Grundy Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grundy Center clear sky chart from 2005-11-06 to 2024-02-25.

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% 8% 4 7% 6% 3 5% 5% 5% 7% 37%	78435
	Night	12% 9% 4 8% 6% 3 4 4 5% 7% 39%	80918
Transparency	Day	47% 4 13% 23% 12% 1	78142
	Night	45% 4 12% 23% 14% 2	80731
Seeing	Day	17% 9% 30% 27% 14% 4	14829
	Night	16% 8% 32% 31% 11% 1	24913

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	7% 11% 8% 20% 11% 6% 5% 5% 4 5% 19%	2527
	11th to 20th	Day	8% 13% 5% 10% 9% 6% 6% 6% 7% 6% 24%	2698
	21st to end	Day	14% 14% 8% 16% 8% 4 5% 4 4 6% 17%	3135
February	1st to 10th	Day	11% 11% 5% 10% 8% 4 5% 5% 5% 8% 29%	1710
	11th to 20th	Day	12% 11% 5% 6% 6% 3 6% 5% 6% 7% 33%	1664
	21st to end	Day	14% 11% 4 9% 7% 3 4 4 5% 6% 33%	1482
March	1st to 10th	Day	14% 10% 5% 8% 6% 3 5% 4 5% 7% 33%	1937
	11th to 20th	Day	14% 8% 4 8% 6% 3 5% 5% 6% 9% 32%	1980
	21st to end	Day	19% 13% 5% 7% 5% 3 5% 4 5% 6% 27%	2204
April	1st to 10th	Day	19% 11% 4 7% 5% 3 4 5% 4 6% 31%	2160
	11th to 20th	Day	18% 8% 3 6% 7% 4 5% 5% 4 6% 33%	2134
	21st to end	Day	22% 7% 3 6% 5% 3 4 4 4 6% 34%	2250
May	1st to 10th	Day	19% 8% 3 7% 5% 4 5% 6% 6% 8% 30%	2375
	11th to 20th	Day	18% 9% 4 6% 5% 3 4 4 4 6% 37%	2479
	21st to end	Day	15% 7% 4 6% 6% 4 5% 5% 6% 8% 36%	2725
June	1st to 10th	Day	10% 6% 2 5% 5% 4 5% 6% 6% 8% 43%	2520
	11th to 20th	Day	10% 5% 2 5% 6% 3 5% 4 5% 9% 45%	2520
	21st to end	Day	11% 6% 2 6% 6% 4 7% 6% 7% 9% 38%	2520
July	1st to 10th	Day	8% 5% 2 4 5% 3 6% 5% 6% 9% 48%	2520
	11th to 20th	Day	6% 4 2 5% 5% 4 5% 5% 6% 8% 49%	2520
	21st to end	Day	6% 4 2 4 5% 3 5% 6% 7% 10% 48%	2620
August	1st to 10th	Day	7% 4 3 6% 6% 5% 6% 6% 7% 8% 42%	2328
	11th to 20th	Day	10% 5% 2 6% 5% 3 5% 5% 6% 7% 44%	2301
	21st to end	Day	10% 6% 2 5% 4 2 4 4 5% 7% 51%	2397
September	1st to 10th	Day	12% 5% 2 5% 4 2 3 4 6% 7% 49%	2160
	11th to 20th	Day	9% 5% 2 5% 5% 3 4 5% 5% 7% 50%	2159
	21st to end	Day	11% 6% 3 4 5% 3 4 4 5% 8% 47%	1877
October	1st to 10th	Day	14% 6% 3 6% 6% 3 3 4 5% 7% 43%	1800
	11th to 20th	Day	17% 9% 3 5% 5% 3 4 3 4 6% 42%	1800
	21st to end	Day	16% 10% 3 5% 5% 3 4 4 4 6% 41%	1980
November	1st to 10th	Day	10% 6% 3 7% 4 3 5% 4 5% 7% 45%	1692
	11th to 20th	Day	13% 10% 4 9% 7% 2 4 4 6% 8% 35%	1520
	21st to end	Day	14% 11% 4 6% 6% 3 4 5% 4 7% 38%	1520
December	1st to 10th	Day	12% 12% 4 8% 7% 4 6% 4 5% 7% 31%	1520
	11th to 20th	Day	12% 11% 5% 9% 7% 4 5% 5% 5% 6% 32%	1918
	21st to end	Day	21% 17% 8% 13% 6% 3 3 3 3 5% 18%	2783
January	1st to 10th	Night	9% 12% 6% 19% 9% 3 5% 4 4 5% 24%	4428
	11th to 20th	Night	10% 15% 6% 10% 8% 3 5% 5% 8% 7% 23%	4271
	21st to end	Night	15% 15% 7% 13% 7% 3 4 4 4 5% 22%	4532
February	1st to 10th	Night	9% 12% 6% 11% 8% 3 4 5% 4 7% 32%	2470
	11th to 20th	Night	12% 8% 4 6% 6% 3 4 5% 5% 6% 40%	2403
