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	19% 12% 4 8% 5% 4 4 3 5% 6% 29%	105241
	Night	20% 14% 4 9% 5% 4 4 3 4 5% 28%	104141
Transparency	Day	54% 6% 13% 19% 7% 1	108715
	Night	57% 6% 11% 17% 8% 1	108756
Seeing	Day	26% 14% 28% 21% 9% 2	14813
	Night	26% 12% 27% 25% 9% 1	23846

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	19% 21% 8% 16% 7% 4 3 2 3 3 15%	2597
	11th to 20th	Day	15% 16% 8% 18% 8% 5% 4 4 3 4 15%	2600
	21st to end	Day	14% 21% 6% 12% 6% 7% 5% 3 4 6% 17%	2780
February	1st to 10th	Day	17% 19% 5% 12% 8% 6% 4 3 3 4 20%	1619
	11th to 20th	Day	14% 18% 7% 13% 6% 5% 4 3 4 5% 20%	1810
	21st to end	Day	17% 19% 5% 9% 7% 5% 5% 3 4 5% 22%	1570
March	1st to 10th	Day	21% 14% 5% 11% 6% 5% 4 3 4 5% 23%	3116
	11th to 20th	Day	17% 21% 5% 10% 5% 4 3 2 4 4 24%	3246
	21st to end	Day	18% 13% 6% 9% 4 4 3 2 4 4 34%	4166
April	1st to 10th	Day	24% 15% 3 8% 4 4 3 3 4 5% 26%	2280
	11th to 20th	Day	21% 12% 3 6% 4 4 3 3 4 5% 35%	2259
	21st to end	Day	26% 11% 3 6% 5% 4 3 3 4 6% 30%	2470
May	1st to 10th	Day	25% 10% 3 7% 5% 4 4 3 6% 6% 28%	4256
	11th to 20th	Day	20% 7% 3 7% 5% 5% 5% 5% 6% 7% 29%	4398
	21st to end	Day	16% 7% 3 7% 5% 5% 5% 4 6% 8% 34%	4837
June	1st to 10th	Day	27% 8% 2 5% 5% 4 4 4 5% 7% 29%	2660
	11th to 20th	Day	18% 6% 2 6% 4 4 5% 5% 6% 8% 36%	2660
	21st to end	Day	24% 8% 3 7% 5% 4 5% 3 5% 6% 30%	2660
July	1st to 10th	Day	15% 6% 2 6% 5% 5% 6% 5% 7% 8% 34%	4440
	11th to 20th	Day	16% 7% 3 7% 5% 5% 5% 5% 7% 8% 33%	4440
	21st to end	Day	15% 7% 2 6% 4 4 4 4 6% 8% 41%	4814
August	1st to 10th	Day	18% 7% 3 8% 5% 5% 5% 4 6% 10% 28%	4423
	11th to 20th	Day	18% 6% 2 7% 5% 5% 4 4 6% 8% 34%	4017
	21st to end	Day	19% 6% 2 6% 3 4 4 4 5% 7% 39%	4180
September	1st to 10th	Day	16% 7% 2 6% 4 4 4 4 5% 8% 41%	2280
	11th to 20th	Day	18% 6% 2 4 4 4 3 4 5% 6% 45%	2161
	21st to end	Day	20% 7% 2 4 4 3 4 4 6% 7% 38%	2090
October	1st to 10th	Day	26% 8% 2 6% 4 4 4 4 6% 7% 29%	2022
	11th to 20th	Day	24% 10% 3 7% 5% 4 5% 3 5% 7% 28%	1817
	21st to end	Day	28% 14% 3 6% 3 5% 4 2 3 5% 26%	1881
November	1st to 10th	Day	21% 13% 3 6% 5% 4 5% 3 4 4 31%	1761
	11th to 20th	Day	19% 17% 5% 9% 5% 4 4 3 4 5% 25%	1785
	21st to end	Day	22% 18% 5% 10% 5% 4 4 3 6% 4 19%	1600
December	1st to 10th	Day	19% 24% 7% 14% 6% 4 4 3 3 3 13%	2600
	11th to 20th	Day	20% 17% 7% 12% 6% 4 4 2 4 4 20%	2986
	21st to end	Day	21% 19% 5% 12% 5% 4 4 3 4 5% 18%	3960
January	1st to 10th	Night	18% 21% 8% 13% 5% 4 3 2 3 4 19%	4569
	11th to 20th	Night	13% 18% 7% 16% 8% 6% 5% 3 4 4 16%	4580
	21st to end	Night	19% 19% 6% 11% 7% 4 3 3 3 3 22%	4899
February	1st to 10th	Night	16% 19% 6% 13% 7% 5% 3 3 4 5% 19%	2556
	11th to 20th	Night	15% 17% 6% 12% 8% 5% 4 3 4 4 23%	2255
	21st to end	Night	16% 18% 5% 9% 6% 4 4 3 3 4 27%	1884
March	1st to 10th	Night	23% 12% 3 8% 4 3 4 2 4 5% 31%	3724
