Rockford
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rockford 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	13% 9% 3 8% 5% 4 4 4 6% 7% 37%	126816
	Night	14% 10% 4 8% 5% 4 4 3 5% 6% 39%	123154
Transparency	Day	45% 5% 12% 24% 13% 1	130310
	Night	46% 4 11% 23% 15% 2	127734
Seeing	Day	18% 11% 28% 24% 14% 5%	17192
	Night	18% 9% 31% 30% 11% 1	26977

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	9% 12% 8% 15% 7% 6% 6% 4 5% 5% 24%	3617
	11th to 20th	Day	9% 12% 6% 12% 4 4 4 7% 7% 5% 30%	3620
	21st to end	Day	13% 15% 8% 9% 4 3 4 3 4 6% 31%	4208
February	1st to 10th	Day	12% 14% 6% 9% 6% 7% 5% 4 5% 7% 26%	1800
	11th to 20th	Day	14% 10% 3 8% 5% 6% 4 3 5% 5% 35%	1915
	21st to end	Day	13% 15% 4 10% 5% 4 4 2 5% 7% 31%	1650
March	1st to 10th	Day	16% 9% 3 6% 4 3 3 2 4 5% 44%	4480
	11th to 20th	Day	12% 6% 2 8% 4 4 4 4 5% 9% 43%	4768
	21st to end	Day	21% 12% 5% 12% 5% 5% 4 3 5% 5% 22%	5921
April	1st to 10th	Day	18% 13% 3 8% 5% 4 4 4 4 6% 31%	2400
	11th to 20th	Day	16% 10% 3 7% 5% 5% 5% 4 6% 6% 34%	2374
	21st to end	Day	20% 9% 3 9% 5% 4 5% 2 5% 6% 33%	2463
May	1st to 10th	Day	15% 8% 3 7% 5% 4 5% 4 5% 7% 37%	5944
	11th to 20th	Day	15% 5% 2 6% 4 4 5% 4 5% 8% 42%	6319
	21st to end	Day	14% 7% 2 6% 4 4 5% 4 7% 9% 37%	6935
June	1st to 10th	Day	10% 5% 2 6% 5% 4 5% 5% 6% 7% 44%	2800
	11th to 20th	Day	11% 6% 2 5% 4 3 5% 4 7% 9% 45%	2800
	21st to end	Day	11% 7% 2 7% 4 4 5% 5% 7% 10% 38%	2800
July	1st to 10th	Day	10% 7% 3 8% 5% 5% 5% 4 7% 10% 37%	6360
	11th to 20th	Day	9% 6% 2 6% 4 5% 5% 4 6% 10% 44%	6360
	21st to end	Day	10% 7% 3 7% 6% 5% 5% 4 6% 9% 38%	6924
August	1st to 10th	Day	10% 5% 2 6% 5% 4 5% 4 7% 9% 42%	4477
	11th to 20th	Day	11% 5% 2 6% 3 4 4 4 7% 10% 45%	3880
	21st to end	Day	9% 5% 2 6% 4 4 4 4 6% 9% 48%	4312
September	1st to 10th	Day	12% 6% 1 5% 5% 4 4 3 5% 8% 47%	2400
	11th to 20th	Day	12% 4 1 4 3 3 3 3 5% 8% 53%	2320
	21st to end	Day	12% 6% 2 7% 4 4 4 3 5% 7% 46%	2200
October	1st to 10th	Day	14% 5% 3 6% 4 3 4 3 5% 5% 49%	2070
	11th to 20th	Day	20% 9% 3 7% 4 4 4 3 5% 6% 36%	1980
	21st to end	Day	21% 9% 3 6% 4 5% 3 2 4 5% 38%	1912
November	1st to 10th	Day	13% 9% 3 6% 4 3 5% 3 4 7% 44%	1762
	11th to 20th	Day	15% 12% 3 9% 6% 4 4 4 4 6% 32%	1800
	21st to end	Day	17% 14% 4 9% 4 3 2 3 4 5% 35%	1645
December	1st to 10th	Day	9% 9% 4 8% 5% 4 5% 4 5% 6% 42%	2610
	11th to 20th	Day	12% 16% 5% 12% 6% 5% 5% 4 5% 6% 23%	3008
	21st to end	Day	22% 22% 9% 11% 5% 4 3 2 4 4 12%	3982
January	1st to 10th	Night	10% 13% 7% 14% 6% 5% 3 2 3 4 32%	6348
	11th to 20th	Night	12% 15% 6% 13% 6% 5% 5% 6% 6% 4 22%	6360
	21st to end	Night	15% 15% 6% 9% 5% 4 4 3 4 5% 30%	6766
February	1st to 10th	Night	14% 14% 5% 8% 5% 5% 4 3 4 4 33%	2600
	11th to 20th	Night	13% 11% 4 7% 6% 4 4 2 5% 6% 38%	2369
	21st to end	Night	17% 12% 4 8% 4 3 3 3 4 6% 36%	1980
