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 2024-11-25.

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% 42%	101103
	Night	11% 6% 2 6% 4 4 4 4 6% 8% 43%	98249
Transparency	Day	37% 9% 15% 20% 15% 5%	104747
	Night	37% 8% 14% 19% 17% 6%	102743
Seeing	Day	13% 6% 20% 29% 24% 8%	15817
	Night	13% 6% 21% 31% 23% 5%	25966

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	14% 6% 2 4 3 4 3 3 6% 8% 47%	2836
	11th to 20th	Day	10% 6% 2 5% 4 5% 4 4 5% 8% 47%	2840
	21st to end	Day	13% 8% 3 7% 5% 4 4 3 4 5% 44%	3124
February	1st to 10th	Day	15% 7% 3 6% 5% 5% 3 4 6% 6% 41%	1710
	11th to 20th	Day	17% 7% 3 6% 4 3 3 3 4 5% 45%	1763
	21st to end	Day	13% 7% 3 7% 4 4 4 4 6% 6% 42%	1693
March	1st to 10th	Day	20% 9% 4 8% 4 3 3 3 4 5% 37%	3435
	11th to 20th	Day	9% 4 2 5% 4 4 4 3 5% 7% 53%	3480
	21st to end	Day	15% 6% 2 7% 4 4 4 3 5% 5% 46%	3815
April	1st to 10th	Day	10% 4 1 5% 3 4 4 4 7% 7% 52%	2232
	11th to 20th	Day	9% 4 2 5% 3 4 4 4 5% 7% 53%	2254
	21st to end	Day	8% 3 2 3 3 4 4 4 6% 8% 56%	2280
May	1st to 10th	Day	6% 5% 2 6% 5% 5% 6% 5% 7% 10% 44%	3787
	11th to 20th	Day	14% 6% 2 7% 5% 5% 4 3 5% 8% 39%	4081
	21st to end	Day	6% 4 1 5% 4 4 5% 5% 8% 11% 47%	4476
June	1st to 10th	Day	10% 7% 2 8% 5% 5% 5% 4 7% 10% 37%	2470
	11th to 20th	Day	8% 5% 2 5% 4 4 5% 5% 8% 13% 41%	2470
	21st to end	Day	7% 5% 2 6% 6% 6% 6% 5% 9% 12% 36%	2470
July	1st to 10th	Day	10% 5% 2 7% 5% 5% 6% 6% 8% 13% 32%	4120
	11th to 20th	Day	7% 6% 3 9% 7% 6% 7% 5% 7% 10% 33%	4120
	21st to end	Day	8% 6% 3 7% 6% 6% 6% 6% 9% 12% 31%	4466
August	1st to 10th	Day	20% 10% 3 8% 5% 5% 4 5% 6% 10% 23%	4104
	11th to 20th	Day	8% 5% 3 6% 4 5% 6% 5% 9% 11% 38%	3747
	21st to end	Day	7% 6% 3 7% 4 5% 5% 4 7% 10% 43%	4180
September	1st to 10th	Day	10% 5% 2 6% 4 5% 6% 4 6% 9% 41%	2280
	11th to 20th	Day	14% 7% 3 7% 4 5% 5% 4 6% 8% 38%	2214
	21st to end	Day	14% 7% 3 8% 5% 5% 5% 3 5% 6% 39%	2014
October	1st to 10th	Day	12% 5% 2 6% 5% 5% 5% 3 5% 6% 45%	1900
	11th to 20th	Day	9% 5% 1 5% 4 4 3 4 3 5% 58%	1900
	21st to end	Day	9% 5% 2 6% 4 3 3 3 5% 5% 57%	2090
November	1st to 10th	Day	11% 6% 2 5% 3 3 3 3 6% 6% 53%	1957
	11th to 20th	Day	12% 5% 2 6% 5% 4 3 3 5% 6% 50%	2000
	21st to end	Day	10% 5% 2 6% 4 4 3 3 5% 6% 53%	1888
December	1st to 10th	Day	15% 6% 3 7% 4 6% 5% 5% 6% 7% 38%	1630
	11th to 20th	Day	15% 5% 1 4 3 3 3 3 4 5% 54%	2153
	21st to end	Day	14% 9% 3 6% 4 4 4 4 6% 6% 41%	3124
January	1st to 10th	Night	13% 7% 2 7% 4 3 4 3 5% 7% 45%	4109
	11th to 20th	Night	11% 7% 2 7% 4 4 5% 5% 6% 7% 41%	4120
	21st to end	Night	12% 8% 3 7% 4 4 4 3 5% 5% 44%	4532
February	1st to 10th	Night	16% 7% 2 5% 4 4 3 3 5% 6% 44%	2470
	11th to 20th	Night	17% 9% 2 6% 3 3 3 3 6% 5% 43%	2301
	21st to end	Night	15% 6% 2 7% 4 4 4 4 5% 6% 41%	1756
March	1st to 10th	Night	21% 12% 4 8% 4 4 3 3 4 6% 31%	3418
