Riverwoods Field Laboratory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Riverwoods Field Laboratory clear sky chart from 2005-11-06 to 2024-11-30.

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	7% 6% 2 8% 6% 6% 7% 5% 8% 10% 36%	101019
	Night	7% 6% 2 7% 5% 5% 5% 5% 7% 9% 41%	98525
Transparency	Day	40% 9% 16% 19% 12% 4	104758
	Night	37% 8% 18% 21% 13% 4	102925
Seeing	Day	7% 4 16% 25% 25% 22%	15847
	Night	8% 4 17% 33% 29% 9%	25929

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% 8% 3 8% 5% 5% 5% 4 6% 6% 42%	2836
	11th to 20th	Day	16% 9% 3 7% 5% 5% 5% 4 6% 10% 31%	2840
	21st to end	Day	8% 8% 3 8% 5% 5% 5% 4 7% 7% 40%	3220
February	1st to 10th	Day	10% 5% 2 6% 4 5% 5% 3 6% 9% 45%	1900
	11th to 20th	Day	5% 4 2 5% 4 5% 4 4 6% 9% 52%	1859
	21st to end	Day	6% 5% 2 6% 4 4 5% 4 6% 9% 49%	1727
March	1st to 10th	Day	6% 6% 2 8% 7% 7% 7% 5% 7% 9% 35%	3435
	11th to 20th	Day	4 3 1 6% 5% 5% 4 4 6% 10% 52%	3480
	21st to end	Day	8% 4 2 5% 4 5% 5% 5% 6% 9% 49%	3815
April	1st to 10th	Day	5% 3 2 5% 5% 6% 6% 6% 8% 9% 46%	2090
	11th to 20th	Day	7% 5% 2 7% 5% 5% 7% 6% 7% 8% 41%	2116
	21st to end	Day	5% 4 1 5% 5% 5% 6% 4 8% 8% 48%	2280
May	1st to 10th	Day	3 2 1 4 4 5% 5% 5% 7% 11% 54%	3795
	11th to 20th	Day	7% 5% 2 6% 5% 6% 6% 5% 9% 11% 36%	4081
	21st to end	Day	4 4 2 6% 5% 5% 6% 4 6% 8% 49%	4476
June	1st to 10th	Day	11% 10% 3 10% 7% 8% 8% 6% 8% 9% 20%	2470
	11th to 20th	Day	6% 7% 3 10% 8% 8% 8% 7% 8% 10% 23%	2470
	21st to end	Day	8% 8% 3 9% 10% 9% 9% 6% 7% 8% 23%	2470
July	1st to 10th	Day	6% 7% 3 10% 8% 8% 8% 7% 9% 11% 23%	4120
	11th to 20th	Day	5% 5% 2 9% 8% 9% 9% 7% 10% 12% 23%	4120
	21st to end	Day	5% 6% 3 9% 8% 9% 9% 7% 10% 12% 22%	4466
August	1st to 10th	Day	4 7% 3 10% 8% 9% 9% 8% 10% 12% 22%	4104
	11th to 20th	Day	3 4 2 8% 6% 7% 7% 6% 9% 13% 36%	3772
	21st to end	Day	4 6% 3 11% 7% 9% 8% 6% 10% 13% 22%	3828
September	1st to 10th	Day	7% 6% 4 11% 9% 8% 8% 7% 9% 10% 22%	2090
	11th to 20th	Day	8% 7% 4 12% 9% 8% 8% 6% 8% 9% 21%	2090
	21st to end	Day	9% 8% 3 9% 7% 7% 8% 6% 8% 10% 26%	2090
October	1st to 10th	Day	10% 6% 3 8% 7% 6% 6% 6% 6% 10% 31%	2043
	11th to 20th	Day	6% 5% 2 8% 6% 7% 7% 5% 7% 9% 38%	1900
	21st to end	Day	8% 5% 2 6% 6% 5% 5% 3 8% 9% 44%	2090
November	1st to 10th	Day	7% 6% 2 7% 6% 5% 5% 5% 6% 8% 43%	1957
	11th to 20th	Day	8% 6% 3 7% 5% 6% 5% 4 6% 10% 40%	2000
	21st to end	Day	8% 6% 2 6% 4 5% 6% 4 7% 9% 44%	1997
December	1st to 10th	Day	7% 6% 2 5% 4 6% 5% 5% 6% 8% 46%	1715
	11th to 20th	Day	10% 6% 2 6% 5% 4 5% 3 6% 9% 42%	2153
	21st to end	Day	10% 7% 3 8% 6% 6% 7% 4 7% 8% 33%	3124
January	1st to 10th	Night	10% 7% 3 7% 5% 5% 5% 4 6% 9% 38%	4109
	11th to 20th	Night	16% 13% 5% 8% 5% 4 4 4 5% 8% 29%	4120
	21st to end	Night	9% 7% 3 7% 5% 5% 5% 4 6% 7% 41%	4432
February	1st to 10th	Night	9% 7% 2 7% 5% 4 4 4 6% 8% 45%	2280
