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 2025-01-22.

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%	105174
	Night	17% 13% 5% 11% 6% 5% 4 3 4 5% 25%	104213
Transparency	Day	57% 4 11% 18% 8% 1	108686
	Night	60% 4 11% 17% 8% 1	108790
Seeing	Day	27% 16% 26% 19% 9% 3	14963
	Night	25% 13% 23% 27% 10% 1	23954

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% 14% 7% 19% 8% 6% 4 3 4 4 16%	2597
	11th to 20th	Day	14% 13% 6% 19% 13% 5% 4 3 4 4 14%	2600
	21st to end	Day	12% 18% 9% 15% 6% 5% 4 4 3 4 19%	2780
February	1st to 10th	Day	13% 16% 6% 14% 10% 6% 5% 3 4 3 20%	1634
	11th to 20th	Day	11% 14% 8% 18% 9% 6% 4 3 4 4 18%	1848
	21st to end	Day	14% 16% 5% 13% 8% 5% 4 3 4 6% 21%	1570
March	1st to 10th	Day	20% 19% 5% 11% 6% 5% 4 3 3 3 21%	3116
	11th to 20th	Day	15% 19% 7% 13% 6% 4 4 2 3 3 24%	3303
	21st to end	Day	19% 13% 6% 11% 4 4 4 2 3 5% 29%	4091
April	1st to 10th	Day	22% 17% 4 8% 5% 4 3 2 3 4 27%	2280
	11th to 20th	Day	17% 13% 3 8% 5% 4 5% 4 5% 5% 31%	2286
	21st to end	Day	23% 13% 4 9% 5% 4 4 3 5% 5% 27%	2470
May	1st to 10th	Day	25% 13% 3 8% 5% 5% 4 3 5% 5% 25%	4106
	11th to 20th	Day	17% 10% 3 10% 7% 6% 6% 4 6% 6% 25%	4385
	21st to end	Day	13% 8% 3 7% 5% 6% 5% 5% 7% 9% 32%	4837
June	1st to 10th	Day	25% 9% 3 8% 5% 5% 4 4 6% 6% 25%	2660
	11th to 20th	Day	17% 7% 3 8% 5% 5% 5% 5% 6% 7% 32%	2660
	21st to end	Day	22% 9% 3 8% 5% 6% 6% 5% 6% 6% 25%	2660
July	1st to 10th	Day	13% 7% 2 7% 6% 6% 6% 5% 8% 11% 29%	4440
	11th to 20th	Day	12% 7% 3 7% 6% 6% 6% 6% 9% 10% 27%	4440
	21st to end	Day	17% 8% 3 7% 5% 5% 5% 4 6% 7% 32%	4814
August	1st to 10th	Day	15% 8% 4 10% 6% 6% 7% 4 6% 8% 26%	4329
	11th to 20th	Day	16% 9% 4 9% 5% 5% 7% 4 8% 9% 23%	4045
	21st to end	Day	13% 8% 3 7% 5% 4 6% 4 6% 9% 35%	4186
September	1st to 10th	Day	15% 8% 3 7% 4 5% 4 4 5% 7% 40%	2280
	11th to 20th	Day	16% 7% 3 5% 4 3 4 3 5% 7% 44%	2118
	21st to end	Day	17% 8% 3 6% 5% 5% 4 4 6% 8% 35%	2090
October	1st to 10th	Day	24% 11% 3 8% 4 4 4 3 5% 6% 28%	2090
	11th to 20th	Day	17% 13% 4 7% 5% 4 4 4 6% 6% 31%	1786
	21st to end	Day	22% 20% 3 9% 5% 3 3 3 3 3 25%	1881
November	1st to 10th	Day	15% 16% 4 8% 5% 4 4 4 5% 6% 29%	1761
	11th to 20th	Day	17% 16% 5% 11% 7% 4 4 4 5% 5% 23%	1800
	21st to end	Day	19% 16% 5% 12% 4 4 4 4 5% 5% 21%	1685
December	1st to 10th	Day	13% 21% 7% 22% 8% 5% 4 3 3 3 12%	2600
	11th to 20th	Day	17% 17% 8% 13% 6% 5% 3 3 3 4 21%	2986
	21st to end	Day	19% 18% 6% 14% 4 4 4 3 3 3 21%	3960
January	1st to 10th	Night	12% 17% 7% 22% 9% 5% 4 3 4 5% 13%	4569
	11th to 20th	Night	10% 17% 6% 19% 13% 6% 4 3 4 4 14%	4580
	21st to end	Night	14% 19% 8% 15% 7% 6% 4 3 4 4 17%	4899
February	1st to 10th	Night	13% 15% 6% 14% 10% 8% 5% 3 4 5% 18%	2546
	11th to 20th	Night	10% 14% 6% 14% 10% 7% 6% 4 4 4 20%	2218
	21st to end	Night	13% 14% 5% 12% 8% 6% 5% 3 4 5% 26%	1884
