The Grainger Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Grainger Observatory clear sky chart from 2005-11-06 to 2024-04-03.

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	17% 6% 3 5% 5% 3 4 4 5% 7% 40%	78698
	Night	18% 7% 3 5% 5% 3 4 4 5% 6% 40%	81419
Transparency	Day	42% 7% 12% 21% 14% 2	78403
	Night	44% 6% 12% 22% 14% 2	81248
Seeing	Day	19% 12% 31% 26% 9% 2	14174
	Night	21% 11% 32% 27% 9% 1	23200

By Month

Percentage Cloud Cover NE (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	16% 7% 2 5% 4 2 4 4 5% 6% 44%	2517
	11th to 20th	Day	17% 6% 3 6% 5% 2 4 5% 4 6% 41%	2520
	21st to end	Day	19% 10% 3 5% 5% 4 4 4 4 5% 36%	2772
February	1st to 10th	Day	17% 7% 4 5% 5% 2 5% 5% 7% 7% 36%	1735
	11th to 20th	Day	14% 7% 3 6% 5% 3 4 5% 6% 7% 41%	1848
	21st to end	Day	17% 10% 3 6% 5% 3 4 4 5% 5% 37%	1570
March	1st to 10th	Day	18% 9% 2 5% 4 3 4 5% 5% 7% 36%	1897
	11th to 20th	Day	16% 7% 3 5% 5% 3 4 4 6% 7% 39%	2016
	21st to end	Day	19% 8% 3 5% 5% 2 4 3 4 5% 40%	2399
April	1st to 10th	Day	19% 9% 2 6% 5% 3 4 3 4 6% 38%	2184
	11th to 20th	Day	17% 6% 3 4 5% 3 5% 5% 6% 7% 38%	2173
	21st to end	Day	21% 6% 4 6% 6% 3 5% 4 4 6% 36%	2340
May	1st to 10th	Day	24% 7% 3 5% 5% 2 5% 4 4 7% 34%	2340
	11th to 20th	Day	18% 6% 3 5% 5% 3 4 4 5% 6% 40%	2338
	21st to end	Day	16% 5% 3 5% 5% 2 4 4 5% 9% 42%	2725
June	1st to 10th	Day	20% 7% 2 5% 5% 3 5% 5% 5% 8% 33%	2520
	11th to 20th	Day	17% 6% 3 5% 4 3 5% 5% 5% 9% 39%	2520
	21st to end	Day	19% 6% 2 6% 6% 4 6% 5% 6% 8% 33%	2520
July	1st to 10th	Day	14% 5% 3 5% 5% 3 5% 5% 7% 8% 40%	2520
	11th to 20th	Day	13% 5% 2 5% 5% 3 5% 6% 7% 10% 38%	2520
	21st to end	Day	12% 5% 3 5% 6% 3 5% 6% 7% 9% 39%	2744
August	1st to 10th	Day	10% 7% 2 5% 6% 4 5% 5% 8% 9% 39%	2355
	11th to 20th	Day	12% 5% 3 5% 5% 4 5% 6% 7% 9% 38%	2304
	21st to end	Day	13% 4 2 3 4 3 4 5% 7% 8% 47%	2391
September	1st to 10th	Day	13% 5% 1 5% 5% 3 4 5% 5% 7% 46%	2143
	11th to 20th	Day	15% 4 2 4 4 3 3 3 4 5% 52%	1980
	21st to end	Day	16% 4 2 5% 4 2 4 4 4 6% 47%	1980
October	1st to 10th	Day	22% 6% 2 5% 4 3 3 4 5% 6% 40%	1980
	11th to 20th	Day	14% 5% 2 5% 4 4 4 4 5% 8% 45%	1747
	21st to end	Day	23% 6% 2 5% 4 3 4 5% 4 6% 39%	1782
November	1st to 10th	Day	16% 5% 3 4 5% 2 4 3 4 5% 49%	1671
	11th to 20th	Day	20% 6% 2 3 4 2 3 4 4 6% 46%	1710
	21st to end	Day	20% 6% 3 4 3 3 3 4 4 5% 44%	1710
December	1st to 10th	Day	19% 7% 3 7% 5% 2 4 4 5% 6% 39%	1549
	11th to 20th	Day	22% 7% 2 5% 4 2 3 4 4 5% 43%	1906
	21st to end	Day	24% 10% 3 5% 5% 3 4 3 5% 5% 33%	2772
January	1st to 10th	Night	18% 7% 2 4 3 2 3 4 5% 6% 45%	4429
	11th to 20th	Night	14% 8% 3 6% 6% 3 5% 4 5% 6% 39%	4440
	21st to end	Night	22% 11% 3 5% 6% 3 4 5% 4 5% 32%	4884
February	1st to 10th	Night	15% 10% 2 6% 5% 3 5% 5% 6% 6% 37%	2438
	11th to 20th	Night	14% 8% 4 6% 5% 3 4 4 5% 5% 41%	2218
	21st to end	Night	17% 8% 3 4 4 2 4 5% 4 6% 42%	1884