	11th to 20th	Night	8% 4 2 5% 3 3 4 3 7% 8% 53%	2204
	21st to end	Night	6% 4 2 4 4 4 4 4 6% 8% 56%	1727
March	1st to 10th	Night	10% 9% 4 9% 5% 5% 5% 4 5% 8% 38%	3418
	11th to 20th	Night	3 3 2 4 4 4 5% 4 8% 12% 51%	3480
	21st to end	Night	9% 5% 2 5% 3 3 3 4 5% 7% 54%	3817
April	1st to 10th	Night	4 4 2 4 3 3 4 4 7% 10% 56%	2090
	11th to 20th	Night	5% 4 2 4 5% 4 5% 4 7% 10% 50%	2010
	21st to end	Night	4 4 2 5% 4 4 4 4 6% 7% 56%	1900
May	1st to 10th	Night	3 4 1 4 3 4 4 4 6% 8% 58%	3113
	11th to 20th	Night	6% 5% 2 5% 4 4 4 5% 7% 11% 47%	2812
	21st to end	Night	4 4 2 6% 4 5% 4 4 7% 8% 52%	3079
June	1st to 10th	Night	10% 8% 3 8% 7% 6% 6% 6% 8% 10% 30%	1710
	11th to 20th	Night	5% 7% 3 7% 7% 6% 6% 6% 10% 12% 31%	1710
	21st to end	Night	6% 6% 4 7% 7% 6% 7% 6% 7% 10% 33%	1710
July	1st to 10th	Night	7% 6% 3 7% 5% 7% 7% 6% 8% 13% 33%	2840
	11th to 20th	Night	4 5% 3 8% 6% 7% 8% 6% 10% 13% 30%	2840
	21st to end	Night	5% 8% 3 10% 7% 6% 7% 6% 10% 11% 26%	3087
August	1st to 10th	Night	3 6% 3 8% 5% 6% 7% 6% 9% 12% 34%	2825
	11th to 20th	Night	5% 5% 2 7% 6% 6% 5% 5% 9% 13% 36%	3017
	21st to end	Night	6% 6% 2 8% 6% 7% 7% 6% 10% 14% 29%	3828
September	1st to 10th	Night	6% 7% 2 9% 7% 7% 7% 7% 9% 12% 26%	2090
	11th to 20th	Night	9% 7% 3 11% 7% 7% 7% 5% 8% 10% 28%	2090
	21st to end	Night	10% 8% 3 10% 6% 6% 6% 5% 7% 9% 31%	2090
October	1st to 10th	Night	11% 8% 2 7% 5% 5% 5% 4 7% 10% 36%	2135
	11th to 20th	Night	6% 6% 3 6% 5% 5% 4 5% 7% 8% 45%	2280
	21st to end	Night	8% 6% 2 5% 4 4 4 4 6% 9% 48%	2508
November	1st to 10th	Night	8% 6% 2 6% 4 4 5% 4 6% 8% 48%	2341
	11th to 20th	Night	8% 5% 2 6% 4 5% 5% 4 6% 8% 45%	2400
	21st to end	Night	9% 6% 2 6% 5% 4 5% 4 6% 8% 46%	2399
December	1st to 10th	Night	7% 5% 2 5% 6% 4 4 5% 6% 8% 50%	2460
	11th to 20th	Night	11% 7% 2 5% 4 4 4 4 5% 7% 47%	3042
	21st to end	Night	11% 8% 3 9% 6% 6% 6% 5% 6% 9% 33%	4532

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	41% 5% 7% 19% 22% 6%	2837
	11th to 20th	Day	50% 4 8% 17% 17% 4	2840
	21st to end	Day	40% 5% 9% 20% 19% 7%	3220
February	1st to 10th	Day	36% 5% 8% 21% 22% 7%	1900
	11th to 20th	Day	28% 4 9% 26% 21% 11%	1869
	21st to end	Day	30% 5% 11% 19% 25% 10%	1727
March	1st to 10th	Day	43% 6% 14% 19% 13% 5%	3435
	11th to 20th	Day	26% 5% 15% 32% 18% 4	3480
	21st to end	Day	30% 6% 9% 29% 17% 9%	3815
April	1st to 10th	Day	29% 8% 14% 26% 17% 6%	2090
	11th to 20th	Day	35% 7% 12% 20% 21% 5%	2116
	21st to end	Day	30% 6% 11% 21% 25% 7%	2280
May	1st to 10th	Day	19% 9% 13% 32% 22% 5%	3830
	11th to 20th	Day	36% 7% 20% 24% 11%	4085
	21st to end	Day	30% 6% 12% 33% 17% 1	4489
June	1st to 10th	Day	55% 9% 18% 14% 4	2470
	11th to 20th	Day	49% 13% 20% 16% 2	2470
	21st to end	Day	54% 11% 21% 11% 2	2470
July	1st to 10th	Day	47% 13% 26% 12% 2	4055
	11th to 20th	Day	46% 14% 31% 8%	4051
	21st to end	Day	45% 18% 25% 11%	4324
