Grout Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grout Observatory clear sky chart from 2005-11-06 to 2024-12-24.

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% 11% 4 10% 6% 5% 5% 4 6% 7% 27%	101853
	Night	15% 12% 4 11% 7% 5% 5% 4 5% 6% 26%	98265
Transparency	Day	54% 6% 12% 19% 8% 1	105339
	Night	57% 6% 11% 17% 8% 1	102855
Seeing	Day	25% 18% 29% 18% 8% 2	14844
	Night	23% 15% 26% 26% 9% 1	23833

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	12% 13% 8% 20% 10% 6% 5% 4 4 4 15%	2517
	11th to 20th	Day	12% 14% 6% 19% 12% 6% 5% 3 4 4 14%	2520
	21st to end	Day	12% 19% 9% 14% 6% 8% 5% 3 4 5% 15%	2772
February	1st to 10th	Day	11% 14% 5% 14% 10% 8% 5% 2 6% 5% 20%	1638
	11th to 20th	Day	9% 15% 7% 16% 10% 7% 6% 4 4 5% 18%	1839
	21st to end	Day	11% 15% 6% 13% 8% 7% 5% 5% 4 5% 22%	1570
March	1st to 10th	Day	19% 17% 6% 12% 7% 5% 5% 3 4 4 19%	3116
	11th to 20th	Day	12% 16% 7% 14% 7% 5% 4 3 4 5% 22%	3242
	21st to end	Day	16% 12% 6% 13% 5% 4 4 3 3 4 30%	4166
April	1st to 10th	Day	17% 16% 4 10% 6% 5% 4 3 4 6% 26%	2280
	11th to 20th	Day	16% 12% 4 8% 6% 5% 4 3 4 6% 32%	2254
	21st to end	Day	21% 12% 3 8% 5% 5% 4 3 5% 6% 27%	2465
May	1st to 10th	Day	22% 13% 4 8% 6% 5% 5% 3 5% 5% 25%	4188
	11th to 20th	Day	15% 9% 4 10% 6% 6% 5% 4 6% 7% 27%	4398
	21st to end	Day	13% 7% 3 7% 5% 5% 5% 5% 7% 9% 33%	4837
June	1st to 10th	Day	22% 8% 3 7% 5% 5% 5% 4 6% 7% 27%	2660
	11th to 20th	Day	15% 8% 2 7% 5% 5% 5% 5% 6% 9% 32%	2660
	21st to end	Day	19% 9% 3 8% 6% 5% 6% 4 7% 6% 27%	2660
July	1st to 10th	Day	11% 7% 3 7% 5% 5% 6% 5% 8% 10% 32%	4440
	11th to 20th	Day	10% 6% 3 7% 6% 6% 7% 5% 8% 11% 31%	4440
	21st to end	Day	13% 8% 3 7% 5% 5% 6% 4 6% 8% 35%	4814
August	1st to 10th	Day	15% 8% 3 8% 6% 6% 7% 5% 7% 9% 27%	4398
	11th to 20th	Day	13% 9% 4 8% 6% 6% 6% 5% 7% 9% 28%	3958
	21st to end	Day	12% 7% 3 7% 5% 5% 5% 5% 7% 9% 36%	4180
September	1st to 10th	Day	15% 6% 2 7% 4 4 4 4 7% 7% 41%	2280
	11th to 20th	Day	14% 8% 2 5% 3 3 4 4 5% 8% 44%	2151
	21st to end	Day	15% 8% 2 7% 5% 5% 5% 4 6% 8% 35%	2090
October	1st to 10th	Day	21% 10% 3 8% 5% 4 4 4 5% 6% 29%	2041
	11th to 20th	Day	14% 12% 4 9% 5% 5% 6% 4 5% 6% 30%	1823
	21st to end	Day	21% 19% 4 8% 5% 4 4 3 4 4 24%	1881
November	1st to 10th	Day	12% 16% 4 8% 6% 5% 5% 4 6% 5% 29%	1761
	11th to 20th	Day	14% 15% 6% 11% 6% 5% 5% 4 4 6% 24%	1800
	21st to end	Day	15% 16% 6% 13% 6% 4 5% 4 6% 6% 20%	1632
December	1st to 10th	Day	15% 19% 7% 17% 8% 6% 4 2 4 5% 14%	1600
	11th to 20th	Day	17% 16% 8% 11% 6% 5% 4 2 5% 5% 21%	1986
	21st to end	Day	19% 15% 7% 16% 7% 6% 6% 3 4 3 15%	2796
January	1st to 10th	Night	13% 14% 6% 21% 9% 6% 6% 4 5% 4 13%	4429
	11th to 20th	Night	10% 14% 5% 20% 14% 7% 5% 3 4 4 13%	4440
	21st to end	Night	13% 19% 8% 15% 7% 7% 5% 3 4 4 16%	4884
February	1st to 10th	Night	10% 14% 6% 15% 11% 8% 6% 4 4 5% 17%	2538
