Grundy Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grundy 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	15% 6% 3 5% 5% 3 5% 5% 6% 8% 40%	78834
	Night	16% 6% 2 5% 5% 3 4 5% 5% 7% 42%	81291
Transparency	Day	42% 5% 12% 24% 15% 3	78547
	Night	41% 4 12% 24% 16% 3	81112
Seeing	Day	18% 9% 25% 25% 16% 7%	14076
	Night	18% 7% 27% 33% 13% 1	23596

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	14% 6% 3 5% 6% 3 4 4 5% 6% 44%	2517
	11th to 20th	Day	13% 11% 4 8% 7% 4 4 4 6% 6% 33%	2715
	21st to end	Day	19% 9% 4 7% 7% 3 5% 5% 4 5% 33%	3124
February	1st to 10th	Day	15% 7% 4 5% 5% 3 4 5% 6% 7% 37%	1710
	11th to 20th	Day	16% 7% 2 4 7% 3 4 5% 5% 8% 40%	1743
	21st to end	Day	19% 9% 4 5% 4 3 4 4 4 5% 37%	1570
March	1st to 10th	Day	14% 5% 3 5% 5% 3 5% 4 5% 7% 44%	1973
	11th to 20th	Day	15% 7% 2 5% 5% 3 5% 4 7% 8% 38%	2090
	21st to end	Day	20% 7% 3 5% 5% 3 4 3 5% 7% 39%	2380
April	1st to 10th	Day	16% 6% 3 5% 5% 3 4 5% 5% 7% 40%	2184
	11th to 20th	Day	15% 6% 2 5% 5% 2 4 4 5% 7% 44%	2134
	21st to end	Day	18% 5% 2 4 5% 3 4 5% 7% 7% 40%	2160
May	1st to 10th	Day	17% 7% 3 6% 4 3 4 6% 6% 8% 36%	2312
	11th to 20th	Day	20% 6% 3 5% 5% 3 4 5% 5% 7% 37%	2478
	21st to end	Day	14% 7% 2 4 5% 3 5% 5% 6% 9% 40%	2725
June	1st to 10th	Day	12% 5% 3 5% 4 3 6% 5% 7% 10% 40%	2520
	11th to 20th	Day	13% 5% 3 6% 5% 3 5% 6% 6% 9% 39%	2520
	21st to end	Day	10% 4 1 5% 6% 4 5% 6% 7% 11% 43%	2520
July	1st to 10th	Day	10% 5% 2 5% 6% 3 6% 6% 7% 11% 40%	2520
	11th to 20th	Day	7% 3 2 5% 5% 3 6% 7% 8% 10% 43%	2520
	21st to end	Day	9% 6% 2 6% 6% 4 6% 6% 7% 9% 38%	2613
August	1st to 10th	Day	10% 6% 2 6% 6% 4 6% 6% 6% 10% 39%	2327
	11th to 20th	Day	11% 6% 3 5% 6% 3 4 5% 7% 9% 40%	2188
	21st to end	Day	12% 5% 3 5% 5% 4 5% 5% 6% 8% 43%	2376
September	1st to 10th	Day	15% 7% 2 4 5% 4 5% 4 5% 6% 44%	2160
	11th to 20th	Day	13% 5% 2 4 4 2 3 4 6% 8% 50%	2160
	21st to end	Day	18% 7% 2 5% 4 3 5% 4 5% 7% 40%	1921
October	1st to 10th	Day	18% 6% 2 5% 4 3 3 4 6% 6% 44%	1800
	11th to 20th	Day	18% 5% 2 5% 5% 3 4 4 4 6% 45%	1800
	21st to end	Day	17% 7% 2 4 4 3 4 5% 5% 7% 43%	1980
November	1st to 10th	Day	12% 7% 2 4 4 2 2 3 5% 8% 50%	1671
	11th to 20th	Day	16% 5% 2 4 4 3 4 4 5% 7% 47%	1705
	21st to end	Day	15% 5% 3 4 6% 3 4 5% 5% 6% 44%	1520
December	1st to 10th	Day	18% 8% 4 6% 5% 2 4 5% 5% 7% 36%	1520
	11th to 20th	Day	19% 6% 3 4 5% 3 4 5% 5% 8% 39%	1906
	21st to end	Day	17% 7% 3 7% 7% 4 5% 6% 5% 7% 33%	2772
January	1st to 10th	Night	14% 7% 3 6% 6% 3 5% 5% 6% 7% 40%	4429
	11th to 20th	Night	18% 8% 2 5% 5% 3 3 4 4 6% 41%	4244
	21st to end	Night	22% 9% 3 5% 5% 2 3 3 4 5% 37%	4532
February	1st to 10th	Night	17% 8% 3 5% 5% 3 4 5% 5% 7% 39%	2470
	11th to 20th	Night	17% 7% 3 6% 5% 3 4 4 5% 6% 41%	2321
	21st to end	Night	17% 8% 3 6% 5% 4 4 5% 4 6% 39%	1884
March	1st to 10th	Night	15% 6% 2 4 4 2 4 4 5% 7% 46%	2189
