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 2026-08-15.

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% 6% 65%	151884
	Night	2 4 2 4 3 3 4 3 5% 7% 64%	147430
Transparency	Day	18% 5% 9% 16% 26% 26%	146850
	Night	18% 5% 10% 18% 27% 21%	143449
Seeing	Day	6% 13% 28% 31% 16% 5%	17298
	Night	4 7% 23% 38% 23% 5%	26796

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	2 8% 3 6% 4 4 5% 3 5% 8% 53%	4722
	11th to 20th	Day	1 4 1 3 2 3 3 3 5% 6% 70%	4740
	21st to end	Day	1 6% 2 5% 3 3 3 4 6% 10% 57%	5647
February	1st to 10th	Day	2 4 2 6% 4 4 4 4 6% 9% 57%	2100
	11th to 20th	Day	2 7% 2 4 5% 4 4 4 7% 9% 52%	2050
	21st to end	Day	2 5% 2 5% 4 4 4 3 7% 8% 56%	1730
March	1st to 10th	Day	1 8% 3 6% 4 4 5% 4 6% 10% 50%	4618
	11th to 20th	Day	5% 2 6% 3 4 4 3 6% 9% 57%	4875
	21st to end	Day	1 6% 2 6% 4 5% 5% 5% 7% 10% 48%	5917
April	1st to 10th	Day	2 4 2 5% 5% 4 4 4 5% 6% 59%	2520
	11th to 20th	Day	2 5% 1 4 4 2 5% 4 5% 7% 61%	2489
	21st to end	Day	2 4 1 4 4 4 4 4 5% 7% 60%	2625
May	1st to 10th	Day	2 3 2 4 3 3 3 3 5% 7% 64%	7626
	11th to 20th	Day	1 3 1 4 3 3 3 2 4 5% 72%	7596
	21st to end	Day	3 5% 2 5% 4 4 4 3 4 6% 59%	8514
June	1st to 10th	Day	3 4 2 5% 4 4 4 3 6% 6% 60%	2940
	11th to 20th	Day	2 3 1 4 3 3 3 3 4 5% 70%	2940
	21st to end	Day	2 2 1 4 3 3 3 2 4 4 73%	2940
July	1st to 10th	Day	1 1 1 2 2 2 2 2 3 3 80%	8219
	11th to 20th	Day	3 3 1 5% 4 4 5% 4 6% 7% 60%	8220
	21st to end	Day	2 2 1 4 3 4 4 3 5% 6% 68%	8663
August	1st to 10th	Day	2 2 1 2 2 2 2 1 3 3 79%	5852
	11th to 20th	Day	1 1 1 2 2 2 2 1 3 3 82%	5413
	21st to end	Day	2 3 2 5% 3 4 4 3 5% 5% 65%	5984
September	1st to 10th	Day	2 3 1 4 2 4 4 3 4 4 69%	2400
	11th to 20th	Day	2 3 2 4 3 3 3 2 4 5% 70%	2212
	21st to end	Day	2 2 1 3 3 2 2 2 4 5% 75%	2200
October	1st to 10th	Day	2 2 1 3 3 3 3 2 4 5% 72%	3600
	11th to 20th	Day	1 2 1 3 2 2 3 2 3 5% 76%	3385
	21st to end	Day	1 2 1 3 2 2 3 2 4 6% 75%	3200
November	1st to 10th	Day	1 4 2 3 3 4 3 3 6% 8% 64%	1854
	11th to 20th	Day	1 4 1 3 2 3 3 2 6% 9% 65%	1890
	21st to end	Day	1 4 2 4 3 4 4 4 6% 7% 62%	1890
December	1st to 10th	Day	1 3 1 3 2 2 3 3 4 8% 70%	3040
	11th to 20th	Day	3 8% 2 5% 4 4 4 3 7% 9% 51%	3181
	21st to end	Day	1 6% 2 6% 4 4 4 4 6% 8% 54%	4092
January	1st to 10th	Night	1 7% 4 6% 4 3 3 3 5% 8% 55%	8172
	11th to 20th	Night	2 1 3 3 3 3 3 4 6% 72%	8220
	21st to end	Night	2 7% 3 4 3 3 4 3 6% 7% 58%	8525
February	1st to 10th	Night	2 6% 2 5% 4 3 3 3 6% 7% 58%	2520
	11th to 20th	Night	2 6% 2 4 2 3 4 4 6% 9% 57%	2458
	21st to end	Night	2 5% 2 5% 3 4 4 4 7% 8% 57%	2076
March	1st to 10th	Night	1 5% 2 6% 4 4 4 4 6% 7% 58%	5496
	11th to 20th	Night	1 4 2 5% 4 4 4 3 6% 10% 56%	5343
	21st to end	Night	1 5% 3 6% 4 4 4 3 5% 8% 56%	5296
April	1st to 10th	Night	3 4 1 4 3 3 4 4 6% 7% 60%	2100
