Gloucester
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Gloucester clear sky chart from 2005-11-06 to 2025-11-18.

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	16% 8% 2 6% 4 4 4 4 5% 7% 40%	126551
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 39%	123811
Transparency	Day	43% 5% 12% 23% 15% 2	130085
	Night	45% 4 11% 21% 16% 2	128364
Seeing	Day	18% 4 19% 33% 21% 5%	16408
	Night	20% 7% 26% 30% 15% 2	25233

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% 6% 3 7% 4 4 4 4 6% 7% 42%	3597
	11th to 20th	Day	14% 8% 3 5% 2 3 3 3 5% 6% 48%	3570
	21st to end	Day	17% 9% 2 7% 3 3 3 3 4 5% 43%	3960
February	1st to 10th	Day	19% 9% 3 6% 5% 4 5% 4 6% 6% 34%	1858
	11th to 20th	Day	14% 8% 2 7% 4 4 4 4 6% 8% 39%	1948
	21st to end	Day	16% 10% 2 6% 5% 4 4 4 6% 6% 37%	1650
March	1st to 10th	Day	20% 11% 3 6% 4 4 4 3 5% 6% 36%	4476
	11th to 20th	Day	15% 11% 3 7% 5% 4 3 3 5% 7% 37%	4838
	21st to end	Day	27% 8% 2 5% 4 4 4 4 5% 6% 31%	5666
April	1st to 10th	Day	20% 8% 2 6% 4 4 4 4 5% 5% 39%	2400
	11th to 20th	Day	17% 8% 2 4 4 4 4 4 5% 7% 41%	2425
	21st to end	Day	18% 6% 2 6% 4 4 4 4 5% 7% 40%	2600
May	1st to 10th	Day	26% 10% 3 6% 4 4 4 3 5% 7% 26%	5886
	11th to 20th	Day	18% 9% 4 9% 5% 5% 4 3 5% 6% 33%	5861
	21st to end	Day	19% 7% 3 6% 4 4 4 4 5% 7% 38%	6948
June	1st to 10th	Day	19% 7% 2 5% 4 5% 4 4 7% 8% 36%	2800
	11th to 20th	Day	14% 7% 3 6% 4 4 4 4 6% 8% 41%	2800
	21st to end	Day	15% 5% 2 6% 5% 5% 4 4 6% 8% 39%	2800
July	1st to 10th	Day	14% 8% 3 6% 4 4 5% 4 7% 10% 35%	6360
	11th to 20th	Day	7% 7% 3 6% 4 5% 6% 4 7% 10% 41%	6360
	21st to end	Day	13% 5% 2 5% 4 3 4 3 6% 8% 47%	6909
August	1st to 10th	Day	14% 6% 3 6% 4 4 5% 5% 7% 8% 39%	4235
	11th to 20th	Day	13% 8% 3 6% 4 4 4 4 5% 7% 41%	4175
	21st to end	Day	7% 3 1 4 3 4 5% 4 6% 9% 51%	4352
September	1st to 10th	Day	12% 6% 2 5% 4 3 4 4 5% 7% 50%	2366
	11th to 20th	Day	13% 7% 2 3 3 3 3 2 5% 5% 54%	2200
	21st to end	Day	16% 6% 2 5% 4 3 4 3 5% 7% 44%	2200
October	1st to 10th	Day	21% 5% 2 5% 3 3 3 3 5% 5% 45%	2200
	11th to 20th	Day	15% 6% 2 4 4 3 5% 3 5% 8% 46%	1958
	21st to end	Day	20% 6% 2 6% 3 3 3 4 5% 5% 42%	1980
November	1st to 10th	Day	16% 7% 2 5% 3 4 3 3 4 6% 47%	1851
	11th to 20th	Day	17% 8% 2 4 3 3 3 3 4 7% 47%	1863
	21st to end	Day	18% 8% 2 4 3 4 3 4 4 7% 42%	1800
December	1st to 10th	Day	16% 9% 2 5% 3 3 3 3 4 6% 47%	2713
	11th to 20th	Day	19% 7% 2 5% 3 3 4 3 4 6% 44%	2986
	21st to end	Day	20% 10% 3 8% 4 4 4 3 4 5% 34%	3960
January	1st to 10th	Night	15% 6% 1 5% 3 4 4 3 5% 7% 47%	6349
	11th to 20th	Night	15% 10% 3 8% 5% 4 4 3 4 5% 38%	6315
	21st to end	Night	16% 10% 3 7% 4 4 5% 3 5% 6% 37%	6996
February	1st to 10th	Night	17% 10% 3 7% 5% 4 4 4 6% 7% 35%	2536
	11th to 20th	Night	16% 10% 3 6% 4 3 4 4 6% 6% 38%	2338
	21st to end	Night	18% 9% 3 6% 4 4 3 3 5% 5% 41%	1980
