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 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	13% 6% 2 6% 4 4 4 3 5% 7% 45%	100751
	Night	13% 6% 2 5% 3 3 3 3 5% 7% 50%	98294
Transparency	Day	36% 6% 11% 23% 19% 5%	104397
	Night	33% 5% 11% 24% 21% 6%	102789
Seeing	Day	15% 10% 24% 26% 19% 6%	15407
	Night	15% 10% 26% 30% 17% 2	25186

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% 9% 2 7% 4 3 3 3 3 4 46%	2847
	11th to 20th	Day	13% 11% 3 5% 4 3 3 2 3 5% 49%	2850
	21st to end	Day	25% 7% 2 3 2 2 2 2 4 5% 46%	3135
February	1st to 10th	Day	21% 9% 2 6% 4 3 3 2 4 4 43%	1710
	11th to 20th	Day	16% 8% 3 7% 4 4 3 3 4 6% 41%	1792
	21st to end	Day	20% 9% 3 7% 4 3 4 3 4 6% 38%	1582
March	1st to 10th	Day	23% 9% 2 6% 4 4 3 3 4 5% 39%	3436
	11th to 20th	Day	15% 6% 2 5% 4 3 4 3 4 5% 49%	3480
	21st to end	Day	17% 7% 3 7% 4 4 4 3 4 5% 43%	3844
April	1st to 10th	Day	15% 7% 2 5% 4 4 4 4 5% 5% 44%	2280
	11th to 20th	Day	16% 7% 3 6% 4 4 4 3 5% 6% 42%	2254
	21st to end	Day	18% 7% 2 6% 4 4 4 3 5% 6% 40%	2280
May	1st to 10th	Day	17% 8% 3 7% 4 4 5% 4 5% 6% 36%	3765
	11th to 20th	Day	15% 5% 2 6% 4 4 5% 4 6% 8% 40%	4046
	21st to end	Day	13% 7% 3 7% 5% 5% 5% 4 6% 8% 36%	4654
June	1st to 10th	Day	7% 5% 2 6% 5% 5% 6% 5% 7% 9% 43%	2660
	11th to 20th	Day	6% 4 1 4 6% 5% 5% 4 7% 11% 46%	2660
	21st to end	Day	4 4 1 4 5% 5% 5% 5% 7% 10% 49%	2660
July	1st to 10th	Day	9% 5% 2 5% 5% 4 5% 4 7% 9% 44%	4122
	11th to 20th	Day	6% 3 2 4 3 3 4 4 6% 8% 57%	4120
	21st to end	Day	10% 8% 3 9% 5% 5% 6% 4 7% 8% 35%	4465
August	1st to 10th	Day	4 3 1 4 3 3 4 4 5% 9% 61%	4104
	11th to 20th	Day	11% 5% 2 6% 5% 4 4 4 6% 9% 43%	3742
	21st to end	Day	6% 3 1 4 4 4 5% 5% 7% 8% 53%	4180
September	1st to 10th	Day	11% 6% 2 4 4 4 4 3 5% 7% 48%	2280
	11th to 20th	Day	12% 5% 2 6% 3 4 4 4 5% 8% 46%	2280
	21st to end	Day	9% 5% 2 5% 4 4 4 4 4 7% 53%	1998
October	1st to 10th	Day	10% 4 2 3 3 3 4 3 4 6% 57%	1900
	11th to 20th	Day	14% 4 2 4 3 3 3 2 4 5% 56%	1900
	21st to end	Day	20% 5% 2 5% 3 3 3 3 4 4 49%	2090
November	1st to 10th	Day	13% 5% 2 4 3 3 3 3 4 5% 56%	1888
	11th to 20th	Day	12% 5% 2 5% 3 3 3 3 5% 6% 53%	1758
	21st to end	Day	20% 7% 3 5% 3 2 2 3 4 5% 44%	1658
December	1st to 10th	Day	20% 6% 2 4 3 3 3 2 4 7% 46%	1520
	11th to 20th	Day	17% 7% 2 6% 4 4 3 3 4 4 46%	1918
	21st to end	Day	16% 11% 3 8% 5% 4 4 2 5% 7% 35%	2893
January	1st to 10th	Night	16% 7% 2 6% 4 4 4 2 4 5% 45%	4108
	11th to 20th	Night	16% 6% 2 5% 3 3 3 3 4 5% 52%	4120
	21st to end	Night	24% 7% 2 5% 3 2 2 2 3 4 46%	4532
February	1st to 10th	Night	21% 9% 2 6% 3 3 3 4 3 5% 42%	2470
	11th to 20th	Night	17% 7% 2 6% 3 3 2 3 4 6% 45%	2274
	21st to end	Night	18% 6% 2 7% 3 3 3 3 3 5% 46%	1884
March	1st to 10th	Night	21% 9% 2 6% 3 3 3 3 4 5% 41%	3422
	11th to 20th	Night	15% 7% 2 5% 3 4 3 3 3 5% 51%	3480