	11th to 20th	Night	3 4 2 4 4 3 3 2 4 6% 65%	2079
	21st to end	Night	3 5% 1 4 4 3 4 3 5% 8% 60%	1993
May	1st to 10th	Night	2 3 1 4 3 3 3 3 5% 7% 65%	5321
	11th to 20th	Night	1 2 1 3 2 2 3 2 4 6% 73%	5296
	21st to end	Night	3 5% 2 5% 4 3 4 3 5% 6% 60%	5667
June	1st to 10th	Night	3 3 1 4 4 3 4 4 5% 7% 63%	1680
	11th to 20th	Night	3 3 1 3 2 3 3 2 4 4 72%	1680
	21st to end	Night	1 2 1 3 3 3 3 2 4 5% 73%	1680
July	1st to 10th	Night	1 2 1 3 3 2 3 2 4 4 76%	4740
	11th to 20th	Night	1 2 1 5% 4 4 5% 4 6% 7% 60%	4740
	21st to end	Night	2 3 1 4 3 3 3 3 4 5% 69%	5425
August	1st to 10th	Night	2 3 1 3 2 2 3 2 4 5% 72%	4166
	11th to 20th	Night	1 1 2 2 2 2 2 4 4 79%	4546
	21st to end	Night	2 2 1 4 3 3 4 3 5% 6% 66%	5015
September	1st to 10th	Night	2 1 1 3 3 2 3 3 4 6% 72%	2000
	11th to 20th	Night	2 3 1 3 3 2 3 2 5% 5% 71%	2186
	21st to end	Night	2 2 1 2 2 2 2 2 3 4 78%	2200
October	1st to 10th	Night	2 2 1 2 2 2 4 3 4 5% 74%	3600
	11th to 20th	Night	1 2 1 3 2 2 3 2 4 6% 76%	3769
	21st to end	Night	1 2 1 3 3 3 3 2 3 5% 75%	4600
November	1st to 10th	Night	2 4 1 3 3 2 3 3 5% 8% 67%	2665
	11th to 20th	Night	2 3 2 4 3 3 3 3 6% 8% 64%	2730
	21st to end	Night	2 3 1 5% 4 4 3 3 5% 7% 63%	2730
December	1st to 10th	Night	1 3 1 3 3 3 4 3 5% 6% 67%	4370
	11th to 20th	Night	2 7% 2 3 2 3 3 4 6% 10% 57%	5240
	21st to end	Night	2 6% 2 6% 5% 5% 5% 5% 8% 10% 46%	7106

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	25% 9% 22% 20% 18% 5%	4723
	11th to 20th	Day	14% 5% 10% 16% 24% 31%	4740
	21st to end	Day	21% 7% 12% 21% 28% 12%	5647
February	1st to 10th	Day	24% 7% 12% 26% 19% 13%	2100
	11th to 20th	Day	25% 7% 12% 23% 25% 9%	2060
	21st to end	Day	22% 9% 14% 28% 21% 7%	1730
March	1st to 10th	Day	24% 6% 11% 22% 27% 9%	4618
	11th to 20th	Day	20% 8% 17% 27% 24% 4	4875
	21st to end	Day	23% 11% 17% 23% 21% 6%	5917
April	1st to 10th	Day	23% 7% 12% 25% 21% 12%	2520
	11th to 20th	Day	21% 6% 11% 23% 28% 11%	2492
	21st to end	Day	21% 6% 10% 28% 28% 7%	2625
May	1st to 10th	Day	17% 5% 8% 22% 35% 12%	7626
	11th to 20th	Day	14% 5% 7% 14% 41% 21%	7599
	21st to end	Day	23% 5% 7% 16% 34% 15%	8515
June	1st to 10th	Day	22% 5% 9% 16% 29% 20%	2940
	11th to 20th	Day	16% 3 6% 11% 27% 37%	2940
	21st to end	Day	13% 3 4 8% 23% 49%	2940
July	1st to 10th	Day	10% 2 3 5% 12% 69%	8151
	11th to 20th	Day	18% 6% 6% 10% 17% 44%	8145
	21st to end	Day	15% 5% 5% 9% 19% 47%	8470
August	1st to 10th	Day	14% 3 4 10% 24% 46%	4293
	11th to 20th	Day	9% 2 4 6% 15% 64%	3929
	21st to end	Day	13% 2 3 7% 26% 48%	4311
September	1st to 10th	Day	17% 3 5% 9% 25% 41%	2400
	11th to 20th	Day	15% 3 6% 14% 28% 34%	2212
	21st to end	Day	13% 3 4 14% 33% 33%	2200
October	1st to 10th	Day	13% 3 6% 10% 32% 35%	3600
	11th to 20th	Day	13% 3 4 12% 37% 32%	3385
