Greers Ferry
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Greers Ferry clear sky chart from 2005-11-06 to 2026-10-29.

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	12% 6% 2 6% 4 4 4 3 5% 7% 46%	148342
	Night	13% 6% 2 5% 3 3 4 3 5% 7% 49%	144012
Transparency	Day	36% 5% 11% 25% 19% 4	151949
	Night	34% 5% 11% 24% 20% 6%	148486
Seeing	Day	15% 9% 25% 26% 19% 6%	18115
	Night	15% 10% 26% 30% 17% 2	27819

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% 7% 2 6% 4 4 4 3 4 5% 46%	5289
	11th to 20th	Day	10% 8% 3 5% 4 3 3 2 3 5% 55%	5310
	21st to end	Day	19% 6% 2 5% 3 2 2 2 4 5% 50%	5812
February	1st to 10th	Day	21% 9% 2 6% 4 3 3 2 3 4 43%	1890
	11th to 20th	Day	17% 8% 2 7% 5% 4 4 3 4 6% 40%	1986
	21st to end	Day	18% 9% 3 6% 4 3 4 3 4 6% 40%	1744
March	1st to 10th	Day	21% 9% 2 7% 4 4 3 3 4 5% 38%	5046
	11th to 20th	Day	10% 5% 2 5% 4 3 3 4 5% 6% 53%	5090
	21st to end	Day	15% 7% 3 7% 4 4 4 3 4 6% 44%	5628
April	1st to 10th	Day	16% 7% 2 5% 4 4 4 4 6% 5% 44%	2520
	11th to 20th	Day	15% 7% 3 6% 4 5% 4 3 5% 6% 42%	2494
	21st to end	Day	18% 7% 2 7% 4 4 4 3 5% 7% 38%	2520
May	1st to 10th	Day	16% 8% 3 7% 4 4 4 4 5% 6% 38%	7021
	11th to 20th	Day	13% 7% 3 8% 5% 5% 5% 4 6% 8% 38%	7564
	21st to end	Day	19% 8% 3 8% 5% 5% 5% 4 6% 7% 29%	8723
June	1st to 10th	Day	7% 5% 2 7% 5% 5% 6% 5% 6% 9% 42%	2940
	11th to 20th	Day	6% 4 1 5% 6% 5% 6% 4 7% 11% 44%	2940
	21st to end	Day	4 4 1 4 5% 5% 5% 5% 7% 10% 49%	2940
July	1st to 10th	Day	6% 4 2 5% 5% 5% 6% 5% 8% 11% 44%	7682
	11th to 20th	Day	4 3 2 5% 4 4 4 5% 6% 10% 53%	7680
	21st to end	Day	6% 5% 2 7% 5% 5% 6% 4 7% 10% 43%	8337
August	1st to 10th	Day	3 3 1 4 3 4 4 4 6% 9% 59%	6014
	11th to 20th	Day	8% 4 1 5% 4 4 4 4 6% 9% 53%	5514
	21st to end	Day	6% 3 1 4 3 4 4 4 7% 9% 56%	6080
September	1st to 10th	Day	10% 6% 2 4 4 4 4 3 5% 8% 49%	2520
	11th to 20th	Day	11% 5% 2 6% 3 4 4 4 5% 9% 47%	2520
	21st to end	Day	9% 6% 2 5% 4 4 4 4 5% 7% 52%	2208
October	1st to 10th	Day	10% 4 2 4 3 3 4 2 4 6% 58%	2100
	11th to 20th	Day	13% 4 2 4 3 3 3 2 4 5% 58%	2100
	21st to end	Day	19% 5% 2 5% 3 3 3 3 4 4 49%	2280
November	1st to 10th	Day	14% 5% 2 4 3 3 3 3 4 5% 56%	2058
	11th to 20th	Day	12% 5% 2 5% 3 3 3 2 5% 6% 53%	1942
	21st to end	Day	20% 7% 2 5% 3 3 2 3 4 5% 46%	1832
December	1st to 10th	Day	17% 7% 2 5% 3 3 3 2 3 5% 51%	2690
	11th to 20th	Day	18% 6% 2 7% 4 3 3 3 3 4 46%	3088
	21st to end	Day	21% 11% 3 7% 4 4 3 2 4 6% 35%	4240
January	1st to 10th	Night	17% 7% 2 6% 4 3 3 2 4 5% 46%	7634
	11th to 20th	Night	10% 5% 2 5% 3 2 3 2 3 4 61%	7680
	21st to end	Night	21% 8% 2 6% 3 3 3 2 4 4 43%	8402
February	1st to 10th	Night	20% 8% 2 5% 3 3 3 3 3 5% 43%	2730
	11th to 20th	Night	18% 7% 2 6% 4 3 3 3 4 6% 44%	2519
	21st to end	Night	17% 6% 2 6% 3 3 3 3 3 5% 48%	2076
March	1st to 10th	Night	19% 8% 2 6% 4 3 3 2 4 4 43%	5032
