De Kalb
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the De Kalb clear sky chart from 2005-11-06 to 2025-10-19.

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% 9% 3 8% 5% 5% 5% 4 6% 7% 37%	126746
	Night	13% 10% 3 8% 5% 4 4 4 5% 6% 39%	123234
Transparency	Day	44% 5% 12% 24% 14% 2	130266
	Night	44% 4 11% 23% 15% 2	127788
Seeing	Day	16% 10% 28% 26% 15% 5%	17165
	Night	17% 9% 31% 30% 12% 1	26946

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	10% 11% 6% 15% 7% 5% 6% 4 5% 6% 24%	3617
	11th to 20th	Day	9% 10% 6% 14% 5% 4 4 5% 8% 5% 29%	3620
	21st to end	Day	13% 15% 7% 9% 4 4 4 3 5% 6% 30%	4188
February	1st to 10th	Day	13% 15% 4 10% 7% 6% 5% 3 6% 6% 26%	1800
	11th to 20th	Day	12% 11% 4 9% 6% 5% 4 4 5% 6% 35%	1947
	21st to end	Day	13% 12% 5% 11% 4 5% 3 4 5% 6% 31%	1650
March	1st to 10th	Day	16% 11% 3 7% 3 3 3 3 3 6% 42%	4480
	11th to 20th	Day	10% 6% 2 7% 5% 5% 5% 4 6% 8% 42%	4718
	21st to end	Day	19% 12% 6% 13% 6% 5% 5% 3 5% 5% 21%	5962
April	1st to 10th	Day	17% 13% 4 8% 6% 5% 4 4 5% 7% 30%	2400
	11th to 20th	Day	16% 9% 3 7% 5% 5% 5% 4 6% 7% 34%	2374
	21st to end	Day	19% 9% 3 8% 5% 5% 5% 4 5% 6% 33%	2473
May	1st to 10th	Day	15% 8% 3 6% 5% 5% 5% 4 6% 8% 35%	5862
	11th to 20th	Day	14% 6% 2 6% 4 5% 5% 4 5% 7% 42%	6307
	21st to end	Day	13% 7% 2 7% 5% 5% 5% 4 6% 10% 37%	6935
June	1st to 10th	Day	10% 5% 2 6% 5% 4 5% 4 6% 8% 46%	2800
	11th to 20th	Day	9% 5% 2 6% 4 4 4 4 6% 9% 45%	2800
	21st to end	Day	10% 7% 2 7% 5% 4 6% 5% 7% 8% 39%	2800
July	1st to 10th	Day	9% 7% 3 7% 5% 5% 5% 5% 8% 10% 37%	6360
	11th to 20th	Day	8% 6% 2 6% 5% 5% 5% 5% 7% 9% 42%	6360
	21st to end	Day	9% 7% 3 8% 5% 5% 6% 5% 6% 8% 38%	6924
August	1st to 10th	Day	8% 6% 2 6% 5% 4 5% 4 7% 9% 43%	4479
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 4 7% 10% 43%	3849
	21st to end	Day	7% 5% 2 6% 5% 4 4 4 6% 9% 48%	4312
September	1st to 10th	Day	11% 4 2 6% 4 4 5% 4 5% 7% 48%	2400
	11th to 20th	Day	12% 4 2 4 3 3 4 4 5% 7% 54%	2302
	21st to end	Day	10% 6% 2 7% 5% 4 4 3 6% 8% 46%	2200
October	1st to 10th	Day	12% 6% 2 5% 3 4 4 4 5% 7% 48%	2100
	11th to 20th	Day	17% 10% 3 7% 5% 4 4 3 6% 6% 37%	1980
	21st to end	Day	20% 9% 3 6% 4 4 4 3 4 6% 37%	1900
November	1st to 10th	Day	11% 10% 3 6% 4 4 4 3 5% 6% 44%	1762
	11th to 20th	Day	15% 13% 4 9% 5% 4 4 4 5% 7% 31%	1800
	21st to end	Day	17% 12% 4 10% 5% 4 4 3 4 5% 34%	1685
December	1st to 10th	Day	8% 9% 4 9% 5% 3 4 3 4 6% 45%	2610
	11th to 20th	Day	11% 15% 5% 12% 6% 6% 6% 5% 6% 7% 23%	3008
	21st to end	Day	21% 23% 9% 12% 6% 4 4 3 3 3 12%	3982
January	1st to 10th	Night	10% 12% 4 14% 7% 5% 5% 3 4 4 32%	6348
	11th to 20th	Night	12% 14% 6% 14% 6% 5% 4 5% 7% 4 23%	6360
	21st to end	Night	15% 14% 6% 9% 4 4 4 3 4 5% 31%	6788
February	1st to 10th	Night	14% 14% 4 8% 5% 5% 4 3 5% 5% 33%	2600
	11th to 20th	Night	13% 11% 3 8% 6% 4 4 3 4 7% 38%	2338
