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 2024-10-24.

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% 63%	100916
	Night	2 5% 2 4 3 4 4 3 5% 7% 61%	98036
Transparency	Day	19% 6% 10% 18% 24% 24%	100662
	Night	20% 6% 11% 19% 24% 19%	97880
Seeing	Day	6% 14% 28% 30% 17% 5%	14944
	Night	5% 7% 23% 37% 23% 5%	24487

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 5% 2 7% 6% 5% 5% 4 6% 9% 49%	2539
	11th to 20th	Day	1 8% 2 5% 5% 5% 5% 5% 8% 9% 49%	2540
	21st to end	Day	2 7% 3 7% 4 4 4 5% 7% 11% 46%	3031
February	1st to 10th	Day	3 5% 2 6% 4 4 4 4 6% 9% 56%	1900
	11th to 20th	Day	2 7% 2 4 5% 4 4 4 7% 10% 52%	1850
	21st to end	Day	2 6% 2 5% 4 5% 4 3 7% 7% 55%	1570
March	1st to 10th	Day	1 8% 3 7% 5% 4 5% 5% 7% 9% 49%	3138
	11th to 20th	Day	1 4 2 4 4 4 4 3 6% 9% 60%	3325
	21st to end	Day	2 8% 3 7% 5% 6% 6% 5% 7% 10% 41%	4037
April	1st to 10th	Day	2 5% 2 6% 5% 4 4 4 5% 6% 57%	2280
	11th to 20th	Day	2 5% 1 4 4 2 4 4 5% 7% 61%	2249
	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	3 3 1 3 3 3 3 2 4 5% 69%	2090
	11th to 20th	Day	2 2 1 3 3 2 3 2 4 6% 72%	1984
	21st to end	Day	2 2 1 3 1 2 3 3 5% 6% 73%	1809
November	1st to 10th	Day	1 4 2 4 3 4 3 3 6% 9% 61%	1674
	11th to 20th	Day	1 3 1 2 2 2 3 2 5% 9% 68%	1710
	21st to end	Day	1 4 2 5% 4 4 4 4 6% 7% 61%	1710
December	1st to 10th	Day	2 5% 2 4 3 2 4 3 5% 8% 61%	1710
	11th to 20th	Day	5% 13% 3 6% 5% 4 4 4 6% 8% 42%	1971
	21st to end	Day	1 8% 3 6% 4 5% 5% 4 6% 9% 48%	2794
January	1st to 10th	Night	2 5% 2 6% 5% 5% 5% 3 7% 9% 52%	4406
	11th to 20th	Night	1 4 2 6% 5% 5% 5% 4 6% 9% 53%	4420
	21st to end	Night	4 11% 3 5% 3 3 4 4 7% 9% 48%	4613
February	1st to 10th	Night	2 6% 2 5% 4 3 3 3 7% 7% 57%	2280
	11th to 20th	Night	2 6% 1 4 2 3 3 4 6% 9% 59%	2218
	21st to end	Night	2 6% 2 5% 3 4 4 5% 7% 8% 55%	1884
March	1st to 10th	Night	1 5% 2 7% 5% 4 5% 4 6% 7% 54%	3736
	11th to 20th	Night	1 4 2 5% 4 5% 5% 4 7% 10% 54%	3653
	21st to end	Night	1 7% 3 6% 5% 5% 5% 4 6% 8% 50%	3612
April	1st to 10th	Night	3 5% 2 5% 4 3 4 4 6% 7% 59%	1900
	11th to 20th	Night	3 4 2 4 4 4 3 2 4 5% 65%	1879
	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	3 3 1 3 3 3 4 3 4 5% 68%	2090
	11th to 20th	Night	2 2 1 3 2 3 3 2 3 6% 73%	2194
	21st to end	Night	1 2 1 3 2 2 2 2 4 6% 75%	2613
November	1st to 10th	Night	2 4 1 3 3 2 3 3 5% 8% 65%	2405
	11th to 20th	Night	2 3 1 3 3 3 3 3 5% 8% 65%	2470
	21st to end	Night	2 3 1 5% 4 4 3 4 6% 7% 61%	2470
December	1st to 10th	Night	2 6% 2 5% 3 4 4 4 6% 7% 57%	2470
	11th to 20th	Night	4 12% 2 5% 3 4 3 4 7% 9% 47%	3228
	21st to end	Night	2 7% 3 7% 5% 6% 6% 5% 7% 9% 44%	4862