March	1st to 10th	Night	20% 17% 5% 8% 5% 6% 4 3 4 4 24%	3724
	11th to 20th	Night	13% 19% 7% 12% 5% 5% 5% 3 4 4 24%	3655
	21st to end	Night	18% 17% 4 12% 5% 4 3 2 3 4 27%	3540
April	1st to 10th	Night	21% 13% 4 8% 5% 4 5% 3 5% 4 28%	1900
	11th to 20th	Night	18% 13% 3 8% 5% 5% 4 3 5% 5% 31%	1842
	21st to end	Night	22% 10% 3 8% 5% 4 3 3 5% 6% 30%	1710
May	1st to 10th	Night	24% 12% 4 9% 5% 4 4 3 4 5% 26%	2834
	11th to 20th	Night	18% 11% 4 8% 6% 4 4 4 6% 6% 29%	2537
	21st to end	Night	16% 7% 3 7% 4 4 5% 4 6% 8% 37%	2738
June	1st to 10th	Night	24% 10% 3 7% 4 4 4 3 5% 6% 30%	1520
	11th to 20th	Night	18% 5% 2 5% 5% 6% 5% 5% 6% 8% 34%	1520
	21st to end	Night	20% 10% 3 8% 4 5% 5% 5% 6% 6% 29%	1520
July	1st to 10th	Night	12% 8% 3 8% 6% 5% 6% 5% 7% 9% 30%	2520
	11th to 20th	Night	18% 8% 2 7% 3 4 4 4 6% 10% 34%	2520
	21st to end	Night	14% 8% 2 7% 5% 5% 5% 4 6% 6% 36%	2739
August	1st to 10th	Night	21% 10% 3 8% 5% 5% 4 4 5% 6% 29%	2570
	11th to 20th	Night	14% 9% 3 9% 6% 6% 6% 4 6% 8% 30%	2782
	21st to end	Night	13% 8% 3 8% 5% 5% 5% 3 6% 8% 37%	3466
September	1st to 10th	Night	15% 6% 2 5% 4 4 5% 4 5% 7% 42%	1900
	11th to 20th	Night	15% 7% 2 6% 3 4 3 2 4 7% 47%	2061
	21st to end	Night	18% 7% 3 6% 4 4 4 4 5% 8% 39%	2090
October	1st to 10th	Night	21% 9% 3 7% 5% 5% 5% 3 5% 7% 30%	2090
	11th to 20th	Night	17% 12% 4 9% 4 6% 5% 4 5% 6% 29%	2389
	21st to end	Night	22% 15% 4 8% 4 5% 4 3 4 5% 24%	2717
November	1st to 10th	Night	17% 14% 4 9% 5% 4 4 4 6% 5% 29%	2537
	11th to 20th	Night	17% 14% 5% 11% 5% 5% 3 4 4 5% 27%	2600
	21st to end	Night	17% 15% 7% 13% 6% 4 4 4 3 5% 21%	2712
December	1st to 10th	Night	13% 18% 7% 19% 10% 6% 5% 4 4 4 9%	4580
	11th to 20th	Night	21% 17% 6% 13% 8% 5% 4 3 3 4 17%	5208
	21st to end	Night	18% 17% 6% 14% 5% 4 5% 3 3 4 20%	6996

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	72% 2 11% 12% 2	2598
	11th to 20th	Day	71% 6% 8% 10% 5% 1	2600
	21st to end	Day	70% 3 10% 11% 5% 1	2780
February	1st to 10th	Day	68% 4 7% 14% 6% 1	1634
	11th to 20th	Day	70% 4 8% 13% 5% 1	1857
	21st to end	Day	65% 3 8% 13% 8% 2	1570
March	1st to 10th	Day	66% 6% 8% 10% 9% 2	3119
	11th to 20th	Day	65% 2 8% 12% 9% 4	3303
	21st to end	Day	60% 3 5% 16% 15% 2	4091
April	1st to 10th	Day	62% 3 7% 16% 9% 2	2280
	11th to 20th	Day	54% 4 9% 20% 12% 1	2286
	21st to end	Day	60% 3 9% 17% 9% 3	2470
May	1st to 10th	Day	62% 3 11% 18% 6% 1	4120
	11th to 20th	Day	57% 5% 16% 15% 7% 1	4389
	21st to end	Day	45% 6% 14% 24% 10% 1	4837
June	1st to 10th	Day	57% 4 12% 19% 7%	2660
	11th to 20th	Day	49% 5% 11% 22% 11% 2	2660
	21st to end	Day	57% 6% 12% 18% 7% 1	2660
July	1st to 10th	Day	47% 4 16% 23% 9%	4370
	11th to 20th	Day	48% 5% 16% 27% 4	4365
	21st to end	Day	50% 4 11% 26% 9%	4661
August	1st to 10th	Day	56% 4 15% 19% 6%	4315
	11th to 20th	Day	55% 5% 16% 20% 5%	4084
