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 2025-07-06.

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% 4 4 4 3 5% 7% 45%	119471
	Night	14% 6% 2 5% 3 3 3 3 4 6% 49%	119487
Transparency	Day	37% 6% 10% 23% 19% 5%	123120
	Night	35% 5% 10% 24% 21% 6%	123982
Seeing	Day	15% 10% 25% 26% 18% 6%	16292
	Night	16% 10% 26% 30% 17% 2	26109

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% 8% 2 6% 4 3 3 2 3 4 49%	4077
	11th to 20th	Day	11% 9% 2 5% 4 3 3 2 4 5% 52%	4080
	21st to end	Day	21% 6% 2 4 2 2 2 2 4 5% 49%	4488
February	1st to 10th	Day	22% 9% 2 6% 4 3 3 2 3 4 42%	1800
	11th to 20th	Day	17% 8% 3 7% 5% 4 4 3 4 6% 40%	1889
	21st to end	Day	19% 9% 3 6% 4 4 4 3 4 6% 39%	1663
March	1st to 10th	Day	21% 9% 2 7% 4 3 3 3 4 5% 39%	4936
	11th to 20th	Day	11% 5% 2 5% 4 3 3 4 5% 6% 53%	4980
	21st to end	Day	16% 7% 3 7% 4 4 4 3 4 6% 43%	5506
April	1st to 10th	Day	16% 8% 2 5% 4 4 3 4 5% 5% 44%	2400
	11th to 20th	Day	15% 7% 3 6% 4 4 4 3 5% 6% 43%	2374
	21st to end	Day	18% 7% 2 7% 4 3 4 3 5% 6% 40%	2400
May	1st to 10th	Day	17% 9% 3 7% 4 4 5% 4 6% 7% 34%	5405
	11th to 20th	Day	14% 7% 3 8% 5% 5% 5% 4 6% 7% 37%	5812
	21st to end	Day	18% 7% 3 7% 5% 4 5% 4 6% 7% 35%	6695
June	1st to 10th	Day	7% 5% 2 7% 5% 5% 6% 5% 7% 8% 44%	2800
	11th to 20th	Day	6% 4 1 5% 6% 5% 6% 4 7% 11% 45%	2800
	21st to end	Day	4 4 1 4 5% 4 5% 5% 7% 10% 50%	2800
July	1st to 10th	Day	9% 5% 2 5% 5% 4 5% 4 8% 9% 44%	4188
	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	15% 5% 2 4 3 3 3 3 4 5% 55%	1958
	11th to 20th	Day	12% 5% 2 5% 3 3 3 2 5% 6% 53%	1850
	21st to end	Day	20% 7% 3 5% 3 2 2 3 4 5% 45%	1745
December	1st to 10th	Day	17% 7% 2 5% 3 3 3 2 3 5% 51%	2610
	11th to 20th	Day	18% 7% 3 7% 4 3 3 3 3 4 46%	3008
	21st to end	Day	21% 11% 3 8% 4 4 3 2 4 6% 34%	4148
January	1st to 10th	Night	17% 6% 2 7% 4 3 3 2 3 4 48%	5888
	11th to 20th	Night	12% 5% 2 5% 3 2 3 3 4 5% 57%	5900
	21st to end	Night	21% 8% 2 5% 3 2 2 2 3 5% 47%	6490
February	1st to 10th	Night	21% 9% 2 5% 3 3 3 3 3 5% 41%	2600
	11th to 20th	Night	18% 7% 2 7% 4 3 2 3 4 6% 43%	2396
	21st to end	Night	18% 6% 2 6% 3 3 3 3 3 5% 47%	1980
March	1st to 10th	Night	20% 8% 2 6% 3 3 3 2 4 4 43%	4922
	11th to 20th	Night	11% 7% 2 5% 3 4 3 3 4 6% 52%	4980
	21st to end	Night	13% 6% 2 7% 4 4 4 3 5% 7% 46%	5422
April	1st to 10th	Night	13% 6% 2 5% 3 3 4 3 5% 8% 48%	2000
	11th to 20th	Night	14% 5% 2 5% 4 4 4 3 5% 6% 49%	1975
	21st to end	Night	15% 6% 2 5% 4 4 3 3 5% 7% 45%	2000
May	1st to 10th	Night	20% 7% 2 6% 4 3 5% 4 5% 7% 39%	4490
	11th to 20th	Night	17% 9% 3 7% 4 3 4 3 5% 7% 36%	4084
	21st to end	Night	18% 8% 2 7% 4 4 4 4 5% 7% 37%	4177
June	1st to 10th	Night	6% 5% 2 5% 3 3 4 4 7% 11% 52%	1600
	11th to 20th	Night	5% 3 1 4 3 3 4 4 7% 9% 58%	1600
	21st to end	Night	3 3 1 3 3 4 3 3 8% 10% 59%	1600
July	1st to 10th	Night	7% 3 2 5% 5% 4 4 4 6% 11% 50%	2886
	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	12% 6% 2 5% 3 3 2 3 3 6% 55%	2340
	11th to 20th	Night	14% 4 1 4 3 3 2 3 5% 5% 57%	2546
	21st to end	Night	16% 6% 2 5% 4 3 3 2 4 6% 50%	2652
December	1st to 10th	Night	14% 7% 2 4 3 3 3 2 4 5% 52%	4580
	11th to 20th	Night	20% 6% 2 5% 4 3 3 3 4 4 46%	5199
	21st to end	Night	23% 7% 2 7% 4 4 3 3 4 5% 37%	6812