	11th to 20th	Night	19% 21% 5% 10% 5% 3 3 2 3 4 25%	3708
	21st to end	Night	16% 16% 5% 10% 5% 3 3 3 3 4 33%	3466
April	1st to 10th	Night	24% 13% 3 8% 4 4 4 4 4 5% 27%	1900
	11th to 20th	Night	25% 9% 3 7% 4 4 4 3 4 5% 33%	1869
	21st to end	Night	23% 11% 3 6% 4 4 3 3 4 5% 33%	1710
May	1st to 10th	Night	25% 9% 3 7% 5% 5% 4 3 5% 6% 28%	2712
	11th to 20th	Night	24% 7% 3 8% 4 4 4 4 4 5% 33%	2495
	21st to end	Night	19% 6% 2 6% 4 4 5% 3 5% 8% 37%	2738
June	1st to 10th	Night	27% 7% 3 5% 3 4 4 4 5% 8% 30%	1520
	11th to 20th	Night	20% 5% 2 6% 4 4 4 4 5% 8% 38%	1520
	21st to end	Night	20% 8% 2 8% 4 4 5% 4 5% 7% 32%	1520
July	1st to 10th	Night	15% 9% 4 7% 6% 4 5% 4 7% 6% 33%	2520
	11th to 20th	Night	17% 7% 2 5% 5% 4 4 4 6% 7% 38%	2520
	21st to end	Night	16% 7% 3 7% 4 4 5% 4 7% 6% 38%	2739
August	1st to 10th	Night	23% 8% 2 7% 3 5% 4 3 5% 6% 32%	2506
	11th to 20th	Night	17% 7% 3 8% 6% 4 5% 4 5% 6% 34%	2776
	21st to end	Night	19% 6% 2 5% 4 4 4 3 5% 7% 41%	3476
September	1st to 10th	Night	18% 7% 2 6% 5% 4 3 3 5% 6% 42%	1900
	11th to 20th	Night	16% 6% 2 5% 3 3 4 4 5% 6% 48%	2019
	21st to end	Night	20% 5% 2 4 5% 5% 4 3 4 7% 41%	2090
October	1st to 10th	Night	21% 8% 3 7% 4 4 4 3 4 7% 36%	2156
	11th to 20th	Night	23% 10% 3 8% 5% 4 4 3 4 5% 30%	2362
	21st to end	Night	27% 11% 3 7% 4 4 4 3 4 4 27%	2717
November	1st to 10th	Night	21% 14% 3 7% 4 4 5% 3 5% 5% 28%	2537
	11th to 20th	Night	21% 14% 4 9% 4 4 3 3 5% 5% 28%	2614
	21st to end	Night	23% 17% 5% 9% 4 4 3 3 4 4 24%	2800
December	1st to 10th	Night	18% 28% 8% 12% 6% 4 3 2 3 4 12%	4580
	11th to 20th	Night	23% 19% 5% 13% 6% 4 3 2 3 3 20%	5208
	21st to end	Night	22% 18% 5% 9% 5% 4 3 3 4 6% 21%	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% 6% 12% 8% 1	2598
	11th to 20th	Day	67% 9% 10% 10% 3	2600
	21st to end	Day	66% 10% 12% 8% 3	2780
February	1st to 10th	Day	69% 5% 9% 12% 4 1	1619
	11th to 20th	Day	67% 5% 13% 10% 5% 1	1819
	21st to end	Day	63% 5% 11% 13% 6% 1	1570
March	1st to 10th	Day	62% 6% 9% 16% 6% 1	3119
	11th to 20th	Day	63% 5% 11% 10% 9% 1	3246
	21st to end	Day	54% 3 9% 18% 14% 1	4166
April	1st to 10th	Day	61% 5% 9% 16% 8% 1	2280
	11th to 20th	Day	52% 5% 9% 20% 12% 1	2259
	21st to end	Day	56% 6% 9% 16% 10% 2	2470
May	1st to 10th	Day	54% 7% 12% 19% 8% 1	4270
	11th to 20th	Day	51% 9% 16% 15% 7% 1	4402
	21st to end	Day	44% 9% 13% 22% 11% 1	4837
June	1st to 10th	Day	53% 7% 12% 18% 9% 1	2660
	11th to 20th	Day	44% 7% 13% 22% 11% 2	2660
	21st to end	Day	55% 7% 10% 18% 9% 1	2660
July	1st to 10th	Day	44% 6% 15% 27% 7%	4370
	11th to 20th	Day	45% 7% 15% 28% 4	4365
	21st to end	Day	41% 7% 14% 31% 7%	4661
August	1st to 10th	Day	51% 7% 16% 20% 6%	4409
	11th to 20th	Day	47% 7% 20% 21% 6%	4057
	21st to end	Day	45% 5% 17% 19% 13% 1	4180
