Mount Pleasant
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mount Pleasant clear sky chart from 2005-11-06 to 2025-10-19.

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% 6% 2 6% 4 4 4 4 6% 8% 41%	126609
	Night	15% 7% 2 6% 4 4 4 4 5% 7% 43%	123110
Transparency	Day	40% 6% 12% 23% 16% 3	130202
	Night	39% 5% 11% 23% 18% 4	127586
Seeing	Day	20% 12% 28% 23% 13% 4	16218
	Night	18% 11% 30% 28% 11% 1	25637

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% 4 2 4 3 3 5% 4 5% 6% 49%	3614
	11th to 20th	Day	13% 9% 3 7% 4 4 4 3 4 5% 44%	4060
	21st to end	Day	16% 8% 3 5% 4 4 4 3 5% 5% 43%	4466
February	1st to 10th	Day	14% 8% 2 7% 5% 4 4 4 6% 7% 38%	1800
	11th to 20th	Day	18% 8% 2 5% 6% 4 4 3 4 6% 40%	1822
	21st to end	Day	18% 9% 3 6% 4 4 3 3 4 6% 40%	1650
March	1st to 10th	Day	18% 5% 1 4 4 4 5% 4 5% 7% 44%	4791
	11th to 20th	Day	11% 6% 2 5% 4 4 4 4 5% 7% 49%	4980
	21st to end	Day	16% 6% 2 6% 5% 4 5% 4 6% 7% 40%	5597
April	1st to 10th	Day	15% 8% 2 6% 5% 4 4 4 7% 7% 39%	2400
	11th to 20th	Day	13% 7% 2 7% 4 5% 3 3 4 7% 46%	2374
	21st to end	Day	18% 6% 2 5% 4 4 3 4 5% 7% 41%	2400
May	1st to 10th	Day	22% 7% 2 6% 4 4 5% 4 6% 9% 31%	5726
	11th to 20th	Day	23% 7% 2 6% 4 4 5% 4 6% 8% 30%	6262
	21st to end	Day	16% 5% 2 6% 5% 5% 4 4 6% 8% 38%	6934
June	1st to 10th	Day	11% 6% 2 4 5% 5% 5% 5% 7% 9% 41%	2800
	11th to 20th	Day	12% 5% 3 6% 5% 5% 5% 4 6% 8% 40%	2800
	21st to end	Day	10% 4 2 5% 4 4 5% 4 7% 11% 45%	2800
July	1st to 10th	Day	7% 4 2 7% 5% 5% 5% 5% 7% 10% 43%	6360
	11th to 20th	Day	9% 6% 3 7% 5% 6% 6% 5% 8% 12% 31%	6281
	21st to end	Day	9% 6% 2 6% 5% 5% 6% 5% 7% 10% 40%	6424
August	1st to 10th	Day	17% 6% 2 6% 4 5% 6% 6% 7% 8% 33%	4234
	11th to 20th	Day	11% 5% 2 6% 4 6% 5% 5% 7% 9% 39%	3981
	21st to end	Day	11% 5% 2 5% 4 5% 4 4 6% 8% 47%	4312
September	1st to 10th	Day	13% 6% 2 5% 4 4 5% 3 4 7% 47%	2400
	11th to 20th	Day	12% 5% 2 6% 4 4 3 3 6% 8% 47%	2400
	21st to end	Day	20% 8% 3 7% 4 4 4 3 5% 6% 36%	2115
October	1st to 10th	Day	16% 6% 2 5% 4 3 4 3 5% 6% 47%	2000
	11th to 20th	Day	17% 5% 2 3 4 3 4 3 5% 5% 49%	1980
	21st to end	Day	14% 5% 2 6% 4 4 4 3 5% 6% 48%	2090
November	1st to 10th	Day	12% 6% 2 4 3 3 4 3 6% 7% 51%	1818
	11th to 20th	Day	14% 6% 2 4 4 3 4 4 5% 5% 50%	1788
	21st to end	Day	15% 6% 1 5% 4 4 4 4 5% 5% 47%	1600
December	1st to 10th	Day	13% 5% 2 7% 4 4 4 3 5% 8% 46%	2600
	11th to 20th	Day	18% 9% 2 5% 4 4 3 3 4 6% 41%	2990
	21st to end	Day	17% 7% 3 6% 4 4 4 3 6% 8% 37%	3960
January	1st to 10th	Night	12% 6% 2 5% 4 4 4 3 5% 6% 49%	6331
	11th to 20th	Night	14% 7% 2 5% 3 3 3 3 5% 6% 50%	5900
	21st to end	Night	16% 9% 2 5% 3 3 3 3 4 5% 47%	6490
February	1st to 10th	Night	16% 7% 2 5% 5% 4 4 3 6% 7% 40%	2600
	11th to 20th	Night	18% 8% 2 6% 4 4 3 3 5% 6% 43%	2463
	21st to end	Night	15% 8% 3 7% 4 4 4 2 5% 6% 42%	1980
