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	17% 12% 4 10% 6% 5% 5% 4 5% 6% 26%	131745
	Night	15% 13% 5% 12% 7% 6% 5% 4 5% 5% 24%	131101
Transparency	Day	58% 4 11% 18% 9% 1	135239
	Night	61% 3 10% 16% 8% 1	135694
Seeing	Day	28% 16% 27% 18% 9% 3	16658
	Night	25% 14% 23% 27% 10% 1	25628

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	10% 12% 8% 20% 11% 10% 5% 3 4 4 13%	4661
	11th to 20th	Day	10% 14% 7% 18% 14% 7% 5% 4 4 4 13%	4650
	21st to end	Day	7% 13% 7% 14% 9% 9% 7% 4 4 6% 20%	5124
February	1st to 10th	Day	12% 15% 6% 15% 10% 6% 5% 3 4 4 21%	1806
	11th to 20th	Day	11% 13% 7% 18% 10% 6% 4 3 4 4 19%	2048
	21st to end	Day	13% 16% 5% 13% 8% 6% 5% 3 5% 5% 21%	1730
March	1st to 10th	Day	18% 17% 5% 14% 7% 6% 4 3 3 3 21%	4576
	11th to 20th	Day	14% 16% 5% 11% 5% 4 4 3 4 4 29%	4823
	21st to end	Day	22% 13% 6% 11% 5% 4 4 3 4 5% 25%	5996
April	1st to 10th	Day	21% 17% 4 8% 5% 4 3 2 4 5% 27%	2520
	11th to 20th	Day	17% 14% 4 8% 5% 4 5% 4 5% 4 30%	2530
	21st to end	Day	22% 12% 4 8% 5% 4 4 3 5% 5% 28%	2730
May	1st to 10th	Day	30% 14% 4 9% 4 4 4 3 4 4 19%	6016
	11th to 20th	Day	17% 11% 3 10% 6% 6% 6% 4 6% 6% 25%	6388
	21st to end	Day	18% 9% 3 7% 5% 5% 5% 5% 7% 8% 27%	6949
June	1st to 10th	Day	25% 9% 3 8% 5% 5% 4 4 6% 6% 25%	2800
	11th to 20th	Day	17% 7% 3 8% 5% 5% 5% 5% 7% 7% 32%	2800
	21st to end	Day	21% 9% 3 8% 5% 6% 6% 5% 6% 6% 25%	2800
July	1st to 10th	Day	14% 8% 3 7% 6% 6% 7% 5% 9% 11% 26%	6360
	11th to 20th	Day	13% 8% 3 7% 5% 6% 6% 5% 8% 10% 30%	6360
	21st to end	Day	15% 8% 2 7% 5% 4 4 4 6% 8% 36%	6926
August	1st to 10th	Day	15% 8% 4 9% 6% 6% 6% 4 6% 8% 27%	4466
	11th to 20th	Day	16% 9% 4 10% 5% 5% 6% 5% 7% 9% 24%	4175
	21st to end	Day	13% 8% 3 7% 5% 4 6% 4 6% 9% 35%	4318
September	1st to 10th	Day	14% 8% 2 7% 4 4 4 4 5% 7% 41%	2400
	11th to 20th	Day	15% 7% 3 5% 4 3 4 3 4 7% 44%	2229
	21st to end	Day	16% 8% 3 6% 5% 4 4 4 6% 8% 36%	2200
October	1st to 10th	Day	23% 10% 3 8% 4 4 4 3 5% 6% 31%	2200
	11th to 20th	Day	17% 13% 4 7% 5% 4 4 3 6% 7% 31%	1880
	21st to end	Day	21% 20% 3 9% 6% 3 3 3 4 3 25%	1980
November	1st to 10th	Day	15% 16% 4 9% 5% 4 4 4 5% 6% 28%	1851
	11th to 20th	Day	16% 16% 5% 12% 7% 4 4 4 5% 4 23%	1890
	21st to end	Day	19% 17% 6% 13% 4 4 3 4 4 5% 20%	1769
December	1st to 10th	Day	12% 21% 6% 22% 7% 5% 5% 3 3 3 13%	2680
	11th to 20th	Day	17% 17% 7% 13% 6% 5% 3 3 3 4 21%	3066
	21st to end	Day	19% 17% 6% 14% 4 4 4 3 3 3 21%	4048
January	1st to 10th	Night	8% 14% 7% 24% 12% 8% 6% 3 3 4 12%	8234
	11th to 20th	Night	9% 17% 7% 20% 14% 7% 4 4 4 3 10%	8235
	21st to end	Night	8% 13% 6% 14% 9% 9% 9% 5% 6% 5% 17%	9057
February	1st to 10th	Night	12% 14% 6% 15% 9% 8% 5% 4 4 5% 19%	2813
	11th to 20th	Night	9% 14% 6% 14% 10% 7% 6% 4 4 5% 19%	2458
	21st to end	Night	12% 14% 5% 12% 8% 6% 5% 3 4 4 26%	2076
