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 2024-01-22.

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	21% 11% 4 7% 6% 3 4 4 5% 6% 30%	74843
	Night	21% 12% 4 8% 6% 3 4 4 4 5% 29%	75087
Transparency	Day	55% 6% 12% 18% 8% 1	74548
	Night	57% 6% 11% 17% 8% 1	74916
Seeing	Day	26% 14% 28% 21% 9% 2	13547
	Night	26% 12% 27% 25% 9% 1	22604

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	22% 22% 7% 12% 7% 4 3 3 3 4 14%	1517
	11th to 20th	Day	20% 18% 7% 14% 6% 3 5% 5% 3 4 16%	1520
	21st to end	Day	12% 15% 5% 11% 8% 5% 7% 5% 4 6% 22%	1592
February	1st to 10th	Day	17% 18% 6% 12% 7% 5% 5% 3 3 4 19%	1534
	11th to 20th	Day	15% 17% 7% 12% 7% 4 5% 4 5% 5% 20%	1712
	21st to end	Day	17% 17% 7% 8% 8% 4 4 4 4 5% 22%	1480
March	1st to 10th	Day	18% 14% 6% 10% 7% 4 5% 4 4 5% 25%	1797
	11th to 20th	Day	19% 13% 5% 8% 6% 3 3 3 4 5% 30%	1844
	21st to end	Day	20% 13% 5% 7% 6% 3 4 3 4 4 31%	2376
April	1st to 10th	Day	24% 14% 4 7% 5% 3 4 4 4 5% 26%	2160
	11th to 20th	Day	22% 11% 3 5% 4 2 4 3 4 5% 36%	2138
	21st to end	Day	26% 10% 3 5% 6% 3 4 4 4 6% 29%	2340
May	1st to 10th	Day	23% 8% 3 6% 5% 3 3 4 5% 6% 33%	2420
	11th to 20th	Day	20% 7% 3 6% 5% 4 4 4 5% 6% 35%	2478
	21st to end	Day	20% 7% 3 5% 6% 3 5% 5% 6% 8% 33%	2725
June	1st to 10th	Day	27% 7% 2 5% 5% 3 4 5% 5% 7% 29%	2520
	11th to 20th	Day	19% 6% 2 5% 5% 3 5% 5% 6% 8% 37%	2520
	21st to end	Day	24% 7% 3 7% 6% 2 5% 4 5% 7% 30%	2520
July	1st to 10th	Day	17% 5% 2 5% 6% 3 6% 5% 6% 8% 38%	2520
	11th to 20th	Day	16% 5% 2 5% 6% 3 5% 6% 7% 10% 34%	2520
	21st to end	Day	17% 7% 3 5% 6% 3 5% 5% 6% 9% 34%	2744
August	1st to 10th	Day	17% 6% 2 6% 7% 4 5% 6% 6% 10% 32%	2506
	11th to 20th	Day	16% 5% 2 5% 6% 4 5% 6% 7% 8% 38%	2291
	21st to end	Day	17% 5% 3 6% 5% 3 4 5% 6% 8% 38%	2376
September	1st to 10th	Day	17% 6% 2 5% 5% 3 4 5% 5% 8% 40%	2160
	11th to 20th	Day	19% 6% 2 3 4 3 3 4 5% 6% 45%	2048
	21st to end	Day	20% 6% 3 4 5% 3 4 4 6% 8% 38%	1980
October	1st to 10th	Day	27% 8% 2 5% 5% 3 4 5% 6% 7% 28%	1916
	11th to 20th	Day	24% 9% 3 5% 6% 3 4 5% 5% 7% 27%	1722
	21st to end	Day	29% 14% 4 6% 4 3 4 3 3 4 25%	1782
November	1st to 10th	Day	21% 13% 4 5% 5% 3 5% 5% 4 4 31%	1671
	11th to 20th	Day	20% 16% 6% 8% 6% 3 4 3 4 5% 24%	1696
	21st to end	Day	22% 17% 5% 8% 6% 2 5% 4 6% 5% 20%	1520
December	1st to 10th	Day	24% 20% 6% 12% 7% 2 4 4 3 4 15%	1520
	11th to 20th	Day	23% 17% 6% 9% 6% 2 5% 3 4 5% 20%	1906
	21st to end	Day	26% 17% 6% 11% 6% 3 4 4 3 3 16%	2772
January	1st to 10th	Night	21% 19% 8% 11% 7% 3 4 3 3 4 17%	2649
	11th to 20th	Night	17% 18% 7% 12% 7% 4 5% 3 4 4 19%	2660
	21st to end	Night	16% 14% 6% 10% 9% 4 4 4 5% 5% 24%	2798
February	1st to 10th	Night	17% 17% 7% 13% 8% 4 4 4 4 5% 18%	2421
	11th to 20th	Night	16% 15% 6% 10% 10% 3 5% 3 4 4 24%	2133
	21st to end	Night	16% 16% 7% 8% 7% 3 4 3 3 4 27%	1776
