Elgin
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Elgin clear sky chart from 2005-11-06 to 2025-11-18.

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	7% 9% 3 7% 5% 4 4 4 5% 7% 46%	135846
	Night	7% 10% 3 8% 5% 5% 5% 4 5% 6% 43%	131818
Transparency	Day	37% 9% 12% 20% 16% 7%	130563
	Night	40% 9% 12% 20% 13% 5%	128103
Seeing	Day	12% 17% 29% 25% 13% 4	15687
	Night	13% 12% 22% 33% 18% 2	24809

By Month

Percentage Cloud Cover SW (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	6% 18% 6% 15% 8% 7% 6% 4 6% 7% 15%	3639
	11th to 20th	Day	6% 15% 5% 11% 6% 5% 5% 4 6% 6% 31%	3640
	21st to end	Day	8% 10% 3 10% 6% 5% 5% 4 5% 6% 38%	4004
February	1st to 10th	Day	9% 19% 6% 12% 7% 5% 6% 4 5% 5% 22%	1694
	11th to 20th	Day	9% 20% 5% 12% 7% 5% 5% 4 4 6% 23%	1899
	21st to end	Day	8% 16% 4 12% 8% 6% 7% 4 5% 6% 24%	1650
March	1st to 10th	Day	6% 12% 4 9% 7% 6% 7% 6% 7% 9% 27%	4518
	11th to 20th	Day	5% 12% 5% 11% 7% 6% 6% 4 6% 7% 32%	4659
	21st to end	Day	8% 16% 5% 13% 8% 8% 8% 5% 6% 7% 17%	5969
April	1st to 10th	Day	10% 16% 4 11% 7% 5% 5% 4 5% 7% 26%	2400
	11th to 20th	Day	9% 13% 4 8% 6% 6% 4 4 5% 8% 34%	2373
	21st to end	Day	11% 13% 4 8% 6% 4 5% 4 6% 7% 33%	2600
May	1st to 10th	Day	8% 9% 3 6% 4 4 4 3 5% 6% 47%	6091
	11th to 20th	Day	10% 11% 3 6% 5% 5% 4 4 5% 6% 41%	6277
	21st to end	Day	8% 7% 3 6% 4 4 4 3 6% 8% 48%	6902
June	1st to 10th	Day	13% 8% 3 7% 5% 4 4 4 6% 7% 38%	2800
	11th to 20th	Day	12% 8% 2 6% 5% 4 4 4 5% 7% 43%	2800
	21st to end	Day	5% 4 1 4 3 3 3 3 5% 6% 63%	2800
July	1st to 10th	Day	2 2 1 2 2 2 3 3 4 5% 75%	6319
	11th to 20th	Day	1 2 1 2 2 1 2 2 3 4 82%	6320
	21st to end	Day	2 1 2 2 3 3 3 5% 7% 72%	6880
August	1st to 10th	Day	4 4 2 5% 3 3 4 4 5% 6% 61%	6210
	11th to 20th	Day	3 2 1 3 2 2 2 2 4 5% 73%	5788
	21st to end	Day	4 3 1 4 3 3 4 3 5% 7% 62%	5984
September	1st to 10th	Day	4 2 1 3 3 3 3 3 4 6% 70%	2400
	11th to 20th	Day	7% 5% 2 5% 3 4 3 3 5% 7% 56%	2300
	21st to end	Day	8% 6% 2 5% 4 3 3 4 4 7% 54%	2200
October	1st to 10th	Day	8% 5% 2 4 3 3 2 2 3 5% 63%	3452
	11th to 20th	Day	7% 8% 3 6% 3 4 4 4 6% 9% 45%	3145
	21st to end	Day	11% 12% 4 9% 5% 7% 6% 4 5% 7% 31%	3200
November	1st to 10th	Day	14% 12% 4 10% 6% 6% 5% 4 4 8% 27%	1854
	11th to 20th	Day	10% 17% 5% 10% 7% 7% 6% 6% 7% 7% 19%	1827
