Belmont Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Belmont Observatory clear sky chart from 2005-11-06 to 2024-03-01.

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	14% 6% 2 5% 5% 3 5% 5% 5% 7% 43%	78540
	Night	15% 6% 2 5% 4 3 4 4 5% 7% 45%	80848
Transparency	Day	39% 7% 11% 23% 17% 4	78257
	Night	39% 6% 11% 22% 18% 4	80650
Seeing	Day	19% 11% 27% 24% 14% 5%	14051
	Night	17% 9% 28% 30% 14% 1	23845

By Month

Percentage Cloud Cover SE (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	19% 5% 2 3 3 2 3 3 5% 6% 50%	2754
	11th to 20th	Day	15% 8% 3 6% 6% 3 4 4 5% 5% 42%	2840
	21st to end	Day	18% 9% 3 5% 5% 3 4 4 4 6% 39%	3124
February	1st to 10th	Day	13% 8% 3 6% 5% 4 4 5% 5% 7% 40%	1710
	11th to 20th	Day	19% 7% 3 4 5% 3 4 3 4 6% 42%	1691
	21st to end	Day	18% 8% 3 5% 5% 4 5% 4 5% 7% 36%	1593
March	1st to 10th	Day	13% 6% 2 5% 5% 3 3 4 3 7% 49%	1980
	11th to 20th	Day	16% 6% 3 4 5% 3 4 4 6% 6% 41%	1980
	21st to end	Day	16% 6% 3 6% 6% 4 5% 5% 5% 7% 38%	2178
April	1st to 10th	Day	11% 5% 2 4 5% 2 5% 3 6% 7% 49%	2158
	11th to 20th	Day	13% 5% 3 5% 5% 2 4 4 4 7% 48%	2134
	21st to end	Day	16% 6% 3 5% 5% 3 4 4 6% 7% 41%	2160
May	1st to 10th	Day	16% 6% 2 5% 4 3 4 5% 6% 9% 40%	2319
	11th to 20th	Day	18% 6% 3 5% 5% 3 6% 5% 5% 7% 38%	2309
	21st to end	Day	15% 5% 2 5% 6% 3 5% 5% 5% 9% 42%	2725
June	1st to 10th	Day	12% 4 2 4 5% 3 5% 6% 6% 9% 44%	2520
	11th to 20th	Day	11% 4 3 5% 5% 3 5% 6% 7% 9% 41%	2520
	21st to end	Day	9% 3 2 4 6% 4 5% 7% 7% 10% 44%	2520
July	1st to 10th	Day	10% 5% 2 5% 7% 3 5% 5% 7% 9% 40%	2497
	11th to 20th	Day	5% 4 2 5% 5% 4 7% 7% 8% 12% 41%	2340
	21st to end	Day	10% 5% 2 5% 6% 4 6% 8% 8% 10% 36%	2547
August	1st to 10th	Day	9% 5% 3 6% 7% 4 7% 6% 7% 10% 36%	2327
	11th to 20th	Day	12% 5% 3 6% 7% 4 6% 6% 7% 9% 35%	2227
	21st to end	Day	11% 4 2 5% 5% 4 5% 6% 8% 9% 41%	2376
September	1st to 10th	Day	14% 5% 2 4 6% 3 4 5% 5% 7% 44%	2160
	11th to 20th	Day	12% 5% 2 5% 4 3 5% 5% 6% 8% 46%	2142
	21st to end	Day	20% 6% 2 4 5% 3 5% 5% 5% 6% 38%	1887
October	1st to 10th	Day	15% 6% 2 4 5% 2 4 4 5% 6% 47%	1800
	11th to 20th	Day	17% 4 2 4 5% 3 4 4 4 5% 50%	1800
	21st to end	Day	14% 6% 2 5% 4 2 3 3 4 6% 50%	1980
November	1st to 10th	Day	13% 4 2 4 4 2 4 3 4 6% 54%	1843
	11th to 20th	Day	14% 5% 2 4 4 2 4 5% 5% 6% 50%	1677
