Lombard
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lombard clear sky chart from 2005-11-06 to 2025-12-18.

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	13% 9% 3 8% 5% 4 5% 4 6% 7% 36%	127148
	Night	14% 10% 4 8% 4 4 4 3 5% 6% 38%	124142
Transparency	Day	45% 7% 12% 22% 13% 2	130671
	Night	45% 6% 11% 22% 14% 2	128693
Seeing	Day	18% 15% 32% 22% 10% 3	17325
	Night	18% 12% 31% 27% 11% 1	27188

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% 11% 7% 12% 6% 6% 5% 4 6% 5% 26%	3617
	11th to 20th	Day	11% 12% 7% 13% 5% 5% 4 6% 6% 4 29%	3620
	21st to end	Day	16% 19% 5% 8% 4 2 2 3 4 7% 29%	4092
February	1st to 10th	Day	14% 15% 5% 9% 7% 5% 5% 4 4 6% 26%	1847
	11th to 20th	Day	14% 11% 5% 9% 6% 4 4 3 5% 5% 34%	1948
	21st to end	Day	14% 13% 5% 10% 5% 4 4 3 5% 7% 31%	1650
March	1st to 10th	Day	17% 12% 3 7% 4 3 3 2 3 5% 40%	4480
	11th to 20th	Day	9% 7% 3 7% 5% 5% 5% 4 5% 8% 42%	4684
	21st to end	Day	19% 14% 6% 12% 6% 4 5% 3 4 5% 20%	5967
April	1st to 10th	Day	17% 14% 3 8% 5% 5% 3 4 5% 6% 30%	2400
	11th to 20th	Day	16% 10% 3 7% 5% 5% 5% 4 5% 7% 35%	2374
	21st to end	Day	19% 9% 3 7% 5% 5% 5% 3 5% 6% 33%	2514
May	1st to 10th	Day	19% 8% 2 6% 5% 4 5% 4 6% 8% 34%	5862
	11th to 20th	Day	15% 6% 2 6% 4 4 5% 4 6% 7% 41%	6169
	21st to end	Day	14% 7% 3 7% 4 4 5% 4 6% 9% 38%	6935
June	1st to 10th	Day	10% 6% 2 6% 4 5% 4 4 6% 7% 46%	2800
	11th to 20th	Day	10% 6% 3 6% 5% 4 4 4 7% 7% 44%	2800
	21st to end	Day	11% 6% 2 7% 4 5% 6% 5% 7% 8% 39%	2800
July	1st to 10th	Day	9% 7% 3 7% 5% 5% 6% 5% 8% 9% 35%	6360
	11th to 20th	Day	9% 6% 2 6% 4 5% 5% 5% 7% 9% 41%	6360
	21st to end	Day	10% 6% 3 7% 5% 5% 5% 4 7% 9% 37%	6924
August	1st to 10th	Day	9% 5% 2 7% 4 5% 5% 4 6% 8% 44%	4464
	11th to 20th	Day	10% 5% 2 6% 4 5% 6% 4 7% 9% 42%	3880
	21st to end	Day	8% 4 2 7% 4 4 4 4 6% 9% 47%	4312
September	1st to 10th	Day	12% 5% 2 6% 4 4 5% 4 5% 7% 48%	2400
	11th to 20th	Day	11% 5% 2 4 3 3 4 3 5% 7% 52%	2269
	21st to end	Day	11% 7% 2 6% 5% 4 6% 4 5% 7% 43%	2200
October	1st to 10th	Day	12% 7% 2 7% 5% 4 3 3 4 6% 47%	2154
	11th to 20th	Day	18% 9% 3 7% 5% 4 4 3 4 7% 36%	1981
	21st to end	Day	19% 9% 3 7% 4 3 3 4 5% 6% 36%	1980
November	1st to 10th	Day	12% 9% 4 6% 4 3 3 3 5% 6% 44%	1852
	11th to 20th	Day	15% 12% 4 9% 5% 3 4 3 5% 7% 33%	1890
	21st to end	Day	18% 12% 4 9% 5% 4 4 2 4 5% 34%	1827
December	1st to 10th	Day	9% 11% 3 9% 5% 4 4 3 5% 6% 42%	2690
	11th to 20th	Day	12% 15% 6% 12% 6% 5% 4 4 5% 6% 24%	3064
	21st to end	Day	24% 23% 7% 12% 5% 4 4 2 3 4 12%	3982
January	1st to 10th	Night	13% 13% 7% 12% 5% 4 4 3 4 4 30%	6348
	11th to 20th	Night	14% 16% 6% 13% 6% 5% 4 4 6% 4 23%	6360
	21st to end	Night	17% 15% 6% 8% 4 4 3 2 3 4 32%	6884
February	1st to 10th	Night	15% 14% 4 8% 6% 5% 4 3 4 6% 31%	2551
	11th to 20th	Night	14% 11% 4 8% 6% 4 4 3 5% 5% 37%	2338