August	1st to 10th	Day	47% 15% 28% 8% 1	4091
	11th to 20th	Day	35% 13% 30% 15% 7%	3811
	21st to end	Day	48% 15% 27% 8% 1	3828
September	1st to 10th	Day	51% 14% 21% 13% 1	2090
	11th to 20th	Day	54% 13% 19% 11% 2	2090
	21st to end	Day	48% 12% 17% 13% 8% 2	2090
October	1st to 10th	Day	52% 11% 15% 12% 7% 3	3394
	11th to 20th	Day	33% 7% 10% 25% 21% 4	3140
	21st to end	Day	33% 6% 9% 23% 23% 6%	3428
November	1st to 10th	Day	37% 6% 11% 19% 17% 10%	1957
	11th to 20th	Day	39% 6% 9% 23% 16% 8%	2000
	21st to end	Day	35% 6% 11% 21% 20% 8%	1997
December	1st to 10th	Day	33% 6% 15% 24% 17% 4	1715
	11th to 20th	Day	37% 6% 11% 20% 16% 11%	2150
	21st to end	Day	45% 7% 11% 18% 15% 4	3124
January	1st to 10th	Night	41% 5% 11% 22% 16% 4	4109
	11th to 20th	Night	53% 4 8% 21% 10% 3	4120
	21st to end	Night	41% 5% 10% 19% 18% 7%	4432
February	1st to 10th	Night	36% 6% 11% 19% 20% 8%	2280
	11th to 20th	Night	27% 5% 11% 24% 23% 10%	2205
	21st to end	Night	26% 5% 12% 22% 28% 8%	1727
March	1st to 10th	Night	45% 6% 13% 19% 14% 4	3418
	11th to 20th	Night	24% 5% 22% 30% 15% 4	3480
	21st to end	Night	28% 6% 12% 25% 23% 6%	3817
April	1st to 10th	Night	23% 6% 19% 29% 16% 7%	2090
	11th to 20th	Night	28% 7% 17% 23% 21% 4	2010
	21st to end	Night	26% 6% 13% 26% 24% 5%	1900
May	1st to 10th	Night	21% 8% 17% 33% 16% 5%	3143
	11th to 20th	Night	28% 8% 21% 32% 10%	2813
	21st to end	Night	26% 8% 16% 34% 16%	3087
June	1st to 10th	Night	47% 8% 23% 17% 5%	1710
	11th to 20th	Night	40% 12% 26% 19% 2	1710
	21st to end	Night	42% 15% 24% 15% 4	1710
July	1st to 10th	Night	39% 13% 30% 16% 1	2795
	11th to 20th	Night	41% 17% 34% 8%	2786
	21st to end	Night	46% 15% 26% 13%	2988
August	1st to 10th	Night	38% 14% 33% 12% 3	2814
	11th to 20th	Night	36% 13% 31% 14% 6%	3044
	21st to end	Night	41% 15% 35% 8% 1	3828
September	1st to 10th	Night	45% 16% 25% 12% 1	2090
	11th to 20th	Night	48% 13% 21% 15% 2	2090
	21st to end	Night	47% 10% 18% 17% 7%	2090
October	1st to 10th	Night	55% 10% 14% 11% 8% 2	3564
	11th to 20th	Night	33% 7% 11% 24% 21% 3	3779
	21st to end	Night	32% 5% 10% 26% 21% 6%	4120
November	1st to 10th	Night	32% 6% 12% 21% 20% 8%	2341
	11th to 20th	Night	34% 7% 12% 24% 16% 7%	2400
	21st to end	Night	34% 5% 10% 22% 21% 7%	2399
December	1st to 10th	Night	31% 5% 13% 30% 15% 6%	2460
	11th to 20th	Night	36% 5% 11% 20% 17% 11%	3044
	21st to end	Night	46% 6% 14% 16% 13% 5%	4532

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	8% 7% 42% 30% 11% 2	270
	11th to 20th	Day	10% 10% 34% 28% 12% 6%	270
	21st to end	Day	14% 13% 41% 23% 7% 1	297
February	1st to 10th	Day	11% 9% 26% 33% 18% 2	270
	11th to 20th	Day	7% 9% 29% 32% 17% 6%	281
	21st to end	Day	7% 7% 23% 35% 24% 4	382
March	1st to 10th	Day	5% 8% 30% 37% 19% 2	460
	11th to 20th	Day	6% 5% 28% 32% 25% 3	460
	21st to end	Day	5% 5% 27% 32% 26% 5%	506
April	1st to 10th	Day	5% 6% 33% 32% 20% 5%	460
