St. James Plantation
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the St. James Plantation clear sky chart from 2005-11-06 to 2026-01-17.

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	11% 6% 2 6% 4 4 5% 4 6% 8% 43%	126984
	Night	11% 6% 2 6% 4 4 4 4 6% 8% 44%	124577
Transparency	Day	37% 8% 16% 19% 16% 5%	130584
	Night	36% 7% 14% 19% 18% 6%	129045
Seeing	Day	13% 6% 21% 30% 23% 7%	17350
	Night	13% 7% 22% 31% 23% 5%	27542

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	11% 5% 2 4 3 3 3 3 5% 6% 54%	4146
	11th to 20th	Day	11% 5% 2 5% 4 4 3 3 4 6% 52%	4114
	21st to end	Day	14% 9% 3 7% 4 4 4 3 4 5% 43%	4466
February	1st to 10th	Day	15% 7% 3 6% 5% 5% 3 4 6% 6% 41%	1800
	11th to 20th	Day	18% 7% 3 6% 4 3 3 3 4 5% 45%	1859
	21st to end	Day	12% 7% 2 6% 3 4 4 4 6% 6% 44%	1780
March	1st to 10th	Day	19% 8% 4 7% 4 3 4 3 5% 5% 39%	4935
	11th to 20th	Day	8% 4 2 5% 4 4 4 4 6% 7% 54%	4980
	21st to end	Day	12% 5% 2 7% 4 4 4 3 4 5% 50%	5465
April	1st to 10th	Day	10% 4 1 5% 3 4 4 4 7% 7% 52%	2350
	11th to 20th	Day	8% 4 2 5% 3 4 4 4 5% 7% 54%	2374
	21st to end	Day	7% 3 2 4 3 4 4 4 7% 8% 55%	2400
May	1st to 10th	Day	7% 5% 3 7% 5% 5% 6% 5% 7% 10% 40%	5441
	11th to 20th	Day	15% 6% 2 7% 5% 5% 5% 4 6% 10% 34%	5861
	21st to end	Day	8% 5% 2 6% 4 5% 5% 5% 7% 10% 44%	6433
June	1st to 10th	Day	10% 7% 2 8% 5% 5% 5% 4 7% 11% 36%	2600
	11th to 20th	Day	8% 5% 2 5% 4 4 5% 5% 8% 13% 41%	2600
	21st to end	Day	6% 5% 2 6% 5% 5% 6% 5% 9% 12% 38%	2600
July	1st to 10th	Day	9% 6% 3 8% 6% 6% 6% 6% 8% 13% 30%	5900
	11th to 20th	Day	5% 5% 2 8% 6% 6% 6% 6% 8% 13% 36%	5900
	21st to end	Day	7% 6% 3 7% 5% 6% 6% 6% 9% 12% 33%	6424
August	1st to 10th	Day	20% 10% 3 8% 5% 5% 4 5% 6% 10% 22%	4234
	11th to 20th	Day	8% 6% 3 6% 4 5% 6% 5% 9% 11% 38%	3868
	21st to end	Day	7% 6% 3 7% 4 5% 5% 4 7% 10% 43%	4312
September	1st to 10th	Day	10% 5% 2 6% 4 5% 6% 4 6% 9% 41%	2400
	11th to 20th	Day	13% 7% 3 7% 4 5% 5% 4 6% 8% 39%	2330
	21st to end	Day	15% 7% 3 8% 5% 5% 5% 3 5% 7% 38%	2120
October	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 4 5% 7% 44%	2000
	11th to 20th	Day	9% 4 2 5% 4 3 3 4 3 5% 58%	2000
	21st to end	Day	9% 5% 2 5% 4 3 3 3 4 5% 57%	2200
November	1st to 10th	Day	10% 6% 2 5% 3 3 3 3 5% 6% 54%	2057
	11th to 20th	Day	11% 5% 2 5% 5% 4 3 3 5% 6% 51%	2100
	21st to end	Day	9% 5% 2 6% 4 4 3 3 5% 6% 52%	2042
December	1st to 10th	Day	11% 5% 2 5% 3 5% 5% 4 5% 6% 50%	2865
	11th to 20th	Day	12% 6% 2 7% 4 4 4 4 4 5% 47%	3463
	21st to end	Day	13% 7% 2 5% 3 4 4 4 6% 6% 45%	4565
January	1st to 10th	Night	10% 6% 2 6% 3 3 3 3 5% 6% 54%	6019
	11th to 20th	Night	12% 6% 2 6% 4 4 4 4 6% 8% 46%	5979
	21st to end	Night	12% 7% 3 6% 4 4 4 3 4 6% 46%	6490
February	1st to 10th	Night	15% 7% 2 6% 5% 4 3 3 5% 6% 44%	2600
	11th to 20th	Night	18% 9% 2 6% 4 3 3 3 5% 5% 42%	2425
	21st to end	Night	14% 6% 2 7% 4 4 4 4 5% 6% 43%	1845