	21st to end	Night	15% 14% 4 8% 4 4 3 2 5% 6% 36%	1980
March	1st to 10th	Night	16% 9% 4 6% 4 4 4 3 4 5% 42%	5373
	11th to 20th	Night	10% 7% 3 6% 4 3 4 4 5% 7% 48%	5284
	21st to end	Night	19% 11% 6% 10% 5% 5% 4 3 4 5% 27%	4967
April	1st to 10th	Night	17% 9% 3 7% 4 4 4 4 4 5% 40%	2000
	11th to 20th	Night	18% 9% 3 5% 4 4 4 3 4 6% 39%	1975
	21st to end	Night	16% 9% 3 7% 5% 4 4 3 5% 6% 38%	1925
May	1st to 10th	Night	16% 8% 3 7% 4 4 4 4 5% 7% 37%	4033
	11th to 20th	Night	11% 6% 1 5% 3 4 5% 4 7% 10% 43%	3605
	21st to end	Night	14% 8% 2 7% 4 4 4 4 6% 7% 40%	3942
June	1st to 10th	Night	9% 5% 2 6% 4 4 4 4 7% 8% 48%	1600
	11th to 20th	Night	8% 5% 2 6% 4 5% 4 5% 7% 9% 46%	1600
	21st to end	Night	8% 5% 3 6% 6% 4 6% 5% 6% 7% 44%	1600
July	1st to 10th	Night	10% 6% 2 6% 5% 5% 6% 4 6% 9% 41%	3618
	11th to 20th	Night	6% 6% 2 6% 4 4 5% 3 6% 9% 49%	3617
	21st to end	Night	9% 6% 2 7% 5% 5% 5% 4 7% 8% 43%	3947
August	1st to 10th	Night	5% 4 2 6% 4 5% 5% 5% 7% 8% 51%	2675
	11th to 20th	Night	7% 6% 2 6% 4 4 5% 4 6% 8% 48%	3200
	21st to end	Night	8% 4 2 4 3 3 4 4 6% 10% 52%	3586
September	1st to 10th	Night	9% 5% 2 4 4 3 3 3 5% 7% 56%	2000
	11th to 20th	Night	9% 4 1 5% 3 4 4 3 5% 8% 54%	2077
	21st to end	Night	10% 5% 2 5% 5% 4 4 4 5% 8% 48%	2200
October	1st to 10th	Night	11% 5% 2 6% 4 4 4 3 5% 5% 52%	2298
	11th to 20th	Night	16% 8% 2 5% 4 3 4 3 4 5% 46%	2377
	21st to end	Night	19% 10% 3 6% 4 4 4 3 5% 7% 37%	2697
November	1st to 10th	Night	13% 8% 2 5% 3 3 3 3 5% 5% 49%	2536
	11th to 20th	Night	15% 9% 3 9% 5% 4 4 3 5% 7% 36%	2600
	21st to end	Night	16% 11% 4 9% 4 4 4 3 4 5% 36%	2713
December	1st to 10th	Night	10% 8% 3 7% 5% 4 4 4 5% 7% 43%	4580
	11th to 20th	Night	15% 15% 4 9% 5% 4 4 3 4 5% 31%	5199
	21st to end	Night	20% 20% 7% 10% 6% 4 4 4 4 5% 16%	6996

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	59% 3 12% 17% 8% 1	3618
	11th to 20th	Day	53% 3 12% 11% 16% 5%	3620
	21st to end	Day	55% 3 15% 18% 9%	4188
February	1st to 10th	Day	57% 4 12% 16% 9% 2	1800
	11th to 20th	Day	49% 4 11% 24% 10% 2	1957
	21st to end	Day	53% 6% 11% 20% 10% 2	1650
March	1st to 10th	Day	45% 4 10% 26% 12% 3	4480
	11th to 20th	Day	38% 5% 12% 32% 12% 1	4718
	21st to end	Day	64% 4 11% 13% 7% 1	5964
April	1st to 10th	Day	54% 5% 11% 21% 9%	2400
	11th to 20th	Day	48% 6% 10% 23% 11% 2	2374
	21st to end	Day	51% 5% 12% 18% 12% 1	2473
May	1st to 10th	Day	45% 5% 11% 23% 15% 1	5897
	11th to 20th	Day	41% 6% 10% 26% 17% 1	6311
	21st to end	Day	41% 5% 12% 25% 13% 3	6949
June	1st to 10th	Day	34% 6% 14% 27% 17% 2	2800
	11th to 20th	Day	35% 5% 12% 25% 20% 3	2800
	21st to end	Day	40% 7% 13% 24% 14% 3	2800
July	1st to 10th	Day	40% 6% 13% 26% 13% 1	6291
	11th to 20th	Day	36% 6% 14% 28% 15% 1	6285
	21st to end	Day	41% 6% 16% 20% 14% 2	6728
August	1st to 10th	Day	35% 6% 17% 29% 12% 1	4464
	11th to 20th	Day	34% 5% 12% 34% 15% 1	3885