	21st to end	Day	11% 3 5% 18% 48% 14%	3200
November	1st to 10th	Day	17% 4 11% 26% 29% 13%	1854
	11th to 20th	Day	15% 5% 12% 26% 30% 12%	1890
	21st to end	Day	21% 7% 9% 23% 27% 14%	1890
December	1st to 10th	Day	13% 4 9% 21% 32% 21%	3040
	11th to 20th	Day	26% 7% 13% 25% 23% 6%	3181
	21st to end	Day	24% 9% 17% 25% 20% 5%	4092
January	1st to 10th	Night	24% 8% 21% 19% 24% 4	8172
	11th to 20th	Night	14% 5% 9% 18% 26% 28%	8220
	21st to end	Night	22% 6% 11% 19% 27% 14%	8525
February	1st to 10th	Night	24% 6% 12% 26% 22% 10%	2520
	11th to 20th	Night	21% 6% 13% 22% 28% 9%	2459
	21st to end	Night	22% 6% 16% 23% 24% 9%	2076
March	1st to 10th	Night	23% 6% 9% 19% 29% 14%	5496
	11th to 20th	Night	19% 7% 13% 33% 22% 4	5343
	21st to end	Night	21% 9% 19% 26% 22% 3	5296
April	1st to 10th	Night	21% 6% 14% 26% 24% 9%	2100
	11th to 20th	Night	20% 5% 10% 28% 26% 11%	2079
	21st to end	Night	21% 6% 10% 31% 27% 5%	1993
May	1st to 10th	Night	17% 6% 10% 20% 38% 10%	5319
	11th to 20th	Night	10% 5% 7% 16% 45% 17%	5297
	21st to end	Night	21% 5% 8% 19% 36% 11%	5668
June	1st to 10th	Night	16% 7% 12% 18% 30% 17%	1680
	11th to 20th	Night	16% 4 5% 13% 31% 31%	1680
	21st to end	Night	13% 3 5% 9% 26% 44%	1680
July	1st to 10th	Night	12% 3 3 6% 11% 66%	4700
	11th to 20th	Night	18% 6% 7% 11% 17% 41%	4692
	21st to end	Night	15% 5% 6% 10% 19% 45%	5306
August	1st to 10th	Night	18% 4 5% 9% 26% 37%	3048
	11th to 20th	Night	9% 4 4 7% 14% 61%	3296
	21st to end	Night	13% 2 4 7% 24% 49%	3608
September	1st to 10th	Night	13% 3 6% 10% 29% 40%	2000
	11th to 20th	Night	14% 3 7% 15% 28% 33%	2186
	21st to end	Night	12% 2 5% 13% 30% 37%	2200
October	1st to 10th	Night	12% 4 6% 11% 33% 34%	3600
	11th to 20th	Night	11% 3 4 16% 36% 30%	3769
	21st to end	Night	14% 2 4 19% 46% 16%	4600
November	1st to 10th	Night	15% 4 11% 28% 28% 14%	2665
	11th to 20th	Night	17% 5% 10% 24% 32% 12%	2730
	21st to end	Night	20% 5% 10% 23% 30% 12%	2730
December	1st to 10th	Night	17% 5% 8% 21% 28% 21%	4370
	11th to 20th	Night	21% 6% 18% 21% 28% 7%	5240
	21st to end	Night	27% 10% 17% 21% 19% 6%	7106

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	13% 14% 35% 24% 10% 3	220
	11th to 20th	Day	5% 14% 32% 18% 24% 8%	220
	21st to end	Day	5% 12% 26% 27% 21% 8%	365
February	1st to 10th	Day	9% 20% 29% 24% 14% 5%	540
	11th to 20th	Day	9% 18% 33% 27% 11% 3	535
	21st to end	Day	5% 14% 36% 34% 9% 2	446
March	1st to 10th	Day	8% 18% 33% 28% 12% 2	540
	11th to 20th	Day	6% 17% 29% 29% 15% 4	540
	21st to end	Day	8% 19% 32% 28% 10% 2	594
April	1st to 10th	Day	8% 18% 26% 29% 15% 4	540
	11th to 20th	Day	10% 23% 29% 26% 10% 2	532
	21st to end	Day	8% 20% 30% 25% 15% 3	540
May	1st to 10th	Day	7% 21% 33% 25% 10% 3	543
	11th to 20th	Day	8% 18% 30% 26% 12% 5%	568
