Cary
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cary clear sky chart from 2005-11-06 to 2025-10-04.

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	13% 6% 2 6% 4 4 5% 4 6% 8% 42%	125697
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 44%	123673
Transparency	Day	39% 5% 12% 21% 18% 5%	129302
	Night	39% 4 10% 20% 21% 6%	128137
Seeing	Day	16% 10% 24% 26% 18% 7%	16569
	Night	16% 8% 26% 32% 17% 2	26877

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	15% 6% 1 6% 3 2 2 2 2 4 56%	4056
	11th to 20th	Day	12% 6% 2 4 2 2 3 3 3 4 57%	4060
	21st to end	Day	13% 7% 3 7% 4 3 4 3 4 5% 46%	4466
February	1st to 10th	Day	14% 6% 2 5% 4 5% 4 4 5% 6% 44%	1800
	11th to 20th	Day	23% 6% 2 4 3 3 3 3 4 4 45%	1773
	21st to end	Day	18% 7% 2 6% 3 4 4 2 5% 5% 43%	1795
March	1st to 10th	Day	20% 6% 2 6% 4 4 4 4 4 6% 40%	4935
	11th to 20th	Day	11% 5% 2 5% 4 4 4 3 4 6% 53%	4980
	21st to end	Day	15% 5% 2 6% 4 4 5% 3 5% 5% 47%	5465
April	1st to 10th	Day	11% 4 2 5% 3 3 4 3 5% 7% 53%	2329
	11th to 20th	Day	10% 5% 2 5% 4 5% 5% 3 5% 7% 48%	2374
	21st to end	Day	14% 5% 2 5% 4 4 5% 4 5% 7% 45%	2400
May	1st to 10th	Day	9% 6% 3 8% 6% 5% 6% 5% 7% 10% 36%	5786
	11th to 20th	Day	19% 7% 3 8% 5% 5% 5% 4 7% 9% 28%	5861
	21st to end	Day	12% 5% 2 5% 4 4 4 5% 6% 9% 42%	6433
June	1st to 10th	Day	9% 6% 2 6% 5% 5% 6% 5% 7% 10% 39%	2600
	11th to 20th	Day	9% 5% 2 5% 5% 5% 5% 5% 7% 10% 42%	2600
	21st to end	Day	8% 4 2 6% 5% 6% 6% 5% 7% 12% 39%	2600
July	1st to 10th	Day	9% 5% 2 7% 6% 6% 7% 6% 9% 11% 32%	5900
	11th to 20th	Day	8% 6% 3 8% 6% 7% 6% 6% 9% 13% 27%	5900
	21st to end	Day	8% 6% 3 7% 6% 6% 7% 6% 8% 11% 33%	6424
August	1st to 10th	Day	19% 9% 3 8% 6% 5% 6% 5% 7% 9% 21%	4234
	11th to 20th	Day	9% 5% 2 8% 6% 6% 6% 5% 7% 10% 35%	4024
	21st to end	Day	6% 5% 2 6% 4 4 4 4 6% 10% 49%	4312
September	1st to 10th	Day	8% 4 2 5% 5% 5% 4 5% 7% 8% 47%	2400
	11th to 20th	Day	15% 6% 2 6% 4 5% 4 4 5% 8% 41%	2273
	21st to end	Day	20% 6% 2 7% 4 4 3 3 5% 7% 37%	2150
October	1st to 10th	Day	14% 6% 2 6% 3 3 3 3 4 6% 49%	1930
	11th to 20th	Day	12% 4 1 4 3 2 3 3 4 5% 58%	1900
	21st to end	Day	11% 4 2 6% 3 3 3 2 3 5% 57%	2090
November	1st to 10th	Day	13% 5% 2 5% 3 3 3 3 4 5% 54%	1957
	11th to 20th	Day	14% 5% 2 4 3 4 3 2 4 7% 52%	2000
	21st to end	Day	13% 5% 2 5% 3 4 4 3 5% 6% 51%	1669
December	1st to 10th	Day	12% 5% 2 6% 4 3 4 3 4 5% 53%	2600
	11th to 20th	Day	16% 6% 2 6% 4 3 4 3 5% 5% 46%	3155
	21st to end	Day	15% 6% 2 6% 4 3 4 3 5% 7% 45%	4466
January	1st to 10th	Night	12% 6% 2 5% 3 3 3 2 4 5% 55%	5889
	11th to 20th	Night	13% 6% 2 5% 3 2 3 2 4 5% 55%	5900
	21st to end	Night	14% 8% 2 6% 3 4 3 3 4 6% 47%	6490
February	1st to 10th	Night	17% 6% 2 5% 4 4 4 3 5% 6% 44%	2600
	11th to 20th	Night	21% 6% 2 6% 3 4 4 3 4 5% 42%	2513
	21st to end	Night	16% 5% 1 4 4 3 4 3 5% 7% 47%	1831
