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 2024-11-05.

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% 38%	101416
	Night	13% 9% 3 7% 5% 4 4 4 5% 7% 39%	97710
Transparency	Day	43% 5% 12% 25% 13% 1	104948
	Night	44% 4 11% 24% 14% 2	102319
Seeing	Day	16% 11% 31% 26% 12% 4	15929
	Night	17% 10% 30% 29% 12% 1	25782

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	12% 13% 5% 9% 5% 5% 5% 4 5% 6% 31%	2527
	11th to 20th	Day	11% 11% 4 12% 6% 5% 6% 6% 7% 8% 23%	2530
	21st to end	Day	17% 15% 7% 12% 5% 4 4 4 4 5% 23%	2783
February	1st to 10th	Day	12% 11% 5% 9% 6% 4 4 4 5% 8% 31%	1686
	11th to 20th	Day	11% 12% 3 7% 5% 4 5% 4 6% 6% 36%	1841
	21st to end	Day	11% 13% 4 8% 5% 5% 4 3 5% 7% 33%	1570
March	1st to 10th	Day	12% 9% 3 7% 5% 4 4 3 6% 6% 42%	3120
	11th to 20th	Day	10% 8% 3 8% 5% 4 4 4 5% 7% 42%	3264
	21st to end	Day	20% 12% 4 10% 5% 4 3 3 4 5% 29%	4165
April	1st to 10th	Day	15% 12% 5% 6% 6% 5% 4 4 5% 6% 31%	2280
	11th to 20th	Day	18% 9% 3 7% 5% 5% 4 4 5% 6% 33%	2254
	21st to end	Day	18% 10% 3 8% 5% 4 5% 3 5% 6% 33%	2432
May	1st to 10th	Day	14% 8% 3 7% 4 4 4 4 5% 7% 39%	4109
	11th to 20th	Day	11% 6% 2 6% 5% 4 5% 4 6% 8% 45%	4399
	21st to end	Day	11% 7% 3 7% 4 4 5% 4 6% 8% 41%	4824
June	1st to 10th	Day	10% 6% 2 6% 4 5% 4 4 6% 9% 44%	2660
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 4 7% 8% 41%	2660
	21st to end	Day	8% 7% 2 6% 5% 4 4 4 7% 9% 45%	2660
July	1st to 10th	Day	13% 6% 2 7% 5% 4 5% 5% 7% 10% 36%	4440
	11th to 20th	Day	4 3 1 4 4 5% 5% 5% 8% 12% 49%	4440
	21st to end	Day	7% 5% 2 6% 5% 4 5% 4 6% 10% 46%	4813
August	1st to 10th	Day	9% 6% 2 7% 4 5% 5% 4 6% 10% 42%	4423
	11th to 20th	Day	11% 5% 2 6% 4 5% 5% 4 7% 9% 45%	3865
	21st to end	Day	9% 6% 2 7% 5% 4 5% 4 6% 8% 43%	4180
September	1st to 10th	Day	11% 5% 2 7% 4 5% 4 4 6% 7% 44%	2280
	11th to 20th	Day	11% 5% 2 5% 4 4 4 3 6% 8% 48%	2161
	21st to end	Day	12% 8% 2 6% 3 5% 5% 4 6% 7% 42%	2090
October	1st to 10th	Day	14% 6% 3 6% 4 4 4 4 5% 6% 45%	2024
	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% 7% 2 6% 5% 5% 4 3 6% 7% 42%	1708
	11th to 20th	Day	10% 10% 5% 9% 6% 6% 5% 4 6% 8% 31%	1710
	21st to end	Day	17% 11% 3 7% 5% 3 4 3 5% 6% 36%	1563
December	1st to 10th	Day	14% 11% 4 8% 5% 5% 5% 4 6% 8% 30%	1520
	11th to 20th	Day	12% 14% 4 9% 5% 4 5% 5% 6% 8% 30%	1918
	21st to end	Day	18% 19% 7% 12% 6% 3 4 3 4 4 20%	2783
January	1st to 10th	Night	14% 13% 7% 10% 5% 4 4 2 3 4 35%	4428
	11th to 20th	Night	12% 13% 4 13% 7% 6% 7% 5% 6% 5% 21%	4440
	21st to end	Night	15% 16% 6% 11% 5% 4 4 3 4 5% 26%	4884
February	1st to 10th	Night	11% 13% 5% 8% 6% 4 5% 3 5% 5% 36%	2489
	11th to 20th	Night	10% 10% 3 8% 6% 4 5% 4 5% 6% 39%	2223
	21st to end	Night	16% 13% 3 6% 4 4 3 3 4 7% 37%	1884