	21st to end	Day	15% 4 2 4 4 2 4 4 4 6% 51%	1520
December	1st to 10th	Day	17% 6% 3 5% 4 3 4 5% 5% 7% 42%	1520
	11th to 20th	Day	17% 6% 2 4 4 2 2 4 4 4 51%	1910
	21st to end	Day	19% 7% 3 6% 5% 3 4 4 5% 7% 38%	2772
January	1st to 10th	Night	15% 7% 2 4 4 2 4 3 4 6% 49%	4173
	11th to 20th	Night	18% 7% 2 4 4 2 4 3 4 5% 48%	4120
	21st to end	Night	20% 8% 3 5% 5% 2 3 3 3 5% 43%	4532
February	1st to 10th	Night	14% 8% 2 5% 5% 3 5% 5% 6% 6% 42%	2470
	11th to 20th	Night	19% 8% 3 5% 4 2 3 3 4 5% 44%	2373
	21st to end	Night	16% 8% 3 4 5% 2 4 4 4 6% 44%	1859
March	1st to 10th	Night	14% 5% 2 4 3 3 4 4 4 6% 51%	1987
	11th to 20th	Night	20% 5% 2 4 4 3 3 4 5% 8% 42%	1980
	21st to end	Night	16% 5% 3 7% 5% 2 4 4 6% 6% 43%	2178
April	1st to 10th	Night	13% 7% 2 5% 5% 2 4 4 5% 7% 46%	1801
	11th to 20th	Night	11% 4 3 5% 4 2 4 4 6% 7% 49%	1775
	21st to end	Night	17% 4 2 4 4 2 3 5% 5% 9% 45%	1800
May	1st to 10th	Night	17% 6% 2 6% 5% 3 4 5% 5% 7% 40%	1656
	11th to 20th	Night	17% 6% 2 5% 5% 3 5% 5% 7% 8% 36%	1579
	21st to end	Night	13% 6% 3 5% 5% 3 4 5% 7% 8% 41%	1550
June	1st to 10th	Night	11% 5% 2 5% 4 3 5% 5% 6% 9% 45%	1440
	11th to 20th	Night	11% 5% 3 5% 6% 4 6% 5% 7% 9% 40%	1440
	21st to end	Night	8% 3 3 5% 5% 3 6% 6% 8% 11% 42%	1440
July	1st to 10th	Night	9% 7% 2 5% 6% 3 5% 4 8% 10% 41%	1458
	11th to 20th	Night	5% 4 3 4 6% 4 6% 6% 7% 12% 43%	1620
	21st to end	Night	8% 3 3 5% 6% 5% 7% 6% 7% 10% 39%	1771
August	1st to 10th	Night	9% 4 2 6% 6% 5% 7% 6% 7% 8% 40%	1611
	11th to 20th	Night	13% 6% 2 6% 7% 4 4 6% 6% 9% 37%	1656
	21st to end	Night	10% 6% 2 7% 6% 4 6% 5% 7% 11% 36%	1980
September	1st to 10th	Night	15% 6% 2 4 4 3 4 4 6% 7% 45%	1800
	11th to 20th	Night	11% 7% 3 5% 5% 2 4 4 4 8% 49%	1814
	21st to end	Night	21% 7% 3 5% 4 2 3 4 4 7% 41%	2071
October	1st to 10th	Night	18% 6% 2 5% 5% 2 3 4 5% 6% 46%	2160
	11th to 20th	Night	17% 5% 3 5% 3 3 4 4 4 6% 48%	2160
	21st to end	Night	14% 4 2 4 4 2 4 4 5% 6% 50%	2376
November	1st to 10th	Night	12% 6% 2 4 4 3 3 4 4 5% 53%	2231
	11th to 20th	Night	14% 5% 3 4 4 2 5% 3 5% 6% 50%	2498
	21st to end	Night	13% 4 2 4 4 3 4 5% 5% 6% 51%	2660
December	1st to 10th	Night	16% 8% 3 5% 5% 2 4 4 5% 6% 43%	2660
	11th to 20th	Night	17% 6% 3 4 4 3 4 4 4 6% 46%	3285
	21st to end	Night	17% 6% 3 5% 4 3 4 5% 6% 7% 40%	4884

