Belmont
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Belmont clear sky chart from 2005-11-06 to 2025-04-11.

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	2 4 2 4 3 3 4 3 5% 7% 64%	117254
	Night	2 4 2 4 3 3 4 3 5% 7% 63%	119862
Transparency	Day	18% 6% 9% 17% 26% 24%	117000
	Night	18% 6% 11% 19% 27% 19%	119706
Seeing	Day	6% 14% 29% 30% 16% 5%	15479
	Night	5% 7% 23% 37% 23% 5%	25165

By Month

Percentage Cloud Cover SW (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	1 4 2 5% 5% 5% 5% 3 6% 9% 57%	3639
	11th to 20th	Day	1 6% 1 3 3 3 4 3 5% 6% 64%	3640
	21st to end	Day	2 6% 3 6% 4 4 4 5% 6% 9% 54%	4353
February	1st to 10th	Day	3 4 2 6% 4 4 4 4 6% 9% 56%	2000
	11th to 20th	Day	2 7% 2 4 5% 4 4 4 7% 10% 52%	1950
	21st to end	Day	2 6% 2 5% 4 5% 4 3 7% 7% 56%	1650
March	1st to 10th	Day	1 8% 3 6% 4 4 5% 4 6% 10% 49%	4518
	11th to 20th	Day	5% 3 6% 3 4 4 3 6% 9% 56%	4770
	21st to end	Day	1 6% 2 6% 5% 5% 5% 5% 7% 10% 48%	5789
April	1st to 10th	Day	2 4 2 6% 5% 4 4 4 5% 6% 58%	2400
	11th to 20th	Day	2 5% 1 4 4 2 4 4 5% 7% 61%	2251
	21st to end	Day	2 4 1 4 3 4 3 4 5% 7% 62%	2375
May	1st to 10th	Day	2 3 1 4 3 3 3 3 4 6% 68%	4110
	11th to 20th	Day	2 3 1 4 3 3 3 2 4 6% 67%	4071
	21st to end	Day	3 5% 2 5% 3 3 3 3 4 5% 64%	4556
June	1st to 10th	Day	4 4 2 5% 4 4 4 3 6% 6% 59%	2660
	11th to 20th	Day	2 3 1 4 3 3 3 3 4 5% 69%	2660
	21st to end	Day	2 2 1 4 3 3 2 2 4 4 72%	2660
July	1st to 10th	Day	2 2 1 3 3 2 2 2 3 3 77%	4420
	11th to 20th	Day	3 3 1 4 3 4 4 4 5% 7% 61%	4420
	21st to end	Day	3 3 2 5% 5% 5% 5% 4 5% 6% 58%	4648
August	1st to 10th	Day	3 3 1 3 3 3 3 2 3 4 72%	4049
	11th to 20th	Day	1 1 1 2 2 1 2 1 3 3 81%	3724
	21st to end	Day	2 2 1 3 2 2 2 2 3 3 77%	4180
September	1st to 10th	Day	2 3 1 4 2 4 4 3 4 4 70%	2280
	11th to 20th	Day	2 3 1 4 3 3 3 2 4 5% 71%	2102
	21st to end	Day	2 2 1 3 2 2 2 2 3 5% 76%	2090
October	1st to 10th	Day	2 2 1 3 3 3 3 2 5% 5% 72%	3490
	11th to 20th	Day	1 2 1 3 2 2 3 2 3 5% 76%	3281
	21st to end	Day	1 2 1 3 2 2 2 2 4 6% 75%	3101
November	1st to 10th	Day	1 4 2 4 3 4 3 3 6% 9% 62%	1764
	11th to 20th	Day	1 3 1 2 2 2 3 2 5% 9% 68%	1800
