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 2026-02-16.

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% 5% 2 7% 6% 6% 6% 5% 7% 10% 38%	130735
	Night	7% 6% 2 7% 5% 5% 5% 4 7% 9% 43%	130110
Transparency	Day	38% 9% 16% 20% 13% 4	134430
	Night	36% 8% 17% 22% 13% 5%	134485
Seeing	Day	7% 4 17% 25% 25% 21%	17458
	Night	8% 4 17% 33% 29% 9%	27602

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	5% 5% 2 5% 3 4 4 4 6% 8% 56%	5258
	11th to 20th	Day	15% 9% 3 8% 5% 5% 5% 4 5% 9% 33%	5280
	21st to end	Day	13% 6% 2 6% 4 4 4 4 6% 8% 42%	5960
February	1st to 10th	Day	10% 5% 2 6% 4 5% 4 3 5% 8% 47%	2100
	11th to 20th	Day	5% 4 2 5% 4 5% 4 4 6% 9% 52%	2010
	21st to end	Day	7% 5% 2 6% 4 4 5% 4 6% 9% 49%	1815
March	1st to 10th	Day	6% 5% 2 7% 6% 6% 6% 4 6% 8% 44%	4935
	11th to 20th	Day	3 2 1 5% 4 4 3 3 5% 8% 62%	4980
	21st to end	Day	7% 4 2 6% 4 5% 5% 4 6% 9% 47%	5465
April	1st to 10th	Day	4 4 2 5% 5% 6% 6% 6% 8% 9% 47%	2200
	11th to 20th	Day	7% 5% 2 6% 5% 5% 6% 6% 7% 8% 43%	2229
	21st to end	Day	5% 3 1 5% 5% 5% 6% 4 8% 9% 49%	2400
May	1st to 10th	Day	2 2 1 4 4 5% 6% 5% 8% 12% 50%	5449
	11th to 20th	Day	7% 4 2 6% 4 5% 5% 4 7% 10% 46%	5861
	21st to end	Day	4 4 2 6% 5% 6% 6% 5% 8% 11% 44%	6433
June	1st to 10th	Day	11% 10% 4 10% 7% 8% 7% 6% 8% 10% 20%	2600
	11th to 20th	Day	6% 7% 3 10% 8% 8% 8% 7% 8% 11% 23%	2600
	21st to end	Day	8% 8% 3 8% 10% 9% 9% 6% 7% 9% 23%	2600
July	1st to 10th	Day	7% 7% 3 10% 8% 7% 9% 7% 9% 12% 21%	5900
	11th to 20th	Day	5% 5% 2 8% 7% 8% 8% 7% 11% 13% 26%	5900
	21st to end	Day	4 4 2 7% 6% 7% 8% 7% 10% 14% 29%	6424
August	1st to 10th	Day	4 6% 3 10% 8% 9% 9% 8% 10% 12% 22%	4234
	11th to 20th	Day	3 4 2 8% 6% 7% 7% 6% 9% 13% 36%	3893
	21st to end	Day	4 6% 3 11% 7% 9% 8% 7% 10% 13% 22%	3949
September	1st to 10th	Day	7% 7% 4 11% 9% 8% 8% 7% 9% 9% 21%	2200
	11th to 20th	Day	8% 7% 4 12% 8% 8% 8% 6% 8% 9% 22%	2200
	21st to end	Day	8% 8% 3 8% 7% 7% 8% 6% 8% 10% 25%	2200
October	1st to 10th	Day	11% 6% 3 8% 7% 6% 5% 6% 7% 11% 31%	2150
	11th to 20th	Day	6% 5% 2 7% 6% 6% 7% 5% 7% 8% 40%	2000
	21st to end	Day	7% 5% 2 6% 6% 6% 5% 3 7% 10% 44%	2200
November	1st to 10th	Day	7% 6% 2 7% 6% 5% 5% 5% 6% 9% 44%	2057
	11th to 20th	Day	8% 6% 3 6% 5% 6% 5% 4 6% 10% 42%	2100
	21st to end	Day	8% 6% 2 6% 4 5% 5% 4 7% 9% 45%	2100
December	1st to 10th	Day	5% 4 1 4 3 4 4 4 5% 6% 58%	3025
	11th to 20th	Day	9% 6% 2 7% 5% 5% 7% 5% 8% 9% 37%	3463
	21st to end	Day	9% 6% 3 8% 6% 6% 7% 5% 7% 9% 35%	4565
January	1st to 10th	Night	6% 4 2 4 4 4 4 3 5% 8% 56%	7635
	11th to 20th	Night	14% 11% 4 9% 5% 4 4 4 5% 8% 33%	7680
	21st to end	Night	12% 6% 2 6% 4 4 4 3 5% 6% 47%	8203
February	1st to 10th	Night	8% 7% 2 6% 4 4 4 4 5% 8% 47%	2520
	11th to 20th	Night	8% 5% 2 6% 3 3 4 3 7% 8% 52%	2384
	21st to end	Night	6% 4 2 4 4 4 4 4 6% 8% 56%	1815