March	1st to 10th	Night	17% 16% 5% 9% 7% 3 4 3 3 5% 26%	2150
	11th to 20th	Night	21% 14% 5% 8% 6% 3 4 3 4 5% 29%	2110
	21st to end	Night	19% 14% 5% 6% 5% 2 4 4 4 5% 34%	1980
April	1st to 10th	Night	25% 12% 4 8% 4 3 4 4 4 5% 27%	1800
	11th to 20th	Night	24% 9% 3 6% 5% 2 4 3 4 5% 34%	1770
	21st to end	Night	24% 10% 3 5% 5% 3 3 4 4 5% 33%	1620
May	1st to 10th	Night	23% 10% 3 6% 5% 3 4 4 5% 6% 32%	1554
	11th to 20th	Night	21% 6% 4 7% 4 3 4 5% 4 6% 36%	1415
	21st to end	Night	20% 6% 2 5% 5% 3 5% 4 5% 7% 38%	1550
June	1st to 10th	Night	27% 6% 3 4 5% 3 5% 5% 5% 8% 30%	1440
	11th to 20th	Night	20% 5% 2 5% 5% 3 4 5% 5% 9% 38%	1440
	21st to end	Night	18% 7% 3 7% 5% 4 5% 5% 6% 8% 33%	1440
July	1st to 10th	Night	15% 6% 3 5% 6% 3 5% 5% 8% 7% 38%	1440
	11th to 20th	Night	13% 5% 2 4 6% 4 5% 7% 8% 8% 39%	1440
	21st to end	Night	15% 5% 3 6% 5% 3 6% 6% 7% 6% 38%	1574
August	1st to 10th	Night	17% 5% 2 6% 4 4 4 4 7% 8% 38%	1432
	11th to 20th	Night	15% 5% 3 7% 6% 3 5% 5% 5% 7% 39%	1588
	21st to end	Night	16% 7% 2 5% 5% 3 5% 4 5% 7% 41%	1980
September	1st to 10th	Night	18% 7% 2 5% 5% 3 3 4 5% 6% 42%	1800
	11th to 20th	Night	17% 6% 2 4 4 2 3 5% 4 6% 47%	1912
	21st to end	Night	21% 5% 3 4 4 3 4 3 4 7% 41%	1980
October	1st to 10th	Night	20% 7% 3 6% 5% 3 4 4 5% 7% 36%	2042
	11th to 20th	Night	24% 9% 4 7% 6% 3 5% 4 4 5% 29%	2237
	21st to end	Night	29% 11% 3 6% 5% 3 4 4 4 5% 26%	2574
November	1st to 10th	Night	22% 12% 4 6% 5% 3 5% 4 5% 6% 29%	2407
	11th to 20th	Night	22% 13% 4 8% 5% 3 4 4 4 5% 27%	2483
	21st to end	Night	23% 16% 5% 9% 5% 3 3 3 4 5% 24%	2660
December	1st to 10th	Night	25% 21% 6% 8% 7% 3 4 3 3 4 15%	2660
	11th to 20th	Night	24% 18% 6% 10% 7% 3 3 3 3 3 19%	3288
	21st to end	Night	27% 18% 6% 9% 6% 3 3 3 3 3 19%	4884

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	75% 5% 10% 9% 2	1518
	11th to 20th	Day	71% 5% 10% 9% 5%	1520
	21st to end	Day	62% 7% 13% 12% 6% 1	1592
February	1st to 10th	Day	70% 5% 9% 12% 4 1	1534
	11th to 20th	Day	66% 5% 13% 10% 5% 1	1721
	21st to end	Day	65% 5% 10% 13% 7% 1	1480
March	1st to 10th	Day	62% 5% 11% 14% 6% 2	1800
	11th to 20th	Day	57% 5% 12% 14% 11% 1	1844
	21st to end	Day	58% 3 10% 19% 9% 2	2376
April	1st to 10th	Day	61% 5% 10% 16% 8% 1	2160
	11th to 20th	Day	52% 5% 9% 20% 12% 1	2138
	21st to end	Day	57% 6% 9% 16% 10% 2	2340
May	1st to 10th	Day	52% 4 9% 19% 13% 2	2420
	11th to 20th	Day	49% 5% 11% 19% 13% 2	2482
	21st to end	Day	49% 8% 12% 18% 12% 1	2725
June	1st to 10th	Day	53% 7% 12% 18% 9% 1	2520
	11th to 20th	Day	44% 7% 12% 23% 11% 2	2520
	21st to end	Day	55% 7% 10% 18% 9% 1	2520
July	1st to 10th	Day	41% 8% 14% 26% 9%	2450
	11th to 20th	Day	43% 8% 15% 27% 6%	2445
	21st to end	Day	45% 9% 16% 23% 7%	2591
August	1st to 10th	Day	47% 9% 17% 23% 5%	2492
	11th to 20th	Day	43% 9% 17% 21% 10%	2291