	21st to end	Night	14% 10% 4 6% 8% 4 4 3 4 5% 38%	1869
March	1st to 10th	Night	14% 10% 4 6% 5% 3 3 4 4 6% 40%	2039
	11th to 20th	Night	14% 7% 3 8% 6% 3 4 5% 4 7% 38%	1980
	21st to end	Night	17% 12% 5% 7% 7% 3 5% 4 5% 6% 30%	2152
April	1st to 10th	Night	14% 9% 3 7% 5% 3 4 4 5% 6% 41%	1800
	11th to 20th	Night	15% 9% 3 6% 6% 3 4 4 5% 7% 38%	1775
	21st to end	Night	18% 6% 3 5% 4 3 4 4 4 7% 42%	1706
May	1st to 10th	Night	16% 7% 2 6% 6% 3 5% 5% 6% 7% 39%	1584
	11th to 20th	Night	14% 7% 3 6% 5% 3 5% 5% 6% 9% 39%	1416
	21st to end	Night	13% 6% 2 5% 5% 3 5% 5% 5% 7% 44%	1551
June	1st to 10th	Night	6% 5% 2 5% 5% 4 6% 5% 5% 10% 48%	1440
	11th to 20th	Night	9% 5% 2 5% 6% 3 4 5% 7% 9% 46%	1440
	21st to end	Night	11% 4 2 5% 4 3 4 4 5% 8% 50%	1440
July	1st to 10th	Night	5% 4 1 4 5% 3 5% 6% 6% 8% 54%	1440
	11th to 20th	Night	5% 3 1 4 3 3 4 4 6% 9% 59%	1440
	21st to end	Night	5% 3 2 4 4 3 5% 4 6% 9% 57%	1694
August	1st to 10th	Night	6% 4 2 5% 4 3 4 6% 6% 9% 52%	1611
	11th to 20th	Night	9% 5% 2 5% 4 2 4 4 4 7% 55%	1600
	21st to end	Night	8% 3 2 4 4 3 4 4 4 6% 57%	1957
September	1st to 10th	Night	9% 5% 3 5% 5% 2 3 3 5% 6% 54%	1800
	11th to 20th	Night	12% 5% 2 4 4 2 4 3 4 6% 55%	1800
	21st to end	Night	10% 5% 1 4 4 2 3 3 5% 6% 57%	2062
October	1st to 10th	Night	11% 6% 2 5% 4 3 3 4 4 7% 50%	2160
	11th to 20th	Night	16% 6% 2 5% 5% 2 4 4 5% 7% 45%	2160
	21st to end	Night	16% 9% 3 5% 4 2 4 4 5% 7% 42%	2376
November	1st to 10th	Night	11% 7% 2 5% 4 2 3 3 4 7% 52%	2384
	11th to 20th	Night	12% 8% 4 7% 6% 3 5% 4 4 5% 41%	2655
	21st to end	Night	14% 9% 3 7% 5% 2 5% 3 5% 6% 42%	2660
December	1st to 10th	Night	11% 10% 4 8% 7% 4 6% 4 5% 7% 33%	2660
	11th to 20th	Night	13% 10% 4 8% 7% 4 5% 5% 4 6% 34%	3279
	21st to end	Night	20% 15% 6% 13% 7% 3 4 4 4 4 21%	4884

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	68% 4 10% 11% 7%	2528
	11th to 20th	Day	59% 3 11% 18% 7%	2698
	21st to end	Day	69% 3 11% 12% 4 1	3135
February	1st to 10th	Day	54% 4 14% 18% 9% 1	1710
	11th to 20th	Day	49% 4 11% 23% 10% 2	1673
	21st to end	Day	52% 4 12% 19% 10% 2	1482
March	1st to 10th	Day	51% 4 15% 20% 8% 2	1937
	11th to 20th	Day	49% 5% 13% 22% 9% 1	1980
	21st to end	Day	57% 5% 12% 18% 7% 1	2204
April	1st to 10th	Day	53% 5% 11% 22% 9% 1	2160
	11th to 20th	Day	52% 4 10% 20% 12% 1	2134
	21st to end	Day	51% 4 10% 22% 12% 1	2250
May	1st to 10th	Day	51% 5% 14% 21% 8% 1	2375
	11th to 20th	Day	48% 5% 12% 23% 11% 1	2483
	21st to end	Day	46% 5% 14% 20% 13% 2	2725
June	1st to 10th	Day	37% 5% 16% 25% 15% 1	2520
	11th to 20th	Day	37% 6% 12% 24% 20% 2	2520
	21st to end	Day	41% 6% 14% 23% 15% 1	2520
July	1st to 10th	Day	33% 5% 13% 32% 14% 2	2451
	11th to 20th	Day	31% 6% 16% 27% 16% 3	2445
	21st to end	Day	30% 5% 20% 27% 16% 2	2474
August	1st to 10th	Day	37% 6% 14% 29% 12% 2	2314