	21st to end	Day	8% 15% 5% 9% 6% 5% 6% 5% 7% 8% 28%	1600
December	1st to 10th	Day	5% 10% 4 8% 4 5% 5% 4 5% 8% 43%	2620
	11th to 20th	Day	8% 15% 5% 12% 9% 7% 7% 5% 7% 7% 18%	3028
	21st to end	Day	9% 15% 6% 12% 7% 7% 7% 4 5% 6% 22%	4004
January	1st to 10th	Night	9% 18% 6% 14% 9% 7% 6% 4 6% 7% 14%	6306
	11th to 20th	Night	8% 17% 4 9% 5% 5% 5% 4 5% 6% 30%	6320
	21st to end	Night	7% 11% 4 9% 6% 5% 5% 4 6% 6% 37%	6952
February	1st to 10th	Night	12% 19% 5% 11% 7% 5% 4 4 6% 5% 22%	2705
	11th to 20th	Night	8% 19% 5% 11% 5% 4 5% 4 5% 6% 28%	2388
	21st to end	Night	8% 17% 6% 11% 8% 5% 5% 4 5% 6% 24%	1980
March	1st to 10th	Night	6% 10% 3 8% 5% 6% 6% 5% 8% 9% 35%	5376
	11th to 20th	Night	6% 15% 5% 10% 5% 5% 4 3 4 5% 37%	5325
	21st to end	Night	9% 14% 4 11% 7% 6% 7% 5% 7% 7% 23%	5006
April	1st to 10th	Night	11% 13% 4 10% 5% 6% 5% 4 5% 5% 32%	2000
	11th to 20th	Night	12% 11% 4 7% 5% 5% 4 3 5% 5% 40%	1974
	21st to end	Night	9% 10% 3 7% 4 4 5% 4 7% 6% 41%	1800
May	1st to 10th	Night	9% 8% 2 4 3 3 3 3 4 6% 54%	3875
	11th to 20th	Night	12% 11% 3 6% 3 3 4 3 5% 5% 46%	3616
	21st to end	Night	10% 7% 2 5% 3 3 3 3 4 6% 53%	3972
June	1st to 10th	Night	12% 6% 2 6% 4 4 4 4 6% 9% 43%	1600
	11th to 20th	Night	12% 6% 2 5% 4 4 4 4 5% 6% 47%	1600
	21st to end	Night	5% 5% 1 4 4 3 3 3 5% 6% 60%	1600
July	1st to 10th	Night	2 2 1 2 2 2 3 2 3 5% 77%	3640
	11th to 20th	Night	1 1 1 1 1 2 2 3 3 85%	3640
	21st to end	Night	3 2 1 1 2 2 2 2 4 8% 74%	3970
August	1st to 10th	Night	3 2 1 3 2 3 3 4 6% 7% 66%	3582
	11th to 20th	Night	2 2 1 3 2 3 3 2 4 5% 73%	4044
	21st to end	Night	3 3 1 4 3 3 3 2 4 5% 68%	5015
September	1st to 10th	Night	4 2 1 3 2 3 2 2 4 7% 71%	2000
	11th to 20th	Night	6% 4 1 5% 3 4 5% 3 5% 6% 58%	2100
	21st to end	Night	8% 5% 2 6% 2 3 3 2 4 6% 59%	2200
October	1st to 10th	Night	6% 5% 2 4 3 3 2 3 4 4 63%	3745
	11th to 20th	Night	9% 6% 3 6% 4 5% 4 3 5% 7% 47%	4008
	21st to end	Night	8% 13% 3 8% 5% 5% 5% 4 6% 7% 37%	4600
November	1st to 10th	Night	14% 11% 3 8% 7% 4 6% 5% 5% 6% 30%	2665
	11th to 20th	Night	13% 14% 4 8% 6% 7% 6% 5% 7% 6% 24%	2726
	21st to end	Night	9% 14% 4 9% 6% 5% 6% 5% 6% 6% 30%	2800
December	1st to 10th	Night	5% 10% 4 8% 5% 5% 4 3 6% 10% 41%	4560
	11th to 20th	Night	9% 16% 5% 12% 8% 8% 7% 5% 7% 7% 15%	5176
	21st to end	Night	10% 17% 5% 12% 7% 8% 8% 4 5% 6% 17%	6952