March	1st to 10th	Night	17% 9% 3 6% 4 3 4 4 4 6% 40%	2270
	11th to 20th	Night	17% 7% 3 6% 5% 3 5% 5% 5% 6% 38%	2161
	21st to end	Night	19% 8% 4 5% 4 2 4 4 4 5% 41%	2195
April	1st to 10th	Night	18% 8% 3 5% 5% 3 5% 4 5% 6% 38%	1821
	11th to 20th	Night	20% 6% 3 4 5% 3 4 4 4 6% 41%	1736
	21st to end	Night	21% 7% 3 6% 5% 3 4 4 5% 6% 36%	1620
May	1st to 10th	Night	22% 8% 3 6% 4 3 4 5% 5% 7% 33%	1620
	11th to 20th	Night	18% 6% 3 7% 6% 4 4 5% 5% 7% 35%	1570
	21st to end	Night	18% 8% 3 6% 5% 2 4 5% 5% 7% 38%	1550
June	1st to 10th	Night	20% 7% 4 6% 6% 3 5% 4 5% 5% 36%	1440
	11th to 20th	Night	20% 5% 2 5% 4 3 4 5% 6% 8% 38%	1440
	21st to end	Night	18% 6% 3 5% 5% 3 4 5% 6% 8% 38%	1440
July	1st to 10th	Night	16% 6% 2 5% 5% 3 6% 5% 6% 8% 39%	1440
	11th to 20th	Night	13% 5% 2 5% 5% 4 5% 6% 8% 10% 38%	1440
	21st to end	Night	14% 5% 2 4 4 3 6% 6% 5% 9% 44%	1574
August	1st to 10th	Night	10% 5% 2 4 5% 3 5% 6% 6% 8% 46%	1566
	11th to 20th	Night	14% 6% 3 5% 6% 4 6% 5% 6% 8% 37%	1596
	21st to end	Night	14% 5% 2 4 4 2 4 5% 6% 8% 46%	1962
September	1st to 10th	Night	13% 5% 2 5% 4 2 4 4 5% 7% 48%	1816
	11th to 20th	Night	14% 6% 2 3 4 2 3 3 4 7% 52%	1980
	21st to end	Night	15% 5% 2 4 5% 3 3 4 5% 8% 47%	1980
October	1st to 10th	Night	22% 5% 3 5% 5% 2 4 4 5% 6% 39%	1980
	11th to 20th	Night	16% 5% 2 4 5% 4 4 5% 5% 6% 44%	2207
	21st to end	Night	23% 7% 2 5% 4 3 4 4 4 6% 38%	2574
November	1st to 10th	Night	15% 5% 3 5% 4 3 5% 4 4 6% 47%	2407
	11th to 20th	Night	17% 7% 2 5% 4 2 4 3 3 6% 46%	2470
	21st to end	Night	17% 7% 3 4 4 2 5% 4 4 7% 45%	2470
December	1st to 10th	Night	20% 8% 3 6% 5% 3 5% 4 4 6% 38%	2629
	11th to 20th	Night	20% 6% 2 5% 5% 3 3 5% 5% 6% 41%	3288
	21st to end	Night	22% 9% 3 5% 6% 3 4 3 4 6% 36%	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	39% 7% 11% 31% 12% 1	2518
	11th to 20th	Day	43% 7% 12% 24% 12% 2	2520
	21st to end	Day	49% 7% 10% 23% 9% 1	2772
February	1st to 10th	Day	46% 6% 14% 20% 11% 3	1735
	11th to 20th	Day	42% 6% 13% 21% 15% 3	1857
	21st to end	Day	47% 6% 11% 19% 13% 3	1570
March	1st to 10th	Day	45% 7% 13% 18% 14% 4	1900
	11th to 20th	Day	42% 6% 12% 20% 15% 5%	2016
	21st to end	Day	46% 5% 8% 18% 17% 7%	2399
April	1st to 10th	Day	47% 6% 9% 18% 17% 4	2184
	11th to 20th	Day	42% 8% 12% 20% 15% 3	2173
	21st to end	Day	47% 8% 8% 16% 17% 3	2340
May	1st to 10th	Day	50% 6% 10% 19% 13% 2	2340
	11th to 20th	Day	43% 7% 11% 19% 17% 3	2342
	21st to end	Day	40% 9% 15% 21% 13% 2	2725
June	1st to 10th	Day	48% 8% 12% 20% 11% 1	2520
	11th to 20th	Day	40% 9% 13% 19% 16% 3	2520
	21st to end	Day	45% 10% 13% 19% 11% 1	2520
July	1st to 10th	Day	38% 9% 14% 22% 16% 2	2450
	11th to 20th	Day	38% 9% 15% 25% 12% 1	2445
	21st to end	Day	36% 10% 15% 23% 13% 3	2591
August	1st to 10th	Day	38% 10% 16% 23% 12% 1	2341
	11th to 20th	Day	39% 9% 15% 24% 12% 1	2304