March	1st to 10th	Night	26% 9% 2 5% 3 3 3 3 4 5% 35%	5364
	11th to 20th	Night	16% 11% 3 7% 5% 4 4 3 5% 6% 36%	5120
	21st to end	Night	26% 9% 2 7% 4 4 4 4 4 4 32%	5249
April	1st to 10th	Night	20% 9% 3 7% 5% 4 4 4 5% 5% 36%	2000
	11th to 20th	Night	19% 8% 2 6% 5% 4 3 3 4 5% 39%	1923
	21st to end	Night	19% 7% 2 8% 5% 5% 5% 3 5% 5% 37%	1800
May	1st to 10th	Night	28% 11% 3 8% 4 4 3 3 4 6% 26%	4054
	11th to 20th	Night	17% 10% 3 8% 5% 5% 6% 4 7% 7% 28%	4033
	21st to end	Night	19% 9% 3 9% 5% 5% 4 4 5% 6% 31%	3955
June	1st to 10th	Night	19% 8% 3 8% 4 4 4 3 5% 6% 37%	1600
	11th to 20th	Night	16% 9% 2 6% 4 4 4 4 5% 9% 37%	1600
	21st to end	Night	17% 7% 2 6% 5% 4 4 3 8% 8% 37%	1600
July	1st to 10th	Night	10% 7% 3 7% 5% 4 5% 4 7% 9% 37%	3600
	11th to 20th	Night	9% 7% 2 8% 5% 6% 5% 5% 6% 10% 38%	3600
	21st to end	Night	12% 7% 3 7% 4 4 4 3 6% 6% 44%	3926
August	1st to 10th	Night	17% 7% 2 8% 5% 5% 6% 4 6% 8% 32%	2900
	11th to 20th	Night	10% 9% 3 8% 5% 5% 6% 5% 7% 8% 34%	2872
	21st to end	Night	9% 5% 2 5% 4 4 4 3 6% 8% 51%	3543
September	1st to 10th	Night	11% 6% 3 5% 4 4 4 3 5% 7% 48%	2033
	11th to 20th	Night	13% 5% 2 5% 3 3 3 3 4 5% 54%	2200
	21st to end	Night	16% 5% 2 5% 4 4 4 4 5% 7% 43%	2200
October	1st to 10th	Night	19% 5% 2 7% 3 3 3 3 5% 6% 44%	2200
	11th to 20th	Night	15% 6% 2 5% 3 4 4 3 5% 7% 46%	2436
	21st to end	Night	22% 7% 2 5% 4 3 3 3 5% 6% 40%	2860
November	1st to 10th	Night	15% 5% 2 6% 4 4 4 3 5% 6% 46%	2667
	11th to 20th	Night	17% 7% 2 5% 4 3 3 3 4 6% 45%	2692
	21st to end	Night	17% 7% 2 5% 4 4 3 4 5% 7% 43%	2600
December	1st to 10th	Night	19% 8% 2 5% 4 4 3 3 4 5% 42%	4466
	11th to 20th	Night	20% 9% 2 5% 3 3 4 3 4 5% 40%	5208
	21st to end	Night	18% 10% 3 8% 4 3 3 3 5% 6% 36%	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	42% 3 9% 27% 19% 1	3598
	11th to 20th	Day	39% 4 12% 24% 19% 3	3600
	21st to end	Day	44% 4 10% 22% 18% 2	3960
February	1st to 10th	Day	49% 6% 13% 18% 13% 2	1858
	11th to 20th	Day	43% 6% 13% 21% 13% 2	1957
	21st to end	Day	45% 6% 13% 20% 14% 3	1650
March	1st to 10th	Day	51% 3 9% 14% 13% 10%	4479
	11th to 20th	Day	46% 5% 13% 24% 11% 2	4838
	21st to end	Day	54% 3 7% 14% 14% 7%	5666
April	1st to 10th	Day	46% 4 9% 19% 18% 4	2400
	11th to 20th	Day	42% 5% 11% 23% 15% 4	2425
	21st to end	Day	43% 6% 10% 22% 17% 4	2600
May	1st to 10th	Day	57% 6% 14% 15% 7% 1	5897
	11th to 20th	Day	53% 4 12% 19% 10% 2	5865
	21st to end	Day	46% 5% 12% 23% 13% 1	6948
June	1st to 10th	Day	45% 7% 13% 22% 13% 1	2800
	11th to 20th	Day	41% 6% 14% 21% 17% 2	2800
	21st to end	Day	41% 7% 15% 22% 14% 1	2800
July	1st to 10th	Day	46% 6% 13% 20% 14% 1	6290
	11th to 20th	Day	39% 5% 17% 26% 13% 1	6285
	21st to end	Day	37% 4 13% 30% 15% 1	6711
August	1st to 10th	Day	42% 7% 18% 25% 8%	4222
	11th to 20th	Day	42% 5% 13% 29% 11% 1	4214
	21st to end	Day	29% 4 13% 33% 20% 1	4352
