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 2025-06-10.

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	14% 11% 4 9% 6% 5% 4 3 5% 7% 33%	119254
	Night	14% 12% 4 10% 5% 5% 4 3 5% 6% 31%	119165
Transparency	Day	50% 6% 11% 20% 12% 1	122838
	Night	52% 5% 11% 19% 11% 1	123722
Seeing	Day	19% 15% 30% 21% 11% 4	16596
	Night	20% 11% 30% 28% 10% 1	26540

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% 20% 7% 19% 8% 6% 4 3 4 4 14%	3617
	11th to 20th	Day	11% 16% 8% 20% 15% 10% 5% 3 4 3 5%	3620
	21st to end	Day	16% 20% 9% 19% 7% 4 5% 2 3 4 11%	3982
February	1st to 10th	Day	12% 15% 6% 13% 12% 7% 5% 3 4 5% 18%	1824
	11th to 20th	Day	12% 16% 6% 15% 9% 5% 4 3 4 5% 21%	1948
	21st to end	Day	15% 16% 6% 13% 5% 4 5% 3 5% 6% 22%	1650
March	1st to 10th	Day	13% 12% 4 9% 5% 5% 4 4 5% 6% 32%	4480
	11th to 20th	Day	11% 8% 4 8% 6% 5% 5% 4 6% 7% 36%	4842
	21st to end	Day	20% 13% 6% 8% 5% 4 4 3 4 5% 28%	5661
April	1st to 10th	Day	18% 15% 4 7% 5% 4 4 3 4 6% 30%	2400
	11th to 20th	Day	17% 11% 3 7% 5% 4 4 3 5% 7% 35%	2440
	21st to end	Day	16% 10% 3 7% 5% 5% 5% 4 5% 6% 34%	2600
May	1st to 10th	Day	19% 8% 2 6% 4 4 5% 3 5% 7% 38%	5862
	11th to 20th	Day	14% 7% 2 7% 5% 4 4 4 6% 8% 39%	5877
	21st to end	Day	14% 7% 2 6% 4 5% 4 3 6% 7% 41%	6935
June	1st to 10th	Day	10% 6% 2 6% 5% 4 5% 4 7% 8% 43%	2787
	11th to 20th	Day	11% 6% 2 6% 4 4 4 4 5% 9% 45%	2660
	21st to end	Day	10% 6% 3 7% 4 5% 4 4 6% 8% 42%	2660
July	1st to 10th	Day	11% 7% 3 7% 5% 5% 5% 4 7% 9% 36%	4440
	11th to 20th	Day	7% 4 2 4 4 5% 5% 4 7% 10% 49%	4440
	21st to end	Day	7% 6% 2 6% 5% 5% 5% 4 7% 9% 45%	4813
August	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 4 7% 8% 40%	4145
	11th to 20th	Day	12% 5% 2 6% 4 4 5% 4 5% 7% 46%	4044
	21st to end	Day	8% 5% 3 6% 5% 5% 5% 4 6% 9% 44%	4252
September	1st to 10th	Day	10% 7% 2 7% 5% 4 5% 3 6% 7% 44%	2247
	11th to 20th	Day	12% 4 2 5% 4 5% 4 4 6% 7% 47%	2090
	21st to end	Day	13% 7% 3 7% 5% 4 4 3 5% 8% 41%	2090
October	1st to 10th	Day	15% 8% 2 8% 5% 4 3 3 4 6% 42%	2090
	11th to 20th	Day	20% 10% 3 9% 5% 5% 5% 3 5% 7% 26%	1844
	21st to end	Day	21% 11% 4 10% 6% 5% 5% 4 5% 6% 24%	1881
November	1st to 10th	Day	14% 12% 3 7% 5% 4 4 4 6% 7% 35%	1762
	11th to 20th	Day	15% 18% 6% 11% 5% 5% 4 4 6% 6% 22%	1800
	21st to end	Day	20% 17% 5% 9% 5% 4 3 3 5% 5% 24%	1800
December	1st to 10th	Day	10% 21% 10% 14% 6% 5% 5% 3 5% 6% 15%	2681
	11th to 20th	Day	15% 21% 12% 13% 9% 4 4 3 4 4 12%	3008
	21st to end	Day	23% 23% 7% 12% 5% 4 3 2 4 4 13%	3982
January	1st to 10th	Night	11% 18% 9% 20% 7% 5% 4 3 4 4 16%	6348
	11th to 20th	Night	12% 17% 8% 21% 11% 9% 4 3 3 4 9%	6360
	21st to end	Night	15% 19% 8% 14% 7% 6% 6% 3 4 4 14%	6996
February	1st to 10th	Night	13% 17% 6% 12% 9% 6% 4 3 5% 5% 20%	2572
	11th to 20th	Night	12% 14% 6% 11% 7% 6% 5% 4 4 5% 24%	2338