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	41% 3 9% 26% 19% 2	4078
	11th to 20th	Day	35% 3 7% 25% 25% 5%	4080
	21st to end	Day	39% 3 6% 18% 26% 8%	4488
February	1st to 10th	Day	48% 4 8% 22% 15% 3	1800
	11th to 20th	Day	45% 5% 8% 19% 18% 4	1899
	21st to end	Day	48% 4 8% 16% 17% 8%	1663
March	1st to 10th	Day	49% 4 6% 16% 13% 13%	4936
	11th to 20th	Day	31% 5% 7% 20% 23% 13%	4980
	21st to end	Day	43% 4 8% 26% 19% 1	5508
April	1st to 10th	Day	40% 6% 9% 20% 21% 3	2400
	11th to 20th	Day	42% 5% 9% 21% 16% 7%	2374
	21st to end	Day	45% 6% 10% 22% 16% 2	2400
May	1st to 10th	Day	47% 8% 11% 19% 14% 1	5440
	11th to 20th	Day	43% 9% 13% 23% 11% 1	5816
	21st to end	Day	46% 8% 11% 18% 15% 2	6709
June	1st to 10th	Day	35% 9% 15% 23% 17% 1	2800
	11th to 20th	Day	30% 9% 17% 26% 16% 3	2800
	21st to end	Day	26% 8% 17% 28% 18% 4	2800
July	1st to 10th	Day	33% 7% 17% 28% 13% 2	4124
	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	32% 4 8% 24% 24% 8%	1958
	11th to 20th	Day	31% 4 8% 23% 24% 10%	1850
	21st to end	Day	42% 4 7% 20% 23% 5%	1745
December	1st to 10th	Day	39% 2 7% 27% 21% 3	2610
	11th to 20th	Day	43% 4 7% 23% 20% 3	3005
	21st to end	Day	50% 6% 8% 20% 13% 3	4148
January	1st to 10th	Night	39% 5% 10% 25% 18% 3	5888
	11th to 20th	Night	30% 5% 7% 22% 30% 5%	5900
	21st to end	Night	42% 3 8% 19% 24% 4	6490
February	1st to 10th	Night	45% 5% 9% 20% 17% 4	2600
	11th to 20th	Night	43% 4 9% 21% 18% 5%	2397
	21st to end	Night	41% 3 8% 19% 21% 8%	1980
March	1st to 10th	Night	44% 5% 6% 14% 18% 13%	4922
	11th to 20th	Night	33% 4 7% 24% 19% 12%	4980
	21st to end	Night	38% 5% 10% 27% 18% 2	5422
April	1st to 10th	Night	35% 5% 11% 23% 20% 6%	2000
	11th to 20th	Night	36% 5% 8% 22% 20% 9%	1975
	21st to end	Night	38% 6% 12% 22% 18% 3	2000
May	1st to 10th	Night	44% 7% 14% 22% 11% 2	4520
	11th to 20th	Night	47% 6% 13% 21% 11% 1	4085
	21st to end	Night	46% 7% 12% 18% 13% 4	4185
June	1st to 10th	Night	24% 9% 13% 30% 21% 2	1600
	11th to 20th	Night	22% 6% 15% 32% 21% 3	1600
	21st to end	Night	19% 8% 14% 31% 22% 7%	1600
July	1st to 10th	Night	29% 7% 19% 26% 19% 1	2841
	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	32% 3 7% 24% 24% 8%	2340
	11th to 20th	Night	30% 3 7% 25% 26% 10%	2546
	21st to end	Night	37% 3 8% 22% 25% 5%	2652
December	1st to 10th	Night	36% 3 7% 34% 17% 3	4580
	11th to 20th	Night	43% 3 6% 22% 20% 7%	5201
	21st to end	Night	47% 6% 9% 23% 12% 3	6812