	11th to 20th	Night	11% 5% 3 6% 4 4 4 4 5% 7% 47%	3480
	21st to end	Night	14% 4 2 5% 3 4 4 3 5% 7% 49%	3817
April	1st to 10th	Night	10% 6% 2 5% 4 4 4 4 6% 7% 49%	1947
	11th to 20th	Night	9% 5% 2 6% 5% 4 5% 4 5% 6% 49%	1875
	21st to end	Night	7% 4 2 4 4 4 4 4 6% 7% 55%	1900
May	1st to 10th	Night	6% 5% 2 7% 5% 5% 5% 4 6% 10% 45%	3130
	11th to 20th	Night	10% 7% 3 8% 4 5% 5% 4 6% 8% 40%	2817
	21st to end	Night	7% 4 3 5% 4 4 5% 4 6% 11% 47%	3079
June	1st to 10th	Night	10% 7% 3 8% 5% 5% 5% 5% 9% 10% 35%	1710
	11th to 20th	Night	8% 5% 2 5% 5% 4 5% 4 7% 11% 43%	1710
	21st to end	Night	8% 5% 2 5% 6% 5% 5% 6% 10% 14% 34%	1710
July	1st to 10th	Night	6% 5% 2 8% 6% 7% 7% 6% 8% 12% 32%	2840
	11th to 20th	Night	8% 6% 3 8% 5% 5% 6% 6% 9% 13% 31%	2840
	21st to end	Night	7% 7% 3 7% 6% 5% 7% 7% 10% 13% 29%	3087
August	1st to 10th	Night	19% 9% 4 7% 4 3 5% 5% 7% 11% 26%	2825
	11th to 20th	Night	6% 5% 3 6% 5% 6% 6% 5% 8% 12% 39%	3037
	21st to end	Night	6% 5% 2 6% 5% 5% 5% 4 7% 10% 46%	3476
September	1st to 10th	Night	10% 5% 2 6% 5% 5% 6% 4 7% 11% 39%	1900
	11th to 20th	Night	12% 6% 3 7% 5% 5% 6% 4 7% 8% 36%	1962
	21st to end	Night	17% 7% 3 6% 4 4 4 3 5% 8% 40%	2166
October	1st to 10th	Night	11% 6% 3 6% 4 5% 4 3 5% 7% 46%	2280
	11th to 20th	Night	8% 5% 2 5% 3 4 4 4 5% 7% 55%	2280
	21st to end	Night	8% 5% 2 5% 4 3 3 2 4 6% 56%	2508
November	1st to 10th	Night	11% 5% 2 5% 3 3 4 4 5% 6% 53%	2341
	11th to 20th	Night	11% 8% 2 6% 4 4 3 4 4 6% 48%	2400
	21st to end	Night	10% 6% 2 5% 4 3 4 3 6% 8% 50%	2382
December	1st to 10th	Night	15% 8% 2 6% 5% 4 4 4 5% 8% 40%	2470
	11th to 20th	Night	15% 7% 2 5% 4 3 3 3 4 5% 49%	3042
	21st to end	Night	15% 7% 2 6% 4 3 4 3 5% 6% 45%	4532

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	34% 5% 10% 20% 25% 6%	2837
	11th to 20th	Day	35% 5% 7% 20% 23% 9%	2840
	21st to end	Day	42% 5% 7% 20% 18% 8%	3124
February	1st to 10th	Day	41% 6% 10% 20% 18% 4	1710
	11th to 20th	Day	42% 5% 7% 17% 21% 8%	1772
	21st to end	Day	39% 8% 11% 18% 16% 9%	1693
March	1st to 10th	Day	48% 6% 10% 14% 14% 7%	3435
	11th to 20th	Day	29% 7% 10% 17% 25% 12%	3480
	21st to end	Day	39% 6% 9% 22% 17% 6%	3815
April	1st to 10th	Day	29% 6% 15% 21% 23% 6%	2232
	11th to 20th	Day	29% 6% 14% 24% 19% 9%	2254
	21st to end	Day	25% 9% 15% 25% 23% 3	2280
May	1st to 10th	Day	32% 11% 21% 24% 10% 2	3822
	11th to 20th	Day	41% 8% 15% 23% 13%	4085
	21st to end	Day	25% 11% 22% 28% 14% 1	4489
June	1st to 10th	Day	40% 11% 17% 22% 10%	2470
	11th to 20th	Day	31% 12% 21% 24% 11%	2470
	21st to end	Day	36% 14% 22% 21% 7% 1	2470
July	1st to 10th	Day	39% 12% 24% 20% 3 1	4055
	11th to 20th	Day	42% 14% 29% 11% 4	4051
	21st to end	Day	41% 14% 24% 15% 6%	4324
August	1st to 10th	Day	55% 10% 22% 11% 2	4091
	11th to 20th	Day	35% 12% 22% 21% 10%	3783
	21st to end	Day	34% 10% 20% 31% 4 1	4180
