South Baymouth
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the South Baymouth clear sky chart from 2005-11-06 to 2025-04-26.

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	14% 10% 4 8% 5% 4 4 4 5% 7% 34%	113230
	Night	15% 13% 5% 10% 5% 5% 4 3 5% 6% 30%	115485
Transparency	Day	48% 7% 13% 20% 11% 1	116789
	Night	53% 7% 12% 17% 9% 1	120090
Seeing	Day	18% 13% 31% 25% 11% 2	15842
	Night	21% 13% 29% 24% 11% 2	25288

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% 17% 8% 12% 5% 3 4 3 4 6% 25%	3203
	11th to 20th	Day	10% 16% 8% 20% 10% 6% 5% 4 5% 4 13%	3589
	21st to end	Day	13% 18% 8% 15% 6% 6% 5% 4 5% 5% 15%	4244
February	1st to 10th	Day	11% 17% 6% 14% 8% 5% 4 4 4 5% 22%	1800
	11th to 20th	Day	11% 14% 5% 12% 5% 5% 5% 5% 5% 6% 27%	1753
	21st to end	Day	12% 15% 4 13% 6% 5% 4 4 6% 6% 26%	1639
March	1st to 10th	Day	16% 10% 3 7% 4 4 5% 4 6% 7% 35%	4679
	11th to 20th	Day	11% 12% 5% 10% 6% 6% 5% 5% 6% 7% 29%	4980
	21st to end	Day	16% 10% 4 8% 5% 4 4 3 5% 6% 35%	5465
April	1st to 10th	Day	17% 11% 3 6% 5% 4 4 3 6% 7% 33%	2347
	11th to 20th	Day	18% 8% 2 7% 5% 4 4 4 5% 7% 39%	2572
	21st to end	Day	19% 9% 3 7% 4 4 3 3 5% 8% 37%	2535
May	1st to 10th	Day	15% 6% 2 7% 5% 4 4 3 5% 7% 42%	4108
	11th to 20th	Day	13% 7% 3 8% 4 4 4 4 5% 7% 41%	4232
	21st to end	Day	14% 6% 2 7% 5% 4 4 3 5% 6% 45%	4990
June	1st to 10th	Day	13% 6% 2 6% 6% 5% 5% 4 6% 8% 39%	2850
	11th to 20th	Day	10% 6% 3 5% 5% 4 5% 5% 6% 9% 42%	2850
	21st to end	Day	14% 7% 2 6% 4 4 5% 4 7% 7% 39%	2850
July	1st to 10th	Day	13% 8% 3 8% 4 5% 4 4 5% 7% 39%	4760
	11th to 20th	Day	8% 5% 2 5% 4 4 4 4 6% 8% 50%	4650
	21st to end	Day	9% 5% 2 5% 4 4 4 3 5% 8% 52%	4656
August	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 4 7% 10% 44%	4104
	11th to 20th	Day	13% 5% 2 6% 4 4 4 4 5% 8% 46%	4044
	21st to end	Day	9% 5% 2 6% 4 4 5% 4 6% 8% 46%	4532
September	1st to 10th	Day	11% 6% 2 7% 5% 4 4 4 6% 8% 44%	2244
	11th to 20th	Day	12% 4 2 5% 4 3 5% 3 6% 8% 49%	2090
	21st to end	Day	16% 6% 3 7% 5% 4 4 4 6% 8% 39%	2090
October	1st to 10th	Day	16% 8% 3 6% 5% 4 4 4 6% 6% 36%	1957
	11th to 20th	Day	23% 11% 3 8% 6% 4 4 4 4 6% 27%	1900
	21st to end	Day	24% 12% 3 8% 7% 5% 5% 3 5% 6% 22%	1946
November	1st to 10th	Day	18% 13% 4 9% 5% 4 4 5% 5% 8% 25%	1738
	11th to 20th	Day	17% 16% 5% 11% 5% 5% 5% 3 5% 6% 20%	1600
	21st to end	Day	23% 21% 6% 10% 6% 4 4 4 5% 5% 13%	1600
December	1st to 10th	Day	16% 24% 7% 11% 6% 5% 5% 3 5% 5% 14%	2536
	11th to 20th	Day	21% 19% 6% 14% 5% 4 5% 3 4 5% 13%	2621
	21st to end	Day	27% 24% 6% 11% 5% 5% 3 3 5% 4 8%	3476
January	1st to 10th	Night	11% 16% 9% 13% 5% 4 4 3 4 6% 24%	6762
	11th to 20th	Night	11% 18% 6% 18% 7% 4 5% 4 5% 5% 17%	6316
	21st to end	Night	11% 19% 8% 14% 7% 7% 4 3 3 5% 19%	6720
February	1st to 10th	Night	11% 17% 7% 13% 7% 6% 5% 4 5% 4 22%	2600
