Granger Lake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Granger Lake clear sky chart from 2005-11-06 to 2025-08-05.

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% 6% 2 5% 4 4 4 3 5% 7% 47%	125717
	Night	13% 5% 2 5% 3 3 3 3 5% 7% 51%	122636
Transparency	Day	35% 5% 10% 21% 20% 10%	129460
	Night	33% 4 10% 20% 20% 12%	126956
Seeing	Day	13% 6% 20% 28% 24% 8%	15943
	Night	14% 6% 26% 36% 16% 2	25792

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	15% 10% 3 5% 3 3 3 3 4 5% 47%	4077
	11th to 20th	Day	14% 5% 2 4 3 3 3 2 6% 6% 54%	4080
	21st to end	Day	26% 9% 3 6% 3 2 2 2 2 5% 41%	4596
February	1st to 10th	Day	20% 7% 2 6% 3 3 4 2 4 5% 45%	2000
	11th to 20th	Day	20% 7% 2 6% 3 3 3 2 5% 5% 45%	1949
	21st to end	Day	19% 8% 2 5% 3 3 3 3 4 4 48%	1672
March	1st to 10th	Day	13% 7% 2 6% 3 3 3 3 4 6% 50%	4936
	11th to 20th	Day	15% 6% 2 5% 4 3 4 3 4 6% 49%	4980
	21st to end	Day	14% 8% 2 6% 4 4 4 3 5% 6% 44%	5462
April	1st to 10th	Day	14% 9% 3 6% 4 4 4 3 5% 7% 41%	2200
	11th to 20th	Day	16% 6% 3 7% 4 5% 3 4 4 6% 43%	2206
	21st to end	Day	14% 6% 2 5% 4 4 3 3 5% 7% 45%	2518
May	1st to 10th	Day	19% 7% 3 7% 4 4 4 4 6% 7% 34%	5862
	11th to 20th	Day	12% 5% 2 6% 4 5% 5% 4 6% 8% 44%	5862
	21st to end	Day	12% 6% 2 7% 5% 5% 6% 5% 8% 10% 34%	6434
June	1st to 10th	Day	5% 4 1 4 4 5% 4 5% 7% 9% 50%	2600
	11th to 20th	Day	7% 4 2 5% 4 5% 5% 4 7% 10% 50%	2600
	21st to end	Day	6% 6% 2 6% 6% 5% 6% 6% 7% 9% 42%	2600
July	1st to 10th	Day	8% 6% 3 7% 6% 5% 5% 4 7% 9% 40%	5900
	11th to 20th	Day	4 4 2 5% 4 4 5% 4 8% 11% 50%	5900
	21st to end	Day	6% 5% 2 6% 4 5% 5% 4 7% 11% 44%	6422
August	1st to 10th	Day	1 1 1 2 2 2 3 2 5% 8% 72%	4157
	11th to 20th	Day	3 2 1 3 2 3 4 3 6% 10% 62%	3927
	21st to end	Day	3 2 1 4 3 5% 5% 5% 8% 10% 53%	3929
September	1st to 10th	Day	8% 5% 2 7% 5% 5% 5% 4 5% 7% 47%	2090
	11th to 20th	Day	10% 6% 2 7% 4 5% 5% 4 5% 7% 45%	2090
	21st to end	Day	7% 4 2 4 4 4 5% 4 6% 7% 54%	2090
October	1st to 10th	Day	6% 3 2 4 3 3 4 3 5% 6% 61%	2090
	11th to 20th	Day	13% 4 2 5% 2 3 3 3 3 5% 58%	1901
	21st to end	Day	13% 4 1 5% 2 3 3 2 4 5% 59%	2090
November	1st to 10th	Day	14% 7% 2 5% 3 5% 3 3 4 6% 49%	1958
	11th to 20th	Day	11% 5% 2 6% 4 3 3 3 4 6% 54%	1920
	21st to end	Day	16% 5% 2 6% 4 4 4 3 4 5% 47%	1800
December	1st to 10th	Day	21% 7% 2 5% 3 3 3 3 4 4 44%	2940
	11th to 20th	Day	19% 7% 3 5% 3 3 4 3 5% 5% 43%	3391
	21st to end	Day	14% 6% 2 5% 3 3 3 2 3 4 55%	4488
January	1st to 10th	Night	18% 8% 2 4 2 2 2 2 3 5% 53%	5888
	11th to 20th	Night	12% 4 1 3 2 2 2 2 4 5% 64%	5900
	21st to end	Night	27% 7% 2 5% 3 2 3 2 3 5% 41%	6426
February	1st to 10th	Night	19% 7% 2 5% 3 3 2 3 4 5% 48%	2400
	11th to 20th	Night	19% 7% 2 5% 3 3 3 3 3 3 49%	2338
	21st to end	Night	19% 6% 2 4 3 3 2 3 5% 6% 47%	1956
March	1st to 10th	Night	15% 7% 2 5% 4 3 4 4 4 7% 45%	4919