September	1st to 10th	Day	37% 13% 19% 17% 12% 3	2280
	11th to 20th	Day	42% 11% 14% 16% 14% 3	2214
	21st to end	Day	44% 8% 13% 17% 15% 3	2014
October	1st to 10th	Day	40% 5% 10% 19% 22% 4	3160
	11th to 20th	Day	20% 4 7% 17% 29% 24%	3140
	21st to end	Day	27% 5% 8% 24% 26% 9%	3428
November	1st to 10th	Day	32% 5% 9% 20% 25% 9%	1957
	11th to 20th	Day	36% 6% 8% 18% 23% 8%	2000
	21st to end	Day	33% 7% 9% 21% 24% 7%	1888
December	1st to 10th	Day	43% 9% 12% 17% 13% 6%	1630
	11th to 20th	Day	34% 5% 7% 21% 24% 9%	2150
	21st to end	Day	42% 6% 12% 19% 18% 3	3124
January	1st to 10th	Night	37% 6% 9% 20% 21% 7%	4109
	11th to 20th	Night	39% 7% 10% 15% 24% 5%	4120
	21st to end	Night	39% 7% 7% 14% 26% 6%	4532
February	1st to 10th	Night	41% 4 9% 19% 20% 7%	2470
	11th to 20th	Night	42% 6% 10% 15% 20% 8%	2303
	21st to end	Night	43% 5% 10% 18% 15% 9%	1756
March	1st to 10th	Night	53% 7% 10% 13% 12% 5%	3418
	11th to 20th	Night	34% 7% 11% 14% 22% 13%	3480
	21st to end	Night	35% 6% 11% 26% 16% 5%	3817
April	1st to 10th	Night	32% 6% 14% 17% 22% 8%	1947
	11th to 20th	Night	35% 6% 12% 21% 17% 9%	1875
	21st to end	Night	26% 7% 15% 28% 20% 4	1900
May	1st to 10th	Night	31% 11% 20% 23% 13% 2	3160
	11th to 20th	Night	40% 10% 15% 22% 14%	2818
	21st to end	Night	28% 11% 18% 27% 15%	3087
June	1st to 10th	Night	41% 9% 20% 21% 9%	1710
	11th to 20th	Night	32% 11% 21% 27% 8%	1710
	21st to end	Night	36% 16% 20% 22% 6%	1710
July	1st to 10th	Night	39% 12% 22% 21% 5%	2795
	11th to 20th	Night	39% 15% 32% 11% 3	2786
	21st to end	Night	39% 16% 21% 19% 5%	2988
August	1st to 10th	Night	50% 11% 24% 13% 2	2814
	11th to 20th	Night	34% 12% 23% 23% 7%	3066
	21st to end	Night	32% 10% 19% 33% 6%	3476
September	1st to 10th	Night	38% 13% 19% 15% 11% 3	1900
	11th to 20th	Night	44% 10% 15% 17% 12% 2	1962
	21st to end	Night	44% 8% 13% 20% 13% 2	2166
October	1st to 10th	Night	38% 5% 11% 17% 22% 6%	3800
	11th to 20th	Night	19% 4 7% 15% 22% 32%	3779
	21st to end	Night	24% 4 7% 26% 28% 11%	4120
November	1st to 10th	Night	32% 7% 9% 18% 24% 10%	2341
	11th to 20th	Night	38% 6% 9% 16% 24% 9%	2400
	21st to end	Night	33% 6% 10% 21% 23% 7%	2382
December	1st to 10th	Night	43% 7% 12% 16% 16% 5%	2470
	11th to 20th	Night	38% 5% 8% 19% 24% 7%	3044
	21st to end	Night	40% 5% 9% 21% 21% 3	4532

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	17% 12% 39% 25% 7%	270
	11th to 20th	Day	13% 10% 30% 37% 9%	270
	21st to end	Day	18% 15% 31% 30% 6% 1	297
February	1st to 10th	Day	16% 12% 33% 34% 6%	270
	11th to 20th	Day	24% 11% 33% 27% 5%	370
	21st to end	Day	13% 13% 37% 28% 8% 1	382
March	1st to 10th	Day	13% 10% 35% 31% 11% 1	460
	11th to 20th	Day	13% 12% 37% 28% 8% 1	460
	21st to end	Day	13% 9% 37% 28% 10% 2	506
April	1st to 10th	Day	12% 12% 29% 30% 14% 2	460
	11th to 20th	Day	11% 8% 26% 29% 21% 5%	452
	21st to end	Day	8% 6% 23% 34% 23% 6%	460
