Middle Fork River Forest Preserve
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Middle Fork River Forest Preserve clear sky chart from 2005-11-06 to 2026-08-30.

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	12% 9% 3 8% 5% 5% 5% 4 6% 8% 36%	143861
	Night	13% 9% 4 8% 5% 4 4 3 5% 6% 38%	138641
Transparency	Day	45% 5% 12% 23% 14% 2	147408
	Night	45% 4 10% 23% 15% 2	143158
Seeing	Day	16% 9% 27% 25% 16% 6%	18219
	Night	18% 9% 29% 30% 13% 1	27959

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% 13% 6% 14% 7% 4 4 3 5% 7% 23%	4691
	11th to 20th	Day	9% 11% 6% 15% 8% 8% 5% 5% 6% 4 23%	4710
	21st to end	Day	16% 17% 4 10% 8% 4 4 3 4 5% 24%	5454
February	1st to 10th	Day	16% 12% 5% 9% 7% 5% 4 3 5% 6% 28%	1997
	11th to 20th	Day	15% 12% 4 9% 5% 6% 4 3 5% 7% 31%	2048
	21st to end	Day	15% 13% 6% 9% 4 4 3 3 4 7% 32%	1730
March	1st to 10th	Day	18% 15% 4 7% 4 3 2 2 2 4 38%	4580
	11th to 20th	Day	8% 10% 4 6% 4 4 5% 4 6% 8% 42%	4772
	21st to end	Day	16% 12% 5% 9% 5% 5% 5% 3 5% 6% 28%	6099
April	1st to 10th	Day	17% 10% 4 7% 5% 3 4 5% 5% 7% 34%	2520
	11th to 20th	Day	15% 9% 3 7% 5% 4 4 4 5% 7% 37%	2494
	21st to end	Day	20% 8% 3 7% 4 5% 4 3 4 7% 35%	2603
May	1st to 10th	Day	17% 8% 3 7% 5% 5% 5% 5% 6% 8% 31%	7616
	11th to 20th	Day	14% 7% 3 8% 5% 5% 5% 4 5% 7% 38%	7763
	21st to end	Day	12% 8% 3 8% 5% 5% 5% 4 6% 9% 35%	9047
June	1st to 10th	Day	10% 6% 3 6% 5% 5% 5% 5% 7% 8% 42%	2940
	11th to 20th	Day	8% 5% 2 6% 4 5% 5% 4 6% 9% 46%	2940
	21st to end	Day	10% 6% 3 7% 5% 5% 5% 4 7% 9% 40%	2940
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 5% 8% 11% 40%	8280
	11th to 20th	Day	8% 5% 2 7% 5% 5% 6% 4 7% 10% 41%	8280
	21st to end	Day	10% 6% 3 7% 5% 5% 6% 4 7% 9% 38%	8990
August	1st to 10th	Day	7% 4 2 6% 4 4 5% 4 6% 9% 51%	4436
	11th to 20th	Day	8% 5% 2 6% 4 5% 5% 5% 7% 12% 41%	3969
	21st to end	Day	5% 4 2 5% 4 4 4 4 7% 11% 50%	4420
September	1st to 10th	Day	10% 5% 3 6% 5% 4 3 4 6% 7% 48%	2400
	11th to 20th	Day	9% 5% 2 4 4 4 4 4 5% 7% 52%	2259
	21st to end	Day	11% 5% 2 5% 5% 4 4 4 6% 7% 47%	2200
October	1st to 10th	Day	10% 5% 2 6% 5% 4 5% 4 4 7% 47%	2179
	11th to 20th	Day	15% 7% 2 6% 4 4 4 3 6% 7% 42%	2000
	21st to end	Day	18% 9% 3 7% 4 4 4 4 5% 7% 35%	1986
November	1st to 10th	Day	11% 8% 2 5% 3 4 4 4 5% 5% 50%	1852
	11th to 20th	Day	16% 11% 3 7% 4 3 4 3 4 5% 38%	1890
	21st to end	Day	17% 11% 4 10% 5% 4 3 3 4 4 36%	1890
December	1st to 10th	Day	10% 11% 5% 8% 5% 4 3 2 4 5% 42%	2728
	11th to 20th	Day	14% 15% 5% 12% 6% 5% 6% 4 5% 6% 23%	3088
	21st to end	Day	26% 18% 6% 13% 5% 4 4 3 4 4 12%	4070
January	1st to 10th	Night	13% 14% 6% 11% 5% 4 4 3 5% 4 29%	8232
	11th to 20th	Night	10% 14% 6% 14% 7% 6% 6% 4 4 4 25%	8280
	21st to end	Night	16% 12% 5% 9% 5% 4 4 3 4 5% 31%	8759
February	1st to 10th	Night	17% 12% 5% 10% 6% 5% 3 3 4 6% 29%	2620