	11th to 20th	Night	15% 6% 2 5% 3 3 4 3 4 5% 49%	4980
	21st to end	Night	16% 8% 3 6% 4 4 4 2 4 6% 44%	5467
April	1st to 10th	Night	12% 6% 2 5% 4 4 3 4 6% 7% 48%	2200
	11th to 20th	Night	15% 6% 3 5% 3 3 4 3 5% 6% 47%	2143
	21st to end	Night	14% 6% 2 5% 3 3 4 3 4 8% 48%	1877
May	1st to 10th	Night	20% 8% 3 6% 4 3 4 3 6% 7% 36%	4053
	11th to 20th	Night	9% 4 2 6% 4 4 5% 4 7% 7% 47%	4053
	21st to end	Night	12% 6% 2 7% 5% 4 4 4 6% 8% 40%	4443
June	1st to 10th	Night	5% 3 1 4 2 3 3 4 6% 10% 61%	1800
	11th to 20th	Night	4 3 2 3 3 3 3 4 7% 9% 59%	1800
	21st to end	Night	4 4 2 4 3 4 4 4 7% 10% 54%	1800
July	1st to 10th	Night	8% 6% 2 6% 5% 4 4 4 7% 11% 43%	4078
	11th to 20th	Night	3 2 2 4 3 4 4 4 7% 12% 57%	4077
	21st to end	Night	5% 5% 2 5% 4 4 5% 4 7% 11% 48%	4449
August	1st to 10th	Night	1 1 1 1 2 3 2 4 8% 77%	2872
	11th to 20th	Night	3 2 1 3 3 3 4 4 7% 10% 61%	2893
	21st to end	Night	3 2 1 3 3 3 4 4 7% 10% 57%	3727
September	1st to 10th	Night	7% 4 2 4 5% 5% 4 4 6% 6% 53%	2090
	11th to 20th	Night	9% 5% 2 6% 4 4 4 4 7% 7% 47%	2090
	21st to end	Night	7% 3 1 3 4 4 4 3 6% 8% 57%	2090
October	1st to 10th	Night	6% 2 1 3 3 3 3 2 4 6% 67%	2090
	11th to 20th	Night	12% 4 1 4 3 3 3 3 4 6% 58%	2258
	21st to end	Night	12% 5% 2 4 2 3 3 2 3 4 60%	2508
November	1st to 10th	Night	13% 5% 2 4 3 3 3 2 3 6% 53%	2340
	11th to 20th	Night	10% 5% 2 5% 3 4 3 2 4 5% 56%	2475
	21st to end	Night	17% 6% 2 5% 3 4 4 3 4 5% 49%	2600
December	1st to 10th	Night	20% 8% 2 5% 3 3 3 3 4 6% 43%	4250
	11th to 20th	Night	19% 9% 2 5% 3 2 3 3 3 5% 47%	4816
	21st to end	Night	16% 6% 2 4 2 2 2 2 3 3 57%	6490

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% 3 4 24% 20% 9%	4078
	11th to 20th	Day	32% 2 5% 19% 27% 16%	4080
	21st to end	Day	50% 2 2 10% 22% 14%	4596
February	1st to 10th	Day	43% 3 5% 12% 22% 14%	2000
	11th to 20th	Day	43% 3 6% 18% 19% 11%	1959
	21st to end	Day	41% 3 5% 17% 19% 14%	1672
March	1st to 10th	Day	37% 3 5% 18% 19% 18%	4936
	11th to 20th	Day	37% 4 7% 9% 16% 27%	4980
	21st to end	Day	41% 4 8% 20% 20% 7%	5464
April	1st to 10th	Day	43% 5% 7% 16% 15% 13%	2200
	11th to 20th	Day	43% 4 7% 15% 19% 11%	2206
	21st to end	Day	40% 4 7% 19% 22% 8%	2518
May	1st to 10th	Day	48% 5% 10% 18% 15% 4	5900
	11th to 20th	Day	38% 5% 11% 17% 24% 5%	5866
	21st to end	Day	42% 6% 11% 23% 14% 3	6447
June	1st to 10th	Day	28% 6% 14% 27% 21% 4	2600
	11th to 20th	Day	29% 6% 12% 29% 19% 6%	2600
	21st to end	Day	35% 8% 12% 23% 19% 3	2600
July	1st to 10th	Day	38% 8% 19% 29% 6%	5836
	11th to 20th	Day	26% 7% 17% 33% 16% 2	5831
	21st to end	Day	32% 7% 17% 27% 15% 3	6240
August	1st to 10th	Day	12% 4 11% 40% 32% 1	4143
	11th to 20th	Day	17% 4 23% 40% 13% 2	3966
	21st to end	Day	23% 9% 21% 27% 18% 2	3928
September	1st to 10th	Day	35% 7% 10% 21% 18% 9%	2090
