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 2024-11-01.

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 6% 4 4 4 4 6% 8% 42%	101028
	Night	15% 6% 2 6% 4 4 4 4 5% 7% 43%	97846
Transparency	Day	40% 6% 12% 23% 16% 3	100835
	Night	40% 6% 11% 22% 17% 4	97708
Seeing	Day	19% 11% 27% 24% 14% 5%	15031
	Night	17% 9% 28% 30% 14% 1	24668

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 4 3 3 3 3 4 6% 50%	2754
	11th to 20th	Day	15% 8% 2 6% 5% 5% 4 3 5% 5% 41%	2840
	21st to end	Day	18% 10% 2 6% 4 4 4 3 4 6% 39%	3124
February	1st to 10th	Day	13% 8% 3 7% 4 5% 5% 4 5% 7% 40%	1710
	11th to 20th	Day	18% 8% 2 5% 4 4 3 3 4 6% 42%	1691
	21st to end	Day	18% 8% 2 6% 4 5% 5% 3 5% 7% 36%	1593
March	1st to 10th	Day	20% 8% 2 7% 4 4 4 3 3 7% 37%	3435
	11th to 20th	Day	10% 5% 2 5% 3 4 4 3 6% 7% 51%	3480
	21st to end	Day	20% 6% 2 6% 4 4 4 3 5% 6% 38%	3815
April	1st to 10th	Day	11% 6% 2 6% 5% 4 4 3 5% 6% 48%	2278
	11th to 20th	Day	13% 6% 2 6% 4 3 4 3 4 7% 48%	2254
	21st to end	Day	16% 7% 2 5% 4 5% 4 3 6% 7% 41%	2280
May	1st to 10th	Day	19% 7% 3 7% 5% 4 5% 4 6% 8% 32%	4048
	11th to 20th	Day	22% 6% 2 6% 4 4 4 4 6% 8% 33%	4103
	21st to end	Day	10% 4 2 5% 4 4 5% 4 6% 10% 45%	4823
June	1st to 10th	Day	11% 5% 2 5% 5% 5% 5% 5% 6% 9% 43%	2660
	11th to 20th	Day	10% 5% 2 6% 5% 4 5% 5% 7% 9% 42%	2660
	21st to end	Day	8% 4 2 5% 6% 5% 5% 5% 7% 10% 44%	2660
July	1st to 10th	Day	6% 4 2 5% 5% 5% 6% 5% 9% 11% 41%	4389
	11th to 20th	Day	10% 5% 2 5% 5% 5% 5% 5% 8% 13% 36%	4120
	21st to end	Day	12% 6% 2 7% 6% 5% 6% 6% 7% 9% 34%	4466
August	1st to 10th	Day	15% 6% 2 6% 5% 5% 5% 4 7% 10% 33%	4104
	11th to 20th	Day	9% 5% 2 6% 6% 6% 6% 4 8% 9% 39%	3898
	21st to end	Day	10% 4 1 5% 4 4 4 4 6% 9% 49%	4180
September	1st to 10th	Day	13% 5% 2 5% 5% 4 4 4 5% 7% 45%	2280
	11th to 20th	Day	13% 5% 2 5% 4 4 5% 4 6% 7% 45%	2260
	21st to end	Day	22% 6% 2 5% 4 4 4 4 5% 6% 37%	1991
October	1st to 10th	Day	15% 6% 2 5% 4 3 4 3 5% 6% 48%	1900
	11th to 20th	Day	16% 4 2 4 4 4 4 3 4 5% 52%	1900
	21st to end	Day	13% 6% 2 5% 3 3 3 3 4 6% 52%	2090
November	1st to 10th	Day	13% 5% 1 5% 4 3 3 3 4 6% 54%	1843
	11th to 20th	Day	14% 6% 2 4 4 3 3 4 5% 6% 50%	1677
	21st to end	Day	15% 5% 2 5% 3 3 4 3 5% 5% 51%	1520
December	1st to 10th	Day	16% 6% 3 5% 3 3 4 4 5% 7% 42%	1520
	11th to 20th	Day	17% 7% 1 4 4 3 3 3 4 5% 50%	1910
	21st to end	Day	18% 7% 3 6% 4 4 4 3 5% 7% 38%	2772
January	1st to 10th	Night	15% 7% 2 4 3 3 3 3 4 5% 49%	4173
	11th to 20th	Night	17% 7% 2 5% 3 3 4 2 4 5% 48%	4120
	21st to end	Night	20% 8% 3 5% 4 3 3 3 3 5% 43%	4532
February	1st to 10th	Night	13% 8% 2 6% 4 4 5% 4 6% 6% 42%	2470
	11th to 20th	Night	19% 9% 3 5% 3 3 3 3 4 5% 44%	2373
	21st to end	Night	16% 8% 3 5% 4 3 4 4 4 6% 44%	1859
March	1st to 10th	Night	21% 7% 2 6% 3 3 4 4 5% 6% 39%	3418
