Lake Zurich
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lake Zurich clear sky chart from 2005-11-06 to 2025-08-05.

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% 9% 3 7% 4 4 4 4 6% 7% 37%	125756
	Night	14% 10% 4 8% 4 4 4 3 5% 6% 39%	122549
Transparency	Day	45% 4 11% 24% 14% 2	129279
	Night	46% 4 10% 23% 15% 2	127100
Seeing	Day	18% 14% 31% 23% 11% 4	16876
	Night	18% 11% 30% 27% 12% 1	26694

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	11% 11% 8% 13% 6% 5% 5% 4 5% 5% 27%	3617
	11th to 20th	Day	10% 13% 7% 11% 4 4 4 6% 6% 5% 32%	3620
	21st to end	Day	15% 19% 6% 7% 3 3 2 2 4 6% 33%	4072
February	1st to 10th	Day	13% 16% 5% 8% 5% 6% 5% 3 5% 6% 28%	1832
	11th to 20th	Day	13% 12% 5% 8% 5% 5% 4 3 5% 5% 36%	1948
	21st to end	Day	13% 13% 4 11% 5% 5% 3 3 5% 5% 32%	1650
March	1st to 10th	Day	15% 11% 3 7% 4 3 3 3 3 5% 42%	4480
	11th to 20th	Day	10% 7% 3 6% 5% 4 5% 4 6% 9% 41%	4671
	21st to end	Day	19% 14% 6% 12% 5% 5% 4 3 5% 6% 22%	5967
April	1st to 10th	Day	18% 14% 3 7% 5% 4 4 3 5% 6% 30%	2400
	11th to 20th	Day	16% 10% 2 6% 6% 5% 4 4 6% 7% 34%	2374
	21st to end	Day	18% 9% 3 7% 5% 6% 4 4 5% 6% 33%	2516
May	1st to 10th	Day	16% 9% 3 6% 4 4 4 4 5% 8% 36%	5862
	11th to 20th	Day	14% 6% 2 6% 4 4 5% 4 6% 8% 41%	6217
	21st to end	Day	16% 7% 2 6% 4 4 4 4 6% 8% 39%	6935
June	1st to 10th	Day	10% 6% 2 6% 4 5% 4 4 6% 7% 45%	2800
	11th to 20th	Day	10% 6% 2 5% 4 4 5% 4 8% 9% 43%	2800
	21st to end	Day	10% 7% 3 6% 5% 4 6% 4 8% 8% 39%	2800
July	1st to 10th	Day	10% 7% 3 7% 4 5% 6% 5% 7% 10% 35%	6360
	11th to 20th	Day	9% 5% 2 5% 4 5% 5% 4 7% 9% 43%	6360
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 4 7% 9% 40%	6924
August	1st to 10th	Day	11% 5% 2 6% 4 4 5% 4 7% 9% 42%	4399
	11th to 20th	Day	10% 5% 3 6% 4 4 5% 4 6% 9% 44%	3766
	21st to end	Day	8% 5% 3 6% 5% 4 4 4 5% 9% 48%	4180
September	1st to 10th	Day	12% 6% 2 5% 4 4 4 4 5% 7% 47%	2280
	11th to 20th	Day	12% 4 1 3 3 4 4 4 7% 7% 52%	2160
	21st to end	Day	12% 6% 2 6% 4 4 5% 4 6% 9% 42%	2090
October	1st to 10th	Day	13% 6% 3 6% 4 3 4 3 5% 6% 47%	2037
	11th to 20th	Day	18% 8% 3 7% 5% 4 4 4 5% 6% 36%	1881
	21st to end	Day	20% 10% 3 6% 4 4 3 3 5% 5% 37%	1881
November	1st to 10th	Day	12% 10% 3 6% 4 4 5% 3 4 6% 44%	1762
	11th to 20th	Day	15% 13% 3 9% 5% 4 4 3 5% 6% 33%	1800
	21st to end	Day	17% 13% 4 7% 4 4 4 2 3 6% 35%	1715
December	1st to 10th	Day	9% 9% 4 8% 5% 4 4 3 5% 7% 41%	2610
	11th to 20th	Day	11% 15% 6% 11% 5% 6% 5% 4 6% 6% 25%	3008
	21st to end	Day	22% 24% 7% 10% 5% 3 3 3 4 4 14%	3982
January	1st to 10th	Night	12% 13% 6% 13% 5% 4 4 3 4 5% 33%	6348
	11th to 20th	Night	13% 14% 6% 13% 6% 5% 4 5% 6% 4 23%	6360
	21st to end	Night	16% 15% 6% 9% 4 3 3 3 4 5% 32%	6904
February	1st to 10th	Night	14% 14% 5% 9% 5% 4 4 3 4 5% 32%	2566
	11th to 20th	Night	13% 11% 3 8% 5% 5% 4 3 5% 6% 37%	2338
