Green Bank Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Green Bank Observatory 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% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	100837
	Night	13% 9% 3 8% 5% 5% 5% 4 6% 7% 35%	98077
Transparency	Day	48% 5% 10% 21% 13% 3	104486
	Night	47% 6% 11% 20% 13% 3	102567
Seeing	Day	26% 14% 25% 19% 11% 5%	14957
	Night	24% 13% 21% 27% 14% 2	24921

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% 13% 4 10% 5% 5% 4 3 5% 6% 31%	2836
	11th to 20th	Day	11% 12% 7% 16% 6% 5% 4 4 4 5% 27%	2840
	21st to end	Day	18% 13% 4 9% 5% 4 3 3 4 6% 32%	3124
February	1st to 10th	Day	12% 13% 4 8% 5% 4 5% 3 6% 6% 33%	1710
	11th to 20th	Day	15% 10% 4 9% 5% 4 5% 4 5% 8% 31%	1665
	21st to end	Day	16% 12% 4 8% 5% 4 5% 3 4 6% 33%	1638
March	1st to 10th	Day	19% 9% 5% 8% 4 4 5% 4 5% 8% 30%	3435
	11th to 20th	Day	10% 9% 3 7% 4 4 5% 4 5% 6% 43%	3480
	21st to end	Day	14% 12% 4 8% 5% 4 5% 3 4 6% 34%	3815
April	1st to 10th	Day	12% 10% 4 8% 6% 4 5% 3 5% 7% 36%	2180
	11th to 20th	Day	14% 9% 3 6% 5% 3 5% 3 5% 7% 39%	2254
	21st to end	Day	19% 9% 3 7% 6% 5% 5% 4 5% 5% 34%	2427
May	1st to 10th	Day	20% 9% 3 9% 6% 5% 5% 4 6% 7% 26%	4082
	11th to 20th	Day	23% 9% 3 7% 4 4 5% 4 6% 8% 28%	4081
	21st to end	Day	11% 6% 3 8% 6% 6% 7% 5% 7% 9% 32%	4476
June	1st to 10th	Day	15% 7% 3 6% 5% 5% 5% 5% 7% 7% 36%	2617
	11th to 20th	Day	12% 7% 2 7% 6% 6% 6% 5% 7% 9% 32%	2660
	21st to end	Day	10% 6% 3 8% 6% 5% 6% 5% 7% 9% 35%	2539
July	1st to 10th	Day	8% 6% 3 8% 6% 6% 6% 6% 7% 8% 36%	4120
	11th to 20th	Day	9% 5% 2 7% 7% 6% 6% 5% 8% 11% 34%	4120
	21st to end	Day	15% 10% 4 9% 7% 6% 6% 5% 7% 8% 23%	4466
August	1st to 10th	Day	13% 8% 3 9% 6% 7% 7% 6% 7% 10% 25%	4104
	11th to 20th	Day	10% 6% 3 10% 7% 7% 7% 5% 7% 9% 27%	4044
	21st to end	Day	7% 5% 2 7% 6% 6% 6% 4 7% 9% 41%	4228
September	1st to 10th	Day	14% 7% 2 7% 5% 5% 4 3 6% 6% 39%	2278
	11th to 20th	Day	13% 8% 2 7% 5% 4 4 4 5% 6% 40%	2090
	21st to end	Day	23% 8% 3 7% 4 4 4 3 5% 6% 33%	2087
October	1st to 10th	Day	15% 7% 3 6% 4 5% 4 3 5% 7% 41%	1900
	11th to 20th	Day	16% 9% 3 6% 4 3 3 3 4 4 44%	1900
	21st to end	Day	16% 9% 2 6% 4 3 4 3 5% 6% 43%	2090
November	1st to 10th	Day	10% 8% 3 5% 4 4 4 4 5% 5% 48%	1887
	11th to 20th	Day	15% 8% 3 6% 3 4 4 4 5% 7% 41%	1704
	21st to end	Day	14% 9% 4 6% 5% 5% 5% 3 5% 5% 40%	1520
December	1st to 10th	Day	16% 13% 5% 8% 5% 5% 4 3 6% 7% 28%	1520
	11th to 20th	Day	16% 9% 4 6% 5% 5% 4 2 5% 7% 38%	1910
	21st to end	Day	18% 11% 5% 9% 6% 5% 5% 3 6% 8% 26%	3010
January	1st to 10th	Night	10% 13% 6% 12% 6% 6% 6% 4 5% 6% 26%	4109
	11th to 20th	Night	13% 13% 4 9% 7% 4 4 3 4 5% 35%	4120
	21st to end	Night	20% 11% 4 7% 4 4 3 3 5% 6% 32%	4532
February	1st to 10th	Night	11% 10% 4 8% 6% 4 5% 4 6% 8% 34%	2470
	11th to 20th	Night	15% 9% 3 9% 6% 5% 5% 5% 6% 6% 30%	2399
