Greenbrier Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Greenbrier Observatory clear sky chart from 2005-11-06 to 2024-11-08.

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% 9% 3 8% 5% 5% 5% 4 6% 8% 35%	100903
	Night	13% 9% 4 8% 5% 4 4 4 6% 7% 37%	98141
Transparency	Day	23% 49% 7% 12% 7% 2	104566
	Night	22% 50% 7% 12% 8% 2	102617
Seeing	Day	22% 15% 24% 21% 13% 5%	15093
	Night	20% 13% 21% 28% 16% 2	25330

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% 15% 5% 8% 5% 4 4 3 4 6% 30%	2836
	11th to 20th	Day	11% 14% 8% 14% 8% 5% 4 3 3 4 26%	2840
	21st to end	Day	17% 16% 4 9% 4 3 3 2 4 6% 31%	3124
February	1st to 10th	Day	12% 10% 4 10% 5% 6% 5% 3 4 7% 35%	1710
	11th to 20th	Day	16% 8% 4 11% 5% 5% 4 3 6% 6% 32%	1715
	21st to end	Day	17% 10% 4 9% 5% 4 4 3 5% 6% 32%	1620
March	1st to 10th	Day	19% 11% 4 7% 5% 4 5% 3 5% 7% 30%	3435
	11th to 20th	Day	12% 8% 3 8% 5% 3 4 3 4 5% 45%	3480
	21st to end	Day	15% 11% 4 8% 5% 5% 4 3 5% 7% 33%	3815
April	1st to 10th	Day	10% 10% 3 7% 5% 5% 5% 3 5% 7% 40%	2150
	11th to 20th	Day	13% 8% 3 7% 6% 4 4 4 5% 7% 40%	2254
	21st to end	Day	15% 9% 2 7% 5% 4 5% 4 5% 6% 37%	2448
May	1st to 10th	Day	18% 10% 4 9% 6% 5% 5% 4 6% 7% 26%	4082
	11th to 20th	Day	20% 9% 4 8% 5% 5% 5% 5% 6% 8% 26%	4081
	21st to end	Day	8% 6% 2 7% 6% 5% 7% 5% 8% 10% 35%	4476
June	1st to 10th	Day	13% 6% 3 7% 5% 6% 5% 5% 6% 8% 37%	2470
	11th to 20th	Day	9% 6% 3 8% 6% 6% 6% 5% 7% 11% 34%	2470
	21st to end	Day	9% 6% 2 7% 7% 5% 6% 5% 8% 10% 36%	2470
July	1st to 10th	Day	7% 5% 2 7% 6% 6% 6% 5% 8% 10% 38%	4120
	11th to 20th	Day	8% 6% 2 6% 5% 5% 6% 5% 8% 12% 36%	4120
	21st to end	Day	14% 10% 4 10% 6% 6% 7% 5% 7% 7% 23%	4466
August	1st to 10th	Day	12% 7% 3 8% 5% 6% 6% 6% 8% 11% 28%	4104
	11th to 20th	Day	9% 6% 3 9% 6% 6% 7% 6% 7% 9% 32%	4044
	21st to end	Day	6% 4 2 7% 5% 5% 5% 5% 7% 10% 44%	4257
September	1st to 10th	Day	11% 6% 3 6% 5% 5% 5% 4 6% 7% 42%	2227
	11th to 20th	Day	13% 8% 3 8% 5% 5% 5% 4 5% 7% 37%	2090
	21st to end	Day	20% 8% 3 7% 5% 4 4 3 6% 6% 34%	2090
October	1st to 10th	Day	15% 6% 3 6% 4 3 4 3 5% 7% 44%	1924
	11th to 20th	Day	15% 8% 3 6% 4 4 4 3 5% 6% 44%	1900
	21st to end	Day	14% 9% 2 7% 4 4 4 2 4 6% 44%	2090
November	1st to 10th	Day	10% 8% 2 5% 4 4 3 3 5% 6% 49%	1927
	11th to 20th	Day	12% 9% 3 6% 5% 5% 5% 4 5% 6% 41%	1710
	21st to end	Day	13% 9% 3 5% 4 5% 4 3 5% 8% 42%	1653
December	1st to 10th	Day	18% 12% 4 9% 5% 5% 4 4 6% 6% 27%	1520
	11th to 20th	Day	16% 12% 3 7% 3 3 3 3 4 7% 38%	2061
	21st to end	Day	17% 12% 3 10% 7% 5% 4 4 6% 7% 24%	3124
January	1st to 10th	Night	11% 15% 8% 11% 5% 5% 5% 4 4 5% 26%	4109
	11th to 20th	Night	13% 11% 4 10% 7% 4 4 3 4 5% 35%	4120
	21st to end	Night	18% 14% 3 6% 6% 5% 4 3 4 5% 31%	4532
February	1st to 10th	Night	13% 10% 4 7% 5% 5% 5% 4 7% 8% 34%	2470
	11th to 20th	Night	18% 9% 3 9% 6% 4 4 4 6% 5% 31%	2349