March	1st to 10th	Night	7% 6% 3 7% 4 4 4 3 5% 8% 49%	4918
	11th to 20th	Night	5% 3 1 4 3 3 4 4 6% 9% 58%	4980
	21st to end	Night	8% 5% 2 6% 3 4 4 4 6% 8% 51%	5467
April	1st to 10th	Night	4 4 2 5% 3 3 4 4 7% 10% 56%	2200
	11th to 20th	Night	4 4 2 4 5% 4 5% 4 7% 9% 52%	2117
	21st to end	Night	4 4 1 5% 3 5% 4 4 6% 7% 57%	2000
May	1st to 10th	Night	3 4 2 5% 4 5% 5% 4 7% 9% 52%	4473
	11th to 20th	Night	7% 5% 2 6% 4 4 4 4 6% 9% 49%	4032
	21st to end	Night	5% 5% 2 7% 5% 5% 4 5% 8% 10% 45%	4421
June	1st to 10th	Night	10% 9% 3 8% 7% 5% 6% 6% 7% 10% 29%	1800
	11th to 20th	Night	5% 8% 3 7% 7% 6% 6% 6% 10% 13% 30%	1800
	21st to end	Night	6% 6% 4 7% 7% 6% 7% 6% 7% 10% 34%	1800
July	1st to 10th	Night	8% 7% 3 8% 6% 7% 7% 6% 9% 13% 27%	4058
	11th to 20th	Night	4 5% 3 8% 6% 7% 7% 5% 9% 14% 32%	4057
	21st to end	Night	4 6% 3 9% 6% 6% 6% 5% 8% 11% 35%	4427
August	1st to 10th	Night	3 6% 3 8% 6% 6% 7% 6% 9% 12% 34%	2915
	11th to 20th	Night	5% 5% 3 7% 6% 6% 5% 5% 9% 13% 37%	3115
	21st to end	Night	6% 6% 2 8% 6% 7% 7% 6% 10% 14% 28%	3949
September	1st to 10th	Night	7% 7% 2 9% 7% 7% 7% 7% 9% 12% 26%	2200
	11th to 20th	Night	9% 7% 3 11% 7% 7% 6% 5% 8% 9% 28%	2200
	21st to end	Night	10% 8% 3 10% 6% 6% 6% 5% 7% 9% 31%	2200
October	1st to 10th	Night	10% 8% 2 7% 5% 5% 5% 4 7% 10% 36%	2248
	11th to 20th	Night	7% 6% 3 6% 5% 5% 4 5% 7% 8% 46%	2400
	21st to end	Night	8% 5% 2 5% 4 4 4 4 6% 9% 48%	2640
November	1st to 10th	Night	8% 6% 2 6% 4 4 5% 4 6% 8% 48%	2461
	11th to 20th	Night	8% 5% 2 6% 4 5% 5% 4 6% 8% 47%	2520
	21st to end	Night	9% 5% 2 6% 4 4 5% 4 6% 8% 47%	2520
December	1st to 10th	Night	6% 3 1 4 4 3 3 4 5% 7% 60%	4370
	11th to 20th	Night	10% 7% 2 7% 4 4 5% 4 5% 8% 42%	4952
	21st to end	Night	10% 8% 3 8% 6% 6% 5% 4 6% 8% 35%	6633

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	27% 4 7% 28% 27% 7%	5259
	11th to 20th	Day	49% 4 9% 18% 14% 6%	5280
	21st to end	Day	40% 4 7% 26% 13% 9%	5960
February	1st to 10th	Day	34% 5% 8% 21% 22% 10%	2100
	11th to 20th	Day	28% 5% 9% 27% 21% 11%	2020
	21st to end	Day	30% 5% 11% 19% 25% 9%	1815
March	1st to 10th	Day	37% 5% 12% 19% 20% 7%	4935
	11th to 20th	Day	21% 4 11% 25% 23% 17%	4980
	21st to end	Day	32% 6% 9% 28% 18% 8%	5465
April	1st to 10th	Day	29% 7% 14% 26% 19% 6%	2200
	11th to 20th	Day	34% 7% 11% 19% 22% 7%	2229
	21st to end	Day	29% 6% 10% 22% 26% 7%	2400
May	1st to 10th	Day	22% 9% 18% 30% 18% 3	5481
	11th to 20th	Day	32% 6% 17% 30% 14% 1	5865
	21st to end	Day	31% 9% 18% 30% 12% 1	6447
June	1st to 10th	Day	55% 8% 19% 14% 4	2600
	11th to 20th	Day	47% 13% 21% 17% 2	2600
	21st to end	Day	53% 11% 22% 11% 2	2600
July	1st to 10th	Day	49% 13% 28% 9% 1	5835
	11th to 20th	Day	42% 15% 32% 10%	5831
	21st to end	Day	37% 15% 26% 21%	6240
August	1st to 10th	Day	47% 15% 28% 8% 1	4221
	11th to 20th	Day	35% 12% 30% 16% 7%	3932
