Grand River
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grand River clear sky chart from 2005-11-06 to 2025-10-19.

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% 8% 3 7% 4 4 4 4 5% 7% 41%	126691
	Night	11% 8% 3 8% 4 4 4 3 5% 6% 44%	123293
Transparency	Day	40% 5% 12% 25% 15% 2	130294
	Night	39% 5% 11% 25% 17% 3	127766
Seeing	Day	15% 10% 29% 28% 14% 4	17124
	Night	15% 10% 33% 31% 11% 1	26904

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	11% 11% 4 13% 6% 6% 6% 4 6% 7% 27%	3817
	11th to 20th	Day	7% 10% 4 10% 6% 5% 9% 3 4 6% 37%	4080
	21st to end	Day	10% 12% 6% 13% 5% 5% 5% 4 5% 6% 29%	4488
February	1st to 10th	Day	12% 11% 4 11% 7% 6% 5% 3 5% 7% 30%	1800
	11th to 20th	Day	11% 10% 4 9% 6% 4 5% 4 5% 7% 35%	1754
	21st to end	Day	12% 8% 3 7% 5% 4 4 3 5% 7% 41%	1629
March	1st to 10th	Day	17% 10% 3 7% 3 3 2 3 4 4 45%	4936
	11th to 20th	Day	10% 7% 2 6% 5% 4 4 3 5% 8% 46%	4980
	21st to end	Day	16% 12% 5% 8% 5% 4 4 4 5% 7% 28%	5462
April	1st to 10th	Day	16% 9% 3 6% 4 4 3 4 6% 6% 39%	2363
	11th to 20th	Day	16% 9% 3 7% 5% 4 4 4 5% 7% 37%	2374
	21st to end	Day	20% 9% 3 9% 3 4 4 3 5% 7% 36%	2540
May	1st to 10th	Day	13% 7% 3 6% 4 4 5% 4 6% 8% 39%	5862
	11th to 20th	Day	16% 6% 2 5% 4 3 4 3 5% 6% 46%	6035
	21st to end	Day	15% 8% 3 6% 5% 4 5% 4 6% 7% 38%	6935
June	1st to 10th	Day	8% 6% 2 7% 5% 5% 5% 4 6% 8% 45%	2800
	11th to 20th	Day	8% 5% 2 6% 4 3 4 5% 7% 10% 46%	2800
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 7% 9% 43%	2800
July	1st to 10th	Day	8% 6% 3 6% 4 5% 5% 5% 7% 9% 42%	6360
	11th to 20th	Day	7% 5% 2 7% 5% 5% 5% 4 6% 8% 46%	6046
	21st to end	Day	8% 5% 2 6% 3 4 5% 4 7% 10% 46%	6422
August	1st to 10th	Day	5% 4 2 5% 5% 5% 5% 4 8% 12% 46%	4234
	11th to 20th	Day	13% 6% 2 5% 4 4 5% 4 6% 8% 43%	4171
	21st to end	Day	7% 5% 2 6% 5% 4 4 4 6% 8% 49%	4393
September	1st to 10th	Day	13% 4 1 5% 3 3 3 3 5% 6% 54%	2400
	11th to 20th	Day	9% 5% 1 5% 4 4 3 4 6% 9% 49%	2297
	21st to end	Day	10% 5% 2 5% 3 3 3 3 5% 8% 52%	2145
October	1st to 10th	Day	14% 7% 2 5% 4 4 3 2 5% 7% 49%	2000
	11th to 20th	Day	12% 9% 3 5% 2 4 3 3 4 6% 50%	1980
	21st to end	Day	13% 9% 3 5% 4 4 4 4 5% 5% 44%	2090
November	1st to 10th	Day	10% 6% 2 5% 4 4 4 3 4 8% 51%	1898
	11th to 20th	Day	12% 8% 3 7% 4 3 4 4 4 6% 44%	1600
	21st to end	Day	16% 10% 4 7% 4 3 3 3 5% 6% 39%	1600
December	1st to 10th	Day	6% 6% 2 6% 4 4 4 3 5% 7% 53%	2610
	11th to 20th	Day	11% 9% 3 11% 5% 5% 4 3 5% 6% 38%	3008
	21st to end	Day	20% 15% 6% 14% 5% 4 4 3 5% 5% 19%	3982
January	1st to 10th	Night	10% 10% 5% 21% 7% 5% 4 3 4 6% 26%	6146
	11th to 20th	Night	9% 9% 4 11% 6% 5% 6% 3 7% 6% 34%	5900
	21st to end	Night	12% 13% 5% 11% 4 4 4 3 4 5% 36%	6490
February	1st to 10th	Night	12% 11% 4 9% 6% 4 4 4 5% 5% 35%	2600
	11th to 20th	Night	11% 11% 4 7% 5% 4 3 3 5% 6% 41%	2533