	11th to 20th	Night	13% 5% 2 4 3 3 3 3 5% 8% 50%	3480
	21st to end	Night	14% 6% 2 6% 3 3 4 3 5% 6% 48%	3817
April	1st to 10th	Night	13% 7% 2 6% 4 4 4 3 5% 6% 46%	1901
	11th to 20th	Night	11% 4 2 6% 4 4 4 3 6% 7% 49%	1875
	21st to end	Night	16% 5% 2 5% 4 3 4 4 6% 8% 45%	1900
May	1st to 10th	Night	20% 9% 3 7% 4 4 4 4 6% 7% 32%	2863
	11th to 20th	Night	24% 8% 2 7% 4 4 4 4 5% 6% 32%	2785
	21st to end	Night	10% 5% 2 5% 4 5% 5% 3 7% 10% 43%	2732
June	1st to 10th	Night	11% 5% 2 5% 4 5% 5% 4 6% 9% 44%	1520
	11th to 20th	Night	11% 5% 2 6% 5% 5% 5% 5% 7% 9% 41%	1520
	21st to end	Night	8% 4 2 5% 4 4 6% 5% 8% 11% 42%	1520
July	1st to 10th	Night	7% 6% 2 6% 5% 5% 5% 4 8% 11% 40%	2566
	11th to 20th	Night	8% 6% 3 7% 5% 6% 6% 5% 8% 10% 36%	2840
	21st to end	Night	10% 8% 3 8% 6% 6% 7% 5% 7% 8% 33%	3087
August	1st to 10th	Night	16% 6% 2 8% 5% 6% 5% 4 6% 8% 33%	2825
	11th to 20th	Night	12% 7% 3 9% 5% 5% 5% 5% 6% 8% 36%	2899
	21st to end	Night	11% 6% 2 6% 4 4 5% 3 6% 9% 44%	3476
September	1st to 10th	Night	14% 6% 2 5% 3 4 5% 4 5% 6% 46%	1900
	11th to 20th	Night	11% 7% 2 5% 4 3 4 3 4 8% 48%	1915
	21st to end	Night	23% 7% 3 5% 4 2 4 3 4 6% 39%	2187
October	1st to 10th	Night	18% 6% 2 5% 4 3 3 3 5% 5% 46%	2280
	11th to 20th	Night	16% 5% 2 5% 2 3 3 3 4 6% 50%	2280
	21st to end	Night	13% 4 2 5% 3 3 4 3 5% 6% 52%	2508
November	1st to 10th	Night	12% 6% 2 4 4 3 3 3 4 5% 53%	2238
	11th to 20th	Night	14% 6% 2 5% 4 3 4 3 5% 6% 49%	2498
	21st to end	Night	13% 4 1 5% 3 4 4 4 5% 6% 50%	2660
December	1st to 10th	Night	16% 8% 3 6% 4 3 4 3 5% 5% 43%	2660
	11th to 20th	Night	17% 7% 2 5% 4 4 3 3 4 6% 46%	3285
	21st to end	Night	17% 7% 2 6% 4 4 4 4 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% 15% 18% 4	1593
March	1st to 10th	Day	48% 5% 9% 19% 13% 6%	3435
	11th to 20th	Day	30% 5% 9% 23% 25% 8%	3480
	21st to end	Day	47% 5% 11% 18% 16% 4	3815
April	1st to 10th	Day	35% 5% 12% 21% 19% 8%	2278
	11th to 20th	Day	35% 5% 12% 20% 22% 6%	2254
	21st to end	Day	42% 6% 11% 23% 13% 5%	2280
May	1st to 10th	Day	48% 9% 12% 19% 11% 2	4085
	11th to 20th	Day	47% 8% 12% 22% 10% 1	4107
	21st to end	Day	32% 9% 15% 22% 20% 2	4837
June	1st to 10th	Day	35% 8% 15% 25% 15% 1	2660
	11th to 20th	Day	36% 9% 14% 22% 17% 2	2660
	21st to end	Day	33% 10% 14% 25% 17% 1	2660
July	1st to 10th	Day	35% 8% 23% 19% 11% 6%	4320
	11th to 20th	Day	38% 7% 19% 27% 8% 1	4051
	21st to end	Day	45% 7% 13% 20% 15% 1	4324
August	1st to 10th	Day	45% 7% 17% 25% 7%	4091
	11th to 20th	Day	39% 7% 14% 24% 16% 1	3937
	21st to end	Day	32% 5% 17% 28% 17% 2	4180
September	1st to 10th	Day	38% 8% 10% 18% 21% 5%	2280
	11th to 20th	Day	37% 6% 9% 25% 20% 3	2260
	21st to end	Day	46% 6% 10% 18% 16% 4	1991
October	1st to 10th	Day	38% 5% 9% 24% 21% 4	1900
	11th to 20th	Day	36% 4 8% 24% 24% 5%	1900
	21st to end	Day	34% 4 8% 25% 23% 6%	2090