	21st to end	Night	18% 11% 3 8% 4 4 3 4 4 5% 37%	1980
March	1st to 10th	Night	16% 10% 5% 7% 3 3 3 3 4 4 41%	5373
	11th to 20th	Night	11% 6% 2 6% 4 4 4 3 5% 7% 47%	5320
	21st to end	Night	19% 11% 5% 10% 6% 5% 4 3 4 5% 28%	4962
April	1st to 10th	Night	17% 8% 3 6% 5% 3 5% 4 5% 6% 38%	2000
	11th to 20th	Night	18% 8% 3 6% 4 4 4 3 6% 6% 37%	1975
	21st to end	Night	17% 8% 3 6% 5% 4 4 4 6% 6% 36%	1885
May	1st to 10th	Night	18% 8% 2 6% 5% 5% 4 4 5% 6% 36%	4033
	11th to 20th	Night	12% 6% 2 6% 3 4 5% 4 6% 8% 43%	3737
	21st to end	Night	17% 8% 3 7% 4 4 4 3 5% 6% 41%	3942
June	1st to 10th	Night	10% 5% 2 6% 4 4 5% 4 6% 7% 49%	1600
	11th to 20th	Night	9% 6% 2 6% 4 4 5% 4 6% 9% 45%	1600
	21st to end	Night	10% 7% 2 5% 4 6% 4 4 8% 9% 42%	1600
July	1st to 10th	Night	10% 5% 2 8% 5% 5% 5% 4 6% 8% 40%	3618
	11th to 20th	Night	7% 5% 2 6% 4 4 4 4 6% 8% 50%	3617
	21st to end	Night	9% 6% 2 6% 4 5% 5% 4 7% 8% 44%	3947
August	1st to 10th	Night	6% 5% 1 6% 4 5% 6% 5% 6% 8% 48%	2690
	11th to 20th	Night	8% 5% 2 5% 4 4 4 4 6% 8% 51%	3164
	21st to end	Night	8% 4 1 5% 3 4 4 3 6% 8% 53%	3586
September	1st to 10th	Night	9% 5% 2 5% 4 4 4 2 4 7% 53%	2000
	11th to 20th	Night	9% 4 2 4 3 4 3 3 4 8% 56%	2110
	21st to end	Night	11% 6% 2 7% 4 4 4 4 5% 7% 46%	2200
October	1st to 10th	Night	11% 5% 3 6% 4 3 3 3 4 6% 51%	2244
	11th to 20th	Night	17% 7% 2 7% 3 4 3 3 4 6% 45%	2417
	21st to end	Night	20% 9% 3 7% 4 3 4 3 5% 6% 37%	2860
November	1st to 10th	Night	13% 8% 2 6% 4 3 3 3 4 7% 46%	2666
	11th to 20th	Night	14% 11% 3 8% 4 4 4 3 6% 6% 37%	2730
	21st to end	Night	16% 11% 4 8% 5% 4 4 3 5% 5% 35%	2791
December	1st to 10th	Night	12% 10% 3 8% 4 3 4 3 5% 6% 42%	4720
	11th to 20th	Night	17% 16% 4 9% 5% 4 4 3 4 5% 29%	5298
	21st to end	Night	24% 19% 6% 9% 5% 4 4 3 4 4 17%	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% 5% 13% 16% 8%	3618
	11th to 20th	Day	56% 4 11% 10% 14% 5%	3620
	21st to end	Day	56% 5% 12% 18% 8% 1	4092
February	1st to 10th	Day	59% 6% 12% 13% 9% 2	1847
	11th to 20th	Day	53% 5% 10% 21% 9% 2	1958
	21st to end	Day	52% 7% 12% 18% 8% 2	1650
March	1st to 10th	Day	46% 6% 8% 24% 12% 4	4480
	11th to 20th	Day	38% 7% 13% 31% 10% 1	4684
	21st to end	Day	64% 8% 11% 10% 6% 1	5969
April	1st to 10th	Day	54% 6% 10% 19% 11%	2400
	11th to 20th	Day	47% 8% 11% 20% 11% 3	2374
	21st to end	Day	49% 7% 13% 17% 11% 2	2514
May	1st to 10th	Day	45% 9% 12% 22% 12% 1	5897
	11th to 20th	Day	41% 8% 11% 25% 14% 1	6173
	21st to end	Day	41% 8% 12% 22% 14% 3	6949
June	1st to 10th	Day	34% 9% 13% 25% 17% 2	2800
	11th to 20th	Day	36% 8% 12% 23% 19% 3	2800
	21st to end	Day	39% 10% 13% 21% 14% 2	2800
July	1st to 10th	Day	42% 7% 14% 22% 13% 2	6291
	11th to 20th	Day	36% 7% 15% 26% 15% 1	6285
	21st to end	Day	40% 8% 16% 21% 15% 2	6728
August	1st to 10th	Day	36% 8% 16% 27% 13% 1	4449
	11th to 20th	Day	35% 8% 12% 29% 15% 1	3917