March	1st to 10th	Night	18% 7% 2 7% 4 4 4 3 4 4 42%	4918
	11th to 20th	Night	12% 6% 2 4 3 3 4 3 4 7% 53%	4980
	21st to end	Night	14% 5% 2 5% 4 4 4 3 5% 6% 50%	5467
April	1st to 10th	Night	14% 5% 2 4 3 4 4 3 5% 7% 48%	2070
	11th to 20th	Night	12% 5% 2 6% 4 4 3 3 5% 7% 50%	1975
	21st to end	Night	11% 4 2 5% 5% 5% 4 5% 5% 7% 48%	2000
May	1st to 10th	Night	12% 7% 3 7% 5% 5% 5% 4 6% 8% 38%	4137
	11th to 20th	Night	20% 8% 3 7% 5% 4 4 3 5% 8% 33%	4032
	21st to end	Night	15% 5% 2 6% 4 4 4 3 6% 9% 43%	4421
June	1st to 10th	Night	10% 6% 3 6% 5% 4 6% 6% 8% 9% 39%	1800
	11th to 20th	Night	10% 7% 2 6% 5% 4 5% 4 7% 10% 41%	1800
	21st to end	Night	7% 6% 2 7% 5% 5% 5% 5% 8% 13% 38%	1800
July	1st to 10th	Night	11% 7% 3 7% 6% 5% 5% 6% 8% 11% 30%	4058
	11th to 20th	Night	13% 7% 3 8% 6% 6% 6% 5% 9% 12% 26%	4057
	21st to end	Night	11% 8% 3 8% 5% 5% 6% 5% 7% 10% 31%	4427
August	1st to 10th	Night	22% 7% 3 7% 6% 6% 5% 5% 7% 9% 23%	2915
	11th to 20th	Night	9% 5% 2 7% 5% 6% 6% 5% 8% 11% 34%	2992
	21st to end	Night	6% 5% 2 6% 4 4 5% 4 7% 11% 47%	3586
September	1st to 10th	Night	11% 6% 2 5% 3 4 4 4 6% 9% 47%	2000
	11th to 20th	Night	13% 6% 3 7% 4 4 4 3 5% 7% 43%	2122
	21st to end	Night	19% 7% 3 7% 5% 4 4 3 6% 6% 37%	2248
October	1st to 10th	Night	13% 6% 2 6% 4 3 3 2 4 6% 49%	2317
	11th to 20th	Night	13% 6% 1 5% 3 3 3 2 4 6% 54%	2280
	21st to end	Night	11% 5% 2 5% 3 3 3 3 5% 5% 56%	2508
November	1st to 10th	Night	13% 6% 2 5% 3 3 3 3 4 5% 52%	2341
	11th to 20th	Night	13% 6% 2 5% 4 3 3 3 5% 5% 50%	2400
	21st to end	Night	13% 5% 2 4 3 4 3 3 5% 6% 52%	2720
December	1st to 10th	Night	12% 6% 2 5% 4 3 3 3 4 4 52%	4580
	11th to 20th	Night	16% 9% 3 8% 4 4 3 3 4 5% 40%	5009
	21st to end	Night	15% 6% 2 6% 3 3 3 3 4 7% 48%	6490

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% 2 3 20% 33% 7%	4057
	11th to 20th	Day	32% 2 3 16% 38% 8%	4060
	21st to end	Day	42% 3 5% 21% 22% 8%	4466
February	1st to 10th	Day	40% 3 11% 22% 20% 5%	1800
	11th to 20th	Day	43% 3 7% 19% 22% 6%	1782
	21st to end	Day	44% 3 9% 17% 21% 6%	1795
March	1st to 10th	Day	46% 2 6% 17% 18% 11%	4935
	11th to 20th	Day	33% 3 7% 16% 25% 16%	4980
	21st to end	Day	40% 4 8% 22% 21% 6%	5465
April	1st to 10th	Day	30% 5% 9% 24% 24% 8%	2329
	11th to 20th	Day	35% 4 10% 23% 21% 7%	2374
	21st to end	Day	39% 4 12% 22% 18% 4	2400
May	1st to 10th	Day	42% 5% 18% 22% 11% 1	5820
	11th to 20th	Day	52% 5% 12% 20% 10% 1	5865
	21st to end	Day	38% 4 12% 23% 19% 3	6447
June	1st to 10th	Day	38% 7% 15% 22% 16% 2	2600
	11th to 20th	Day	35% 9% 15% 24% 15% 2	2600
	21st to end	Day	35% 9% 16% 23% 16% 1	2600
July	1st to 10th	Day	41% 10% 17% 20% 10% 2	5835
	11th to 20th	Day	44% 12% 27% 13% 3 1	5831
	21st to end	Day	42% 9% 18% 22% 10%	6240
August	1st to 10th	Day	56% 7% 17% 14% 5%	4221
	11th to 20th	Day	41% 8% 19% 23% 8% 1	4063
	21st to end	Day	30% 5% 13% 33% 16% 2	4312