	11th to 20th	Night	11% 7% 2 5% 3 4 3 3 5% 6% 52%	5090
	21st to end	Night	13% 6% 2 6% 4 4 4 3 5% 7% 46%	5542
April	1st to 10th	Night	13% 6% 2 5% 3 3 4 3 5% 8% 48%	2100
	11th to 20th	Night	14% 5% 2 5% 4 4 4 3 5% 6% 48%	2075
	21st to end	Night	15% 6% 2 6% 4 4 3 3 5% 7% 45%	2100
May	1st to 10th	Night	18% 7% 2 6% 4 4 4 4 5% 7% 40%	5832
	11th to 20th	Night	15% 9% 3 7% 4 4 4 3 5% 7% 38%	5308
	21st to end	Night	20% 9% 3 8% 4 5% 4 4 5% 7% 31%	5443
June	1st to 10th	Night	6% 5% 2 5% 4 3 4 4 7% 10% 51%	1680
	11th to 20th	Night	5% 3 1 5% 3 4 4 4 6% 9% 56%	1680
	21st to end	Night	3 3 1 4 3 4 3 3 7% 10% 58%	1680
July	1st to 10th	Night	5% 3 2 5% 4 4 5% 5% 6% 11% 49%	5299
	11th to 20th	Night	5% 3 2 4 3 4 5% 4 6% 10% 55%	5307
	21st to end	Night	5% 5% 2 6% 4 4 5% 4 6% 10% 49%	5770
August	1st to 10th	Night	2 3 1 4 3 4 5% 4 6% 9% 59%	4156
	11th to 20th	Night	4 3 2 4 3 3 4 4 6% 9% 59%	4530
	21st to end	Night	4 2 1 4 3 4 4 4 6% 10% 58%	5046
September	1st to 10th	Night	7% 4 2 4 3 3 4 4 5% 8% 56%	2100
	11th to 20th	Night	8% 5% 3 5% 4 3 4 4 5% 7% 53%	2100
	21st to end	Night	8% 5% 1 4 3 3 3 3 5% 5% 59%	2384
October	1st to 10th	Night	8% 3 1 3 3 2 3 3 5% 7% 62%	2520
	11th to 20th	Night	11% 4 1 4 2 2 3 2 4 5% 61%	2520
	21st to end	Night	17% 6% 2 5% 2 3 3 2 4 4 53%	2737
November	1st to 10th	Night	11% 6% 2 5% 3 3 3 3 3 6% 56%	2460
	11th to 20th	Night	13% 4 1 4 3 3 3 3 4 5% 57%	2674
	21st to end	Night	16% 6% 2 5% 4 3 3 2 4 6% 49%	2785
December	1st to 10th	Night	14% 7% 2 4 3 3 3 2 4 5% 51%	4720
	11th to 20th	Night	20% 6% 2 5% 4 3 3 3 4 4 47%	5339
	21st to end	Night	23% 7% 2 6% 4 4 3 3 4 6% 38%	6962

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% 3 9% 28% 16% 2	5290
	11th to 20th	Day	34% 3 6% 23% 29% 6%	5310
	21st to end	Day	39% 3 6% 21% 24% 8%	5812
February	1st to 10th	Day	47% 4 8% 22% 16% 3	1890
	11th to 20th	Day	46% 4 8% 20% 17% 4	1996
	21st to end	Day	47% 3 8% 16% 18% 8%	1744
March	1st to 10th	Day	49% 4 6% 16% 13% 12%	5046
	11th to 20th	Day	31% 5% 7% 21% 23% 13%	5090
	21st to end	Day	42% 4 8% 26% 19% 1	5630
April	1st to 10th	Day	40% 6% 10% 20% 21% 3	2520
	11th to 20th	Day	43% 5% 9% 20% 15% 7%	2494
	21st to end	Day	46% 6% 11% 21% 15% 2	2520
May	1st to 10th	Day	45% 7% 9% 19% 17% 3	7056
	11th to 20th	Day	43% 7% 12% 22% 13% 3	7568
	21st to end	Day	51% 8% 12% 15% 12% 1	8737
June	1st to 10th	Day	36% 9% 14% 24% 16% 1	2940
	11th to 20th	Day	32% 9% 17% 25% 15% 3	2940
	21st to end	Day	27% 7% 17% 27% 18% 4	2940
July	1st to 10th	Day	32% 6% 18% 31% 12% 1	7618
	11th to 20th	Day	25% 7% 19% 33% 15% 1	7611
	21st to end	Day	36% 6% 16% 32% 11%	8155
August	1st to 10th	Day	21% 5% 15% 31% 23% 4	6000
	11th to 20th	Day	29% 5% 13% 36% 17% 2	5550
	21st to end	Day	24% 6% 16% 31% 21% 3	6079
September	1st to 10th	Day	33% 7% 11% 23% 19% 7%	2520