	21st to end	Night	17% 12% 3 8% 4 3 4 3 4 6% 36%	1980
March	1st to 10th	Night	16% 10% 3 7% 3 3 3 3 4 5% 41%	5373
	11th to 20th	Night	9% 7% 2 6% 4 4 4 3 5% 8% 48%	5317
	21st to end	Night	20% 13% 5% 9% 5% 4 4 3 4 5% 28%	4962
April	1st to 10th	Night	16% 9% 3 6% 5% 4 5% 4 4 5% 38%	2000
	11th to 20th	Night	19% 8% 2 7% 4 4 4 3 5% 5% 38%	1975
	21st to end	Night	18% 7% 3 7% 4 3 4 3 5% 6% 39%	1882
May	1st to 10th	Night	18% 8% 3 7% 4 4 4 4 6% 7% 36%	4033
	11th to 20th	Night	13% 7% 3 6% 4 4 4 4 6% 8% 42%	3690
	21st to end	Night	17% 8% 3 7% 4 4 3 3 5% 6% 41%	3942
June	1st to 10th	Night	10% 5% 2 6% 4 5% 5% 5% 6% 7% 48%	1600
	11th to 20th	Night	11% 5% 2 5% 5% 5% 4 4 6% 8% 46%	1600
	21st to end	Night	10% 6% 2 6% 5% 4 6% 3 7% 9% 42%	1600
July	1st to 10th	Night	11% 5% 2 7% 5% 5% 5% 4 7% 8% 41%	3618
	11th to 20th	Night	7% 5% 2 5% 4 4 5% 4 6% 8% 51%	3617
	21st to end	Night	9% 5% 3 6% 4 5% 5% 4 7% 8% 46%	3947
August	1st to 10th	Night	7% 4 2 5% 4 4 5% 4 6% 9% 50%	2623
	11th to 20th	Night	9% 5% 2 6% 4 4 5% 3 6% 7% 51%	3061
	21st to end	Night	8% 4 2 5% 4 3 4 4 5% 9% 52%	3476
September	1st to 10th	Night	10% 6% 2 6% 3 4 3 3 4 6% 53%	1900
	11th to 20th	Night	10% 4 2 5% 3 3 3 3 4 7% 56%	2000
	21st to end	Night	12% 6% 2 5% 5% 4 4 4 5% 7% 47%	2090
October	1st to 10th	Night	12% 6% 2 5% 4 3 3 3 4 7% 51%	2141
	11th to 20th	Night	17% 7% 2 6% 4 3 4 3 4 5% 44%	2298
	21st to end	Night	20% 9% 3 6% 3 3 4 3 4 6% 38%	2717
November	1st to 10th	Night	12% 8% 3 7% 4 4 4 3 4 6% 47%	2536
	11th to 20th	Night	14% 11% 4 8% 5% 4 4 3 4 6% 36%	2600
	21st to end	Night	16% 11% 4 7% 5% 4 3 4 4 5% 36%	2680
December	1st to 10th	Night	11% 10% 3 7% 5% 4 4 4 5% 6% 42%	4580
	11th to 20th	Night	16% 16% 4 9% 5% 4 4 3 5% 5% 29%	5199
	21st to end	Night	22% 19% 6% 10% 4 4 4 3 4 5% 18%	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 11% 19% 9%	3618
	11th to 20th	Day	52% 4 11% 12% 15% 5%	3620
	21st to end	Day	55% 2 11% 22% 11%	4072
February	1st to 10th	Day	57% 4 12% 15% 10% 1	1832
	11th to 20th	Day	51% 4 10% 23% 10% 2	1958
	21st to end	Day	54% 5% 10% 18% 11% 2	1650
March	1st to 10th	Day	46% 3 10% 24% 12% 5%	4480
	11th to 20th	Day	40% 5% 12% 31% 11% 1	4671
	21st to end	Day	65% 4 11% 12% 7% 1	5969
April	1st to 10th	Day	55% 4 10% 19% 11%	2400
	11th to 20th	Day	49% 6% 9% 21% 12% 3	2374
	21st to end	Day	51% 6% 12% 18% 12% 1	2516
May	1st to 10th	Day	47% 5% 10% 23% 13% 2	5897
	11th to 20th	Day	41% 6% 11% 26% 14% 1	6221
	21st to end	Day	42% 5% 12% 25% 14% 3	6949
June	1st to 10th	Day	36% 7% 12% 25% 18% 2	2800
	11th to 20th	Day	35% 8% 13% 23% 19% 3	2800
	21st to end	Day	39% 8% 13% 23% 15% 3	2800
July	1st to 10th	Day	43% 4 12% 26% 13% 2	6291
	11th to 20th	Day	38% 3 14% 28% 15% 1	6285
	21st to end	Day	41% 3 16% 22% 17% 2	6728
August	1st to 10th	Day	38% 5% 15% 29% 13% 1	4384
	11th to 20th	Day	37% 4 11% 33% 15% 1	3803
