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 2024-02-20.

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	3 4 2 4 4 3 4 4 5% 7% 60%	78475
	Night	2 4 2 4 4 3 4 4 5% 7% 60%	80722
Transparency	Day	22% 6% 10% 19% 24% 20%	78184
	Night	22% 6% 12% 20% 24% 16%	80535
Seeing	Day	6% 14% 28% 30% 16% 5%	13961
	Night	5% 7% 23% 36% 23% 5%	23667

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 5% 7% 4 5% 5% 6% 9% 49%	2539
	11th to 20th	Day	1 8% 2 4 5% 4 6% 6% 7% 9% 49%	2540
	21st to end	Day	2 7% 3 6% 5% 3 4 6% 7% 11% 46%	3031
February	1st to 10th	Day	3 4 2 5% 4 3 4 4 5% 9% 56%	1900
	11th to 20th	Day	2 6% 3 4 5% 3 5% 5% 7% 10% 52%	1841
	21st to end	Day	2 6% 2 5% 5% 3 4 4 6% 8% 54%	1480
March	1st to 10th	Day	1 7% 4 6% 5% 2 5% 5% 5% 9% 50%	1800
	11th to 20th	Day	1 4 4 5% 5% 3 5% 5% 7% 9% 53%	1880
	21st to end	Day	2 5% 2 5% 5% 4 5% 5% 5% 9% 52%	2300
April	1st to 10th	Day	2 4 2 6% 5% 3 5% 5% 5% 6% 56%	2160
	11th to 20th	Day	3 4 2 4 4 2 4 5% 5% 7% 60%	2129
	21st to end	Day	3 4 2 3 4 3 4 4 5% 7% 62%	2250
May	1st to 10th	Day	3 3 2 4 4 3 4 4 5% 7% 60%	2340
	11th to 20th	Day	3 5% 2 5% 5% 3 4 4 5% 8% 56%	2301
	21st to end	Day	5% 5% 3 5% 6% 4 5% 5% 5% 7% 50%	2570
June	1st to 10th	Day	4 4 2 4 4 2 4 4 6% 6% 59%	2520
	11th to 20th	Day	2 3 2 4 4 2 4 3 4 5% 68%	2520
	21st to end	Day	2 2 1 3 3 2 3 3 4 4 72%	2520
July	1st to 10th	Day	3 3 2 4 5% 3 4 4 5% 6% 63%	2520
	11th to 20th	Day	5% 3 2 4 4 2 4 5% 4 5% 62%	2520
	21st to end	Day	5% 3 3 5% 5% 4 5% 5% 5% 6% 53%	2661
August	1st to 10th	Day	3 4 2 3 4 2 3 3 4 4 68%	2311
	11th to 20th	Day	2 2 1 3 4 2 3 3 5% 5% 69%	2122
	21st to end	Day	3 2 1 3 3 2 3 3 4 5% 73%	2376
September	1st to 10th	Day	2 3 2 3 3 3 4 4 4 4 69%	2160
	11th to 20th	Day	2 3 1 3 4 2 3 3 4 4 72%	1992
	21st to end	Day	2 2 1 3 3 2 2 3 3 5% 75%	1980
October	1st to 10th	Day	3 3 2 3 3 2 4 3 4 5% 69%	1980
	11th to 20th	Day	2 2 1 3 3 1 3 3 4 6% 72%	1881
	21st to end	Day	2 2 1 2 2 2 3 3 5% 6% 73%	1782
November	1st to 10th	Day	1 4 2 3 4 3 3 4 5% 9% 61%	1674
	11th to 20th	Day	1 3 1 2 2 2 3 3 5% 9% 68%	1710
	21st to end	Day	1 4 2 5% 4 3 4 5% 5% 7% 61%	1710
December	1st to 10th	Day	2 5% 2 3 3 2 4 4 5% 9% 61%	1710
	11th to 20th	Day	5% 12% 3 5% 5% 3 5% 4 6% 8% 43%	1971
	21st to end	Day	1 7% 3 6% 5% 4 5% 6% 6% 9% 49%	2794
January	1st to 10th	Night	2 5% 2 5% 6% 4 5% 4 6% 9% 52%	4406
	11th to 20th	Night	1 4 2 5% 6% 4 5% 5% 6% 9% 53%	4420
	21st to end	Night	4 11% 4 4 4 2 4 4 6% 9% 49%	4613
February	1st to 10th	Night	2 6% 2 4 4 2 4 4 7% 8% 57%	2280
	11th to 20th	Night	2 5% 2 3 3 2 3 5% 6% 9% 60%	2211
	21st to end	Night	2 5% 2 5% 4 2 4 6% 7% 8% 54%	1776
March	1st to 10th	Night	1 5% 3 5% 5% 4 5% 6% 7% 8% 51%	2160
	11th to 20th	Night	2 4 3 5% 5% 3 5% 5% 6% 9% 52%	2078
	21st to end	Night	3 5% 3 4 4 2 4 4 5% 8% 58%	2052
April	1st to 10th	Night	4 4 2 4 4 2 4 5% 6% 8% 58%	1800
	11th to 20th	Night	3 4 2 4 4 3 3 3 4 5% 66%	1779
	21st to end	Night	3 5% 2 4 3 2 4 4 5% 8% 61%	1709
May	1st to 10th	Night	2 3 2 4 5% 3 4 5% 5% 6% 61%	1620
	11th to 20th	Night	4 3 2 3 3 2 4 5% 6% 7% 60%	1594
	21st to end	Night	3 5% 2 5% 5% 3 4 6% 5% 8% 54%	1702