	21st to end	Day	32% 9% 14% 24% 19% 3	2391
September	1st to 10th	Day	35% 8% 14% 24% 19% 1	2143
	11th to 20th	Day	34% 6% 9% 26% 23% 2	1980
	21st to end	Day	37% 6% 11% 23% 20% 2	1980
October	1st to 10th	Day	45% 6% 10% 21% 15% 4	1980
	11th to 20th	Day	37% 7% 12% 30% 13% 2	1747
	21st to end	Day	45% 6% 11% 21% 14% 3	1782
November	1st to 10th	Day	38% 5% 14% 25% 17% 2	1671
	11th to 20th	Day	39% 5% 11% 22% 21% 3	1710
	21st to end	Day	42% 6% 16% 21% 14% 1	1710
December	1st to 10th	Day	46% 6% 15% 17% 15% 2	1549
	11th to 20th	Day	43% 5% 13% 19% 15% 5%	1906
	21st to end	Day	52% 7% 11% 19% 10% 2	2772
January	1st to 10th	Night	39% 6% 13% 28% 11% 3	4429
	11th to 20th	Night	45% 7% 12% 22% 11% 3	4440
	21st to end	Night	54% 6% 11% 18% 10% 1	4884
February	1st to 10th	Night	46% 7% 13% 18% 13% 3	2438
	11th to 20th	Night	44% 5% 12% 20% 15% 3	2220
	21st to end	Night	42% 6% 10% 24% 13% 5%	1884
March	1st to 10th	Night	45% 5% 10% 21% 15% 4	2280
	11th to 20th	Night	46% 6% 11% 16% 17% 4	2162
	21st to end	Night	45% 4 9% 15% 21% 5%	2195
April	1st to 10th	Night	46% 6% 11% 18% 16% 4	1821
	11th to 20th	Night	44% 4 10% 21% 18% 2	1736
	21st to end	Night	48% 7% 10% 17% 15% 3	1620
May	1st to 10th	Night	51% 6% 8% 18% 13% 3	1620
	11th to 20th	Night	48% 6% 13% 16% 14% 4	1571
	21st to end	Night	45% 8% 14% 19% 12% 2	1551
June	1st to 10th	Night	50% 6% 11% 21% 12% 1	1440
	11th to 20th	Night	43% 9% 13% 18% 14% 3	1440
	21st to end	Night	43% 9% 14% 22% 12% 1	1440
July	1st to 10th	Night	42% 9% 14% 21% 14% 1	1400
	11th to 20th	Night	38% 9% 17% 26% 10%	1392
	21st to end	Night	35% 8% 15% 29% 11% 2	1486
August	1st to 10th	Night	34% 10% 15% 25% 14% 2	1556
	11th to 20th	Night	41% 9% 16% 19% 13% 1	1596
	21st to end	Night	35% 8% 11% 25% 20% 1	1962
September	1st to 10th	Night	35% 6% 14% 25% 19% 1	1816
	11th to 20th	Night	34% 5% 11% 26% 23% 2	1980
	21st to end	Night	37% 6% 10% 26% 19% 2	1980
October	1st to 10th	Night	45% 7% 12% 20% 13% 3	1980
	11th to 20th	Night	38% 8% 13% 27% 12% 1	2207
	21st to end	Night	47% 5% 12% 21% 13% 1	2574
November	1st to 10th	Night	38% 7% 11% 24% 17% 3	2407
	11th to 20th	Night	42% 4 11% 22% 18% 3	2470
	21st to end	Night	41% 5% 15% 26% 12% 1	2470
December	1st to 10th	Night	48% 6% 13% 21% 11% 1	2629
	11th to 20th	Night	43% 6% 15% 18% 18% 1	3288
	21st to end	Night	50% 6% 9% 24% 10% 1	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	25% 22% 28% 22% 4	180
	11th to 20th	Day	22% 27% 29% 18% 3 1	180
	21st to end	Day	17% 21% 38% 19% 5%	198
February	1st to 10th	Day	25% 20% 37% 17% 1	355
	11th to 20th	Day	22% 25% 36% 15% 2	456
	21st to end	Day	22% 19% 36% 19% 4	382
March	1st to 10th	Day	24% 17% 36% 19% 3	460
	11th to 20th	Day	20% 23% 38% 16% 3 1	460
	21st to end	Day	23% 19% 36% 15% 5% 2	506
April	1st to 10th	Day	22% 20% 32% 19% 6% 1	428
	11th to 20th	Day	19% 18% 36% 19% 8%	412