March	1st to 10th	Night	16% 7% 2 5% 4 3 3 3 4 5% 48%	5057
	11th to 20th	Night	10% 5% 3 5% 3 3 4 3 5% 7% 51%	4980
	21st to end	Night	12% 8% 3 6% 4 3 4 4 5% 6% 45%	5334
April	1st to 10th	Night	15% 8% 2 5% 5% 5% 4 4 6% 6% 40%	2000
	11th to 20th	Night	13% 6% 2 6% 4 4 4 4 6% 6% 44%	1975
	21st to end	Night	17% 6% 2 5% 3 3 4 3 6% 7% 44%	2000
May	1st to 10th	Night	23% 9% 3 7% 4 4 5% 3 5% 8% 29%	4198
	11th to 20th	Night	24% 9% 2 6% 4 3 4 4 5% 8% 31%	3630
	21st to end	Night	19% 6% 2 6% 4 4 5% 4 6% 9% 35%	3920
June	1st to 10th	Night	12% 5% 2 5% 4 5% 4 4 8% 10% 44%	1600
	11th to 20th	Night	12% 5% 3 7% 5% 4 5% 5% 6% 9% 40%	1600
	21st to end	Night	10% 5% 2 5% 4 6% 6% 3 7% 10% 42%	1600
July	1st to 10th	Night	8% 6% 3 7% 5% 6% 6% 4 7% 10% 38%	3598
	11th to 20th	Night	9% 8% 3 8% 6% 6% 6% 5% 8% 10% 30%	3658
	21st to end	Night	8% 6% 2 6% 4 5% 5% 4 6% 11% 42%	4427
August	1st to 10th	Night	18% 6% 2 6% 4 4 5% 5% 7% 10% 33%	2915
	11th to 20th	Night	10% 8% 3 7% 6% 6% 6% 4 6% 7% 36%	3034
	21st to end	Night	11% 5% 2 5% 3 4 4 4 5% 9% 47%	3586
September	1st to 10th	Night	14% 6% 2 5% 4 3 4 3 5% 7% 47%	2000
	11th to 20th	Night	10% 8% 2 5% 4 3 4 2 3 8% 51%	2000
	21st to end	Night	20% 7% 2 7% 5% 4 5% 3 5% 6% 38%	2281
October	1st to 10th	Night	19% 6% 2 4 3 3 3 3 4 5% 46%	2400
	11th to 20th	Night	15% 5% 2 4 3 3 3 3 4 6% 52%	2377
	21st to end	Night	15% 6% 2 4 3 2 4 3 5% 6% 49%	2508
November	1st to 10th	Night	11% 5% 2 6% 3 3 4 3 4 6% 53%	2477
	11th to 20th	Night	14% 7% 2 5% 4 3 3 3 4 5% 49%	2610
	21st to end	Night	13% 5% 2 5% 5% 4 4 3 6% 7% 47%	2800
December	1st to 10th	Night	11% 5% 2 5% 4 4 4 3 4 6% 51%	4580
	11th to 20th	Night	21% 6% 2 5% 4 4 4 3 5% 6% 40%	5205
	21st to end	Night	17% 7% 2 6% 4 4 4 4 6% 8% 37%	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	35% 3 7% 28% 22% 5%	3615
	11th to 20th	Day	42% 4 8% 21% 18% 8%	4060
	21st to end	Day	42% 5% 11% 26% 14% 3	4466
February	1st to 10th	Day	44% 6% 13% 25% 11% 2	1800
	11th to 20th	Day	45% 4 10% 24% 14% 3	1831
	21st to end	Day	46% 4 11% 21% 14% 4	1650
March	1st to 10th	Day	40% 4 9% 22% 18% 7%	4791
	11th to 20th	Day	35% 5% 10% 26% 19% 6%	4980
	21st to end	Day	44% 4 9% 18% 19% 6%	5597
April	1st to 10th	Day	41% 7% 12% 19% 17% 5%	2400
	11th to 20th	Day	39% 5% 11% 21% 18% 5%	2374
	21st to end	Day	42% 5% 10% 21% 19% 3	2400
May	1st to 10th	Day	49% 6% 14% 18% 10% 2	5759
	11th to 20th	Day	49% 8% 11% 22% 10% 1	6266
	21st to end	Day	42% 6% 13% 22% 16% 2	6949
June	1st to 10th	Day	37% 8% 14% 24% 16% 2	2800
	11th to 20th	Day	39% 9% 11% 23% 14% 2	2800
	21st to end	Day	33% 8% 15% 24% 18% 2	2800
July	1st to 10th	Day	37% 8% 16% 20% 16% 4	6290
	11th to 20th	Day	43% 8% 18% 23% 8% 1	6206
	21st to end	Day	38% 6% 11% 21% 22% 1	6240
August	1st to 10th	Day	46% 7% 14% 24% 9%	4221
	11th to 20th	Day	39% 7% 12% 27% 15% 1	4020
	21st to end	Day	35% 5% 12% 28% 18% 2	4312