September	1st to 10th	Day	34% 7% 15% 21% 20% 5%	2400
	11th to 20th	Day	40% 5% 11% 19% 21% 3	2273
	21st to end	Day	46% 5% 9% 21% 16% 4	2150
October	1st to 10th	Day	33% 4 9% 21% 25% 9%	3190
	11th to 20th	Day	20% 2 5% 21% 27% 25%	3140
	21st to end	Day	25% 2 6% 26% 24% 17%	3428
November	1st to 10th	Day	34% 3 7% 23% 27% 7%	1957
	11th to 20th	Day	34% 3 8% 21% 27% 7%	2000
	21st to end	Day	34% 3 8% 23% 25% 6%	1669
December	1st to 10th	Day	35% 3 7% 22% 26% 7%	2600
	11th to 20th	Day	41% 3 10% 19% 21% 6%	3152
	21st to end	Day	40% 3 10% 25% 20% 2	4466
January	1st to 10th	Night	34% 2 4 17% 33% 10%	5889
	11th to 20th	Night	34% 2 5% 16% 35% 8%	5900
	21st to end	Night	40% 2 5% 18% 29% 6%	6490
February	1st to 10th	Night	41% 4 8% 22% 20% 5%	2600
	11th to 20th	Night	45% 3 7% 16% 21% 7%	2515
	21st to end	Night	37% 3 8% 21% 23% 9%	1831
March	1st to 10th	Night	47% 2 5% 13% 22% 10%	4918
	11th to 20th	Night	32% 3 6% 15% 25% 19%	4980
	21st to end	Night	36% 3 8% 24% 23% 6%	5467
April	1st to 10th	Night	36% 4 9% 18% 24% 8%	2070
	11th to 20th	Night	34% 4 9% 24% 21% 7%	1975
	21st to end	Night	35% 6% 11% 26% 18% 4	2000
May	1st to 10th	Night	44% 5% 17% 19% 13% 2	4165
	11th to 20th	Night	50% 5% 11% 19% 13% 1	4033
	21st to end	Night	38% 5% 14% 23% 18% 2	4429
June	1st to 10th	Night	39% 7% 15% 21% 17% 1	1800
	11th to 20th	Night	38% 7% 17% 20% 16% 2	1800
	21st to end	Night	36% 9% 17% 22% 13% 3	1800
July	1st to 10th	Night	44% 10% 18% 17% 9% 1	4015
	11th to 20th	Night	48% 14% 21% 12% 4 1	4006
	21st to end	Night	47% 8% 16% 20% 9%	4297
August	1st to 10th	Night	55% 8% 17% 15% 4 1	2904
	11th to 20th	Night	41% 9% 19% 24% 6% 1	3019
	21st to end	Night	31% 6% 11% 29% 21% 2	3586
September	1st to 10th	Night	35% 7% 13% 23% 17% 5%	2000
	11th to 20th	Night	40% 5% 11% 19% 21% 3	2122
	21st to end	Night	48% 4 12% 16% 16% 4	2248
October	1st to 10th	Night	38% 3 7% 21% 25% 6%	3837
	11th to 20th	Night	25% 2 6% 18% 28% 21%	3779
	21st to end	Night	23% 2 7% 28% 24% 17%	4120
November	1st to 10th	Night	35% 3 7% 20% 27% 8%	2341
	11th to 20th	Night	36% 3 7% 17% 26% 11%	2400
	21st to end	Night	34% 3 7% 23% 26% 7%	2720
December	1st to 10th	Night	37% 2 6% 19% 28% 8%	4580
	11th to 20th	Night	48% 3 8% 17% 17% 8%	5011
	21st to end	Night	38% 2 11% 25% 21% 2	6490

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	20% 26% 36% 15% 3	300
	11th to 20th	Day	24% 25% 31% 16% 4	300
	21st to end	Day	18% 21% 42% 16% 3	330
February	1st to 10th	Day	19% 16% 41% 19% 4	300
	11th to 20th	Day	38% 16% 33% 12% 2	320
	21st to end	Day	25% 16% 33% 21% 4	414
March	1st to 10th	Day	22% 16% 35% 21% 7%	500
	11th to 20th	Day	21% 18% 38% 17% 6% 1	500
	21st to end	Day	21% 17% 35% 21% 5% 1	550
April	1st to 10th	Day	17% 18% 34% 23% 8% 1	500
	11th to 20th	Day	14% 16% 30% 24% 13% 3	492
	21st to end	Day	17% 13% 26% 27% 16% 2	500