	21st to end	Day	8% 14% 28% 29% 15% 6%	617
June	1st to 10th	Day	5% 14% 26% 31% 17% 8%	570
	11th to 20th	Day	7% 16% 28% 29% 15% 5%	570
	21st to end	Day	3 11% 28% 35% 19% 4	570
July	1st to 10th	Day	3 5% 31% 41% 15% 5%	566
	11th to 20th	Day	5% 4 22% 42% 20% 7%	564
	21st to end	Day	4 3 24% 39% 23% 7%	619
August	1st to 10th	Day	4 7% 29% 35% 18% 8%	567
	11th to 20th	Day	3 5% 28% 35% 20% 9%	534
	21st to end	Day	4 6% 24% 37% 21% 8%	583
September	1st to 10th	Day	3 6% 28% 31% 23% 9%	530
	11th to 20th	Day	4 12% 28% 32% 18% 5%	530
	21st to end	Day	4 10% 25% 34% 21% 5%	530
October	1st to 10th	Day	5% 15% 27% 27% 18% 7%	530
	11th to 20th	Day	5% 13% 25% 32% 19% 5%	525
	21st to end	Day	5% 13% 25% 28% 25% 4	363
November	1st to 10th	Day	4 15% 27% 25% 21% 8%	330
	11th to 20th	Day	4 14% 31% 28% 16% 8%	330
	21st to end	Day	4 15% 26% 28% 20% 8%	330
December	1st to 10th	Day	6% 10% 28% 32% 18% 6%	330
	11th to 20th	Day	4 15% 27% 31% 16% 7%	245
	21st to end	Day	7% 20% 29% 26% 14% 3	242
January	1st to 10th	Night	8% 8% 28% 31% 17% 8%	840
	11th to 20th	Night	4 6% 28% 37% 20% 6%	840
	21st to end	Night	7% 6% 27% 35% 19% 6%	924
February	1st to 10th	Night	6% 11% 26% 34% 18% 4	840
	11th to 20th	Night	5% 10% 29% 35% 17% 4	824
	21st to end	Night	4 8% 32% 40% 13% 3	692
March	1st to 10th	Night	5% 11% 27% 41% 14% 3	840
	11th to 20th	Night	4 10% 25% 36% 19% 5%	840
	21st to end	Night	6% 13% 29% 34% 16% 2	924
April	1st to 10th	Night	5% 12% 27% 37% 18% 2	840
	11th to 20th	Night	5% 13% 30% 34% 15% 3	831
	21st to end	Night	5% 12% 30% 35% 15% 4	733
May	1st to 10th	Night	5% 12% 34% 33% 14% 2	630
	11th to 20th	Night	6% 10% 27% 34% 20% 3	621
	21st to end	Night	6% 11% 26% 32% 23% 2	630
June	1st to 10th	Night	4 4 21% 43% 26% 3	420
	11th to 20th	Night	4 7% 23% 42% 22% 2	420
	21st to end	Night	2 4 20% 49% 24% 1	420
July	1st to 10th	Night	3 18% 50% 26% 2	410
	11th to 20th	Night	3 1 14% 45% 33% 4	408
	21st to end	Night	4 1 12% 49% 31% 3	553
August	1st to 10th	Night	5% 3 17% 46% 29% 1	644
	11th to 20th	Night	2 1 14% 44% 34% 6%	808
	21st to end	Night	4 2 12% 40% 37% 5%	880
September	1st to 10th	Night	3 3 13% 35% 39% 7%	800
	11th to 20th	Night	3 5% 16% 37% 31% 8%	800
	21st to end	Night	3 5% 14% 37% 30% 11%	800
October	1st to 10th	Night	4 6% 21% 37% 26% 6%	800
	11th to 20th	Night	4 6% 18% 38% 27% 7%	800
	21st to end	Night	2 8% 21% 34% 26% 9%	880
November	1st to 10th	Night	4 5% 22% 32% 29% 8%	820
	11th to 20th	Night	3 6% 24% 40% 21% 6%	840
	21st to end	Night	3 9% 24% 35% 23% 5%	840
December	1st to 10th	Night	5% 7% 24% 40% 19% 5%	840
	11th to 20th	Night	7% 7% 26% 38% 19% 3	840
	21st to end	Night	5% 12% 30% 36% 14% 3	924

Page generated at 2026-08-12 00:37:15. Version ac07

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