March	1st to 10th	Night	15% 7% 3 6% 5% 4 4 3 5% 6% 43%	3733
	11th to 20th	Night	9% 7% 3 6% 5% 5% 5% 4 6% 9% 42%	3724
	21st to end	Night	18% 9% 4 12% 5% 4 4 3 5% 5% 31%	3466
April	1st to 10th	Night	18% 10% 3 6% 5% 4 4 3 6% 5% 36%	1900
	11th to 20th	Night	20% 8% 3 6% 4 4 4 3 5% 4 39%	1875
	21st to end	Night	16% 9% 3 6% 4 4 4 3 6% 6% 39%	1748
May	1st to 10th	Night	14% 6% 3 8% 5% 5% 5% 3 5% 8% 39%	2783
	11th to 20th	Night	12% 6% 2 6% 5% 5% 5% 5% 7% 8% 39%	2506
	21st to end	Night	15% 8% 3 7% 4 4 4 3 4 6% 41%	2743
June	1st to 10th	Night	11% 5% 2 4 4 4 5% 4 7% 8% 46%	1520
	11th to 20th	Night	11% 6% 2 6% 4 4 4 3 6% 9% 46%	1520
	21st to end	Night	7% 5% 3 6% 4 3 4 4 6% 8% 49%	1520
July	1st to 10th	Night	11% 7% 3 6% 4 4 4 3 6% 9% 41%	2530
	11th to 20th	Night	4 3 1 4 3 4 5% 4 7% 9% 56%	2530
	21st to end	Night	6% 5% 2 5% 5% 4 4 4 6% 8% 52%	2750
August	1st to 10th	Night	11% 4 2 5% 5% 4 4 3 6% 9% 48%	2517
	11th to 20th	Night	9% 5% 2 6% 4 4 4 4 6% 7% 48%	2960
	21st to end	Night	9% 6% 2 5% 4 4 4 3 7% 8% 48%	3476
September	1st to 10th	Night	8% 5% 2 6% 3 4 4 3 6% 6% 53%	1900
	11th to 20th	Night	11% 4 2 5% 3 3 4 3 5% 7% 54%	2000
	21st to end	Night	11% 7% 2 6% 4 4 3 3 5% 7% 47%	2090
October	1st to 10th	Night	12% 6% 2 6% 4 4 3 3 4 6% 50%	2152
	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%	2470
	11th to 20th	Night	13% 9% 3 8% 5% 5% 5% 4 6% 7% 34%	2470
	21st to end	Night	16% 10% 3 6% 4 4 4 4 5% 6% 38%	2612
December	1st to 10th	Night	14% 12% 4 6% 6% 5% 5% 4 6% 7% 30%	2660
	11th to 20th	Night	13% 12% 4 9% 5% 6% 5% 4 5% 6% 31%	3279
	21st to end	Night	20% 18% 5% 10% 5% 4 4 3 4 5% 22%	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	54% 4 16% 19% 6%	2528
	11th to 20th	Day	58% 4 12% 21% 5%	2530
	21st to end	Day	64% 3 9% 18% 6%	2783
February	1st to 10th	Day	50% 4 12% 22% 10% 1	1686
	11th to 20th	Day	47% 5% 11% 23% 12% 2	1851
	21st to end	Day	51% 3 11% 21% 11% 3	1570
March	1st to 10th	Day	42% 5% 10% 25% 15% 3	3120
	11th to 20th	Day	41% 4 11% 25% 17% 2	3264
	21st to end	Day	58% 4 11% 20% 7% 1	4165
April	1st to 10th	Day	53% 5% 11% 20% 11%	2280
	11th to 20th	Day	51% 4 9% 21% 13% 2	2254
	21st to end	Day	51% 4 12% 19% 12% 2	2432
May	1st to 10th	Day	43% 5% 10% 23% 17% 1	4147
	11th to 20th	Day	37% 5% 11% 32% 14% 1	4403
	21st to end	Day	40% 5% 11% 24% 17% 4	4837
June	1st to 10th	Day	36% 5% 13% 28% 16% 2	2660
	11th to 20th	Day	39% 5% 14% 25% 16% 1	2660
	21st to end	Day	35% 6% 15% 27% 15% 2	2660
July	1st to 10th	Day	42% 6% 14% 26% 10% 2	4371
	11th to 20th	Day	25% 8% 16% 34% 15% 2	4365
	21st to end	Day	33% 7% 14% 24% 20% 3	4662
August	1st to 10th	Day	36% 6% 15% 30% 12% 1	4408
	11th to 20th	Day	34% 6% 13% 35% 11% 1	3904