March	1st to 10th	Night	18% 15% 5% 12% 7% 7% 6% 3 4 4 19%	5484
	11th to 20th	Night	15% 15% 5% 11% 5% 5% 5% 3 5% 5% 26%	5355
	21st to end	Night	17% 17% 4 13% 6% 5% 4 2 3 4 24%	5176
April	1st to 10th	Night	21% 13% 4 8% 5% 4 5% 3 5% 4 27%	2100
	11th to 20th	Night	18% 13% 4 9% 5% 5% 4 3 5% 5% 30%	2038
	21st to end	Night	21% 10% 3 8% 5% 4 3 3 5% 6% 31%	1890
May	1st to 10th	Night	28% 13% 4 9% 5% 4 3 3 4 5% 21%	4144
	11th to 20th	Night	17% 13% 4 8% 5% 4 4 4 5% 6% 29%	3681
	21st to end	Night	18% 9% 3 8% 5% 4 5% 4 5% 7% 32%	3926
June	1st to 10th	Night	24% 10% 3 8% 4 4 4 3 4 6% 30%	1600
	11th to 20th	Night	18% 6% 2 5% 5% 6% 5% 5% 6% 8% 34%	1600
	21st to end	Night	19% 10% 3 8% 4 5% 5% 5% 6% 6% 30%	1600
July	1st to 10th	Night	12% 7% 3 7% 6% 5% 7% 5% 7% 10% 30%	3600
	11th to 20th	Night	17% 8% 3 8% 4 4 4 4 6% 9% 33%	3600
	21st to end	Night	11% 7% 2 7% 5% 5% 5% 4 6% 7% 41%	3926
August	1st to 10th	Night	20% 10% 3 8% 5% 5% 4 4 5% 6% 30%	2652
	11th to 20th	Night	13% 9% 3 8% 6% 6% 6% 4 6% 8% 31%	2872
	21st to end	Night	13% 8% 3 8% 5% 5% 5% 3 6% 8% 37%	3576
September	1st to 10th	Night	15% 7% 2 5% 4 4 5% 4 5% 7% 43%	2000
	11th to 20th	Night	14% 7% 2 5% 3 4 4 2 4 7% 47%	2170
	21st to end	Night	17% 7% 3 6% 4 4 4 4 5% 8% 39%	2200
October	1st to 10th	Night	20% 9% 3 7% 5% 5% 5% 3 5% 7% 33%	2200
	11th to 20th	Night	16% 12% 3 9% 4 5% 4 4 5% 6% 30%	2515
	21st to end	Night	22% 15% 4 8% 4 5% 5% 3 4 5% 24%	2860
November	1st to 10th	Night	17% 14% 4 9% 5% 4 4 3 6% 5% 28%	2667
	11th to 20th	Night	16% 14% 6% 12% 6% 5% 3 4 4 5% 26%	2730
	21st to end	Night	17% 15% 7% 13% 7% 4 4 4 3 5% 20%	2848
December	1st to 10th	Night	13% 18% 7% 19% 10% 6% 5% 4 5% 4 10%	4720
	11th to 20th	Night	21% 17% 6% 13% 8% 6% 4 3 3 4 17%	5348
	21st to end	Night	18% 17% 6% 14% 6% 4 5% 3 3 4 20%	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% 2 8% 11% 2	4662
	11th to 20th	Day	72% 4 7% 10% 6% 1	4680
	21st to end	Day	65% 4 8% 13% 10% 1	5124
February	1st to 10th	Day	67% 4 7% 14% 7% 2	1806
	11th to 20th	Day	68% 3 8% 13% 5% 1	2057
	21st to end	Day	65% 3 7% 14% 9% 2	1730
March	1st to 10th	Day	68% 4 7% 8% 8% 4	4579
	11th to 20th	Day	60% 2 12% 15% 8% 3	4823
	21st to end	Day	62% 3 4 15% 14% 1	5996
April	1st to 10th	Day	62% 3 7% 17% 9% 2	2520
	11th to 20th	Day	56% 4 8% 19% 12% 1	2530
	21st to end	Day	59% 3 8% 18% 10% 3	2730
May	1st to 10th	Day	69% 3 8% 15% 4 1	6027
	11th to 20th	Day	58% 4 14% 16% 7% 1	6392
	21st to end	Day	51% 5% 13% 22% 8% 1	6949
June	1st to 10th	Day	58% 4 12% 19% 7%	2800
	11th to 20th	Day	49% 5% 11% 23% 11% 2	2800
	21st to end	Day	56% 6% 11% 19% 7% 1	2800
July	1st to 10th	Day	50% 3 18% 20% 8%	6290
	11th to 20th	Day	48% 4 15% 25% 8%	6285
	21st to end	Day	45% 4 12% 31% 9%	6728
August	1st to 10th	Day	55% 4 15% 20% 6%	4452
	11th to 20th	Day	54% 5% 15% 20% 5%	4214
