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 2026-04-17.

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	14% 11% 4 10% 7% 6% 5% 4 6% 7% 26%	131388
	Night	13% 12% 5% 12% 7% 6% 5% 4 5% 6% 24%	130751
Transparency	Day	56% 6% 11% 18% 8% 1	134856
	Night	59% 6% 11% 16% 8% 1	135357
Seeing	Day	25% 18% 29% 18% 8% 2	16477
	Night	23% 15% 27% 25% 9% 1	25534

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	9% 11% 8% 20% 14% 10% 5% 4 4 4 11%	4661
	11th to 20th	Day	8% 14% 6% 17% 14% 9% 7% 4 5% 4 12%	4650
	21st to end	Day	8% 13% 6% 12% 10% 11% 9% 4 5% 6% 16%	5124
February	1st to 10th	Day	10% 13% 5% 15% 11% 8% 6% 2 6% 5% 20%	1811
	11th to 20th	Day	8% 14% 6% 16% 11% 7% 5% 4 5% 5% 18%	2039
	21st to end	Day	10% 14% 7% 13% 9% 7% 5% 4 4 5% 21%	1730
March	1st to 10th	Day	16% 15% 6% 15% 7% 5% 5% 3 4 4 20%	4576
	11th to 20th	Day	11% 13% 6% 12% 6% 5% 5% 4 5% 6% 27%	4732
	21st to end	Day	17% 12% 6% 13% 6% 4 4 3 4 5% 26%	6102
April	1st to 10th	Day	17% 16% 4 10% 6% 5% 4 3 4 6% 26%	2520
	11th to 20th	Day	16% 13% 4 8% 5% 5% 4 3 4 5% 31%	2446
	21st to end	Day	21% 12% 3 8% 5% 5% 4 4 5% 6% 27%	2595
May	1st to 10th	Day	27% 15% 4 8% 6% 5% 5% 3 4 4 19%	5996
	11th to 20th	Day	15% 10% 4 10% 7% 6% 5% 5% 6% 7% 26%	6318
	21st to end	Day	19% 9% 3 7% 5% 4 5% 5% 7% 8% 28%	6949
June	1st to 10th	Day	22% 8% 3 8% 5% 5% 6% 4 6% 7% 27%	2800
	11th to 20th	Day	15% 8% 2 7% 5% 5% 5% 5% 6% 9% 32%	2800
	21st to end	Day	19% 9% 3 8% 6% 5% 6% 4 7% 7% 28%	2800
July	1st to 10th	Day	10% 8% 3 8% 6% 5% 7% 5% 9% 11% 29%	6360
	11th to 20th	Day	9% 6% 3 7% 6% 6% 7% 5% 8% 10% 32%	6360
	21st to end	Day	12% 7% 3 6% 4 4 5% 4 7% 9% 38%	6926
August	1st to 10th	Day	14% 7% 3 8% 6% 6% 7% 5% 7% 9% 28%	4537
	11th to 20th	Day	12% 9% 4 8% 6% 6% 6% 5% 7% 9% 29%	4085
	21st to end	Day	12% 7% 3 7% 5% 5% 5% 5% 7% 9% 36%	4312
September	1st to 10th	Day	14% 6% 2 7% 4 3 4 4 7% 7% 42%	2400
	11th to 20th	Day	13% 8% 2 5% 3 3 4 4 5% 8% 45%	2264
	21st to end	Day	15% 8% 2 7% 5% 5% 5% 4 6% 8% 36%	2200
October	1st to 10th	Day	20% 9% 3 8% 5% 4 4 3 5% 7% 31%	2148
	11th to 20th	Day	14% 12% 3 9% 5% 4 5% 4 5% 6% 30%	1919
	21st to end	Day	20% 19% 4 9% 5% 4 4 3 4 4 23%	1980
November	1st to 10th	Day	12% 17% 4 9% 6% 5% 5% 4 6% 5% 28%	1851
	11th to 20th	Day	14% 16% 6% 12% 6% 5% 5% 4 4 6% 23%	1890
	21st to end	Day	15% 16% 6% 13% 6% 4 4 4 6% 6% 19%	1713
December	1st to 10th	Day	10% 16% 7% 25% 10% 7% 5% 3 4 4 10%	2680
	11th to 20th	Day	15% 12% 7% 13% 7% 6% 5% 3 5% 5% 20%	3066
	21st to end	Day	16% 14% 8% 15% 6% 5% 7% 3 4 5% 18%	4048
January	1st to 10th	Night	9% 12% 7% 23% 12% 9% 6% 3 4 4 11%	8234
	11th to 20th	Night	7% 14% 7% 20% 14% 9% 6% 4 5% 4 10%	8235
	21st to end	Night	8% 13% 5% 12% 8% 11% 10% 6% 6% 6% 15%	9057
February	1st to 10th	Night	9% 14% 6% 15% 12% 8% 6% 4 4 5% 17%	2804
	11th to 20th	Night	9% 12% 6% 15% 10% 8% 7% 4 5% 5% 19%	2466