	21st to end	Day	26% 9% 36% 20% 8% 1	420
May	1st to 10th	Day	25% 13% 36% 18% 7% 1	423
	11th to 20th	Day	16% 11% 36% 29% 6% 2	447
	21st to end	Day	19% 6% 32% 31% 11% 1	485
June	1st to 10th	Day	21% 6% 31% 30% 11% 1	450
	11th to 20th	Day	14% 8% 36% 33% 8% 2	450
	21st to end	Day	18% 4 35% 30% 11% 2	450
July	1st to 10th	Day	16% 5% 20% 38% 18% 4	445
	11th to 20th	Day	13% 7% 21% 41% 16% 2	444
	21st to end	Day	10% 3 27% 43% 16% 2	488
August	1st to 10th	Day	11% 4 24% 39% 19% 3	446
	11th to 20th	Day	14% 4 24% 35% 18% 5%	439
	21st to end	Day	15% 3 22% 39% 16% 4	495
September	1st to 10th	Day	16% 5% 24% 32% 18% 6%	450
	11th to 20th	Day	17% 8% 28% 27% 15% 6%	450
	21st to end	Day	15% 9% 30% 30% 11% 5%	450
October	1st to 10th	Day	25% 10% 24% 25% 12% 3	450
	11th to 20th	Day	15% 13% 29% 28% 12% 2	387
	21st to end	Day	27% 10% 33% 21% 9%	396
November	1st to 10th	Day	15% 15% 31% 26% 10% 2	365
	11th to 20th	Day	25% 21% 27% 22% 4 1	370
	21st to end	Day	24% 19% 29% 22% 6% 1	370
December	1st to 10th	Day	20% 20% 34% 21% 6%	209
	11th to 20th	Day	23% 19% 39% 17% 1	180
	21st to end	Day	24% 22% 30% 18% 5% 1	198
January	1st to 10th	Night	21% 18% 40% 18% 3	760
	11th to 20th	Night	25% 17% 38% 17% 3	760
	21st to end	Night	19% 20% 38% 18% 4	836
February	1st to 10th	Night	22% 16% 38% 22% 1	760
	11th to 20th	Night	21% 20% 41% 16% 3	743
	21st to end	Night	22% 15% 36% 23% 3	628
March	1st to 10th	Night	25% 17% 37% 19% 3	760
	11th to 20th	Night	21% 16% 38% 19% 6%	760
	21st to end	Night	23% 15% 35% 21% 6%	836
April	1st to 10th	Night	23% 14% 34% 23% 5%	729
	11th to 20th	Night	21% 11% 34% 26% 7%	672
	21st to end	Night	24% 10% 32% 29% 4 1	540
May	1st to 10th	Night	26% 7% 33% 25% 8% 1	540
	11th to 20th	Night	21% 7% 34% 29% 7% 1	509
	21st to end	Night	21% 6% 25% 36% 12%	389
June	1st to 10th	Night	23% 5% 25% 34% 11% 2	360
	11th to 20th	Night	21% 5% 24% 38% 12%	360
	21st to end	Night	20% 3 23% 42% 13%	360
July	1st to 10th	Night	17% 2 18% 45% 16% 1	350
	11th to 20th	Night	13% 2 16% 47% 20% 1	348
	21st to end	Night	13% 3 15% 49% 18% 2	394
August	1st to 10th	Night	14% 3 17% 42% 24% 1	492
	11th to 20th	Night	14% 5% 19% 42% 18% 2	532
	21st to end	Night	15% 4 21% 34% 22% 3	774
September	1st to 10th	Night	14% 3 26% 34% 22% 2	720
	11th to 20th	Night	16% 5% 27% 29% 19% 4	720
	21st to end	Night	17% 4 28% 31% 18% 3	720
October	1st to 10th	Night	23% 7% 26% 28% 15% 2	720
	11th to 20th	Night	18% 10% 30% 30% 12% 1	720
	21st to end	Night	26% 10% 31% 25% 7%	792
November	1st to 10th	Night	16% 10% 35% 29% 8% 1	740
	11th to 20th	Night	21% 16% 34% 22% 7% 1	760
	21st to end	Night	21% 18% 34% 23% 4 1	760
December	1st to 10th	Night	24% 14% 39% 18% 4	760
	11th to 20th	Night	21% 19% 34% 22% 3	760
	21st to end	Night	26% 16% 37% 20% 1	836

Page generated at 2024-03-31 06:08:54. Version ac07

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