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 2024-11-04.

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% 2 6% 4 4 4 4 6% 8% 40%	101339
	Night	16% 7% 2 6% 4 4 4 4 5% 7% 41%	97685
Transparency	Day	41% 5% 12% 24% 16% 3	104941
	Night	41% 4 12% 24% 16% 3	102171
Seeing	Day	18% 9% 25% 25% 16% 7%	14933
	Night	18% 7% 27% 34% 13% 1	24279

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 6% 5% 4 4 3 5% 6% 43%	2517
	11th to 20th	Day	13% 11% 4 9% 5% 4 4 3 6% 6% 33%	2715
	21st to end	Day	19% 9% 4 8% 6% 4 4 3 5% 5% 33%	3124
February	1st to 10th	Day	15% 8% 3 6% 5% 4 4 4 7% 7% 37%	1710
	11th to 20th	Day	16% 8% 2 6% 6% 4 4 4 5% 7% 40%	1743
	21st to end	Day	19% 10% 4 5% 4 4 4 4 5% 5% 37%	1570
March	1st to 10th	Day	24% 6% 2 5% 4 4 4 3 5% 7% 36%	3244
	11th to 20th	Day	10% 6% 2 6% 5% 5% 5% 4 7% 8% 44%	3480
	21st to end	Day	21% 8% 2 5% 4 3 3 2 4 5% 43%	3960
April	1st to 10th	Day	15% 6% 3 6% 5% 4 6% 4 5% 7% 39%	2280
	11th to 20th	Day	14% 7% 2 5% 5% 4 4 4 5% 7% 43%	2254
	21st to end	Day	17% 6% 2 5% 4 4 4 4 6% 7% 40%	2280
May	1st to 10th	Day	27% 7% 3 6% 3 4 4 4 5% 7% 30%	4080
	11th to 20th	Day	24% 8% 3 8% 5% 4 4 4 5% 7% 29%	4398
	21st to end	Day	11% 5% 2 5% 4 4 5% 4 7% 9% 46%	4837
June	1st to 10th	Day	12% 6% 2 5% 4 5% 5% 5% 7% 9% 40%	2660
	11th to 20th	Day	12% 5% 2 6% 5% 5% 5% 5% 7% 9% 39%	2660
	21st to end	Day	10% 4 1 6% 5% 5% 5% 5% 7% 11% 43%	2660
July	1st to 10th	Day	6% 4 2 6% 5% 6% 6% 6% 8% 11% 39%	4440
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 5% 8% 10% 40%	4440
	21st to end	Day	11% 7% 3 8% 6% 6% 6% 5% 6% 8% 36%	4574
August	1st to 10th	Day	18% 6% 2 7% 4 5% 5% 4 6% 9% 34%	4104
	11th to 20th	Day	13% 5% 2 6% 4 4 5% 4 7% 9% 41%	3831
	21st to end	Day	11% 5% 2 5% 4 4 4 4 5% 8% 48%	4180
September	1st to 10th	Day	15% 7% 2 5% 4 5% 4 3 5% 5% 46%	2280
	11th to 20th	Day	12% 5% 2 4 3 3 4 3 6% 8% 50%	2280
	21st to end	Day	21% 7% 2 6% 3 4 4 3 5% 7% 37%	2027
October	1st to 10th	Day	18% 6% 2 6% 3 3 3 3 6% 6% 45%	1900
	11th to 20th	Day	17% 5% 2 5% 4 3 4 3 4 6% 47%	1900
	21st to end	Day	16% 6% 2 4 3 3 3 4 5% 7% 45%	2090
November	1st to 10th	Day	12% 7% 2 5% 3 3 2 2 5% 8% 50%	1698
	11th to 20th	Day	15% 6% 2 4 4 4 4 4 4 7% 46%	1705
	21st to end	Day	15% 6% 2 5% 5% 3 4 4 5% 6% 43%	1520
December	1st to 10th	Day	18% 8% 3 7% 4 4 4 4 6% 7% 36%	1520
	11th to 20th	Day	19% 6% 2 4 4 4 4 4 6% 8% 39%	1906
	21st to end	Day	16% 7% 2 9% 5% 6% 5% 4 5% 7% 33%	2772
January	1st to 10th	Night	14% 7% 2 7% 5% 4 5% 4 6% 6% 39%	4429
	11th to 20th	Night	18% 9% 2 5% 4 4 3 3 5% 6% 41%	4244
	21st to end	Night	22% 10% 2 6% 4 3 3 3 5% 5% 37%	4532
February	1st to 10th	Night	17% 8% 3 6% 5% 4 4 3 5% 7% 39%	2470
	11th to 20th	Night	17% 7% 3 6% 4 4 4 3 5% 5% 41%	2321
	21st to end	Night	17% 8% 2 6% 4 5% 3 4 5% 6% 39%	1884
March	1st to 10th	Night	21% 10% 2 6% 4 4 3 3 4 5% 38%	3578
	11th to 20th	Night	11% 5% 2 6% 4 4 4 3 5% 7% 50%	3479
	21st to end	Night	15% 8% 3 7% 3 3 3 2 4 5% 46%	3671
April	1st to 10th	Night	17% 7% 2 6% 4 5% 4 4 5% 9% 38%	1900
	11th to 20th	Night	15% 6% 2 6% 4 4 4 3 5% 8% 43%	1875