	21st to end	Night	14% 9% 4 9% 6% 5% 5% 4 6% 5% 32%	1832
March	1st to 10th	Night	20% 12% 4 8% 5% 4 4 3 5% 6% 29%	3418
	11th to 20th	Night	12% 8% 5% 7% 5% 3 4 4 5% 6% 42%	3480
	21st to end	Night	14% 8% 2 6% 4 4 4 4 6% 7% 42%	3817
April	1st to 10th	Night	10% 10% 4 8% 6% 4 5% 3 5% 7% 39%	2028
	11th to 20th	Night	12% 7% 3 6% 4 5% 4 4 7% 8% 41%	1875
	21st to end	Night	14% 7% 2 5% 4 4 5% 4 6% 7% 41%	1748
May	1st to 10th	Night	18% 10% 3 9% 5% 4 4 3 6% 7% 30%	2813
	11th to 20th	Night	20% 11% 3 7% 3 4 4 4 5% 7% 32%	2812
	21st to end	Night	10% 7% 3 8% 5% 5% 5% 4 7% 10% 37%	3079
June	1st to 10th	Night	8% 5% 2 5% 5% 5% 5% 4 8% 10% 42%	1710
	11th to 20th	Night	9% 6% 2 7% 4 4 4 5% 6% 9% 44%	1710
	21st to end	Night	8% 5% 2 6% 4 5% 6% 4 7% 10% 45%	1710
July	1st to 10th	Night	6% 4 2 7% 5% 5% 6% 5% 9% 11% 40%	2840
	11th to 20th	Night	6% 6% 2 7% 4 5% 4 5% 7% 11% 44%	2840
	21st to end	Night	9% 9% 4 11% 7% 6% 5% 4 7% 9% 28%	3087
August	1st to 10th	Night	13% 7% 3 8% 5% 5% 6% 4 7% 8% 34%	2825
	11th to 20th	Night	9% 7% 3 9% 5% 5% 6% 4 7% 8% 38%	2785
	21st to end	Night	6% 5% 3 6% 4 3 4 3 6% 9% 51%	3366
September	1st to 10th	Night	10% 6% 2 6% 4 4 4 4 6% 6% 46%	1949
	11th to 20th	Night	11% 7% 3 7% 3 4 3 3 5% 8% 45%	2090
	21st to end	Night	14% 10% 3 6% 4 4 5% 4 5% 7% 38%	2090
October	1st to 10th	Night	14% 7% 2 5% 4 4 5% 3 5% 6% 46%	2255
	11th to 20th	Night	13% 8% 3 8% 4 3 4 3 5% 6% 45%	2280
	21st to end	Night	13% 9% 2 5% 4 4 4 3 5% 7% 46%	2508
November	1st to 10th	Night	10% 7% 3 7% 4 4 3 3 5% 5% 48%	2312
	11th to 20th	Night	12% 9% 3 6% 4 3 4 4 6% 7% 42%	2467
	21st to end	Night	12% 8% 3 6% 5% 5% 4 4 6% 6% 41%	2519
December	1st to 10th	Night	16% 11% 4 9% 4 5% 4 4 6% 6% 30%	2660
	11th to 20th	Night	13% 10% 5% 10% 5% 5% 5% 3 5% 6% 33%	3124
	21st to end	Night	15% 13% 7% 9% 6% 5% 6% 4 5% 6% 25%	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	30% 43% 4 13% 8% 2	2837
	11th to 20th	Day	32% 45% 5% 9% 7% 2	2840
	21st to end	Day	32% 43% 4 11% 9% 1	3124
February	1st to 10th	Day	18% 67% 3 6% 6% 1	1710
	11th to 20th	Day	18% 66% 3 6% 6% 1	1724
	21st to end	Day	19% 65% 3 5% 7% 2	1620
March	1st to 10th	Day	32% 47% 7% 10% 3 1	3435
	11th to 20th	Day	18% 43% 6% 15% 16% 2	3480
	21st to end	Day	27% 44% 5% 12% 9% 2	3815
April	1st to 10th	Day	16% 67% 4 8% 5% 1	2150
	11th to 20th	Day	15% 67% 5% 9% 4 1	2254
	21st to end	Day	16% 66% 5% 9% 4 1	2448
May	1st to 10th	Day	34% 45% 6% 11% 4	4120
	11th to 20th	Day	31% 44% 8% 12% 5%	4085
	21st to end	Day	19% 47% 11% 12% 8% 3	4489
June	1st to 10th	Day	15% 67% 5% 8% 5%	2470
	11th to 20th	Day	14% 68% 5% 7% 6%	2470
	21st to end	Day	17% 66% 5% 8% 4	2470
July	1st to 10th	Day	22% 42% 15% 12% 5% 4	4055
	11th to 20th	Day	22% 40% 12% 21% 4	4051
	21st to end	Day	38% 39% 7% 11% 5%	4324
August	1st to 10th	Day	30% 41% 11% 15% 4	4091