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	36% 4 7% 31% 17% 5%	2755
	11th to 20th	Day	43% 5% 9% 19% 18% 6%	2840
	21st to end	Day	47% 5% 9% 21% 15% 3	3124
February	1st to 10th	Day	43% 5% 10% 26% 13% 3	1710
	11th to 20th	Day	44% 4 7% 21% 19% 5%	1700
	21st to end	Day	47% 5% 11% 14% 18% 5%	1593
March	1st to 10th	Day	37% 4 9% 23% 22% 5%	1980
	11th to 20th	Day	42% 5% 10% 18% 18% 7%	1980
	21st to end	Day	46% 6% 9% 17% 17% 5%	2178
April	1st to 10th	Day	35% 4 12% 22% 19% 8%	2158
	11th to 20th	Day	36% 5% 12% 19% 22% 6%	2134
	21st to end	Day	42% 6% 11% 23% 13% 5%	2160
May	1st to 10th	Day	40% 7% 15% 20% 15% 3	2319
	11th to 20th	Day	45% 8% 12% 22% 11% 2	2313
	21st to end	Day	40% 8% 14% 25% 12%	2725
June	1st to 10th	Day	36% 8% 15% 25% 15% 1	2520
	11th to 20th	Day	37% 9% 14% 21% 16% 2	2520
	21st to end	Day	33% 10% 14% 25% 16% 1	2520
July	1st to 10th	Day	39% 10% 12% 20% 16% 3	2428
	11th to 20th	Day	33% 10% 17% 26% 13% 2	2271
	21st to end	Day	39% 10% 13% 22% 16% 1	2405
August	1st to 10th	Day	41% 10% 14% 24% 10% 1	2314
	11th to 20th	Day	43% 10% 13% 21% 12% 1	2227
	21st to end	Day	36% 9% 16% 23% 13% 3	2376
September	1st to 10th	Day	39% 8% 11% 19% 19% 4	2160
	11th to 20th	Day	36% 6% 10% 25% 20% 4	2142
	21st to end	Day	45% 6% 10% 19% 17% 3	1887
October	1st to 10th	Day	38% 5% 10% 25% 19% 4	1800
	11th to 20th	Day	37% 4 8% 24% 23% 4	1800
	21st to end	Day	36% 4 8% 24% 23% 5%	1980
November	1st to 10th	Day	33% 5% 8% 23% 26% 5%	1843
	11th to 20th	Day	34% 5% 10% 22% 22% 7%	1677
	21st to end	Day	35% 5% 9% 24% 23% 4	1520
December	1st to 10th	Day	42% 5% 10% 24% 14% 4	1520
	11th to 20th	Day	37% 5% 7% 30% 15% 6%	1906
	21st to end	Day	44% 6% 11% 26% 12% 1	2772
January	1st to 10th	Night	37% 3 9% 25% 21% 5%	4173
	11th to 20th	Night	40% 4 8% 21% 22% 5%	4120
	21st to end	Night	45% 4 7% 24% 17% 3	4532
February	1st to 10th	Night	41% 5% 10% 24% 16% 3	2470
	11th to 20th	Night	43% 4 6% 20% 19% 7%	2375
	21st to end	Night	42% 4 10% 20% 17% 6%	1859
March	1st to 10th	Night	35% 5% 10% 24% 23% 4	1987
	11th to 20th	Night	42% 5% 9% 20% 20% 4	1980
	21st to end	Night	42% 5% 10% 21% 16% 6%	2178
April	1st to 10th	Night	38% 5% 10% 18% 23% 6%	1801
	11th to 20th	Night	33% 7% 10% 21% 22% 6%	1775
	21st to end	Night	37% 6% 12% 25% 14% 6%	1800
May	1st to 10th	Night	42% 7% 13% 21% 14% 4	1656
	11th to 20th	Night	43% 9% 12% 19% 14% 2	1580
	21st to end	Night	40% 9% 14% 23% 12% 1	1551
June	1st to 10th	Night	34% 8% 14% 27% 15% 1	1440
	11th to 20th	Night	39% 8% 16% 22% 13% 1	1440
	21st to end	Night	33% 10% 16% 19% 19% 3	1440
July	1st to 10th	Night	37% 10% 17% 18% 14% 4	1417
	11th to 20th	Night	32% 10% 19% 25% 12% 2	1566
	21st to end	Night	35% 10% 16% 24% 13% 1	1672
August	1st to 10th	Night	39% 11% 15% 22% 12% 1	1600
	11th to 20th	Night	42% 9% 15% 21% 12% 1	1656
	21st to end	Night	40% 8% 15% 22% 12% 2	1980
September	1st to 10th	Night	38% 8% 13% 20% 16% 6%	1800
	11th to 20th	Night	35% 6% 11% 24% 22% 3	1814
	21st to end	Night	45% 6% 10% 20% 17% 3	2071
October	1st to 10th	Night	39% 5% 10% 23% 19% 3	2160
	11th to 20th	Night	38% 5% 7% 23% 21% 5%	2160
	21st to end	Night	34% 5% 10% 22% 23% 5%	2376
November	1st to 10th	Night	34% 5% 9% 23% 23% 6%	2231
	11th to 20th	Night	37% 4 8% 20% 24% 7%	2498
	21st to end	Night	34% 5% 9% 25% 22% 5%	2660
December	1st to 10th	Night	43% 5% 10% 23% 15% 4	2660
	11th to 20th	Night	40% 5% 9% 21% 20% 6%	3288
	21st to end	Night	41% 6% 13% 23% 15% 2	4884