September	1st to 10th	Day	41% 8% 14% 26% 10% 1	2280
	11th to 20th	Day	39% 6% 13% 30% 12%	2161
	21st to end	Day	44% 7% 15% 24% 10%	2090
October	1st to 10th	Day	47% 6% 14% 27% 6%	3360
	11th to 20th	Day	50% 6% 15% 24% 4	2994
	21st to end	Day	51% 5% 17% 20% 7% 1	3081
November	1st to 10th	Day	55% 7% 12% 18% 7% 1	1761
	11th to 20th	Day	62% 6% 11% 13% 8% 1	1785
	21st to end	Day	67% 7% 13% 10% 3	1600
December	1st to 10th	Day	76% 5% 8% 9% 2	2600
	11th to 20th	Day	67% 5% 8% 15% 5%	2986
	21st to end	Day	69% 5% 9% 10% 7%	3960
January	1st to 10th	Night	69% 7% 13% 8% 3	4569
	11th to 20th	Night	70% 7% 11% 9% 3	4580
	21st to end	Night	68% 6% 11% 10% 5%	4899
February	1st to 10th	Night	68% 5% 9% 12% 5% 1	2556
	11th to 20th	Night	64% 5% 10% 14% 6%	2257
	21st to end	Night	62% 5% 8% 14% 10% 1	1884
March	1st to 10th	Night	56% 6% 10% 19% 8% 3	3734
	11th to 20th	Night	64% 5% 8% 12% 10% 1	3709
	21st to end	Night	55% 4 8% 20% 11% 2	3466
April	1st to 10th	Night	59% 5% 9% 16% 11% 1	1900
	11th to 20th	Night	54% 5% 9% 20% 12% 1	1869
	21st to end	Night	53% 5% 10% 18% 12% 2	1710
May	1st to 10th	Night	55% 8% 9% 18% 8% 2	2718
	11th to 20th	Night	49% 8% 13% 19% 9% 2	2496
	21st to end	Night	44% 8% 10% 20% 16% 1	2739
June	1st to 10th	Night	52% 7% 10% 21% 9% 1	1520
	11th to 20th	Night	44% 7% 12% 21% 13% 3	1520
	21st to end	Night	50% 7% 13% 19% 11% 1	1520
July	1st to 10th	Night	48% 7% 13% 20% 11%	2480
	11th to 20th	Night	43% 7% 16% 27% 7%	2472
	21st to end	Night	44% 7% 15% 23% 11%	2651
August	1st to 10th	Night	51% 6% 14% 23% 6%	2496
	11th to 20th	Night	50% 7% 17% 20% 7%	2802
	21st to end	Night	43% 6% 13% 23% 14% 1	3476
September	1st to 10th	Night	42% 7% 12% 26% 13% 1	1900
	11th to 20th	Night	37% 5% 16% 29% 12% 1	2019
	21st to end	Night	43% 7% 11% 25% 14% 1	2090
October	1st to 10th	Night	51% 5% 11% 24% 9%	3598
	11th to 20th	Night	50% 5% 11% 27% 8%	3924
	21st to end	Night	53% 5% 12% 18% 10% 2	4467
November	1st to 10th	Night	57% 5% 12% 18% 7% 1	2537
	11th to 20th	Night	58% 6% 10% 15% 9% 2	2614
	21st to end	Night	64% 5% 12% 14% 4	2800
December	1st to 10th	Night	79% 4 8% 7% 2	4580
	11th to 20th	Night	72% 4 6% 11% 7%	5208
	21st to end	Night	66% 5% 9% 13% 8%	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	32% 18% 28% 16% 7%	200
	11th to 20th	Day	28% 24% 32% 13% 4 1	200
	21st to end	Day	23% 18% 32% 21% 7%	200
February	1st to 10th	Day	31% 28% 24% 15% 2	227
	11th to 20th	Day	27% 23% 33% 15% 2	418
	21st to end	Day	26% 23% 28% 21% 2	382
March	1st to 10th	Day	30% 15% 28% 22% 5%	460
	11th to 20th	Day	27% 19% 31% 17% 4 1	460
	21st to end	Day	30% 19% 24% 19% 7% 1	506
April	1st to 10th	Day	33% 14% 29% 16% 7% 1	460
	11th to 20th	Day	33% 17% 25% 16% 6% 3	452
	21st to end	Day	32% 14% 30% 13% 8% 2	460
May	1st to 10th	Day	31% 13% 27% 20% 7% 2	463