	21st to end	Night	13% 7% 2 7% 5% 4 4 3 5% 6% 45%	3786
April	1st to 10th	Night	12% 6% 2 5% 3 3 4 3 5% 8% 49%	1900
	11th to 20th	Night	14% 6% 2 5% 4 4 4 3 4 6% 48%	1875
	21st to end	Night	15% 6% 2 5% 4 4 3 3 5% 7% 46%	1900
May	1st to 10th	Night	20% 6% 2 5% 4 3 4 4 5% 7% 40%	3130
	11th to 20th	Night	15% 7% 2 5% 3 3 3 3 5% 8% 45%	2841
	21st to end	Night	15% 7% 2 7% 4 4 4 4 6% 8% 40%	2908
June	1st to 10th	Night	6% 4 2 5% 3 3 4 4 7% 11% 52%	1520
	11th to 20th	Night	4 3 1 4 3 3 3 4 6% 9% 60%	1520
	21st to end	Night	3 3 1 3 3 4 3 3 8% 10% 59%	1520
July	1st to 10th	Night	7% 3 2 5% 5% 4 4 4 6% 11% 50%	2841
	11th to 20th	Night	5% 3 1 3 2 3 4 3 5% 9% 62%	2850
	21st to end	Night	8% 7% 3 7% 5% 5% 5% 5% 5% 8% 42%	3098
August	1st to 10th	Night	2 2 1 3 3 3 4 3 6% 9% 65%	2836
	11th to 20th	Night	5% 3 2 5% 3 4 4 4 6% 10% 53%	3084
	21st to end	Night	4 2 1 4 4 4 4 4 6% 10% 58%	3476
September	1st to 10th	Night	7% 5% 2 4 3 4 4 4 6% 8% 55%	1900
	11th to 20th	Night	9% 6% 3 5% 4 3 4 4 5% 7% 52%	1900
	21st to end	Night	8% 5% 1 4 3 3 3 3 4 5% 60%	2156
October	1st to 10th	Night	8% 3 1 3 3 3 3 3 5% 7% 62%	2280
	11th to 20th	Night	12% 4 1 4 2 2 2 2 4 5% 60%	2280
	21st to end	Night	17% 6% 2 5% 2 3 3 2 3 4 53%	2508
November	1st to 10th	Night	11% 6% 2 5% 3 3 2 3 3 6% 56%	2263
	11th to 20th	Night	14% 4 1 4 3 3 3 3 5% 5% 56%	2418
	21st to end	Night	17% 6% 2 5% 4 3 3 2 4 6% 49%	2519
December	1st to 10th	Night	18% 7% 2 5% 3 4 3 2 3 5% 46%	2660
	11th to 20th	Night	17% 6% 2 5% 5% 4 4 3 4 4 47%	3279
	21st to end	Night	18% 8% 3 8% 4 5% 4 3 4 6% 38%	4756

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	43% 4 6% 24% 19% 3	2848
	11th to 20th	Day	39% 4 9% 24% 21% 3	2850
	21st to end	Day	42% 4 8% 18% 22% 6%	3135
February	1st to 10th	Day	48% 4 8% 21% 16% 3	1710
	11th to 20th	Day	45% 5% 8% 19% 18% 5%	1802
	21st to end	Day	49% 4 8% 15% 15% 8%	1582
March	1st to 10th	Day	49% 5% 7% 14% 15% 10%	3436
	11th to 20th	Day	37% 6% 10% 23% 19% 5%	3480
	21st to end	Day	44% 5% 9% 21% 20% 1	3844
April	1st to 10th	Day	40% 6% 9% 21% 21% 3	2280
	11th to 20th	Day	43% 6% 9% 21% 16% 7%	2254
	21st to end	Day	45% 6% 9% 22% 16% 2	2280
May	1st to 10th	Day	46% 10% 11% 18% 15% 2	3800
	11th to 20th	Day	38% 11% 14% 24% 12% 1	4050
	21st to end	Day	42% 11% 13% 21% 12% 2	4667
June	1st to 10th	Day	35% 9% 15% 23% 16% 1	2660
	11th to 20th	Day	29% 9% 17% 27% 16% 3	2660
	21st to end	Day	26% 8% 17% 27% 18% 4	2660
July	1st to 10th	Day	33% 7% 17% 28% 13% 2	4058
	11th to 20th	Day	23% 7% 10% 34% 24% 2	4051
	21st to end	Day	45% 7% 15% 24% 8%	4325
August	1st to 10th	Day	20% 6% 17% 32% 20% 6%	4090
	11th to 20th	Day	37% 6% 14% 30% 12% 1	3778
	21st to end	Day	25% 7% 14% 33% 18% 3	4179
September	1st to 10th	Day	35% 7% 11% 22% 18% 7%	2280
	11th to 20th	Day	35% 7% 13% 18% 23% 4	2280
