Effingham
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Effingham clear sky chart from 2005-11-06 to 2026-05-17.

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% 8% 3 7% 4 4 4 4 6% 7% 40%	131813
	Night	14% 8% 3 7% 4 4 4 3 5% 7% 41%	131265
Transparency	Day	41% 5% 11% 25% 15% 2	135385
	Night	42% 4 10% 24% 17% 2	135768
Seeing	Day	16% 8% 26% 26% 17% 6%	17636
	Night	17% 8% 28% 31% 14% 2	27545

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	12% 10% 6% 12% 6% 5% 4 3 5% 6% 33%	4691
	11th to 20th	Day	9% 9% 4 10% 6% 7% 5% 3 6% 7% 35%	4710
	21st to end	Day	18% 15% 4 9% 6% 4 3 3 4 5% 30%	5729
February	1st to 10th	Day	16% 11% 5% 8% 4 4 4 3 6% 7% 33%	2000
	11th to 20th	Day	16% 11% 3 8% 5% 4 4 4 5% 7% 34%	2048
	21st to end	Day	15% 12% 3 9% 5% 4 3 4 5% 7% 35%	1730
March	1st to 10th	Day	18% 13% 2 7% 3 3 3 2 3 5% 41%	4580
	11th to 20th	Day	10% 10% 2 5% 4 3 3 3 5% 7% 47%	4818
	21st to end	Day	16% 10% 3 7% 5% 4 5% 4 6% 7% 33%	6099
April	1st to 10th	Day	15% 8% 3 7% 4 4 4 4 5% 8% 39%	2520
	11th to 20th	Day	16% 8% 3 6% 5% 4 4 3 5% 5% 41%	2494
	21st to end	Day	18% 6% 3 7% 4 4 4 3 5% 6% 39%	2526
May	1st to 10th	Day	18% 8% 2 7% 5% 5% 6% 4 6% 9% 30%	5992
	11th to 20th	Day	17% 7% 2 6% 4 4 4 3 6% 7% 38%	6002
	21st to end	Day	11% 6% 2 7% 5% 5% 6% 5% 7% 9% 37%	6935
June	1st to 10th	Day	9% 6% 2 5% 4 4 5% 4 6% 7% 47%	2800
	11th to 20th	Day	8% 4 2 6% 4 5% 5% 5% 5% 8% 49%	2800
	21st to end	Day	7% 5% 2 5% 5% 5% 5% 5% 7% 9% 45%	2800
July	1st to 10th	Day	9% 4 2 6% 4 4 4 4 7% 10% 45%	6360
	11th to 20th	Day	8% 6% 2 7% 5% 5% 6% 5% 8% 11% 37%	6360
	21st to end	Day	10% 6% 2 7% 5% 5% 6% 6% 8% 10% 36%	6924
August	1st to 10th	Day	8% 4 1 4 3 3 4 3 6% 9% 54%	4099
	11th to 20th	Day	8% 5% 2 7% 5% 5% 5% 4 7% 10% 43%	3849
	21st to end	Day	5% 4 2 5% 4 4 5% 3 6% 10% 53%	4312
September	1st to 10th	Day	12% 5% 2 5% 4 4 4 3 5% 7% 49%	2400
	11th to 20th	Day	8% 4 2 5% 4 4 5% 4 6% 7% 51%	2280
	21st to end	Day	10% 5% 2 6% 4 4 4 3 5% 7% 52%	2200
October	1st to 10th	Day	11% 4 2 4 3 3 3 3 5% 6% 56%	2150
	11th to 20th	Day	14% 5% 2 5% 3 4 5% 3 5% 5% 48%	2000
	21st to end	Day	19% 7% 2 5% 3 3 4 3 4 6% 44%	2045
November	1st to 10th	Day	10% 6% 2 5% 3 3 3 3 5% 5% 53%	1852
	11th to 20th	Day	16% 9% 3 5% 4 4 3 3 4 6% 43%	1890
	21st to end	Day	17% 11% 3 7% 4 4 3 3 5% 5% 38%	1890
December	1st to 10th	Day	10% 9% 3 7% 4 3 3 3 5% 5% 47%	2770
	11th to 20th	Day	14% 11% 5% 11% 4 5% 4 3 4 5% 33%	3088
	21st to end	Day	27% 16% 5% 11% 5% 4 3 3 3 4 19%	4070
January	1st to 10th	Night	14% 11% 5% 9% 3 4 4 3 4 6% 35%	8232
	11th to 20th	Night	12% 11% 5% 11% 5% 5% 6% 4 4 5% 33%	8280
	21st to end	Night	17% 11% 5% 7% 5% 4 4 3 4 6% 32%	8483
February	1st to 10th	Night	17% 11% 4 9% 5% 3 4 3 5% 7% 32%	2618
	11th to 20th	Night	17% 8% 3 8% 5% 4 4 3 4 6% 38%	2458
	21st to end	Night	19% 9% 3 7% 4 3 3 3 4 6% 39%	2076