	11th to 20th	Night	14% 11% 4 10% 5% 3 4 3 5% 6% 34%	2458
	21st to end	Night	17% 10% 5% 9% 5% 4 3 3 4 6% 35%	2076
March	1st to 10th	Night	17% 14% 4 6% 4 3 3 2 4 5% 38%	5493
	11th to 20th	Night	11% 8% 3 7% 4 4 4 3 5% 7% 44%	5436
	21st to end	Night	17% 9% 4 10% 6% 5% 5% 4 5% 6% 30%	5072
April	1st to 10th	Night	13% 10% 3 6% 4 5% 4 4 5% 6% 40%	2100
	11th to 20th	Night	16% 8% 3 7% 4 4 4 2 4 7% 41%	2075
	21st to end	Night	18% 8% 3 7% 4 4 5% 3 5% 6% 37%	2016
May	1st to 10th	Night	18% 10% 3 8% 5% 4 5% 3 5% 6% 33%	5237
	11th to 20th	Night	13% 6% 2 5% 4 3 4 3 6% 9% 43%	5089
	21st to end	Night	12% 8% 3 8% 5% 5% 5% 4 6% 8% 35%	5139
June	1st to 10th	Night	10% 5% 3 6% 3 3 5% 3 5% 8% 49%	1680
	11th to 20th	Night	8% 6% 3 6% 4 4 4 4 6% 9% 48%	1680
	21st to end	Night	8% 5% 3 6% 5% 5% 4 4 8% 9% 43%	1680
July	1st to 10th	Night	9% 5% 2 6% 4 4 4 4 6% 9% 45%	4708
	11th to 20th	Night	6% 5% 2 6% 5% 5% 5% 4 7% 9% 47%	4707
	21st to end	Night	7% 6% 3 8% 5% 6% 5% 4 7% 8% 42%	5117
August	1st to 10th	Night	7% 4 1 4 4 4 4 3 7% 9% 52%	2930
	11th to 20th	Night	6% 5% 2 6% 4 4 5% 4 7% 8% 48%	3300
	21st to end	Night	6% 4 1 5% 3 4 4 4 5% 8% 55%	3676
September	1st to 10th	Night	12% 4 2 5% 4 2 3 3 4 7% 55%	2000
	11th to 20th	Night	7% 4 1 4 4 3 4 3 6% 8% 56%	2120
	21st to end	Night	10% 6% 3 5% 4 3 4 4 5% 7% 49%	2200
October	1st to 10th	Night	11% 5% 1 5% 4 3 3 3 5% 5% 55%	2218
	11th to 20th	Night	15% 6% 2 5% 3 3 3 2 5% 6% 49%	2400
	21st to end	Night	18% 8% 3 7% 4 4 4 3 4 6% 39%	2852
November	1st to 10th	Night	11% 7% 2 6% 4 3 4 3 5% 6% 49%	2666
	11th to 20th	Night	13% 8% 3 7% 5% 4 4 3 5% 6% 41%	2730
	21st to end	Night	18% 11% 4 9% 4 3 3 3 4 6% 35%	2730
December	1st to 10th	Night	12% 9% 3 8% 4 4 3 3 5% 5% 43%	4676
	11th to 20th	Night	17% 11% 4 9% 5% 5% 4 3 4 5% 32%	5339
	21st to end	Night	26% 17% 6% 10% 5% 4 4 3 4 4 16%	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	60% 4 13% 16% 6% 1	4692
	11th to 20th	Day	63% 3 8% 12% 12% 2	4710
	21st to end	Day	64% 2 9% 18% 6% 1	5454
February	1st to 10th	Day	57% 4 12% 17% 8% 2	1997
	11th to 20th	Day	54% 3 10% 22% 9% 1	2058
	21st to end	Day	53% 3 10% 23% 9% 2	1730
March	1st to 10th	Day	53% 3 6% 19% 16% 3	4580
	11th to 20th	Day	38% 5% 10% 34% 12% 1	4772
	21st to end	Day	55% 5% 13% 18% 8% 1	6101
April	1st to 10th	Day	49% 5% 12% 21% 13% 1	2520
	11th to 20th	Day	46% 5% 12% 22% 12% 3	2494
	21st to end	Day	49% 5% 12% 21% 11% 2	2603
May	1st to 10th	Day	49% 6% 12% 21% 12%	7651
	11th to 20th	Day	45% 6% 11% 21% 15% 1	7767
	21st to end	Day	45% 6% 11% 19% 17% 3	9061
June	1st to 10th	Day	37% 7% 13% 23% 17% 3	2940
	11th to 20th	Day	34% 6% 11% 24% 21% 4	2940
	21st to end	Day	39% 8% 14% 21% 16% 3	2940
July	1st to 10th	Day	36% 6% 14% 28% 13% 2	8211
	11th to 20th	Day	36% 7% 14% 29% 14%	8205
	21st to end	Day	41% 6% 14% 25% 12% 1	8794
August	1st to 10th	Day	30% 6% 17% 32% 15% 1	4421