	21st to end	Day	48% 15% 27% 8% 1	3949
September	1st to 10th	Day	52% 14% 21% 12% 1	2200
	11th to 20th	Day	54% 13% 19% 12% 2	2200
	21st to end	Day	48% 12% 18% 13% 8% 2	2200
October	1st to 10th	Day	52% 11% 15% 13% 7% 3	3501
	11th to 20th	Day	32% 7% 10% 25% 22% 4	3240
	21st to end	Day	33% 6% 9% 23% 23% 6%	3538
November	1st to 10th	Day	36% 6% 11% 19% 18% 10%	2057
	11th to 20th	Day	37% 5% 9% 23% 18% 8%	2100
	21st to end	Day	35% 5% 10% 23% 19% 7%	2100
December	1st to 10th	Day	25% 5% 16% 29% 16% 9%	3025
	11th to 20th	Day	38% 7% 11% 19% 16% 8%	3460
	21st to end	Day	43% 7% 12% 20% 15% 3	4565
January	1st to 10th	Night	27% 4 9% 32% 21% 8%	7636
	11th to 20th	Night	50% 4 9% 20% 11% 6%	7680
	21st to end	Night	37% 4 10% 26% 15% 8%	8203
February	1st to 10th	Night	34% 6% 11% 19% 20% 10%	2520
	11th to 20th	Night	28% 5% 11% 25% 22% 9%	2385
	21st to end	Night	26% 5% 12% 22% 27% 8%	1815
March	1st to 10th	Night	34% 5% 12% 22% 21% 6%	4918
	11th to 20th	Night	22% 4 17% 26% 17% 15%	4980
	21st to end	Night	31% 5% 12% 27% 22% 4	5467
April	1st to 10th	Night	23% 6% 18% 29% 17% 7%	2200
	11th to 20th	Night	27% 6% 16% 22% 22% 6%	2117
	21st to end	Night	26% 6% 12% 27% 25% 5%	2000
May	1st to 10th	Night	25% 9% 21% 29% 13% 3	4503
	11th to 20th	Night	29% 7% 20% 32% 11%	4033
	21st to end	Night	31% 8% 21% 28% 11%	4429
June	1st to 10th	Night	48% 8% 23% 17% 4	1800
	11th to 20th	Night	40% 12% 26% 19% 2	1800
	21st to end	Night	42% 15% 24% 15% 4	1800
July	1st to 10th	Night	46% 14% 29% 11% 1	4015
	11th to 20th	Night	39% 18% 34% 9% 1	4006
	21st to end	Night	39% 12% 27% 20% 1	4297
August	1st to 10th	Night	39% 14% 32% 12% 3	2904
	11th to 20th	Night	35% 13% 31% 14% 6%	3142
	21st to end	Night	41% 16% 35% 8% 1	3949
September	1st to 10th	Night	46% 16% 25% 11% 1	2200
	11th to 20th	Night	49% 13% 21% 15% 2	2200
	21st to end	Night	48% 10% 18% 17% 7%	2200
October	1st to 10th	Night	54% 10% 14% 11% 8% 2	3677
	11th to 20th	Night	33% 7% 11% 24% 22% 3	3899
	21st to end	Night	32% 5% 10% 26% 21% 6%	4252
November	1st to 10th	Night	32% 6% 12% 21% 21% 9%	2461
	11th to 20th	Night	32% 7% 11% 25% 18% 7%	2520
	21st to end	Night	34% 5% 10% 23% 21% 7%	2520
December	1st to 10th	Night	23% 5% 16% 32% 13% 11%	4370
	11th to 20th	Night	39% 6% 12% 19% 17% 8%	4954
	21st to end	Night	45% 7% 15% 17% 12% 4	6633

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	7% 8% 40% 31% 11% 3	330
	11th to 20th	Day	11% 12% 35% 27% 11% 5%	330
	21st to end	Day	15% 12% 40% 24% 7% 1	363
February	1st to 10th	Day	10% 9% 24% 32% 20% 4	330
	11th to 20th	Day	7% 9% 30% 32% 16% 5%	327
	21st to end	Day	8% 6% 24% 35% 23% 4	414
March	1st to 10th	Day	5% 8% 30% 36% 18% 2	500
	11th to 20th	Day	5% 7% 29% 32% 24% 4	500
	21st to end	Day	4 5% 28% 33% 26% 4	550
April	1st to 10th	Day	5% 6% 31% 32% 21% 5%	500
	11th to 20th	Day	7% 7% 22% 34% 25% 6%	492