	11th to 20th	Night	9% 12% 6% 15% 9% 8% 7% 4 5% 5% 19%	2226
	21st to end	Night	10% 13% 5% 14% 9% 6% 5% 3 5% 5% 25%	1884
March	1st to 10th	Night	18% 15% 5% 9% 7% 5% 4 3 4 5% 23%	3724
	11th to 20th	Night	12% 17% 7% 12% 7% 5% 5% 2 4 5% 26%	3698
	21st to end	Night	13% 16% 6% 13% 7% 4 4 2 3 3 29%	3466
April	1st to 10th	Night	18% 14% 4 9% 6% 5% 4 4 5% 5% 27%	1900
	11th to 20th	Night	17% 11% 3 8% 7% 6% 4 3 5% 5% 31%	1875
	21st to end	Night	18% 12% 3 7% 5% 4 5% 3 5% 6% 31%	1715
May	1st to 10th	Night	22% 13% 3 7% 5% 4 4 4 4 6% 27%	2782
	11th to 20th	Night	18% 8% 4 8% 6% 5% 4 3 5% 6% 31%	2495
	21st to end	Night	14% 7% 2 7% 5% 4 5% 4 6% 9% 37%	2738
June	1st to 10th	Night	21% 9% 3 8% 5% 6% 4 4 4 6% 30%	1520
	11th to 20th	Night	16% 7% 2 7% 5% 5% 5% 4 7% 8% 36%	1520
	21st to end	Night	20% 7% 3 6% 5% 4 4 5% 6% 8% 31%	1520
July	1st to 10th	Night	12% 7% 2 8% 6% 5% 7% 5% 7% 8% 33%	2520
	11th to 20th	Night	13% 6% 3 8% 5% 5% 6% 4 6% 8% 35%	2520
	21st to end	Night	12% 7% 3 8% 5% 5% 6% 5% 7% 6% 37%	2739
August	1st to 10th	Night	15% 10% 3 9% 5% 5% 4 3 5% 8% 32%	2506
	11th to 20th	Night	14% 8% 3 8% 6% 5% 5% 3 5% 7% 36%	2859
	21st to end	Night	13% 6% 3 7% 4 5% 5% 5% 7% 9% 37%	3476
September	1st to 10th	Night	13% 6% 2 6% 4 5% 5% 4 5% 6% 44%	1900
	11th to 20th	Night	14% 8% 2 5% 4 3 4 3 5% 7% 47%	2028
	21st to end	Night	16% 7% 3 6% 4 4 4 3 5% 7% 41%	2090
October	1st to 10th	Night	19% 9% 3 8% 4 6% 4 4 5% 7% 31%	2137
	11th to 20th	Night	16% 12% 3 10% 5% 5% 5% 4 5% 7% 30%	2356
	21st to end	Night	21% 15% 3 8% 6% 4 5% 3 5% 5% 24%	2717
November	1st to 10th	Night	14% 14% 3 9% 6% 4 5% 4 5% 5% 30%	2537
	11th to 20th	Night	14% 14% 6% 11% 6% 5% 4 3 5% 5% 27%	2600
	21st to end	Night	14% 13% 7% 14% 8% 5% 4 3 5% 5% 20%	2765
December	1st to 10th	Night	16% 18% 7% 15% 8% 6% 7% 4 4 4 12%	2800
	11th to 20th	Night	17% 17% 7% 15% 7% 6% 4 4 4 4 15%	3428
	21st to end	Night	19% 15% 5% 19% 8% 5% 4 3 4 4 14%	4933

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	71% 5% 10% 12% 2	2518
	11th to 20th	Day	72% 6% 8% 10% 4 1	2520
	21st to end	Day	72% 4 10% 9% 5% 1	2772
February	1st to 10th	Day	65% 5% 9% 13% 6% 1	1638
	11th to 20th	Day	68% 5% 9% 13% 5% 1	1848
	21st to end	Day	64% 5% 8% 13% 9% 2	1570
March	1st to 10th	Day	69% 5% 7% 10% 8% 1	3119
	11th to 20th	Day	64% 5% 10% 11% 9% 2	3242
	21st to end	Day	58% 4 5% 15% 16% 2	4166
April	1st to 10th	Day	60% 4 8% 18% 9% 2	2280
	11th to 20th	Day	54% 5% 8% 19% 12% 1	2254
	21st to end	Day	58% 5% 10% 16% 9% 2	2465
May	1st to 10th	Day	60% 5% 10% 18% 6% 1	4202
	11th to 20th	Day	55% 7% 14% 15% 8% 1	4402
	21st to end	Day	43% 8% 13% 25% 10% 1	4837
June	1st to 10th	Day	55% 7% 11% 17% 9%	2660
	11th to 20th	Day	48% 6% 11% 22% 12% 1	2660
	21st to end	Day	56% 7% 10% 19% 7% 1	2660
July	1st to 10th	Day	42% 8% 15% 24% 10%	4370
	11th to 20th	Day	42% 9% 16% 28% 5%	4365
	21st to end	Day	45% 7% 11% 27% 9%	4661
