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 2026-10-14.

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% 10% 4 9% 5% 5% 4 4 5% 7% 34%	149188
	Night	13% 12% 4 10% 5% 5% 4 3 5% 6% 32%	142848
Transparency	Day	48% 5% 11% 21% 13% 1	152727
	Night	51% 5% 11% 20% 12% 1	147386
Seeing	Day	19% 15% 30% 21% 11% 4	18537
	Night	19% 11% 30% 28% 10% 1	28307

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	13% 21% 8% 18% 8% 6% 4 3 4 3 12%	4691
	11th to 20th	Day	9% 16% 10% 22% 14% 11% 4 2 3 3 6%	4710
	21st to end	Day	13% 18% 8% 16% 11% 6% 7% 3 3 3 13%	5156
February	1st to 10th	Day	12% 15% 6% 13% 11% 7% 5% 2 4 5% 19%	1916
	11th to 20th	Day	11% 16% 7% 15% 9% 5% 4 3 4 5% 22%	2048
	21st to end	Day	15% 15% 6% 13% 5% 4 5% 3 5% 6% 22%	1730
March	1st to 10th	Day	13% 12% 4 9% 5% 5% 4 3 5% 6% 32%	4580
	11th to 20th	Day	11% 8% 4 9% 6% 5% 4 4 5% 7% 35%	4949
	21st to end	Day	20% 13% 6% 8% 5% 4 4 3 4 5% 28%	5786
April	1st to 10th	Day	18% 15% 4 7% 5% 4 4 3 4 6% 29%	2520
	11th to 20th	Day	18% 11% 3 7% 4 3 4 3 5% 7% 34%	2563
	21st to end	Day	16% 10% 3 7% 5% 5% 5% 3 5% 6% 35%	2730
May	1st to 10th	Day	16% 7% 2 7% 4 4 5% 3 5% 8% 38%	7616
	11th to 20th	Day	14% 8% 2 8% 5% 4 4 4 6% 7% 37%	7642
	21st to end	Day	14% 6% 2 5% 4 4 4 3 5% 7% 44%	9047
June	1st to 10th	Day	10% 6% 2 6% 5% 4 5% 4 7% 9% 43%	2940
	11th to 20th	Day	12% 6% 3 6% 4 4 4 4 5% 9% 43%	2940
	21st to end	Day	10% 6% 3 6% 4 5% 4 4 6% 8% 43%	2940
July	1st to 10th	Day	10% 6% 2 6% 5% 5% 5% 4 7% 10% 40%	8280
	11th to 20th	Day	6% 5% 2 5% 3 4 4 4 6% 9% 50%	8280
	21st to end	Day	8% 5% 2 5% 4 4 5% 4 6% 9% 47%	8990
August	1st to 10th	Day	14% 7% 2 6% 4 4 4 4 6% 9% 39%	6084
	11th to 20th	Day	12% 6% 3 7% 4 4 5% 4 6% 8% 41%	5951
	21st to end	Day	7% 5% 2 7% 5% 6% 5% 5% 7% 9% 42%	6180
September	1st to 10th	Day	10% 7% 2 7% 5% 5% 5% 4 6% 7% 43%	2483
	11th to 20th	Day	12% 4 2 5% 3 5% 4 4 6% 7% 46%	2310
	21st to end	Day	13% 7% 3 7% 5% 4 4 3 5% 7% 42%	2310
October	1st to 10th	Day	14% 8% 2 7% 4 4 3 3 4 6% 44%	2310
	11th to 20th	Day	20% 10% 3 9% 5% 5% 5% 3 5% 7% 27%	1973
	21st to end	Day	20% 11% 4 9% 6% 5% 5% 4 4 6% 26%	1980
November	1st to 10th	Day	13% 12% 3 7% 5% 4 4 4 6% 7% 35%	1852
	11th to 20th	Day	15% 18% 6% 11% 5% 4 4 4 5% 5% 23%	1890
	21st to end	Day	20% 17% 5% 9% 5% 4 3 3 5% 5% 23%	1890
December	1st to 10th	Day	10% 21% 10% 14% 6% 5% 5% 3 5% 6% 15%	2763
	11th to 20th	Day	15% 21% 12% 13% 9% 4 4 3 4 4 12%	3088
	21st to end	Day	23% 23% 7% 12% 5% 4 3 2 4 4 13%	4070
January	1st to 10th	Night	12% 18% 9% 20% 7% 5% 4 3 4 4 16%	8232
	11th to 20th	Night	10% 19% 9% 21% 10% 9% 4 3 3 3 9%	8280
	21st to end	Night	12% 17% 7% 16% 9% 7% 7% 4 5% 5% 13%	9058
February	1st to 10th	Night	12% 17% 6% 12% 9% 6% 4 3 5% 5% 21%	2700
	11th to 20th	Night	12% 14% 6% 11% 7% 6% 5% 4 4 5% 25%	2458
	21st to end	Night	15% 15% 4 10% 7% 5% 5% 4 4 6% 25%	2076