	21st to end	Day	32% 7% 13% 26% 20% 2	4311
September	1st to 10th	Day	34% 7% 13% 25% 18% 4	2400
	11th to 20th	Day	30% 6% 11% 28% 22% 3	2269
	21st to end	Day	38% 8% 10% 24% 19% 1	2200
October	1st to 10th	Day	26% 4 8% 31% 29% 2	3505
	11th to 20th	Day	35% 4 12% 26% 23% 1	3209
	21st to end	Day	43% 4 9% 27% 14% 3	3180
November	1st to 10th	Day	40% 6% 13% 24% 16% 2	1852
	11th to 20th	Day	51% 5% 11% 21% 10% 1	1890
	21st to end	Day	54% 5% 10% 23% 8%	1827
December	1st to 10th	Day	45% 4 17% 26% 8% 1	2690
	11th to 20th	Day	59% 5% 12% 17% 6% 1	3061
	21st to end	Day	78% 4 6% 8% 4	3982
January	1st to 10th	Night	57% 5% 9% 19% 10%	6348
	11th to 20th	Night	62% 5% 10% 9% 13% 1	6360
	21st to end	Night	57% 4 13% 15% 11% 1	6884
February	1st to 10th	Night	55% 5% 12% 16% 11% 2	2551
	11th to 20th	Night	49% 5% 11% 21% 13% 1	2339
	21st to end	Night	50% 5% 12% 19% 11% 3	1980
March	1st to 10th	Night	47% 5% 10% 24% 11% 3	5373
	11th to 20th	Night	36% 5% 12% 30% 14% 3	5320
	21st to end	Night	57% 7% 10% 17% 9% 1	4962
April	1st to 10th	Night	44% 7% 13% 22% 12% 1	2000
	11th to 20th	Night	46% 7% 9% 19% 15% 4	1975
	21st to end	Night	45% 8% 11% 22% 12% 2	1885
May	1st to 10th	Night	47% 7% 13% 16% 14% 3	4060
	11th to 20th	Night	36% 9% 15% 24% 13% 3	3738
	21st to end	Night	42% 7% 10% 20% 16% 4	3950
June	1st to 10th	Night	33% 8% 13% 24% 19% 3	1600
	11th to 20th	Night	33% 8% 12% 24% 22% 2	1600
	21st to end	Night	37% 9% 14% 20% 17% 2	1600
July	1st to 10th	Night	40% 6% 14% 25% 12% 3	3580
	11th to 20th	Night	30% 7% 14% 28% 19% 2	3572
	21st to end	Night	35% 7% 15% 23% 18% 3	3832
August	1st to 10th	Night	32% 8% 15% 27% 17% 1	2681
	11th to 20th	Night	30% 7% 14% 28% 19% 1	3193
	21st to end	Night	29% 6% 13% 33% 17% 4	3586
September	1st to 10th	Night	31% 6% 13% 25% 23% 3	2000
	11th to 20th	Night	27% 5% 8% 30% 27% 2	2110
	21st to end	Night	36% 7% 11% 28% 17% 2	2200
October	1st to 10th	Night	22% 4 7% 37% 28% 2	3673
	11th to 20th	Night	37% 3 8% 23% 27% 2	3928
	21st to end	Night	40% 4 9% 25% 18% 3	4610
November	1st to 10th	Night	39% 6% 9% 26% 18% 3	2666
	11th to 20th	Night	47% 6% 11% 22% 12% 2	2730
	21st to end	Night	51% 5% 11% 22% 11%	2791
December	1st to 10th	Night	43% 4 14% 27% 10% 2	4720
	11th to 20th	Night	57% 5% 12% 15% 11% 1	5300
	21st to end	Night	70% 6% 8% 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	22% 19% 39% 16% 3	300
	11th to 20th	Day	25% 24% 36% 12% 2	300
	21st to end	Day	27% 22% 35% 13% 3	330
February	1st to 10th	Day	26% 22% 38% 12% 2	300
	11th to 20th	Day	28% 25% 31% 15% 1	300
	21st to end	Day	28% 25% 36% 9% 2	249
March	1st to 10th	Day	22% 31% 35% 11% 1	300
	11th to 20th	Day	28% 22% 36% 11% 3	330
	21st to end	Day	29% 20% 30% 16% 4 1	550
April	1st to 10th	Day	28% 20% 31% 17% 4	500
	11th to 20th	Day	25% 24% 23% 19% 8% 1	492
	21st to end	Day	23% 20% 33% 16% 6% 1	500