	11th to 20th	Night	10% 16% 5% 11% 7% 5% 5% 4 5% 5% 27%	2533
	21st to end	Night	14% 17% 4 9% 6% 4 5% 4 6% 6% 25%	1989
March	1st to 10th	Night	16% 9% 4 8% 5% 4 4 3 4 6% 39%	5161
	11th to 20th	Night	11% 10% 3 8% 6% 5% 5% 4 6% 7% 34%	4979
	21st to end	Night	18% 10% 5% 10% 5% 4 4 3 4 5% 34%	5467
April	1st to 10th	Night	19% 9% 3 7% 4 4 4 4 5% 6% 35%	2070
	11th to 20th	Night	18% 8% 3 6% 3 4 4 4 5% 8% 37%	1777
	21st to end	Night	18% 11% 4 8% 5% 5% 4 4 5% 6% 33%	1761
May	1st to 10th	Night	15% 6% 3 9% 6% 5% 4 4 5% 5% 39%	2833
	11th to 20th	Night	10% 6% 3 8% 5% 5% 5% 5% 6% 8% 39%	2659
	21st to end	Night	13% 7% 3 7% 4 4 3 4 5% 7% 41%	2587
June	1st to 10th	Night	11% 8% 2 6% 5% 4 5% 5% 7% 7% 39%	1330
	11th to 20th	Night	10% 6% 2 5% 4 4 7% 4 7% 8% 44%	1330
	21st to end	Night	12% 6% 2 7% 5% 5% 5% 4 6% 7% 40%	1330
July	1st to 10th	Night	12% 6% 2 6% 5% 4 4 4 5% 8% 45%	2210
	11th to 20th	Night	10% 4 2 5% 4 3 6% 4 6% 7% 51%	2303
	21st to end	Night	8% 5% 3 7% 4 4 4 3 5% 6% 51%	2900
August	1st to 10th	Night	8% 6% 2 7% 4 4 5% 5% 6% 8% 45%	2825
	11th to 20th	Night	11% 5% 2 6% 3 4 5% 3 7% 8% 45%	2785
	21st to end	Night	8% 6% 3 6% 4 4 4 3 5% 7% 50%	3124
September	1st to 10th	Night	12% 5% 2 6% 3 4 4 4 5% 8% 47%	1914
	11th to 20th	Night	10% 5% 2 5% 4 5% 4 3 6% 7% 50%	2090
	21st to end	Night	17% 6% 2 6% 5% 4 5% 3 5% 8% 39%	2090
October	1st to 10th	Night	18% 8% 3 7% 6% 4 4 4 5% 6% 36%	2223
	11th to 20th	Night	23% 9% 3 8% 6% 5% 4 3 5% 6% 26%	2280
	21st to end	Night	24% 12% 4 8% 6% 4 4 3 5% 6% 23%	2650
November	1st to 10th	Night	19% 14% 3 7% 4 5% 4 4 5% 7% 28%	2558
	11th to 20th	Night	17% 18% 5% 10% 7% 6% 5% 3 5% 6% 19%	2800
	21st to end	Night	21% 20% 6% 9% 6% 5% 4 3 5% 6% 16%	2800
December	1st to 10th	Night	17% 21% 6% 10% 5% 5% 4 3 5% 5% 18%	4641
	11th to 20th	Night	18% 20% 6% 16% 6% 5% 4 3 4 4 14%	5586
	21st to end	Night	25% 22% 7% 11% 4 4 4 3 5% 4 12%	7502

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	61% 9% 11% 14% 2 2	3204
	11th to 20th	Day	77% 7% 6% 6% 4	3620
	21st to end	Day	69% 8% 9% 9% 4	4244
February	1st to 10th	Day	65% 7% 11% 13% 3	1800
	11th to 20th	Day	58% 8% 14% 13% 6% 1	1762
	21st to end	Day	56% 8% 13% 18% 4	1639
March	1st to 10th	Day	48% 7% 14% 13% 15% 3	4682
	11th to 20th	Day	53% 7% 15% 16% 8% 1	4980
	21st to end	Day	48% 8% 16% 15% 11% 2	5465
April	1st to 10th	Day	49% 6% 13% 20% 11% 1	2347
	11th to 20th	Day	44% 5% 10% 25% 14% 2	2572
	21st to end	Day	46% 5% 13% 20% 13% 3	2535
May	1st to 10th	Day	39% 7% 12% 23% 17% 2	4120
	11th to 20th	Day	38% 8% 14% 25% 14% 1	4236
	21st to end	Day	39% 7% 13% 17% 22% 3	4990
June	1st to 10th	Day	40% 7% 13% 25% 13% 2	2850
	11th to 20th	Day	36% 7% 13% 26% 17% 2	2850
	21st to end	Day	41% 7% 12% 24% 13% 2	2850
July	1st to 10th	Day	43% 6% 14% 23% 13% 1	4686
	11th to 20th	Day	29% 8% 16% 34% 13% 1	4569
	21st to end	Day	29% 7% 12% 29% 23% 1	4509