	21st to end	Day	45% 4 16% 20% 14% 2	4318
September	1st to 10th	Day	41% 5% 12% 29% 13% 1	2400
	11th to 20th	Day	40% 4 11% 29% 16% 1	2229
	21st to end	Day	46% 5% 13% 22% 13% 1	2200
October	1st to 10th	Day	52% 3 10% 23% 11%	3590
	11th to 20th	Day	48% 4 12% 26% 10%	3044
	21st to end	Day	52% 3 13% 22% 10% 1	3180
November	1st to 10th	Day	55% 6% 12% 17% 10% 1	1851
	11th to 20th	Day	63% 4 9% 11% 10% 2	1890
	21st to end	Day	65% 5% 12% 14% 3	1769
December	1st to 10th	Day	77% 2 10% 8% 3	2680
	11th to 20th	Day	67% 4 5% 15% 10%	3066
	21st to end	Day	69% 3 5% 10% 10% 3	4048
January	1st to 10th	Night	78% 2 9% 9% 3	8234
	11th to 20th	Night	78% 3 7% 7% 4 1	8280
	21st to end	Night	67% 3 8% 13% 9% 1	9057
February	1st to 10th	Night	68% 4 8% 13% 7% 1	2813
	11th to 20th	Night	67% 3 8% 14% 7% 1	2460
	21st to end	Night	61% 3 9% 16% 9% 1	2076
March	1st to 10th	Night	68% 5% 7% 11% 8% 1	5494
	11th to 20th	Night	60% 2 14% 14% 8% 1	5356
	21st to end	Night	65% 2 4 14% 13% 1	5176
April	1st to 10th	Night	59% 2 9% 18% 10% 1	2100
	11th to 20th	Night	55% 4 8% 21% 11%	2038
	21st to end	Night	53% 4 10% 17% 13% 2	1890
May	1st to 10th	Night	67% 3 10% 14% 6% 1	4150
	11th to 20th	Night	56% 5% 12% 21% 6% 1	3682
	21st to end	Night	51% 3 15% 20% 9% 1	3927
June	1st to 10th	Night	55% 4 10% 20% 10% 1	1600
	11th to 20th	Night	46% 7% 15% 21% 10% 2	1600
	21st to end	Night	51% 8% 13% 20% 8% 1	1600
July	1st to 10th	Night	49% 3 19% 21% 7%	3560
	11th to 20th	Night	49% 4 13% 25% 8%	3552
	21st to end	Night	42% 4 15% 30% 8%	3809
August	1st to 10th	Night	54% 3 14% 22% 6%	2642
	11th to 20th	Night	52% 5% 16% 18% 9% 1	2899
	21st to end	Night	46% 4 15% 23% 12%	3576
September	1st to 10th	Night	40% 6% 14% 28% 11%	2000
	11th to 20th	Night	38% 4 12% 31% 14% 1	2170
	21st to end	Night	44% 4 15% 25% 12% 1	2200
October	1st to 10th	Night	54% 3 13% 20% 9% 1	3590
	11th to 20th	Night	51% 3 10% 23% 12%	4090
	21st to end	Night	53% 3 13% 21% 10%	4610
November	1st to 10th	Night	57% 4 12% 19% 7% 1	2667
	11th to 20th	Night	60% 3 10% 15% 9% 2	2730
	21st to end	Night	66% 4 12% 14% 5%	2848
December	1st to 10th	Night	79% 2 8% 9% 2	4720
	11th to 20th	Night	72% 3 5% 11% 9%	5348
	21st to end	Night	69% 2 6% 12% 7% 3	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	28% 20% 30% 19% 4	220
	11th to 20th	Day	35% 25% 25% 10% 5% 1	220
	21st to end	Day	29% 19% 30% 15% 7%	242
February	1st to 10th	Day	35% 24% 26% 11% 4	306
	11th to 20th	Day	29% 22% 31% 15% 3	536
	21st to end	Day	29% 22% 28% 17% 4	446
March	1st to 10th	Day	34% 20% 27% 17% 3	540
	11th to 20th	Day	32% 21% 28% 13% 5% 1	540
	21st to end	Day	37% 18% 24% 13% 6% 2	594
April	1st to 10th	Day	42% 15% 22% 14% 4 2	540
	11th to 20th	Day	36% 22% 21% 13% 6% 2	532
	21st to end	Day	37% 19% 23% 13% 7% 2	540
May	1st to 10th	Day	41% 16% 23% 13% 5% 2	543