	21st to end	Day	32% 5% 12% 31% 17% 2	4311
September	1st to 10th	Day	35% 5% 12% 26% 18% 4	2400
	11th to 20th	Day	30% 4 11% 29% 23% 3	2302
	21st to end	Day	37% 4 10% 28% 20% 2	2200
October	1st to 10th	Day	25% 4 7% 31% 33% 1	3425
	11th to 20th	Day	32% 3 11% 26% 26% 2	3220
	21st to end	Day	43% 3 10% 27% 14% 3	3112
November	1st to 10th	Day	41% 4 13% 25% 15% 2	1762
	11th to 20th	Day	52% 4 12% 21% 10% 1	1800
	21st to end	Day	54% 4 11% 23% 8%	1685
December	1st to 10th	Day	41% 4 17% 30% 8% 1	2610
	11th to 20th	Day	59% 3 12% 17% 7% 2	3005
	21st to end	Day	78% 3 6% 8% 4	3982
January	1st to 10th	Night	55% 3 10% 20% 11% 1	6348
	11th to 20th	Night	61% 3 10% 10% 15% 1	6360
	21st to end	Night	56% 2 12% 17% 11% 1	6788
February	1st to 10th	Night	53% 4 11% 18% 13% 1	2600
	11th to 20th	Night	47% 4 10% 23% 14% 1	2339
	21st to end	Night	49% 4 12% 19% 12% 4	1980
March	1st to 10th	Night	46% 2 11% 24% 12% 5%	5373
	11th to 20th	Night	36% 4 11% 32% 14% 3	5284
	21st to end	Night	58% 3 11% 18% 9% 1	4967
April	1st to 10th	Night	45% 4 12% 23% 14% 2	2000
	11th to 20th	Night	46% 3 9% 22% 17% 4	1975
	21st to end	Night	46% 5% 11% 25% 10% 3	1925
May	1st to 10th	Night	46% 4 12% 18% 16% 3	4060
	11th to 20th	Night	35% 7% 16% 24% 14% 4	3606
	21st to end	Night	41% 6% 11% 24% 16% 3	3950
June	1st to 10th	Night	33% 5% 13% 27% 20% 3	1600
	11th to 20th	Night	33% 6% 13% 25% 22% 3	1600
	21st to end	Night	37% 7% 14% 22% 17% 3	1600
July	1st to 10th	Night	40% 5% 13% 24% 15% 2	3580
	11th to 20th	Night	31% 5% 13% 30% 20% 1	3572
	21st to end	Night	37% 7% 15% 22% 17% 3	3832
August	1st to 10th	Night	29% 6% 15% 30% 18% 1	2666
	11th to 20th	Night	31% 6% 14% 31% 18% 1	3230
	21st to end	Night	28% 5% 13% 33% 18% 3	3586
September	1st to 10th	Night	29% 5% 11% 29% 24% 3	2000
	11th to 20th	Night	28% 4 9% 34% 23% 2	2077
	21st to end	Night	34% 4 12% 29% 19% 2	2200
October	1st to 10th	Night	23% 3 7% 35% 30% 2	3753
	11th to 20th	Night	34% 3 7% 25% 29% 3	3876
	21st to end	Night	42% 3 8% 26% 18% 3	4435
November	1st to 10th	Night	37% 5% 9% 28% 19% 3	2536
	11th to 20th	Night	48% 3 12% 22% 13% 2	2600
	21st to end	Night	50% 4 11% 23% 12%	2713
December	1st to 10th	Night	41% 3 14% 29% 11% 2	4580
	11th to 20th	Night	56% 4 12% 18% 10% 1	5201
	21st to end	Night	69% 4 9% 11% 6%	6996

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	20% 12% 34% 25% 8% 1	300
	11th to 20th	Day	17% 16% 42% 19% 4 1	300
	21st to end	Day	22% 13% 35% 23% 5% 1	330
February	1st to 10th	Day	17% 17% 39% 20% 6% 1	300
	11th to 20th	Day	21% 16% 37% 22% 4	300
	21st to end	Day	22% 19% 39% 17% 3	249
March	1st to 10th	Day	22% 16% 37% 21% 3	300
	11th to 20th	Day	22% 18% 37% 15% 8% 1	300
	21st to end	Day	27% 14% 31% 19% 6% 2	545
April	1st to 10th	Day	24% 12% 36% 23% 5%	500
	11th to 20th	Day	20% 18% 30% 20% 9% 2	492
	21st to end	Day	21% 14% 31% 23% 8% 2	500