September	1st to 10th	Day	37% 7% 12% 21% 19% 4	2400
	11th to 20th	Day	35% 5% 10% 25% 21% 3	2400
	21st to end	Day	47% 6% 9% 18% 18% 2	2115
October	1st to 10th	Day	38% 4 7% 18% 28% 5%	3260
	11th to 20th	Day	27% 4 9% 26% 24% 10%	3220
	21st to end	Day	24% 3 10% 29% 29% 5%	3428
November	1st to 10th	Day	32% 5% 12% 25% 21% 4	1818
	11th to 20th	Day	35% 5% 11% 23% 23% 3	1788
	21st to end	Day	39% 5% 11% 28% 16% 2	1600
December	1st to 10th	Day	37% 5% 15% 27% 12% 4	2600
	11th to 20th	Day	45% 3 12% 28% 11% 1	2986
	21st to end	Day	44% 5% 16% 19% 15% 1	3960
January	1st to 10th	Night	37% 4 8% 25% 23% 4	6331
	11th to 20th	Night	36% 4 10% 25% 20% 7%	5900
	21st to end	Night	40% 4 8% 28% 19% 2	6490
February	1st to 10th	Night	43% 5% 13% 24% 13% 1	2600
	11th to 20th	Night	43% 3 9% 22% 17% 4	2465
	21st to end	Night	44% 4 9% 21% 16% 5%	1980
March	1st to 10th	Night	39% 4 7% 26% 13% 10%	5057
	11th to 20th	Night	33% 5% 12% 29% 16% 6%	4980
	21st to end	Night	40% 4 9% 19% 23% 4	5334
April	1st to 10th	Night	43% 6% 10% 19% 19% 4	2000
	11th to 20th	Night	39% 5% 12% 19% 19% 5%	1975
	21st to end	Night	38% 6% 11% 22% 18% 5%	2000
May	1st to 10th	Night	54% 5% 11% 16% 10% 3	4227
	11th to 20th	Night	50% 7% 10% 21% 11% 1	3631
	21st to end	Night	44% 7% 14% 22% 11% 2	3927
June	1st to 10th	Night	35% 7% 11% 26% 18% 2	1600
	11th to 20th	Night	38% 9% 13% 23% 14% 2	1600
	21st to end	Night	36% 7% 13% 24% 18% 2	1600
July	1st to 10th	Night	42% 8% 17% 17% 14% 3	3560
	11th to 20th	Night	47% 7% 18% 18% 8% 1	3613
	21st to end	Night	37% 5% 13% 23% 21% 1	4297
August	1st to 10th	Night	45% 7% 17% 21% 9% 1	2904
	11th to 20th	Night	44% 6% 10% 25% 13% 1	3061
	21st to end	Night	34% 5% 11% 31% 16% 2	3586
September	1st to 10th	Night	37% 6% 11% 21% 20% 5%	2000
	11th to 20th	Night	35% 4 9% 26% 23% 3	2000
	21st to end	Night	47% 6% 9% 17% 18% 3	2281
October	1st to 10th	Night	42% 4 10% 20% 20% 5%	3920
	11th to 20th	Night	25% 3 10% 24% 30% 7%	3876
	21st to end	Night	24% 3 11% 31% 25% 6%	4120
November	1st to 10th	Night	33% 4 10% 25% 22% 5%	2477
	11th to 20th	Night	37% 5% 8% 23% 20% 5%	2610
	21st to end	Night	36% 6% 11% 24% 20% 3	2800
December	1st to 10th	Night	35% 4 14% 30% 16% 2	4580
	11th to 20th	Night	46% 3 12% 18% 17% 4	5208
	21st to end	Night	43% 5% 14% 20% 17% 1	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	31% 23% 31% 13% 2 1	203
	11th to 20th	Day	31% 24% 31% 12% 2	300
	21st to end	Day	24% 26% 33% 14% 3 1	330
February	1st to 10th	Day	27% 21% 39% 10% 3	300
	11th to 20th	Day	36% 21% 35% 7% 1	369
	21st to end	Day	30% 20% 32% 16% 2	414
March	1st to 10th	Day	23% 24% 31% 18% 3	500
	11th to 20th	Day	27% 21% 32% 15% 4 1	500
	21st to end	Day	24% 20% 34% 17% 5% 1	550
April	1st to 10th	Day	24% 21% 35% 14% 4 2	500
	11th to 20th	Day	21% 18% 32% 17% 10% 3	492
	21st to end	Day	25% 14% 34% 18% 7% 1	500