	11th to 20th	Day	24% 43% 8% 16% 8% 1	4083
	21st to end	Day	16% 41% 11% 22% 9%	4257
September	1st to 10th	Day	15% 63% 6% 7% 6% 2	2227
	11th to 20th	Day	19% 61% 4 8% 7%	2090
	21st to end	Day	22% 62% 4 8% 4	2090
October	1st to 10th	Day	17% 39% 5% 10% 22% 6%	3197
	11th to 20th	Day	14% 38% 5% 16% 9% 17%	3140
	21st to end	Day	13% 37% 6% 21% 10% 12%	3428
November	1st to 10th	Day	18% 58% 5% 11% 7% 1	1927
	11th to 20th	Day	16% 60% 3 8% 10% 3	1710
	21st to end	Day	15% 61% 6% 10% 7%	1653
December	1st to 10th	Day	23% 61% 5% 6% 3 2	1520
	11th to 20th	Day	19% 52% 4 11% 13% 2	2058
	21st to end	Day	34% 43% 7% 11% 5%	3124
January	1st to 10th	Night	32% 46% 4 7% 8% 1	4109
	11th to 20th	Night	28% 40% 5% 15% 10% 3	4120
	21st to end	Night	31% 43% 6% 13% 7% 1	4532
February	1st to 10th	Night	14% 67% 7% 7% 5% 1	2470
	11th to 20th	Night	18% 65% 4 6% 5% 1	2351
	21st to end	Night	16% 66% 4 8% 4 2	1832
March	1st to 10th	Night	35% 46% 7% 8% 3 1	3418
	11th to 20th	Night	18% 45% 4 17% 15%	3480
	21st to end	Night	21% 44% 8% 14% 12% 2	3817
April	1st to 10th	Night	16% 68% 4 7% 6%	2028
	11th to 20th	Night	14% 67% 6% 8% 5% 1	1875
	21st to end	Night	12% 66% 6% 11% 5%	1748
May	1st to 10th	Night	33% 45% 6% 12% 3 1	2840
	11th to 20th	Night	30% 44% 9% 13% 4	2813
	21st to end	Night	19% 50% 13% 12% 3 4	3087
June	1st to 10th	Night	12% 69% 5% 8% 5%	1710
	11th to 20th	Night	11% 66% 6% 10% 6% 1	1710
	21st to end	Night	12% 67% 7% 9% 5%	1710
July	1st to 10th	Night	20% 44% 15% 13% 6% 1	2795
	11th to 20th	Night	20% 39% 13% 24% 3	2786
	21st to end	Night	35% 42% 8% 11% 4	2988
August	1st to 10th	Night	30% 41% 12% 15% 1	2814
	11th to 20th	Night	24% 41% 12% 17% 6%	2812
	21st to end	Night	17% 39% 10% 25% 10%	3366
September	1st to 10th	Night	14% 62% 7% 9% 6% 2	1949
	11th to 20th	Night	17% 62% 4 10% 8%	2090
	21st to end	Night	21% 63% 5% 8% 3	2090
October	1st to 10th	Night	21% 38% 4 14% 20% 3	3762
	11th to 20th	Night	14% 40% 5% 13% 15% 12%	3779
	21st to end	Night	9% 37% 8% 21% 15% 11%	4120
November	1st to 10th	Night	18% 57% 6% 10% 7% 1	2312
	11th to 20th	Night	13% 60% 5% 8% 12% 3	2467
	21st to end	Night	15% 62% 6% 10% 7% 1	2519
December	1st to 10th	Night	22% 62% 6% 6% 2 2	2660
	11th to 20th	Night	22% 53% 5% 11% 9% 1	3126
	21st to end	Night	32% 45% 7% 9% 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	39% 26% 20% 13% 1 1	270
	11th to 20th	Day	40% 24% 28% 7% 1	270
	21st to end	Day	40% 24% 25% 9% 2	297
February	1st to 10th	Day	36% 22% 29% 11% 3	270
	11th to 20th	Day	45% 27% 20% 7%	270
	21st to end	Day	40% 24% 25% 9% 1	275
March	1st to 10th	Day	31% 24% 23% 17% 5%	460
	11th to 20th	Day	32% 27% 23% 15% 3 1	460
	21st to end	Day	27% 25% 25% 18% 5%	506
April	1st to 10th	Day	26% 25% 25% 15% 10%	460
	11th to 20th	Day	24% 18% 28% 18% 9% 3	452
	21st to end	Day	26% 20% 27% 15% 11% 2	460
May	1st to 10th	Day	26% 19% 30% 12% 10% 3	463