	21st to end	Day	1 4 2 4 4 5% 4 4 5% 7% 61%	1800
December	1st to 10th	Day	1 3 1 3 2 2 3 3 5% 8% 70%	2950
	11th to 20th	Day	3 9% 2 5% 4 4 4 3 7% 9% 51%	3099
	21st to end	Day	1 6% 2 6% 4 4 5% 4 6% 8% 55%	4004
January	1st to 10th	Night	1 4 2 5% 4 4 4 3 6% 9% 61%	6306
	11th to 20th	Night	1 3 1 4 4 4 4 3 4 7% 65%	6320
	21st to end	Night	3 8% 3 5% 3 3 3 3 5% 7% 57%	6586
February	1st to 10th	Night	2 6% 2 5% 4 3 3 3 6% 7% 58%	2400
	11th to 20th	Night	2 6% 2 4 2 3 4 4 6% 9% 59%	2338
	21st to end	Night	2 6% 2 5% 3 4 4 4 6% 7% 57%	1980
March	1st to 10th	Night	1 5% 2 7% 4 4 4 4 6% 7% 57%	5376
	11th to 20th	Night	1 4 2 5% 4 4 4 3 7% 10% 55%	5228
	21st to end	Night	1 6% 3 6% 4 4 4 3 5% 8% 56%	5182
April	1st to 10th	Night	3 4 1 5% 4 3 4 4 6% 7% 59%	2000
	11th to 20th	Night	3 4 2 4 4 4 3 2 4 5% 65%	1883
	21st to end	Night	3 5% 1 4 4 3 3 3 5% 8% 60%	1803
May	1st to 10th	Night	3 4 2 4 3 3 3 3 5% 6% 66%	2860
	11th to 20th	Night	3 3 2 4 2 3 4 3 5% 6% 65%	2834
	21st to end	Night	3 5% 2 5% 3 3 3 3 4 6% 62%	3024
June	1st to 10th	Night	3 3 1 4 4 2 4 4 5% 7% 63%	1520
	11th to 20th	Night	3 3 1 3 2 3 3 2 4 4 71%	1520
	21st to end	Night	1 2 1 3 3 3 3 2 4 5% 72%	1520
July	1st to 10th	Night	2 2 1 3 3 2 2 2 4 4 75%	2540
	11th to 20th	Night	2 3 1 5% 4 4 5% 4 7% 7% 58%	2540
	21st to end	Night	3 4 2 5% 5% 5% 5% 4 6% 6% 56%	2907
August	1st to 10th	Night	3 4 1 5% 3 3 5% 3 5% 7% 62%	2875
	11th to 20th	Night	1 1 2 2 2 2 2 3 4 79%	3126
	21st to end	Night	2 2 1 3 2 2 2 2 3 4 77%	3498
September	1st to 10th	Night	2 1 1 3 2 2 3 2 4 6% 73%	1900
	11th to 20th	Night	2 3 1 3 3 2 3 2 4 5% 71%	2076
	21st to end	Night	2 2 1 2 2 2 1 2 3 3 79%	2090
October	1st to 10th	Night	2 2 1 2 2 2 4 3 4 5% 73%	3490
	11th to 20th	Night	2 2 1 3 2 2 3 2 4 6% 76%	3654
	21st to end	Night	1 2 1 3 3 3 3 2 3 5% 75%	4457
November	1st to 10th	Night	2 4 1 3 3 2 3 3 5% 8% 66%	2535
	11th to 20th	Night	2 3 1 3 3 3 3 3 5% 8% 66%	2600
	21st to end	Night	2 3 1 5% 4 4 3 4 6% 7% 62%	2600
December	1st to 10th	Night	1 3 1 3 3 3 4 3 5% 6% 67%	4240
	11th to 20th	Night	2 7% 2 3 2 3 3 4 6% 10% 56%	5102
	21st to end	Night	2 6% 2 6% 5% 5% 5% 5% 8% 10% 47%	6952