September	1st to 10th	Day	34% 5% 12% 27% 21% 1	2366
	11th to 20th	Day	33% 4 10% 28% 23% 3	2200
	21st to end	Day	40% 4 10% 23% 21% 2	2200
October	1st to 10th	Day	40% 3 9% 26% 19% 2	3590
	11th to 20th	Day	32% 3 7% 31% 23% 4	3161
	21st to end	Day	32% 3 11% 31% 20% 2	3180
November	1st to 10th	Day	40% 4 14% 25% 16% 2	1851
	11th to 20th	Day	39% 3 9% 24% 22% 3	1863
	21st to end	Day	43% 6% 15% 23% 13% 1	1800
December	1st to 10th	Day	41% 3 15% 28% 13% 1	2713
	11th to 20th	Day	42% 4 15% 15% 19% 5%	2986
	21st to end	Day	53% 4 8% 20% 12% 4	3960
January	1st to 10th	Night	38% 3 10% 32% 15% 2	6349
	11th to 20th	Night	49% 3 12% 20% 13% 3	6360
	21st to end	Night	50% 3 9% 16% 17% 4	6996
February	1st to 10th	Night	48% 6% 13% 18% 12% 2	2536
	11th to 20th	Night	46% 5% 10% 20% 15% 3	2340
	21st to end	Night	45% 3 10% 24% 13% 4	1980
March	1st to 10th	Night	52% 3 8% 15% 15% 7%	5374
	11th to 20th	Night	48% 5% 14% 17% 13% 2	5121
	21st to end	Night	56% 3 5% 12% 18% 6%	5249
April	1st to 10th	Night	49% 5% 10% 17% 16% 3	2000
	11th to 20th	Night	47% 4 9% 20% 18% 2	1923
	21st to end	Night	48% 5% 9% 17% 18% 3	1800
May	1st to 10th	Night	60% 5% 12% 13% 7% 3	4060
	11th to 20th	Night	53% 6% 15% 15% 9% 2	4034
	21st to end	Night	54% 5% 12% 17% 11% 1	3956
June	1st to 10th	Night	48% 7% 11% 21% 13% 1	1600
	11th to 20th	Night	45% 7% 13% 20% 15% 1	1600
	21st to end	Night	42% 9% 12% 22% 14% 2	1600
July	1st to 10th	Night	43% 5% 12% 21% 18%	3560
	11th to 20th	Night	42% 4 18% 21% 14% 1	3552
	21st to end	Night	40% 4 11% 30% 14% 1	3809
August	1st to 10th	Night	50% 5% 13% 20% 10% 1	2889
	11th to 20th	Night	45% 7% 14% 24% 10%	2899
	21st to end	Night	31% 4 14% 27% 23% 1	3543
September	1st to 10th	Night	35% 6% 12% 26% 20% 1	2033
	11th to 20th	Night	33% 5% 10% 25% 25% 2	2200
	21st to end	Night	39% 5% 9% 25% 20% 2	2200
October	1st to 10th	Night	42% 3 12% 26% 15% 2	3590
	11th to 20th	Night	29% 3 11% 27% 27% 3	3972
	21st to end	Night	35% 3 11% 30% 19% 2	4610
November	1st to 10th	Night	39% 5% 11% 24% 18% 3	2667
	11th to 20th	Night	40% 4 11% 22% 20% 3	2692
	21st to end	Night	42% 5% 15% 24% 12% 2	2600
December	1st to 10th	Night	45% 3 11% 28% 12% 1	4466
	11th to 20th	Night	47% 4 15% 15% 18% 1	5208
	21st to end	Night	50% 4 10% 20% 15%	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	23% 8% 34% 29% 7% 1	200
	11th to 20th	Day	19% 11% 32% 30% 8% 1	200
	21st to end	Day	20% 5% 38% 27% 10% 1	220
February	1st to 10th	Day	24% 10% 34% 27% 5%	420
	11th to 20th	Day	21% 9% 42% 23% 4	496
	21st to end	Day	18% 8% 30% 35% 7% 1	414
March	1st to 10th	Day	20% 8% 30% 32% 10%	500
	11th to 20th	Day	16% 9% 31% 35% 9%	500
	21st to end	Day	25% 5% 27% 31% 9% 3	550
April	1st to 10th	Day	25% 7% 26% 29% 11% 2	500
	11th to 20th	Day	21% 4 20% 42% 11% 1	492
	21st to end	Day	21% 3 18% 41% 15% 2	500
