Green Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Green Mountain Observatory clear sky chart from 2005-11-06 to 2025-02-21.

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 9% 5% 5% 5% 4 5% 6% 29%	108527
	Night	18% 14% 5% 11% 6% 5% 4 3 4 5% 26%	110060
Transparency	Day	53% 7% 12% 19% 8% 1	112056
	Night	59% 6% 11% 16% 8% 1	114695
Seeing	Day	26% 18% 29% 18% 7% 2	14981
	Night	25% 15% 27% 23% 8% 1	24096

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	13% 17% 8% 20% 9% 5% 4 2 4 4 15%	3597
	11th to 20th	Day	11% 15% 6% 19% 9% 5% 5% 3 4 4 20%	3570
	21st to end	Day	12% 17% 8% 12% 6% 6% 5% 3 4 5% 21%	3960
February	1st to 10th	Day	14% 15% 6% 14% 8% 5% 4 4 6% 5% 20%	1760
	11th to 20th	Day	12% 17% 5% 15% 8% 5% 4 3 5% 5% 21%	1918
	21st to end	Day	14% 16% 5% 12% 7% 6% 4 2 4 4 25%	1570
March	1st to 10th	Day	23% 18% 5% 10% 5% 4 4 3 4 4 19%	3116
	11th to 20th	Day	14% 22% 6% 11% 6% 5% 4 3 3 4 22%	3265
	21st to end	Day	18% 13% 5% 11% 5% 3 4 3 3 4 31%	4166
April	1st to 10th	Day	22% 15% 3 8% 5% 3 3 3 4 6% 29%	2280
	11th to 20th	Day	19% 12% 3 7% 4 4 4 2 5% 6% 33%	2254
	21st to end	Day	24% 12% 3 7% 4 3 4 2 4 6% 30%	2437
May	1st to 10th	Day	25% 12% 3 8% 5% 5% 4 4 4 5% 27%	4221
	11th to 20th	Day	17% 9% 3 8% 6% 5% 6% 4 5% 8% 29%	4398
	21st to end	Day	14% 7% 3 6% 5% 4 5% 4 7% 9% 36%	4837
June	1st to 10th	Day	25% 7% 3 6% 5% 5% 5% 4 6% 8% 27%	2660
	11th to 20th	Day	18% 6% 2 7% 5% 5% 5% 5% 6% 8% 34%	2660
	21st to end	Day	21% 8% 3 7% 6% 4 5% 4 7% 7% 28%	2660
July	1st to 10th	Day	13% 6% 2 7% 5% 5% 5% 4 8% 9% 35%	4440
	11th to 20th	Day	12% 6% 3 6% 5% 5% 6% 5% 7% 10% 34%	4440
	21st to end	Day	13% 7% 2 7% 5% 5% 5% 5% 6% 8% 36%	4814
August	1st to 10th	Day	17% 8% 3 7% 5% 4 5% 4 6% 8% 32%	4423
	11th to 20th	Day	16% 8% 3 8% 5% 4 5% 4 5% 8% 33%	3897
	21st to end	Day	14% 7% 2 5% 4 4 4 4 6% 8% 41%	4180
September	1st to 10th	Day	17% 6% 1 5% 4 3 3 3 5% 7% 45%	2280
	11th to 20th	Day	15% 7% 2 4 3 4 4 2 5% 7% 47%	2170
	21st to end	Day	16% 8% 2 6% 4 4 4 5% 6% 7% 38%	2090
October	1st to 10th	Day	24% 10% 3 7% 4 4 4 3 4 6% 31%	2014
	11th to 20th	Day	18% 13% 3 7% 5% 4 4 3 5% 6% 32%	1848
	21st to end	Day	26% 14% 4 8% 5% 4 4 3 4 4 26%	1881
November	1st to 10th	Day	17% 14% 4 8% 5% 5% 4 3 4 6% 31%	1761
	11th to 20th	Day	19% 16% 5% 9% 5% 4 4 3 4 5% 26%	1800
	21st to end	Day	19% 16% 6% 10% 5% 4 4 4 5% 5% 22%	1614
December	1st to 10th	Day	15% 18% 7% 20% 8% 6% 5% 3 4 4 11%	2600
	11th to 20th	Day	18% 16% 7% 13% 5% 4 4 3 4 4 21%	2986
	21st to end	Day	18% 17% 6% 12% 6% 5% 6% 3 4 5% 18%	3960
January	1st to 10th	Night	13% 17% 11% 21% 8% 4 3 2 3 3 14%	6349
	11th to 20th	Night	10% 19% 8% 21% 9% 6% 5% 3 4 3 12%	6315
	21st to end	Night	13% 15% 6% 15% 9% 7% 5% 4 4 5% 17%	6996
February	1st to 10th	Night	15% 16% 6% 15% 8% 5% 4 3 4 4 19%	2635
	11th to 20th	Night	13% 16% 7% 14% 10% 5% 3 3 4 4 20%	2367