	21st to end	Night	18% 6% 2 5% 3 3 4 3 5% 6% 44%	1900
May	1st to 10th	Night	24% 7% 2 7% 4 4 5% 3 5% 7% 31%	2889
	11th to 20th	Night	24% 9% 4 7% 4 4 3 3 6% 7% 28%	2495
	21st to end	Night	12% 6% 2 6% 4 4 4 3 6% 11% 42%	2738
June	1st to 10th	Night	12% 6% 2 5% 3 4 5% 3 7% 8% 44%	1520
	11th to 20th	Night	12% 5% 2 6% 4 4 5% 4 7% 11% 40%	1520
	21st to end	Night	10% 5% 2 6% 3 4 6% 5% 7% 10% 42%	1520
July	1st to 10th	Night	6% 6% 2 8% 6% 5% 5% 5% 8% 10% 40%	2520
	11th to 20th	Night	8% 4 2 7% 5% 6% 6% 4 8% 10% 37%	2520
	21st to end	Night	14% 7% 2 7% 5% 4 5% 4 6% 9% 38%	2976
August	1st to 10th	Night	20% 5% 2 5% 5% 5% 5% 4 7% 8% 35%	2825
	11th to 20th	Night	13% 6% 2 7% 5% 4 4 4 6% 7% 42%	2966
	21st to end	Night	12% 5% 1 5% 4 4 4 4 6% 7% 46%	3476
September	1st to 10th	Night	13% 6% 2 6% 4 4 4 3 5% 5% 47%	1900
	11th to 20th	Night	12% 6% 2 4 3 3 3 3 5% 7% 51%	1900
	21st to end	Night	20% 6% 2 5% 4 5% 4 4 4 6% 40%	2151
October	1st to 10th	Night	20% 7% 3 7% 3 3 3 3 4 5% 44%	2280
	11th to 20th	Night	14% 5% 2 5% 4 4 3 3 4 7% 49%	2280
	21st to end	Night	18% 5% 2 4 3 3 3 3 5% 7% 48%	2508
November	1st to 10th	Night	11% 5% 2 5% 3 4 3 4 5% 6% 53%	2452
	11th to 20th	Night	14% 8% 2 6% 4 3 4 3 4 7% 46%	2474
	21st to end	Night	14% 5% 2 5% 5% 4 4 4 5% 7% 45%	2660
December	1st to 10th	Night	19% 7% 2 6% 5% 4 4 4 5% 7% 37%	2660
	11th to 20th	Night	18% 5% 2 6% 4 4 4 4 6% 7% 40%	3288
	21st to end	Night	17% 7% 3 7% 4 5% 5% 5% 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	48% 4 9% 27% 10% 3	3247
	11th to 20th	Day	36% 4 10% 28% 18% 4	3480
	21st to end	Day	46% 3 9% 21% 18% 3	3960
April	1st to 10th	Day	43% 4 11% 20% 17% 5%	2280
	11th to 20th	Day	39% 4 11% 24% 17% 5%	2254
	21st to end	Day	42% 5% 10% 22% 18% 4	2280
May	1st to 10th	Day	53% 3 10% 21% 11% 2	4092
	11th to 20th	Day	54% 4 14% 18% 8% 1	4402
	21st to end	Day	34% 5% 15% 26% 17% 2	4837
June	1st to 10th	Day	38% 7% 14% 25% 15% 2	2660
	11th to 20th	Day	39% 6% 13% 25% 13% 3	2660
	21st to end	Day	34% 6% 16% 26% 17% 1	2660
July	1st to 10th	Day	36% 8% 16% 22% 14% 3	4370
	11th to 20th	Day	36% 8% 16% 26% 14% 1	4365
	21st to end	Day	44% 6% 11% 20% 17% 1	4428
August	1st to 10th	Day	47% 6% 11% 25% 10%	4091
	11th to 20th	Day	37% 6% 11% 29% 16% 1	3870
	21st to end	Day	34% 6% 10% 28% 20% 2	4180
September	1st to 10th	Day	41% 3 11% 21% 20% 5%	2280
	11th to 20th	Day	33% 4 9% 27% 23% 4	2280
	21st to end	Day	46% 4 8% 19% 19% 3	2027
October	1st to 10th	Day	37% 3 9% 19% 27% 5%	3160
	11th to 20th	Day	28% 3 12% 28% 22% 7%	3140
	21st to end	Day	25% 3 12% 28% 28% 3	3428
November	1st to 10th	Day	34% 3 13% 26% 20% 4	1698
	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	48% 3 10% 24% 13% 1	3588
	11th to 20th	Night	35% 4 11% 31% 17% 3	3480
	21st to end	Night	42% 3 7% 21% 23% 4	3671
April	1st to 10th	Night	44% 4 10% 21% 17% 4	1900
	11th to 20th	Night	39% 4 11% 22% 20% 4	1875
	21st to end	Night	40% 5% 9% 22% 18% 5%	1900
May	1st to 10th	Night	53% 4 9% 19% 13% 2	2896
	11th to 20th	Night	55% 5% 14% 14% 10% 2	2496
	21st to end	Night	37% 5% 16% 24% 14% 4	2739