March	1st to 10th	Night	15% 9% 4 9% 4 4 4 3 5% 7% 36%	5493
	11th to 20th	Night	12% 9% 2 6% 4 4 4 3 6% 9% 42%	5231
	21st to end	Night	17% 13% 4 7% 5% 4 4 3 5% 6% 32%	5430
April	1st to 10th	Night	17% 10% 3 7% 5% 4 5% 3 5% 6% 35%	2100
	11th to 20th	Night	20% 9% 2 7% 4 4 4 4 5% 6% 37%	2004
	21st to end	Night	16% 9% 3 7% 3 4 5% 3 6% 7% 39%	1890
May	1st to 10th	Night	14% 8% 3 6% 5% 4 4 4 5% 7% 40%	5237
	11th to 20th	Night	13% 7% 2 7% 4 4 3 4 6% 8% 42%	5204
	21st to end	Night	15% 6% 2 6% 4 4 4 3 4 6% 46%	5139
June	1st to 10th	Night	10% 6% 2 6% 5% 5% 4 4 6% 8% 45%	1680
	11th to 20th	Night	11% 5% 2 6% 4 4 5% 3 7% 9% 43%	1680
	21st to end	Night	9% 5% 2 5% 5% 5% 4 4 6% 9% 46%	1680
July	1st to 10th	Night	10% 6% 3 6% 4 4 5% 4 6% 9% 43%	4708
	11th to 20th	Night	6% 4 2 5% 3 4 4 4 6% 9% 56%	4707
	21st to end	Night	10% 6% 2 5% 4 4 3 4 5% 7% 50%	5117
August	1st to 10th	Night	14% 7% 2 5% 3 3 3 3 5% 7% 47%	4068
	11th to 20th	Night	12% 5% 2 6% 4 4 4 4 6% 8% 45%	4095
	21st to end	Night	8% 6% 2 6% 5% 5% 4 4 6% 8% 46%	4930
September	1st to 10th	Night	10% 4 2 6% 4 4 4 3 7% 6% 51%	2116
	11th to 20th	Night	10% 4 2 5% 3 3 3 4 4 6% 55%	2310
	21st to end	Night	13% 7% 2 7% 4 4 4 3 4 7% 44%	2310
October	1st to 10th	Night	14% 6% 2 5% 3 3 3 3 5% 7% 49%	2310
	11th to 20th	Night	19% 10% 3 7% 5% 4 4 3 5% 6% 35%	2488
	21st to end	Night	22% 10% 3 6% 4 3 4 3 5% 7% 34%	2860
November	1st to 10th	Night	14% 10% 3 7% 4 5% 4 3 6% 6% 37%	2666
	11th to 20th	Night	15% 14% 4 8% 5% 5% 5% 4 7% 7% 25%	2730
	21st to end	Night	17% 17% 6% 9% 4 4 4 3 4 6% 25%	2730
December	1st to 10th	Night	12% 19% 7% 14% 6% 5% 5% 3 5% 6% 19%	4642
	11th to 20th	Night	17% 22% 7% 14% 8% 5% 4 3 4 4 13%	5339
	21st to end	Night	25% 21% 6% 10% 4 4 4 3 4 5% 14%	7150

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	76% 6% 8% 9% 1	4692
	11th to 20th	Day	85% 4 5% 4 2	4710
	21st to end	Day	78% 4 8% 8% 3	5156
February	1st to 10th	Day	69% 5% 8% 14% 5%	1916
	11th to 20th	Day	65% 5% 9% 14% 8%	2058
	21st to end	Day	62% 6% 11% 14% 6%	1730
March	1st to 10th	Day	53% 4 14% 18% 10% 1	4580
	11th to 20th	Day	47% 6% 15% 21% 10% 1	4949
	21st to end	Day	59% 4 10% 15% 11% 1	5788
April	1st to 10th	Day	56% 5% 10% 18% 11% 1	2520
	11th to 20th	Day	49% 5% 10% 21% 14% 2	2563
	21st to end	Day	48% 6% 11% 20% 13% 2	2730
May	1st to 10th	Day	44% 6% 10% 23% 17% 2	7651
	11th to 20th	Day	44% 7% 10% 21% 16% 1	7646
	21st to end	Day	38% 6% 9% 22% 23% 3	9061
June	1st to 10th	Day	36% 7% 11% 27% 17% 2	2940
	11th to 20th	Day	37% 6% 13% 23% 20% 1	2940
	21st to end	Day	37% 7% 13% 23% 17% 2	2940
July	1st to 10th	Day	38% 6% 13% 23% 17% 2	8211
	11th to 20th	Day	28% 6% 11% 30% 23% 1	8205
	21st to end	Day	32% 6% 13% 26% 23% 1	8794
August	1st to 10th	Day	41% 5% 12% 24% 17% 1	6069
	11th to 20th	Day	39% 6% 14% 27% 13% 1	5990