	11th to 20th	Day	22% 14% 28% 23% 11% 3	487
	21st to end	Day	21% 15% 33% 22% 8% 2	529
June	1st to 10th	Day	28% 12% 30% 20% 8% 2	490
	11th to 20th	Day	20% 12% 31% 26% 11% 1	490
	21st to end	Day	28% 11% 30% 19% 9% 3	490
July	1st to 10th	Day	17% 8% 32% 27% 14% 2	485
	11th to 20th	Day	19% 8% 31% 30% 8% 4	484
	21st to end	Day	20% 8% 30% 30% 11% 2	532
August	1st to 10th	Day	18% 8% 28% 29% 14% 3	486
	11th to 20th	Day	18% 6% 26% 28% 16% 4	479
	21st to end	Day	19% 11% 24% 24% 14% 7%	539
September	1st to 10th	Day	17% 11% 32% 23% 12% 6%	490
	11th to 20th	Day	18% 8% 22% 24% 19% 10%	490
	21st to end	Day	19% 10% 26% 23% 16% 6%	490
October	1st to 10th	Day	26% 13% 27% 18% 11% 4	490
	11th to 20th	Day	29% 16% 24% 20% 10% 2	445
	21st to end	Day	38% 15% 22% 19% 6%	429
November	1st to 10th	Day	31% 19% 27% 19% 5%	395
	11th to 20th	Day	30% 19% 26% 15% 8% 1	385
	21st to end	Day	33% 23% 31% 12% 3	200
December	1st to 10th	Day	35% 16% 26% 16% 6% 2	200
	11th to 20th	Day	32% 23% 31% 11% 4 1	200
	21st to end	Day	35% 20% 25% 14% 5% 1	220
January	1st to 10th	Night	31% 14% 31% 20% 4	800
	11th to 20th	Night	27% 18% 31% 18% 6%	800
	21st to end	Night	23% 14% 30% 27% 6%	841
February	1st to 10th	Night	27% 16% 33% 21% 4	760
	11th to 20th	Night	23% 18% 33% 22% 4	743
	21st to end	Night	25% 17% 31% 25% 3	628
March	1st to 10th	Night	26% 15% 29% 25% 5%	760
	11th to 20th	Night	27% 16% 27% 23% 6%	760
	21st to end	Night	26% 14% 26% 25% 7% 1	836
April	1st to 10th	Night	30% 14% 27% 23% 5%	760
	11th to 20th	Night	29% 15% 27% 23% 7%	745
	21st to end	Night	28% 15% 26% 23% 8%	570
May	1st to 10th	Night	29% 9% 30% 23% 8% 1	500
	11th to 20th	Night	24% 11% 28% 27% 9% 1	374
	21st to end	Night	21% 6% 31% 33% 8%	411
June	1st to 10th	Night	29% 7% 27% 27% 10% 1	380
	11th to 20th	Night	21% 7% 27% 31% 14%	380
	21st to end	Night	19% 8% 26% 33% 13% 1	380
July	1st to 10th	Night	15% 6% 28% 38% 12% 1	370
	11th to 20th	Night	15% 4 22% 41% 17% 1	368
	21st to end	Night	15% 5% 24% 41% 15%	416
August	1st to 10th	Night	21% 4 18% 37% 18% 1	378
	11th to 20th	Night	17% 6% 20% 34% 22% 2	554
	21st to end	Night	19% 5% 22% 33% 19% 2	836
September	1st to 10th	Night	20% 6% 24% 34% 15% 1	760
	11th to 20th	Night	18% 7% 20% 30% 23% 2	760
	21st to end	Night	21% 6% 28% 27% 16% 2	760
October	1st to 10th	Night	24% 10% 25% 27% 13% 1	760
	11th to 20th	Night	29% 11% 26% 24% 9% 1	760
	21st to end	Night	35% 12% 27% 21% 6%	836
November	1st to 10th	Night	29% 15% 29% 20% 6% 1	780
	11th to 20th	Night	27% 17% 26% 19% 10% 1	800
	21st to end	Night	32% 17% 27% 18% 6% 1	800
December	1st to 10th	Night	34% 16% 28% 17% 4	800
	11th to 20th	Night	32% 16% 28% 18% 6% 1	800
	21st to end	Night	36% 15% 26% 19% 5%	880

Page generated at 2025-01-19 03:06:49. Version ac07

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