	21st to end	Night	11% 8% 3 8% 5% 4 4 2 5% 5% 46%	2017
March	1st to 10th	Night	16% 7% 3 6% 3 3 3 3 4 5% 47%	4919
	11th to 20th	Night	10% 7% 2 5% 3 3 3 3 4 6% 53%	4980
	21st to end	Night	14% 11% 4 11% 5% 4 4 4 5% 5% 31%	5467
April	1st to 10th	Night	11% 8% 2 6% 5% 4 3 3 6% 6% 47%	2036
	11th to 20th	Night	13% 7% 3 7% 5% 4 4 3 5% 5% 44%	1975
	21st to end	Night	17% 7% 3 6% 4 5% 4 3 4 7% 41%	1855
May	1st to 10th	Night	13% 6% 2 6% 4 4 4 3 5% 7% 45%	4053
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 4 7% 51%	3878
	21st to end	Night	15% 7% 3 7% 4 4 4 3 5% 6% 43%	3942
June	1st to 10th	Night	6% 4 2 5% 3 3 4 4 7% 8% 54%	1600
	11th to 20th	Night	6% 5% 1 4 4 3 4 3 8% 11% 50%	1600
	21st to end	Night	6% 4 1 6% 3 4 5% 3 7% 8% 52%	1600
July	1st to 10th	Night	8% 4 2 5% 4 3 4 4 7% 8% 51%	3618
	11th to 20th	Night	5% 3 2 6% 4 4 5% 3 6% 8% 55%	3927
	21st to end	Night	8% 4 2 6% 4 4 4 4 6% 9% 48%	4449
August	1st to 10th	Night	4 3 1 4 3 4 4 4 7% 8% 58%	2926
	11th to 20th	Night	10% 6% 2 4 3 4 3 3 5% 7% 52%	2888
	21st to end	Night	6% 4 2 5% 3 3 4 3 5% 9% 58%	3507
September	1st to 10th	Night	10% 4 2 4 2 3 3 3 5% 6% 57%	2000
	11th to 20th	Night	9% 5% 1 5% 4 4 4 3 6% 7% 53%	2099
	21st to end	Night	9% 4 1 4 3 3 3 3 5% 7% 58%	2235
October	1st to 10th	Night	12% 7% 2 6% 2 3 3 3 4 6% 53%	2400
	11th to 20th	Night	11% 6% 2 6% 4 4 3 3 4 5% 53%	2377
	21st to end	Night	14% 8% 3 6% 4 3 3 2 4 5% 48%	2508
November	1st to 10th	Night	9% 5% 1 5% 3 3 4 3 6% 7% 55%	2393
	11th to 20th	Night	11% 6% 2 6% 4 3 3 3 5% 7% 49%	2800
	21st to end	Night	14% 10% 3 6% 4 3 3 3 4 5% 44%	2800
December	1st to 10th	Night	6% 9% 3 6% 4 3 4 3 5% 6% 50%	4580
	11th to 20th	Night	11% 8% 3 9% 4 4 4 3 5% 7% 41%	5199
	21st to end	Night	18% 17% 6% 12% 5% 4 4 3 4 4 24%	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	54% 5% 13% 17% 10% 1	3818
	11th to 20th	Day	49% 4 10% 20% 15% 3	4080
	21st to end	Day	56% 4 11% 21% 8% 1	4488
February	1st to 10th	Day	54% 4 11% 19% 10% 2	1800
	11th to 20th	Day	48% 5% 10% 21% 13% 2	1763
	21st to end	Day	43% 4 13% 23% 16% 2	1629
March	1st to 10th	Day	44% 3 7% 24% 19% 3	4936
	11th to 20th	Day	37% 4 9% 25% 22% 3	4980
	21st to end	Day	53% 8% 16% 17% 4 1	5464
April	1st to 10th	Day	45% 6% 11% 25% 12% 1	2363
	11th to 20th	Day	46% 6% 11% 23% 13% 1	2374
	21st to end	Day	49% 5% 12% 22% 11% 1	2540
May	1st to 10th	Day	40% 7% 12% 23% 17% 1	5900
	11th to 20th	Day	38% 5% 9% 29% 16% 3	6039
	21st to end	Day	44% 6% 11% 23% 15% 1	6949
June	1st to 10th	Day	35% 9% 15% 27% 13% 3	2800
	11th to 20th	Day	32% 9% 13% 26% 19% 1	2800
	21st to end	Day	35% 9% 15% 24% 15% 2	2800
July	1st to 10th	Day	36% 6% 14% 30% 12% 1	6291
	11th to 20th	Day	35% 6% 14% 27% 16% 1	5973
	21st to end	Day	32% 8% 16% 32% 11% 1	6240
August	1st to 10th	Day	30% 6% 15% 31% 16% 1	4220
	11th to 20th	Day	38% 6% 13% 26% 16% 1	4210