June	1st to 10th	Night	37% 7% 11% 24% 20% 2	1520
	11th to 20th	Night	37% 6% 15% 23% 17% 2	1520
	21st to end	Night	35% 6% 14% 26% 17% 2	1520
July	1st to 10th	Night	39% 8% 17% 18% 14% 5%	2480
	11th to 20th	Night	40% 7% 15% 23% 14% 1	2472
	21st to end	Night	42% 7% 14% 19% 18% 1	2881
August	1st to 10th	Night	47% 5% 17% 22% 9%	2814
	11th to 20th	Night	40% 5% 12% 26% 15% 1	2993
	21st to end	Night	35% 5% 11% 29% 18% 2	3476
September	1st to 10th	Night	38% 5% 10% 21% 21% 4	1900
	11th to 20th	Night	32% 4 9% 26% 25% 4	1900
	21st to end	Night	45% 5% 8% 20% 20% 3	2151
October	1st to 10th	Night	44% 4 9% 21% 22% 2	3800
	11th to 20th	Night	27% 3 10% 28% 28% 4	3779
	21st to end	Night	26% 3 11% 33% 24% 3	4120
November	1st to 10th	Night	32% 4 12% 27% 21% 5%	2452
	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% 23% 6% 2	506
April	1st to 10th	Day	22% 12% 34% 21% 9% 2	460
	11th to 20th	Day	16% 15% 30% 21% 12% 5%	452
	21st to end	Day	24% 11% 29% 22% 13% 2	460
May	1st to 10th	Day	25% 12% 25% 21% 14% 3	463
	11th to 20th	Day	21% 8% 25% 25% 14% 7%	487
	21st to end	Day	18% 5% 18% 24% 23% 12%	529
June	1st to 10th	Day	15% 5% 26% 27% 18% 10%	490
	11th to 20th	Day	15% 7% 21% 29% 20% 8%	490
	21st to end	Day	13% 3 24% 30% 20% 10%	490
July	1st to 10th	Day	12% 3 13% 27% 29% 16%	485
	11th to 20th	Day	5% 2 13% 33% 32% 15%	484
	21st to end	Day	15% 3 17% 31% 27% 8%	532
August	1st to 10th	Day	12% 4 14% 28% 28% 13%	486
	11th to 20th	Day	12% 4 14% 29% 26% 16%	479
	21st to end	Day	12% 4 13% 29% 25% 16%	539
September	1st to 10th	Day	18% 4 17% 21% 23% 16%	490
	11th to 20th	Day	13% 3 16% 26% 25% 16%	490
	21st to end	Day	26% 8% 18% 26% 16% 7%	490
October	1st to 10th	Day	23% 5% 20% 24% 20% 8%	490
	11th to 20th	Day	17% 10% 24% 28% 17% 4	490
	21st to end	Day	17% 7% 32% 30% 11% 3	539
November	1st to 10th	Day	15% 9% 28% 28% 17% 3	374
	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	20% 8% 36% 29% 6%	836
April	1st to 10th	Night	21% 12% 37% 26% 5%	760
	11th to 20th	Night	19% 9% 31% 32% 9%	751
	21st to end	Night	20% 7% 31% 33% 9% 1	760
May	1st to 10th	Night	22% 6% 25% 38% 10% 1	613
	11th to 20th	Night	20% 5% 27% 36% 12%	374
	21st to end	Night	13% 2 18% 44% 22% 2	411
June	1st to 10th	Night	14% 3 19% 42% 21% 1	380
	11th to 20th	Night	13% 2 18% 46% 21%	380
	21st to end	Night	12% 3 15% 52% 18% 1	380
July	1st to 10th	Night	10% 3 14% 42% 27% 3	370
	11th to 20th	Night	7% 2 10% 46% 33% 3	368
	21st to end	Night	12% 2 11% 45% 28% 2	552
August	1st to 10th	Night	12% 1 6% 48% 30% 3	567
	11th to 20th	Night	13% 2 11% 40% 31% 3	667
	21st to end	Night	13% 2 11% 40% 29% 4	836
September	1st to 10th	Night	15% 4 17% 36% 24% 4	760
	11th to 20th	Night	14% 4 10% 39% 30% 4	760
	21st to end	Night	22% 3 16% 36% 21% 2	760
October	1st to 10th	Night	22% 4 18% 33% 19% 4	760
	11th to 20th	Night	17% 6% 26% 36% 14% 1	760
	21st to end	Night	20% 5% 31% 31% 13% 1	836
November	1st to 10th	Night	13% 4 31% 37% 15% 1	755
	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 2024-11-01 22:47:45. Version ac07

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