May	1st to 10th	Day	5% 8% 30% 31% 20% 6%	463
	11th to 20th	Day	14% 3 14% 36% 27% 6%	487
	21st to end	Day	12% 4 12% 29% 29% 15%	529
June	1st to 10th	Day	12% 4 14% 31% 30% 8%	490
	11th to 20th	Day	11% 1 13% 27% 32% 15%	490
	21st to end	Day	9% 2 10% 32% 37% 10%	490
July	1st to 10th	Day	9% 2 5% 26% 42% 15%	485
	11th to 20th	Day	10% 2 7% 25% 42% 14%	484
	21st to end	Day	8% 2 7% 31% 42% 10%	532
August	1st to 10th	Day	15% 3 7% 26% 37% 13%	486
	11th to 20th	Day	9% 2 6% 26% 38% 18%	479
	21st to end	Day	11% 3 8% 22% 40% 17%	539
September	1st to 10th	Day	10% 3 9% 27% 35% 17%	490
	11th to 20th	Day	19% 3 11% 26% 30% 10%	490
	21st to end	Day	18% 3 12% 28% 29% 11%	490
October	1st to 10th	Day	14% 6% 17% 27% 25% 11%	490
	11th to 20th	Day	9% 3 20% 34% 26% 7%	490
	21st to end	Day	9% 6% 21% 33% 26% 5%	539
November	1st to 10th	Day	14% 5% 20% 32% 23% 5%	495
	11th to 20th	Day	14% 8% 31% 31% 15% 1	500
	21st to end	Day	10% 9% 29% 32% 17% 3	424
December	1st to 10th	Day	22% 7% 26% 29% 15% 1	231
	11th to 20th	Day	17% 10% 28% 29% 14% 2	270
	21st to end	Day	18% 14% 27% 27% 11% 4	297
January	1st to 10th	Night	16% 13% 34% 30% 8%	760
	11th to 20th	Night	14% 11% 29% 35% 11%	760
	21st to end	Night	16% 10% 36% 33% 5%	836
February	1st to 10th	Night	19% 8% 32% 33% 8%	760
	11th to 20th	Night	22% 13% 31% 31% 4	743
	21st to end	Night	18% 9% 31% 32% 10%	628
March	1st to 10th	Night	14% 10% 32% 32% 11% 1	760
	11th to 20th	Night	13% 11% 30% 35% 10% 1	760
	21st to end	Night	15% 8% 31% 31% 14% 1	836
April	1st to 10th	Night	12% 9% 30% 34% 14% 1	760
	11th to 20th	Night	12% 8% 25% 31% 21% 4	751
	21st to end	Night	8% 5% 28% 35% 22% 3	760
May	1st to 10th	Night	7% 7% 24% 37% 22% 3	760
	11th to 20th	Night	15% 4 18% 30% 28% 4	566
	21st to end	Night	11% 3 9% 28% 35% 15%	617
June	1st to 10th	Night	9% 4 12% 35% 34% 7%	570
	11th to 20th	Night	11% 2 9% 28% 37% 12%	570
	21st to end	Night	9% 3 8% 34% 36% 9%	570
July	1st to 10th	Night	7% 3 6% 27% 47% 10%	555
	11th to 20th	Night	10% 1 3 27% 46% 13%	552
	21st to end	Night	8% 3 5% 31% 42% 10%	624
August	1st to 10th	Night	12% 3 7% 26% 40% 12%	567
	11th to 20th	Night	10% 1 7% 26% 43% 12%	714
	21st to end	Night	12% 2 6% 24% 42% 15%	836
September	1st to 10th	Night	11% 2 8% 26% 41% 13%	760
	11th to 20th	Night	15% 3 9% 30% 34% 9%	760
	21st to end	Night	19% 3 12% 27% 30% 8%	760
October	1st to 10th	Night	13% 4 18% 31% 27% 8%	760
	11th to 20th	Night	11% 4 22% 34% 23% 6%	760
	21st to end	Night	12% 6% 19% 31% 26% 6%	836
November	1st to 10th	Night	13% 6% 23% 33% 21% 4	780
	11th to 20th	Night	14% 10% 28% 29% 18% 2	800
	21st to end	Night	12% 8% 28% 32% 18% 2	779
December	1st to 10th	Night	18% 8% 28% 32% 13% 1	760
	11th to 20th	Night	17% 9% 28% 31% 14% 1	760
	21st to end	Night	19% 9% 29% 29% 13% 1	836

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