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	19% 9% 25% 21% 20% 6%	3639
	11th to 20th	Day	18% 7% 12% 17% 25% 20%	3640
	21st to end	Day	24% 8% 14% 18% 24% 12%	4353
February	1st to 10th	Day	23% 7% 12% 26% 18% 12%	2000
	11th to 20th	Day	24% 7% 12% 23% 25% 9%	1960
	21st to end	Day	22% 9% 13% 27% 22% 8%	1650
March	1st to 10th	Day	25% 7% 11% 22% 27% 9%	4518
	11th to 20th	Day	20% 8% 18% 27% 23% 4	4770
	21st to end	Day	23% 11% 17% 23% 20% 6%	5789
April	1st to 10th	Day	24% 7% 12% 25% 21% 11%	2400
	11th to 20th	Day	21% 7% 11% 24% 27% 10%	2254
	21st to end	Day	20% 6% 10% 30% 27% 7%	2375
May	1st to 10th	Day	16% 5% 8% 26% 32% 13%	4110
	11th to 20th	Day	16% 6% 9% 17% 37% 15%	4074
	21st to end	Day	21% 5% 7% 17% 37% 13%	4556
June	1st to 10th	Day	22% 5% 9% 17% 29% 18%	2660
	11th to 20th	Day	17% 4 6% 12% 28% 34%	2660
	21st to end	Day	14% 3 4 9% 22% 48%	2660
July	1st to 10th	Day	12% 2 3 5% 8% 69%	4351
	11th to 20th	Day	17% 5% 5% 9% 16% 47%	4345
	21st to end	Day	20% 7% 7% 11% 17% 38%	4501
August	1st to 10th	Day	15% 3 4 9% 25% 45%	4035
	11th to 20th	Day	9% 2 3 6% 15% 65%	3760
	21st to end	Day	12% 2 3 6% 27% 50%	4179
September	1st to 10th	Day	16% 3 5% 9% 25% 43%	2280
	11th to 20th	Day	15% 3 6% 15% 28% 35%	2102
	21st to end	Day	13% 2 4 13% 34% 34%	2090
October	1st to 10th	Day	13% 4 6% 10% 32% 36%	3490
	11th to 20th	Day	12% 2 4 12% 37% 32%	3281
	21st to end	Day	11% 3 5% 18% 48% 14%	3101
November	1st to 10th	Day	18% 4 11% 26% 28% 13%	1764
	11th to 20th	Day	13% 5% 12% 26% 31% 13%	1800
	21st to end	Day	22% 7% 9% 23% 25% 14%	1800
December	1st to 10th	Day	14% 4 9% 20% 32% 22%	2950
	11th to 20th	Day	27% 7% 13% 25% 23% 5%	3099
	21st to end	Day	24% 9% 17% 26% 19% 5%	4004
January	1st to 10th	Night	18% 7% 24% 22% 25% 5%	6306
	11th to 20th	Night	17% 6% 12% 19% 29% 16%	6320
	21st to end	Night	24% 6% 13% 16% 27% 14%	6586
February	1st to 10th	Night	24% 6% 12% 27% 22% 10%	2400
	11th to 20th	Night	20% 6% 13% 22% 29% 10%	2339
	21st to end	Night	23% 6% 14% 23% 25% 9%	1980
March	1st to 10th	Night	23% 7% 10% 19% 28% 14%	5376
	11th to 20th	Night	20% 8% 13% 34% 22% 4	5228
	21st to end	Night	21% 9% 20% 26% 21% 3	5182
April	1st to 10th	Night	22% 6% 14% 26% 23% 9%	2000
	11th to 20th	Night	21% 5% 10% 28% 24% 11%	1883
	21st to end	Night	21% 6% 11% 31% 26% 6%	1803
May	1st to 10th	Night	17% 5% 11% 21% 36% 10%	2860
	11th to 20th	Night	14% 6% 10% 18% 38% 13%	2835
	21st to end	Night	19% 6% 9% 20% 37% 9%	3024
June	1st to 10th	Night	16% 7% 13% 18% 30% 16%	1520
	11th to 20th	Night	17% 4 6% 14% 31% 28%	1520
	21st to end	Night	14% 4 5% 9% 26% 43%	1520
July	1st to 10th	Night	12% 2 4 6% 10% 66%	2500
	11th to 20th	Night	19% 6% 7% 9% 18% 41%	2492
	21st to end	Night	23% 7% 7% 12% 14% 38%	2816
August	1st to 10th	Night	18% 5% 6% 10% 27% 35%	2866
	11th to 20th	Night	9% 4 4 7% 14% 63%	3156
	21st to end	Night	12% 2 4 7% 24% 51%	3498
September	1st to 10th	Night	12% 3 6% 9% 28% 42%	1900
	11th to 20th	Night	14% 3 7% 15% 27% 33%	2076
	21st to end	Night	12% 2 4 13% 30% 38%	2090
October	1st to 10th	Night	12% 4 6% 11% 33% 34%	3490
	11th to 20th	Night	11% 3 4 16% 36% 30%	3654
	21st to end	Night	14% 2 4 19% 45% 16%	4457
November	1st to 10th	Night	15% 5% 12% 27% 27% 14%	2535
	11th to 20th	Night	15% 5% 10% 24% 33% 12%	2600
	21st to end	Night	21% 6% 10% 22% 29% 12%	2600
December	1st to 10th	Night	17% 5% 8% 20% 27% 22%	4240
	11th to 20th	Night	22% 6% 18% 20% 28% 6%	5102
	21st to end	Night	27% 10% 17% 20% 19% 6%	6952