	21st to end	Day	34% 5% 13% 25% 21% 2	4179
September	1st to 10th	Day	35% 7% 12% 24% 18% 3	2280
	11th to 20th	Day	30% 6% 11% 31% 19% 3	2160
	21st to end	Day	39% 6% 10% 27% 18% 1	2090
October	1st to 10th	Day	26% 3 7% 38% 25% 1	3388
	11th to 20th	Day	36% 4 10% 27% 23% 1	3109
	21st to end	Day	45% 3 7% 28% 13% 4	3081
November	1st to 10th	Day	42% 4 13% 23% 16% 2	1762
	11th to 20th	Day	53% 4 11% 21% 10% 1	1800
	21st to end	Day	53% 4 10% 24% 8%	1715
December	1st to 10th	Day	44% 3 17% 28% 7% 1	2610
	11th to 20th	Day	59% 4 12% 18% 7% 1	3005
	21st to end	Day	76% 2 7% 10% 5%	3982
January	1st to 10th	Night	57% 3 7% 21% 11% 1	6348
	11th to 20th	Night	63% 3 9% 9% 14% 1	6360
	21st to end	Night	57% 3 13% 16% 11% 1	6904
February	1st to 10th	Night	55% 4 10% 17% 13% 2	2566
	11th to 20th	Night	49% 4 11% 21% 14% 1	2339
	21st to end	Night	50% 4 10% 21% 10% 4	1980
March	1st to 10th	Night	47% 3 11% 24% 11% 3	5373
	11th to 20th	Night	36% 4 12% 31% 14% 3	5317
	21st to end	Night	59% 4 8% 17% 10% 1	4962
April	1st to 10th	Night	47% 5% 10% 22% 13% 1	2000
	11th to 20th	Night	47% 5% 8% 19% 16% 4	1975
	21st to end	Night	46% 5% 10% 23% 14% 2	1882
May	1st to 10th	Night	47% 5% 12% 17% 16% 3	4060
	11th to 20th	Night	39% 6% 14% 23% 14% 4	3691
	21st to end	Night	45% 5% 8% 22% 16% 4	3950
June	1st to 10th	Night	33% 7% 13% 27% 19% 3	1600
	11th to 20th	Night	36% 6% 10% 26% 22% 2	1600
	21st to end	Night	37% 7% 15% 22% 17% 2	1600
July	1st to 10th	Night	41% 4 13% 28% 12% 3	3580
	11th to 20th	Night	32% 4 13% 31% 18% 2	3572
	21st to end	Night	36% 4 14% 24% 20% 2	3832
August	1st to 10th	Night	31% 5% 15% 29% 19% 1	2614
	11th to 20th	Night	32% 5% 13% 31% 18% 1	3090
	21st to end	Night	31% 4 12% 33% 17% 4	3476
September	1st to 10th	Night	33% 5% 10% 27% 24% 2	1900
	11th to 20th	Night	29% 5% 7% 33% 24% 2	2000
	21st to end	Night	37% 6% 10% 30% 16% 2	2090
October	1st to 10th	Night	23% 3 7% 39% 26% 2	3570
	11th to 20th	Night	37% 3 7% 24% 26% 2	3809
	21st to end	Night	42% 3 9% 25% 18% 3	4467
November	1st to 10th	Night	39% 5% 8% 27% 18% 3	2536
	11th to 20th	Night	49% 4 10% 23% 12% 1	2600
	21st to end	Night	50% 4 13% 22% 11%	2680
December	1st to 10th	Night	44% 3 13% 28% 10% 2	4580
	11th to 20th	Night	58% 4 11% 15% 11% 1	5201
	21st to end	Night	70% 3 9% 12% 7%	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	21% 19% 36% 21% 3 1	300
	11th to 20th	Day	24% 25% 34% 15% 2 1	300
	21st to end	Day	26% 20% 35% 15% 4 1	330
February	1st to 10th	Day	25% 21% 39% 13% 2	300
	11th to 20th	Day	27% 24% 35% 12% 2	300
	21st to end	Day	26% 26% 37% 8% 3 1	249
March	1st to 10th	Day	24% 25% 38% 12% 2	300
	11th to 20th	Day	29% 19% 35% 12% 5%	325
	21st to end	Day	28% 17% 31% 17% 6% 1	550
April	1st to 10th	Day	28% 18% 30% 20% 4	500
	11th to 20th	Day	25% 23% 23% 20% 7% 2	492
	21st to end	Day	24% 18% 29% 18% 9% 1	500