	11th to 20th	Day	34% 6% 13% 21% 22% 4	2520
	21st to end	Day	32% 4 10% 22% 27% 4	2208
October	1st to 10th	Day	17% 3 6% 14% 40% 20%	3360
	11th to 20th	Day	19% 2 4 22% 39% 15%	3340
	21st to end	Day	33% 4 5% 30% 23% 5%	3618
November	1st to 10th	Day	32% 4 7% 25% 24% 7%	2058
	11th to 20th	Day	31% 4 8% 24% 24% 9%	1942
	21st to end	Day	42% 4 7% 21% 22% 4	1832
December	1st to 10th	Day	39% 2 7% 27% 21% 3	2690
	11th to 20th	Day	43% 4 7% 24% 20% 3	3085
	21st to end	Day	49% 6% 8% 21% 13% 3	4240
January	1st to 10th	Night	40% 4 9% 30% 15% 2	7635
	11th to 20th	Night	27% 4 6% 20% 35% 8%	7680
	21st to end	Night	45% 3 7% 18% 22% 5%	8402
February	1st to 10th	Night	44% 5% 9% 21% 17% 4	2730
	11th to 20th	Night	43% 4 9% 22% 18% 5%	2520
	21st to end	Night	40% 3 9% 18% 21% 9%	2076
March	1st to 10th	Night	44% 5% 6% 14% 18% 13%	5032
	11th to 20th	Night	33% 4 8% 25% 19% 12%	5090
	21st to end	Night	37% 5% 10% 27% 18% 2	5542
April	1st to 10th	Night	35% 5% 11% 24% 20% 6%	2100
	11th to 20th	Night	36% 5% 9% 21% 20% 8%	2075
	21st to end	Night	38% 6% 12% 22% 18% 3	2100
May	1st to 10th	Night	44% 5% 13% 21% 14% 2	5862
	11th to 20th	Night	45% 5% 13% 21% 12% 4	5309
	21st to end	Night	53% 7% 13% 15% 10% 3	5454
June	1st to 10th	Night	25% 9% 13% 30% 21% 2	1680
	11th to 20th	Night	23% 6% 15% 32% 20% 3	1680
	21st to end	Night	20% 8% 14% 31% 21% 6%	1680
July	1st to 10th	Night	29% 6% 20% 26% 19% 1	5256
	11th to 20th	Night	24% 7% 19% 33% 15% 2	5256
	21st to end	Night	32% 5% 18% 32% 13%	5641
August	1st to 10th	Night	21% 5% 17% 32% 22% 2	4146
	11th to 20th	Night	21% 5% 17% 37% 17% 3	4560
	21st to end	Night	23% 5% 15% 30% 24% 3	5046
September	1st to 10th	Night	27% 6% 11% 28% 21% 8%	2100
	11th to 20th	Night	31% 6% 11% 23% 23% 5%	2100
	21st to end	Night	26% 5% 11% 22% 30% 7%	2384
October	1st to 10th	Night	14% 3 6% 14% 36% 25%	4040
	11th to 20th	Night	17% 2 4 16% 39% 21%	4019
	21st to end	Night	30% 3 5% 24% 29% 10%	4349
November	1st to 10th	Night	31% 3 8% 24% 25% 8%	2460
	11th to 20th	Night	29% 3 7% 27% 25% 9%	2674
	21st to end	Night	38% 3 8% 22% 24% 5%	2785
December	1st to 10th	Night	36% 3 8% 33% 17% 3	4720
	11th to 20th	Night	42% 3 6% 22% 20% 7%	5341
	21st to end	Night	46% 5% 9% 23% 13% 3	6962

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	25% 16% 38% 16% 4	330
	11th to 20th	Day	23% 23% 40% 12% 2	330
	21st to end	Day	19% 23% 40% 15% 3	363
February	1st to 10th	Day	29% 17% 35% 14% 5% 1	330
	11th to 20th	Day	27% 25% 34% 11% 3	330
	21st to end	Day	30% 22% 33% 11% 4 1	273
March	1st to 10th	Day	25% 18% 35% 18% 5%	330
	11th to 20th	Day	26% 20% 36% 17%	330
	21st to end	Day	22% 21% 40% 14% 3	383
April	1st to 10th	Day	23% 16% 31% 21% 8% 1	540
	11th to 20th	Day	22% 18% 35% 17% 6% 1	532
	21st to end	Day	20% 15% 36% 18% 10% 1	540
May	1st to 10th	Day	19% 16% 33% 22% 9% 1	543