	11th to 20th	Day	23% 12% 31% 17% 13% 5%	487
	21st to end	Day	17% 13% 22% 22% 18% 7%	529
June	1st to 10th	Day	15% 12% 27% 24% 15% 7%	490
	11th to 20th	Day	12% 10% 28% 24% 18% 7%	490
	21st to end	Day	10% 12% 29% 29% 13% 7%	490
July	1st to 10th	Day	13% 6% 23% 28% 20% 10%	485
	11th to 20th	Day	6% 5% 22% 36% 20% 10%	484
	21st to end	Day	14% 6% 25% 27% 18% 10%	532
August	1st to 10th	Day	16% 7% 25% 21% 21% 10%	486
	11th to 20th	Day	14% 7% 24% 26% 18% 11%	479
	21st to end	Day	11% 4 19% 27% 23% 17%	539
September	1st to 10th	Day	16% 6% 21% 25% 22% 11%	490
	11th to 20th	Day	15% 6% 19% 29% 24% 8%	490
	21st to end	Day	28% 11% 19% 24% 15% 2	490
October	1st to 10th	Day	21% 10% 20% 27% 19% 4	490
	11th to 20th	Day	19% 17% 24% 23% 15% 2	490
	21st to end	Day	21% 17% 26% 22% 12% 2	539
November	1st to 10th	Day	19% 16% 27% 23% 12% 2	483
	11th to 20th	Day	33% 15% 26% 19% 6%	270
	21st to end	Day	25% 21% 30% 17% 6% 1	243
December	1st to 10th	Day	34% 28% 19% 13% 4 1	180
	11th to 20th	Day	37% 21% 23% 13% 6%	227
	21st to end	Day	33% 21% 26% 13% 6% 1	297
January	1st to 10th	Night	29% 24% 24% 19% 4	760
	11th to 20th	Night	29% 20% 28% 21% 3	760
	21st to end	Night	29% 20% 30% 20% 1	836
February	1st to 10th	Night	25% 20% 25% 27% 3	760
	11th to 20th	Night	32% 27% 24% 17% 1	743
	21st to end	Night	26% 24% 30% 18% 2	628
March	1st to 10th	Night	24% 19% 28% 25% 3	760
	11th to 20th	Night	28% 20% 24% 21% 7% 1	760
	21st to end	Night	21% 19% 27% 25% 8%	836
April	1st to 10th	Night	21% 22% 27% 25% 5%	760
	11th to 20th	Night	19% 17% 26% 27% 10% 1	751
	21st to end	Night	18% 14% 27% 32% 9%	608
May	1st to 10th	Night	18% 12% 25% 30% 14% 1	570
	11th to 20th	Night	20% 10% 20% 30% 18% 1	561
	21st to end	Night	14% 4 16% 32% 27% 6%	617
June	1st to 10th	Night	9% 4 18% 39% 27% 3	570
	11th to 20th	Night	11% 2 16% 33% 35% 2	570
	21st to end	Night	9% 4 17% 39% 27% 4	570
July	1st to 10th	Night	11% 1 13% 38% 30% 6%	555
	11th to 20th	Night	7% 3 11% 34% 42% 5%	552
	21st to end	Night	6% 3 14% 38% 36% 4	624
August	1st to 10th	Night	14% 2 13% 34% 33% 4	567
	11th to 20th	Night	13% 2 12% 34% 34% 5%	564
	21st to end	Night	10% 2 10% 33% 39% 6%	765
September	1st to 10th	Night	13% 4 13% 30% 34% 6%	760
	11th to 20th	Night	18% 4 12% 29% 33% 4	760
	21st to end	Night	21% 9% 17% 26% 24% 2	760
October	1st to 10th	Night	16% 10% 18% 27% 26% 3	760
	11th to 20th	Night	18% 14% 20% 29% 17% 1	760
	21st to end	Night	19% 13% 21% 33% 13% 1	836
November	1st to 10th	Night	18% 10% 24% 33% 13% 2	771
	11th to 20th	Night	24% 15% 20% 31% 10% 1	760
	21st to end	Night	20% 16% 23% 31% 8% 1	760
December	1st to 10th	Night	29% 20% 23% 22% 6%	760
	11th to 20th	Night	22% 19% 28% 26% 5%	760
	21st to end	Night	25% 18% 23% 31% 3	836

Page generated at 2024-11-05 08:15:51. Version ac07

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