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	12% 15% 35% 26% 11% 3	200
	11th to 20th	Day	5% 16% 35% 18% 21% 7%	200
	21st to end	Day	5% 13% 25% 27% 21% 9%	335
February	1st to 10th	Day	9% 22% 29% 23% 13% 5%	500
	11th to 20th	Day	8% 17% 33% 27% 12% 3	495
	21st to end	Day	6% 14% 37% 33% 9% 2	414
March	1st to 10th	Day	8% 17% 34% 28% 12% 2	500
	11th to 20th	Day	6% 18% 29% 29% 15% 4	500
	21st to end	Day	9% 20% 32% 28% 10% 2	550
April	1st to 10th	Day	8% 19% 28% 28% 15% 3	500
	11th to 20th	Day	10% 23% 29% 26% 10% 2	453
	21st to end	Day	8% 19% 31% 25% 14% 3	460
May	1st to 10th	Day	8% 22% 33% 24% 9% 3	463
	11th to 20th	Day	8% 18% 30% 26% 13% 5%	488
	21st to end	Day	9% 15% 28% 29% 14% 5%	529
June	1st to 10th	Day	6% 15% 26% 31% 17% 6%	490
	11th to 20th	Day	7% 17% 29% 29% 13% 5%	490
	21st to end	Day	2 11% 29% 34% 20% 3	490
July	1st to 10th	Day	3 5% 29% 42% 15% 6%	486
	11th to 20th	Day	5% 5% 23% 42% 19% 7%	484
	21st to end	Day	4 3 24% 39% 22% 7%	531
August	1st to 10th	Day	5% 7% 31% 34% 18% 6%	487
	11th to 20th	Day	3 5% 28% 35% 20% 8%	477
	21st to end	Day	4 7% 25% 36% 21% 7%	539
September	1st to 10th	Day	3 6% 27% 31% 24% 9%	490
	11th to 20th	Day	4 13% 28% 33% 18% 5%	490
	21st to end	Day	4 10% 26% 34% 21% 5%	490
October	1st to 10th	Day	5% 15% 27% 28% 18% 7%	490
	11th to 20th	Day	5% 13% 25% 33% 19% 5%	485
	21st to end	Day	4 13% 26% 28% 24% 5%	330
November	1st to 10th	Day	3 17% 28% 24% 21% 6%	300
	11th to 20th	Day	2 13% 32% 27% 17% 9%	300
	21st to end	Day	4 16% 24% 27% 21% 8%	300
December	1st to 10th	Day	6% 10% 28% 31% 18% 7%	300
	11th to 20th	Day	4 14% 28% 32% 14% 7%	223
	21st to end	Day	5% 20% 31% 27% 13% 3	220
January	1st to 10th	Night	8% 8% 28% 31% 18% 9%	800
	11th to 20th	Night	4 6% 29% 36% 18% 7%	800
	21st to end	Night	8% 6% 27% 34% 19% 7%	880
February	1st to 10th	Night	6% 12% 27% 33% 18% 4	800
	11th to 20th	Night	5% 9% 28% 36% 18% 4	784
	21st to end	Night	5% 8% 31% 39% 14% 3	660
March	1st to 10th	Night	5% 11% 26% 41% 14% 4	800
	11th to 20th	Night	4 11% 25% 35% 20% 5%	800
	21st to end	Night	7% 14% 29% 32% 16% 2	880
April	1st to 10th	Night	5% 12% 28% 37% 17% 2	800
	11th to 20th	Night	5% 13% 30% 32% 16% 4	753
	21st to end	Night	6% 13% 29% 33% 16% 4	663
May	1st to 10th	Night	5% 13% 34% 32% 13% 2	570
	11th to 20th	Night	7% 11% 25% 34% 21% 3	561
	21st to end	Night	7% 11% 27% 31% 23% 2	570
June	1st to 10th	Night	3 4 21% 42% 26% 3	380
	11th to 20th	Night	4 8% 23% 39% 24% 2	380
	21st to end	Night	2 3 19% 49% 25% 1	380
July	1st to 10th	Night	3 1 18% 48% 28% 2	370
	11th to 20th	Night	3 1 14% 44% 33% 5%	368
	21st to end	Night	4 1 13% 46% 33% 3	500
August	1st to 10th	Night	5% 3 16% 45% 29% 1	582
	11th to 20th	Night	2 1 14% 43% 35% 6%	752
	21st to end	Night	4 1 12% 40% 38% 5%	836
September	1st to 10th	Night	3 3 13% 34% 40% 7%	760
	11th to 20th	Night	3 5% 17% 36% 32% 8%	760
	21st to end	Night	3 5% 14% 35% 30% 12%	760
October	1st to 10th	Night	4 5% 21% 37% 27% 6%	760
	11th to 20th	Night	4 5% 17% 38% 28% 7%	760
	21st to end	Night	2 8% 21% 34% 25% 10%	836
November	1st to 10th	Night	4 5% 22% 32% 29% 8%	780
	11th to 20th	Night	3 5% 25% 40% 21% 6%	800
	21st to end	Night	3 9% 24% 35% 24% 6%	800
December	1st to 10th	Night	5% 7% 23% 40% 20% 6%	800
	11th to 20th	Night	7% 7% 25% 39% 19% 3	800
	21st to end	Night	4 12% 30% 35% 15% 3	880

Page generated at 2025-04-08 06:17:25. Version ac07

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