	21st to end	Night	14% 10% 4 10% 5% 4 5% 4 5% 6% 33%	1814
March	1st to 10th	Night	21% 15% 4 7% 4 5% 4 4 4 5% 27%	3418
	11th to 20th	Night	9% 7% 3 11% 5% 4 5% 3 5% 7% 41%	3480
	21st to end	Night	14% 10% 3 6% 5% 4 4 4 6% 7% 40%	3817
April	1st to 10th	Night	12% 11% 4 7% 6% 4 5% 4 5% 8% 35%	1999
	11th to 20th	Night	11% 7% 2 7% 6% 5% 5% 3 6% 9% 39%	1875
	21st to end	Night	13% 8% 3 7% 4 5% 5% 4 6% 7% 39%	1767
May	1st to 10th	Night	22% 12% 4 8% 5% 5% 4 3 4 6% 28%	2813
	11th to 20th	Night	22% 12% 3 7% 5% 4 4 3 5% 6% 29%	2812
	21st to end	Night	11% 8% 3 7% 5% 5% 6% 5% 7% 9% 34%	3079
June	1st to 10th	Night	10% 6% 3 5% 5% 4 5% 4 8% 10% 41%	1553
	11th to 20th	Night	11% 6% 2 7% 4 5% 4 4 7% 10% 40%	1520
	21st to end	Night	9% 5% 2 5% 5% 5% 4 5% 8% 11% 41%	1631
July	1st to 10th	Night	7% 5% 2 7% 5% 5% 5% 5% 8% 11% 39%	2840
	11th to 20th	Night	8% 7% 3 7% 5% 4 5% 4 7% 11% 40%	2840
	21st to end	Night	12% 10% 5% 10% 7% 6% 5% 4 6% 8% 27%	3087
August	1st to 10th	Night	13% 9% 3 9% 5% 5% 5% 5% 6% 9% 31%	2825
	11th to 20th	Night	13% 8% 3 7% 5% 5% 6% 4 7% 7% 35%	2785
	21st to end	Night	7% 6% 2 6% 4 4 4 5% 5% 9% 47%	3380
September	1st to 10th	Night	13% 7% 2 6% 4 4 5% 3 5% 7% 44%	1900
	11th to 20th	Night	13% 6% 2 6% 4 5% 4 2 5% 8% 45%	2088
	21st to end	Night	20% 8% 2 6% 4 5% 5% 3 5% 6% 35%	2092
October	1st to 10th	Night	16% 9% 2 4 3 4 4 4 5% 6% 43%	2280
	11th to 20th	Night	13% 10% 3 6% 3 3 4 3 4 7% 42%	2280
	21st to end	Night	14% 8% 3 6% 4 4 5% 3 5% 6% 42%	2508
November	1st to 10th	Night	9% 9% 3 7% 4 4 4 4 5% 6% 45%	2264
	11th to 20th	Night	13% 8% 4 7% 4 5% 5% 5% 5% 7% 39%	2467
	21st to end	Night	14% 8% 2 6% 6% 5% 5% 4 6% 7% 37%	2660
December	1st to 10th	Night	14% 12% 5% 10% 6% 5% 4 4 5% 7% 27%	2660
	11th to 20th	Night	13% 9% 3 10% 5% 5% 4 4 5% 7% 34%	3285
	21st to end	Night	16% 9% 6% 8% 5% 5% 5% 5% 5% 8% 28%	4628

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	55% 5% 4 19% 14% 3	2837
	11th to 20th	Day	61% 6% 5% 13% 12% 3	2840
	21st to end	Day	57% 3 6% 17% 15% 2	3124
February	1st to 10th	Day	52% 4 8% 20% 13% 3	1710
	11th to 20th	Day	53% 4 7% 17% 15% 3	1674
	21st to end	Day	54% 4 8% 17% 13% 4	1638
March	1st to 10th	Day	52% 6% 8% 21% 11% 2	3435
	11th to 20th	Day	41% 6% 7% 25% 18% 2	3480
	21st to end	Day	54% 4 8% 18% 13% 3	3815
April	1st to 10th	Day	49% 4 11% 18% 15% 4	2180
	11th to 20th	Day	46% 3 9% 21% 17% 3	2254
	21st to end	Day	52% 3 10% 20% 12% 2	2427
May	1st to 10th	Day	58% 5% 10% 20% 7% 1	4120
	11th to 20th	Day	55% 5% 12% 21% 6% 1	4085
	21st to end	Day	46% 6% 15% 19% 11% 3	4489
June	1st to 10th	Day	46% 6% 13% 23% 12%	2617
	11th to 20th	Day	48% 6% 15% 19% 12% 1	2660
	21st to end	Day	44% 6% 14% 23% 11% 1	2539
July	1st to 10th	Day	43% 8% 15% 21% 8% 4	4055
	11th to 20th	Day	41% 7% 15% 28% 9% 1	4051
	21st to end	Day	57% 7% 9% 18% 9% 1	4324
August	1st to 10th	Day	51% 7% 15% 19% 7%	4091