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% 39% 15% 4	300
	11th to 20th	Day	24% 23% 39% 12% 2	300
	21st to end	Day	19% 24% 38% 16% 3	330
February	1st to 10th	Day	30% 18% 33% 13% 5% 1	300
	11th to 20th	Day	27% 26% 33% 10% 3	300
	21st to end	Day	31% 22% 32% 11% 3	249
March	1st to 10th	Day	25% 18% 34% 17% 5%	300
	11th to 20th	Day	27% 20% 36% 16%	300
	21st to end	Day	24% 22% 38% 12% 3	349
April	1st to 10th	Day	23% 17% 32% 20% 7% 1	500
	11th to 20th	Day	23% 18% 34% 18% 6% 1	492
	21st to end	Day	21% 14% 35% 18% 10% 1	500
May	1st to 10th	Day	19% 16% 33% 22% 9% 1	503
	11th to 20th	Day	17% 8% 30% 29% 14% 3	701
	21st to end	Day	12% 7% 28% 32% 17% 5%	789
June	1st to 10th	Day	9% 4 20% 32% 28% 7%	730
	11th to 20th	Day	7% 3 18% 38% 27% 7%	730
	21st to end	Day	5% 2 23% 35% 27% 8%	730
July	1st to 10th	Day	6% 3 14% 33% 32% 12%	697
	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	18% 11% 32% 27% 12%	300
	11th to 20th	Day	13% 16% 42% 22% 5% 2	300
	21st to end	Day	30% 13% 30% 19% 6% 2	300
December	1st to 10th	Day	24% 17% 33% 17% 8% 1	300
	11th to 20th	Day	25% 16% 39% 14% 6%	300
	21st to end	Day	28% 20% 37% 14% 2	330
January	1st to 10th	Night	22% 17% 32% 24% 4	1000
	11th to 20th	Night	21% 19% 36% 21% 4	1000
	21st to end	Night	20% 16% 37% 24% 3	1100
February	1st to 10th	Night	27% 16% 30% 24% 4	1000
	11th to 20th	Night	22% 19% 35% 22% 2	841
	21st to end	Night	20% 22% 34% 22% 2	660
March	1st to 10th	Night	21% 13% 33% 29% 4	603
	11th to 20th	Night	20% 14% 34% 28% 5%	600
	21st to end	Night	19% 15% 37% 23% 7%	660
April	1st to 10th	Night	16% 15% 37% 26% 7%	600
	11th to 20th	Night	17% 14% 32% 28% 8% 1	593
	21st to end	Night	18% 9% 38% 26% 9% 1	600
May	1st to 10th	Night	16% 8% 27% 39% 9% 1	600
	11th to 20th	Night	16% 7% 28% 29% 18% 2	591
	21st to end	Night	11% 4 22% 37% 20% 5%	650
June	1st to 10th	Night	7% 4 15% 37% 34% 5%	600
	11th to 20th	Night	5% 1 12% 46% 33% 4	600
	21st to end	Night	3 2 15% 36% 40% 5%	600
July	1st to 10th	Night	3 2 9% 35% 41% 10%	570
	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	13% 10% 28% 35% 14% 1	780
	11th to 20th	Night	16% 11% 31% 33% 10%	800
	21st to end	Night	17% 16% 29% 27% 10%	852
December	1st to 10th	Night	22% 11% 36% 25% 6%	1000
	11th to 20th	Night	23% 12% 28% 32% 5%	1000
	21st to end	Night	25% 15% 30% 25% 5%	1100

Page generated at 2025-07-03 00:30:16. Version ac07

Copyright © 2025 Attilla Danko.