Pisgah Astronomical Research Institute
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Pisgah Astronomical Research Institute clear sky chart from 2005-11-06 to 2024-11-04.

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	14% 6% 2 6% 5% 5% 5% 4 6% 7% 39%	101091
	Night	15% 6% 2 5% 4 4 4 4 5% 7% 43%	97878
Transparency	Day	40% 9% 11% 18% 16% 6%	104859
	Night	38% 8% 11% 18% 17% 8%	102249
Seeing	Day	20% 14% 23% 22% 14% 7%	15023
	Night	19% 12% 20% 26% 19% 4	25059

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% 5% 2 3 2 3 3 2 6% 7% 51%	2836
	11th to 20th	Day	13% 6% 2 7% 4 3 3 2 3 4 52%	2840
	21st to end	Day	24% 5% 1 4 3 2 3 2 4 5% 48%	3124
February	1st to 10th	Day	15% 6% 3 6% 4 4 4 3 6% 6% 44%	1828
	11th to 20th	Day	17% 8% 2 5% 4 4 4 3 4 5% 43%	1848
	21st to end	Day	17% 7% 1 6% 5% 5% 3 4 5% 5% 40%	1570
March	1st to 10th	Day	24% 6% 2 5% 3 3 3 3 4 6% 42%	3336
	11th to 20th	Day	14% 6% 2 4 4 3 3 2 4 5% 53%	3480
	21st to end	Day	19% 7% 2 6% 4 5% 3 3 5% 5% 41%	3815
April	1st to 10th	Day	12% 6% 2 6% 4 3 5% 5% 7% 8% 43%	2090
	11th to 20th	Day	15% 6% 2 6% 4 3 5% 4 6% 7% 42%	2267
	21st to end	Day	13% 5% 2 6% 4 4 5% 4 5% 7% 47%	2470
May	1st to 10th	Day	12% 8% 3 8% 5% 4 6% 4 6% 7% 37%	4082
	11th to 20th	Day	18% 7% 3 8% 4 5% 5% 4 6% 7% 33%	4081
	21st to end	Day	9% 6% 3 6% 4 5% 6% 4 8% 9% 39%	4476
June	1st to 10th	Day	12% 6% 3 7% 6% 6% 7% 5% 9% 10% 29%	2470
	11th to 20th	Day	7% 6% 2 6% 7% 7% 7% 7% 9% 10% 32%	2470
	21st to end	Day	8% 7% 2 7% 8% 7% 6% 6% 8% 10% 32%	2470
July	1st to 10th	Day	9% 6% 2 8% 7% 7% 7% 6% 8% 11% 30%	4120
	11th to 20th	Day	11% 6% 3 9% 7% 8% 7% 6% 7% 9% 27%	4120
	21st to end	Day	12% 8% 4 12% 9% 8% 8% 7% 8% 8% 16%	4466
August	1st to 10th	Day	12% 7% 3 9% 7% 7% 8% 7% 8% 10% 24%	4104
	11th to 20th	Day	11% 5% 2 8% 6% 6% 6% 5% 8% 11% 31%	4044
	21st to end	Day	8% 4 2 7% 5% 6% 6% 5% 6% 8% 42%	4150
September	1st to 10th	Day	12% 7% 3 8% 5% 5% 6% 5% 7% 8% 33%	2090
	11th to 20th	Day	17% 8% 3 7% 5% 5% 6% 5% 6% 7% 31%	2090
	21st to end	Day	18% 6% 2 6% 5% 5% 4 3 6% 7% 38%	2090
October	1st to 10th	Day	15% 5% 2 4 3 4 3 3 5% 6% 49%	2079
	11th to 20th	Day	12% 5% 2 3 2 3 3 3 5% 5% 57%	1900
	21st to end	Day	15% 4 1 4 3 2 3 3 4 5% 56%	2090
November	1st to 10th	Day	11% 7% 2 4 2 2 3 2 4 6% 56%	1788
	11th to 20th	Day	13% 6% 1 4 4 3 3 2 4 6% 54%	1710
	21st to end	Day	15% 5% 2 4 3 3 3 3 5% 6% 50%	1710
December	1st to 10th	Day	18% 8% 2 7% 4 4 4 4 5% 5% 38%	1710
	11th to 20th	Day	15% 6% 2 4 3 3 3 3 5% 6% 50%	2153
	21st to end	Day	20% 4 1 4 2 3 4 3 5% 6% 46%	3124
January	1st to 10th	Night	17% 5% 1 4 3 3 3 3 4 5% 52%	4109
	11th to 20th	Night	10% 5% 2 6% 4 4 4 3 5% 6% 50%	4120
	21st to end	Night	24% 6% 1 4 3 3 3 3 4 5% 45%	4532
February	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 6% 5% 48%	2350
	11th to 20th	Night	17% 7% 2 5% 4 4 4 3 4 5% 45%	2218