	21st to end	Day	45% 5% 16% 20% 14% 2	4186
September	1st to 10th	Day	42% 5% 12% 28% 13% 1	2280
	11th to 20th	Day	40% 3 11% 30% 14% 1	2118
	21st to end	Day	46% 5% 13% 23% 12% 1	2090
October	1st to 10th	Day	53% 3 10% 22% 10%	3480
	11th to 20th	Day	48% 4 12% 25% 11%	2950
	21st to end	Day	51% 3 13% 22% 9% 1	3081
November	1st to 10th	Day	54% 6% 13% 17% 10% 1	1761
	11th to 20th	Day	63% 5% 9% 12% 10% 2	1800
	21st to end	Day	64% 5% 13% 14% 4	1685
December	1st to 10th	Day	78% 2 10% 7% 2	2600
	11th to 20th	Day	66% 4 5% 15% 10%	2986
	21st to end	Day	69% 4 5% 10% 10% 3	3960
January	1st to 10th	Night	74% 3 12% 8% 2	4569
	11th to 20th	Night	75% 3 10% 9% 3 1	4580
	21st to end	Night	72% 3 10% 11% 4 1	4899
February	1st to 10th	Night	68% 4 9% 12% 6% 1	2546
	11th to 20th	Night	67% 4 8% 14% 6% 1	2220
	21st to end	Night	61% 3 10% 16% 9% 1	1884
March	1st to 10th	Night	62% 5% 9% 15% 7% 2	3734
	11th to 20th	Night	64% 3 10% 14% 9% 1	3656
	21st to end	Night	63% 2 5% 15% 13% 2	3540
April	1st to 10th	Night	59% 3 9% 19% 10% 1	1900
	11th to 20th	Night	53% 5% 9% 22% 11%	1842
	21st to end	Night	54% 5% 11% 16% 13% 2	1710
May	1st to 10th	Night	60% 3 12% 17% 8% 1	2840
	11th to 20th	Night	54% 5% 14% 18% 8% 1	2538
	21st to end	Night	44% 5% 18% 20% 13% 1	2739
June	1st to 10th	Night	54% 4 11% 20% 9% 1	1520
	11th to 20th	Night	46% 6% 15% 20% 10% 2	1520
	21st to end	Night	52% 8% 13% 19% 8% 1	1520
July	1st to 10th	Night	50% 4 16% 20% 9%	2480
	11th to 20th	Night	46% 4 15% 26% 8%	2472
	21st to end	Night	47% 5% 12% 26% 9%	2651
August	1st to 10th	Night	56% 4 14% 21% 6%	2560
	11th to 20th	Night	52% 6% 17% 17% 9%	2809
	21st to end	Night	46% 4 15% 22% 12%	3466
September	1st to 10th	Night	40% 7% 15% 28% 11%	1900
	11th to 20th	Night	39% 4 13% 31% 13% 1	2061
	21st to end	Night	43% 4 15% 25% 11% 1	2090
October	1st to 10th	Night	55% 3 13% 20% 8% 1	3480
	11th to 20th	Night	51% 4 10% 23% 12%	3964
	21st to end	Night	52% 3 14% 21% 10%	4467
November	1st to 10th	Night	56% 4 13% 19% 7% 1	2537
	11th to 20th	Night	59% 4 10% 16% 10% 2	2600
	21st to end	Night	65% 4 12% 14% 5%	2712
December	1st to 10th	Night	79% 2 8% 9% 1	4580
	11th to 20th	Night	72% 3 5% 11% 9%	5208
	21st to end	Night	69% 2 6% 12% 7% 3	6996

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	29% 21% 29% 19% 4	200
	11th to 20th	Day	34% 24% 25% 11% 6% 1	200
	21st to end	Day	25% 19% 32% 16% 8% 1	200
February	1st to 10th	Day	37% 22% 26% 11% 4	254
	11th to 20th	Day	29% 24% 30% 14% 3	456
	21st to end	Day	30% 21% 27% 18% 4	382
March	1st to 10th	Day	33% 17% 27% 18% 3	460
	11th to 20th	Day	31% 21% 28% 14% 5% 1	460
	21st to end	Day	33% 19% 24% 15% 7% 2	506
April	1st to 10th	Day	41% 15% 22% 16% 5% 2	460
	11th to 20th	Day	32% 23% 21% 14% 7% 2	452
	21st to end	Day	37% 18% 24% 13% 7% 1	460
May	1st to 10th	Day	38% 16% 23% 14% 6% 2	463