	21st to end	Day	31% 4 9% 22% 28% 5%	1998
October	1st to 10th	Day	17% 3 6% 14% 40% 21%	3160
	11th to 20th	Day	20% 2 3 22% 39% 15%	3140
	21st to end	Day	32% 4 5% 30% 23% 5%	3428
November	1st to 10th	Day	31% 5% 8% 25% 24% 8%	1888
	11th to 20th	Day	32% 4 8% 23% 25% 9%	1758
	21st to end	Day	43% 4 7% 19% 23% 5%	1658
December	1st to 10th	Day	41% 3 9% 24% 18% 5%	1520
	11th to 20th	Day	43% 4 6% 21% 22% 4	1915
	21st to end	Day	46% 7% 11% 17% 15% 5%	2893
January	1st to 10th	Night	40% 5% 7% 21% 24% 3	4108
	11th to 20th	Night	34% 5% 10% 24% 23% 4	4120
	21st to end	Night	43% 4 10% 19% 19% 4	4532
February	1st to 10th	Night	45% 5% 9% 20% 17% 4	2470
	11th to 20th	Night	41% 4 9% 22% 19% 6%	2275
	21st to end	Night	41% 4 9% 19% 19% 8%	1884
March	1st to 10th	Night	45% 7% 8% 13% 17% 10%	3422
	11th to 20th	Night	37% 5% 9% 26% 19% 4	3480
	21st to end	Night	38% 6% 10% 27% 18% 1	3786
April	1st to 10th	Night	34% 5% 11% 23% 20% 7%	1900
	11th to 20th	Night	36% 5% 8% 21% 21% 8%	1875
	21st to end	Night	37% 6% 12% 23% 18% 3	1900
May	1st to 10th	Night	42% 8% 13% 21% 13% 3	3160
	11th to 20th	Night	37% 7% 16% 25% 14% 2	2842
	21st to end	Night	40% 10% 14% 22% 13% 2	2916
June	1st to 10th	Night	24% 9% 14% 31% 20% 2	1520
	11th to 20th	Night	19% 6% 15% 34% 22% 3	1520
	21st to end	Night	19% 8% 14% 30% 22% 7%	1520
July	1st to 10th	Night	29% 7% 19% 25% 19% 1	2796
	11th to 20th	Night	19% 7% 12% 36% 23% 4	2796
	21st to end	Night	40% 6% 15% 27% 12%	2999
August	1st to 10th	Night	15% 6% 18% 36% 22% 3	2826
	11th to 20th	Night	26% 6% 21% 31% 16% 1	3114
	21st to end	Night	23% 5% 14% 32% 23% 3	3476
September	1st to 10th	Night	28% 7% 11% 27% 20% 9%	1900
	11th to 20th	Night	32% 7% 11% 20% 24% 6%	1900
	21st to end	Night	26% 5% 10% 22% 31% 7%	2156
October	1st to 10th	Night	13% 4 7% 15% 36% 26%	3800
	11th to 20th	Night	17% 2 4 16% 40% 21%	3779
	21st to end	Night	29% 3 5% 24% 29% 10%	4120
November	1st to 10th	Night	31% 4 7% 25% 25% 8%	2263
	11th to 20th	Night	30% 3 7% 25% 26% 8%	2418
	21st to end	Night	38% 4 8% 21% 25% 5%	2519
December	1st to 10th	Night	41% 4 9% 20% 21% 5%	2660
	11th to 20th	Night	40% 4 7% 22% 23% 5%	3281
	21st to end	Night	44% 7% 11% 20% 16% 3	4756

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	26% 17% 36% 15% 4	270
	11th to 20th	Day	26% 24% 36% 11% 3	270
	21st to end	Day	20% 24% 37% 16% 3	297
February	1st to 10th	Day	28% 20% 34% 13% 5% 1	270
	11th to 20th	Day	26% 27% 32% 11% 3	270
	21st to end	Day	34% 24% 32% 7% 3	225
March	1st to 10th	Day	24% 19% 35% 17% 5%	270
	11th to 20th	Day	28% 20% 34% 17%	270
	21st to end	Day	26% 23% 36% 11% 3	315
April	1st to 10th	Day	22% 18% 31% 20% 7% 1	460
	11th to 20th	Day	23% 18% 34% 18% 6% 1	452
	21st to end	Day	22% 15% 36% 16% 10% 1	460
May	1st to 10th	Day	20% 17% 32% 22% 9% 1	463