	11th to 20th	Day	37% 6% 13% 29% 14% 1	2301
	21st to end	Day	34% 4 12% 28% 20% 3	2397
September	1st to 10th	Day	34% 4 14% 26% 20% 3	2160
	11th to 20th	Day	32% 5% 13% 33% 16% 1	2159
	21st to end	Day	36% 5% 11% 31% 17% 1	1877
October	1st to 10th	Day	42% 4 10% 27% 16% 1	1800
	11th to 20th	Day	45% 3 10% 24% 17% 1	1800
	21st to end	Day	45% 3 12% 24% 14% 1	1980
November	1st to 10th	Day	38% 4 13% 26% 16% 2	1692
	11th to 20th	Day	48% 4 13% 22% 13% 1	1520
	21st to end	Day	47% 5% 16% 23% 9% 1	1520
December	1st to 10th	Day	54% 4 14% 19% 8% 2	1520
	11th to 20th	Day	53% 4 18% 17% 8%	1915
	21st to end	Day	71% 4 8% 13% 4	2783
January	1st to 10th	Night	63% 3 11% 14% 8% 1	4428
	11th to 20th	Night	58% 3 12% 17% 9%	4271
	21st to end	Night	65% 3 11% 15% 6% 1	4532
February	1st to 10th	Night	52% 4 11% 20% 12% 1	2470
	11th to 20th	Night	44% 5% 11% 26% 13% 2	2404
	21st to end	Night	49% 3 10% 20% 15% 3	1869
March	1st to 10th	Night	46% 3 12% 27% 10% 1	2039
	11th to 20th	Night	46% 3 12% 23% 15% 1	1980
	21st to end	Night	56% 4 14% 17% 8% 1	2152
April	1st to 10th	Night	45% 3 11% 26% 13% 2	1800
	11th to 20th	Night	47% 4 10% 21% 16% 3	1775
	21st to end	Night	42% 4 12% 27% 13% 2	1706
May	1st to 10th	Night	44% 5% 13% 22% 14% 1	1584
	11th to 20th	Night	43% 5% 12% 27% 12% 1	1417
	21st to end	Night	39% 6% 13% 24% 15% 3	1552
June	1st to 10th	Night	32% 7% 15% 27% 16% 3	1440
	11th to 20th	Night	33% 5% 14% 21% 21% 6%	1440
	21st to end	Night	32% 4 15% 29% 16% 4	1440
July	1st to 10th	Night	27% 6% 15% 31% 18% 3	1400
	11th to 20th	Night	23% 5% 16% 35% 17% 4	1392
	21st to end	Night	24% 3 16% 33% 20% 3	1600
August	1st to 10th	Night	28% 6% 15% 34% 15% 3	1601
	11th to 20th	Night	29% 4 13% 34% 17% 2	1600
	21st to end	Night	29% 4 13% 25% 24% 6%	1957
September	1st to 10th	Night	31% 4 9% 27% 26% 3	1800
	11th to 20th	Night	31% 4 10% 32% 22% 1	1800
	21st to end	Night	29% 3 12% 32% 22% 2	2062
October	1st to 10th	Night	34% 4 11% 27% 22% 3	2160
	11th to 20th	Night	40% 4 11% 24% 18% 3	2160
	21st to end	Night	43% 3 12% 23% 17% 2	2376
November	1st to 10th	Night	34% 4 14% 26% 18% 4	2384
	11th to 20th	Night	45% 3 12% 25% 13% 1	2655
	21st to end	Night	44% 3 13% 27% 11% 1	2660
December	1st to 10th	Night	50% 4 13% 23% 9% 1	2660
	11th to 20th	Night	50% 5% 13% 25% 7% 1	3281
	21st to end	Night	67% 4 8% 14% 7%	4884

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	19% 11% 40% 21% 8% 1	270
	11th to 20th	Day	21% 12% 42% 20% 4	270
	21st to end	Day	20% 12% 34% 27% 5% 2	297
February	1st to 10th	Day	16% 19% 39% 21% 4 1	270
	11th to 20th	Day	19% 18% 35% 21% 6% 1	270
	21st to end	Day	24% 13% 37% 24% 1	210
March	1st to 10th	Day	15% 20% 42% 17% 6% 1	240
	11th to 20th	Day	26% 11% 30% 25% 7% 1	240
	21st to end	Day	33% 15% 27% 18% 6% 1	290
April	1st to 10th	Day	28% 12% 36% 17% 7%	420
	11th to 20th	Day	24% 18% 29% 21% 8% 1	412