May	1st to 10th	Day	26% 16% 30% 18% 7% 3	503
	11th to 20th	Day	23% 12% 29% 23% 10% 3	527
	21st to end	Day	19% 7% 24% 26% 20% 4	573
June	1st to 10th	Day	15% 10% 30% 22% 18% 6%	530
	11th to 20th	Day	16% 8% 27% 28% 17% 4	530
	21st to end	Day	11% 6% 28% 32% 17% 6%	530
July	1st to 10th	Day	11% 3 22% 32% 22% 10%	525
	11th to 20th	Day	6% 3 21% 34% 28% 8%	524
	21st to end	Day	12% 3 23% 31% 25% 6%	576
August	1st to 10th	Day	14% 3 21% 31% 22% 9%	526
	11th to 20th	Day	14% 4 20% 31% 21% 9%	519
	21st to end	Day	10% 4 24% 31% 21% 10%	583
September	1st to 10th	Day	15% 7% 20% 27% 20% 11%	530
	11th to 20th	Day	14% 3 22% 31% 20% 10%	530
	21st to end	Day	26% 9% 22% 27% 12% 4	530
October	1st to 10th	Day	21% 8% 23% 24% 17% 7%	530
	11th to 20th	Day	17% 16% 27% 25% 12% 3	522
	21st to end	Day	15% 14% 35% 26% 9% 2	539
November	1st to 10th	Day	16% 14% 33% 25% 11% 2	425
	11th to 20th	Day	18% 15% 36% 20% 10% 1	388
	21st to end	Day	25% 18% 33% 19% 6%	200
December	1st to 10th	Day	24% 20% 39% 14% 3 2	200
	11th to 20th	Day	28% 18% 31% 18% 6%	200
	21st to end	Day	25% 16% 35% 18% 5%	220
January	1st to 10th	Night	21% 19% 39% 19% 3	800
	11th to 20th	Night	23% 18% 35% 21% 4	800
	21st to end	Night	20% 19% 40% 20% 1	880
February	1st to 10th	Night	21% 16% 37% 24% 2	800
	11th to 20th	Night	28% 21% 36% 15% 1	783
	21st to end	Night	25% 17% 38% 17% 3	660
March	1st to 10th	Night	20% 17% 39% 21% 3	800
	11th to 20th	Night	22% 17% 36% 20% 5% 1	800
	21st to end	Night	20% 15% 41% 18% 4 1	880
April	1st to 10th	Night	21% 18% 38% 19% 4	800
	11th to 20th	Night	16% 15% 35% 27% 6% 1	791
	21st to end	Night	21% 11% 34% 26% 7%	800
May	1st to 10th	Night	22% 8% 31% 30% 9% 1	676
	11th to 20th	Night	22% 8% 27% 32% 10%	423
	21st to end	Night	17% 5% 22% 35% 19% 3	433
June	1st to 10th	Night	13% 2 28% 37% 20%	400
	11th to 20th	Night	15% 3 23% 40% 20% 1	400
	21st to end	Night	11% 2 25% 45% 16% 1	400
July	1st to 10th	Night	10% 3 18% 39% 26% 4	390
	11th to 20th	Night	8% 1 16% 38% 34% 2	410
	21st to end	Night	9% 2 15% 44% 26% 3	657
August	1st to 10th	Night	13% 2 12% 42% 28% 3	597
	11th to 20th	Night	14% 3 16% 39% 25% 2	688
	21st to end	Night	12% 3 17% 37% 27% 4	880
September	1st to 10th	Night	16% 3 20% 34% 25% 3	800
	11th to 20th	Night	13% 3 16% 37% 26% 4	800
	21st to end	Night	23% 4 21% 34% 16% 2	800
October	1st to 10th	Night	21% 6% 22% 28% 19% 4	800
	11th to 20th	Night	19% 8% 29% 32% 12% 1	793
	21st to end	Night	20% 10% 31% 28% 12%	836
November	1st to 10th	Night	15% 8% 33% 32% 10% 2	780
	11th to 20th	Night	18% 17% 31% 28% 6% 1	800
	21st to end	Night	18% 16% 33% 26% 7% 1	800
December	1st to 10th	Night	23% 12% 38% 20% 7% 1	800
	11th to 20th	Night	19% 12% 41% 23% 5%	800
	21st to end	Night	20% 14% 36% 28% 2	880

Page generated at 2025-10-16 01:50:28. Version ac07

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