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	62% 14% 10% 12% 1	3639
	11th to 20th	Day	50% 9% 14% 20% 6% 1	3640
	21st to end	Day	46% 10% 9% 19% 15% 1	4004
February	1st to 10th	Day	62% 11% 12% 11% 4	1694
	11th to 20th	Day	61% 11% 10% 11% 7%	1909
	21st to end	Day	59% 12% 11% 11% 6% 1	1650
March	1st to 10th	Day	46% 12% 13% 18% 9% 1	4518
	11th to 20th	Day	45% 12% 12% 26% 5%	4659
	21st to end	Day	59% 15% 12% 12% 2	5969
April	1st to 10th	Day	57% 10% 14% 16% 3	2400
	11th to 20th	Day	48% 10% 14% 16% 10% 1	2376
	21st to end	Day	50% 10% 11% 19% 10%	2600
May	1st to 10th	Day	34% 10% 11% 25% 20% 1	6091
	11th to 20th	Day	40% 10% 15% 22% 11% 2	6280
	21st to end	Day	33% 10% 17% 28% 11% 1	6904
June	1st to 10th	Day	45% 9% 13% 23% 9% 1	2800
	11th to 20th	Day	39% 9% 14% 22% 14% 3	2800
	21st to end	Day	22% 6% 12% 33% 21% 6%	2800
July	1st to 10th	Day	12% 3 7% 17% 29% 32%	6251
	11th to 20th	Day	9% 2 5% 11% 32% 40%	6245
	21st to end	Day	11% 4 6% 27% 34% 19%	6681
August	1st to 10th	Day	18% 4 8% 14% 37% 20%	4524
	11th to 20th	Day	15% 4 7% 29% 34% 12%	4188
	21st to end	Day	17% 5% 11% 28% 26% 14%	4311
September	1st to 10th	Day	17% 5% 11% 24% 29% 15%	2400
	11th to 20th	Day	27% 7% 12% 21% 22% 11%	2300
	21st to end	Day	30% 8% 12% 24% 21% 5%	2200
October	1st to 10th	Day	26% 5% 10% 26% 27% 6%	3452
	11th to 20th	Day	34% 8% 17% 26% 15% 1	3145
	21st to end	Day	50% 9% 10% 22% 8%	3200
November	1st to 10th	Day	54% 10% 13% 17% 6% 1	1854
	11th to 20th	Day	61% 12% 14% 11% 3	1827
	21st to end	Day	52% 12% 14% 13% 8% 1	1600
December	1st to 10th	Day	40% 10% 19% 27% 4	2620
	11th to 20th	Day	59% 14% 16% 9% 1	3028
	21st to end	Day	57% 12% 18% 12% 2	4004
January	1st to 10th	Night	63% 14% 11% 10% 1	6306
	11th to 20th	Night	50% 9% 15% 18% 8%	6320
	21st to end	Night	43% 13% 12% 18% 13% 1	6952
February	1st to 10th	Night	63% 10% 12% 11% 3	2705
	11th to 20th	Night	56% 10% 14% 14% 6% 1	2389
	21st to end	Night	59% 9% 10% 15% 5% 1	1980
March	1st to 10th	Night	40% 10% 14% 22% 13% 1	5376
	11th to 20th	Night	47% 9% 10% 30% 5%	5325
	21st to end	Night	54% 14% 14% 15% 3	5006
April	1st to 10th	Night	52% 11% 15% 18% 4	2000
	11th to 20th	Night	44% 9% 15% 19% 11% 1	1974
	21st to end	Night	43% 12% 14% 19% 12% 1	1800
May	1st to 10th	Night	29% 9% 13% 28% 19% 2	3872
	11th to 20th	Night	37% 9% 13% 29% 12%	3616
	21st to end	Night	31% 10% 16% 32% 11%	3972
June	1st to 10th	Night	37% 11% 12% 27% 12% 1	1600
	11th to 20th	Night	36% 10% 13% 27% 12% 3	1600
	21st to end	Night	24% 7% 13% 30% 20% 6%	1600
July	1st to 10th	Night	11% 4 6% 18% 29% 32%	3600
	11th to 20th	Night	6% 2 5% 15% 28% 42%	3592
	21st to end	Night	11% 2 4 30% 31% 21%	3857
August	1st to 10th	Night	12% 4 8% 17% 43% 15%	2601
	11th to 20th	Night	15% 5% 7% 29% 34% 10%	2920
	21st to end	Night	14% 4 9% 33% 28% 12%	3608
September	1st to 10th	Night	15% 4 11% 27% 28% 16%	2000
	11th to 20th	Night	26% 7% 11% 23% 23% 10%	2100
	21st to end	Night	27% 7% 11% 28% 19% 8%	2200
October	1st to 10th	Night	26% 5% 8% 24% 31% 5%	3745
	11th to 20th	Night	34% 9% 16% 20% 19% 1	4008
	21st to end	Night	42% 10% 10% 25% 11% 1	4600
November	1st to 10th	Night	52% 11% 14% 17% 6% 1	2665
	11th to 20th	Night	57% 10% 14% 15% 4	2726
	21st to end	Night	51% 9% 12% 17% 10% 1	2800
December	1st to 10th	Night	37% 12% 22% 24% 5% 1	4560
	11th to 20th	Night	63% 12% 14% 11% 1	5176
	21st to end	Night	59% 12% 16% 9% 3	6952