	21st to end	Day	26% 13% 34% 17% 10% 1	510
May	1st to 10th	Day	24% 12% 30% 23% 9% 2	603
	11th to 20th	Day	25% 9% 29% 24% 11% 3	625
	21st to end	Day	16% 9% 25% 30% 17% 4	679
June	1st to 10th	Day	8% 5% 27% 35% 20% 4	630
	11th to 20th	Day	13% 5% 29% 34% 16% 3	630
	21st to end	Day	11% 7% 32% 31% 14% 5%	630
July	1st to 10th	Day	10% 1 18% 36% 24% 11%	621
	11th to 20th	Day	8% 3 27% 35% 21% 6%	618
	21st to end	Day	7% 2 24% 37% 24% 6%	684
August	1st to 10th	Day	9% 1 23% 34% 26% 8%	626
	11th to 20th	Day	13% 3 20% 31% 25% 8%	615
	21st to end	Day	13% 2 21% 34% 25% 5%	516
September	1st to 10th	Day	15% 6% 22% 28% 21% 7%	450
	11th to 20th	Day	10% 3 26% 32% 23% 5%	449
	21st to end	Day	15% 12% 26% 28% 16% 3	270
October	1st to 10th	Day	17% 6% 31% 25% 17% 3	270
	11th to 20th	Day	22% 21% 33% 16% 9%	270
	21st to end	Day	23% 15% 33% 18% 8% 2	297
November	1st to 10th	Day	15% 11% 43% 21% 8% 2	270
	11th to 20th	Day	18% 19% 34% 21% 7% 1	270
	21st to end	Day	23% 11% 40% 23% 1 1	270
December	1st to 10th	Day	15% 13% 43% 20% 8% 1	270
	11th to 20th	Day	18% 13% 35% 24% 8% 1	270
	21st to end	Day	25% 14% 42% 17% 2	297
January	1st to 10th	Night	15% 12% 39% 27% 7%	950
	11th to 20th	Night	17% 12% 36% 29% 5% 1	950
	21st to end	Night	20% 11% 35% 29% 5%	1045
February	1st to 10th	Night	17% 12% 38% 27% 6% 1	950
	11th to 20th	Night	16% 13% 39% 26% 5% 1	929
	21st to end	Night	19% 13% 37% 24% 5%	649
March	1st to 10th	Night	19% 13% 36% 23% 7% 1	599
	11th to 20th	Night	19% 8% 35% 28% 10%	540
	21st to end	Night	23% 10% 37% 23% 7% 1	594
April	1st to 10th	Night	19% 12% 39% 25% 4	540
	11th to 20th	Night	21% 9% 36% 27% 7%	533
	21st to end	Night	22% 9% 34% 26% 7% 1	540
May	1st to 10th	Night	19% 7% 35% 31% 8% 1	540
	11th to 20th	Night	17% 5% 34% 34% 9%	531
	21st to end	Night	17% 5% 27% 36% 16%	584
June	1st to 10th	Night	9% 3 28% 41% 18% 1	540
	11th to 20th	Night	10% 2 31% 44% 12% 1	540
	21st to end	Night	11% 4 27% 40% 17% 1	540
July	1st to 10th	Night	7% 2 13% 47% 29% 2	525
	11th to 20th	Night	5% 2 24% 41% 25% 3	522
	21st to end	Night	7% 1 19% 47% 24% 2	591
August	1st to 10th	Night	8% 2 16% 45% 28% 2	537
	11th to 20th	Night	10% 2 17% 39% 27% 4	534
	21st to end	Night	11% 2 21% 39% 25% 3	594
September	1st to 10th	Night	11% 3 22% 39% 22% 3	540
	11th to 20th	Night	14% 2 23% 38% 20% 3	540
	21st to end	Night	12% 7% 30% 35% 15% 1	624
October	1st to 10th	Night	15% 5% 32% 33% 14% 1	720
	11th to 20th	Night	19% 10% 34% 28% 8%	720
	21st to end	Night	23% 11% 30% 27% 8% 1	792
November	1st to 10th	Night	15% 10% 39% 28% 8% 1	740
	11th to 20th	Night	17% 13% 42% 22% 7%	945
	21st to end	Night	18% 11% 39% 24% 8%	950
December	1st to 10th	Night	16% 12% 38% 27% 5% 1	950
	11th to 20th	Night	22% 8% 36% 27% 6% 1	950
	21st to end	Night	26% 12% 33% 24% 5%	1045

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