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 2025-04-26.

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%	113447
	Night	14% 9% 4 8% 5% 4 4 3 5% 6% 38%	115351
Transparency	Day	45% 5% 12% 23% 14% 2	117041
	Night	45% 4 10% 23% 15% 2	119887
Seeing	Day	16% 9% 27% 25% 16% 6%	16259
	Night	18% 9% 29% 30% 13% 1	26235

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	14% 12% 7% 16% 8% 4 4 3 5% 6% 22%	3617
	11th to 20th	Day	11% 12% 7% 14% 8% 9% 5% 5% 6% 4 20%	3620
	21st to end	Day	19% 19% 5% 10% 6% 4 3 3 4 4 25%	4211
February	1st to 10th	Day	16% 13% 5% 9% 7% 5% 4 3 5% 6% 28%	1902
	11th to 20th	Day	15% 12% 3 9% 5% 6% 5% 3 5% 7% 31%	1948
	21st to end	Day	16% 13% 6% 9% 4 4 2 3 4 7% 32%	1650
March	1st to 10th	Day	18% 15% 4 8% 4 3 2 2 2 4 38%	4480
	11th to 20th	Day	8% 10% 4 5% 4 4 5% 4 6% 8% 42%	4669
	21st to end	Day	16% 12% 5% 9% 5% 5% 5% 3 5% 6% 28%	5967
April	1st to 10th	Day	16% 10% 4 7% 5% 3 4 4 5% 7% 34%	2400
	11th to 20th	Day	15% 9% 3 7% 5% 4 4 4 5% 7% 37%	2374
	21st to end	Day	20% 8% 3 8% 4 5% 4 3 5% 6% 35%	2415
May	1st to 10th	Day	16% 9% 2 6% 5% 5% 5% 5% 7% 9% 29%	4082
	11th to 20th	Day	17% 7% 3 6% 5% 4 5% 4 5% 8% 37%	4162
	21st to end	Day	8% 5% 2 6% 4 4 5% 4 6% 9% 45%	4824
June	1st to 10th	Day	9% 5% 3 5% 5% 5% 6% 5% 7% 9% 42%	2660
	11th to 20th	Day	8% 5% 2 6% 4 5% 5% 4 6% 9% 47%	2660
	21st to end	Day	9% 6% 3 7% 5% 5% 5% 5% 7% 9% 40%	2660
July	1st to 10th	Day	12% 6% 3 6% 5% 4 5% 5% 7% 11% 36%	4440
	11th to 20th	Day	6% 5% 2 5% 4 5% 6% 4 7% 11% 44%	4440
	21st to end	Day	9% 6% 2 7% 5% 5% 6% 4 7% 9% 41%	4813
August	1st to 10th	Day	7% 4 2 5% 4 4 5% 4 6% 9% 51%	4172
	11th to 20th	Day	8% 5% 2 5% 4 5% 5% 5% 7% 12% 42%	3729
	21st to end	Day	5% 4 2 5% 4 4 4 4 7% 10% 49%	4180
September	1st to 10th	Day	10% 5% 3 6% 5% 4 3 4 6% 7% 46%	2280
	11th to 20th	Day	10% 5% 2 4 3 4 4 4 5% 7% 51%	2146
	21st to end	Day	12% 6% 2 5% 5% 3 4 4 6% 7% 47%	2090
October	1st to 10th	Day	11% 5% 2 6% 5% 4 5% 4 4 7% 47%	2070
	11th to 20th	Day	15% 8% 2 6% 4 3 4 3 5% 7% 42%	1900
	21st to end	Day	18% 9% 3 7% 4 3 4 4 5% 7% 35%	1887
November	1st to 10th	Day	11% 7% 2 5% 3 4 4 4 5% 5% 49%	1762
	11th to 20th	Day	16% 11% 3 7% 4 3 4 3 4 5% 38%	1800
	21st to end	Day	17% 10% 4 10% 5% 4 3 3 5% 5% 36%	1800
December	1st to 10th	Day	11% 11% 5% 8% 4 4 3 2 4 5% 42%	2647
	11th to 20th	Day	14% 15% 5% 12% 6% 5% 6% 4 5% 6% 23%	3008
	21st to end	Day	27% 18% 6% 12% 5% 4 4 3 4 4 12%	3982
January	1st to 10th	Night	15% 15% 7% 12% 5% 4 4 3 5% 4 26%	6348
	11th to 20th	Night	13% 16% 7% 13% 7% 5% 5% 3 4 4 22%	6360
	21st to end	Night	19% 14% 6% 10% 5% 4 3 3 3 4 30%	6766
February	1st to 10th	Night	18% 12% 5% 9% 6% 5% 3 3 4 6% 29%	2495
	11th to 20th	Night	14% 11% 4 10% 6% 3 4 3 5% 6% 34%	2338
	21st to end	Night	17% 10% 4 9% 4 4 3 3 4 6% 35%	1980