	21st to end	Night	15% 15% 4 10% 7% 5% 5% 4 4 6% 25%	1980
March	1st to 10th	Night	15% 9% 4 9% 4 4 4 3 5% 7% 36%	5373
	11th to 20th	Night	11% 9% 2 6% 4 4 4 3 6% 9% 42%	5118
	21st to end	Night	18% 13% 4 7% 5% 4 4 3 5% 6% 32%	5312
April	1st to 10th	Night	17% 10% 3 7% 5% 3 4 3 5% 6% 36%	2000
	11th to 20th	Night	19% 9% 2 7% 4 4 4 4 5% 6% 38%	1907
	21st to end	Night	16% 9% 3 7% 3 4 5% 3 6% 6% 39%	1800
May	1st to 10th	Night	17% 9% 3 7% 5% 4 4 4 5% 7% 36%	4033
	11th to 20th	Night	15% 8% 3 7% 4 4 3 4 5% 6% 41%	4002
	21st to end	Night	18% 7% 2 6% 4 3 3 3 4 6% 44%	3942
June	1st to 10th	Night	10% 6% 2 7% 5% 5% 4 4 6% 8% 44%	1597
	11th to 20th	Night	10% 5% 2 6% 4 4 5% 3 7% 9% 44%	1520
	21st to end	Night	9% 5% 2 5% 4 5% 4 4 6% 9% 46%	1520
July	1st to 10th	Night	9% 7% 2 7% 5% 5% 5% 4 6% 9% 40%	2530
	11th to 20th	Night	7% 4 2 5% 3 4 4 4 6% 6% 56%	2530
	21st to end	Night	7% 5% 2 6% 4 5% 4 4 6% 8% 49%	2750
August	1st to 10th	Night	10% 6% 2 5% 3 3 4 3 5% 8% 50%	2785
	11th to 20th	Night	11% 4 2 5% 4 5% 4 3 5% 7% 50%	2785
	21st to end	Night	9% 6% 2 6% 5% 5% 5% 4 6% 8% 45%	3401
September	1st to 10th	Night	10% 4 2 5% 4 4 4 3 7% 6% 51%	1914
	11th to 20th	Night	10% 5% 2 5% 3 3 3 4 5% 6% 55%	2090
	21st to end	Night	13% 7% 2 6% 4 3 5% 3 4 8% 44%	2090
October	1st to 10th	Night	15% 6% 2 6% 3 3 3 3 5% 7% 47%	2090
	11th to 20th	Night	19% 11% 3 7% 5% 4 4 3 5% 6% 34%	2330
	21st to end	Night	23% 10% 3 6% 4 3 5% 3 5% 7% 32%	2717
November	1st to 10th	Night	14% 10% 3 6% 4 5% 4 3 6% 6% 38%	2536
	11th to 20th	Night	15% 15% 4 9% 5% 5% 5% 4 7% 7% 24%	2600
	21st to end	Night	17% 16% 6% 9% 5% 4 4 3 5% 6% 25%	2600
December	1st to 10th	Night	12% 19% 7% 14% 6% 5% 4 3 5% 6% 19%	4504
	11th to 20th	Night	17% 22% 7% 13% 8% 5% 3 3 4 4 13%	5199
	21st to end	Night	25% 21% 6% 10% 4 4 4 3 4 5% 14%	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	73% 6% 8% 11% 1	3618
	11th to 20th	Day	84% 5% 6% 4 2	3620
	21st to end	Day	79% 4 8% 6% 2	3982
February	1st to 10th	Day	70% 5% 8% 13% 5%	1824
	11th to 20th	Day	66% 5% 8% 14% 7%	1958
	21st to end	Day	62% 6% 11% 14% 7%	1650
March	1st to 10th	Day	53% 4 14% 18% 10% 1	4480
	11th to 20th	Day	46% 6% 15% 22% 10% 1	4842
	21st to end	Day	59% 4 10% 15% 11% 1	5663
April	1st to 10th	Day	55% 5% 10% 19% 11% 1	2400
	11th to 20th	Day	49% 5% 10% 21% 14% 2	2440
	21st to end	Day	48% 6% 11% 19% 14% 2	2600
May	1st to 10th	Day	45% 6% 10% 20% 16% 2	5897
	11th to 20th	Day	42% 8% 11% 22% 15% 2	5881
	21st to end	Day	40% 7% 10% 23% 19% 2	6949
June	1st to 10th	Day	36% 7% 11% 28% 16% 2	2787
	11th to 20th	Day	36% 6% 12% 23% 21% 1	2660
	21st to end	Day	39% 7% 13% 22% 18% 2	2660
July	1st to 10th	Day	43% 6% 13% 19% 15% 3	4371
	11th to 20th	Day	28% 8% 14% 30% 18% 1	4365
	21st to end	Day	34% 8% 13% 22% 22% 2	4662
August	1st to 10th	Day	38% 6% 11% 27% 16% 1	4130