March	1st to 10th	Night	17% 9% 4 6% 4 4 3 3 4 5% 42%	5373
	11th to 20th	Night	9% 7% 2 6% 4 4 4 3 5% 7% 49%	5222
	21st to end	Night	21% 13% 5% 9% 4 4 3 3 4 5% 28%	5008
April	1st to 10th	Night	18% 10% 3 6% 4 4 4 3 4 6% 39%	2000
	11th to 20th	Night	18% 9% 3 6% 5% 4 3 2 4 5% 40%	1975
	21st to end	Night	17% 8% 3 6% 4 4 4 3 6% 7% 37%	1935
May	1st to 10th	Night	18% 7% 3 6% 5% 4 4 3 6% 6% 38%	3952
	11th to 20th	Night	12% 5% 2 6% 4 4 5% 4 6% 8% 44%	3596
	21st to end	Night	15% 8% 2 7% 5% 4 4 4 6% 7% 39%	3942
June	1st to 10th	Night	8% 4 1 5% 4 4 5% 5% 7% 8% 48%	1600
	11th to 20th	Night	9% 5% 2 6% 3 5% 5% 4 6% 8% 47%	1600
	21st to end	Night	10% 5% 3 7% 5% 5% 5% 4 6% 7% 45%	1600
July	1st to 10th	Night	12% 6% 3 6% 5% 4 5% 3 5% 8% 41%	3618
	11th to 20th	Night	7% 5% 2 6% 4 4 4 4 5% 8% 51%	3617
	21st to end	Night	9% 6% 2 7% 4 4 5% 4 6% 8% 45%	3947
August	1st to 10th	Night	6% 4 2 5% 4 4 4 3 7% 9% 52%	2679
	11th to 20th	Night	9% 6% 2 6% 4 3 4 4 6% 8% 49%	3170
	21st to end	Night	8% 5% 1 5% 4 4 4 4 5% 9% 51%	3586
September	1st to 10th	Night	9% 5% 2 6% 3 4 4 2 5% 6% 55%	2000
	11th to 20th	Night	10% 4 2 4 4 3 4 3 5% 7% 53%	2060
	21st to end	Night	11% 5% 2 6% 4 4 4 3 5% 7% 49%	2200
October	1st to 10th	Night	11% 6% 2 5% 3 4 4 3 4 5% 52%	2326
	11th to 20th	Night	17% 7% 2 6% 4 3 3 3 5% 5% 45%	2377
	21st to end	Night	20% 8% 3 5% 4 4 4 3 5% 6% 37%	2684
November	1st to 10th	Night	12% 8% 3 6% 3 4 3 3 4 5% 49%	2536
	11th to 20th	Night	16% 10% 4 9% 5% 4 4 3 5% 5% 37%	2600
	21st to end	Night	17% 12% 3 7% 5% 4 3 3 5% 5% 36%	2753
December	1st to 10th	Night	10% 9% 3 7% 5% 5% 5% 3 5% 6% 41%	4580
	11th to 20th	Night	17% 17% 4 9% 5% 3 4 3 4 5% 29%	5199
	21st to end	Night	23% 19% 6% 10% 5% 4 5% 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	62% 4 10% 17% 6%	3618
	11th to 20th	Day	52% 5% 12% 10% 18% 3	3620
	21st to end	Day	55% 3 14% 19% 9%	4208
February	1st to 10th	Day	57% 5% 12% 16% 9% 1	1800
	11th to 20th	Day	50% 5% 11% 24% 9% 2	1925
	21st to end	Day	53% 5% 10% 20% 10% 1	1650
March	1st to 10th	Day	43% 5% 10% 28% 10% 4	4480
	11th to 20th	Day	39% 5% 12% 32% 11% 1	4768
	21st to end	Day	63% 5% 10% 13% 7% 1	5923
April	1st to 10th	Day	54% 6% 10% 21% 9%	2400
	11th to 20th	Day	49% 6% 10% 22% 11% 2	2374
	21st to end	Day	51% 6% 12% 18% 12% 1	2463
May	1st to 10th	Day	46% 5% 10% 21% 17% 1	5979
	11th to 20th	Day	41% 6% 10% 27% 16% 1	6323
	21st to end	Day	41% 6% 12% 26% 12% 3	6949
June	1st to 10th	Day	36% 8% 12% 26% 16% 1	2800
	11th to 20th	Day	35% 7% 13% 25% 19% 2	2800
	21st to end	Day	39% 8% 14% 24% 13% 2	2800
July	1st to 10th	Day	43% 3 13% 27% 12% 1	6291
	11th to 20th	Day	37% 3 15% 28% 15% 1	6285
	21st to end	Day	42% 4 16% 21% 15% 2	6728
August	1st to 10th	Day	37% 5% 16% 29% 12% 1	4462
	11th to 20th	Day	34% 4 13% 35% 14% 1	3916
	21st to end	Day	34% 4 13% 30% 17% 1	4311