	11th to 20th	Day	51% 6% 10% 22% 10%	4083
	21st to end	Day	38% 7% 14% 27% 13% 1	4228
September	1st to 10th	Day	45% 4 10% 18% 17% 5%	2278
	11th to 20th	Day	44% 3 6% 24% 19% 3	2090
	21st to end	Day	52% 3 7% 20% 15% 3	2087
October	1st to 10th	Day	31% 4 8% 23% 26% 8%	3160
	11th to 20th	Day	35% 3 9% 24% 14% 15%	3140
	21st to end	Day	32% 2 8% 27% 19% 12%	3428
November	1st to 10th	Day	39% 3 8% 23% 22% 5%	1887
	11th to 20th	Day	43% 4 7% 18% 22% 5%	1704
	21st to end	Day	48% 3 8% 20% 19% 2	1520
December	1st to 10th	Day	58% 4 8% 17% 12% 3	1520
	11th to 20th	Day	49% 4 5% 20% 17% 5%	1906
	21st to end	Day	58% 5% 8% 16% 13% 1	3010
January	1st to 10th	Night	57% 9% 4 13% 13% 4	4109
	11th to 20th	Night	54% 3 7% 17% 15% 4	4120
	21st to end	Night	55% 4 6% 18% 16% 2	4532
February	1st to 10th	Night	49% 5% 11% 19% 13% 2	2470
	11th to 20th	Night	55% 5% 7% 17% 12% 4	2401
	21st to end	Night	53% 4 10% 18% 9% 5%	1814
March	1st to 10th	Night	59% 5% 6% 17% 10% 2	3418
	11th to 20th	Night	43% 6% 9% 25% 15% 1	3480
	21st to end	Night	45% 5% 9% 20% 17% 4	3817
April	1st to 10th	Night	49% 6% 10% 19% 14% 2	1999
	11th to 20th	Night	43% 5% 15% 20% 14% 3	1875
	21st to end	Night	45% 5% 13% 24% 12% 1	1767
May	1st to 10th	Night	58% 5% 11% 18% 7% 1	2840
	11th to 20th	Night	55% 7% 12% 21% 5% 1	2813
	21st to end	Night	44% 9% 19% 20% 4 4	3087
June	1st to 10th	Night	38% 8% 16% 27% 11%	1553
	11th to 20th	Night	38% 7% 19% 24% 10% 1	1520
	21st to end	Night	35% 8% 19% 26% 12% 1	1631
July	1st to 10th	Night	37% 9% 20% 21% 11% 2	2795
	11th to 20th	Night	37% 8% 19% 26% 8% 1	2786
	21st to end	Night	51% 8% 14% 20% 8%	2988
August	1st to 10th	Night	49% 8% 17% 21% 6%	2814
	11th to 20th	Night	45% 6% 15% 23% 10% 1	2812
	21st to end	Night	33% 6% 17% 32% 12% 1	3380
September	1st to 10th	Night	41% 3 16% 19% 18% 3	1900
	11th to 20th	Night	39% 4 10% 26% 18% 2	2088
	21st to end	Night	49% 5% 9% 21% 14% 1	2092
October	1st to 10th	Night	39% 4 8% 26% 19% 5%	3800
	11th to 20th	Night	38% 3 9% 17% 18% 14%	3779
	21st to end	Night	31% 4 9% 26% 19% 12%	4120
November	1st to 10th	Night	40% 4 10% 19% 20% 6%	2264
	11th to 20th	Night	46% 5% 6% 18% 20% 5%	2467
	21st to end	Night	48% 4 9% 20% 16% 3	2660
December	1st to 10th	Night	59% 4 8% 16% 11% 3	2660
	11th to 20th	Night	51% 4 8% 15% 19% 2	3288
	21st to end	Night	54% 6% 9% 18% 11% 1	4628

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	44% 21% 23% 7% 3 1	270
	11th to 20th	Day	41% 23% 27% 7% 1	270
	21st to end	Day	41% 23% 27% 8% 2	297
February	1st to 10th	Day	42% 21% 26% 7% 3	270
	11th to 20th	Day	49% 23% 22% 5%	270
	21st to end	Day	45% 22% 23% 9% 1	293
March	1st to 10th	Day	36% 22% 24% 15% 3	460
	11th to 20th	Day	35% 24% 23% 13% 5% 1	460
	21st to end	Day	33% 21% 27% 14% 5%	506
April	1st to 10th	Day	32% 23% 27% 11% 6%	460
	11th to 20th	Day	28% 19% 27% 14% 10% 2	452
	21st to end	Day	33% 18% 28% 13% 7% 3	460