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	23% 11% 26% 23% 16% 2	2539
	11th to 20th	Day	26% 10% 18% 24% 14% 8%	2540
	21st to end	Day	28% 9% 18% 23% 18% 4	3031
February	1st to 10th	Day	24% 7% 13% 27% 19% 10%	1900
	11th to 20th	Day	25% 7% 12% 23% 25% 9%	1860
	21st to end	Day	23% 9% 13% 28% 22% 5%	1570
March	1st to 10th	Day	24% 8% 14% 28% 18% 8%	3138
	11th to 20th	Day	17% 7% 19% 30% 22% 4	3325
	21st to end	Day	28% 12% 19% 23% 15% 2	4037
April	1st to 10th	Day	25% 7% 12% 26% 19% 11%	2280
	11th to 20th	Day	21% 7% 11% 24% 27% 10%	2252
	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	17% 3 7% 15% 33% 24%	2090
	11th to 20th	Day	14% 4 5% 17% 34% 27%	1984
	21st to end	Day	12% 4 8% 20% 41% 15%	1809
November	1st to 10th	Day	19% 5% 11% 26% 27% 12%	1674
	11th to 20th	Day	13% 5% 12% 25% 31% 13%	1710
	21st to end	Day	22% 7% 10% 22% 26% 13%	1710
December	1st to 10th	Day	19% 6% 15% 24% 25% 11%	1710
	11th to 20th	Day	36% 7% 11% 21% 19% 6%	1971
	21st to end	Day	27% 11% 17% 20% 20% 6%	2794
January	1st to 10th	Night	24% 8% 22% 24% 19% 3	4406
	11th to 20th	Night	25% 9% 16% 25% 18% 8%	4420
	21st to end	Night	29% 8% 16% 22% 20% 5%	4613
February	1st to 10th	Night	24% 6% 12% 27% 22% 8%	2280
	11th to 20th	Night	20% 6% 14% 22% 29% 10%	2219
	21st to end	Night	24% 6% 15% 24% 25% 7%	1884
March	1st to 10th	Night	24% 8% 13% 23% 20% 11%	3736
	11th to 20th	Night	19% 8% 16% 30% 21% 5%	3653
	21st to end	Night	24% 11% 17% 26% 19% 3	3612
April	1st to 10th	Night	23% 6% 14% 27% 21% 9%	1900
	11th to 20th	Night	21% 5% 10% 28% 24% 11%	1879
	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	17% 4 8% 17% 29% 25%	2090
	11th to 20th	Night	14% 4 5% 19% 33% 26%	2194
	21st to end	Night	12% 2 6% 24% 40% 16%	2613
November	1st to 10th	Night	16% 5% 12% 27% 27% 14%	2405
	11th to 20th	Night	15% 5% 10% 24% 32% 13%	2470
	21st to end	Night	21% 6% 11% 21% 30% 12%	2470
December	1st to 10th	Night	24% 7% 13% 20% 25% 10%	2470
	11th to 20th	Night	31% 7% 14% 20% 20% 8%	3228
	21st to end	Night	30% 11% 15% 18% 19% 8%	4862

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% 14% 35% 26% 11% 3	180
	11th to 20th	Day	6% 14% 34% 17% 22% 7%	180
	21st to end	Day	6% 12% 24% 27% 21% 10%	305
February	1st to 10th	Day	7% 20% 29% 24% 14% 5%	460
	11th to 20th	Day	7% 17% 31% 29% 13% 4	455
	21st to end	Day	6% 14% 37% 31% 9% 2	382
March	1st to 10th	Day	7% 18% 33% 28% 12% 2	460
	11th to 20th	Day	5% 18% 29% 28% 15% 5%	460
	21st to end	Day	8% 22% 32% 27% 9% 1	506
April	1st to 10th	Day	9% 19% 27% 27% 15% 3	460
	11th to 20th	Day	10% 23% 29% 26% 10% 2	452
	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% 26% 33% 19% 5%	485
	21st to end	Day	5% 13% 25% 27% 25% 5%	306
November	1st to 10th	Day	4 17% 26% 24% 22% 6%	270
	11th to 20th	Day	2 13% 30% 28% 19% 10%	270
	21st to end	Day	4 17% 23% 27% 20% 9%	270
December	1st to 10th	Day	7% 11% 29% 31% 17% 5%	270
	11th to 20th	Day	4 15% 28% 32% 14% 6%	201
	21st to end	Day	4 22% 30% 27% 14% 4	198
January	1st to 10th	Night	8% 7% 26% 31% 19% 9%	760
	11th to 20th	Night	4 6% 27% 37% 19% 7%	760
	21st to end	Night	8% 6% 26% 34% 19% 7%	836
February	1st to 10th	Night	6% 10% 27% 34% 19% 4	760
	11th to 20th	Night	5% 9% 26% 37% 19% 4	744
	21st to end	Night	5% 9% 32% 38% 14% 3	628
March	1st to 10th	Night	5% 11% 26% 40% 14% 4	760
	11th to 20th	Night	4 11% 23% 35% 21% 5%	760
	21st to end	Night	7% 14% 29% 32% 16% 2	836
April	1st to 10th	Night	5% 12% 28% 36% 17% 2	760
	11th to 20th	Night	5% 13% 30% 32% 16% 4	751
	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% 33% 26% 10%	804
November	1st to 10th	Night	4 5% 21% 31% 29% 9%	740
	11th to 20th	Night	3 5% 24% 39% 22% 6%	760
	21st to end	Night	3 9% 23% 35% 24% 6%	760
December	1st to 10th	Night	5% 7% 23% 40% 19% 6%	760
	11th to 20th	Night	7% 8% 24% 39% 19% 3	760
	21st to end	Night	4 12% 29% 36% 15% 3	836

Page generated at 2024-10-20 23:19:59. Version ac07

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