Granby
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Granby clear sky chart from 2005-11-06 to 2026-05-17.

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	19% 12% 4 9% 5% 5% 4 3 5% 6% 28%	131890
	Night	19% 14% 5% 10% 6% 4 4 3 4 5% 27%	130948
Transparency	Day	56% 6% 12% 18% 8% 1	135346
	Night	59% 5% 10% 17% 8% 1	135579
Seeing	Day	26% 14% 28% 20% 9% 2	16500
	Night	26% 12% 27% 25% 9% 1	25510

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% 21% 8% 17% 7% 5% 3 2 3 3 14%	4661
	11th to 20th	Day	11% 18% 9% 22% 8% 6% 5% 3 3 4 11%	4650
	21st to end	Day	10% 16% 6% 13% 10% 9% 5% 4 4 5% 16%	5124
February	1st to 10th	Day	16% 18% 5% 12% 8% 6% 4 3 3 4 20%	1791
	11th to 20th	Day	14% 18% 7% 13% 6% 5% 4 3 4 5% 19%	2006
	21st to end	Day	16% 19% 5% 9% 7% 6% 5% 3 4 5% 21%	1730
March	1st to 10th	Day	20% 15% 7% 12% 6% 5% 5% 3 4 4 20%	4576
	11th to 20th	Day	15% 16% 4 9% 5% 5% 4 3 5% 6% 29%	4750
	21st to end	Day	20% 14% 5% 9% 5% 4 4 2 4 4 28%	6102
April	1st to 10th	Day	23% 15% 4 8% 4 4 3 3 4 5% 26%	2520
	11th to 20th	Day	22% 13% 3 6% 4 4 3 3 4 4 34%	2499
	21st to end	Day	25% 11% 3 6% 5% 4 3 3 4 6% 30%	2730
May	1st to 10th	Day	29% 13% 3 8% 5% 4 4 3 5% 5% 22%	6240
	11th to 20th	Day	21% 7% 3 7% 5% 5% 5% 4 6% 7% 29%	6402
	21st to end	Day	22% 9% 3 7% 5% 5% 4 4 5% 7% 30%	6949
June	1st to 10th	Day	26% 8% 2 5% 5% 4 4 4 5% 7% 30%	2800
	11th to 20th	Day	18% 7% 2 6% 4 4 5% 5% 6% 7% 36%	2800
	21st to end	Day	24% 8% 3 7% 5% 4 5% 3 5% 6% 30%	2800
July	1st to 10th	Day	15% 7% 3 7% 6% 6% 6% 5% 7% 9% 29%	6360
	11th to 20th	Day	14% 6% 2 6% 4 5% 5% 5% 7% 8% 37%	6360
	21st to end	Day	14% 7% 2 5% 4 4 4 4 6% 10% 42%	6926
August	1st to 10th	Day	18% 7% 3 8% 5% 5% 5% 4 6% 10% 30%	4563
	11th to 20th	Day	18% 6% 2 7% 5% 5% 4 4 6% 8% 34%	4146
	21st to end	Day	19% 6% 2 6% 3 4 4 4 5% 7% 38%	4312
September	1st to 10th	Day	16% 7% 2 5% 4 4 4 4 5% 8% 42%	2400
	11th to 20th	Day	17% 6% 2 4 4 4 3 4 5% 6% 46%	2274
	21st to end	Day	20% 7% 2 4 5% 3 4 4 6% 7% 38%	2200
October	1st to 10th	Day	25% 8% 2 6% 4 4 4 4 6% 7% 32%	2128
	11th to 20th	Day	23% 10% 3 7% 5% 5% 5% 3 5% 7% 29%	1912
	21st to end	Day	28% 14% 3 7% 3 5% 4 2 3 5% 26%	1980
November	1st to 10th	Day	20% 15% 3 6% 5% 4 4 3 4 4 30%	1851
	11th to 20th	Day	19% 18% 5% 9% 5% 4 4 2 4 5% 24%	1874
	21st to end	Day	22% 19% 6% 10% 5% 4 4 3 6% 4 19%	1680
December	1st to 10th	Day	19% 24% 7% 15% 6% 4 4 3 3 3 13%	2680
	11th to 20th	Day	20% 17% 6% 12% 6% 4 4 3 4 4 20%	3066
	21st to end	Day	21% 18% 5% 12% 5% 4 4 3 4 5% 18%	4048
January	1st to 10th	Night	15% 23% 9% 15% 6% 4 3 2 3 4 18%	8234
	11th to 20th	Night	12% 21% 9% 19% 8% 5% 4 3 3 3 12%	8235
	21st to end	Night	11% 15% 5% 15% 11% 6% 4 3 4 4 21%	9057
February	1st to 10th	Night	15% 18% 6% 14% 7% 5% 3 3 4 5% 20%	2824
	11th to 20th	Night	15% 17% 6% 13% 8% 5% 4 3 4 4 23%	2498
