Raleigh
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Raleigh clear sky chart from 2005-11-06 to 2024-01-15.

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% 5% 2 5% 5% 3 5% 5% 6% 8% 43%	74481
	Night	14% 5% 2 5% 5% 3 4 5% 5% 7% 45%	75307
Transparency	Day	38% 7% 12% 21% 18% 4	74202
	Night	38% 7% 11% 20% 19% 5%	75105
Seeing	Day	16% 9% 23% 25% 19% 7%	14208
	Night	15% 7% 26% 33% 17% 2	24602

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	16% 6% 3 5% 5% 3 4 4 3 5% 47%	1706
	11th to 20th	Day	18% 6% 3 4 4 3 4 4 5% 5% 44%	1662
	21st to end	Day	17% 6% 2 5% 3 2 3 5% 4 5% 47%	1782
February	1st to 10th	Day	15% 5% 2 5% 4 4 5% 4 6% 6% 44%	1620
	11th to 20th	Day	21% 6% 2 4 4 2 3 3 4 5% 47%	1599
	21st to end	Day	19% 6% 4 4 4 3 4 4 5% 5% 42%	1599
March	1st to 10th	Day	15% 5% 1 4 3 2 3 5% 4 5% 51%	1980
	11th to 20th	Day	15% 6% 2 6% 5% 3 4 5% 5% 7% 43%	1980
	21st to end	Day	16% 7% 2 5% 5% 2 4 5% 5% 7% 40%	2178
April	1st to 10th	Day	11% 3 2 4 4 2 3 4 4 8% 54%	2102
	11th to 20th	Day	11% 4 2 5% 4 4 4 5% 5% 8% 48%	2134
	21st to end	Day	14% 5% 3 5% 5% 3 4 5% 6% 7% 45%	2160
May	1st to 10th	Day	11% 5% 3 5% 5% 3 4 6% 7% 9% 43%	2292
	11th to 20th	Day	13% 7% 3 6% 6% 4 4 6% 6% 7% 39%	2301
	21st to end	Day	11% 5% 3 5% 5% 4 5% 6% 6% 9% 40%	2531
June	1st to 10th	Day	10% 6% 2 5% 6% 4 6% 6% 7% 10% 39%	2340
	11th to 20th	Day	9% 4 2 5% 5% 3 6% 6% 7% 10% 41%	2340
	21st to end	Day	8% 4 2 6% 6% 4 6% 7% 7% 11% 38%	2340
July	1st to 10th	Day	8% 5% 2 6% 7% 5% 7% 7% 8% 12% 33%	2340
	11th to 20th	Day	8% 5% 2 6% 7% 4 6% 7% 9% 11% 33%	2340
	21st to end	Day	6% 5% 3 5% 8% 5% 8% 8% 8% 11% 34%	2547
August	1st to 10th	Day	9% 6% 3 8% 7% 4 7% 7% 9% 10% 31%	2327
	11th to 20th	Day	11% 6% 3 7% 7% 4 6% 7% 8% 9% 32%	2218
	21st to end	Day	10% 7% 3 7% 6% 4 6% 6% 7% 10% 36%	2376
September	1st to 10th	Day	9% 4 2 5% 5% 3 5% 6% 7% 9% 46%	2160
	11th to 20th	Day	14% 5% 2 5% 5% 3 5% 4 5% 8% 43%	2060
	21st to end	Day	20% 6% 3 5% 5% 2 4 4 5% 8% 38%	1930
October	1st to 10th	Day	15% 7% 2 5% 4 3 3 3 4 7% 49%	1800
	11th to 20th	Day	12% 4 2 4 3 3 3 4 4 6% 56%	1800
	21st to end	Day	12% 4 2 4 4 3 3 3 4 5% 56%	1980
November	1st to 10th	Day	14% 5% 2 4 4 3 3 4 3 5% 55%	1857
	11th to 20th	Day	14% 5% 2 3 3 3 3 3 4 7% 52%	1900
	21st to end	Day	13% 4 2 5% 3 1 4 4 5% 6% 51%	1575
December	1st to 10th	Day	17% 7% 2 6% 4 3 4 5% 4 7% 42%	1520
	11th to 20th	Day	16% 4 2 4 3 2 2 3 4 5% 56%	1981
	21st to end	Day	16% 6% 3 5% 4 3 4 4 5% 6% 43%	3124
January	1st to 10th	Night	16% 6% 2 5% 4 2 4 4 4 6% 47%	2459
	11th to 20th	Night	15% 7% 3 5% 5% 3 4 4 5% 6% 44%	2404
	21st to end	Night	16% 5% 2 4 4 2 4 4 4 6% 50%	2574
February	1st to 10th	Night	18% 6% 2 4 4 3 4 4 5% 6% 44%	2340
	11th to 20th	Night	20% 6% 2 5% 4 2 4 4 4 6% 43%	2246
	21st to end	Night	17% 6% 2 5% 4 3 3 4 6% 7% 45%	1654