	21st to end	Day	38% 6% 14% 24% 16% 1	4179
September	1st to 10th	Day	38% 6% 12% 29% 15% 1	2280
	11th to 20th	Day	34% 5% 14% 33% 13% 1	2161
	21st to end	Day	40% 4 13% 31% 11% 1	2090
October	1st to 10th	Day	24% 3 8% 38% 24% 2	3362
	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	41% 4 13% 26% 14% 2	1708
	11th to 20th	Day	51% 4 14% 22% 8% 1	1710
	21st to end	Day	50% 3 15% 24% 8% 1	1563
December	1st to 10th	Day	51% 4 16% 21% 7% 1	1520
	11th to 20th	Day	51% 4 17% 18% 8% 2	1915
	21st to end	Day	69% 3 8% 14% 5%	2783
January	1st to 10th	Night	55% 3 9% 21% 10% 1	4428
	11th to 20th	Night	63% 4 10% 15% 7%	4440
	21st to end	Night	61% 4 11% 18% 6%	4884
February	1st to 10th	Night	51% 4 9% 22% 13% 2	2489
	11th to 20th	Night	46% 4 10% 25% 14% 2	2224
	21st to end	Night	49% 3 12% 21% 13% 3	1884
March	1st to 10th	Night	43% 3 14% 27% 10% 3	3733
	11th to 20th	Night	37% 6% 11% 24% 20% 2	3724
	21st to end	Night	56% 3 8% 19% 11% 2	3466
April	1st to 10th	Night	48% 5% 8% 20% 17% 2	1900
	11th to 20th	Night	47% 4 8% 21% 17% 3	1875
	21st to end	Night	45% 4 11% 24% 15% 2	1748
May	1st to 10th	Night	45% 4 11% 19% 18% 2	2810
	11th to 20th	Night	40% 5% 12% 28% 13% 1	2507
	21st to end	Night	44% 5% 9% 20% 14% 7%	2751
June	1st to 10th	Night	33% 6% 13% 29% 17% 1	1520
	11th to 20th	Night	35% 4 12% 27% 19% 2	1520
	21st to end	Night	32% 6% 16% 26% 17% 3	1520
July	1st to 10th	Night	39% 6% 14% 23% 12% 5%	2490
	11th to 20th	Night	23% 6% 17% 34% 19% 2	2482
	21st to end	Night	29% 7% 15% 24% 22% 4	2662
August	1st to 10th	Night	33% 7% 14% 27% 17% 2	2508
	11th to 20th	Night	33% 6% 13% 30% 17% 1	2987
	21st to end	Night	32% 6% 13% 24% 22% 3	3476
September	1st to 10th	Night	31% 4 11% 27% 24% 2	1900
	11th to 20th	Night	31% 3 11% 37% 17% 2	2000
	21st to end	Night	37% 4 10% 31% 17% 1	2090
October	1st to 10th	Night	27% 3 7% 40% 22% 1	3594
	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	39% 4 12% 26% 17% 2	2470
	11th to 20th	Night	49% 4 13% 21% 11% 2	2470
	21st to end	Night	46% 4 16% 25% 8% 1	2612
December	1st to 10th	Night	52% 4 13% 22% 8% 1	2660
	11th to 20th	Night	54% 4 13% 18% 10% 1	3281
	21st to end	Night	64% 5% 10% 15% 6%	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	18% 13% 36% 25% 6% 2	270
	11th to 20th	Day	18% 16% 35% 23% 6% 1	270
	21st to end	Day	20% 11% 35% 27% 5% 2	297
February	1st to 10th	Day	13% 20% 42% 17% 6% 1	270
	11th to 20th	Day	21% 23% 34% 17% 5%	270
	21st to end	Day	21% 24% 37% 15% 2	225
March	1st to 10th	Day	19% 21% 43% 14% 3	270
	11th to 20th	Day	22% 18% 30% 21% 6% 2	298
	21st to end	Day	27% 12% 30% 23% 7% 1	506
April	1st to 10th	Day	25% 10% 37% 19% 8%	460
	11th to 20th	Day	23% 19% 28% 18% 9% 2	452
	21st to end	Day	23% 13% 29% 22% 12% 2	460