	21st to end	Night	14% 17% 6% 11% 6% 5% 4 3 4 5% 26%	1885
March	1st to 10th	Night	22% 16% 4 8% 6% 4 3 2 4 4 25%	3724
	11th to 20th	Night	15% 22% 5% 9% 5% 4 3 2 4 4 27%	3690
	21st to end	Night	18% 18% 4 11% 4 3 3 2 3 3 31%	3466
April	1st to 10th	Night	21% 16% 3 8% 4 3 4 3 4 5% 27%	1900
	11th to 20th	Night	24% 13% 3 7% 5% 4 3 3 3 5% 30%	1875
	21st to end	Night	23% 12% 3 7% 4 3 4 3 4 4 32%	1742
May	1st to 10th	Night	26% 12% 3 7% 4 4 4 4 5% 6% 25%	2748
	11th to 20th	Night	21% 11% 3 8% 5% 4 3 3 4 5% 31%	2495
	21st to end	Night	17% 7% 2 5% 4 5% 4 4 6% 9% 37%	2738
June	1st to 10th	Night	26% 7% 2 6% 5% 4 6% 3 4 6% 31%	1520
	11th to 20th	Night	20% 5% 2 7% 6% 3 4 4 6% 8% 36%	1520
	21st to end	Night	22% 8% 2 7% 5% 4 4 4 7% 6% 31%	1520
July	1st to 10th	Night	16% 6% 2 6% 5% 5% 7% 5% 5% 8% 34%	2520
	11th to 20th	Night	17% 7% 3 5% 4 4 5% 4 6% 7% 37%	2520
	21st to end	Night	15% 8% 3 8% 5% 5% 5% 4 6% 7% 36%	2739
August	1st to 10th	Night	20% 10% 2 7% 5% 4 4 3 5% 6% 35%	2506
	11th to 20th	Night	16% 8% 3 8% 5% 4 5% 3 5% 7% 36%	2896
	21st to end	Night	15% 6% 2 5% 3 4 4 4 6% 9% 42%	3476
September	1st to 10th	Night	16% 7% 2 6% 4 4 4 3 5% 7% 45%	1900
	11th to 20th	Night	15% 6% 2 5% 4 3 4 3 4 7% 48%	2009
	21st to end	Night	17% 7% 2 6% 4 5% 4 3 5% 6% 42%	2090
October	1st to 10th	Night	24% 9% 2 6% 4 5% 4 4 5% 5% 31%	2164
	11th to 20th	Night	22% 12% 2 7% 5% 4 5% 3 5% 5% 31%	2332
	21st to end	Night	27% 14% 3 7% 4 4 4 3 4 5% 25%	2717
November	1st to 10th	Night	21% 14% 4 8% 4 4 4 3 4 5% 30%	2537
	11th to 20th	Night	19% 15% 5% 9% 5% 4 4 3 4 5% 27%	2600
	21st to end	Night	19% 18% 6% 11% 5% 4 4 3 5% 5% 21%	2785
December	1st to 10th	Night	16% 22% 11% 18% 6% 5% 4 2 3 3 10%	4580
	11th to 20th	Night	22% 18% 5% 12% 6% 5% 4 3 3 3 19%	5208
	21st to end	Night	18% 18% 6% 13% 6% 6% 5% 3 3 4 18%	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% 5% 10% 11% 2	3598
	11th to 20th	Day	67% 5% 11% 10% 6% 1	3600
	21st to end	Day	63% 5% 12% 10% 9% 1	3960
February	1st to 10th	Day	63% 8% 9% 14% 5% 2	1760
	11th to 20th	Day	65% 4 11% 14% 6%	1927
	21st to end	Day	62% 5% 10% 12% 9% 2	1570
March	1st to 10th	Day	66% 6% 10% 11% 5% 1	3119
	11th to 20th	Day	63% 7% 10% 11% 8% 1	3265
	21st to end	Day	55% 5% 8% 16% 15% 2	4166
April	1st to 10th	Day	57% 4 9% 19% 9% 2	2280
	11th to 20th	Day	53% 5% 9% 18% 15% 1	2254
	21st to end	Day	56% 5% 10% 17% 10% 2	2437
May	1st to 10th	Day	57% 7% 10% 18% 7% 1	4235
	11th to 20th	Day	50% 9% 14% 17% 9% 1	4402
	21st to end	Day	40% 10% 14% 25% 10% 1	4837
June	1st to 10th	Day	52% 8% 13% 17% 10%	2660
	11th to 20th	Day	46% 8% 11% 21% 12% 2	2660
	21st to end	Day	53% 8% 12% 18% 9% 1	2660
July	1st to 10th	Day	42% 7% 15% 25% 10%	4370
	11th to 20th	Day	41% 9% 15% 29% 6%	4365
	21st to end	Day	44% 8% 12% 28% 7%	4661
August	1st to 10th	Day	48% 8% 15% 21% 8%	4409