	21st to end	Night	10% 12% 5% 14% 9% 6% 5% 3 5% 5% 25%	2076
March	1st to 10th	Night	15% 14% 5% 13% 8% 8% 5% 3 4 5% 20%	5484
	11th to 20th	Night	12% 14% 5% 11% 7% 5% 5% 3 5% 6% 26%	5414
	21st to end	Night	14% 15% 5% 13% 7% 5% 5% 3 3 4 25%	5072
April	1st to 10th	Night	17% 13% 4 9% 6% 5% 4 4 5% 5% 27%	2100
	11th to 20th	Night	17% 12% 3 8% 7% 6% 4 3 5% 4 30%	2036
	21st to end	Night	18% 12% 3 7% 5% 4 5% 3 5% 7% 31%	1805
May	1st to 10th	Night	26% 16% 4 8% 5% 4 3 3 4 6% 22%	3974
	11th to 20th	Night	16% 11% 4 8% 5% 5% 5% 4 5% 6% 30%	3575
	21st to end	Night	15% 11% 3 9% 5% 5% 5% 4 5% 8% 30%	3926
June	1st to 10th	Night	21% 9% 3 8% 5% 6% 4 4 4 6% 31%	1600
	11th to 20th	Night	16% 7% 2 7% 5% 5% 5% 4 7% 8% 36%	1600
	21st to end	Night	19% 7% 3 6% 5% 5% 4 5% 6% 8% 32%	1600
July	1st to 10th	Night	10% 7% 3 7% 6% 5% 7% 5% 7% 9% 33%	3600
	11th to 20th	Night	13% 7% 3 9% 6% 5% 5% 4 6% 8% 35%	3600
	21st to end	Night	9% 6% 2 7% 5% 5% 5% 5% 6% 8% 41%	3926
August	1st to 10th	Night	14% 9% 3 9% 5% 5% 4 3 5% 8% 33%	2586
	11th to 20th	Night	14% 8% 3 8% 5% 5% 5% 3 5% 7% 37%	2952
	21st to end	Night	13% 6% 3 8% 4 5% 5% 5% 7% 9% 37%	3586
September	1st to 10th	Night	13% 6% 2 7% 4 5% 5% 4 5% 6% 44%	2000
	11th to 20th	Night	13% 7% 2 6% 4 3 4 3 5% 7% 47%	2135
	21st to end	Night	16% 7% 3 6% 5% 4 4 3 4 7% 41%	2200
October	1st to 10th	Night	19% 9% 2 8% 4 6% 4 4 5% 7% 33%	2250
	11th to 20th	Night	16% 11% 3 9% 5% 5% 5% 4 5% 7% 31%	2480
	21st to end	Night	21% 15% 3 8% 6% 4 5% 3 5% 5% 23%	2860
November	1st to 10th	Night	14% 14% 4 10% 6% 4 4 4 5% 5% 29%	2667
	11th to 20th	Night	14% 13% 7% 12% 6% 5% 4 3 5% 5% 26%	2730
	21st to end	Night	14% 13% 7% 14% 8% 6% 4 3 5% 5% 20%	2903
December	1st to 10th	Night	11% 14% 7% 21% 10% 8% 6% 4 4 4 11%	4720
	11th to 20th	Night	16% 17% 6% 15% 7% 6% 5% 3 4 5% 15%	5348
	21st to end	Night	14% 14% 5% 18% 8% 6% 5% 3 4 6% 16%	7150

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	76% 4 8% 10% 2	4662
	11th to 20th	Day	71% 6% 8% 9% 5% 2	4680
	21st to end	Day	67% 4 9% 10% 9%	5124
February	1st to 10th	Day	64% 5% 9% 13% 7% 2	1811
	11th to 20th	Day	67% 5% 9% 14% 5% 1	2048
	21st to end	Day	64% 5% 8% 12% 9% 2	1730
March	1st to 10th	Day	68% 4 7% 9% 8% 4	4579
	11th to 20th	Day	56% 5% 14% 14% 8% 3	4732
	21st to end	Day	60% 4 6% 14% 15% 1	6102
April	1st to 10th	Day	60% 4 8% 17% 9% 2	2520
	11th to 20th	Day	56% 5% 8% 18% 13% 1	2446
	21st to end	Day	57% 5% 10% 16% 10% 2	2595
May	1st to 10th	Day	68% 5% 8% 14% 5% 1	6007
	11th to 20th	Day	56% 7% 13% 16% 7% 1	6322
	21st to end	Day	51% 8% 12% 22% 8% 1	6949
June	1st to 10th	Day	55% 7% 11% 17% 10%	2800
	11th to 20th	Day	47% 6% 12% 22% 12% 1	2800
	21st to end	Day	55% 7% 10% 20% 8% 1	2800
July	1st to 10th	Day	45% 8% 16% 22% 9%	6290
	11th to 20th	Day	41% 8% 15% 26% 9%	6285
	21st to end	Day	41% 7% 12% 32% 9%	6728
August	1st to 10th	Day	50% 8% 15% 20% 7%	4523