	11th to 20th	Day	36% 6% 11% 28% 18% 1	4083
	21st to end	Day	36% 6% 13% 27% 16% 2	4251
September	1st to 10th	Day	39% 6% 13% 26% 15% 2	2247
	11th to 20th	Day	34% 7% 11% 29% 17% 2	2090
	21st to end	Day	42% 5% 12% 24% 16%	2090
October	1st to 10th	Day	30% 4 7% 36% 20% 2	3480
	11th to 20th	Day	43% 5% 11% 26% 15%	3047
	21st to end	Day	51% 4 13% 23% 8% 1	3081
November	1st to 10th	Day	47% 6% 13% 21% 12% 1	1762
	11th to 20th	Day	62% 6% 12% 15% 6% 1	1800
	21st to end	Day	63% 5% 15% 13% 4	1800
December	1st to 10th	Day	70% 9% 12% 8% 2	2681
	11th to 20th	Day	78% 4 8% 9% 1	3005
	21st to end	Day	76% 5% 6% 10% 4	3982
January	1st to 10th	Night	71% 5% 8% 11% 5% 1	6348
	11th to 20th	Night	80% 4 8% 5% 4	6360
	21st to end	Night	74% 5% 10% 7% 3	6996
February	1st to 10th	Night	66% 5% 10% 13% 6%	2572
	11th to 20th	Night	63% 5% 9% 16% 8% 1	2339
	21st to end	Night	59% 4 13% 15% 7% 1	1980
March	1st to 10th	Night	48% 5% 13% 24% 9% 1	5373
	11th to 20th	Night	38% 5% 16% 26% 13% 2	5118
	21st to end	Night	53% 5% 10% 18% 12% 1	5312
April	1st to 10th	Night	49% 5% 10% 20% 14% 1	2000
	11th to 20th	Night	47% 5% 8% 20% 17% 3	1907
	21st to end	Night	45% 6% 12% 19% 15% 3	1800
May	1st to 10th	Night	48% 6% 12% 15% 16% 3	4060
	11th to 20th	Night	43% 6% 8% 22% 18% 3	4003
	21st to end	Night	42% 5% 8% 22% 20% 3	3950
June	1st to 10th	Night	37% 6% 10% 28% 16% 3	1597
	11th to 20th	Night	35% 7% 13% 24% 19% 1	1520
	21st to end	Night	35% 7% 15% 25% 16% 2	1520
July	1st to 10th	Night	39% 6% 10% 25% 12% 7%	2490
	11th to 20th	Night	26% 7% 12% 29% 25% 1	2482
	21st to end	Night	33% 8% 13% 20% 23% 3	2662
August	1st to 10th	Night	32% 5% 14% 29% 18% 1	2776
	11th to 20th	Night	33% 7% 9% 34% 16% 1	2812
	21st to end	Night	37% 6% 10% 28% 18% 2	3401
September	1st to 10th	Night	31% 8% 12% 29% 19% 2	1914
	11th to 20th	Night	29% 6% 11% 33% 19% 2	2090
	21st to end	Night	41% 5% 11% 28% 14% 1	2090
October	1st to 10th	Night	28% 4 9% 36% 21% 2	3480
	11th to 20th	Night	40% 5% 13% 23% 18%	3866
	21st to end	Night	44% 3 10% 29% 14%	4467
November	1st to 10th	Night	45% 6% 11% 23% 13% 1	2536
	11th to 20th	Night	57% 6% 13% 16% 7% 1	2600
	21st to end	Night	59% 5% 15% 16% 4	2600
December	1st to 10th	Night	67% 7% 13% 11% 2	4504
	11th to 20th	Night	75% 4 9% 8% 3	5201
	21st to end	Night	73% 4 9% 9% 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	18% 20% 38% 17% 5% 1	300
	11th to 20th	Day	21% 25% 40% 11% 2	300
	21st to end	Day	29% 18% 33% 16% 3	330
February	1st to 10th	Day	24% 30% 27% 16% 2 2	300
	11th to 20th	Day	23% 31% 31% 15% 1	300
	21st to end	Day	31% 29% 31% 8% 2	249
March	1st to 10th	Day	24% 28% 38% 8% 2	300
	11th to 20th	Day	25% 16% 33% 18% 8%	440
	21st to end	Day	30% 18% 28% 18% 4 2	550
April	1st to 10th	Day	28% 19% 30% 17% 5% 1	500
	11th to 20th	Day	28% 23% 27% 14% 6% 3	492
	21st to end	Day	25% 14% 31% 20% 8% 1	500