	21st to end	Day	32% 5% 16% 28% 15% 4	4392
September	1st to 10th	Day	31% 5% 12% 29% 19% 4	2400
	11th to 20th	Day	31% 6% 13% 30% 19% 1	2297
	21st to end	Day	31% 5% 9% 32% 22% 1	2145
October	1st to 10th	Day	23% 3 5% 20% 47% 2	3260
	11th to 20th	Day	23% 2 7% 29% 30% 8%	3220
	21st to end	Day	36% 4 10% 27% 20% 3	3428
November	1st to 10th	Day	33% 5% 14% 25% 20% 3	1898
	11th to 20th	Day	41% 4 12% 25% 17% 1	1600
	21st to end	Day	46% 4 13% 24% 13%	1600
December	1st to 10th	Day	32% 4 19% 31% 13% 1	2610
	11th to 20th	Day	47% 4 14% 25% 10%	3005
	21st to end	Day	68% 4 11% 12% 5% 1	3982
January	1st to 10th	Night	60% 4 10% 16% 9% 1	6146
	11th to 20th	Night	49% 4 12% 18% 16% 2	5900
	21st to end	Night	52% 3 10% 18% 14% 3	6490
February	1st to 10th	Night	50% 5% 11% 20% 12% 2	2600
	11th to 20th	Night	44% 5% 10% 25% 14% 2	2534
	21st to end	Night	40% 5% 10% 22% 19% 4	2017
March	1st to 10th	Night	41% 3 7% 25% 22% 2	4919
	11th to 20th	Night	33% 4 7% 30% 21% 6%	4980
	21st to end	Night	53% 6% 15% 17% 7% 2	5467
April	1st to 10th	Night	37% 5% 12% 26% 17% 3	2036
	11th to 20th	Night	41% 5% 12% 25% 14% 4	1975
	21st to end	Night	45% 6% 11% 23% 14% 2	1855
May	1st to 10th	Night	38% 5% 11% 26% 18% 2	4080
	11th to 20th	Night	34% 6% 11% 26% 19% 4	3879
	21st to end	Night	42% 7% 10% 23% 17% 2	3950
June	1st to 10th	Night	24% 8% 19% 30% 16% 3	1600
	11th to 20th	Night	26% 7% 15% 27% 21% 4	1600
	21st to end	Night	28% 7% 15% 26% 20% 4	1600
July	1st to 10th	Night	30% 5% 14% 30% 17% 4	3580
	11th to 20th	Night	28% 5% 18% 28% 20% 1	3880
	21st to end	Night	31% 7% 19% 29% 13% 1	4320
August	1st to 10th	Night	23% 6% 16% 34% 20% 1	2916
	11th to 20th	Night	32% 5% 13% 29% 20% 2	2915
	21st to end	Night	25% 5% 17% 29% 18% 6%	3507
September	1st to 10th	Night	27% 6% 11% 26% 24% 6%	2000
	11th to 20th	Night	30% 5% 11% 31% 20% 2	2099
	21st to end	Night	27% 5% 9% 29% 27% 3	2235
October	1st to 10th	Night	23% 3 5% 19% 47% 3	3920
	11th to 20th	Night	27% 3 8% 25% 30% 7%	3876
	21st to end	Night	40% 3 8% 25% 22% 2	4120
November	1st to 10th	Night	28% 5% 17% 25% 21% 4	2393
	11th to 20th	Night	35% 4 11% 29% 18% 3	2800
	21st to end	Night	42% 5% 9% 27% 15% 2	2800
December	1st to 10th	Night	36% 4 10% 38% 12% 1	4580
	11th to 20th	Night	43% 5% 13% 26% 12%	5201
	21st to end	Night	64% 4 10% 15% 5% 1	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% 12% 34% 23% 7% 1	300
	11th to 20th	Day	18% 20% 37% 21% 4	300
	21st to end	Day	18% 13% 43% 23% 2	330
February	1st to 10th	Day	19% 19% 38% 21% 3 1	300
	11th to 20th	Day	18% 17% 40% 18% 6% 1	300
	21st to end	Day	18% 15% 44% 19% 3 1	249
March	1st to 10th	Day	20% 17% 43% 17% 3	300
	11th to 20th	Day	25% 15% 34% 22% 5%	300
	21st to end	Day	36% 19% 28% 15% 2	330
April	1st to 10th	Day	26% 16% 34% 18% 5%	463
	11th to 20th	Day	20% 17% 35% 21% 7% 1	492
	21st to end	Day	23% 15% 36% 18% 7% 1	640