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	16% 28% 28% 17% 10% 3	200
	11th to 20th	Day	15% 22% 35% 19% 7% 4	200
	21st to end	Day	16% 17% 34% 24% 8% 1	220
February	1st to 10th	Day	17% 28% 36% 14% 4	294
	11th to 20th	Day	19% 29% 30% 17% 5%	470
	21st to end	Day	21% 23% 30% 21% 5%	414
March	1st to 10th	Day	18% 24% 34% 18% 5%	500
	11th to 20th	Day	16% 23% 29% 20% 9% 1	500
	21st to end	Day	19% 26% 33% 16% 5% 1	550
April	1st to 10th	Day	18% 24% 28% 18% 9% 4	500
	11th to 20th	Day	17% 23% 31% 17% 8% 3	492
	21st to end	Day	15% 19% 30% 25% 10% 2	500
May	1st to 10th	Day	13% 16% 34% 24% 11% 2	503
	11th to 20th	Day	18% 16% 27% 23% 12% 4	528
	21st to end	Day	12% 15% 24% 28% 16% 6%	573
June	1st to 10th	Day	15% 14% 22% 28% 15% 7%	530
	11th to 20th	Day	12% 19% 31% 25% 10% 2	530
	21st to end	Day	4 10% 32% 32% 17% 5%	530
July	1st to 10th	Day	3 8% 30% 35% 20% 5%	526
	11th to 20th	Day	2 11% 33% 32% 18% 5%	524
	21st to end	Day	1 8% 30% 30% 21% 9%	575
August	1st to 10th	Day	4 8% 28% 33% 19% 8%	527
	11th to 20th	Day	3 8% 23% 35% 22% 10%	517
	21st to end	Day	6% 12% 26% 30% 17% 9%	583
September	1st to 10th	Day	4 12% 24% 28% 22% 11%	530
	11th to 20th	Day	8% 11% 25% 29% 20% 7%	530
	21st to end	Day	9% 11% 26% 31% 17% 5%	530
October	1st to 10th	Day	11% 14% 30% 24% 18% 4	530
	11th to 20th	Day	11% 18% 27% 24% 16% 5%	465
	21st to end	Day	15% 20% 25% 28% 9% 4	363
November	1st to 10th	Day	21% 19% 28% 24% 8%	330
	11th to 20th	Day	17% 21% 27% 24% 10% 1	303
	21st to end	Day	18% 15% 33% 27% 9%	200
December	1st to 10th	Day	14% 20% 30% 18% 15% 4	200
	11th to 20th	Day	21% 31% 31% 15% 2 1	200
	21st to end	Day	14% 30% 27% 23% 5% 2	220
January	1st to 10th	Night	18% 19% 31% 20% 11% 1	800
	11th to 20th	Night	18% 17% 26% 27% 11% 1	800
	21st to end	Night	14% 15% 25% 30% 15% 1	880
February	1st to 10th	Night	20% 18% 26% 29% 6% 2	800
	11th to 20th	Night	15% 17% 23% 32% 11% 1	784
	21st to end	Night	18% 20% 26% 24% 10% 2	660
March	1st to 10th	Night	15% 18% 25% 31% 11% 1	800
	11th to 20th	Night	13% 17% 25% 30% 15% 1	800
	21st to end	Night	16% 16% 29% 28% 10% 1	880
April	1st to 10th	Night	14% 14% 26% 29% 16% 2	800
	11th to 20th	Night	12% 14% 28% 28% 15% 2	786
	21st to end	Night	10% 13% 23% 36% 16% 1	600
May	1st to 10th	Night	9% 11% 24% 36% 18% 2	505
	11th to 20th	Night	12% 8% 14% 31% 32% 4	394
	21st to end	Night	13% 8% 15% 34% 27% 2	433
June	1st to 10th	Night	12% 9% 15% 36% 26% 3	400
	11th to 20th	Night	12% 7% 21% 39% 20% 1	400
	21st to end	Night	5% 4 16% 44% 30% 2	400
July	1st to 10th	Night	2 3 13% 44% 35% 2	390
	11th to 20th	Night	1 2 12% 55% 29% 1	388
	21st to end	Night	3 2 10% 49% 34% 3	438
August	1st to 10th	Night	3 2 10% 44% 38% 3	398
	11th to 20th	Night	2 2 12% 42% 38% 5%	585
	21st to end	Night	5% 3 18% 39% 32% 3	880
September	1st to 10th	Night	5% 3 13% 42% 33% 5%	800
	11th to 20th	Night	5% 4 14% 36% 35% 7%	800
	21st to end	Night	10% 5% 18% 37% 27% 5%	800
October	1st to 10th	Night	12% 6% 17% 40% 23% 2	800
	11th to 20th	Night	13% 10% 19% 34% 21% 4	800
	21st to end	Night	14% 11% 22% 35% 15% 3	880
November	1st to 10th	Night	18% 14% 27% 31% 8% 1	820
	11th to 20th	Night	22% 13% 28% 26% 11% 1	828
	21st to end	Night	14% 16% 27% 27% 14% 3	800
December	1st to 10th	Night	14% 16% 27% 30% 12% 2	800
	11th to 20th	Night	17% 19% 31% 24% 9% 1	800
	21st to end	Night	20% 23% 28% 24% 5% 1	880

Page generated at 2025-11-14 21:55:58. Version ac07

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