September	1st to 10th	Day	35% 6% 11% 27% 18% 2	2400
	11th to 20th	Day	30% 5% 12% 32% 19% 2	2320
	21st to end	Day	37% 5% 10% 27% 19% 1	2200
October	1st to 10th	Day	25% 3 6% 37% 28% 1	3369
	11th to 20th	Day	35% 3 10% 27% 24% 2	3220
	21st to end	Day	44% 3 8% 28% 13% 3	3124
November	1st to 10th	Day	41% 4 14% 24% 14% 2	1762
	11th to 20th	Day	53% 5% 11% 20% 9% 1	1800
	21st to end	Day	52% 4 13% 22% 8%	1645
December	1st to 10th	Day	43% 3 18% 27% 7% 1	2610
	11th to 20th	Day	62% 4 11% 16% 6% 1	3005
	21st to end	Day	77% 3 6% 9% 5%	3982
January	1st to 10th	Night	57% 3 9% 20% 10%	6348
	11th to 20th	Night	63% 4 9% 9% 15% 1	6360
	21st to end	Night	57% 4 12% 16% 10%	6766
February	1st to 10th	Night	54% 4 11% 18% 12% 1	2600
	11th to 20th	Night	48% 5% 11% 22% 14% 1	2370
	21st to end	Night	49% 4 12% 20% 11% 4	1980
March	1st to 10th	Night	46% 4 11% 22% 11% 6%	5373
	11th to 20th	Night	35% 4 11% 34% 13% 3	5222
	21st to end	Night	60% 3 9% 17% 10% 1	5008
April	1st to 10th	Night	47% 5% 11% 22% 14% 2	2000
	11th to 20th	Night	47% 4 7% 21% 17% 4	1975
	21st to end	Night	45% 6% 12% 24% 11% 2	1935
May	1st to 10th	Night	45% 6% 13% 17% 17% 2	3979
	11th to 20th	Night	35% 8% 14% 23% 15% 3	3597
	21st to end	Night	41% 7% 11% 23% 15% 2	3950
June	1st to 10th	Night	31% 7% 15% 25% 19% 2	1600
	11th to 20th	Night	34% 6% 11% 24% 23% 2	1600
	21st to end	Night	38% 7% 14% 21% 18% 3	1600
July	1st to 10th	Night	43% 3 13% 25% 14% 2	3580
	11th to 20th	Night	32% 3 13% 31% 20% 1	3572
	21st to end	Night	38% 4 14% 24% 18% 2	3832
August	1st to 10th	Night	30% 4 15% 31% 18% 2	2670
	11th to 20th	Night	33% 4 14% 31% 17% 1	3200
	21st to end	Night	31% 5% 12% 32% 18% 3	3586
September	1st to 10th	Night	31% 5% 10% 28% 23% 3	2000
	11th to 20th	Night	29% 6% 9% 33% 20% 3	2060
	21st to end	Night	35% 5% 10% 29% 20% 2	2200
October	1st to 10th	Night	25% 3 6% 35% 29% 1	3807
	11th to 20th	Night	35% 3 8% 25% 26% 3	3876
	21st to end	Night	43% 3 9% 26% 16% 3	4422
November	1st to 10th	Night	38% 5% 10% 27% 18% 3	2536
	11th to 20th	Night	49% 5% 12% 21% 12% 1	2600
	21st to end	Night	50% 5% 11% 23% 11% 1	2753
December	1st to 10th	Night	44% 3 14% 28% 9% 1	4580
	11th to 20th	Night	58% 4 12% 16% 9% 1	5201
	21st to end	Night	71% 5% 8% 11% 5%	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% 14% 33% 24% 8% 1	300
	11th to 20th	Day	22% 16% 37% 21% 3 1	300
	21st to end	Day	23% 14% 36% 24% 2 2	330
February	1st to 10th	Day	20% 18% 41% 15% 6%	300
	11th to 20th	Day	22% 20% 33% 21% 3	300
	21st to end	Day	29% 18% 35% 17% 1	249
March	1st to 10th	Day	21% 22% 38% 15% 4	300
	11th to 20th	Day	26% 17% 36% 13% 6% 1	300
	21st to end	Day	29% 16% 30% 17% 7% 1	530
April	1st to 10th	Day	26% 15% 33% 21% 5%	500
	11th to 20th	Day	23% 21% 25% 20% 8% 3	492
	21st to end	Day	23% 14% 30% 23% 9% 1	500
May	1st to 10th	Day	21% 15% 29% 23% 9% 3	533