May	1st to 10th	Day	23% 19% 30% 20% 6% 2	503
	11th to 20th	Day	19% 13% 30% 24% 10% 4	530
	21st to end	Day	15% 14% 29% 24% 14% 4	789
June	1st to 10th	Day	12% 13% 35% 26% 10% 3	726
	11th to 20th	Day	14% 12% 33% 25% 12% 3	680
	21st to end	Day	15% 10% 36% 23% 12% 4	680
July	1st to 10th	Day	8% 7% 29% 28% 19% 8%	671
	11th to 20th	Day	11% 8% 28% 30% 15% 9%	668
	21st to end	Day	9% 5% 31% 31% 17% 7%	739
August	1st to 10th	Day	11% 6% 23% 26% 22% 12%	515
	11th to 20th	Day	13% 5% 21% 28% 21% 12%	479
	21st to end	Day	12% 7% 23% 28% 21% 8%	539
September	1st to 10th	Day	15% 7% 23% 27% 19% 8%	490
	11th to 20th	Day	10% 5% 20% 29% 27% 9%	490
	21st to end	Day	17% 12% 22% 27% 16% 6%	490
October	1st to 10th	Day	19% 14% 24% 24% 16% 3	490
	11th to 20th	Day	24% 20% 31% 21% 4 1	396
	21st to end	Day	30% 18% 37% 10% 4 1	330
November	1st to 10th	Day	25% 19% 35% 14% 6% 1	300
	11th to 20th	Day	26% 24% 36% 10% 5%	300
	21st to end	Day	30% 16% 39% 12% 1	300
December	1st to 10th	Day	26% 31% 24% 14% 5% 1	300
	11th to 20th	Day	25% 28% 32% 12% 3	300
	21st to end	Day	30% 25% 32% 9% 3 1	330
January	1st to 10th	Night	21% 18% 33% 22% 6% 1	1000
	11th to 20th	Night	24% 16% 34% 21% 5% 1	1000
	21st to end	Night	23% 12% 34% 28% 4	1100
February	1st to 10th	Night	22% 15% 35% 24% 3	830
	11th to 20th	Night	21% 16% 36% 23% 4 1	783
	21st to end	Night	24% 17% 36% 20% 2	660
March	1st to 10th	Night	21% 14% 34% 24% 7%	800
	11th to 20th	Night	17% 16% 36% 24% 6% 1	800
	21st to end	Night	23% 13% 31% 27% 7%	815
April	1st to 10th	Night	21% 13% 35% 26% 5% 1	600
	11th to 20th	Night	26% 12% 32% 23% 6% 1	593
	21st to end	Night	23% 12% 32% 26% 6% 1	600
May	1st to 10th	Night	17% 10% 36% 30% 8% 1	600
	11th to 20th	Night	18% 9% 31% 29% 11% 1	591
	21st to end	Night	15% 7% 28% 33% 16% 1	650
June	1st to 10th	Night	12% 6% 31% 37% 14% 1	599
	11th to 20th	Night	12% 5% 31% 35% 16% 1	570
	21st to end	Night	11% 6% 25% 40% 17% 1	570
July	1st to 10th	Night	9% 2 22% 39% 25% 3	555
	11th to 20th	Night	7% 3 24% 40% 23% 3	552
	21st to end	Night	8% 2 23% 40% 26% 1	624
August	1st to 10th	Night	9% 2 20% 41% 25% 2	567
	11th to 20th	Night	10% 3 18% 40% 24% 4	564
	21st to end	Night	11% 3 21% 38% 25% 2	627
September	1st to 10th	Night	10% 4 22% 38% 22% 4	584
	11th to 20th	Night	12% 5% 21% 36% 24% 3	760
	21st to end	Night	18% 8% 23% 30% 18% 3	760
October	1st to 10th	Night	18% 6% 27% 32% 15% 1	760
	11th to 20th	Night	26% 11% 28% 25% 9% 1	760
	21st to end	Night	28% 11% 32% 21% 7%	836
November	1st to 10th	Night	22% 11% 33% 25% 8% 1	780
	11th to 20th	Night	24% 15% 35% 20% 6%	800
	21st to end	Night	26% 14% 31% 25% 4	800
December	1st to 10th	Night	25% 16% 30% 23% 5%	950
	11th to 20th	Night	27% 16% 35% 18% 4 1	1000
	21st to end	Night	32% 13% 37% 17% 1	1100

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