August	1st to 10th	Day	51% 8% 15% 19% 7%	4384
	11th to 20th	Day	49% 8% 17% 21% 5%	3997
	21st to end	Day	42% 8% 17% 20% 13% 1	4180
September	1st to 10th	Day	40% 6% 13% 29% 12% 1	2280
	11th to 20th	Day	39% 4 12% 29% 15% 1	2151
	21st to end	Day	45% 7% 14% 22% 10% 2	2090
October	1st to 10th	Day	48% 6% 12% 23% 10%	3392
	11th to 20th	Day	49% 5% 15% 22% 10%	3000
	21st to end	Day	50% 5% 12% 24% 8% 1	3081
November	1st to 10th	Day	53% 7% 12% 19% 9%	1761
	11th to 20th	Day	61% 6% 10% 11% 9% 2	1800
	21st to end	Day	62% 6% 14% 13% 4	1632
December	1st to 10th	Day	73% 5% 10% 9% 3	1600
	11th to 20th	Day	64% 4 10% 16% 7%	1986
	21st to end	Day	74% 4 7% 10% 5%	2796
January	1st to 10th	Night	71% 6% 13% 8% 2	4429
	11th to 20th	Night	72% 7% 10% 7% 4	4440
	21st to end	Night	73% 4 8% 10% 4 1	4884
February	1st to 10th	Night	66% 7% 9% 12% 6% 1	2538
	11th to 20th	Night	65% 6% 8% 14% 7%	2228
	21st to end	Night	60% 6% 10% 15% 8% 1	1884
March	1st to 10th	Night	63% 4 9% 14% 8% 2	3734
	11th to 20th	Night	61% 5% 8% 15% 10% 1	3699
	21st to end	Night	60% 5% 5% 14% 14% 1	3466
April	1st to 10th	Night	57% 6% 9% 17% 10% 1	1900
	11th to 20th	Night	54% 5% 8% 21% 12%	1875
	21st to end	Night	53% 6% 10% 16% 13% 3	1715
May	1st to 10th	Night	57% 5% 10% 18% 8% 1	2788
	11th to 20th	Night	52% 7% 14% 17% 8% 1	2496
	21st to end	Night	42% 7% 16% 19% 14% 1	2739
June	1st to 10th	Night	53% 7% 9% 22% 8% 1	1520
	11th to 20th	Night	45% 7% 14% 21% 10% 2	1520
	21st to end	Night	48% 9% 15% 19% 9%	1520
July	1st to 10th	Night	45% 8% 16% 20% 11%	2480
	11th to 20th	Night	43% 8% 14% 27% 8%	2472
	21st to end	Night	43% 8% 14% 24% 10%	2651
August	1st to 10th	Night	50% 6% 14% 23% 6%	2496
	11th to 20th	Night	46% 7% 18% 19% 9%	2886
	21st to end	Night	41% 8% 15% 23% 13%	3476
September	1st to 10th	Night	39% 7% 15% 28% 11% 1	1900
	11th to 20th	Night	38% 5% 14% 31% 12% 1	2028
	21st to end	Night	42% 6% 14% 24% 12% 2	2090
October	1st to 10th	Night	52% 6% 13% 21% 8%	3566
	11th to 20th	Night	50% 5% 11% 21% 12%	3905
	21st to end	Night	52% 4 14% 21% 8% 1	4467
November	1st to 10th	Night	53% 7% 13% 18% 8% 1	2537
	11th to 20th	Night	58% 5% 9% 17% 9% 2	2600
	21st to end	Night	64% 6% 12% 14% 4	2765
December	1st to 10th	Night	75% 6% 9% 8% 2	2800
	11th to 20th	Night	72% 5% 8% 9% 6%	3428
	21st to end	Night	74% 5% 7% 11% 3	4933

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% 23% 36% 14% 6%	180
	11th to 20th	Day	31% 26% 27% 13% 3 1	180
	21st to end	Day	23% 23% 34% 15% 5%	198
February	1st to 10th	Day	32% 32% 25% 8% 3	236
	11th to 20th	Day	30% 24% 33% 11% 2	447
	21st to end	Day	29% 24% 29% 14% 3	382
March	1st to 10th	Day	32% 18% 31% 16% 3	460
	11th to 20th	Day	29% 24% 28% 15% 4	460
	21st to end	Day	31% 24% 24% 13% 7% 1	506
April	1st to 10th	Day	33% 20% 25% 15% 6% 1	460
	11th to 20th	Day	32% 23% 21% 15% 6% 2	452
	21st to end	Day	34% 19% 26% 13% 7% 2	460