	11th to 20th	Night	16% 6% 2 6% 5% 3 4 4 4 5% 45%	2089
	21st to end	Night	18% 6% 4 5% 4 3 4 3 5% 6% 42%	2217
April	1st to 10th	Night	18% 6% 3 6% 4 4 4 4 5% 8% 38%	1821
	11th to 20th	Night	14% 5% 2 6% 5% 3 4 4 5% 8% 44%	1775
	21st to end	Night	19% 6% 2 4 4 2 3 4 5% 7% 44%	1800
May	1st to 10th	Night	18% 6% 3 5% 5% 3 5% 5% 5% 7% 38%	1662
	11th to 20th	Night	18% 7% 3 5% 5% 3 4 4 5% 7% 37%	1415
	21st to end	Night	13% 6% 2 6% 5% 3 4 4 5% 10% 42%	1550
June	1st to 10th	Night	13% 6% 2 4 4 3 4 5% 6% 8% 44%	1440
	11th to 20th	Night	13% 5% 2 5% 5% 3 5% 5% 7% 11% 40%	1440
	21st to end	Night	10% 4 3 4 4 3 6% 6% 7% 11% 43%	1440
July	1st to 10th	Night	8% 7% 3 7% 7% 4 5% 5% 8% 9% 37%	1440
	11th to 20th	Night	5% 3 2 5% 5% 5% 6% 6% 6% 11% 46%	1440
	21st to end	Night	10% 4 2 4 6% 4 6% 6% 7% 11% 42%	1702
August	1st to 10th	Night	9% 5% 2 4 6% 4 6% 6% 7% 9% 42%	1611
	11th to 20th	Night	11% 4 3 6% 7% 4 5% 5% 7% 8% 40%	1695
	21st to end	Night	12% 5% 2 5% 5% 3 5% 5% 7% 8% 44%	1980
September	1st to 10th	Night	14% 6% 2 6% 5% 3 4 5% 5% 6% 46%	1800
	11th to 20th	Night	12% 6% 2 3 4 2 4 3 6% 7% 52%	1800
	21st to end	Night	17% 6% 3 4 5% 3 5% 4 4 6% 42%	2037
October	1st to 10th	Night	20% 6% 3 6% 3 2 3 3 4 5% 43%	2160
	11th to 20th	Night	15% 5% 2 5% 4 3 3 3 4 7% 48%	2160
	21st to end	Night	19% 5% 2 3 4 2 3 4 4 7% 47%	2376
November	1st to 10th	Night	11% 4 2 4 4 3 3 5% 5% 6% 53%	2406
	11th to 20th	Night	14% 7% 2 5% 4 2 4 4 4 7% 46%	2474
	21st to end	Night	14% 4 2 5% 5% 3 5% 5% 5% 7% 45%	2660
December	1st to 10th	Night	19% 7% 2 5% 5% 3 4 5% 5% 7% 37%	2660
	11th to 20th	Night	18% 5% 2 5% 5% 3 5% 5% 5% 7% 40%	3288
	21st to end	Night	17% 6% 3 6% 5% 4 5% 6% 6% 8% 34%	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	42% 3 13% 27% 11% 3	2518
	11th to 20th	Day	50% 3 11% 19% 14% 3	2715
	21st to end	Day	52% 4 11% 22% 9% 1	3124
February	1st to 10th	Day	45% 4 13% 25% 11% 2	1710
	11th to 20th	Day	44% 3 10% 26% 14% 3	1752
	21st to end	Day	49% 3 9% 22% 13% 4	1570
March	1st to 10th	Day	41% 3 11% 28% 14% 2	1976
	11th to 20th	Day	43% 4 11% 20% 17% 5%	2090
	21st to end	Day	47% 4 9% 20% 18% 3	2380
April	1st to 10th	Day	43% 4 11% 20% 17% 5%	2184
	11th to 20th	Day	39% 4 11% 24% 17% 5%	2134
	21st to end	Day	42% 5% 10% 21% 17% 4	2160
May	1st to 10th	Day	45% 5% 10% 22% 15% 3	2312
	11th to 20th	Day	47% 5% 12% 21% 13% 2	2482
	21st to end	Day	39% 7% 16% 23% 13% 2	2725
June	1st to 10th	Day	38% 7% 14% 25% 15% 2	2520
	11th to 20th	Day	41% 6% 13% 24% 13% 3	2520
	21st to end	Day	34% 6% 16% 26% 16% 1	2520
July	1st to 10th	Day	38% 7% 13% 24% 15% 3	2450
	11th to 20th	Day	31% 7% 15% 28% 16% 3	2445
	21st to end	Day	40% 6% 12% 22% 16% 3	2467
August	1st to 10th	Day	41% 6% 12% 25% 15% 1	2314
	11th to 20th	Day	39% 6% 14% 23% 17% 2	2188
	21st to end	Day	38% 6% 13% 25% 16% 4	2376