	11th to 20th	Day	49% 8% 16% 22% 5%	4124
	21st to end	Day	42% 8% 17% 20% 13% 1	4312
September	1st to 10th	Day	39% 6% 12% 30% 13% 1	2400
	11th to 20th	Day	38% 4 12% 29% 16% 1	2264
	21st to end	Day	45% 7% 14% 22% 11% 2	2200
October	1st to 10th	Day	46% 6% 12% 24% 11%	3499
	11th to 20th	Day	49% 5% 15% 22% 10%	3096
	21st to end	Day	50% 5% 12% 23% 8% 1	3180
November	1st to 10th	Day	55% 7% 12% 18% 8%	1851
	11th to 20th	Day	62% 6% 11% 11% 9% 2	1890
	21st to end	Day	64% 6% 14% 12% 4	1713
December	1st to 10th	Day	78% 4 9% 7% 2	2680
	11th to 20th	Day	64% 5% 9% 16% 6%	3066
	21st to end	Day	69% 4 6% 11% 9% 1	4048
January	1st to 10th	Night	75% 5% 10% 7% 3	8234
	11th to 20th	Night	75% 6% 8% 6% 5%	8280
	21st to end	Night	67% 5% 8% 10% 9% 1	9057
February	1st to 10th	Night	66% 6% 8% 13% 6%	2804
	11th to 20th	Night	65% 6% 8% 14% 6%	2468
	21st to end	Night	60% 5% 10% 16% 8% 1	2076
March	1st to 10th	Night	67% 4 8% 11% 8% 1	5494
	11th to 20th	Night	57% 5% 15% 14% 8% 1	5415
	21st to end	Night	62% 5% 5% 14% 12% 1	5072
April	1st to 10th	Night	57% 6% 9% 16% 10% 1	2100
	11th to 20th	Night	55% 5% 8% 20% 12%	2036
	21st to end	Night	52% 6% 10% 17% 13% 3	1805
May	1st to 10th	Night	64% 5% 9% 15% 6% 1	3980
	11th to 20th	Night	53% 8% 13% 19% 7% 1	3576
	21st to end	Night	51% 6% 13% 18% 10% 1	3927
June	1st to 10th	Night	53% 7% 9% 22% 9% 1	1600
	11th to 20th	Night	45% 7% 14% 22% 11% 2	1600
	21st to end	Night	48% 9% 14% 20% 9%	1600
July	1st to 10th	Night	43% 8% 19% 21% 8%	3560
	11th to 20th	Night	45% 8% 13% 26% 9%	3552
	21st to end	Night	38% 8% 16% 29% 9%	3809
August	1st to 10th	Night	49% 6% 15% 23% 6%	2576
	11th to 20th	Night	46% 7% 18% 20% 10% 1	2979
	21st to end	Night	41% 8% 16% 23% 13%	3586
September	1st to 10th	Night	39% 7% 14% 28% 11% 1	2000
	11th to 20th	Night	37% 5% 14% 32% 12% 1	2135
	21st to end	Night	43% 6% 13% 24% 13% 2	2200
October	1st to 10th	Night	51% 6% 13% 21% 9%	3679
	11th to 20th	Night	49% 5% 12% 21% 12%	4029
	21st to end	Night	52% 5% 14% 21% 8% 1	4610
November	1st to 10th	Night	54% 7% 13% 18% 8% 1	2667
	11th to 20th	Night	59% 5% 9% 16% 8% 2	2730
	21st to end	Night	64% 6% 12% 14% 5%	2903
December	1st to 10th	Night	76% 6% 9% 7% 1	4720
	11th to 20th	Night	70% 5% 7% 10% 8%	5348
	21st to end	Night	70% 4 7% 11% 7% 1	7150

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	20% 24% 37% 14% 5%	220
	11th to 20th	Day	29% 27% 29% 11% 3 1	220
	21st to end	Day	26% 22% 33% 14% 4	242
February	1st to 10th	Day	30% 30% 27% 9% 3	289
	11th to 20th	Day	29% 24% 33% 12% 2	527
	21st to end	Day	28% 25% 30% 14% 3	446
March	1st to 10th	Day	32% 19% 31% 15% 3	540
	11th to 20th	Day	31% 23% 28% 14% 3	540
	21st to end	Day	34% 24% 23% 11% 7% 1	594
April	1st to 10th	Day	35% 21% 23% 14% 6% 1	540
	11th to 20th	Day	34% 23% 22% 14% 5% 2	516
	21st to end	Day	34% 20% 26% 13% 6% 2	500