	11th to 20th	Day	22% 11% 27% 24% 12% 3	725
	21st to end	Day	20% 9% 28% 22% 15% 7%	789
June	1st to 10th	Day	10% 8% 31% 28% 17% 5%	730
	11th to 20th	Day	14% 6% 34% 26% 14% 5%	730
	21st to end	Day	13% 8% 28% 31% 16% 3	730
July	1st to 10th	Day	10% 3 18% 32% 24% 12%	721
	11th to 20th	Day	11% 4 25% 31% 21% 8%	718
	21st to end	Day	9% 4 23% 34% 23% 7%	794
August	1st to 10th	Day	10% 3 20% 32% 25% 11%	726
	11th to 20th	Day	13% 5% 18% 25% 23% 15%	538
	21st to end	Day	10% 6% 18% 32% 25% 9%	583
September	1st to 10th	Day	17% 7% 20% 28% 21% 8%	530
	11th to 20th	Day	11% 3 19% 30% 25% 12%	530
	21st to end	Day	15% 11% 24% 26% 16% 8%	530
October	1st to 10th	Day	17% 10% 26% 26% 18% 3	400
	11th to 20th	Day	24% 19% 29% 21% 6%	324
	21st to end	Day	32% 14% 28% 20% 5% 1	330
November	1st to 10th	Day	19% 15% 35% 19% 9% 3	300
	11th to 20th	Day	23% 16% 39% 15% 7%	300
	21st to end	Day	30% 10% 38% 16% 5%	300
December	1st to 10th	Day	19% 18% 42% 14% 5% 2	300
	11th to 20th	Day	22% 13% 38% 18% 8% 1	300
	21st to end	Day	29% 15% 36% 17% 3	330
January	1st to 10th	Night	19% 14% 36% 26% 5%	1000
	11th to 20th	Night	20% 14% 36% 24% 5% 1	1000
	21st to end	Night	22% 12% 34% 27% 6%	1100
February	1st to 10th	Night	22% 12% 39% 23% 5%	1000
	11th to 20th	Night	20% 15% 36% 26% 4	814
	21st to end	Night	23% 14% 39% 20% 4	660
March	1st to 10th	Night	21% 11% 37% 25% 6%	800
	11th to 20th	Night	16% 13% 36% 29% 6%	712
	21st to end	Night	25% 12% 31% 25% 5% 1	660
April	1st to 10th	Night	23% 13% 36% 24% 5%	600
	11th to 20th	Night	22% 13% 31% 27% 7%	593
	21st to end	Night	21% 10% 32% 29% 8% 1	600
May	1st to 10th	Night	19% 8% 33% 31% 9%	600
	11th to 20th	Night	18% 7% 32% 32% 10%	591
	21st to end	Night	16% 4 29% 35% 16%	650
June	1st to 10th	Night	10% 4 27% 44% 15%	600
	11th to 20th	Night	11% 4 31% 40% 13% 1	600
	21st to end	Night	10% 4 31% 38% 17% 1	600
July	1st to 10th	Night	10% 1 17% 39% 29% 3	585
	11th to 20th	Night	10% 3 20% 41% 25% 2	582
	21st to end	Night	8% 3 17% 46% 25% 1	657
August	1st to 10th	Night	6% 2 14% 46% 29% 2	597
	11th to 20th	Night	9% 3 13% 41% 30% 6%	594
	21st to end	Night	11% 2 17% 42% 26% 3	660
September	1st to 10th	Night	9% 5% 22% 35% 25% 5%	600
	11th to 20th	Night	12% 4 20% 33% 28% 3	660
	21st to end	Night	13% 8% 23% 33% 21% 2	800
October	1st to 10th	Night	14% 7% 30% 31% 17% 1	800
	11th to 20th	Night	21% 11% 32% 26% 10%	793
	21st to end	Night	25% 10% 33% 24% 8%	836
November	1st to 10th	Night	17% 10% 39% 24% 10% 1	780
	11th to 20th	Night	21% 14% 36% 24% 6%	800
	21st to end	Night	23% 13% 33% 24% 7%	953
December	1st to 10th	Night	22% 12% 38% 22% 6%	1000
	11th to 20th	Night	24% 13% 32% 27% 4 1	1000
	21st to end	Night	30% 12% 33% 22% 3	1100

Page generated at 2025-10-16 00:50:23. Version ac07

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