May	1st to 10th	Day	15% 14% 32% 23% 12% 5%	503
	11th to 20th	Day	16% 8% 23% 30% 19% 5%	527
	21st to end	Day	20% 6% 19% 26% 20% 9%	573
June	1st to 10th	Day	12% 6% 22% 31% 22% 8%	530
	11th to 20th	Day	9% 5% 19% 29% 26% 12%	530
	21st to end	Day	9% 5% 13% 33% 30% 11%	530
July	1st to 10th	Day	10% 2 10% 30% 34% 15%	525
	11th to 20th	Day	6% 1 8% 28% 39% 17%	524
	21st to end	Day	8% 2 11% 34% 32% 13%	576
August	1st to 10th	Day	15% 3 14% 29% 26% 11%	526
	11th to 20th	Day	12% 2 12% 26% 31% 16%	519
	21st to end	Day	11% 3 11% 29% 27% 19%	583
September	1st to 10th	Day	11% 3 16% 28% 28% 15%	530
	11th to 20th	Day	16% 4 12% 27% 29% 11%	530
	21st to end	Day	23% 5% 18% 30% 18% 5%	530
October	1st to 10th	Day	18% 5% 20% 24% 23% 11%	502
	11th to 20th	Day	12% 9% 24% 31% 20% 5%	490
	21st to end	Day	12% 9% 24% 32% 18% 6%	539
November	1st to 10th	Day	18% 8% 28% 28% 15% 3	495
	11th to 20th	Day	17% 11% 30% 29% 11% 2	500
	21st to end	Day	14% 17% 32% 22% 13% 2	234
December	1st to 10th	Day	20% 16% 40% 17% 8%	200
	11th to 20th	Day	18% 14% 36% 22% 10% 1	237
	21st to end	Day	22% 15% 32% 20% 9% 1	330
January	1st to 10th	Night	19% 15% 36% 26% 5%	800
	11th to 20th	Night	18% 13% 35% 27% 6%	800
	21st to end	Night	19% 9% 41% 28% 4	880
February	1st to 10th	Night	20% 9% 40% 28% 3	800
	11th to 20th	Night	24% 15% 39% 21% 1	783
	21st to end	Night	19% 14% 37% 26% 4	660
March	1st to 10th	Night	17% 14% 34% 30% 5%	800
	11th to 20th	Night	21% 11% 36% 24% 8%	800
	21st to end	Night	18% 12% 40% 26% 4	880
April	1st to 10th	Night	17% 15% 35% 26% 6%	800
	11th to 20th	Night	15% 12% 35% 28% 11% 1	791
	21st to end	Night	13% 6% 36% 36% 9% 1	800
May	1st to 10th	Night	13% 9% 33% 36% 9%	653
	11th to 20th	Night	19% 5% 23% 35% 17% 1	591
	21st to end	Night	14% 2 14% 41% 26% 2	650
June	1st to 10th	Night	13% 4 17% 43% 23% 1	600
	11th to 20th	Night	13% 4 15% 37% 26% 5%	600
	21st to end	Night	8% 3 14% 41% 33% 2	600
July	1st to 10th	Night	9% 3 10% 36% 39% 3	585
	11th to 20th	Night	9% 2 6% 41% 36% 6%	582
	21st to end	Night	8% 2 10% 40% 35% 5%	657
August	1st to 10th	Night	14% 1 8% 40% 33% 4	597
	11th to 20th	Night	13% 2 9% 36% 35% 5%	659
	21st to end	Night	12% 3 10% 32% 38% 6%	880
September	1st to 10th	Night	13% 3 11% 33% 36% 6%	800
	11th to 20th	Night	15% 4 12% 34% 32% 4	800
	21st to end	Night	23% 4 15% 34% 22% 2	800
October	1st to 10th	Night	17% 3 17% 33% 26% 5%	773
	11th to 20th	Night	15% 8% 24% 32% 20% 2	760
	21st to end	Night	14% 7% 24% 32% 21% 2	836
November	1st to 10th	Night	15% 6% 25% 35% 15% 3	780
	11th to 20th	Night	15% 10% 29% 35% 11%	800
	21st to end	Night	14% 9% 30% 32% 14% 1	800
December	1st to 10th	Night	20% 8% 34% 30% 7%	800
	11th to 20th	Night	19% 7% 32% 34% 7%	800
	21st to end	Night	20% 11% 29% 31% 9%	880

Page generated at 2025-10-01 01:39:02. Version ac07

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