June	1st to 10th	Night	3 2 2 3 4 2 4 4 5% 6% 65%	1440
	11th to 20th	Night	4 3 1 3 3 2 3 3 4 4 70%	1440
	21st to end	Night	2 2 1 3 4 2 4 3 4 5% 72%	1440
July	1st to 10th	Night	3 4 2 4 5% 2 4 3 5% 7% 61%	1440
	11th to 20th	Night	3 3 2 5% 5% 3 5% 5% 5% 6% 58%	1440
	21st to end	Night	5% 5% 3 5% 5% 3 5% 5% 6% 7% 51%	1655
August	1st to 10th	Night	4 4 2 4 4 3 3 3 5% 6% 62%	1625
	11th to 20th	Night	2 2 1 2 3 2 3 4 4 6% 71%	1780
	21st to end	Night	3 1 1 2 3 2 3 3 3 5% 73%	1980
September	1st to 10th	Night	2 1 1 3 3 2 3 3 4 6% 73%	1800
	11th to 20th	Night	2 3 1 3 3 1 3 3 4 4 72%	1966
	21st to end	Night	3 3 1 2 2 2 2 2 3 4 78%	1980
October	1st to 10th	Night	4 2 1 2 3 2 4 4 4 5% 68%	1980
	11th to 20th	Night	2 2 1 3 3 2 3 3 3 6% 73%	2077
	21st to end	Night	1 2 1 2 2 1 3 2 4 6% 75%	2574
November	1st to 10th	Night	2 4 1 2 3 1 3 4 5% 8% 66%	2405
	11th to 20th	Night	2 2 1 3 3 2 4 4 5% 8% 66%	2470
	21st to end	Night	2 3 2 4 4 3 4 4 6% 7% 61%	2470
December	1st to 10th	Night	2 6% 2 4 4 3 4 5% 6% 8% 57%	2470
	11th to 20th	Night	4 11% 3 4 4 3 4 4 6% 10% 47%	3228
	21st to end	Night	2 6% 3 6% 6% 4 6% 6% 7% 10% 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%	1851
	21st to end	Day	24% 9% 13% 27% 21% 5%	1480
March	1st to 10th	Day	29% 7% 12% 27% 19% 5%	1800
	11th to 20th	Day	25% 6% 14% 25% 23% 7%	1880
	21st to end	Day	26% 8% 19% 26% 17% 3	2300
April	1st to 10th	Day	26% 7% 12% 25% 18% 11%	2160
	11th to 20th	Day	22% 6% 12% 23% 27% 10%	2132
	21st to end	Day	20% 6% 10% 30% 27% 7%	2250
May	1st to 10th	Day	22% 5% 8% 23% 31% 11%	2340
	11th to 20th	Day	26% 6% 9% 23% 28% 9%	2304
	21st to end	Day	31% 7% 10% 20% 25% 8%	2570
June	1st to 10th	Day	23% 5% 10% 16% 28% 19%	2520
	11th to 20th	Day	17% 4 6% 12% 27% 34%	2520
	21st to end	Day	14% 3 4 9% 23% 46%	2520
July	1st to 10th	Day	20% 3 6% 9% 14% 48%	2451
	11th to 20th	Day	21% 5% 5% 10% 18% 41%	2445
	21st to end	Day	26% 6% 8% 10% 19% 31%	2513
August	1st to 10th	Day	19% 3 5% 8% 18% 47%	2296
	11th to 20th	Day	15% 4 6% 10% 22% 44%	2122
	21st to end	Day	14% 2 4 9% 30% 41%	2376
September	1st to 10th	Day	17% 3 5% 9% 23% 43%	2160
	11th to 20th	Day	15% 3 5% 15% 27% 35%	1992
	21st to end	Day	13% 2 4 14% 34% 32%	1980
October	1st to 10th	Day	17% 3 7% 16% 33% 23%	1980
	11th to 20th	Day	14% 4 6% 18% 33% 26%	1881
	21st to end	Day	12% 4 8% 20% 40% 16%	1782
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%	2212
	21st to end	Night	24% 7% 15% 24% 24% 7%	1776
March	1st to 10th	Night	26% 7% 15% 23% 22% 7%	2160
	11th to 20th	Night	26% 7% 13% 23% 22% 9%	2078
	21st to end	Night	23% 6% 18% 31% 17% 5%	2052
April	1st to 10th	Night	24% 7% 15% 26% 20% 9%	1800
	11th to 20th	Night	22% 5% 10% 27% 25% 11%	1779
	21st to end	Night	21% 5% 11% 31% 26% 6%	1709
May	1st to 10th	Night	21% 5% 10% 23% 31% 10%	1620
	11th to 20th	Night	20% 5% 10% 22% 33% 9%	1595
	21st to end	Night	26% 7% 12% 24% 25% 7%	1702
June	1st to 10th	Night	17% 6% 12% 18% 30% 17%	1440
	11th to 20th	Night	18% 4 5% 14% 31% 28%	1440
	21st to end	Night	14% 4 5% 9% 26% 42%	1440
July	1st to 10th	Night	21% 4 6% 9% 16% 45%	1400