	11th to 20th	Day	37% 8% 11% 18% 20% 5%	2090
	21st to end	Day	28% 7% 8% 23% 19% 15%	2090
October	1st to 10th	Day	15% 3 6% 13% 24% 39%	3480
	11th to 20th	Day	19% 2 6% 18% 35% 20%	3141
	21st to end	Day	25% 2 5% 22% 33% 13%	3428
November	1st to 10th	Day	38% 5% 8% 18% 21% 11%	1958
	11th to 20th	Day	33% 4 8% 18% 21% 16%	1920
	21st to end	Day	40% 4 6% 18% 20% 13%	1800
December	1st to 10th	Day	45% 3 7% 17% 19% 9%	2940
	11th to 20th	Day	43% 4 6% 11% 24% 12%	3389
	21st to end	Day	34% 3 4 18% 28% 14%	4488
January	1st to 10th	Night	37% 3 4 21% 24% 11%	5888
	11th to 20th	Night	25% 2 4 18% 33% 17%	5900
	21st to end	Night	48% 2 3 12% 20% 15%	6426
February	1st to 10th	Night	40% 3 5% 13% 21% 17%	2400
	11th to 20th	Night	40% 4 5% 17% 20% 14%	2338
	21st to end	Night	40% 4 6% 15% 18% 16%	1956
March	1st to 10th	Night	39% 4 6% 15% 21% 14%	4919
	11th to 20th	Night	38% 3 8% 11% 15% 24%	4980
	21st to end	Night	43% 4 7% 17% 20% 9%	5467
April	1st to 10th	Night	36% 6% 9% 18% 17% 15%	2200
	11th to 20th	Night	38% 4 10% 15% 18% 14%	2143
	21st to end	Night	36% 5% 10% 20% 20% 9%	1877
May	1st to 10th	Night	46% 5% 9% 22% 13% 5%	4080
	11th to 20th	Night	34% 6% 12% 20% 21% 7%	4054
	21st to end	Night	40% 6% 13% 26% 12% 3	4452
June	1st to 10th	Night	19% 7% 18% 31% 22% 4	1800
	11th to 20th	Night	20% 5% 16% 32% 21% 6%	1800
	21st to end	Night	24% 7% 15% 30% 20% 4	1800
July	1st to 10th	Night	34% 8% 26% 27% 5%	4035
	11th to 20th	Night	19% 6% 24% 36% 14% 1	4026
	21st to end	Night	28% 8% 21% 26% 12% 4	4320
August	1st to 10th	Night	8% 4 14% 41% 32% 2	2862
	11th to 20th	Night	18% 6% 27% 34% 14% 2	2920
	21st to end	Night	20% 9% 23% 30% 16% 2	3727
September	1st to 10th	Night	30% 6% 13% 26% 15% 10%	2090
	11th to 20th	Night	33% 7% 15% 21% 16% 7%	2090
	21st to end	Night	25% 5% 13% 24% 19% 14%	2090
October	1st to 10th	Night	14% 3 6% 16% 20% 42%	3480
	11th to 20th	Night	17% 2 7% 16% 33% 24%	3744
	21st to end	Night	24% 3 5% 21% 31% 17%	4120
November	1st to 10th	Night	34% 4 8% 19% 23% 13%	2340
	11th to 20th	Night	31% 4 8% 19% 22% 17%	2475
	21st to end	Night	39% 3 7% 17% 19% 15%	2600
December	1st to 10th	Night	44% 3 5% 19% 20% 10%	4250
	11th to 20th	Night	42% 3 7% 9% 24% 16%	4817
	21st to end	Night	35% 2 3 16% 29% 15%	6490

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	21% 11% 37% 23% 8% 1	300
	11th to 20th	Day	20% 13% 31% 25% 9% 2	300
	21st to end	Day	22% 11% 37% 21% 8%	330
February	1st to 10th	Day	25% 13% 29% 25% 8%	300
	11th to 20th	Day	26% 14% 33% 22% 4	300
	21st to end	Day	24% 13% 39% 20% 2	249
March	1st to 10th	Day	21% 14% 32% 25% 7%	300
	11th to 20th	Day	16% 18% 40% 19% 5% 2	300
	21st to end	Day	19% 11% 38% 24% 6% 2	330
April	1st to 10th	Day	21% 16% 36% 21% 5% 1	300
	11th to 20th	Day	19% 13% 36% 24% 7% 1	324
	21st to end	Day	18% 11% 34% 27% 9% 1	618