	21st to end	Day	37% 6% 14% 29% 13% 1	6179
September	1st to 10th	Day	39% 6% 13% 27% 14% 2	2483
	11th to 20th	Day	34% 7% 12% 28% 17% 2	2310
	21st to end	Day	42% 5% 12% 24% 17% 1	2310
October	1st to 10th	Day	30% 4 7% 37% 20% 2	3700
	11th to 20th	Day	43% 4 11% 26% 15%	3176
	21st to end	Day	51% 4 13% 23% 9% 1	3180
November	1st to 10th	Day	47% 6% 13% 21% 12% 1	1852
	11th to 20th	Day	62% 5% 11% 15% 5% 1	1890
	21st to end	Day	63% 5% 14% 14% 4	1890
December	1st to 10th	Day	70% 9% 12% 8% 2	2763
	11th to 20th	Day	78% 4 8% 9% 1	3085
	21st to end	Day	76% 4 6% 10% 4	4070
January	1st to 10th	Night	72% 4 9% 10% 5% 1	8233
	11th to 20th	Night	81% 3 7% 4 4	8280
	21st to end	Night	73% 5% 10% 9% 3	9058
February	1st to 10th	Night	65% 5% 10% 14% 6% 1	2700
	11th to 20th	Night	62% 5% 8% 17% 8% 1	2459
	21st to end	Night	59% 4 13% 16% 7% 1	2076
March	1st to 10th	Night	48% 5% 13% 24% 9% 1	5493
	11th to 20th	Night	39% 4 16% 26% 13% 2	5231
	21st to end	Night	53% 5% 10% 18% 12% 1	5430
April	1st to 10th	Night	50% 5% 10% 20% 14% 1	2100
	11th to 20th	Night	48% 5% 8% 19% 18% 3	2004
	21st to end	Night	44% 6% 13% 20% 14% 3	1890
May	1st to 10th	Night	43% 6% 12% 19% 18% 2	5264
	11th to 20th	Night	40% 6% 10% 24% 18% 2	5205
	21st to end	Night	40% 4 8% 21% 22% 4	5149
June	1st to 10th	Night	36% 6% 10% 28% 17% 3	1680
	11th to 20th	Night	36% 7% 13% 24% 18% 1	1680
	21st to end	Night	34% 7% 16% 25% 16% 2	1680
July	1st to 10th	Night	37% 6% 12% 26% 15% 4	4670
	11th to 20th	Night	25% 5% 11% 30% 26% 2	4662
	21st to end	Night	34% 5% 11% 25% 23% 3	5002
August	1st to 10th	Night	37% 5% 12% 27% 19% 1	4059
	11th to 20th	Night	36% 8% 13% 30% 12% 1	4122
	21st to end	Night	35% 6% 12% 31% 15% 1	4930
September	1st to 10th	Night	32% 7% 12% 29% 18% 2	2116
	11th to 20th	Night	30% 5% 11% 33% 19% 2	2310
	21st to end	Night	40% 5% 11% 28% 15% 2	2310
October	1st to 10th	Night	28% 4 9% 37% 21% 2	3700
	11th to 20th	Night	40% 5% 12% 23% 19%	4024
	21st to end	Night	43% 3 10% 29% 14%	4610
November	1st to 10th	Night	46% 6% 10% 23% 13% 1	2666
	11th to 20th	Night	56% 6% 13% 18% 6% 1	2730
	21st to end	Night	60% 5% 15% 16% 4	2730
December	1st to 10th	Night	67% 7% 13% 11% 2	4642
	11th to 20th	Night	75% 4 9% 8% 3	5341
	21st to end	Night	73% 4 9% 9% 5%	7150

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% 22% 36% 17% 5% 1	330
	11th to 20th	Day	22% 26% 39% 10% 2	330
	21st to end	Day	31% 18% 33% 16% 3	363
February	1st to 10th	Day	23% 30% 28% 15% 2 2	330
	11th to 20th	Day	23% 31% 31% 15% 1	330
	21st to end	Day	32% 29% 30% 7% 1	273
March	1st to 10th	Day	25% 26% 38% 8% 2	330
	11th to 20th	Day	27% 17% 31% 17% 7%	477
	21st to end	Day	30% 20% 28% 18% 4 2	594
April	1st to 10th	Day	30% 19% 30% 16% 4 1	540
	11th to 20th	Day	29% 22% 27% 14% 6% 2	532
	21st to end	Day	26% 14% 31% 20% 8% 1	540
May	1st to 10th	Day	23% 19% 31% 19% 6% 2	543