	11th to 20th	Day	17% 8% 30% 28% 14% 3	750
	21st to end	Day	12% 7% 27% 32% 16% 5%	844
June	1st to 10th	Day	9% 4 21% 32% 27% 7%	780
	11th to 20th	Day	7% 3 19% 37% 26% 7%	780
	21st to end	Day	6% 2 24% 35% 26% 8%	780
July	1st to 10th	Day	6% 2 14% 32% 33% 13%	771
	11th to 20th	Day	5% 2 14% 32% 35% 12%	768
	21st to end	Day	8% 2 11% 33% 33% 14%	849
August	1st to 10th	Day	6% 3 11% 38% 32% 10%	776
	11th to 20th	Day	12% 3 10% 29% 31% 16%	574
	21st to end	Day	9% 1 7% 30% 34% 17%	627
September	1st to 10th	Day	11% 2 9% 30% 38% 11%	570
	11th to 20th	Day	11% 2 13% 25% 34% 16%	570
	21st to end	Day	12% 4 19% 34% 23% 8%	402
October	1st to 10th	Day	8% 4 24% 37% 21% 6%	360
	11th to 20th	Day	13% 13% 29% 26% 18% 2	360
	21st to end	Day	27% 12% 27% 21% 11% 2	387
November	1st to 10th	Day	17% 11% 32% 28% 12%	330
	11th to 20th	Day	13% 15% 42% 23% 5% 2	330
	21st to end	Day	29% 13% 30% 20% 6% 2	330
December	1st to 10th	Day	24% 17% 34% 17% 7% 1	330
	11th to 20th	Day	24% 15% 40% 15% 6%	330
	21st to end	Day	27% 20% 36% 15% 2	363
January	1st to 10th	Night	22% 16% 32% 24% 4	1050
	11th to 20th	Night	20% 19% 37% 21% 3	1050
	21st to end	Night	20% 16% 37% 24% 3	1155
February	1st to 10th	Night	25% 15% 30% 25% 4	1050
	11th to 20th	Night	22% 19% 35% 22% 2	884
	21st to end	Night	20% 22% 33% 23% 2	692
March	1st to 10th	Night	21% 12% 34% 29% 4	633
	11th to 20th	Night	20% 15% 34% 27% 4	630
	21st to end	Night	18% 14% 37% 23% 7%	693
April	1st to 10th	Night	16% 14% 36% 27% 7%	630
	11th to 20th	Night	17% 14% 32% 29% 8%	623
	21st to end	Night	17% 8% 38% 26% 10%	630
May	1st to 10th	Night	16% 8% 27% 40% 9%	630
	11th to 20th	Night	16% 7% 28% 29% 18% 2	621
	21st to end	Night	12% 5% 21% 37% 21% 5%	683
June	1st to 10th	Night	7% 4 15% 36% 34% 4	630
	11th to 20th	Night	5% 1 13% 46% 32% 3	630
	21st to end	Night	4 2 15% 36% 38% 5%	630
July	1st to 10th	Night	4 3 8% 33% 42% 10%	615
	11th to 20th	Night	6% 1 7% 31% 44% 10%	612
	21st to end	Night	7% 1 7% 32% 43% 10%	690
August	1st to 10th	Night	3 10% 35% 43% 8%	627
	11th to 20th	Night	7% 1 10% 37% 38% 7%	624
	21st to end	Night	6% 1 8% 34% 43% 8%	693
September	1st to 10th	Night	8% 2 10% 35% 41% 4	630
	11th to 20th	Night	10% 2 13% 30% 39% 7%	630
	21st to end	Night	9% 4 17% 35% 32% 3	751
October	1st to 10th	Night	9% 4 18% 34% 31% 4	840
	11th to 20th	Night	13% 8% 25% 33% 20% 1	840
	21st to end	Night	20% 8% 27% 27% 17% 1	913
November	1st to 10th	Night	12% 10% 28% 35% 14% 1	820
	11th to 20th	Night	15% 10% 31% 34% 10%	840
	21st to end	Night	18% 16% 29% 27% 10%	895
December	1st to 10th	Night	21% 12% 36% 25% 6%	1050
	11th to 20th	Night	23% 12% 30% 31% 5%	1050
	21st to end	Night	25% 15% 30% 25% 5%	1155

Page generated at 2026-10-26 03:37:47. Version ac07

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