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% 21% 30% 17% 4	249
	11th to 20th	Day	26% 28% 28% 14% 4	270
	21st to end	Day	27% 21% 37% 11% 4	297
February	1st to 10th	Day	25% 21% 36% 14% 4	270
	11th to 20th	Day	36% 21% 32% 10% 1	298
	21st to end	Day	31% 19% 28% 19% 3	382
March	1st to 10th	Day	23% 19% 35% 18% 4	420
	11th to 20th	Day	24% 21% 32% 16% 6% 1	420
	21st to end	Day	25% 17% 33% 18% 6% 1	462
April	1st to 10th	Day	24% 19% 28% 21% 7% 1	420
	11th to 20th	Day	20% 15% 29% 21% 11% 4	412
	21st to end	Day	24% 12% 34% 20% 9% 1	420
May	1st to 10th	Day	21% 13% 33% 21% 9% 3	423
	11th to 20th	Day	22% 10% 32% 23% 10% 3	447
	21st to end	Day	21% 4 24% 27% 20% 4	485
June	1st to 10th	Day	13% 3 28% 33% 18% 6%	450
	11th to 20th	Day	13% 8% 24% 30% 16% 9%	450
	21st to end	Day	11% 4 28% 32% 19% 6%	450
July	1st to 10th	Day	13% 2 18% 31% 25% 10%	445
	11th to 20th	Day	5% 3 17% 36% 28% 11%	444
	21st to end	Day	12% 2 20% 30% 24% 10%	488
August	1st to 10th	Day	11% 5% 20% 29% 24% 11%	446
	11th to 20th	Day	11% 4 15% 31% 28% 11%	439
	21st to end	Day	12% 4 18% 27% 25% 14%	495
September	1st to 10th	Day	16% 5% 18% 25% 20% 16%	450
	11th to 20th	Day	15% 5% 15% 30% 24% 11%	450
	21st to end	Day	27% 7% 22% 27% 11% 5%	450
October	1st to 10th	Day	22% 7% 22% 24% 18% 7%	450
	11th to 20th	Day	18% 12% 25% 25% 18% 2	450
	21st to end	Day	18% 13% 33% 25% 9% 2	495
November	1st to 10th	Day	18% 9% 31% 26% 14% 3	449
	11th to 20th	Day	20% 11% 33% 24% 10% 2	337
	21st to end	Day	20% 14% 33% 24% 8% 1	180
December	1st to 10th	Day	22% 21% 33% 16% 7% 1	180
	11th to 20th	Day	23% 14% 33% 19% 9% 1	180
	21st to end	Day	27% 15% 33% 17% 7% 1	198
January	1st to 10th	Night	21% 15% 40% 20% 3	760
	11th to 20th	Night	22% 14% 33% 27% 4	760
	21st to end	Night	20% 16% 37% 25% 2	836
February	1st to 10th	Night	19% 14% 36% 27% 3	760
	11th to 20th	Night	26% 17% 37% 18% 2	743
	21st to end	Night	22% 18% 36% 22% 2	628
March	1st to 10th	Night	19% 17% 36% 26% 3	723
	11th to 20th	Night	23% 15% 35% 20% 6%	720
	21st to end	Night	19% 15% 36% 26% 5%	792
April	1st to 10th	Night	17% 17% 37% 23% 6%	720
	11th to 20th	Night	14% 13% 36% 27% 10%	711
	21st to end	Night	19% 10% 33% 31% 7%	720
May	1st to 10th	Night	20% 9% 33% 25% 12% 1	576
	11th to 20th	Night	19% 4 28% 33% 15% 1	518
	21st to end	Night	15% 2 20% 39% 22% 2	389
June	1st to 10th	Night	11% 2 22% 41% 24% 1	360
	11th to 20th	Night	12% 4 18% 42% 21% 2	360
	21st to end	Night	8% 3 22% 44% 21% 2	360
July	1st to 10th	Night	12% 3 16% 35% 30% 4	367
	11th to 20th	Night	7% 2 11% 39% 35% 6%	522
	21st to end	Night	7% 1 18% 38% 31% 4	591
August	1st to 10th	Night	11% 1 14% 40% 31% 4	537
	11th to 20th	Night	15% 3 12% 38% 29% 4	592
	21st to end	Night	12% 3 14% 33% 32% 6%	792
September	1st to 10th	Night	17% 2 13% 33% 30% 5%	720
	11th to 20th	Night	13% 4 15% 36% 28% 4	720
	21st to end	Night	24% 5% 20% 29% 21% 1	720
October	1st to 10th	Night	21% 5% 19% 30% 22% 3	720
	11th to 20th	Night	19% 7% 26% 30% 16% 1	720
	21st to end	Night	17% 10% 31% 30% 11% 1	792
November	1st to 10th	Night	14% 9% 31% 30% 14% 2	740
	11th to 20th	Night	18% 11% 34% 28% 8%	760
	21st to end	Night	14% 11% 34% 29% 11% 1	760
December	1st to 10th	Night	21% 11% 37% 23% 9%	760
	11th to 20th	Night	19% 9% 36% 29% 6%	760
	21st to end	Night	21% 12% 29% 34% 5%	836

Page generated at 2024-02-27 06:30:19. Version ac07

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