May	1st to 10th	Day	26% 3 18% 36% 14% 3	503
	11th to 20th	Day	17% 2 16% 41% 20% 5%	527
	21st to end	Day	18% 2 10% 36% 28% 6%	573
June	1st to 10th	Day	19% 2 7% 39% 26% 6%	530
	11th to 20th	Day	15% 3 9% 38% 29% 6%	530
	21st to end	Day	15% 2 8% 40% 29% 6%	530
July	1st to 10th	Day	17% 2 4 31% 38% 9%	525
	11th to 20th	Day	10% 2 7% 32% 39% 10%	524
	21st to end	Day	12% 1 5% 35% 43% 4	576
August	1st to 10th	Day	10% 2 6% 31% 43% 8%	526
	11th to 20th	Day	13% 2 5% 34% 33% 14%	519
	21st to end	Day	11% 1 7% 35% 36% 10%	583
September	1st to 10th	Day	16% 1 11% 31% 28% 12%	530
	11th to 20th	Day	18% 2 11% 27% 30% 12%	530
	21st to end	Day	20% 1 12% 36% 23% 8%	530
October	1st to 10th	Day	22% 1 17% 26% 25% 8%	530
	11th to 20th	Day	16% 4 24% 28% 24% 4	448
	21st to end	Day	24% 4 24% 32% 14% 2	462
November	1st to 10th	Day	15% 6% 27% 34% 16% 2	425
	11th to 20th	Day	21% 8% 29% 31% 10%	421
	21st to end	Day	23% 8% 31% 25% 11% 2	400
December	1st to 10th	Day	20% 7% 37% 26% 9% 1	274
	11th to 20th	Day	21% 7% 38% 30% 6%	200
	21st to end	Day	24% 7% 30% 29% 8% 1	220
January	1st to 10th	Night	22% 13% 41% 21% 4	800
	11th to 20th	Night	26% 12% 36% 22% 4 1	800
	21st to end	Night	18% 13% 42% 23% 5%	880
February	1st to 10th	Night	22% 12% 37% 26% 3	800
	11th to 20th	Night	21% 13% 44% 19% 3	783
	21st to end	Night	22% 10% 38% 24% 7%	660
March	1st to 10th	Night	23% 10% 37% 25% 6%	800
	11th to 20th	Night	22% 7% 35% 27% 9% 1	800
	21st to end	Night	24% 7% 31% 26% 10% 1	880
April	1st to 10th	Night	25% 5% 29% 32% 9%	800
	11th to 20th	Night	20% 3 28% 37% 12%	739
	21st to end	Night	21% 3 25% 36% 14% 1	600
May	1st to 10th	Night	26% 3 19% 38% 13% 2	600
	11th to 20th	Night	22% 1 18% 39% 17% 3	592
	21st to end	Night	22% 2 10% 43% 21% 2	449
June	1st to 10th	Night	22% 2 13% 39% 22% 4	400
	11th to 20th	Night	17% 3 11% 43% 25% 2	400
	21st to end	Night	19% 1 10% 42% 26% 2	400
July	1st to 10th	Night	15% 2 6% 40% 33% 4	390
	11th to 20th	Night	13% 2 6% 39% 37% 3	388
	21st to end	Night	14% 2 8% 40% 35% 2	438
August	1st to 10th	Night	11% 2 8% 40% 36% 3	583
	11th to 20th	Night	12% 2 8% 40% 32% 5%	592
	21st to end	Night	14% 1 12% 31% 35% 7%	850
September	1st to 10th	Night	14% 1 14% 38% 30% 5%	800
	11th to 20th	Night	15% 3 15% 32% 27% 9%	800
	21st to end	Night	19% 2 15% 33% 26% 6%	800
October	1st to 10th	Night	22% 4 19% 29% 21% 6%	800
	11th to 20th	Night	18% 5% 27% 33% 16% 2	800
	21st to end	Night	24% 6% 31% 29% 9% 1	880
November	1st to 10th	Night	17% 5% 30% 34% 12% 2	820
	11th to 20th	Night	20% 11% 35% 26% 7% 1	829
	21st to end	Night	19% 14% 33% 26% 7% 1	800
December	1st to 10th	Night	24% 10% 38% 22% 6%	800
	11th to 20th	Night	24% 13% 37% 22% 4 1	800
	21st to end	Night	23% 12% 37% 23% 5%	880

Page generated at 2025-11-15 00:55:57. Version ac07

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