	21st to end	Day	43% 8% 14% 24% 11%	2376
September	1st to 10th	Day	42% 8% 14% 25% 11% 1	2160
	11th to 20th	Day	39% 6% 12% 30% 12%	2048
	21st to end	Day	43% 7% 15% 24% 10% 1	1980
October	1st to 10th	Day	54% 7% 13% 18% 8%	1916
	11th to 20th	Day	55% 8% 15% 17% 5%	1722
	21st to end	Day	64% 5% 10% 14% 5% 1	1782
November	1st to 10th	Day	55% 7% 12% 17% 7% 1	1671
	11th to 20th	Day	63% 6% 11% 12% 8% 1	1696
	21st to end	Day	66% 7% 14% 10% 3	1520
December	1st to 10th	Day	74% 6% 10% 8% 2	1520
	11th to 20th	Day	67% 5% 10% 13% 4	1906
	21st to end	Day	73% 6% 11% 7% 3	2772
January	1st to 10th	Night	73% 5% 10% 9% 3	2649
	11th to 20th	Night	71% 5% 9% 10% 5% 1	2660
	21st to end	Night	63% 5% 11% 12% 8% 1	2798
February	1st to 10th	Night	70% 5% 8% 11% 5% 1	2421
	11th to 20th	Night	64% 5% 10% 14% 6%	2135
	21st to end	Night	63% 4 8% 13% 11% 1	1776
March	1st to 10th	Night	61% 4 10% 16% 8% 1	2160
	11th to 20th	Night	59% 4 10% 14% 10% 2	2111
	21st to end	Night	54% 4 7% 20% 11% 3	1980
April	1st to 10th	Night	59% 5% 9% 15% 10% 1	1800
	11th to 20th	Night	53% 5% 9% 20% 13% 1	1770
	21st to end	Night	54% 5% 9% 18% 12% 3	1620
May	1st to 10th	Night	54% 6% 6% 17% 14% 3	1554
	11th to 20th	Night	49% 6% 9% 18% 15% 3	1416
	21st to end	Night	45% 8% 10% 21% 14% 2	1551
June	1st to 10th	Night	52% 7% 10% 21% 9% 1	1440
	11th to 20th	Night	44% 8% 11% 21% 13% 3	1440
	21st to end	Night	48% 8% 13% 20% 11%	1440
July	1st to 10th	Night	41% 10% 13% 23% 13% 1	1400
	11th to 20th	Night	38% 10% 14% 28% 9%	1392
	21st to end	Night	41% 10% 14% 25% 9%	1486
August	1st to 10th	Night	41% 9% 19% 24% 8%	1422
	11th to 20th	Night	43% 8% 16% 23% 11%	1588
	21st to end	Night	41% 9% 14% 24% 11% 1	1980
September	1st to 10th	Night	42% 7% 12% 26% 13% 1	1800
	11th to 20th	Night	38% 6% 15% 29% 11% 1	1912
	21st to end	Night	43% 7% 11% 25% 13% 1	1980
October	1st to 10th	Night	48% 7% 11% 21% 11%	2042
	11th to 20th	Night	57% 6% 11% 20% 6%	2237
	21st to end	Night	61% 6% 10% 14% 8% 1	2574
November	1st to 10th	Night	57% 5% 12% 18% 7% 1	2407
	11th to 20th	Night	59% 6% 11% 14% 8% 2	2483
	21st to end	Night	64% 5% 12% 14% 5%	2660
December	1st to 10th	Night	74% 4 9% 10% 2	2660
	11th to 20th	Night	71% 5% 8% 11% 4	3288
	21st to end	Night	71% 5% 9% 12% 2	4884

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% 16% 27% 17% 8%	180
	11th to 20th	Day	29% 22% 31% 13% 4 1	180
	21st to end	Day	22% 19% 33% 19% 7%	178
February	1st to 10th	Day	33% 30% 23% 12% 2	202
	11th to 20th	Day	28% 22% 33% 14% 3	380
	21st to end	Day	26% 23% 27% 22% 3	346
March	1st to 10th	Day	30% 15% 29% 21% 5%	420
	11th to 20th	Day	27% 19% 31% 17% 5% 1	420
	21st to end	Day	30% 18% 24% 19% 8% 1	462
April	1st to 10th	Day	33% 13% 30% 16% 7% 1	420
	11th to 20th	Day	33% 16% 24% 17% 7% 3	412
	21st to end	Day	32% 13% 31% 14% 8% 2	420