March	1st to 10th	Night	19% 14% 3 6% 3 4 3 3 4 5% 36%	5493
	11th to 20th	Night	13% 6% 3 6% 4 3 3 2 5% 6% 49%	5406
	21st to end	Night	18% 8% 3 8% 5% 5% 5% 4 5% 6% 34%	5072
April	1st to 10th	Night	13% 7% 3 7% 4 4 5% 3 4 5% 44%	2100
	11th to 20th	Night	16% 7% 3 5% 4 3 4 3 4 7% 44%	2075
	21st to end	Night	17% 8% 3 6% 4 4 4 4 5% 6% 40%	2091
May	1st to 10th	Night	17% 8% 3 7% 5% 4 5% 4 5% 8% 33%	4123
	11th to 20th	Night	15% 7% 2 6% 4 4 4 4 7% 9% 37%	4011
	21st to end	Night	13% 6% 2 7% 4 4 5% 4 5% 9% 39%	3942
June	1st to 10th	Night	9% 6% 2 6% 5% 3 4 3 5% 8% 51%	1600
	11th to 20th	Night	6% 4 1 5% 4 5% 3 4 6% 9% 51%	1600
	21st to end	Night	6% 4 1 6% 4 4 5% 4 7% 11% 47%	1600
July	1st to 10th	Night	10% 6% 2 6% 4 4 4 3 5% 9% 48%	3618
	11th to 20th	Night	6% 5% 2 6% 5% 6% 6% 5% 7% 8% 46%	3617
	21st to end	Night	8% 6% 3 7% 5% 4 6% 5% 8% 10% 40%	3947
August	1st to 10th	Night	5% 4 2 4 4 3 4 4 6% 9% 56%	3049
	11th to 20th	Night	7% 4 2 5% 4 5% 4 4 6% 9% 50%	3200
	21st to end	Night	5% 3 1 4 3 3 4 4 6% 9% 59%	3586
September	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 5% 8% 54%	2000
	11th to 20th	Night	6% 4 2 6% 4 3 4 3 5% 8% 54%	2099
	21st to end	Night	11% 5% 2 5% 4 4 4 3 5% 6% 52%	2200
October	1st to 10th	Night	8% 4 1 4 3 3 3 3 5% 6% 59%	2246
	11th to 20th	Night	14% 5% 2 5% 3 3 3 3 4 6% 54%	2400
	21st to end	Night	18% 8% 2 5% 4 3 3 3 4 6% 43%	2793
November	1st to 10th	Night	11% 7% 2 5% 3 3 3 3 5% 6% 53%	2666
	11th to 20th	Night	14% 7% 2 6% 4 3 3 3 4 6% 48%	2730
	21st to end	Night	20% 9% 4 6% 4 3 3 2 4 6% 39%	2730
December	1st to 10th	Night	14% 9% 3 7% 4 4 3 3 4 5% 45%	4635
	11th to 20th	Night	17% 7% 3 7% 4 4 4 3 5% 7% 38%	5339
	21st to end	Night	30% 15% 5% 7% 4 4 3 3 4 4 21%	7150

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	52% 4 10% 24% 9% 1	4692
	11th to 20th	Day	48% 5% 11% 19% 15% 3	4710
	21st to end	Day	59% 3 7% 19% 9% 4	5729
February	1st to 10th	Day	51% 4 11% 23% 10% 1	2000
	11th to 20th	Day	51% 3 9% 22% 12% 2	2058
	21st to end	Day	50% 4 8% 23% 12% 3	1730
March	1st to 10th	Day	48% 2 5% 21% 19% 4	4580
	11th to 20th	Day	36% 4 11% 31% 16% 2	4818
	21st to end	Day	49% 5% 12% 24% 9% 1	6101
April	1st to 10th	Day	44% 5% 12% 24% 15% 1	2520
	11th to 20th	Day	45% 4 10% 26% 13% 2	2494
	21st to end	Day	46% 3 11% 26% 11% 2	2526
May	1st to 10th	Day	49% 6% 13% 21% 10%	6027
	11th to 20th	Day	45% 5% 11% 26% 12% 1	6006
	21st to end	Day	40% 7% 13% 23% 16% 1	6949
June	1st to 10th	Day	34% 5% 14% 27% 16% 3	2800
	11th to 20th	Day	34% 5% 11% 26% 21% 3	2800
	21st to end	Day	34% 6% 16% 24% 16% 4	2800
July	1st to 10th	Day	33% 6% 11% 30% 18% 2	6291
	11th to 20th	Day	38% 9% 14% 25% 12% 1	6285
	21st to end	Day	39% 10% 18% 24% 9% 1	6728
August	1st to 10th	Day	26% 5% 14% 38% 16% 1	4084
	11th to 20th	Day	34% 6% 16% 27% 16% 1	3885
	21st to end	Day	26% 5% 16% 34% 15% 4	4311