March	1st to 10th	Night	16% 14% 4 6% 4 3 3 2 4 5% 38%	5373
	11th to 20th	Night	11% 7% 3 7% 4 4 4 3 5% 7% 45%	5318
	21st to end	Night	17% 9% 4 10% 6% 4 5% 4 5% 6% 30%	4962
April	1st to 10th	Night	13% 10% 3 6% 4 5% 4 3 5% 6% 41%	2000
	11th to 20th	Night	16% 8% 3 6% 4 4 4 2 4 7% 41%	1975
	21st to end	Night	18% 8% 3 7% 4 5% 5% 3 4 5% 37%	1880
May	1st to 10th	Night	17% 9% 2 8% 4 4 5% 4 6% 8% 34%	2813
	11th to 20th	Night	17% 8% 2 5% 5% 4 4 3 6% 7% 39%	2732
	21st to end	Night	9% 6% 3 6% 4 4 4 4 7% 8% 44%	2743
June	1st to 10th	Night	10% 5% 3 6% 3 3 4 3 5% 8% 50%	1520
	11th to 20th	Night	7% 6% 2 6% 4 4 4 4 6% 9% 49%	1520
	21st to end	Night	7% 5% 2 6% 5% 5% 4 4 8% 9% 43%	1520
July	1st to 10th	Night	11% 4 2 6% 5% 4 5% 4 6% 10% 44%	2530
	11th to 20th	Night	4 3 1 4 4 4 4 4 7% 10% 56%	2530
	21st to end	Night	6% 6% 3 7% 5% 6% 4 4 6% 9% 45%	2750
August	1st to 10th	Night	7% 4 1 5% 4 4 4 3 7% 9% 52%	2754
	11th to 20th	Night	6% 4 2 6% 4 4 5% 4 7% 8% 49%	3100
	21st to end	Night	7% 4 1 5% 3 4 4 4 5% 8% 54%	3476
September	1st to 10th	Night	12% 4 2 4 4 2 3 3 4 7% 54%	1900
	11th to 20th	Night	8% 4 1 4 4 3 3 3 6% 8% 55%	2014
	21st to end	Night	11% 6% 3 5% 4 3 4 4 5% 7% 48%	2090
October	1st to 10th	Night	11% 5% 1 5% 4 3 3 3 5% 5% 55%	2107
	11th to 20th	Night	15% 6% 2 5% 3 3 3 2 5% 6% 50%	2280
	21st to end	Night	18% 8% 3 7% 4 4 4 3 4 6% 39%	2709
November	1st to 10th	Night	11% 7% 2 6% 4 3 4 3 5% 6% 49%	2536
	11th to 20th	Night	13% 9% 3 7% 5% 4 4 3 5% 7% 41%	2600
	21st to end	Night	17% 11% 4 9% 4 3 3 3 4 6% 35%	2600
December	1st to 10th	Night	12% 8% 2 8% 4 4 3 3 5% 5% 44%	4537
	11th to 20th	Night	17% 11% 4 9% 5% 5% 4 3 4 5% 32%	5199
	21st to end	Night	26% 17% 6% 10% 5% 4 4 3 4 4 16%	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	63% 4 9% 16% 7% 1	3618
	11th to 20th	Day	65% 4 9% 9% 10% 3	3620
	21st to end	Day	64% 2 10% 16% 7% 1	4211
February	1st to 10th	Day	58% 4 12% 16% 8% 2	1902
	11th to 20th	Day	54% 4 11% 21% 9% 1	1958
	21st to end	Day	53% 3 10% 22% 9% 2	1650
March	1st to 10th	Day	54% 3 6% 19% 16% 3	4480
	11th to 20th	Day	38% 5% 10% 34% 12% 1	4669
	21st to end	Day	55% 5% 13% 17% 8% 1	5969
April	1st to 10th	Day	49% 5% 12% 21% 13% 1	2400
	11th to 20th	Day	46% 5% 12% 22% 12% 3	2374
	21st to end	Day	50% 5% 12% 19% 12% 2	2415
May	1st to 10th	Day	47% 8% 13% 23% 8%	4120
	11th to 20th	Day	45% 7% 12% 26% 10% 1	4166
	21st to end	Day	33% 7% 12% 21% 23% 4	4837
June	1st to 10th	Day	36% 7% 14% 23% 17% 3	2660
	11th to 20th	Day	33% 6% 11% 25% 21% 4	2660
	21st to end	Day	39% 8% 13% 21% 16% 3	2660
July	1st to 10th	Day	41% 6% 12% 27% 11% 2	4371
	11th to 20th	Day	32% 7% 15% 29% 16% 1	4365
	21st to end	Day	38% 7% 16% 20% 16% 2	4662
August	1st to 10th	Day	29% 6% 17% 32% 15% 1	4157
	11th to 20th	Day	34% 6% 15% 28% 16% 1	3765
	21st to end	Day	29% 6% 15% 31% 15% 4	4179