	11th to 20th	Day	16% 8% 29% 30% 14% 3	652
	21st to end	Day	10% 7% 28% 32% 17% 5%	734
June	1st to 10th	Day	9% 4 20% 31% 29% 7%	680
	11th to 20th	Day	6% 3 18% 38% 28% 8%	680
	21st to end	Day	5% 2 25% 35% 25% 8%	680
July	1st to 10th	Day	6% 3 15% 33% 32% 11%	671
	11th to 20th	Day	5% 2 15% 32% 34% 12%	668
	21st to end	Day	8% 2 12% 32% 33% 14%	739
August	1st to 10th	Day	6% 3 12% 38% 32% 10%	676
	11th to 20th	Day	14% 3 11% 29% 30% 13%	492
	21st to end	Day	9% 2 6% 29% 34% 19%	539
September	1st to 10th	Day	12% 2 10% 31% 36% 10%	490
	11th to 20th	Day	12% 2 14% 27% 32% 13%	490
	21st to end	Day	12% 5% 21% 32% 22% 8%	338
October	1st to 10th	Day	7% 5% 25% 37% 20% 5%	300
	11th to 20th	Day	14% 15% 30% 25% 14% 1	300
	21st to end	Day	26% 13% 30% 21% 8% 1	330
November	1st to 10th	Day	15% 11% 33% 28% 13%	279
	11th to 20th	Day	13% 16% 41% 22% 6% 1	270
	21st to end	Day	30% 13% 29% 20% 6% 1	270
December	1st to 10th	Day	24% 18% 31% 17% 9% 1	270
	11th to 20th	Day	25% 16% 38% 15% 6%	270
	21st to end	Day	26% 21% 38% 14% 1	297
January	1st to 10th	Night	22% 17% 33% 23% 5%	950
	11th to 20th	Night	22% 18% 35% 21% 4	950
	21st to end	Night	20% 16% 37% 24% 3	1045
February	1st to 10th	Night	26% 16% 30% 24% 4	950
	11th to 20th	Night	21% 18% 36% 23% 2	799
	21st to end	Night	21% 22% 35% 20% 2	628
March	1st to 10th	Night	21% 13% 31% 30% 4	573
	11th to 20th	Night	20% 14% 34% 28% 5%	570
	21st to end	Night	19% 14% 37% 22% 7%	627
April	1st to 10th	Night	15% 15% 38% 25% 7%	570
	11th to 20th	Night	17% 14% 31% 28% 8% 1	563
	21st to end	Night	17% 9% 39% 25% 9% 1	570
May	1st to 10th	Night	17% 8% 26% 39% 9% 1	570
	11th to 20th	Night	15% 7% 27% 30% 19% 2	561
	21st to end	Night	11% 4 22% 37% 21% 5%	617
June	1st to 10th	Night	7% 4 15% 35% 34% 5%	570
	11th to 20th	Night	4 1 11% 46% 34% 3	570
	21st to end	Night	3 2 15% 37% 38% 5%	570
July	1st to 10th	Night	3 2 9% 35% 41% 10%	555
	11th to 20th	Night	6% 1 8% 33% 42% 10%	552
	21st to end	Night	7% 1 7% 34% 42% 9%	624
August	1st to 10th	Night	3 1 10% 35% 43% 9%	567
	11th to 20th	Night	7% 1 11% 38% 35% 7%	564
	21st to end	Night	6% 1 9% 33% 44% 8%	627
September	1st to 10th	Night	9% 2 10% 36% 41% 3	570
	11th to 20th	Night	11% 2 14% 33% 36% 5%	570
	21st to end	Night	10% 4 17% 33% 32% 3	679
October	1st to 10th	Night	8% 4 19% 33% 31% 4	760
	11th to 20th	Night	13% 9% 27% 33% 17% 1	760
	21st to end	Night	20% 9% 28% 27% 16% 1	836
November	1st to 10th	Night	12% 9% 28% 35% 14% 1	755
	11th to 20th	Night	16% 11% 31% 32% 10%	760
	21st to end	Night	18% 15% 30% 27% 11%	809
December	1st to 10th	Night	22% 11% 35% 25% 6%	950
	11th to 20th	Night	23% 12% 28% 32% 5%	950
	21st to end	Night	24% 15% 31% 25% 5%	1045

Page generated at 2024-11-01 21:47:49. Version ac07

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