	21st to end	Night	17% 8% 2 5% 3 4 5% 4 5% 7% 40%	1884
March	1st to 10th	Night	23% 10% 2 6% 3 3 3 3 4 5% 37%	3515
	11th to 20th	Night	11% 5% 2 4 3 3 4 4 5% 8% 51%	3480
	21st to end	Night	20% 6% 2 5% 4 4 4 3 5% 5% 42%	3817
April	1st to 10th	Night	14% 7% 2 5% 2 3 3 3 5% 7% 49%	2090
	11th to 20th	Night	14% 5% 2 5% 3 2 3 3 5% 7% 51%	1873
	21st to end	Night	12% 5% 2 4 4 3 4 4 4 8% 51%	1710
May	1st to 10th	Night	13% 9% 2 5% 4 4 5% 4 6% 8% 39%	2813
	11th to 20th	Night	20% 8% 3 6% 3 3 4 3 5% 7% 38%	2812
	21st to end	Night	9% 5% 2 5% 4 4 4 5% 7% 10% 45%	3079
June	1st to 10th	Night	11% 7% 4 6% 5% 4 5% 6% 9% 10% 33%	1710
	11th to 20th	Night	8% 6% 2 7% 5% 4 5% 4 8% 9% 41%	1710
	21st to end	Night	9% 5% 2 5% 4 4 7% 4 8% 11% 40%	1710
July	1st to 10th	Night	11% 6% 3 8% 5% 5% 7% 6% 8% 9% 31%	2840
	11th to 20th	Night	9% 6% 2 7% 6% 5% 6% 6% 8% 12% 33%	2840
	21st to end	Night	13% 9% 4 9% 7% 6% 6% 6% 7% 10% 23%	3087
August	1st to 10th	Night	12% 8% 3 8% 6% 6% 6% 4 7% 9% 33%	2825
	11th to 20th	Night	15% 8% 3 8% 5% 5% 5% 4 7% 7% 33%	2785
	21st to end	Night	10% 5% 2 6% 4 4 4 4 6% 9% 46%	3469
September	1st to 10th	Night	13% 7% 2 7% 4 5% 6% 5% 7% 9% 37%	2090
	11th to 20th	Night	20% 7% 2 7% 5% 4 4 4 6% 7% 34%	2090
	21st to end	Night	19% 7% 2 6% 4 4 4 2 4 6% 43%	2090
October	1st to 10th	Night	15% 6% 2 4 3 3 4 4 4 7% 49%	2099
	11th to 20th	Night	15% 5% 2 4 3 3 3 3 4 5% 53%	2280
	21st to end	Night	15% 5% 1 3 3 3 3 2 3 4 57%	2508
November	1st to 10th	Night	14% 6% 1 3 3 2 4 3 4 5% 55%	2359
	11th to 20th	Night	15% 6% 1 4 3 2 3 3 4 6% 53%	2470
	21st to end	Night	16% 4 2 4 4 3 4 4 5% 6% 48%	2470
December	1st to 10th	Night	18% 7% 3 5% 5% 4 3 4 5% 5% 40%	2470
	11th to 20th	Night	14% 6% 2 4 3 4 3 3 4 5% 53%	3042
	21st to end	Night	22% 6% 2 6% 3 4 4 4 5% 7% 38%	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	33% 5% 5% 18% 28% 12%	2837
	11th to 20th	Day	37% 4 5% 14% 26% 13%	2840
	21st to end	Day	40% 4 5% 15% 24% 12%	3124
February	1st to 10th	Day	40% 5% 8% 19% 19% 8%	1828
	11th to 20th	Day	43% 5% 8% 16% 20% 8%	1858
	21st to end	Day	45% 6% 7% 15% 17% 9%	1570
March	1st to 10th	Day	44% 5% 8% 17% 16% 10%	3336
	11th to 20th	Day	34% 6% 6% 17% 25% 12%	3480
	21st to end	Day	44% 7% 8% 23% 14% 5%	3815
April	1st to 10th	Day	36% 8% 11% 20% 19% 6%	2090
	11th to 20th	Day	39% 8% 9% 17% 19% 7%	2267
	21st to end	Day	36% 8% 11% 25% 17% 3	2470
May	1st to 10th	Day	43% 10% 13% 19% 12% 2	4120
	11th to 20th	Day	48% 11% 11% 19% 10% 1	4085
	21st to end	Day	35% 14% 15% 20% 15% 1	4489
June	1st to 10th	Day	46% 12% 17% 18% 6% 1	2470
	11th to 20th	Day	40% 13% 18% 18% 9% 1	2470
	21st to end	Day	42% 13% 16% 17% 11% 2	2470
July	1st to 10th	Day	43% 15% 19% 18% 5%	4055
	11th to 20th	Day	47% 14% 22% 13% 3 1	4051
	21st to end	Day	62% 14% 12% 9% 3	4324
August	1st to 10th	Day	47% 14% 17% 18% 3	4091
	11th to 20th	Day	40% 13% 23% 12% 8% 3	4083