	11th to 20th	Day	48% 6% 17% 24% 5%	3936
	21st to end	Day	39% 6% 18% 23% 13% 1	4180
September	1st to 10th	Day	38% 6% 13% 29% 12% 1	2280
	11th to 20th	Day	37% 5% 12% 30% 15% 1	2170
	21st to end	Day	42% 8% 17% 21% 10% 2	2090
October	1st to 10th	Day	47% 5% 10% 27% 10%	3352
	11th to 20th	Day	48% 5% 15% 21% 11%	3051
	21st to end	Day	48% 5% 13% 26% 7%	3081
November	1st to 10th	Day	55% 7% 13% 17% 8%	1761
	11th to 20th	Day	59% 6% 12% 13% 9% 2	1800
	21st to end	Day	62% 7% 15% 13% 3	1614
December	1st to 10th	Day	76% 7% 8% 7% 1	2600
	11th to 20th	Day	66% 5% 10% 14% 5%	2986
	21st to end	Day	67% 7% 10% 9% 7% 1	3960
January	1st to 10th	Night	76% 5% 11% 6% 2	6349
	11th to 20th	Night	75% 5% 10% 7% 3 1	6360
	21st to end	Night	69% 4 9% 8% 9% 1	6996
February	1st to 10th	Night	66% 6% 10% 11% 6% 1	2635
	11th to 20th	Night	65% 4 9% 13% 7%	2369
	21st to end	Night	60% 5% 11% 16% 7% 1	1885
March	1st to 10th	Night	63% 4 10% 15% 7% 1	3734
	11th to 20th	Night	60% 7% 9% 15% 8% 1	3691
	21st to end	Night	57% 4 7% 15% 15% 2	3466
April	1st to 10th	Night	59% 5% 8% 16% 10% 1	1900
	11th to 20th	Night	57% 6% 8% 17% 12%	1875
	21st to end	Night	54% 5% 11% 15% 12% 3	1742
May	1st to 10th	Night	57% 8% 9% 16% 9% 1	2754
	11th to 20th	Night	53% 7% 12% 18% 9% 1	2496
	21st to end	Night	42% 9% 13% 20% 15% 2	2739
June	1st to 10th	Night	53% 8% 11% 18% 9% 1	1520
	11th to 20th	Night	45% 7% 15% 21% 10% 2	1520
	21st to end	Night	49% 9% 12% 20% 9% 1	1520
July	1st to 10th	Night	45% 8% 15% 19% 12% 1	2480
	11th to 20th	Night	43% 8% 14% 25% 9%	2472
	21st to end	Night	45% 8% 12% 25% 10%	2651
August	1st to 10th	Night	49% 7% 13% 22% 8%	2496
	11th to 20th	Night	48% 6% 15% 21% 9% 1	2923
	21st to end	Night	38% 7% 16% 25% 12% 1	3476
September	1st to 10th	Night	40% 6% 15% 27% 11% 1	1900
	11th to 20th	Night	38% 5% 14% 30% 12%	2009
	21st to end	Night	42% 7% 12% 24% 13% 2	2090
October	1st to 10th	Night	55% 6% 10% 21% 8%	3606
	11th to 20th	Night	52% 5% 10% 21% 12%	3868
	21st to end	Night	54% 4 12% 22% 7%	4467
November	1st to 10th	Night	56% 6% 12% 17% 8% 1	2537
	11th to 20th	Night	58% 6% 11% 15% 9% 2	2600
	21st to end	Night	65% 6% 12% 14% 4	2785
December	1st to 10th	Night	80% 5% 7% 6% 1	4580
	11th to 20th	Night	70% 5% 8% 11% 7%	5208
	21st to end	Night	69% 6% 8% 10% 5% 2	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	27% 29% 28% 15% 3	200
	11th to 20th	Day	33% 24% 30% 12% 1 1	200
	21st to end	Day	27% 28% 33% 8% 4	220
February	1st to 10th	Day	33% 34% 26% 6% 1	280
	11th to 20th	Day	33% 27% 30% 9% 2	466
	21st to end	Day	31% 22% 35% 10% 2	382
March	1st to 10th	Day	35% 20% 27% 16% 2	460
	11th to 20th	Day	33% 24% 26% 15% 3	460
	21st to end	Day	35% 22% 26% 12% 5% 2	506
April	1st to 10th	Day	37% 19% 25% 14% 5% 1	460
	11th to 20th	Day	35% 22% 22% 13% 5% 2	452
	21st to end	Day	33% 21% 27% 12% 5% 2	460