September	1st to 10th	Day	35% 5% 10% 26% 20% 4	2400
	11th to 20th	Day	31% 5% 10% 26% 26% 3	2280
	21st to end	Day	33% 4 7% 27% 26% 4	2200
October	1st to 10th	Day	19% 2 7% 25% 41% 5%	3501
	11th to 20th	Day	26% 4 8% 22% 36% 4	3240
	21st to end	Day	35% 3 7% 25% 28% 2	3283
November	1st to 10th	Day	33% 4 11% 29% 19% 5%	1852
	11th to 20th	Day	43% 3 8% 24% 17% 5%	1890
	21st to end	Day	50% 2 9% 24% 14% 1	1890
December	1st to 10th	Day	39% 4 14% 29% 14%	2770
	11th to 20th	Day	54% 3 9% 24% 8% 1	3085
	21st to end	Day	69% 3 10% 12% 5% 1	4070
January	1st to 10th	Night	51% 4 12% 24% 8%	8233
	11th to 20th	Night	53% 4 8% 17% 15% 3	8280
	21st to end	Night	53% 3 10% 21% 11% 1	8483
February	1st to 10th	Night	53% 3 11% 21% 10% 2	2618
	11th to 20th	Night	49% 2 10% 22% 16% 2	2459
	21st to end	Night	47% 3 10% 24% 14% 2	2076
March	1st to 10th	Night	52% 3 6% 21% 15% 3	5493
	11th to 20th	Night	36% 3 10% 27% 21% 2	5406
	21st to end	Night	49% 5% 10% 26% 8% 1	5072
April	1st to 10th	Night	43% 4 9% 25% 16% 3	2100
	11th to 20th	Night	40% 4 9% 24% 19% 3	2075
	21st to end	Night	45% 4 10% 25% 13% 3	2091
May	1st to 10th	Night	47% 7% 15% 21% 9% 2	4150
	11th to 20th	Night	42% 7% 11% 24% 15% 1	4012
	21st to end	Night	41% 6% 13% 22% 16% 2	3950
June	1st to 10th	Night	33% 4 11% 27% 22% 3	1600
	11th to 20th	Night	29% 5% 11% 26% 23% 6%	1600
	21st to end	Night	31% 6% 14% 24% 18% 7%	1600
July	1st to 10th	Night	33% 5% 10% 26% 23% 3	3580
	11th to 20th	Night	32% 9% 14% 27% 16% 2	3572
	21st to end	Night	35% 9% 18% 24% 13% 1	3832
August	1st to 10th	Night	24% 6% 15% 37% 16% 1	3040
	11th to 20th	Night	30% 6% 13% 30% 19% 1	3230
	21st to end	Night	22% 6% 15% 34% 20% 4	3586
September	1st to 10th	Night	30% 4 8% 30% 22% 6%	2000
	11th to 20th	Night	28% 4 6% 29% 27% 5%	2099
	21st to end	Night	33% 4 7% 23% 26% 6%	2200
October	1st to 10th	Night	18% 2 7% 23% 45% 5%	3675
	11th to 20th	Night	24% 2 7% 26% 36% 4	3899
	21st to end	Night	34% 3 7% 25% 25% 5%	4505
November	1st to 10th	Night	34% 3 10% 25% 23% 5%	2666
	11th to 20th	Night	39% 3 8% 26% 22% 3	2730
	21st to end	Night	48% 2 9% 24% 16% 1	2730
December	1st to 10th	Night	43% 3 11% 27% 16%	4635
	11th to 20th	Night	46% 5% 13% 24% 13%	5341
	21st to end	Night	68% 3 10% 13% 5% 1	7150

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	22% 15% 38% 18% 7%	330
	11th to 20th	Day	24% 21% 33% 18% 3	330
	21st to end	Day	22% 18% 37% 18% 3	363
February	1st to 10th	Day	24% 15% 42% 16% 2	330
	11th to 20th	Day	29% 22% 30% 16% 3	330
	21st to end	Day	25% 16% 38% 18% 3	273
March	1st to 10th	Day	23% 20% 38% 16% 3	330
	11th to 20th	Day	27% 18% 37% 13% 4 1	335
	21st to end	Day	30% 12% 36% 16% 6% 1	594
April	1st to 10th	Day	23% 13% 34% 23% 6% 1	540
	11th to 20th	Day	22% 15% 27% 21% 13% 2	532
	21st to end	Day	22% 9% 36% 22% 10% 2	540