	11th to 20th	Day	19% 14% 31% 23% 9% 4	570
	21st to end	Day	15% 14% 30% 23% 14% 4	844
June	1st to 10th	Day	12% 14% 35% 27% 9% 3	780
	11th to 20th	Day	16% 14% 33% 24% 11% 3	780
	21st to end	Day	14% 10% 36% 24% 12% 3	780
July	1st to 10th	Day	8% 8% 30% 29% 18% 8%	771
	11th to 20th	Day	11% 9% 27% 30% 15% 8%	768
	21st to end	Day	10% 6% 32% 30% 16% 7%	849
August	1st to 10th	Day	12% 6% 23% 27% 22% 11%	598
	11th to 20th	Day	12% 6% 23% 29% 20% 10%	559
	21st to end	Day	12% 8% 24% 28% 20% 8%	627
September	1st to 10th	Day	15% 8% 23% 28% 18% 8%	570
	11th to 20th	Day	12% 5% 20% 28% 26% 9%	570
	21st to end	Day	17% 11% 23% 26% 16% 6%	570
October	1st to 10th	Day	17% 14% 26% 25% 15% 3	570
	11th to 20th	Day	23% 18% 33% 21% 5% 1	443
	21st to end	Day	29% 18% 37% 12% 4 1	363
November	1st to 10th	Day	25% 19% 35% 14% 6% 1	330
	11th to 20th	Day	27% 24% 34% 10% 5%	330
	21st to end	Day	30% 19% 38% 11% 2	330
December	1st to 10th	Day	26% 29% 25% 14% 5% 1	330
	11th to 20th	Day	28% 27% 31% 11% 3	330
	21st to end	Day	30% 26% 31% 9% 3 1	363
January	1st to 10th	Night	21% 18% 33% 22% 5%	1050
	11th to 20th	Night	24% 17% 33% 20% 4	1050
	21st to end	Night	23% 12% 34% 28% 3	1155
February	1st to 10th	Night	21% 15% 35% 25% 3	871
	11th to 20th	Night	21% 16% 35% 23% 4	823
	21st to end	Night	25% 17% 36% 20% 2	692
March	1st to 10th	Night	21% 14% 34% 24% 7%	840
	11th to 20th	Night	19% 16% 36% 23% 5% 1	840
	21st to end	Night	22% 14% 31% 26% 6%	856
April	1st to 10th	Night	21% 13% 35% 25% 5% 1	630
	11th to 20th	Night	27% 13% 31% 22% 6% 1	623
	21st to end	Night	22% 12% 33% 27% 6%	630
May	1st to 10th	Night	16% 10% 37% 30% 7%	630
	11th to 20th	Night	18% 9% 33% 29% 11% 1	621
	21st to end	Night	15% 7% 28% 34% 16% 1	683
June	1st to 10th	Night	12% 5% 31% 37% 14% 1	630
	11th to 20th	Night	13% 5% 31% 36% 14%	630
	21st to end	Night	11% 6% 26% 40% 17% 1	630
July	1st to 10th	Night	8% 3 23% 39% 24% 2	615
	11th to 20th	Night	7% 3 23% 42% 23% 3	612
	21st to end	Night	9% 2 23% 40% 25% 1	690
August	1st to 10th	Night	8% 3 20% 42% 25% 2	627
	11th to 20th	Night	10% 3 19% 41% 22% 4	624
	21st to end	Night	10% 3 21% 38% 25% 2	693
September	1st to 10th	Night	10% 4 21% 38% 23% 4	646
	11th to 20th	Night	12% 5% 21% 36% 24% 3	840
	21st to end	Night	16% 8% 24% 31% 18% 3	840
October	1st to 10th	Night	18% 6% 28% 33% 15% 1	840
	11th to 20th	Night	25% 11% 28% 27% 9% 1	813
	21st to end	Night	27% 11% 33% 22% 7%	880
November	1st to 10th	Night	22% 12% 33% 25% 8% 1	820
	11th to 20th	Night	24% 16% 34% 21% 6%	840
	21st to end	Night	27% 15% 31% 24% 4	840
December	1st to 10th	Night	24% 17% 30% 23% 5%	998
	11th to 20th	Night	26% 16% 35% 18% 4 1	1050
	21st to end	Night	32% 13% 37% 17% 1	1155

Page generated at 2026-10-11 05:06:26. Version ac07

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