March	1st to 10th	Night	13% 4 2 5% 5% 2 3 4 4 5% 52%	1980
	11th to 20th	Night	17% 6% 2 5% 5% 3 4 6% 5% 7% 41%	1980
	21st to end	Night	16% 6% 2 4 5% 3 4 4 5% 6% 46%	2178
April	1st to 10th	Night	15% 4 2 5% 4 2 3 4 5% 7% 49%	1856
	11th to 20th	Night	13% 5% 2 4 4 3 3 5% 5% 7% 49%	1775
	21st to end	Night	11% 3 2 4 6% 3 5% 6% 5% 7% 48%	1800
May	1st to 10th	Night	9% 7% 2 5% 5% 3 5% 5% 6% 9% 46%	1682
	11th to 20th	Night	14% 6% 2 6% 5% 3 5% 4 5% 9% 40%	1592
	21st to end	Night	10% 5% 2 4 6% 3 5% 6% 6% 9% 43%	1744
June	1st to 10th	Night	10% 5% 3 5% 5% 3 6% 6% 7% 9% 40%	1620
	11th to 20th	Night	11% 6% 2 5% 6% 3 5% 5% 8% 8% 40%	1620
	21st to end	Night	7% 5% 3 7% 4 4 5% 6% 10% 11% 37%	1620
July	1st to 10th	Night	6% 5% 2 7% 5% 4 6% 8% 9% 10% 37%	1620
	11th to 20th	Night	6% 5% 2 5% 6% 4 7% 7% 9% 11% 38%	1620
	21st to end	Night	7% 5% 2 6% 6% 5% 6% 7% 7% 12% 37%	1771
August	1st to 10th	Night	10% 5% 2 5% 7% 4 6% 7% 9% 10% 34%	1611
	11th to 20th	Night	11% 6% 3 6% 6% 4 6% 7% 7% 12% 33%	1664
	21st to end	Night	10% 6% 3 6% 6% 4 4 6% 6% 11% 38%	1980
September	1st to 10th	Night	11% 5% 2 5% 4 3 4 5% 6% 8% 46%	1800
	11th to 20th	Night	12% 5% 2 6% 5% 3 4 4 5% 7% 45%	1896
	21st to end	Night	19% 7% 3 6% 6% 4 4 4 5% 6% 38%	2029
October	1st to 10th	Night	14% 5% 2 5% 5% 2 3 3 5% 6% 49%	2160
	11th to 20th	Night	13% 5% 2 4 4 2 3 3 4 6% 53%	2160
	21st to end	Night	12% 4 2 4 4 2 3 3 5% 5% 55%	2376
November	1st to 10th	Night	14% 6% 2 4 4 2 3 4 4 5% 53%	2221
	11th to 20th	Night	13% 5% 2 5% 4 2 4 5% 4 6% 51%	2280
	21st to end	Night	13% 4 2 4 4 3 3 4 5% 7% 51%	2596
December	1st to 10th	Night	18% 6% 3 5% 6% 3 4 4 5% 4 42%	2660
	11th to 20th	Night	17% 6% 2 4 3 2 3 3 4 6% 48%	3207
	21st to end	Night	16% 6% 3 5% 4 2 3 3 4 7% 47%	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	42% 5% 7% 21% 20% 6%	1707
	11th to 20th	Day	42% 5% 7% 20% 22% 5%	1662
	21st to end	Day	39% 4 7% 21% 22% 7%	1782
February	1st to 10th	Day	40% 5% 12% 21% 18% 5%	1620
	11th to 20th	Day	41% 4 7% 19% 22% 6%	1608
	21st to end	Day	44% 5% 10% 15% 19% 6%	1599
March	1st to 10th	Day	35% 5% 8% 22% 23% 7%	1980
	11th to 20th	Day	40% 6% 9% 20% 17% 7%	1980
	21st to end	Day	44% 6% 10% 19% 18% 3	2178
April	1st to 10th	Day	28% 6% 9% 24% 24% 8%	2102
	11th to 20th	Day	34% 6% 10% 22% 20% 7%	2134
	21st to end	Day	38% 7% 11% 21% 19% 5%	2160
May	1st to 10th	Day	36% 8% 15% 20% 17% 3	2292
	11th to 20th	Day	43% 8% 13% 21% 13% 2	2305
	21st to end	Day	39% 8% 15% 23% 13% 1	2531
June	1st to 10th	Day	38% 10% 14% 21% 15% 1	2340
	11th to 20th	Day	34% 11% 16% 22% 14% 2	2340
	21st to end	Day	37% 11% 15% 20% 15% 1	2340
July	1st to 10th	Day	39% 12% 16% 19% 11% 3	2275
	11th to 20th	Day	39% 12% 20% 19% 8% 2	2271
	21st to end	Day	38% 13% 19% 21% 8% 1	2405
August	1st to 10th	Day	43% 11% 17% 19% 9% 1	2314
	11th to 20th	Day	44% 12% 15% 19% 8% 1	2218
	21st to end	Day	40% 10% 16% 20% 10% 3	2376