May	1st to 10th	Day	31% 12% 27% 19% 7% 2	423
	11th to 20th	Day	22% 15% 28% 22% 10% 2	447
	21st to end	Day	21% 15% 33% 21% 8% 2	485
June	1st to 10th	Day	29% 12% 30% 20% 7% 2	450
	11th to 20th	Day	20% 12% 30% 26% 12% 1	450
	21st to end	Day	26% 11% 30% 20% 9% 3	450
July	1st to 10th	Day	18% 7% 30% 27% 15% 2	445
	11th to 20th	Day	18% 8% 31% 31% 9% 4	444
	21st to end	Day	20% 9% 30% 30% 11% 2	488
August	1st to 10th	Day	18% 8% 28% 28% 15% 3	446
	11th to 20th	Day	17% 7% 26% 29% 16% 5%	439
	21st to end	Day	19% 12% 23% 24% 15% 7%	495
September	1st to 10th	Day	16% 10% 33% 23% 12% 6%	450
	11th to 20th	Day	19% 9% 24% 23% 17% 8%	450
	21st to end	Day	19% 11% 26% 23% 15% 6%	450
October	1st to 10th	Day	26% 14% 26% 18% 11% 4	450
	11th to 20th	Day	28% 17% 24% 19% 11% 2	410
	21st to end	Day	40% 14% 22% 19% 6%	396
November	1st to 10th	Day	30% 18% 27% 19% 5%	365
	11th to 20th	Day	30% 20% 25% 15% 8% 1	356
	21st to end	Day	31% 22% 32% 13% 3	180
December	1st to 10th	Day	35% 17% 23% 17% 6% 2	180
	11th to 20th	Day	31% 23% 31% 11% 3 1	180
	21st to end	Day	37% 22% 23% 13% 5% 1	198
January	1st to 10th	Night	32% 14% 29% 21% 4	760
	11th to 20th	Night	27% 19% 31% 18% 6%	760
	21st to end	Night	23% 14% 30% 27% 6% 1	800
February	1st to 10th	Night	27% 16% 33% 20% 4	720
	11th to 20th	Night	24% 18% 32% 22% 4	703
	21st to end	Night	25% 16% 31% 26% 3	592
March	1st to 10th	Night	26% 15% 28% 26% 5%	720
	11th to 20th	Night	28% 15% 27% 23% 6%	720
	21st to end	Night	26% 13% 26% 26% 8% 1	792
April	1st to 10th	Night	30% 14% 27% 23% 5%	720
	11th to 20th	Night	28% 14% 26% 24% 7%	706
	21st to end	Night	29% 13% 26% 23% 8%	540
May	1st to 10th	Night	29% 9% 30% 23% 9% 1	474
	11th to 20th	Night	24% 10% 28% 27% 9% 1	354
	21st to end	Night	22% 6% 31% 33% 8% 1	389
June	1st to 10th	Night	29% 7% 28% 26% 10% 1	360
	11th to 20th	Night	21% 8% 27% 31% 14%	360
	21st to end	Night	17% 8% 27% 34% 13% 1	360
July	1st to 10th	Night	15% 6% 29% 37% 12% 1	350
	11th to 20th	Night	14% 5% 22% 41% 18% 1	348
	21st to end	Night	14% 5% 25% 40% 15% 1	394
August	1st to 10th	Night	20% 4 18% 37% 19% 1	358
	11th to 20th	Night	17% 6% 20% 34% 22% 2	524
	21st to end	Night	19% 5% 22% 33% 19% 2	792
September	1st to 10th	Night	20% 6% 23% 35% 15% 1	720
	11th to 20th	Night	19% 7% 21% 30% 21% 2	720
	21st to end	Night	22% 7% 28% 26% 15% 2	720
October	1st to 10th	Night	24% 10% 25% 27% 13% 1	720
	11th to 20th	Night	28% 11% 25% 24% 9% 2	720
	21st to end	Night	35% 11% 27% 21% 5%	792
November	1st to 10th	Night	29% 14% 29% 20% 6% 1	740
	11th to 20th	Night	27% 18% 26% 18% 10% 1	760
	21st to end	Night	32% 16% 26% 19% 6% 1	760
December	1st to 10th	Night	36% 16% 27% 17% 4	760
	11th to 20th	Night	31% 16% 28% 18% 6% 1	760
	21st to end	Night	36% 15% 27% 18% 4	836

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