May	1st to 10th	Day	32% 19% 28% 12% 7% 2	463
	11th to 20th	Day	20% 17% 33% 22% 6% 3	487
	21st to end	Day	19% 17% 35% 20% 8% 2	529
June	1st to 10th	Day	29% 16% 29% 18% 8% 1	490
	11th to 20th	Day	21% 16% 28% 23% 10% 2	490
	21st to end	Day	22% 19% 34% 17% 7% 1	490
July	1st to 10th	Day	18% 13% 29% 27% 11% 2	485
	11th to 20th	Day	15% 11% 35% 26% 12% 2	484
	21st to end	Day	16% 11% 36% 24% 11% 2	532
August	1st to 10th	Day	17% 13% 27% 27% 15% 2	486
	11th to 20th	Day	18% 11% 28% 24% 12% 6%	479
	21st to end	Day	16% 14% 26% 25% 13% 6%	539
September	1st to 10th	Day	18% 13% 28% 20% 14% 7%	490
	11th to 20th	Day	19% 9% 23% 21% 17% 10%	490
	21st to end	Day	17% 12% 31% 20% 14% 6%	490
October	1st to 10th	Day	27% 13% 24% 18% 13% 5%	490
	11th to 20th	Day	23% 20% 27% 19% 8% 3	449
	21st to end	Day	34% 19% 24% 16% 6%	429
November	1st to 10th	Day	29% 18% 23% 23% 7% 1	395
	11th to 20th	Day	27% 24% 26% 16% 7% 1	400
	21st to end	Day	34% 18% 28% 17% 4	232
December	1st to 10th	Day	29% 21% 31% 13% 7%	200
	11th to 20th	Day	32% 24% 32% 12% 1	200
	21st to end	Day	33% 21% 30% 14% 2	204
January	1st to 10th	Night	22% 23% 32% 19% 4	760
	11th to 20th	Night	26% 21% 29% 20% 4	760
	21st to end	Night	18% 21% 35% 21% 4 1	836
February	1st to 10th	Night	19% 21% 35% 21% 4	760
	11th to 20th	Night	22% 22% 34% 19% 3	743
	21st to end	Night	21% 21% 32% 23% 3	628
March	1st to 10th	Night	25% 17% 31% 24% 3	760
	11th to 20th	Night	25% 19% 28% 22% 6%	760
	21st to end	Night	28% 17% 25% 21% 7% 1	836
April	1st to 10th	Night	28% 17% 27% 23% 5%	760
	11th to 20th	Night	26% 17% 26% 22% 7% 1	751
	21st to end	Night	29% 15% 27% 23% 6%	575
May	1st to 10th	Night	28% 14% 26% 23% 8% 1	544
	11th to 20th	Night	18% 11% 31% 30% 8% 2	374
	21st to end	Night	18% 10% 26% 35% 11%	411
June	1st to 10th	Night	27% 10% 23% 28% 11% 1	380
	11th to 20th	Night	21% 5% 23% 35% 15% 1	380
	21st to end	Night	21% 9% 19% 38% 12% 1	380
July	1st to 10th	Night	15% 5% 21% 41% 16% 1	370
	11th to 20th	Night	13% 5% 21% 40% 20% 1	368
	21st to end	Night	14% 4 20% 45% 16%	416
August	1st to 10th	Night	16% 4 16% 41% 22%	378
	11th to 20th	Night	15% 6% 22% 35% 21% 2	600
	21st to end	Night	17% 6% 19% 34% 21% 2	836
September	1st to 10th	Night	16% 8% 24% 33% 17% 2	760
	11th to 20th	Night	17% 7% 20% 29% 24% 4	760
	21st to end	Night	18% 11% 24% 26% 18% 2	760
October	1st to 10th	Night	25% 11% 20% 27% 15% 2	760
	11th to 20th	Night	24% 12% 23% 29% 11% 1	760
	21st to end	Night	30% 15% 26% 24% 5%	836
November	1st to 10th	Night	24% 17% 27% 23% 8% 1	780
	11th to 20th	Night	23% 20% 28% 19% 9% 1	800
	21st to end	Night	26% 19% 27% 20% 7% 1	800
December	1st to 10th	Night	26% 21% 30% 19% 4	800
	11th to 20th	Night	28% 19% 26% 22% 4	800
	21st to end	Night	28% 22% 27% 19% 4	851

Page generated at 2024-12-21 04:13:46. Version ac07

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