end	Day	33% 19% 26% 15% 7%	305
December	1st to 10th	Day	27% 25% 26% 17% 5%	180
	11th to 20th	Day	36% 21% 29% 12% 2	180
	21st to end	Day	32% 19% 27% 17% 4 1	198
January	1st to 10th	Night	22% 21% 28% 26% 3	760
	11th to 20th	Night	26% 19% 24% 27% 3	729
	21st to end	Night	18% 25% 29% 23% 4	792
February	1st to 10th	Night	23% 21% 32% 22% 2	720
	11th to 20th	Night	21% 24% 31% 20% 3	703
	21st to end	Night	23% 21% 27% 26% 4	592
March	1st to 10th	Night	26% 17% 30% 24% 3	720
	11th to 20th	Night	25% 21% 25% 23% 6%	720
	21st to end	Night	26% 19% 24% 24% 8% 1	792
April	1st to 10th	Night	27% 17% 27% 25% 4	720
	11th to 20th	Night	24% 18% 26% 26% 6%	689
	21st to end	Night	28% 16% 27% 23% 6%	540
May	1st to 10th	Night	29% 13% 24% 26% 8% 1	540
	11th to 20th	Night	21% 13% 25% 31% 10%	422
	21st to end	Night	21% 7% 21% 37% 13% 1	389
June	1st to 10th	Night	22% 9% 20% 35% 13% 1	360
	11th to 20th	Night	21% 9% 24% 31% 15%	360
	21st to end	Night	20% 4 23% 38% 14% 1	360
July	1st to 10th	Night	15% 6% 20% 41% 17% 1	350
	11th to 20th	Night	16% 3 20% 38% 22% 2	348
	21st to end	Night	15% 3 19% 41% 20% 1	394
August	1st to 10th	Night	16% 4 17% 43% 20% 1	421
	11th to 20th	Night	17% 7% 26% 30% 18% 3	540
	21st to end	Night	17% 6% 17% 33% 23% 3	792
September	1st to 10th	Night	13% 8% 22% 35% 21% 2	720
	11th to 20th	Night	17% 10% 19% 30% 19% 4	720
	21st to end	Night	19% 10% 20% 31% 17% 3	720
October	1st to 10th	Night	25% 12% 18% 28% 15% 2	720
	11th to 20th	Night	22% 16% 22% 27% 13% 1	720
	21st to end	Night	30% 16% 21% 25% 8%	792
November	1st to 10th	Night	20% 15% 23% 30% 10% 1	740
	11th to 20th	Night	24% 18% 24% 25% 8% 1	760
	21st to end	Night	24% 19% 23% 28% 6% 1	760
December	1st to 10th	Night	27% 20% 27% 21% 5%	760
	11th to 20th	Night	28% 20% 26% 22% 4 1	760
	21st to end	Night	29% 22% 23% 23% 3	836

Page generated at 2024-01-10 01:25:00. Version ac07

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