	21st to end	Night	15% 18% 5% 10% 6% 4 4 3 3 4 27%	2076
March	1st to 10th	Night	21% 14% 5% 9% 6% 4 4 2 3 5% 27%	5484
	11th to 20th	Night	17% 16% 4 9% 5% 4 4 3 5% 6% 26%	5423
	21st to end	Night	17% 18% 5% 10% 5% 4 4 3 3 4 27%	5072
April	1st to 10th	Night	25% 13% 4 8% 4 4 4 3 4 5% 27%	2100
	11th to 20th	Night	25% 10% 3 7% 4 4 4 2 4 5% 32%	2068
	21st to end	Night	22% 11% 3 6% 4 4 3 3 5% 5% 34%	1890
May	1st to 10th	Night	28% 11% 3 8% 5% 4 4 3 5% 6% 23%	3962
	11th to 20th	Night	25% 7% 3 8% 5% 4 4 4 5% 5% 30%	3623
	21st to end	Night	20% 10% 3 8% 4 5% 4 3 5% 7% 32%	3926
June	1st to 10th	Night	27% 7% 2 5% 4 4 4 4 5% 8% 30%	1600
	11th to 20th	Night	19% 6% 2 6% 4 4 5% 4 6% 8% 37%	1600
	21st to end	Night	20% 8% 2 8% 4 4 5% 4 5% 7% 32%	1600
July	1st to 10th	Night	15% 8% 3 7% 6% 5% 6% 5% 7% 8% 30%	3600
	11th to 20th	Night	19% 7% 2 5% 4 4 4 4 6% 7% 38%	3600
	21st to end	Night	13% 6% 3 7% 5% 4 6% 4 7% 7% 38%	3926
August	1st to 10th	Night	23% 8% 2 7% 3 5% 4 3 5% 6% 34%	2586
	11th to 20th	Night	16% 7% 3 8% 6% 4 5% 4 5% 6% 35%	2866
	21st to end	Night	18% 6% 2 5% 4 4 4 3 5% 7% 41%	3586
September	1st to 10th	Night	17% 7% 2 6% 5% 4 3 3 5% 6% 43%	2000
	11th to 20th	Night	15% 6% 2 5% 3 3 4 4 5% 6% 48%	2125
	21st to end	Night	21% 6% 2 5% 5% 5% 4 3 4 6% 41%	2200
October	1st to 10th	Night	20% 7% 3 7% 4 4 3 3 4 7% 38%	2270
	11th to 20th	Night	22% 10% 3 8% 5% 4 4 3 4 5% 31%	2487
	21st to end	Night	27% 11% 3 7% 4 4 4 3 4 5% 27%	2860
November	1st to 10th	Night	22% 14% 3 7% 4 4 5% 3 4 5% 28%	2667
	11th to 20th	Night	21% 16% 4 8% 4 4 3 3 5% 5% 27%	2745
	21st to end	Night	23% 17% 5% 9% 4 4 3 3 4 4 23%	2940
December	1st to 10th	Night	18% 28% 8% 12% 6% 4 3 2 3 3 12%	4720
	11th to 20th	Night	22% 19% 5% 13% 6% 4 3 2 3 3 20%	5348
	21st to end	Night	22% 19% 5% 10% 5% 4 3 3 3 6% 21%	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	74% 4 10% 10% 2	4662
	11th to 20th	Day	75% 7% 6% 7% 4	4680
	21st to end	Day	67% 8% 8% 9% 8%	5124
February	1st to 10th	Day	68% 6% 8% 13% 5% 1	1791
	11th to 20th	Day	67% 5% 12% 11% 5% 1	2015
	21st to end	Day	65% 5% 10% 13% 6% 1	1730
March	1st to 10th	Day	67% 5% 7% 12% 7% 2	4579
	11th to 20th	Day	56% 5% 15% 13% 10% 1	4750
	21st to end	Day	60% 4 8% 15% 13% 1	6102
April	1st to 10th	Day	61% 4 9% 17% 8% 1	2520
	11th to 20th	Day	53% 5% 9% 19% 12% 1	2499
	21st to end	Day	56% 6% 9% 17% 10% 2	2730
May	1st to 10th	Day	63% 5% 9% 16% 6% 1	6251
	11th to 20th	Day	53% 8% 15% 15% 8% 1	6406
	21st to end	Day	52% 6% 12% 19% 10% 1	6949
June	1st to 10th	Day	53% 7% 12% 18% 10% 1	2800
	11th to 20th	Day	44% 7% 13% 23% 11% 2	2800
	21st to end	Day	55% 7% 10% 19% 9% 1	2800
July	1st to 10th	Day	50% 5% 15% 24% 6%	6290
	11th to 20th	Day	42% 6% 14% 30% 8%	6285
	21st to end	Day	39% 6% 14% 34% 7%	6728
August	1st to 10th	Day	50% 7% 16% 21% 6%	4549
	11th to 20th	Day	46% 6% 19% 21% 6%	4186