May	1st to 10th	Day	16% 10% 34% 30% 10%	703
	11th to 20th	Day	19% 6% 27% 31% 14% 3	725
	21st to end	Day	16% 6% 23% 35% 17% 4	789
June	1st to 10th	Day	7% 2 15% 41% 28% 8%	730
	11th to 20th	Day	7% 1 16% 43% 28% 5%	730
	21st to end	Day	6% 2 12% 33% 36% 12%	730
July	1st to 10th	Day	7% 2 8% 35% 34% 14%	721
	11th to 20th	Day	3 1 5% 33% 48% 11%	718
	21st to end	Day	5% 1 5% 28% 46% 15%	794
August	1st to 10th	Day	4 3 25% 48% 19%	697
	11th to 20th	Day	5% 2 25% 49% 18%	603
	21st to end	Day	6% 1 4 19% 36% 34%	392
September	1st to 10th	Day	8% 5% 18% 38% 31%	300
	11th to 20th	Day	5% 1 6% 24% 36% 28%	300
	21st to end	Day	8% 2 4 24% 37% 25%	300
October	1st to 10th	Day	3 2 10% 26% 42% 17%	300
	11th to 20th	Day	10% 3 21% 29% 28% 9%	300
	21st to end	Day	18% 11% 20% 22% 21% 7%	330
November	1st to 10th	Day	21% 5% 21% 32% 15% 6%	300
	11th to 20th	Day	16% 7% 26% 27% 18% 5%	300
	21st to end	Day	17% 16% 30% 22% 13% 3	300
December	1st to 10th	Day	19% 6% 28% 28% 14% 4	300
	11th to 20th	Day	23% 11% 31% 23% 11% 1	300
	21st to end	Day	22% 6% 36% 24% 9% 2	330
January	1st to 10th	Night	20% 9% 32% 31% 8% 1	1000
	11th to 20th	Night	20% 11% 30% 32% 7%	1000
	21st to end	Night	20% 10% 33% 30% 7%	1075
February	1st to 10th	Night	24% 10% 32% 28% 6%	800
	11th to 20th	Night	25% 11% 38% 23% 3	784
	21st to end	Night	24% 12% 42% 20% 3	636
March	1st to 10th	Night	20% 12% 35% 28% 4	600
	11th to 20th	Night	18% 10% 37% 30% 5%	600
	21st to end	Night	14% 12% 43% 27% 3	660
April	1st to 10th	Night	14% 11% 41% 31% 3	600
	11th to 20th	Night	16% 12% 38% 29% 5%	593
	21st to end	Night	18% 8% 38% 29% 7% 1	600
May	1st to 10th	Night	13% 10% 40% 32% 5% 1	600
	11th to 20th	Night	11% 5% 33% 37% 12% 1	591
	21st to end	Night	10% 4 32% 40% 14% 1	650
June	1st to 10th	Night	5% 2 21% 44% 27% 2	600
	11th to 20th	Night	5% 1 28% 49% 17% 1	600
	21st to end	Night	5% 1 14% 47% 29% 4	600
July	1st to 10th	Night	6% 2 9% 48% 29% 6%	585
	11th to 20th	Night	3 8% 55% 31% 2	582
	21st to end	Night	4 1 7% 48% 37% 3	657
August	1st to 10th	Night	2 5% 54% 36% 4	579
	11th to 20th	Night	7% 1 4 48% 36% 5%	564
	21st to end	Night	5% 4 39% 46% 6%	627
September	1st to 10th	Night	8% 1 7% 38% 40% 6%	570
	11th to 20th	Night	11% 2 7% 33% 44% 3	570
	21st to end	Night	8% 1 9% 40% 38% 4	570
October	1st to 10th	Night	5% 1 11% 50% 29% 5%	570
	11th to 20th	Night	12% 3 20% 40% 22% 2	738
	21st to end	Night	14% 7% 25% 34% 19% 2	836
November	1st to 10th	Night	14% 4 22% 40% 19% 2	780
	11th to 20th	Night	13% 6% 28% 37% 15% 1	875
	21st to end	Night	19% 9% 28% 30% 13% 1	1000
December	1st to 10th	Night	19% 7% 28% 35% 10% 1	1000
	11th to 20th	Night	23% 8% 28% 34% 7%	1000
	21st to end	Night	21% 8% 31% 33% 8%	1100

Page generated at 2025-08-02 00:32:23. Version ac07

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