May	1st to 10th	Day	20% 12% 30% 26% 10% 2	503
	11th to 20th	Day	20% 12% 26% 26% 14% 4	713
	21st to end	Day	16% 10% 27% 24% 16% 7%	789
June	1st to 10th	Day	10% 7% 29% 29% 18% 6%	730
	11th to 20th	Day	10% 6% 29% 31% 18% 5%	730
	21st to end	Day	12% 9% 28% 31% 15% 5%	730
July	1st to 10th	Day	9% 4 18% 30% 22% 16%	721
	11th to 20th	Day	11% 4 22% 31% 22% 11%	718
	21st to end	Day	7% 4 21% 34% 26% 7%	794
August	1st to 10th	Day	9% 2 19% 32% 27% 12%	715
	11th to 20th	Day	11% 6% 17% 26% 23% 17%	519
	21st to end	Day	7% 4 17% 32% 30% 11%	583
September	1st to 10th	Day	12% 5% 20% 29% 25% 8%	530
	11th to 20th	Day	11% 3 17% 28% 28% 13%	530
	21st to end	Day	12% 10% 25% 26% 19% 7%	530
October	1st to 10th	Day	14% 6% 28% 27% 20% 6%	430
	11th to 20th	Day	20% 16% 32% 23% 9%	324
	21st to end	Day	31% 12% 30% 20% 5% 1	330
November	1st to 10th	Day	16% 13% 36% 22% 10% 3	300
	11th to 20th	Day	23% 17% 34% 18% 6% 2	300
	21st to end	Day	25% 11% 34% 25% 5%	300
December	1st to 10th	Day	17% 15% 40% 21% 6% 1	300
	11th to 20th	Day	17% 14% 39% 21% 8%	300
	21st to end	Day	26% 17% 38% 16% 3	330
January	1st to 10th	Night	18% 13% 40% 24% 5%	1000
	11th to 20th	Night	19% 14% 38% 23% 5% 1	1000
	21st to end	Night	20% 12% 35% 29% 4	1100
February	1st to 10th	Night	21% 13% 39% 23% 5%	1000
	11th to 20th	Night	20% 14% 35% 26% 4	783
	21st to end	Night	22% 12% 41% 21% 4	660
March	1st to 10th	Night	21% 12% 38% 24% 6%	800
	11th to 20th	Night	16% 11% 38% 28% 7% 1	735
	21st to end	Night	23% 13% 33% 24% 5% 1	660
April	1st to 10th	Night	20% 14% 36% 25% 5%	600
	11th to 20th	Night	22% 13% 33% 25% 7%	593
	21st to end	Night	19% 10% 34% 30% 7%	600
May	1st to 10th	Night	16% 11% 33% 32% 8% 1	600
	11th to 20th	Night	17% 7% 33% 30% 12% 1	591
	21st to end	Night	14% 4 29% 34% 18%	650
June	1st to 10th	Night	10% 5% 27% 44% 15%	600
	11th to 20th	Night	9% 3 30% 46% 12% 1	600
	21st to end	Night	10% 4 28% 37% 19% 1	600
July	1st to 10th	Night	8% 2 15% 41% 29% 4	585
	11th to 20th	Night	9% 3 21% 37% 26% 4	582
	21st to end	Night	6% 2 16% 49% 25% 2	657
August	1st to 10th	Night	5% 1 16% 44% 30% 4	597
	11th to 20th	Night	7% 3 14% 42% 30% 5%	594
	21st to end	Night	10% 1 18% 40% 29% 2	660
September	1st to 10th	Night	10% 4 21% 34% 27% 4	600
	11th to 20th	Night	9% 4 19% 35% 30% 4	677
	21st to end	Night	12% 7% 23% 36% 19% 3	800
October	1st to 10th	Night	14% 5% 28% 35% 17% 2	800
	11th to 20th	Night	21% 11% 31% 26% 10%	793
	21st to end	Night	24% 11% 33% 23% 8%	836
November	1st to 10th	Night	17% 10% 39% 23% 10% 1	780
	11th to 20th	Night	19% 16% 34% 24% 7%	800
	21st to end	Night	20% 12% 37% 24% 6%	913
December	1st to 10th	Night	20% 13% 38% 23% 5%	1000
	11th to 20th	Night	22% 12% 34% 27% 5% 1	1000
	21st to end	Night	27% 14% 33% 22% 3	1100

Page generated at 2025-10-16 00:50:25. Version ac07

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