September	1st to 10th	Day	36% 6% 11% 25% 18% 5%	2280
	11th to 20th	Day	32% 5% 11% 25% 23% 5%	2146
	21st to end	Day	36% 5% 10% 26% 22% 2	2090
October	1st to 10th	Day	26% 4 7% 25% 35% 3	3434
	11th to 20th	Day	30% 4 10% 24% 30% 2	3140
	21st to end	Day	43% 3 9% 21% 22% 2	3087
November	1st to 10th	Day	34% 5% 15% 25% 17% 4	1762
	11th to 20th	Day	48% 3 11% 21% 14% 3	1800
	21st to end	Day	52% 4 11% 20% 12% 2	1800
December	1st to 10th	Day	46% 4 11% 29% 11%	2647
	11th to 20th	Day	63% 4 10% 18% 5% 1	3005
	21st to end	Day	76% 4 8% 8% 4	3982
January	1st to 10th	Night	60% 5% 9% 18% 8%	6348
	11th to 20th	Night	67% 3 7% 11% 10% 2	6360
	21st to end	Night	60% 3 8% 18% 10% 1	6766
February	1st to 10th	Night	57% 4 11% 17% 9% 2	2495
	11th to 20th	Night	51% 3 11% 21% 12% 2	2339
	21st to end	Night	51% 5% 10% 20% 11% 3	1980
March	1st to 10th	Night	50% 3 8% 24% 12% 4	5373
	11th to 20th	Night	39% 4 11% 27% 17% 3	5318
	21st to end	Night	53% 6% 10% 22% 7% 1	4962
April	1st to 10th	Night	44% 5% 11% 23% 15% 2	2000
	11th to 20th	Night	44% 4 9% 23% 16% 4	1975
	21st to end	Night	49% 5% 10% 21% 11% 4	1880
May	1st to 10th	Night	48% 6% 12% 24% 10%	2840
	11th to 20th	Night	43% 6% 15% 25% 10% 1	2733
	21st to end	Night	35% 8% 12% 21% 19% 5%	2751
June	1st to 10th	Night	33% 5% 11% 28% 19% 4	1520
	11th to 20th	Night	31% 5% 13% 26% 21% 4	1520
	21st to end	Night	34% 7% 16% 22% 16% 4	1520
July	1st to 10th	Night	37% 5% 14% 27% 12% 5%	2490
	11th to 20th	Night	23% 6% 14% 33% 23% 2	2482
	21st to end	Night	35% 6% 16% 21% 17% 4	2662
August	1st to 10th	Night	27% 6% 18% 30% 17% 2	2745
	11th to 20th	Night	30% 6% 15% 29% 18% 1	3130
	21st to end	Night	28% 6% 11% 33% 18% 4	3476
September	1st to 10th	Night	31% 5% 10% 27% 21% 7%	1900
	11th to 20th	Night	27% 4 9% 27% 28% 5%	2014
	21st to end	Night	35% 5% 8% 25% 22% 4	2090
October	1st to 10th	Night	21% 3 6% 31% 37% 3	3510
	11th to 20th	Night	27% 3 7% 27% 33% 3	3779
	21st to end	Night	37% 3 8% 23% 24% 5%	4459
November	1st to 10th	Night	35% 4 10% 27% 19% 4	2536
	11th to 20th	Night	44% 4 11% 23% 16% 3	2600
	21st to end	Night	52% 3 9% 23% 13% 1	2600
December	1st to 10th	Night	42% 3 12% 31% 12% 1	4537
	11th to 20th	Night	55% 3 12% 17% 12% 1	5201
	21st to end	Night	72% 4 9% 10% 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	23% 14% 33% 23% 5% 1	300
	11th to 20th	Day	26% 16% 38% 17% 4 1	300
	21st to end	Day	25% 14% 36% 21% 5%	330
February	1st to 10th	Day	24% 14% 38% 20% 4	300
	11th to 20th	Day	26% 15% 38% 16% 5% 1	300
	21st to end	Day	27% 13% 44% 14% 2	249
March	1st to 10th	Day	22% 18% 39% 18% 3	300
	11th to 20th	Day	25% 17% 37% 17% 5%	333
	21st to end	Day	30% 13% 34% 14% 7% 1	550
April	1st to 10th	Day	24% 13% 32% 22% 7% 1	500
	11th to 20th	Day	22% 15% 30% 19% 11% 2	492
	21st to end	Day	22% 12% 36% 19% 8% 2	480
May	1st to 10th	Day	20% 9% 34% 25% 9% 3	463