May	1st to 10th	Day	32% 17% 27% 15% 6% 2	463
	11th to 20th	Day	22% 16% 34% 21% 6% 2	487
	21st to end	Day	20% 18% 34% 21% 6% 1	529
June	1st to 10th	Day	30% 15% 31% 18% 6% 1	490
	11th to 20th	Day	22% 14% 35% 20% 7% 2	490
	21st to end	Day	25% 17% 33% 18% 7% 1	490
July	1st to 10th	Day	17% 10% 35% 26% 10% 2	485
	11th to 20th	Day	14% 12% 36% 26% 11% 1	484
	21st to end	Day	17% 9% 37% 29% 7% 1	532
August	1st to 10th	Day	16% 12% 30% 27% 14% 2	486
	11th to 20th	Day	18% 8% 31% 25% 14% 4	479
	21st to end	Day	15% 14% 29% 25% 12% 4	539
September	1st to 10th	Day	18% 14% 30% 20% 12% 6%	490
	11th to 20th	Day	16% 14% 22% 22% 19% 7%	490
	21st to end	Day	19% 13% 30% 21% 12% 4	490
October	1st to 10th	Day	27% 15% 26% 15% 13% 3	490
	11th to 20th	Day	28% 19% 26% 20% 7% 1	463
	21st to end	Day	38% 16% 26% 17% 3	429
November	1st to 10th	Day	32% 19% 26% 17% 5% 1	395
	11th to 20th	Day	31% 25% 25% 15% 4 1	400
	21st to end	Day	36% 18% 31% 14% 1	214
December	1st to 10th	Day	29% 23% 31% 14% 4	200
	11th to 20th	Day	34% 28% 29% 9% 1	200
	21st to end	Day	31% 24% 31% 10% 3	220
January	1st to 10th	Night	25% 25% 33% 15% 2	800
	11th to 20th	Night	28% 23% 30% 16% 4	800
	21st to end	Night	20% 20% 35% 21% 4	880
February	1st to 10th	Night	26% 18% 36% 18% 2	800
	11th to 20th	Night	24% 22% 35% 16% 2	783
	21st to end	Night	28% 20% 30% 19% 4	629
March	1st to 10th	Night	28% 17% 29% 24% 2	760
	11th to 20th	Night	27% 22% 25% 21% 4	760
	21st to end	Night	31% 17% 26% 19% 6% 1	836
April	1st to 10th	Night	30% 16% 28% 22% 4	760
	11th to 20th	Night	32% 19% 23% 18% 7% 1	751
	21st to end	Night	30% 15% 27% 22% 6%	602
May	1st to 10th	Night	30% 13% 24% 25% 8%	523
	11th to 20th	Night	21% 10% 35% 26% 8% 1	374
	21st to end	Night	21% 9% 30% 31% 9% 1	411
June	1st to 10th	Night	27% 8% 28% 28% 8% 1	380
	11th to 20th	Night	21% 7% 26% 35% 11%	380
	21st to end	Night	24% 6% 24% 36% 9% 1	380
July	1st to 10th	Night	15% 5% 26% 41% 13% 1	370
	11th to 20th	Night	14% 4 25% 39% 17% 1	368
	21st to end	Night	16% 5% 22% 46% 12%	416
August	1st to 10th	Night	18% 4 18% 41% 18% 1	378
	11th to 20th	Night	16% 5% 23% 35% 19% 2	623
	21st to end	Night	16% 6% 24% 32% 20% 2	836
September	1st to 10th	Night	18% 9% 24% 32% 15% 2	760
	11th to 20th	Night	19% 7% 20% 30% 20% 3	760
	21st to end	Night	19% 10% 27% 27% 16% 2	760
October	1st to 10th	Night	29% 10% 23% 22% 15% 1	760
	11th to 20th	Night	29% 13% 23% 25% 9% 1	760
	21st to end	Night	35% 13% 28% 19% 4	836
November	1st to 10th	Night	29% 18% 28% 17% 8% 1	780
	11th to 20th	Night	27% 19% 28% 17% 8% 1	800
	21st to end	Night	30% 22% 28% 15% 5% 1	800
December	1st to 10th	Night	30% 24% 27% 17% 3	800
	11th to 20th	Night	28% 22% 28% 19% 3	800
	21st to end	Night	32% 21% 29% 15% 3	880

Page generated at 2025-02-18 03:26:32. Version ac07

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