	11th to 20th	Day	34% 6% 15% 27% 16% 1	4005
	21st to end	Day	28% 6% 15% 32% 16% 4	4419
September	1st to 10th	Day	35% 6% 11% 25% 19% 4	2400
	11th to 20th	Day	32% 5% 11% 25% 24% 4	2259
	21st to end	Day	36% 5% 10% 25% 23% 2	2200
October	1st to 10th	Day	26% 4 7% 25% 36% 3	3543
	11th to 20th	Day	30% 4 10% 24% 30% 2	3240
	21st to end	Day	43% 4 8% 22% 23% 2	3186
November	1st to 10th	Day	34% 4 14% 26% 17% 4	1852
	11th to 20th	Day	48% 3 11% 21% 13% 3	1890
	21st to end	Day	53% 3 11% 20% 12% 1	1890
December	1st to 10th	Day	46% 4 10% 29% 11%	2728
	11th to 20th	Day	63% 4 9% 18% 5% 1	3085
	21st to end	Day	76% 4 8% 8% 4	4070
January	1st to 10th	Night	57% 4 12% 20% 7%	8233
	11th to 20th	Night	63% 3 8% 14% 11% 2	8280
	21st to end	Night	57% 3 9% 21% 9% 1	8759
February	1st to 10th	Night	57% 4 11% 18% 9% 2	2620
	11th to 20th	Night	51% 3 11% 22% 11% 2	2459
	21st to end	Night	51% 5% 10% 20% 11% 2	2076
March	1st to 10th	Night	50% 3 8% 23% 11% 4	5493
	11th to 20th	Night	39% 4 11% 27% 16% 3	5436
	21st to end	Night	53% 6% 10% 22% 8% 1	5072
April	1st to 10th	Night	44% 5% 11% 23% 14% 2	2100
	11th to 20th	Night	43% 5% 9% 22% 17% 4	2075
	21st to end	Night	48% 5% 10% 22% 11% 4	2016
May	1st to 10th	Night	52% 5% 8% 21% 12% 2	5264
	11th to 20th	Night	38% 6% 12% 24% 19% 1	5090
	21st to end	Night	45% 7% 11% 19% 16% 3	5149
June	1st to 10th	Night	35% 4 10% 27% 20% 4	1680
	11th to 20th	Night	32% 6% 12% 26% 21% 4	1680
	21st to end	Night	35% 7% 16% 23% 17% 4	1680
July	1st to 10th	Night	35% 6% 12% 28% 16% 3	4670
	11th to 20th	Night	32% 7% 13% 29% 18% 1	4662
	21st to end	Night	38% 6% 14% 25% 15% 3	5002
August	1st to 10th	Night	28% 6% 17% 31% 17% 1	2921
	11th to 20th	Night	31% 6% 15% 28% 18% 1	3330
	21st to end	Night	27% 6% 11% 34% 19% 4	3676
September	1st to 10th	Night	30% 5% 10% 28% 20% 7%	2000
	11th to 20th	Night	27% 4 9% 27% 29% 5%	2120
	21st to end	Night	35% 5% 9% 24% 24% 4	2200
October	1st to 10th	Night	21% 3 6% 30% 38% 3	3621
	11th to 20th	Night	27% 3 7% 27% 32% 3	3899
	21st to end	Night	37% 3 8% 23% 24% 5%	4602
November	1st to 10th	Night	35% 4 10% 27% 19% 4	2666
	11th to 20th	Night	44% 4 11% 24% 15% 3	2730
	21st to end	Night	52% 3 9% 23% 12% 1	2730
December	1st to 10th	Night	42% 3 12% 30% 12% 1	4676
	11th to 20th	Night	55% 3 12% 17% 12% 1	5341
	21st to end	Night	72% 4 9% 10% 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	23% 14% 32% 24% 5% 1	330
	11th to 20th	Day	24% 18% 38% 16% 3 1	330
	21st to end	Day	23% 16% 36% 20% 4	363
February	1st to 10th	Day	23% 14% 39% 20% 5%	330
	11th to 20th	Day	25% 16% 38% 16% 5% 1	330
	21st to end	Day	25% 14% 45% 13% 2	273
March	1st to 10th	Day	23% 18% 39% 17% 2	330
	11th to 20th	Day	25% 16% 37% 16% 4	365
	21st to end	Day	28% 14% 35% 15% 7% 1	594
April	1st to 10th	Day	25% 14% 33% 20% 7% 1	540
	11th to 20th	Day	21% 17% 31% 19% 10% 2	532
	21st to end	Day	22% 12% 36% 20% 8% 2	540