May	1st to 10th	Day	24% 16% 33% 18% 7% 2	503
	11th to 20th	Day	20% 15% 34% 21% 8% 2	653
	21st to end	Day	16% 14% 34% 21% 11% 4	789
June	1st to 10th	Day	11% 13% 35% 27% 11% 3	730
	11th to 20th	Day	13% 9% 35% 29% 11% 2	730
	21st to end	Day	12% 12% 36% 25% 10% 4	730
July	1st to 10th	Day	8% 7% 29% 29% 17% 9%	721
	11th to 20th	Day	12% 8% 31% 28% 14% 6%	718
	21st to end	Day	8% 7% 34% 32% 13% 5%	794
August	1st to 10th	Day	7% 4 29% 34% 18% 8%	666
	11th to 20th	Day	10% 8% 21% 33% 20% 8%	519
	21st to end	Day	7% 6% 25% 35% 21% 7%	583
September	1st to 10th	Day	15% 7% 27% 29% 17% 5%	530
	11th to 20th	Day	10% 5% 22% 32% 24% 7%	530
	21st to end	Day	14% 15% 25% 25% 15% 5%	530
October	1st to 10th	Day	15% 13% 30% 25% 14% 3	484
	11th to 20th	Day	23% 26% 30% 17% 4	330
	21st to end	Day	31% 18% 35% 12% 3	363
November	1st to 10th	Day	21% 20% 34% 18% 5% 2	330
	11th to 20th	Day	29% 24% 28% 14% 5%	330
	21st to end	Day	27% 20% 36% 15% 2	330
December	1st to 10th	Day	20% 27% 36% 12% 4 1	330
	11th to 20th	Day	24% 25% 36% 13% 3	321
	21st to end	Day	29% 26% 32% 11% 2	330
January	1st to 10th	Night	20% 17% 40% 20% 3	1000
	11th to 20th	Night	23% 20% 34% 19% 4 1	1000
	21st to end	Night	22% 15% 36% 23% 3	1100
February	1st to 10th	Night	23% 16% 36% 21% 4	951
	11th to 20th	Night	21% 17% 35% 24% 3	783
	21st to end	Night	22% 17% 39% 18% 3	660
March	1st to 10th	Night	22% 16% 34% 21% 6%	800
	11th to 20th	Night	18% 14% 39% 23% 6%	788
	21st to end	Night	24% 13% 32% 23% 7%	660
April	1st to 10th	Night	21% 16% 35% 22% 6% 1	600
	11th to 20th	Night	23% 15% 30% 24% 7%	593
	21st to end	Night	20% 14% 34% 25% 7%	600
May	1st to 10th	Night	19% 11% 34% 29% 7% 1	600
	11th to 20th	Night	17% 8% 30% 32% 13% 1	591
	21st to end	Night	15% 6% 29% 32% 18%	650
June	1st to 10th	Night	11% 6% 29% 39% 16%	600
	11th to 20th	Night	11% 6% 31% 40% 12% 1	600
	21st to end	Night	11% 7% 31% 35% 17% 1	600
July	1st to 10th	Night	8% 4 18% 44% 23% 3	585
	11th to 20th	Night	8% 4 25% 37% 24% 2	582
	21st to end	Night	6% 2 23% 45% 22% 2	657
August	1st to 10th	Night	5% 3 18% 45% 26% 3	597
	11th to 20th	Night	6% 3 19% 39% 29% 4	594
	21st to end	Night	8% 3 19% 39% 28% 2	660
September	1st to 10th	Night	11% 6% 23% 32% 25% 3	600
	11th to 20th	Night	9% 4 21% 34% 27% 4	710
	21st to end	Night	13% 9% 25% 31% 20% 3	800
October	1st to 10th	Night	13% 7% 27% 35% 17% 2	800
	11th to 20th	Night	20% 15% 30% 26% 8%	800
	21st to end	Night	25% 15% 30% 22% 8%	880
November	1st to 10th	Night	17% 15% 38% 21% 8% 1	820
	11th to 20th	Night	21% 18% 33% 22% 5%	840
	21st to end	Night	23% 16% 37% 19% 5%	901
December	1st to 10th	Night	24% 16% 36% 20% 4	1050
	11th to 20th	Night	24% 15% 37% 21% 4	1036
	21st to end	Night	32% 16% 33% 18% 2	1100

Page generated at 2025-12-15 00:13:51. Version ac07

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