May	1st to 10th	Day	34% 19% 26% 12% 6% 2	503
	11th to 20th	Day	21% 17% 32% 22% 5% 3	527
	21st to end	Day	21% 16% 34% 20% 8% 1	573
June	1st to 10th	Day	28% 17% 29% 18% 8% 1	530
	11th to 20th	Day	21% 16% 29% 22% 9% 2	530
	21st to end	Day	21% 19% 35% 17% 7% 1	530
July	1st to 10th	Day	18% 14% 30% 26% 11% 2	525
	11th to 20th	Day	15% 11% 35% 27% 11% 2	524
	21st to end	Day	16% 11% 36% 24% 11% 1	576
August	1st to 10th	Day	15% 12% 26% 29% 15% 3	526
	11th to 20th	Day	18% 11% 28% 24% 12% 8%	519
	21st to end	Day	15% 15% 27% 25% 12% 6%	583
September	1st to 10th	Day	17% 13% 29% 20% 15% 7%	530
	11th to 20th	Day	18% 9% 24% 21% 17% 11%	530
	21st to end	Day	18% 12% 31% 20% 14% 5%	530
October	1st to 10th	Day	26% 13% 24% 18% 13% 5%	530
	11th to 20th	Day	23% 20% 26% 20% 8% 2	485
	21st to end	Day	34% 20% 24% 17% 6%	462
November	1st to 10th	Day	30% 18% 24% 21% 6%	425
	11th to 20th	Day	27% 23% 27% 15% 7% 1	430
	21st to end	Day	34% 20% 27% 15% 4	253
December	1st to 10th	Day	29% 20% 31% 14% 6%	220
	11th to 20th	Day	31% 25% 33% 10% 1	220
	21st to end	Day	32% 20% 31% 14% 3	242
January	1st to 10th	Night	21% 25% 32% 18% 4	840
	11th to 20th	Night	26% 21% 30% 19% 5%	840
	21st to end	Night	18% 20% 35% 21% 4 1	924
February	1st to 10th	Night	19% 21% 35% 22% 4	840
	11th to 20th	Night	22% 21% 34% 19% 3	823
	21st to end	Night	20% 20% 32% 25% 3	692
March	1st to 10th	Night	24% 18% 32% 23% 3	840
	11th to 20th	Night	26% 19% 28% 21% 5%	840
	21st to end	Night	28% 18% 26% 20% 7% 1	924
April	1st to 10th	Night	28% 18% 27% 22% 5%	840
	11th to 20th	Night	27% 17% 27% 22% 7% 1	816
	21st to end	Night	29% 15% 27% 23% 5%	605
May	1st to 10th	Night	28% 14% 27% 23% 8% 1	572
	11th to 20th	Night	19% 11% 31% 30% 8% 2	394
	21st to end	Night	19% 10% 25% 35% 11%	433
June	1st to 10th	Night	27% 10% 23% 29% 11% 1	400
	11th to 20th	Night	21% 7% 23% 34% 15% 1	400
	21st to end	Night	20% 10% 19% 39% 12% 1	400
July	1st to 10th	Night	15% 5% 22% 41% 16% 1	390
	11th to 20th	Night	14% 5% 21% 40% 19% 1	388
	21st to end	Night	14% 4 20% 45% 16%	438
August	1st to 10th	Night	15% 4 16% 42% 22%	398
	11th to 20th	Night	14% 6% 21% 35% 22% 2	633
	21st to end	Night	17% 5% 20% 35% 21% 2	880
September	1st to 10th	Night	15% 8% 24% 33% 18% 2	800
	11th to 20th	Night	16% 7% 19% 29% 25% 4	800
	21st to end	Night	18% 11% 25% 27% 17% 2	800
October	1st to 10th	Night	24% 11% 20% 28% 16% 2	800
	11th to 20th	Night	24% 12% 23% 29% 10% 1	800
	21st to end	Night	29% 15% 26% 25% 5%	880
November	1st to 10th	Night	25% 17% 26% 23% 8%	820
	11th to 20th	Night	23% 20% 28% 19% 9% 1	840
	21st to end	Night	26% 19% 28% 20% 7% 1	840
December	1st to 10th	Night	26% 21% 30% 19% 4	840
	11th to 20th	Night	28% 19% 27% 21% 4	840
	21st to end	Night	28% 22% 26% 19% 4	924

Page generated at 2026-04-14 02:44:30. Version ac07

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