May	1st to 10th	Day	21% 10% 33% 24% 9% 3	543
	11th to 20th	Day	20% 8% 27% 26% 14% 5%	613
	21st to end	Day	13% 6% 32% 25% 16% 9%	844
June	1st to 10th	Day	11% 7% 25% 31% 20% 6%	780
	11th to 20th	Day	11% 6% 27% 31% 19% 5%	780
	21st to end	Day	12% 6% 27% 32% 19% 6%	780
July	1st to 10th	Day	11% 2 15% 30% 25% 16%	771
	11th to 20th	Day	8% 3 17% 33% 27% 13%	768
	21st to end	Day	11% 3 16% 32% 28% 10%	849
August	1st to 10th	Day	7% 2 15% 31% 29% 16%	651
	11th to 20th	Day	10% 3 16% 30% 29% 11%	559
	21st to end	Day	7% 2 13% 30% 33% 15%	619
September	1st to 10th	Day	11% 5% 16% 28% 29% 10%	530
	11th to 20th	Day	9% 2 12% 28% 31% 18%	530
	21st to end	Day	14% 6% 23% 24% 20% 12%	530
October	1st to 10th	Day	11% 5% 26% 32% 20% 8%	509
	11th to 20th	Day	13% 17% 29% 28% 10% 2	330
	21st to end	Day	28% 13% 34% 19% 5% 1	363
November	1st to 10th	Day	14% 12% 33% 24% 13% 4	330
	11th to 20th	Day	25% 16% 32% 18% 9% 1	330
	21st to end	Day	25% 11% 34% 25% 5%	330
December	1st to 10th	Day	18% 15% 43% 18% 6% 1	330
	11th to 20th	Day	18% 16% 40% 21% 5% 1	330
	21st to end	Day	26% 17% 36% 17% 3	363
January	1st to 10th	Night	22% 14% 38% 24% 3	1050
	11th to 20th	Night	22% 15% 38% 20% 4	1050
	21st to end	Night	21% 14% 38% 24% 3	1155
February	1st to 10th	Night	25% 12% 36% 24% 3	940
	11th to 20th	Night	20% 15% 37% 24% 4	823
	21st to end	Night	24% 12% 40% 21% 2	692
March	1st to 10th	Night	22% 14% 35% 24% 4	840
	11th to 20th	Night	19% 13% 37% 25% 6%	830
	21st to end	Night	23% 9% 38% 24% 5% 1	693
April	1st to 10th	Night	17% 13% 38% 26% 5%	630
	11th to 20th	Night	21% 11% 35% 24% 9%	623
	21st to end	Night	21% 8% 35% 28% 7% 1	630
May	1st to 10th	Night	20% 4 36% 31% 8%	630
	11th to 20th	Night	18% 5% 29% 34% 13% 1	621
	21st to end	Night	11% 4 27% 38% 18% 2	683
June	1st to 10th	Night	12% 3 23% 43% 18%	630
	11th to 20th	Night	10% 4 25% 43% 17% 1	630
	21st to end	Night	11% 3 22% 42% 21% 1	630
July	1st to 10th	Night	11% 1 13% 37% 33% 5%	615
	11th to 20th	Night	6% 2 15% 42% 30% 5%	612
	21st to end	Night	6% 2 13% 46% 32% 2	690
August	1st to 10th	Night	8% 2 9% 45% 33% 3	627
	11th to 20th	Night	8% 2 10% 47% 29% 4	624
	21st to end	Night	7% 3 12% 36% 39% 3	687
September	1st to 10th	Night	14% 2 13% 38% 30% 4	600
	11th to 20th	Night	8% 3 13% 40% 32% 4	720
	21st to end	Night	13% 6% 22% 30% 27% 3	800
October	1st to 10th	Night	12% 5% 22% 38% 21% 2	800
	11th to 20th	Night	18% 10% 30% 29% 13%	800
	21st to end	Night	21% 12% 36% 24% 6%	880
November	1st to 10th	Night	14% 9% 41% 26% 9% 1	820
	11th to 20th	Night	18% 13% 35% 27% 6% 1	840
	21st to end	Night	24% 15% 30% 25% 6%	840
December	1st to 10th	Night	22% 12% 37% 25% 4	1019
	11th to 20th	Night	24% 12% 32% 25% 6%	1050
	21st to end	Night	31% 13% 31% 22% 2	1155

Page generated at 2026-08-27 03:07:16. Version ac07

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