	11th to 20th	Day	7% 7% 23% 33% 24% 6%	452
	21st to end	Day	7% 3 18% 33% 28% 10%	460
May	1st to 10th	Day	5% 2 16% 32% 31% 15%	463
	11th to 20th	Day	9% 3 17% 25% 29% 17%	487
	21st to end	Day	7% 2 7% 22% 34% 28%	529
June	1st to 10th	Day	13% 3 9% 17% 31% 27%	490
	11th to 20th	Day	8% 2 6% 17% 28% 38%	490
	21st to end	Day	7% 3 3 11% 24% 52%	490
July	1st to 10th	Day	6% 1 3 9% 26% 55%	485
	11th to 20th	Day	3 1 2 13% 27% 54%	484
	21st to end	Day	9% 2 2 13% 25% 48%	532
August	1st to 10th	Day	4 1 6% 14% 25% 50%	486
	11th to 20th	Day	3 2 9% 27% 57%	479
	21st to end	Day	8% 3 6% 17% 27% 38%	539
September	1st to 10th	Day	7% 3 7% 10% 27% 46%	490
	11th to 20th	Day	6% 2 6% 20% 28% 38%	490
	21st to end	Day	8% 4 7% 19% 33% 31%	490
October	1st to 10th	Day	13% 3 11% 31% 29% 12%	490
	11th to 20th	Day	6% 4 17% 34% 29% 11%	490
	21st to end	Day	8% 4 16% 35% 28% 9%	539
November	1st to 10th	Day	9% 7% 19% 28% 28% 8%	495
	11th to 20th	Day	13% 5% 23% 33% 22% 5%	500
	21st to end	Day	7% 4 21% 38% 24% 5%	499
December	1st to 10th	Day	5% 8% 28% 33% 19% 7%	275
	11th to 20th	Day	8% 8% 27% 36% 18% 4	270
	21st to end	Day	6% 9% 29% 35% 16% 6%	297
January	1st to 10th	Night	9% 6% 29% 37% 18% 1	760
	11th to 20th	Night	9% 6% 27% 42% 14% 2	760
	21st to end	Night	13% 7% 31% 39% 11%	836
February	1st to 10th	Night	11% 8% 26% 41% 14%	760
	11th to 20th	Night	8% 6% 28% 41% 16% 1	743
	21st to end	Night	9% 3 28% 46% 14%	628
March	1st to 10th	Night	6% 5% 33% 43% 14%	760
	11th to 20th	Night	7% 5% 27% 42% 19% 1	760
	21st to end	Night	6% 5% 28% 45% 15%	836
April	1st to 10th	Night	4 5% 27% 46% 17%	760
	11th to 20th	Night	7% 4 26% 43% 19% 1	751
	21st to end	Night	6% 3 24% 46% 20% 1	760
May	1st to 10th	Night	5% 3 17% 48% 25% 2	756
	11th to 20th	Night	7% 1 11% 44% 33% 4	561
	21st to end	Night	6% 1 6% 35% 44% 7%	617
June	1st to 10th	Night	11% 2 5% 25% 47% 9%	570
	11th to 20th	Night	7% 1 4 19% 48% 21%	570
	21st to end	Night	7% 2 2 13% 44% 31%	570
July	1st to 10th	Night	8% 2 2 7% 49% 32%	555
	11th to 20th	Night	5% 1 1 10% 44% 39%	552
	21st to end	Night	5% 2 3 10% 45% 36%	624
August	1st to 10th	Night	6% 1 3 9% 51% 31%	567
	11th to 20th	Night	5% 2 3 9% 48% 33%	665
	21st to end	Night	11% 2 6% 16% 40% 24%	836
September	1st to 10th	Night	9% 3 5% 15% 44% 24%	760
	11th to 20th	Night	9% 3 5% 20% 44% 20%	760
	21st to end	Night	14% 2 7% 23% 41% 13%	760
October	1st to 10th	Night	14% 3 11% 34% 33% 6%	760
	11th to 20th	Night	8% 4 11% 36% 36% 5%	760
	21st to end	Night	11% 3 16% 37% 29% 4	836
November	1st to 10th	Night	9% 3 20% 38% 26% 3	780
	11th to 20th	Night	8% 4 20% 43% 22% 3	800
	21st to end	Night	9% 3 22% 43% 22% 1	800
December	1st to 10th	Night	8% 3 21% 41% 26% 2	760
	11th to 20th	Night	10% 5% 21% 39% 24% 2	760
	21st to end	Night	9% 4 23% 39% 24% 1	836

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