May	1st to 10th	Day	21% 13% 29% 25% 10% 2	543
	11th to 20th	Day	18% 7% 26% 27% 17% 6%	575
	21st to end	Day	10% 6% 28% 30% 18% 8%	789
June	1st to 10th	Day	12% 4 20% 31% 25% 8%	730
	11th to 20th	Day	9% 4 22% 33% 25% 6%	730
	21st to end	Day	7% 4 26% 32% 23% 8%	730
July	1st to 10th	Day	11% 2 13% 29% 26% 18%	721
	11th to 20th	Day	8% 3 14% 34% 28% 12%	718
	21st to end	Day	10% 2 12% 32% 32% 11%	794
August	1st to 10th	Day	10% 2 12% 30% 33% 12%	601
	11th to 20th	Day	9% 3 14% 31% 28% 15%	519
	21st to end	Day	7% 2 12% 30% 34% 16%	583
September	1st to 10th	Day	12% 3 15% 28% 32% 10%	530
	11th to 20th	Day	9% 1 10% 32% 31% 17%	530
	21st to end	Day	13% 6% 21% 25% 23% 12%	530
October	1st to 10th	Day	10% 4 23% 34% 25% 6%	480
	11th to 20th	Day	15% 11% 32% 26% 12% 4	330
	21st to end	Day	28% 12% 33% 23% 4 1	363
November	1st to 10th	Day	13% 12% 36% 22% 14% 3	330
	11th to 20th	Day	20% 15% 34% 21% 8% 2	330
	21st to end	Day	27% 11% 30% 22% 9%	330
December	1st to 10th	Day	19% 15% 41% 17% 5% 3	330
	11th to 20th	Day	23% 12% 38% 21% 5%	330
	21st to end	Day	31% 14% 35% 15% 5%	363
January	1st to 10th	Night	19% 14% 38% 24% 4	1050
	11th to 20th	Night	23% 16% 36% 22% 4	1050
	21st to end	Night	21% 13% 38% 26% 2	1155
February	1st to 10th	Night	23% 13% 35% 27% 2	938
	11th to 20th	Night	21% 16% 35% 26% 2	823
	21st to end	Night	25% 11% 39% 23% 2	692
March	1st to 10th	Night	24% 13% 37% 22% 4	840
	11th to 20th	Night	20% 13% 35% 27% 5% 1	782
	21st to end	Night	26% 8% 32% 29% 6%	693
April	1st to 10th	Night	18% 12% 34% 30% 6%	630
	11th to 20th	Night	19% 11% 30% 29% 10% 1	623
	21st to end	Night	22% 6% 33% 30% 8% 1	630
May	1st to 10th	Night	17% 5% 34% 34% 9%	630
	11th to 20th	Night	15% 4 25% 36% 17% 3	609
	21st to end	Night	10% 3 24% 38% 23% 3	650
June	1st to 10th	Night	11% 2 21% 42% 23% 1	600
	11th to 20th	Night	9% 2 18% 44% 26% 1	600
	21st to end	Night	8% 2 21% 39% 29% 2	600
July	1st to 10th	Night	11% 2 8% 34% 37% 8%	585
	11th to 20th	Night	5% 4 13% 40% 34% 5%	582
	21st to end	Night	8% 2 11% 40% 35% 4	657
August	1st to 10th	Night	7% 3 10% 40% 36% 5%	597
	11th to 20th	Night	7% 2 9% 43% 34% 4	594
	21st to end	Night	7% 2 11% 34% 41% 6%	660
September	1st to 10th	Night	13% 2 11% 38% 33% 5%	600
	11th to 20th	Night	7% 2 13% 37% 36% 5%	699
	21st to end	Night	13% 3 20% 30% 29% 5%	800
October	1st to 10th	Night	10% 4 21% 39% 22% 4	800
	11th to 20th	Night	16% 10% 29% 29% 17% 1	800
	21st to end	Night	21% 11% 33% 29% 6%	880
November	1st to 10th	Night	14% 7% 36% 32% 9% 1	820
	11th to 20th	Night	17% 11% 33% 30% 8%	840
	21st to end	Night	24% 13% 30% 26% 8%	840
December	1st to 10th	Night	23% 10% 36% 26% 5%	991
	11th to 20th	Night	24% 9% 32% 29% 5% 1	1050
	21st to end	Night	32% 11% 33% 21% 3	1155

Page generated at 2026-05-14 00:58:59. Version ac07

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