	21st to end	Day	35% 11% 14% 27% 12% 1	4150
September	1st to 10th	Day	44% 12% 11% 17% 14% 3	2090
	11th to 20th	Day	49% 10% 10% 16% 14% 2	2090
	21st to end	Day	44% 8% 11% 16% 15% 6%	2090
October	1st to 10th	Day	28% 4 8% 18% 31% 11%	3456
	11th to 20th	Day	20% 4 5% 17% 26% 28%	3140
	21st to end	Day	29% 5% 6% 21% 22% 17%	3428
November	1st to 10th	Day	30% 6% 6% 19% 28% 12%	1788
	11th to 20th	Day	32% 4 6% 21% 26% 11%	1710
	21st to end	Day	36% 5% 6% 19% 25% 9%	1710
December	1st to 10th	Day	46% 7% 6% 17% 17% 7%	1710
	11th to 20th	Day	35% 4 7% 16% 21% 17%	2150
	21st to end	Day	37% 6% 8% 20% 21% 7%	3124
January	1st to 10th	Night	36% 5% 6% 14% 24% 15%	4109
	11th to 20th	Night	36% 5% 7% 11% 28% 12%	4120
	21st to end	Night	43% 3 7% 14% 20% 13%	4532
February	1st to 10th	Night	38% 5% 9% 20% 21% 8%	2350
	11th to 20th	Night	42% 5% 7% 18% 21% 7%	2219
	21st to end	Night	43% 7% 8% 15% 19% 8%	1884
March	1st to 10th	Night	49% 8% 7% 13% 16% 8%	3515
	11th to 20th	Night	30% 8% 9% 21% 19% 14%	3480
	21st to end	Night	43% 7% 7% 21% 13% 8%	3817
April	1st to 10th	Night	34% 6% 10% 21% 24% 4	2090
	11th to 20th	Night	34% 5% 10% 21% 23% 7%	1873
	21st to end	Night	32% 8% 14% 25% 18% 2	1710
May	1st to 10th	Night	41% 11% 14% 22% 11% 1	2840
	11th to 20th	Night	43% 10% 16% 18% 12%	2813
	21st to end	Night	29% 13% 19% 27% 10% 1	3087
June	1st to 10th	Night	40% 13% 18% 21% 6% 1	1710
	11th to 20th	Night	35% 11% 23% 23% 8% 1	1710
	21st to end	Night	34% 13% 19% 18% 12% 3	1710
July	1st to 10th	Night	44% 14% 20% 16% 5% 1	2795
	11th to 20th	Night	38% 15% 26% 16% 3 1	2786
	21st to end	Night	52% 16% 17% 11% 4 1	2988
August	1st to 10th	Night	45% 11% 24% 16% 5%	2814
	11th to 20th	Night	46% 13% 17% 13% 10% 1	2812
	21st to end	Night	33% 9% 18% 29% 10% 1	3469
September	1st to 10th	Night	41% 11% 14% 18% 12% 3	2090
	11th to 20th	Night	46% 11% 11% 17% 13% 1	2090
	21st to end	Night	42% 5% 10% 21% 16% 5%	2090
October	1st to 10th	Night	28% 6% 12% 16% 29% 9%	3502
	11th to 20th	Night	21% 4 5% 17% 28% 25%	3779
	21st to end	Night	32% 3 4 19% 25% 16%	4120
November	1st to 10th	Night	33% 5% 8% 18% 24% 12%	2359
	11th to 20th	Night	34% 4 6% 17% 25% 14%	2470
	21st to end	Night	37% 5% 5% 20% 24% 9%	2470
December	1st to 10th	Night	45% 6% 7% 16% 18% 9%	2470
	11th to 20th	Night	35% 4 8% 14% 25% 14%	3044
	21st to end	Night	43% 6% 9% 14% 21% 7%	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	29% 25% 32% 12% 1	270
	11th to 20th	Day	31% 29% 26% 13% 2	270
	21st to end	Day	29% 33% 31% 4 2 1	297
February	1st to 10th	Day	28% 27% 36% 9% 1	270
	11th to 20th	Day	39% 29% 27% 5%	270
	21st to end	Day	41% 34% 18% 5% 1	225
March	1st to 10th	Day	27% 23% 29% 15% 6%	395
	11th to 20th	Day	26% 25% 30% 15% 4 1	460
	21st to end	Day	27% 25% 27% 15% 5% 1	506
April	1st to 10th	Day	24% 20% 26% 20% 8% 1	460
	11th to 20th	Day	19% 23% 28% 19% 10% 1	452