September	1st to 10th	Day	42% 3 11% 21% 18% 5%	2160
	11th to 20th	Day	33% 4 10% 27% 22% 4	2160
	21st to end	Day	44% 4 9% 20% 20% 3	1921
October	1st to 10th	Day	40% 4 13% 24% 17% 2	1800
	11th to 20th	Day	41% 4 10% 24% 19% 2	1800
	21st to end	Day	41% 3 10% 27% 18% 2	1980
November	1st to 10th	Day	34% 3 13% 26% 20% 4	1671
	11th to 20th	Day	38% 4 11% 23% 21% 2	1705
	21st to end	Day	41% 4 11% 31% 11% 2	1520
December	1st to 10th	Day	47% 4 16% 20% 12% 1	1520
	11th to 20th	Day	44% 3 12% 30% 10% 1	1906
	21st to end	Day	49% 5% 15% 20% 10% 1	2772
January	1st to 10th	Night	43% 5% 11% 27% 11% 3	4429
	11th to 20th	Night	44% 4 12% 25% 13% 2	4244
	21st to end	Night	49% 3 8% 28% 10% 2	4532
February	1st to 10th	Night	45% 3 13% 25% 12% 2	2470
	11th to 20th	Night	44% 4 9% 22% 18% 3	2323
	21st to end	Night	47% 3 10% 21% 15% 5%	1884
March	1st to 10th	Night	38% 4 12% 26% 18% 2	2199
	11th to 20th	Night	42% 4 10% 24% 17% 4	2090
	21st to end	Night	44% 3 9% 21% 19% 5%	2217
April	1st to 10th	Night	44% 4 10% 20% 17% 4	1821
	11th to 20th	Night	39% 4 10% 23% 20% 4	1775
	21st to end	Night	41% 5% 9% 22% 18% 5%	1800
May	1st to 10th	Night	44% 6% 10% 20% 16% 4	1662
	11th to 20th	Night	46% 6% 12% 15% 18% 3	1416
	21st to end	Night	39% 7% 17% 20% 15% 3	1551
June	1st to 10th	Night	38% 7% 11% 23% 19% 2	1440
	11th to 20th	Night	38% 6% 16% 21% 17% 2	1440
	21st to end	Night	34% 6% 14% 26% 17% 2	1440
July	1st to 10th	Night	43% 7% 14% 19% 14% 3	1400
	11th to 20th	Night	33% 7% 16% 26% 16% 2	1392
	21st to end	Night	33% 8% 18% 21% 18% 2	1607
August	1st to 10th	Night	37% 6% 16% 26% 15% 1	1600
	11th to 20th	Night	40% 6% 15% 20% 17% 2	1695
	21st to end	Night	37% 5% 15% 22% 18% 3	1980
September	1st to 10th	Night	39% 5% 11% 22% 20% 4	1800
	11th to 20th	Night	32% 4 9% 26% 25% 4	1800
	21st to end	Night	42% 5% 8% 20% 21% 3	2037
October	1st to 10th	Night	44% 3 10% 24% 17% 2	2160
	11th to 20th	Night	38% 3 11% 28% 18% 2	2160
	21st to end	Night	38% 4 10% 29% 18% 2	2376
November	1st to 10th	Night	32% 4 11% 27% 21% 5%	2406
	11th to 20th	Night	39% 4 9% 25% 18% 5%	2474
	21st to end	Night	38% 4 13% 28% 16% 2	2660
December	1st to 10th	Night	47% 4 12% 24% 13% 1	2660
	11th to 20th	Night	43% 4 13% 23% 15% 3	3288
	21st to end	Night	47% 5% 15% 23% 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	26% 16% 33% 21% 3 2	180
	11th to 20th	Day	27% 22% 32% 16% 3	236
	21st to end	Day	22% 16% 36% 18% 6% 1	297
February	1st to 10th	Day	23% 16% 35% 20% 5%	270
	11th to 20th	Day	23% 20% 41% 14% 2	350
	21st to end	Day	28% 14% 32% 21% 4	382
March	1st to 10th	Day	19% 18% 30% 26% 6%	460
	11th to 20th	Day	20% 18% 35% 19% 5% 2	460
	21st to end	Day	23% 13% 33% 22% 6% 2	506
April	1st to 10th	Day	22% 12% 34% 21% 8% 2	428
	11th to 20th	Day	16% 15% 29% 22% 13% 6%	412
	21st to end	Day	25% 10% 30% 21% 11% 2	420