May	1st to 10th	Day	34% 15% 25% 14% 9% 3	463
	11th to 20th	Day	27% 14% 27% 18% 10% 4	487
	21st to end	Day	17% 11% 25% 22% 16% 7%	529
June	1st to 10th	Day	22% 11% 24% 22% 15% 5%	490
	11th to 20th	Day	20% 9% 29% 19% 16% 8%	490
	21st to end	Day	14% 13% 29% 26% 13% 5%	490
July	1st to 10th	Day	15% 6% 26% 24% 19% 10%	485
	11th to 20th	Day	8% 6% 29% 32% 15% 9%	484
	21st to end	Day	16% 5% 24% 28% 19% 9%	532
August	1st to 10th	Day	17% 8% 23% 24% 19% 10%	486
	11th to 20th	Day	17% 5% 23% 26% 17% 12%	479
	21st to end	Day	12% 4 23% 28% 20% 13%	539
September	1st to 10th	Day	18% 5% 19% 27% 19% 12%	490
	11th to 20th	Day	14% 6% 20% 29% 21% 11%	490
	21st to end	Day	30% 11% 21% 21% 14% 3	490
October	1st to 10th	Day	20% 10% 23% 24% 17% 5%	490
	11th to 20th	Day	26% 16% 21% 23% 12% 3	490
	21st to end	Day	27% 15% 30% 18% 10% 1	539
November	1st to 10th	Day	21% 18% 27% 22% 10% 3	467
	11th to 20th	Day	36% 14% 26% 17% 6%	272
	21st to end	Day	31% 14% 34% 17% 2 1	180
December	1st to 10th	Day	33% 22% 27% 14% 2 1	180
	11th to 20th	Day	39% 17% 27% 14% 3 1	180
	21st to end	Day	35% 23% 24% 12% 5%	267
January	1st to 10th	Night	34% 22% 26% 15% 3	760
	11th to 20th	Night	34% 17% 24% 23% 2	760
	21st to end	Night	32% 21% 30% 16% 2	836
February	1st to 10th	Night	28% 20% 27% 23% 3	760
	11th to 20th	Night	36% 25% 24% 13% 1	743
	21st to end	Night	34% 21% 29% 14% 1	628
March	1st to 10th	Night	27% 23% 25% 22% 3	760
	11th to 20th	Night	34% 20% 21% 19% 5% 1	760
	21st to end	Night	26% 22% 25% 21% 5% 1	836
April	1st to 10th	Night	28% 20% 29% 20% 4	760
	11th to 20th	Night	22% 17% 25% 25% 11% 1	751
	21st to end	Night	23% 12% 26% 30% 8% 1	627
May	1st to 10th	Night	25% 8% 22% 28% 16% 1	570
	11th to 20th	Night	26% 8% 19% 31% 14% 1	561
	21st to end	Night	16% 5% 18% 33% 23% 5%	617
June	1st to 10th	Night	14% 4 22% 31% 26% 3	413
	11th to 20th	Night	14% 5% 17% 40% 21% 2	380
	21st to end	Night	12% 4 19% 41% 20% 3	491
July	1st to 10th	Night	13% 3 14% 34% 29% 6%	555
	11th to 20th	Night	9% 3 13% 37% 32% 6%	552
	21st to end	Night	12% 3 12% 37% 33% 4	624
August	1st to 10th	Night	12% 2 12% 38% 33% 3	567
	11th to 20th	Night	15% 3 10% 34% 32% 5%	564
	21st to end	Night	14% 2 12% 33% 33% 6%	779
September	1st to 10th	Night	17% 3 13% 32% 30% 6%	760
	11th to 20th	Night	17% 4 13% 31% 31% 4	760
	21st to end	Night	24% 7% 18% 26% 22% 2	760
October	1st to 10th	Night	24% 8% 15% 27% 21% 5%	760
	11th to 20th	Night	23% 14% 19% 28% 15% 1	760
	21st to end	Night	25% 14% 20% 28% 13% 1	836
November	1st to 10th	Night	18% 13% 24% 31% 13% 2	755
	11th to 20th	Night	27% 16% 21% 26% 9% 2	760
	21st to end	Night	28% 16% 23% 26% 6% 1	760
December	1st to 10th	Night	31% 22% 22% 21% 4	760
	11th to 20th	Night	27% 19% 25% 25% 4	760
	21st to end	Night	28% 15% 26% 27% 4	836

Page generated at 2024-11-01 22:47:39. Version ac07

Copyright © 2024 Attilla Danko.