	11th to 20th	Day	25% 16% 30% 20% 6% 3	487
	21st to end	Day	22% 17% 32% 19% 8% 2	529
June	1st to 10th	Day	32% 15% 29% 15% 8% 2	490
	11th to 20th	Day	22% 15% 30% 20% 10% 2	490
	21st to end	Day	26% 18% 28% 19% 8% 2	490
July	1st to 10th	Day	20% 10% 31% 27% 10% 2	485
	11th to 20th	Day	17% 10% 35% 24% 11% 4	484
	21st to end	Day	20% 11% 31% 24% 11% 2	532
August	1st to 10th	Day	18% 11% 27% 23% 18% 3	486
	11th to 20th	Day	21% 9% 28% 23% 14% 6%	479
	21st to end	Day	17% 15% 25% 23% 14% 6%	539
September	1st to 10th	Day	18% 13% 26% 20% 17% 6%	490
	11th to 20th	Day	20% 9% 23% 20% 18% 10%	490
	21st to end	Day	20% 12% 24% 23% 13% 7%	490
October	1st to 10th	Day	28% 13% 22% 19% 13% 6%	490
	11th to 20th	Day	27% 15% 24% 20% 12% 2	430
	21st to end	Day	36% 14% 22% 20% 8%	429
November	1st to 10th	Day	32% 16% 19% 25% 8%	395
	11th to 20th	Day	31% 19% 26% 19% 5% 2	400
	21st to end	Day	35% 18% 24% 16% 5% 1	285
December	1st to 10th	Day	34% 19% 24% 17% 7%	200
	11th to 20th	Day	42% 16% 29% 11% 4	200
	21st to end	Day	36% 15% 26% 16% 6%	220
January	1st to 10th	Night	25% 20% 28% 22% 4	800
	11th to 20th	Night	28% 18% 26% 23% 5%	800
	21st to end	Night	21% 20% 27% 27% 5%	841
February	1st to 10th	Night	20% 20% 32% 24% 4	760
	11th to 20th	Night	23% 20% 31% 22% 4	743
	21st to end	Night	25% 16% 29% 26% 4	628
March	1st to 10th	Night	27% 16% 28% 25% 4	760
	11th to 20th	Night	29% 16% 25% 23% 8%	760
	21st to end	Night	30% 15% 23% 24% 7% 1	836
April	1st to 10th	Night	31% 14% 24% 24% 7%	760
	11th to 20th	Night	27% 17% 25% 24% 7% 1	718
	21st to end	Night	31% 12% 27% 23% 7%	570
May	1st to 10th	Night	29% 13% 25% 24% 9% 1	570
	11th to 20th	Night	21% 11% 26% 30% 10% 1	403
	21st to end	Night	22% 10% 17% 37% 14%	411
June	1st to 10th	Night	28% 7% 20% 34% 11%	380
	11th to 20th	Night	20% 9% 18% 36% 18%	380
	21st to end	Night	22% 7% 20% 36% 14% 1	380
July	1st to 10th	Night	15% 7% 14% 45% 18% 1	370
	11th to 20th	Night	16% 5% 18% 39% 19% 2	368
	21st to end	Night	17% 3 16% 44% 20% 1	416
August	1st to 10th	Night	19% 3 11% 40% 26% 1	416
	11th to 20th	Night	18% 6% 17% 36% 20% 2	562
	21st to end	Night	19% 6% 17% 32% 23% 4	826
September	1st to 10th	Night	18% 6% 20% 34% 19% 2	760
	11th to 20th	Night	19% 8% 17% 28% 25% 4	760
	21st to end	Night	22% 9% 20% 26% 20% 3	760
October	1st to 10th	Night	28% 11% 17% 28% 16% 2	760
	11th to 20th	Night	24% 13% 21% 27% 13% 2	760
	21st to end	Night	32% 12% 24% 26% 7%	836
November	1st to 10th	Night	26% 17% 22% 26% 9% 1	780
	11th to 20th	Night	26% 16% 24% 24% 9% 1	800
	21st to end	Night	28% 20% 23% 23% 5% 2	800
December	1st to 10th	Night	32% 19% 25% 20% 5%	800
	11th to 20th	Night	30% 18% 24% 23% 5%	800
	21st to end	Night	32% 16% 24% 24% 4	880

Page generated at 2025-01-19 03:06:47. Version ac07

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