August	1st to 10th	Day	33% 8% 17% 29% 13% 1	4091
	11th to 20th	Day	35% 7% 18% 29% 11%	4083
	21st to end	Day	33% 9% 15% 33% 10% 1	4531
September	1st to 10th	Day	37% 8% 16% 28% 10% 1	2244
	11th to 20th	Day	33% 7% 17% 31% 11% 1	2090
	21st to end	Day	43% 9% 15% 23% 9%	2090
October	1st to 10th	Day	37% 6% 11% 28% 19%	3256
	11th to 20th	Day	46% 6% 15% 25% 7%	3140
	21st to end	Day	57% 6% 14% 16% 6%	3183
November	1st to 10th	Day	57% 8% 13% 17% 5%	1738
	11th to 20th	Day	63% 7% 13% 11% 5%	1600
	21st to end	Day	73% 6% 12% 8% 2	1600
December	1st to 10th	Day	72% 6% 14% 6% 1	2536
	11th to 20th	Day	72% 6% 10% 10% 2	2621
	21st to end	Day	81% 8% 6% 5%	3476
January	1st to 10th	Night	61% 9% 13% 13% 3 1	6762
	11th to 20th	Night	69% 8% 9% 9% 5%	6360
	21st to end	Night	70% 6% 9% 8% 5% 1	6720
February	1st to 10th	Night	65% 5% 11% 14% 4	2600
	11th to 20th	Night	58% 8% 12% 15% 6% 1	2535
	21st to end	Night	57% 7% 13% 16% 7%	1989
March	1st to 10th	Night	46% 6% 14% 17% 13% 3	5171
	11th to 20th	Night	47% 6% 17% 20% 9% 1	4980
	21st to end	Night	52% 6% 10% 17% 13% 2	5467
April	1st to 10th	Night	50% 5% 10% 20% 14% 2	2070
	11th to 20th	Night	43% 5% 9% 22% 18% 2	1777
	21st to end	Night	51% 6% 9% 16% 15% 3	1761
May	1st to 10th	Night	44% 6% 12% 21% 15% 2	2840
	11th to 20th	Night	39% 7% 12% 22% 17% 2	2660
	21st to end	Night	40% 8% 12% 19% 16% 5%	2588
June	1st to 10th	Night	39% 8% 10% 23% 18% 2	1330
	11th to 20th	Night	34% 8% 13% 23% 20% 2	1330
	21st to end	Night	41% 7% 14% 22% 13% 2	1330
July	1st to 10th	Night	37% 5% 13% 24% 16% 4	2175
	11th to 20th	Night	30% 7% 11% 38% 13% 1	2261
	21st to end	Night	33% 6% 12% 23% 26% 1	2808
August	1st to 10th	Night	34% 8% 14% 30% 13% 1	2814
	11th to 20th	Night	35% 8% 18% 25% 13%	2812
	21st to end	Night	33% 7% 14% 32% 13% 1	3124
September	1st to 10th	Night	35% 7% 14% 28% 14% 2	1914
	11th to 20th	Night	33% 7% 16% 33% 11% 1	2090
	21st to end	Night	43% 8% 15% 23% 10%	2090
October	1st to 10th	Night	37% 5% 10% 30% 18%	3704
	11th to 20th	Night	46% 6% 16% 23% 9%	3779
	21st to end	Night	52% 8% 18% 17% 5%	4363
November	1st to 10th	Night	55% 7% 13% 19% 6%	2558
	11th to 20th	Night	66% 7% 12% 10% 4	2800
	21st to end	Night	70% 7% 12% 9% 2	2800
December	1st to 10th	Night	68% 7% 15% 8% 2	4641
	11th to 20th	Night	74% 7% 8% 8% 3	5585
	21st to end	Night	76% 9% 8% 5% 2	7502

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	26% 19% 33% 18% 4	210
	11th to 20th	Day	21% 17% 38% 19% 5%	300
	21st to end	Day	26% 14% 33% 18% 9%	330
February	1st to 10th	Day	22% 26% 34% 13% 4	300
	11th to 20th	Day	18% 23% 39% 16% 3	300
	21st to end	Day	24% 27% 36% 11% 2	249
March	1st to 10th	Day	21% 18% 37% 19% 4 1	385
	11th to 20th	Day	19% 16% 36% 23% 5% 1	500
	21st to end	Day	22% 17% 32% 22% 6% 1	550
April	1st to 10th	Day	23% 15% 31% 23% 8% 1	500
	11th to 20th	Day	26% 16% 28% 18% 10% 3	492