	11th to 20th	Day	27% 15% 30% 19% 5% 3	551
	21st to end	Day	24% 17% 31% 19% 8% 2	573
June	1st to 10th	Day	32% 14% 29% 15% 8% 2	530
	11th to 20th	Day	24% 15% 30% 19% 10% 2	530
	21st to end	Day	26% 17% 29% 18% 8% 2	530
July	1st to 10th	Day	19% 11% 32% 26% 9% 2	525
	11th to 20th	Day	17% 11% 34% 24% 10% 3	524
	21st to end	Day	21% 11% 31% 24% 11% 2	576
August	1st to 10th	Day	17% 11% 26% 25% 18% 3	526
	11th to 20th	Day	20% 9% 28% 22% 14% 7%	519
	21st to end	Day	17% 15% 26% 22% 14% 6%	583
September	1st to 10th	Day	17% 13% 27% 21% 16% 7%	530
	11th to 20th	Day	19% 9% 23% 20% 19% 10%	530
	21st to end	Day	20% 13% 25% 22% 13% 7%	530
October	1st to 10th	Day	27% 13% 22% 18% 13% 6%	530
	11th to 20th	Day	27% 15% 24% 20% 12% 2	464
	21st to end	Day	35% 14% 24% 19% 7%	462
November	1st to 10th	Day	32% 16% 20% 23% 8%	425
	11th to 20th	Day	29% 19% 27% 18% 5% 2	430
	21st to end	Day	35% 18% 25% 16% 5% 1	309
December	1st to 10th	Day	33% 18% 26% 17% 6%	220
	11th to 20th	Day	41% 17% 28% 10% 4	220
	21st to end	Day	36% 16% 26% 16% 6%	242
January	1st to 10th	Night	25% 20% 27% 23% 4	840
	11th to 20th	Night	29% 18% 26% 22% 5%	840
	21st to end	Night	21% 20% 28% 27% 4 1	924
February	1st to 10th	Night	21% 19% 32% 24% 5%	840
	11th to 20th	Night	23% 20% 31% 22% 4	823
	21st to end	Night	24% 16% 30% 26% 4	692
March	1st to 10th	Night	28% 16% 29% 23% 4	840
	11th to 20th	Night	30% 16% 24% 22% 8%	840
	21st to end	Night	31% 15% 24% 23% 6% 1	924
April	1st to 10th	Night	32% 15% 24% 23% 6%	840
	11th to 20th	Night	28% 17% 25% 23% 6% 1	794
	21st to end	Night	30% 13% 28% 23% 7%	630
May	1st to 10th	Night	30% 13% 26% 23% 8% 1	630
	11th to 20th	Night	20% 11% 28% 30% 10% 1	438
	21st to end	Night	22% 10% 17% 37% 14%	433
June	1st to 10th	Night	29% 7% 20% 34% 10%	400
	11th to 20th	Night	20% 10% 18% 36% 17%	400
	21st to end	Night	21% 8% 21% 36% 14% 1	400
July	1st to 10th	Night	15% 7% 14% 46% 17% 1	390
	11th to 20th	Night	16% 5% 17% 39% 20% 2	388
	21st to end	Night	16% 3 16% 45% 19% 1	438
August	1st to 10th	Night	18% 3 11% 40% 26% 1	438
	11th to 20th	Night	18% 7% 17% 36% 21% 2	592
	21st to end	Night	19% 6% 17% 32% 23% 4	870
September	1st to 10th	Night	18% 6% 20% 34% 20% 2	800
	11th to 20th	Night	18% 8% 17% 28% 26% 4	800
	21st to end	Night	21% 9% 21% 27% 20% 3	800
October	1st to 10th	Night	27% 11% 17% 28% 16% 2	800
	11th to 20th	Night	24% 13% 22% 27% 13% 2	800
	21st to end	Night	31% 12% 24% 26% 7%	880
November	1st to 10th	Night	27% 17% 22% 26% 8% 1	820
	11th to 20th	Night	26% 16% 25% 23% 9% 1	840
	21st to end	Night	27% 20% 23% 23% 5% 2	840
December	1st to 10th	Night	31% 19% 25% 20% 5%	840
	11th to 20th	Night	30% 19% 24% 22% 5%	840
	21st to end	Night	32% 17% 24% 24% 4	924

Page generated at 2026-05-14 01:58:57. Version ac07

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