May	1st to 10th	Day	19% 14% 31% 27% 7% 2	703
	11th to 20th	Day	21% 12% 29% 23% 12% 3	725
	21st to end	Day	17% 9% 28% 29% 14% 3	789
June	1st to 10th	Day	8% 6% 23% 32% 24% 7%	730
	11th to 20th	Day	9% 4 29% 35% 19% 4	730
	21st to end	Day	8% 8% 33% 34% 14% 3	730
July	1st to 10th	Day	8% 5% 17% 34% 24% 11%	721
	11th to 20th	Day	7% 3 25% 37% 21% 7%	718
	21st to end	Day	9% 2 21% 39% 24% 5%	794
August	1st to 10th	Day	8% 3 21% 39% 24% 5%	726
	11th to 20th	Day	12% 4 17% 38% 25% 5%	715
	21st to end	Day	11% 3 18% 35% 26% 7%	638
September	1st to 10th	Day	13% 4 22% 28% 26% 7%	530
	11th to 20th	Day	11% 3 19% 38% 23% 7%	427
	21st to end	Day	13% 7% 27% 30% 15% 7%	330
October	1st to 10th	Day	18% 8% 30% 24% 15% 5%	330
	11th to 20th	Day	14% 21% 37% 19% 6% 2	324
	21st to end	Day	18% 18% 32% 21% 9% 2	330
November	1st to 10th	Day	14% 12% 44% 19% 9% 2	300
	11th to 20th	Day	18% 19% 37% 18% 6% 1	300
	21st to end	Day	22% 14% 38% 20% 6% 1	300
December	1st to 10th	Day	9% 17% 40% 25% 7% 2	300
	11th to 20th	Day	15% 16% 36% 26% 6% 1	300
	21st to end	Day	26% 18% 34% 19% 3	330
January	1st to 10th	Night	13% 13% 41% 26% 8%	1000
	11th to 20th	Night	15% 15% 40% 23% 6%	1000
	21st to end	Night	16% 13% 39% 27% 5%	1100
February	1st to 10th	Night	19% 13% 39% 24% 5%	1000
	11th to 20th	Night	17% 14% 38% 26% 4	979
	21st to end	Night	17% 13% 43% 21% 4 1	697
March	1st to 10th	Night	16% 13% 39% 25% 6%	600
	11th to 20th	Night	20% 13% 36% 25% 6%	600
	21st to end	Night	24% 14% 38% 19% 5%	660
April	1st to 10th	Night	15% 16% 41% 25% 4	600
	11th to 20th	Night	17% 16% 37% 24% 6%	593
	21st to end	Night	21% 10% 36% 25% 8%	600
May	1st to 10th	Night	15% 11% 34% 34% 6% 1	600
	11th to 20th	Night	15% 10% 29% 33% 12% 1	591
	21st to end	Night	16% 6% 29% 35% 14%	650
June	1st to 10th	Night	8% 4 27% 39% 22% 1	600
	11th to 20th	Night	8% 4 26% 44% 17% 1	600
	21st to end	Night	8% 3 32% 42% 14%	600
July	1st to 10th	Night	6% 2 17% 40% 32% 3	585
	11th to 20th	Night	5% 2 18% 49% 24% 2	582
	21st to end	Night	7% 2 17% 49% 24% 2	657
August	1st to 10th	Night	5% 2 21% 45% 24% 2	597
	11th to 20th	Night	9% 2 16% 42% 28% 3	594
	21st to end	Night	8% 2 17% 41% 30% 2	660
September	1st to 10th	Night	12% 2 21% 37% 25% 3	600
	11th to 20th	Night	13% 2 23% 38% 23% 2	600
	21st to end	Night	9% 5% 27% 36% 21% 1	635
October	1st to 10th	Night	15% 7% 28% 33% 15% 2	800
	11th to 20th	Night	14% 11% 39% 28% 8%	793
	21st to end	Night	20% 11% 36% 25% 7% 1	836
November	1st to 10th	Night	11% 11% 39% 31% 8% 1	795
	11th to 20th	Night	14% 16% 39% 24% 6% 1	1000
	21st to end	Night	18% 13% 37% 25% 7%	1000
December	1st to 10th	Night	16% 13% 38% 27% 6% 1	1000
	11th to 20th	Night	20% 9% 37% 28% 6%	1000
	21st to end	Night	22% 14% 37% 24% 4	1100

Page generated at 2025-10-16 00:50:24. Version ac07

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