March	1st to 10th	Night	16% 10% 3 8% 4 4 4 3 4 5% 39%	4918
	11th to 20th	Night	12% 4 2 5% 3 3 4 4 5% 8% 49%	4980
	21st to end	Night	12% 4 2 5% 3 3 4 3 5% 6% 53%	5467
April	1st to 10th	Night	10% 6% 2 5% 4 4 4 4 6% 7% 49%	2049
	11th to 20th	Night	9% 5% 2 6% 5% 4 5% 4 5% 6% 51%	1975
	21st to end	Night	6% 4 2 4 4 4 4 4 6% 7% 54%	2000
May	1st to 10th	Night	7% 6% 2 6% 4 5% 5% 4 6% 11% 43%	4490
	11th to 20th	Night	13% 7% 2 8% 4 5% 5% 4 7% 9% 35%	4037
	21st to end	Night	11% 5% 3 6% 4 4 5% 4 6% 10% 41%	4421
June	1st to 10th	Night	10% 7% 2 7% 5% 5% 5% 5% 8% 11% 35%	1800
	11th to 20th	Night	8% 5% 2 5% 5% 4 6% 4 7% 12% 42%	1800
	21st to end	Night	8% 5% 2 5% 6% 5% 5% 5% 9% 13% 36%	1800
July	1st to 10th	Night	6% 5% 2 8% 6% 6% 6% 6% 9% 12% 32%	4058
	11th to 20th	Night	7% 5% 2 7% 5% 5% 6% 6% 10% 16% 30%	4057
	21st to end	Night	6% 6% 2 7% 6% 6% 7% 6% 10% 13% 30%	4427
August	1st to 10th	Night	19% 9% 4 8% 4 3 5% 5% 7% 11% 25%	2915
	11th to 20th	Night	6% 5% 2 7% 5% 6% 6% 5% 8% 12% 38%	3135
	21st to end	Night	6% 5% 2 6% 5% 5% 5% 4 7% 10% 45%	3586
September	1st to 10th	Night	10% 5% 2 6% 5% 5% 6% 4 7% 11% 39%	2000
	11th to 20th	Night	12% 6% 3 7% 5% 5% 6% 4 7% 9% 36%	2065
	21st to end	Night	17% 7% 3 6% 4 3 4 3 5% 8% 40%	2280
October	1st to 10th	Night	11% 6% 3 6% 4 5% 5% 4 5% 7% 45%	2400
	11th to 20th	Night	10% 4 2 4 3 4 4 4 5% 7% 54%	2400
	21st to end	Night	9% 5% 2 5% 4 3 3 2 4 6% 56%	2640
November	1st to 10th	Night	11% 5% 2 5% 3 3 4 4 5% 6% 54%	2461
	11th to 20th	Night	10% 7% 2 6% 4 4 3 3 4 6% 49%	2520
	21st to end	Night	9% 5% 2 5% 4 3 4 3 6% 8% 49%	2573
December	1st to 10th	Night	11% 6% 2 5% 4 3 3 3 4 6% 52%	4380
	11th to 20th	Night	14% 8% 3 7% 4 3 4 3 5% 6% 44%	4952
	21st to end	Night	13% 6% 2 6% 4 3 4 4 5% 7% 46%	6633

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	30% 4 8% 18% 34% 7%	4147
	11th to 20th	Day	34% 4 6% 17% 31% 9%	4114
	21st to end	Day	43% 4 6% 17% 20% 9%	4466
February	1st to 10th	Day	42% 6% 11% 21% 17% 4	1800
	11th to 20th	Day	43% 5% 7% 17% 21% 7%	1868
	21st to end	Day	37% 7% 11% 19% 17% 9%	1780
March	1st to 10th	Day	47% 5% 9% 13% 19% 7%	4935
	11th to 20th	Day	28% 5% 9% 15% 26% 16%	4980
	21st to end	Day	36% 5% 10% 22% 18% 9%	5465
April	1st to 10th	Day	29% 7% 15% 22% 22% 6%	2350
	11th to 20th	Day	28% 6% 14% 26% 19% 9%	2374
	21st to end	Day	25% 8% 16% 25% 23% 3	2400
May	1st to 10th	Day	35% 10% 22% 24% 8% 1	5473
	11th to 20th	Day	44% 8% 18% 21% 9%	5865
	21st to end	Day	32% 9% 19% 24% 15% 1	6447
June	1st to 10th	Day	40% 10% 17% 22% 10%	2600
	11th to 20th	Day	31% 12% 22% 24% 11%	2600
	21st to end	Day	34% 13% 23% 22% 7% 1	2600
July	1st to 10th	Day	41% 13% 28% 15% 2	5835
	11th to 20th	Day	35% 14% 38% 10% 2	5831
	21st to end	Day	40% 12% 23% 19% 5%	6240
August	1st to 10th	Day	56% 10% 22% 11% 2	4221
	11th to 20th	Day	35% 13% 22% 21% 9%	3904
	21st to end	Day	35% 10% 20% 31% 4 1	4312