May	1st to 10th	Day	22% 17% 33% 19% 7% 2	503
	11th to 20th	Day	22% 13% 31% 22% 9% 3	675
	21st to end	Day	15% 13% 33% 22% 11% 5%	789
June	1st to 10th	Day	12% 12% 35% 25% 13% 3	730
	11th to 20th	Day	14% 10% 31% 30% 12% 3	730
	21st to end	Day	13% 9% 34% 29% 11% 4	730
July	1st to 10th	Day	9% 5% 28% 29% 19% 10%	721
	11th to 20th	Day	12% 6% 31% 29% 14% 8%	718
	21st to end	Day	8% 7% 28% 35% 16% 6%	794
August	1st to 10th	Day	11% 6% 24% 33% 18% 9%	655
	11th to 20th	Day	12% 5% 21% 32% 20% 10%	479
	21st to end	Day	8% 5% 24% 32% 22% 9%	539
September	1st to 10th	Day	15% 7% 24% 29% 20% 5%	490
	11th to 20th	Day	10% 5% 22% 30% 21% 11%	490
	21st to end	Day	17% 13% 25% 24% 15% 6%	490
October	1st to 10th	Day	16% 14% 25% 27% 14% 4	437
	11th to 20th	Day	23% 22% 30% 20% 5%	300
	21st to end	Day	34% 15% 33% 13% 5% 1	330
November	1st to 10th	Day	20% 17% 35% 19% 8% 1	300
	11th to 20th	Day	26% 21% 33% 15% 4	300
	21st to end	Day	26% 14% 41% 17% 2 1	300
December	1st to 10th	Day	19% 23% 36% 17% 5% 1	300
	11th to 20th	Day	19% 23% 37% 15% 4	300
	21st to end	Day	29% 22% 33% 13% 2 1	330
January	1st to 10th	Night	19% 18% 38% 21% 4	1000
	11th to 20th	Night	22% 19% 36% 18% 4 1	1000
	21st to end	Night	22% 14% 37% 22% 5%	1100
February	1st to 10th	Night	23% 14% 37% 20% 5%	966
	11th to 20th	Night	21% 17% 34% 24% 3	783
	21st to end	Night	25% 16% 36% 19% 2	660
March	1st to 10th	Night	20% 15% 36% 21% 7%	800
	11th to 20th	Night	17% 13% 39% 23% 7% 1	781
	21st to end	Night	25% 12% 28% 26% 8% 1	660
April	1st to 10th	Night	21% 14% 35% 24% 6% 1	600
	11th to 20th	Night	23% 15% 31% 23% 8% 1	593
	21st to end	Night	23% 13% 27% 30% 7% 1	600
May	1st to 10th	Night	18% 10% 33% 30% 8% 1	600
	11th to 20th	Night	20% 6% 27% 31% 15% 1	591
	21st to end	Night	16% 7% 25% 33% 18% 2	650
June	1st to 10th	Night	11% 5% 27% 40% 17% 1	600
	11th to 20th	Night	13% 6% 29% 39% 14% 1	600
	21st to end	Night	11% 5% 29% 33% 21% 1	600
July	1st to 10th	Night	9% 2 18% 38% 29% 3	585
	11th to 20th	Night	9% 4 22% 37% 24% 3	582
	21st to end	Night	7% 2 19% 45% 26% 2	657
August	1st to 10th	Night	6% 1 17% 44% 28% 4	579
	11th to 20th	Night	8% 3 15% 38% 31% 5%	564
	21st to end	Night	7% 3 19% 38% 29% 3	627
September	1st to 10th	Night	12% 6% 19% 32% 26% 4	570
	11th to 20th	Night	11% 4 21% 32% 27% 5%	670
	21st to end	Night	13% 9% 26% 28% 21% 4	760
October	1st to 10th	Night	15% 8% 24% 32% 19% 3	760
	11th to 20th	Night	21% 14% 31% 25% 9%	760
	21st to end	Night	26% 12% 31% 22% 8%	836
November	1st to 10th	Night	18% 14% 35% 23% 9% 1	780
	11th to 20th	Night	21% 19% 32% 23% 5% 1	800
	21st to end	Night	23% 14% 37% 21% 6%	880
December	1st to 10th	Night	22% 17% 35% 21% 5%	1000
	11th to 20th	Night	24% 15% 34% 23% 4 1	1000
	21st to end	Night	30% 16% 33% 18% 3	1100

Page generated at 2025-08-02 00:32:29. Version ac07

Copyright © 2025 Attilla Danko.