May	1st to 10th	Day	21% 10% 37% 21% 8% 2	477
	11th to 20th	Day	17% 12% 32% 26% 10% 3	675
	21st to end	Day	15% 8% 32% 26% 13% 6%	734
June	1st to 10th	Day	12% 9% 33% 31% 11% 3	680
	11th to 20th	Day	15% 7% 34% 27% 13% 4	680
	21st to end	Day	12% 8% 31% 33% 11% 4	680
July	1st to 10th	Day	9% 4 25% 33% 21% 9%	671
	11th to 20th	Day	8% 6% 29% 31% 18% 7%	668
	21st to end	Day	6% 4 28% 35% 19% 8%	739
August	1st to 10th	Day	8% 4 26% 34% 19% 8%	676
	11th to 20th	Day	13% 3 20% 28% 25% 10%	479
	21st to end	Day	13% 7% 22% 28% 22% 8%	539
September	1st to 10th	Day	14% 6% 26% 28% 18% 9%	490
	11th to 20th	Day	10% 3 22% 36% 20% 10%	490
	21st to end	Day	19% 9% 25% 26% 17% 5%	490
October	1st to 10th	Day	20% 11% 27% 26% 13% 4	424
	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	15% 17% 32% 25% 9% 2	282
	11th to 20th	Day	16% 19% 38% 20% 7%	270
	21st to end	Day	23% 11% 39% 19% 6% 1	270
December	1st to 10th	Day	22% 18% 36% 20% 4	270
	11th to 20th	Day	24% 19% 36% 16% 5% 1	270
	21st to end	Day	26% 19% 35% 16% 4	297
January	1st to 10th	Night	18% 15% 38% 24% 5%	950
	11th to 20th	Night	20% 14% 36% 23% 5% 1	950
	21st to end	Night	19% 10% 37% 27% 7%	1045
February	1st to 10th	Night	19% 16% 35% 25% 5% 1	950
	11th to 20th	Night	18% 18% 34% 24% 6%	748
	21st to end	Night	23% 14% 39% 21% 4	628
March	1st to 10th	Night	19% 12% 34% 26% 9% 1	760
	11th to 20th	Night	17% 12% 37% 27% 6%	751
	21st to end	Night	21% 7% 27% 33% 10% 1	627
April	1st to 10th	Night	21% 10% 37% 25% 6% 1	570
	11th to 20th	Night	25% 8% 33% 25% 8% 1	563
	21st to end	Night	21% 8% 29% 31% 10% 1	570
May	1st to 10th	Night	18% 7% 31% 34% 9% 1	570
	11th to 20th	Night	19% 7% 29% 32% 12% 1	561
	21st to end	Night	15% 4 22% 41% 16% 2	617
June	1st to 10th	Night	13% 5% 25% 39% 17% 1	570
	11th to 20th	Night	13% 4 29% 36% 16% 2	570
	21st to end	Night	8% 5% 26% 42% 19% 1	570
July	1st to 10th	Night	9% 1 17% 43% 27% 3	555
	11th to 20th	Night	6% 3 22% 40% 26% 3	552
	21st to end	Night	6% 2 20% 45% 25% 2	624
August	1st to 10th	Night	9% 1 15% 44% 28% 3	567
	11th to 20th	Night	9% 3 16% 38% 29% 6%	564
	21st to end	Night	9% 3 19% 39% 26% 4	627
September	1st to 10th	Night	10% 3 22% 34% 25% 6%	570
	11th to 20th	Night	13% 4 18% 36% 25% 5%	670
	21st to end	Night	14% 8% 24% 28% 21% 4	760
October	1st to 10th	Night	16% 8% 27% 29% 18% 3	760
	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% 10% 1	760
	11th to 20th	Night	21% 14% 34% 24% 6% 1	760
	21st to end	Night	22% 14% 35% 22% 8%	902
December	1st to 10th	Night	21% 15% 35% 23% 5%	950
	11th to 20th	Night	23% 14% 35% 21% 6% 1	950
	21st to end	Night	27% 15% 34% 21% 4	1045

Page generated at 2024-11-03 01:11:18. Version ac07

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