	11th to 20th	Day	22% 6% 24% 29% 14% 5%	529
	21st to end	Day	11% 5% 31% 26% 16% 9%	734
June	1st to 10th	Day	11% 7% 24% 32% 20% 6%	680
	11th to 20th	Day	11% 5% 26% 32% 21% 6%	680
	21st to end	Day	11% 6% 28% 30% 19% 6%	680
July	1st to 10th	Day	11% 2 16% 29% 24% 17%	671
	11th to 20th	Day	7% 3 18% 33% 26% 13%	668
	21st to end	Day	11% 2 15% 33% 29% 11%	739
August	1st to 10th	Day	6% 2 14% 32% 30% 16%	563
	11th to 20th	Day	10% 3 17% 29% 29% 12%	479
	21st to end	Day	7% 2 13% 29% 34% 15%	539
September	1st to 10th	Day	12% 5% 16% 29% 30% 9%	490
	11th to 20th	Day	9% 2 12% 29% 30% 17%	490
	21st to end	Day	14% 6% 24% 24% 20% 11%	490
October	1st to 10th	Day	11% 5% 27% 32% 18% 8%	470
	11th to 20th	Day	14% 17% 31% 26% 9% 3	300
	21st to end	Day	29% 14% 33% 18% 5% 1	330
November	1st to 10th	Day	15% 12% 32% 24% 14% 4	300
	11th to 20th	Day	24% 16% 32% 18% 9% 1	300
	21st to end	Day	24% 11% 35% 26% 4	300
December	1st to 10th	Day	18% 14% 43% 17% 6% 1	300
	11th to 20th	Day	18% 16% 40% 20% 5% 1	300
	21st to end	Day	27% 17% 35% 18% 3	330
January	1st to 10th	Night	22% 14% 38% 24% 3	1000
	11th to 20th	Night	23% 14% 37% 21% 5%	1000
	21st to end	Night	21% 13% 38% 24% 3	1100
February	1st to 10th	Night	25% 12% 36% 23% 3	895
	11th to 20th	Night	20% 15% 36% 24% 4	783
	21st to end	Night	25% 12% 39% 21% 3	660
March	1st to 10th	Night	21% 14% 35% 25% 5%	800
	11th to 20th	Night	18% 13% 37% 26% 6%	790
	21st to end	Night	24% 9% 37% 25% 5% 1	660
April	1st to 10th	Night	17% 13% 37% 27% 6%	600
	11th to 20th	Night	21% 10% 35% 24% 9% 1	593
	21st to end	Night	22% 9% 34% 27% 7% 1	587
May	1st to 10th	Night	20% 4 36% 32% 9%	570
	11th to 20th	Night	19% 4 27% 34% 14% 1	561
	21st to end	Night	11% 4 26% 38% 19% 2	617
June	1st to 10th	Night	12% 3 23% 43% 19%	570
	11th to 20th	Night	10% 3 25% 43% 19% 1	570
	21st to end	Night	10% 2 23% 42% 22% 1	570
July	1st to 10th	Night	11% 1 14% 37% 32% 6%	555
	11th to 20th	Night	6% 2 17% 41% 30% 5%	552
	21st to end	Night	6% 2 13% 46% 32% 2	624
August	1st to 10th	Night	8% 2 9% 46% 32% 4	567
	11th to 20th	Night	7% 2 10% 46% 30% 4	564
	21st to end	Night	8% 3 12% 33% 41% 3	627
September	1st to 10th	Night	15% 2 13% 38% 29% 4	570
	11th to 20th	Night	8% 3 14% 40% 32% 4	684
	21st to end	Night	13% 6% 22% 30% 26% 3	760
October	1st to 10th	Night	13% 5% 23% 37% 21% 3	760
	11th to 20th	Night	18% 10% 30% 28% 14%	760
	21st to end	Night	22% 12% 35% 24% 7%	836
November	1st to 10th	Night	14% 9% 40% 26% 9% 1	780
	11th to 20th	Night	18% 13% 34% 28% 6% 1	800
	21st to end	Night	23% 15% 31% 25% 6%	800
December	1st to 10th	Night	22% 12% 36% 25% 4	970
	11th to 20th	Night	24% 11% 32% 26% 6%	1000
	21st to end	Night	31% 13% 31% 22% 2	1100

Page generated at 2025-04-23 08:25:04. Version ac07

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