September	1st to 10th	Day	33% 10% 15% 18% 19% 5%	2160
	11th to 20th	Day	39% 7% 10% 19% 21% 4	2060
	21st to end	Day	45% 6% 10% 20% 16% 4	1930
October	1st to 10th	Day	37% 5% 11% 22% 22% 4	1800
	11th to 20th	Day	31% 4 8% 28% 22% 7%	1800
	21st to end	Day	32% 4 7% 22% 26% 9%	1980
November	1st to 10th	Day	33% 4 8% 21% 27% 7%	1857
	11th to 20th	Day	33% 4 8% 21% 27% 7%	1900
	21st to end	Day	34% 4 8% 24% 24% 7%	1575
December	1st to 10th	Day	43% 4 11% 20% 17% 4	1520
	11th to 20th	Day	32% 2 8% 25% 24% 9%	1977
	21st to end	Day	41% 5% 10% 21% 21% 2	3124
January	1st to 10th	Night	39% 5% 8% 19% 24% 6%	2459
	11th to 20th	Night	42% 5% 9% 18% 20% 5%	2404
	21st to end	Night	37% 3 5% 22% 26% 6%	2574
February	1st to 10th	Night	40% 5% 9% 20% 20% 6%	2340
	11th to 20th	Night	44% 4 8% 16% 21% 7%	2248
	21st to end	Night	38% 5% 9% 19% 21% 8%	1654
March	1st to 10th	Night	35% 4 8% 20% 23% 9%	1980
	11th to 20th	Night	42% 6% 9% 18% 18% 6%	1980
	21st to end	Night	39% 6% 10% 21% 19% 5%	2178
April	1st to 10th	Night	36% 6% 9% 16% 25% 8%	1856
	11th to 20th	Night	35% 5% 10% 21% 21% 8%	1775
	21st to end	Night	35% 7% 11% 25% 18% 4	1800
May	1st to 10th	Night	34% 8% 15% 21% 18% 4	1682
	11th to 20th	Night	41% 8% 13% 21% 16% 1	1593
	21st to end	Night	35% 11% 16% 23% 14% 1	1745
June	1st to 10th	Night	38% 11% 15% 20% 15% 1	1620
	11th to 20th	Night	38% 9% 17% 18% 16% 2	1620
	21st to end	Night	36% 13% 17% 19% 13% 3	1620
July	1st to 10th	Night	36% 14% 18% 18% 12% 2	1575
	11th to 20th	Night	34% 15% 19% 20% 8% 3	1566
	21st to end	Night	37% 12% 19% 22% 10%	1672
August	1st to 10th	Night	39% 14% 18% 21% 7% 1	1600
	11th to 20th	Night	41% 12% 18% 19% 8% 1	1664
	21st to end	Night	38% 11% 17% 22% 9% 2	1980
September	1st to 10th	Night	35% 10% 13% 21% 16% 5%	1800
	11th to 20th	Night	38% 6% 12% 19% 21% 3	1896
	21st to end	Night	47% 7% 12% 15% 16% 4	2029
October	1st to 10th	Night	36% 5% 10% 21% 23% 5%	2160
	11th to 20th	Night	33% 5% 9% 22% 24% 7%	2160
	21st to end	Night	31% 4 8% 21% 26% 10%	2376
November	1st to 10th	Night	34% 4 9% 17% 28% 8%	2221
	11th to 20th	Night	35% 5% 8% 16% 25% 11%	2280
	21st to end	Night	34% 5% 7% 22% 26% 7%	2596
December	1st to 10th	Night	45% 6% 8% 19% 18% 4	2660
	11th to 20th	Night	38% 4 7% 20% 22% 8%	3210
	21st to end	Night	39% 4 7% 25% 23% 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	20% 20% 38% 17% 4	270
	11th to 20th	Day	21% 26% 32% 16% 6%	254
	21st to end	Day	19% 22% 39% 17% 3	264
February	1st to 10th	Day	20% 17% 39% 19% 5%	240
	11th to 20th	Day	36% 12% 38% 12% 2	266
	21st to end	Day	24% 18% 32% 22% 4	346
March	1st to 10th	Day	20% 16% 34% 24% 6%	420
	11th to 20th	Day	22% 17% 38% 17% 6% 1	420
	21st to end	Day	21% 14% 35% 22% 7% 2	462
April	1st to 10th	Day	17% 17% 32% 24% 10% 1	420
	11th to 20th	Day	13% 17% 27% 24% 16% 3	412