	21st to end	Day	6% 3 17% 33% 30% 10%	500
May	1st to 10th	Day	4 2 16% 33% 31% 15%	503
	11th to 20th	Day	9% 3 17% 25% 30% 16%	527
	21st to end	Day	6% 2 8% 23% 34% 28%	573
June	1st to 10th	Day	14% 4 8% 16% 30% 28%	530
	11th to 20th	Day	8% 2 5% 17% 28% 39%	530
	21st to end	Day	6% 3 3 12% 24% 51%	530
July	1st to 10th	Day	6% 1 4 9% 26% 54%	525
	11th to 20th	Day	3 1 2 14% 28% 52%	524
	21st to end	Day	9% 2 2 12% 26% 49%	576
August	1st to 10th	Day	4 1 6% 13% 25% 51%	526
	11th to 20th	Day	3 2 9% 28% 57%	519
	21st to end	Day	8% 3 6% 17% 28% 37%	583
September	1st to 10th	Day	7% 3 7% 10% 28% 44%	530
	11th to 20th	Day	6% 3 6% 22% 28% 36%	530
	21st to end	Day	7% 4 7% 19% 32% 31%	530
October	1st to 10th	Day	13% 3 10% 32% 29% 12%	530
	11th to 20th	Day	6% 4 16% 34% 31% 10%	530
	21st to end	Day	7% 4 17% 35% 28% 9%	583
November	1st to 10th	Day	9% 7% 19% 29% 28% 8%	535
	11th to 20th	Day	12% 4 22% 33% 23% 5%	540
	21st to end	Day	8% 4 21% 37% 25% 5%	540
December	1st to 10th	Day	5% 7% 33% 32% 18% 5%	335
	11th to 20th	Day	6% 8% 28% 35% 18% 3	330
	21st to end	Day	6% 9% 28% 34% 17% 6%	363
January	1st to 10th	Night	9% 6% 30% 37% 18% 1	840
	11th to 20th	Night	9% 6% 30% 41% 13% 1	840
	21st to end	Night	13% 6% 31% 39% 10%	924
February	1st to 10th	Night	11% 8% 26% 41% 14%	840
	11th to 20th	Night	9% 6% 28% 40% 16%	804
	21st to end	Night	9% 4 28% 46% 13%	660
March	1st to 10th	Night	6% 5% 33% 43% 13%	800
	11th to 20th	Night	7% 6% 27% 41% 18% 1	800
	21st to end	Night	6% 5% 28% 46% 15%	880
April	1st to 10th	Night	4 5% 27% 47% 17%	800
	11th to 20th	Night	6% 4 26% 44% 19% 1	791
	21st to end	Night	5% 3 24% 46% 21% 1	800
May	1st to 10th	Night	5% 3 18% 48% 25% 2	796
	11th to 20th	Night	7% 1 11% 44% 33% 4	591
	21st to end	Night	7% 1 6% 35% 44% 8%	650
June	1st to 10th	Night	11% 3 6% 25% 47% 9%	600
	11th to 20th	Night	7% 2 4 19% 49% 20%	600
	21st to end	Night	7% 2 2 14% 45% 30%	600
July	1st to 10th	Night	8% 2 2 7% 50% 31%	585
	11th to 20th	Night	5% 1 1 9% 44% 39%	582
	21st to end	Night	5% 2 2 10% 46% 35%	657
August	1st to 10th	Night	6% 1 3 9% 51% 31%	597
	11th to 20th	Night	5% 2 2 9% 49% 32%	701
	21st to end	Night	11% 2 6% 16% 41% 24%	880
September	1st to 10th	Night	10% 3 5% 15% 44% 23%	800
	11th to 20th	Night	10% 3 5% 21% 43% 19%	800
	21st to end	Night	13% 2 7% 24% 40% 13%	800
October	1st to 10th	Night	14% 3 11% 34% 33% 6%	800
	11th to 20th	Night	8% 4 11% 37% 36% 5%	800
	21st to end	Night	11% 3 16% 38% 28% 4	880
November	1st to 10th	Night	9% 3 21% 39% 26% 2	820
	11th to 20th	Night	8% 4 19% 44% 23% 2	840
	21st to end	Night	9% 3 21% 43% 23% 1	840
December	1st to 10th	Night	8% 3 22% 41% 24% 2	840
	11th to 20th	Night	9% 5% 21% 39% 23% 2	840
	21st to end	Night	9% 4 23% 38% 24% 1	924

Page generated at 2026-02-13 01:42:15. Version ac07

Copyright © 2026 Attilla Danko.