	21st to end	Day	21% 17% 26% 21% 11% 3	460
May	1st to 10th	Day	18% 20% 32% 16% 10% 5%	463
	11th to 20th	Day	21% 11% 24% 24% 13% 8%	487
	21st to end	Day	18% 9% 19% 28% 15% 11%	529
June	1st to 10th	Day	16% 8% 22% 28% 16% 11%	490
	11th to 20th	Day	12% 8% 20% 23% 23% 14%	490
	21st to end	Day	11% 6% 25% 29% 16% 12%	490
July	1st to 10th	Day	10% 7% 15% 24% 27% 17%	485
	11th to 20th	Day	10% 3 12% 29% 25% 21%	484
	21st to end	Day	11% 5% 13% 27% 26% 20%	532
August	1st to 10th	Day	19% 6% 16% 22% 19% 17%	486
	11th to 20th	Day	13% 5% 15% 28% 24% 14%	479
	21st to end	Day	14% 2 12% 28% 29% 15%	539
September	1st to 10th	Day	11% 7% 14% 30% 28% 11%	490
	11th to 20th	Day	20% 5% 15% 26% 25% 10%	490
	21st to end	Day	22% 8% 16% 30% 19% 5%	490
October	1st to 10th	Day	20% 7% 22% 25% 18% 8%	490
	11th to 20th	Day	13% 15% 26% 27% 15% 4	490
	21st to end	Day	19% 11% 28% 29% 10% 3	539
November	1st to 10th	Day	20% 15% 28% 23% 12% 2	368
	11th to 20th	Day	23% 18% 33% 21% 4	270
	21st to end	Day	16% 19% 42% 17% 5% 1	270
December	1st to 10th	Day	30% 26% 30% 11% 2	270
	11th to 20th	Day	23% 20% 39% 14% 4	270
	21st to end	Day	32% 19% 34% 11% 3	297
January	1st to 10th	Night	23% 20% 29% 23% 5%	760
	11th to 20th	Night	21% 22% 29% 22% 6%	760
	21st to end	Night	21% 21% 32% 24% 3	836
February	1st to 10th	Night	17% 20% 28% 30% 4	760
	11th to 20th	Night	23% 22% 31% 21% 2	743
	21st to end	Night	25% 25% 25% 21% 3	628
March	1st to 10th	Night	21% 19% 29% 25% 6%	760
	11th to 20th	Night	20% 18% 29% 23% 8% 1	760
	21st to end	Night	21% 18% 28% 23% 10%	836
April	1st to 10th	Night	20% 20% 26% 24% 9% 1	760
	11th to 20th	Night	22% 14% 24% 24% 14% 2	634
	21st to end	Night	15% 12% 23% 31% 18% 2	570
May	1st to 10th	Night	17% 14% 24% 24% 19% 2	570
	11th to 20th	Night	19% 7% 20% 27% 24% 3	561
	21st to end	Night	15% 4 16% 24% 33% 8%	617
June	1st to 10th	Night	14% 4 14% 32% 30% 7%	570
	11th to 20th	Night	11% 3 15% 26% 34% 12%	570
	21st to end	Night	9% 4 14% 27% 38% 8%	570
July	1st to 10th	Night	13% 2 9% 23% 43% 10%	555
	11th to 20th	Night	12% 2 6% 24% 42% 13%	552
	21st to end	Night	8% 4 7% 28% 40% 13%	624
August	1st to 10th	Night	15% 3 10% 21% 39% 12%	567
	11th to 20th	Night	17% 4 8% 24% 37% 9%	564
	21st to end	Night	14% 4 7% 23% 41% 11%	665
September	1st to 10th	Night	15% 4 10% 26% 36% 9%	760
	11th to 20th	Night	24% 4 11% 22% 32% 7%	760
	21st to end	Night	22% 5% 13% 30% 26% 4	760
October	1st to 10th	Night	17% 8% 18% 24% 24% 8%	760
	11th to 20th	Night	18% 11% 18% 29% 20% 3	760
	21st to end	Night	19% 10% 18% 33% 17% 3	836
November	1st to 10th	Night	17% 8% 19% 32% 20% 3	755
	11th to 20th	Night	18% 14% 22% 32% 12% 2	760
	21st to end	Night	18% 14% 22% 30% 14% 2	760
December	1st to 10th	Night	23% 16% 24% 28% 8%	760
	11th to 20th	Night	20% 14% 27% 31% 8% 1	760
	21st to end	Night	29% 14% 25% 27% 6%	836

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