	21st to end	Day	19% 14% 24% 24% 18% 2	420
May	1st to 10th	Day	14% 15% 30% 23% 12% 5%	423
	11th to 20th	Day	16% 6% 20% 31% 20% 6%	447
	21st to end	Day	19% 5% 16% 28% 21% 12%	485
June	1st to 10th	Day	12% 5% 19% 31% 24% 8%	450
	11th to 20th	Day	10% 3 23% 23% 27% 13%	450
	21st to end	Day	9% 4 16% 31% 30% 10%	450
July	1st to 10th	Day	8% 3 9% 29% 35% 17%	445
	11th to 20th	Day	6% 2 9% 25% 39% 19%	444
	21st to end	Day	9% 1 12% 32% 33% 12%	488
August	1st to 10th	Day	13% 3 13% 30% 28% 13%	446
	11th to 20th	Day	11% 3 13% 25% 30% 19%	439
	21st to end	Day	12% 3 9% 28% 26% 22%	495
September	1st to 10th	Day	10% 2 15% 25% 30% 18%	450
	11th to 20th	Day	14% 3 12% 29% 28% 13%	450
	21st to end	Day	23% 6% 18% 28% 19% 6%	450
October	1st to 10th	Day	17% 6% 23% 21% 24% 10%	450
	11th to 20th	Day	12% 7% 24% 30% 21% 6%	450
	21st to end	Day	14% 9% 25% 32% 16% 4	495
November	1st to 10th	Day	18% 9% 26% 28% 15% 4	455
	11th to 20th	Day	16% 11% 30% 30% 10% 3	460
	21st to end	Day	15% 15% 27% 25% 16% 2	213
December	1st to 10th	Day	20% 15% 36% 22% 6% 1	180
	11th to 20th	Day	18% 12% 36% 22% 10%	202
	21st to end	Day	24% 12% 33% 21% 9% 1	297
January	1st to 10th	Night	20% 13% 36% 26% 5% 1	760
	11th to 20th	Night	18% 12% 35% 29% 6%	740
	21st to end	Night	19% 9% 41% 28% 4	792
February	1st to 10th	Night	20% 9% 41% 27% 4	720
	11th to 20th	Night	23% 14% 40% 22% 1	703
	21st to end	Night	19% 14% 37% 27% 3	592
March	1st to 10th	Night	15% 13% 35% 32% 4	720
	11th to 20th	Night	20% 10% 36% 25% 8% 1	720
	21st to end	Night	18% 11% 37% 29% 5% 1	792
April	1st to 10th	Night	17% 14% 33% 28% 7%	720
	11th to 20th	Night	15% 11% 32% 29% 13%	711
	21st to end	Night	14% 5% 36% 35% 9%	720
May	1st to 10th	Night	10% 11% 33% 37% 9%	602
	11th to 20th	Night	17% 4 23% 38% 17% 1	531
	21st to end	Night	12% 2 13% 43% 28% 2	584
June	1st to 10th	Night	11% 3 17% 44% 23% 1	540
	11th to 20th	Night	13% 3 16% 37% 27% 4	540
	21st to end	Night	7% 3 14% 42% 32% 3	540
July	1st to 10th	Night	8% 2 10% 37% 40% 3	525
	11th to 20th	Night	8% 2 8% 39% 38% 6%	522
	21st to end	Night	7% 1 13% 40% 36% 4	591
August	1st to 10th	Night	11% 1 8% 43% 33% 4	537
	11th to 20th	Night	12% 3 11% 35% 35% 5%	600
	21st to end	Night	11% 2 11% 31% 39% 6%	792
September	1st to 10th	Night	13% 1 9% 32% 38% 6%	720
	11th to 20th	Night	14% 2 13% 34% 33% 3	720
	21st to end	Night	22% 4 16% 32% 24% 2	720
October	1st to 10th	Night	16% 4 18% 31% 26% 5%	720
	11th to 20th	Night	14% 8% 25% 29% 22% 2	720
	21st to end	Night	14% 6% 27% 32% 19% 2	792
November	1st to 10th	Night	16% 6% 25% 34% 15% 3	740
	11th to 20th	Night	15% 9% 29% 35% 12%	760
	21st to end	Night	14% 7% 31% 32% 15% 1	760
December	1st to 10th	Night	19% 10% 32% 33% 6%	760
	11th to 20th	Night	20% 7% 32% 35% 8%	760
	21st to end	Night	20% 11% 29% 31% 9%	836

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