	21st to end	Day	26% 13% 34% 18% 8% 1	480
May	1st to 10th	Day	20% 9% 37% 24% 8% 2	463
	11th to 20th	Day	17% 10% 29% 30% 13% 2	487
	21st to end	Day	13% 8% 33% 29% 15% 3	617
June	1st to 10th	Day	14% 9% 32% 31% 13% 1	680
	11th to 20th	Day	12% 8% 31% 34% 13% 1	680
	21st to end	Day	17% 8% 30% 31% 11% 2	680
July	1st to 10th	Day	9% 4 28% 39% 17% 2	671
	11th to 20th	Day	10% 6% 28% 38% 14% 4	668
	21st to end	Day	11% 3 30% 36% 16% 4	644
August	1st to 10th	Day	12% 6% 25% 33% 20% 5%	486
	11th to 20th	Day	12% 5% 21% 35% 20% 8%	479
	21st to end	Day	13% 8% 28% 29% 18% 4	539
September	1st to 10th	Day	13% 10% 28% 26% 18% 5%	490
	11th to 20th	Day	13% 7% 23% 31% 19% 7%	490
	21st to end	Day	19% 11% 24% 26% 17% 4	490
October	1st to 10th	Day	19% 19% 24% 24% 12% 2	490
	11th to 20th	Day	24% 19% 34% 17% 6% 1	490
	21st to end	Day	28% 23% 33% 13% 3 1	395
November	1st to 10th	Day	29% 18% 33% 15% 4	287
	11th to 20th	Day	26% 22% 34% 16% 3	200
	21st to end	Day	31% 25% 33% 11% 2	200
December	1st to 10th	Day	28% 24% 29% 15% 6% 1	200
	11th to 20th	Day	32% 24% 34% 10% 2	200
	21st to end	Day	33% 25% 31% 7% 2 1	220
January	1st to 10th	Night	22% 21% 32% 20% 4	1000
	11th to 20th	Night	25% 18% 31% 20% 5% 1	1000
	21st to end	Night	22% 13% 35% 22% 7% 1	980
February	1st to 10th	Night	19% 21% 35% 20% 4	800
	11th to 20th	Night	19% 18% 33% 24% 6%	783
	21st to end	Night	21% 17% 37% 21% 5%	660
March	1st to 10th	Night	18% 17% 37% 22% 6% 1	800
	11th to 20th	Night	17% 15% 36% 23% 8% 1	800
	21st to end	Night	20% 11% 31% 29% 8% 1	880
April	1st to 10th	Night	24% 12% 33% 25% 5% 1	738
	11th to 20th	Night	22% 11% 30% 23% 11% 3	593
	21st to end	Night	22% 11% 28% 25% 13% 1	587
May	1st to 10th	Night	15% 6% 28% 34% 14% 3	570
	11th to 20th	Night	16% 7% 26% 33% 16% 1	473
	21st to end	Night	14% 5% 26% 34% 19% 2	411
June	1st to 10th	Night	15% 4 31% 34% 13% 2	380
	11th to 20th	Night	11% 8% 22% 38% 20% 2	380
	21st to end	Night	14% 3 21% 37% 22% 2	380
July	1st to 10th	Night	10% 3 20% 42% 23% 1	370
	11th to 20th	Night	8% 3 20% 39% 25% 4	422
	21st to end	Night	9% 4 23% 34% 27% 4	624
August	1st to 10th	Night	10% 4 21% 37% 24% 4	567
	11th to 20th	Night	11% 4 17% 31% 32% 5%	564
	21st to end	Night	12% 4 22% 32% 23% 6%	627
September	1st to 10th	Night	14% 6% 26% 27% 20% 7%	708
	11th to 20th	Night	12% 6% 24% 30% 18% 10%	760
	21st to end	Night	18% 9% 24% 26% 19% 4	760
October	1st to 10th	Night	21% 12% 26% 26% 13% 2	760
	11th to 20th	Night	28% 15% 28% 19% 10%	760
	21st to end	Night	28% 17% 31% 19% 4 1	836
November	1st to 10th	Night	26% 16% 29% 21% 7% 1	780
	11th to 20th	Night	27% 22% 28% 17% 7%	800
	21st to end	Night	32% 18% 29% 16% 4	800
December	1st to 10th	Night	31% 23% 29% 12% 6%	835
	11th to 20th	Night	29% 22% 32% 12% 4 1	1000
	21st to end	Night	34% 17% 30% 14% 5%	1100

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