	21st to end	Day	45% 5% 17% 20% 13% 1	4312
September	1st to 10th	Day	40% 7% 14% 28% 10% 1	2400
	11th to 20th	Day	38% 6% 12% 31% 13%	2274
	21st to end	Day	44% 7% 14% 24% 11%	2200
October	1st to 10th	Day	45% 6% 14% 28% 7%	3466
	11th to 20th	Day	50% 6% 15% 25% 5%	3089
	21st to end	Day	52% 5% 17% 19% 7% 1	3180
November	1st to 10th	Day	57% 7% 11% 17% 7%	1851
	11th to 20th	Day	63% 6% 10% 12% 8% 1	1874
	21st to end	Day	68% 6% 12% 9% 3	1680
December	1st to 10th	Day	76% 5% 8% 9% 2	2680
	11th to 20th	Day	67% 5% 8% 15% 5%	3066
	21st to end	Day	69% 5% 9% 11% 7%	4048
January	1st to 10th	Night	72% 5% 9% 10% 3	8234
	11th to 20th	Night	77% 5% 7% 7% 4	8280
	21st to end	Night	67% 5% 7% 11% 10%	9057
February	1st to 10th	Night	67% 5% 8% 14% 6%	2824
	11th to 20th	Night	64% 5% 10% 14% 6%	2500
	21st to end	Night	62% 4 8% 14% 10% 1	2076
March	1st to 10th	Night	60% 5% 7% 15% 10% 4	5494
	11th to 20th	Night	59% 5% 12% 14% 9% 1	5424
	21st to end	Night	61% 4 7% 18% 9% 1	5072
April	1st to 10th	Night	59% 5% 8% 16% 10% 1	2100
	11th to 20th	Night	55% 5% 8% 19% 12% 1	2068
	21st to end	Night	52% 5% 10% 18% 13% 2	1890
May	1st to 10th	Night	62% 6% 9% 16% 6% 1	3968
	11th to 20th	Night	53% 7% 11% 19% 8% 1	3624
	21st to end	Night	52% 6% 9% 19% 13% 1	3927
June	1st to 10th	Night	52% 7% 10% 20% 10% 1	1600
	11th to 20th	Night	45% 7% 11% 21% 13% 3	1600
	21st to end	Night	50% 7% 12% 20% 11% 1	1600
July	1st to 10th	Night	49% 6% 17% 20% 8%	3560
	11th to 20th	Night	44% 6% 15% 27% 8%	3552
	21st to end	Night	42% 6% 15% 26% 10%	3809
August	1st to 10th	Night	49% 6% 14% 24% 6%	2576
	11th to 20th	Night	49% 7% 16% 20% 8%	2892
	21st to end	Night	43% 6% 13% 23% 14% 1	3586
September	1st to 10th	Night	41% 7% 11% 27% 13% 1	2000
	11th to 20th	Night	37% 5% 15% 30% 13% 1	2125
	21st to end	Night	44% 6% 10% 24% 14% 1	2200
October	1st to 10th	Night	50% 5% 11% 24% 10%	3712
	11th to 20th	Night	49% 5% 11% 27% 8%	4049
	21st to end	Night	53% 5% 12% 18% 10% 2	4610
November	1st to 10th	Night	57% 5% 12% 18% 6% 1	2667
	11th to 20th	Night	59% 6% 11% 15% 9% 2	2745
	21st to end	Night	64% 5% 12% 14% 4	2940
December	1st to 10th	Night	78% 4 8% 8% 2	4720
	11th to 20th	Night	72% 4 6% 11% 7%	5348
	21st to end	Night	66% 5% 9% 13% 8%	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	32% 17% 28% 15% 8%	220
	11th to 20th	Day	26% 26% 33% 11% 3	220
	21st to end	Day	26% 18% 31% 19% 6%	242
February	1st to 10th	Day	28% 28% 28% 14% 2 1	279
	11th to 20th	Day	28% 22% 33% 15% 2	494
	21st to end	Day	26% 22% 31% 19% 2	446
March	1st to 10th	Day	30% 18% 28% 19% 4	540
	11th to 20th	Day	30% 18% 31% 16% 4 1	540
	21st to end	Day	31% 20% 25% 17% 6% 1	594
April	1st to 10th	Day	34% 14% 30% 15% 6% 1	540
	11th to 20th	Day	36% 17% 24% 16% 5% 3	532
	21st to end	Day	32% 15% 31% 13% 7% 2	540
May	1st to 10th	Day	33% 13% 27% 19% 6% 2	543