September	1st to 10th	Day	36% 12% 18% 17% 13% 3	2400
	11th to 20th	Day	42% 11% 14% 17% 14% 3	2330
	21st to end	Day	45% 8% 13% 18% 14% 2	2120
October	1st to 10th	Day	40% 5% 10% 19% 22% 4	3260
	11th to 20th	Day	20% 4 7% 17% 29% 23%	3240
	21st to end	Day	28% 5% 8% 24% 26% 10%	3538
November	1st to 10th	Day	31% 5% 10% 20% 26% 9%	2057
	11th to 20th	Day	35% 6% 8% 19% 24% 8%	2100
	21st to end	Day	33% 7% 10% 21% 24% 6%	2042
December	1st to 10th	Day	35% 6% 9% 16% 23% 11%	2865
	11th to 20th	Day	39% 6% 10% 18% 20% 6%	3460
	21st to end	Day	37% 5% 14% 24% 16% 2	4565
January	1st to 10th	Night	31% 4 7% 17% 34% 7%	6019
	11th to 20th	Night	37% 5% 8% 14% 31% 6%	5979
	21st to end	Night	39% 5% 6% 15% 27% 9%	6490
February	1st to 10th	Night	41% 4 9% 20% 19% 6%	2600
	11th to 20th	Night	43% 6% 9% 15% 20% 7%	2427
	21st to end	Night	41% 5% 10% 18% 16% 10%	1845
March	1st to 10th	Night	47% 5% 8% 14% 20% 6%	4918
	11th to 20th	Night	32% 5% 10% 12% 24% 16%	4980
	21st to end	Night	34% 5% 11% 23% 19% 7%	5467
April	1st to 10th	Night	33% 6% 14% 18% 21% 8%	2049
	11th to 20th	Night	33% 6% 12% 22% 17% 9%	1975
	21st to end	Night	27% 7% 15% 28% 19% 4	2000
May	1st to 10th	Night	33% 10% 20% 26% 10% 1	4520
	11th to 20th	Night	43% 9% 19% 19% 10%	4038
	21st to end	Night	36% 9% 18% 22% 16%	4429
June	1st to 10th	Night	40% 9% 21% 21% 8%	1800
	11th to 20th	Night	33% 12% 21% 26% 8%	1800
	21st to end	Night	34% 15% 21% 23% 6%	1800
July	1st to 10th	Night	40% 12% 28% 16% 3	4015
	11th to 20th	Night	35% 16% 38% 9% 2	4006
	21st to end	Night	38% 15% 21% 22% 3	4297
August	1st to 10th	Night	51% 11% 24% 12% 2	2904
	11th to 20th	Night	35% 12% 23% 23% 7%	3164
	21st to end	Night	33% 10% 19% 32% 6%	3586
September	1st to 10th	Night	38% 13% 19% 16% 12% 3	2000
	11th to 20th	Night	44% 10% 15% 17% 12% 2	2065
	21st to end	Night	44% 8% 14% 20% 12% 2	2280
October	1st to 10th	Night	39% 5% 11% 17% 22% 6%	3920
	11th to 20th	Night	20% 4 7% 15% 23% 31%	3899
	21st to end	Night	25% 4 7% 25% 28% 11%	4252
November	1st to 10th	Night	32% 7% 9% 18% 24% 10%	2461
	11th to 20th	Night	37% 5% 9% 16% 24% 9%	2520
	21st to end	Night	33% 6% 11% 22% 22% 7%	2573
December	1st to 10th	Night	34% 4 8% 15% 27% 12%	4380
	11th to 20th	Night	41% 5% 12% 17% 18% 7%	4954
	21st to end	Night	37% 5% 12% 25% 18% 2	6633

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	16% 12% 39% 26% 7%	330
	11th to 20th	Day	13% 11% 31% 36% 8%	318
	21st to end	Day	18% 14% 33% 28% 6% 1	330
February	1st to 10th	Day	16% 12% 33% 33% 6%	300
	11th to 20th	Day	25% 12% 33% 26% 5%	406
	21st to end	Day	12% 12% 37% 29% 9% 1	414
March	1st to 10th	Day	14% 10% 35% 30% 10% 1	500
	11th to 20th	Day	14% 12% 38% 28% 8% 1	500
	21st to end	Day	12% 10% 37% 29% 10% 1	550
April	1st to 10th	Day	12% 12% 29% 31% 14% 2	500
	11th to 20th	Day	10% 8% 26% 30% 20% 5%	492
	21st to end	Day	8% 6% 23% 34% 24% 6%	500