May	1st to 10th	Day	24% 12% 26% 21% 13% 4	423
	11th to 20th	Day	21% 9% 26% 24% 14% 7%	447
	21st to end	Day	20% 5% 17% 24% 22% 12%	485
June	1st to 10th	Day	15% 4 25% 27% 19% 10%	450
	11th to 20th	Day	16% 7% 22% 27% 20% 8%	450
	21st to end	Day	13% 3 23% 31% 20% 10%	450
July	1st to 10th	Day	13% 2 12% 26% 29% 18%	445
	11th to 20th	Day	5% 2 12% 32% 33% 17%	444
	21st to end	Day	15% 2 16% 31% 27% 8%	488
August	1st to 10th	Day	11% 3 14% 28% 29% 14%	446
	11th to 20th	Day	12% 4 12% 28% 27% 17%	439
	21st to end	Day	13% 3 13% 30% 25% 16%	495
September	1st to 10th	Day	19% 4 16% 20% 24% 16%	450
	11th to 20th	Day	14% 4 16% 26% 25% 16%	450
	21st to end	Day	22% 8% 19% 26% 17% 8%	450
October	1st to 10th	Day	23% 4 19% 25% 21% 8%	450
	11th to 20th	Day	18% 9% 25% 28% 18% 3	450
	21st to end	Day	18% 7% 33% 30% 9% 3	495
November	1st to 10th	Day	15% 8% 28% 27% 18% 4	365
	11th to 20th	Day	15% 15% 30% 28% 11% 1	365
	21st to end	Day	19% 17% 32% 19% 11% 2	180
December	1st to 10th	Day	24% 15% 33% 19% 6% 2	180
	11th to 20th	Day	24% 14% 37% 16% 8% 1	180
	21st to end	Day	21% 12% 35% 22% 10% 1	198
January	1st to 10th	Night	17% 11% 41% 27% 4	760
	11th to 20th	Night	24% 12% 31% 29% 4	760
	21st to end	Night	19% 10% 40% 29% 2	836
February	1st to 10th	Night	21% 11% 36% 30% 2	760
	11th to 20th	Night	21% 15% 40% 23% 2	743
	21st to end	Night	22% 14% 38% 23% 3	628
March	1st to 10th	Night	19% 11% 39% 27% 5%	760
	11th to 20th	Night	20% 13% 35% 25% 6% 1	760
	21st to end	Night	21% 8% 36% 29% 6%	836
April	1st to 10th	Night	21% 12% 37% 26% 5%	729
	11th to 20th	Night	18% 8% 32% 33% 10%	711
	21st to end	Night	21% 7% 30% 33% 8% 1	720
May	1st to 10th	Night	21% 6% 25% 37% 10% 1	582
	11th to 20th	Night	20% 5% 28% 35% 12%	354
	21st to end	Night	14% 2 17% 42% 23% 2	389
June	1st to 10th	Night	15% 3 19% 40% 22% 1	360
	11th to 20th	Night	14% 1 18% 46% 21%	360
	21st to end	Night	11% 3 14% 53% 18% 1	360
July	1st to 10th	Night	11% 3 14% 41% 28% 3	350
	11th to 20th	Night	7% 1 9% 47% 33% 3	348
	21st to end	Night	11% 2 11% 46% 27% 2	522
August	1st to 10th	Night	11% 1 5% 48% 31% 3	537
	11th to 20th	Night	13% 2 10% 40% 32% 3	631
	21st to end	Night	14% 2 11% 40% 30% 4	792
September	1st to 10th	Night	16% 3 16% 35% 25% 4	720
	11th to 20th	Night	14% 4 10% 38% 30% 4	720
	21st to end	Night	19% 3 17% 37% 22% 2	720
October	1st to 10th	Night	23% 3 18% 33% 19% 4	720
	11th to 20th	Night	18% 6% 25% 36% 15% 1	720
	21st to end	Night	21% 5% 32% 30% 12% 1	792
November	1st to 10th	Night	13% 4 31% 37% 15% 1	740
	11th to 20th	Night	18% 11% 28% 34% 9% 1	760
	21st to end	Night	16% 10% 33% 32% 10%	760
December	1st to 10th	Night	23% 10% 33% 27% 6% 1	760
	11th to 20th	Night	18% 9% 38% 29% 6%	760
	21st to end	Night	20% 8% 36% 33% 3	836

Page generated at 2024-03-31 06:08:50. Version ac07

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