	11th to 20th	Night	23% 7% 8% 9% 17% 37%	1392
	21st to end	Night	26% 8% 8% 12% 16% 29%	1563
August	1st to 10th	Night	21% 4 7% 9% 18% 42%	1616
	11th to 20th	Night	12% 5% 7% 12% 22% 42%	1780
	21st to end	Night	14% 3 5% 10% 24% 43%	1980
September	1st to 10th	Night	13% 3 6% 9% 27% 42%	1800
	11th to 20th	Night	14% 3 7% 15% 27% 34%	1966
	21st to end	Night	13% 2 5% 13% 31% 37%	1980
October	1st to 10th	Night	17% 4 8% 17% 29% 24%	1980
	11th to 20th	Night	14% 4 6% 19% 32% 25%	2077
	21st to end	Night	12% 2 6% 24% 39% 17%	2574
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% 29% 29% 13% 4	452
	21st to end	Day	7% 14% 37% 31% 9% 3	346
March	1st to 10th	Day	6% 18% 33% 29% 12% 2	420
	11th to 20th	Day	5% 18% 29% 27% 16% 4	420
	21st to end	Day	8% 21% 32% 28% 9% 1	462
April	1st to 10th	Day	9% 20% 27% 28% 15% 3	420
	11th to 20th	Day	11% 24% 29% 25% 9% 2	412
	21st to end	Day	9% 19% 30% 25% 14% 3	420
May	1st to 10th	Day	7% 22% 34% 25% 9% 3	423
	11th to 20th	Day	9% 20% 30% 26% 11% 4	448
	21st to end	Day	10% 15% 27% 30% 13% 5%	485
June	1st to 10th	Day	6% 16% 26% 29% 16% 6%	450
	11th to 20th	Day	7% 18% 29% 28% 14% 4	450
	21st to end	Day	3 12% 28% 34% 20% 4	450
July	1st to 10th	Day	4 5% 29% 43% 14% 4	446
	11th to 20th	Day	5% 5% 24% 43% 19% 5%	444
	21st to end	Day	5% 2 24% 41% 21% 7%	487
August	1st to 10th	Day	5% 7% 33% 32% 18% 5%	447
	11th to 20th	Day	3 4 26% 36% 22% 9%	437
	21st to end	Day	4 6% 25% 38% 21% 6%	495
September	1st to 10th	Day	3 7% 28% 30% 23% 8%	450
	11th to 20th	Day	3 12% 27% 33% 19% 5%	450
	21st to end	Day	4 10% 26% 34% 20% 4	450
October	1st to 10th	Day	5% 16% 28% 28% 17% 6%	450
	11th to 20th	Day	4 13% 24% 34% 20% 5%	446
	21st to end	Day	5% 13% 25% 27% 25% 5%	297
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% 25% 37% 19% 4	742
	21st to end	Night	5% 8% 32% 37% 14% 4	592
March	1st to 10th	Night	5% 11% 25% 40% 15% 4	720
	11th to 20th	Night	4 11% 22% 36% 21% 6%	720
	21st to end	Night	7% 14% 29% 32% 16% 2	792
April	1st to 10th	Night	5% 11% 28% 37% 18% 2	720
	11th to 20th	Night	5% 14% 29% 32% 16% 4	711
	21st to end	Night	6% 13% 29% 33% 15% 4	629
May	1st to 10th	Night	5% 12% 33% 33% 14% 3	540
	11th to 20th	Night	7% 11% 24% 34% 21% 3	531
	21st to end	Night	7% 11% 27% 30% 23% 3	540
June	1st to 10th	Night	4 5% 21% 41% 26% 3	360
	11th to 20th	Night	4 8% 23% 38% 25% 2	360
	21st to end	Night	2 4 20% 47% 26% 1	360
July	1st to 10th	Night	3 1 17% 47% 29% 3	350
	11th to 20th	Night	3 1 14% 44% 32% 5%	348
	21st to end	Night	4 1 14% 44% 33% 3	473
August	1st to 10th	Night	5% 3 17% 44% 29% 1	551
	11th to 20th	Night	2 1 12% 42% 37% 6%	712
	21st to end	Night	4 2 11% 40% 38% 6%	792
September	1st to 10th	Night	3 3 13% 33% 40% 7%	720
	11th to 20th	Night	3 4 17% 35% 33% 9%	720
	21st to end	Night	4 5% 14% 34% 31% 12%	720
October	1st to 10th	Night	4 5% 22% 37% 25% 7%	720
	11th to 20th	Night	4 5% 17% 37% 29% 8%	720
	21st to end	Night	2 8% 21% 32% 26% 10%	792
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 2024-02-17 03:28:41. Version ac07

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