	11th to 20th	Day	23% 14% 27% 23% 10% 2	551
	21st to end	Day	23% 14% 32% 21% 7% 2	573
June	1st to 10th	Day	28% 12% 31% 20% 8% 2	530
	11th to 20th	Day	21% 11% 32% 24% 11% 1	530
	21st to end	Day	28% 11% 32% 19% 8% 3	530
July	1st to 10th	Day	17% 9% 33% 27% 13% 2	525
	11th to 20th	Day	18% 8% 32% 30% 8% 3	524
	21st to end	Day	19% 8% 31% 29% 10% 1	576
August	1st to 10th	Day	17% 8% 28% 30% 15% 2	526
	11th to 20th	Day	18% 7% 27% 28% 17% 4	519
	21st to end	Day	20% 11% 24% 24% 14% 7%	583
September	1st to 10th	Day	16% 11% 32% 23% 13% 5%	530
	11th to 20th	Day	17% 8% 22% 25% 19% 9%	530
	21st to end	Day	20% 11% 26% 22% 15% 5%	530
October	1st to 10th	Day	24% 14% 27% 19% 11% 4	530
	11th to 20th	Day	28% 17% 24% 20% 10% 2	480
	21st to end	Day	39% 15% 22% 18% 6%	462
November	1st to 10th	Day	33% 19% 26% 18% 4	425
	11th to 20th	Day	32% 18% 26% 15% 8% 1	414
	21st to end	Day	35% 22% 30% 11% 2	220
December	1st to 10th	Day	35% 17% 26% 16% 5% 1	220
	11th to 20th	Day	32% 23% 31% 10% 4	220
	21st to end	Day	34% 21% 24% 14% 5% 1	242
January	1st to 10th	Night	31% 14% 31% 20% 3	840
	11th to 20th	Night	28% 18% 30% 18% 5%	840
	21st to end	Night	23% 15% 30% 26% 6%	924
February	1st to 10th	Night	25% 16% 32% 22% 5%	840
	11th to 20th	Night	23% 18% 34% 21% 4	823
	21st to end	Night	25% 15% 32% 24% 3	692
March	1st to 10th	Night	27% 16% 28% 24% 5%	840
	11th to 20th	Night	28% 17% 27% 23% 6%	840
	21st to end	Night	27% 14% 27% 25% 6% 1	924
April	1st to 10th	Night	31% 14% 28% 22% 5%	840
	11th to 20th	Night	30% 14% 27% 22% 6%	824
	21st to end	Night	28% 14% 27% 22% 8%	630
May	1st to 10th	Night	29% 9% 30% 23% 8% 1	552
	11th to 20th	Night	24% 11% 28% 27% 9% 1	406
	21st to end	Night	22% 6% 30% 33% 8%	433
June	1st to 10th	Night	30% 7% 28% 26% 9% 1	400
	11th to 20th	Night	21% 8% 28% 31% 13%	400
	21st to end	Night	19% 8% 27% 33% 13% 1	400
July	1st to 10th	Night	15% 6% 28% 38% 12% 1	390
	11th to 20th	Night	15% 4 22% 41% 16% 1	388
	21st to end	Night	15% 5% 24% 41% 14%	438
August	1st to 10th	Night	20% 4 18% 38% 19% 1	398
	11th to 20th	Night	16% 5% 20% 35% 22% 2	584
	21st to end	Night	19% 5% 22% 34% 18% 2	880
September	1st to 10th	Night	19% 6% 24% 34% 15% 1	800
	11th to 20th	Night	17% 7% 20% 30% 24% 2	800
	21st to end	Night	22% 6% 28% 27% 16% 2	800
October	1st to 10th	Night	24% 10% 25% 27% 13% 1	800
	11th to 20th	Night	28% 11% 26% 25% 9% 1	800
	21st to end	Night	35% 11% 27% 21% 6%	880
November	1st to 10th	Night	30% 15% 29% 20% 6% 1	820
	11th to 20th	Night	28% 16% 26% 19% 10% 1	840
	21st to end	Night	32% 17% 28% 17% 5% 1	840
December	1st to 10th	Night	34% 17% 28% 17% 4	840
	11th to 20th	Night	32% 16% 28% 18% 5% 1	840
	21st to end	Night	36% 15% 26% 19% 5%	924

Page generated at 2026-05-14 01:58:59. Version ac07

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