May	1st to 10th	Day	5% 8% 31% 31% 19% 6%	503
	11th to 20th	Day	14% 3 15% 36% 25% 5%	527
	21st to end	Day	12% 5% 13% 29% 27% 13%	573
June	1st to 10th	Day	12% 4 16% 31% 29% 8%	530
	11th to 20th	Day	10% 2 12% 29% 32% 14%	530
	21st to end	Day	8% 2 9% 33% 38% 10%	530
July	1st to 10th	Day	9% 2 6% 27% 41% 14%	525
	11th to 20th	Day	9% 2 6% 24% 44% 15%	524
	21st to end	Day	7% 2 7% 30% 43% 11%	576
August	1st to 10th	Day	15% 3 8% 26% 36% 12%	526
	11th to 20th	Day	11% 2 6% 25% 39% 18%	519
	21st to end	Day	11% 3 9% 23% 39% 15%	583
September	1st to 10th	Day	9% 3 10% 28% 34% 16%	530
	11th to 20th	Day	18% 4 11% 28% 30% 10%	530
	21st to end	Day	19% 3 11% 29% 28% 11%	530
October	1st to 10th	Day	14% 6% 18% 28% 24% 10%	530
	11th to 20th	Day	10% 3 21% 33% 25% 7%	530
	21st to end	Day	10% 7% 21% 33% 24% 5%	583
November	1st to 10th	Day	13% 6% 21% 31% 23% 5%	535
	11th to 20th	Day	13% 9% 31% 31% 15% 1	540
	21st to end	Day	10% 10% 31% 30% 17% 3	482
December	1st to 10th	Day	23% 7% 27% 28% 14% 1	281
	11th to 20th	Day	16% 10% 27% 32% 13% 2	330
	21st to end	Day	16% 13% 30% 28% 10% 3	363
January	1st to 10th	Night	14% 13% 34% 30% 8%	840
	11th to 20th	Night	14% 11% 31% 34% 10%	825
	21st to end	Night	16% 10% 36% 33% 5%	880
February	1st to 10th	Night	19% 9% 31% 33% 7%	800
	11th to 20th	Night	22% 13% 31% 30% 4	783
	21st to end	Night	17% 10% 30% 33% 10%	660
March	1st to 10th	Night	14% 11% 33% 31% 11% 1	800
	11th to 20th	Night	13% 10% 30% 36% 10% 1	800
	21st to end	Night	15% 9% 32% 31% 13% 1	880
April	1st to 10th	Night	12% 9% 30% 34% 14% 1	800
	11th to 20th	Night	11% 8% 25% 32% 21% 4	791
	21st to end	Night	7% 5% 27% 35% 23% 3	800
May	1st to 10th	Night	7% 7% 24% 38% 21% 3	800
	11th to 20th	Night	15% 4 20% 30% 27% 4	596
	21st to end	Night	11% 3 10% 27% 34% 14%	650
June	1st to 10th	Night	9% 4 13% 35% 33% 6%	600
	11th to 20th	Night	11% 2 10% 30% 37% 11%	600
	21st to end	Night	9% 3 8% 34% 38% 10%	600
July	1st to 10th	Night	7% 3 7% 27% 46% 10%	585
	11th to 20th	Night	9% 1 3 27% 46% 13%	582
	21st to end	Night	8% 3 6% 30% 42% 11%	657
August	1st to 10th	Night	12% 3 7% 27% 40% 11%	597
	11th to 20th	Night	9% 2 7% 26% 43% 13%	752
	21st to end	Night	12% 3 6% 25% 41% 14%	880
September	1st to 10th	Night	11% 2 8% 27% 40% 12%	800
	11th to 20th	Night	15% 3 10% 31% 34% 9%	800
	21st to end	Night	20% 3 12% 27% 30% 8%	800
October	1st to 10th	Night	13% 4 18% 31% 26% 8%	800
	11th to 20th	Night	12% 4 22% 34% 23% 6%	800
	21st to end	Night	13% 7% 19% 31% 25% 5%	880
November	1st to 10th	Night	13% 6% 24% 33% 21% 4	820
	11th to 20th	Night	14% 10% 28% 29% 17% 2	840
	21st to end	Night	11% 7% 30% 32% 17% 2	840
December	1st to 10th	Night	19% 9% 28% 31% 12% 1	840
	11th to 20th	Night	17% 9% 27% 32% 13% 1	840
	21st to end	Night	18% 9% 29% 31% 12% 1	924

Page generated at 2026-01-14 01:14:26. Version ac07

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