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	45% 5% 9% 22% 16% 3	3418
	11th to 20th	Night	31% 5% 8% 22% 27% 6%	3480
	21st to end	Night	38% 5% 10% 21% 21% 6%	3817
April	1st to 10th	Night	38% 5% 10% 17% 23% 6%	1901
	11th to 20th	Night	33% 7% 11% 22% 21% 6%	1875
	21st to end	Night	37% 6% 12% 25% 14% 6%	1900
May	1st to 10th	Night	49% 7% 13% 19% 10% 2	2890
	11th to 20th	Night	49% 9% 11% 20% 9% 1	2786
	21st to end	Night	34% 9% 17% 24% 13% 2	2739
June	1st to 10th	Night	34% 9% 14% 26% 15% 1	1520
	11th to 20th	Night	38% 8% 15% 24% 14% 1	1520
	21st to end	Night	33% 10% 16% 19% 19% 3	1520
July	1st to 10th	Night	36% 9% 24% 16% 12% 3	2525
	11th to 20th	Night	41% 7% 21% 22% 7% 1	2786
	21st to end	Night	47% 8% 14% 18% 12%	2988
August	1st to 10th	Night	48% 7% 15% 22% 7% 1	2814
	11th to 20th	Night	44% 6% 14% 22% 13%	2926
	21st to end	Night	37% 6% 12% 28% 16% 1	3476
September	1st to 10th	Night	38% 8% 12% 19% 16% 7%	1900
	11th to 20th	Night	35% 6% 11% 23% 22% 3	1915
	21st to end	Night	47% 5% 9% 19% 16% 3	2187
October	1st to 10th	Night	40% 5% 10% 22% 19% 4	2280
	11th to 20th	Night	36% 5% 7% 23% 22% 6%	2280
	21st to end	Night	33% 4 10% 24% 24% 6%	2508
November	1st to 10th	Night	34% 5% 9% 23% 23% 6%	2238
	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% 18% 29% 19% 3	382
March	1st to 10th	Day	24% 18% 35% 18% 4 1	460
	11th to 20th	Day	23% 22% 32% 16% 6% 1	460
	21st to end	Day	25% 16% 33% 19% 6% 1	506
April	1st to 10th	Day	24% 20% 28% 20% 6% 1	460
	11th to 20th	Day	20% 16% 30% 21% 10% 4	452
	21st to end	Day	23% 12% 35% 20% 9% 1	460
May	1st to 10th	Day	22% 13% 32% 20% 10% 3	463
	11th to 20th	Day	22% 10% 31% 22% 11% 3	487
	21st to end	Day	19% 4 26% 27% 19% 4	529
June	1st to 10th	Day	12% 4 29% 32% 17% 6%	490
	11th to 20th	Day	12% 8% 24% 31% 17% 8%	490
	21st to end	Day	12% 4 27% 32% 19% 6%	490
July	1st to 10th	Day	12% 2 18% 33% 25% 10%	485
	11th to 20th	Day	6% 2 17% 36% 27% 11%	484
	21st to end	Day	13% 3 20% 31% 24% 10%	532
August	1st to 10th	Day	12% 5% 21% 29% 23% 10%	486
	11th to 20th	Day	11% 4 18% 32% 26% 10%	479
	21st to end	Day	11% 4 18% 27% 25% 14%	539
September	1st to 10th	Day	15% 5% 18% 26% 21% 15%	490
	11th to 20th	Day	16% 5% 16% 30% 23% 11%	490
	21st to end	Day	29% 7% 23% 26% 10% 5%	490
October	1st to 10th	Day	21% 7% 22% 25% 18% 7%	490
	11th to 20th	Day	17% 12% 25% 26% 18% 2	490
	21st to end	Day	16% 12% 33% 26% 10% 3	539
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	20% 17% 34% 26% 3	760
	11th to 20th	Night	22% 16% 35% 20% 6%	760
	21st to end	Night	19% 15% 35% 26% 5%	836
April	1st to 10th	Night	18% 17% 37% 23% 5%	760
	11th to 20th	Night	14% 13% 37% 26% 9%	751
	21st to end	Night	18% 10% 34% 30% 7%	760
May	1st to 10th	Night	21% 8% 33% 26% 12%	607
	11th to 20th	Night	19% 5% 28% 34% 14% 1	547
	21st to end	Night	15% 2 20% 39% 22% 2	411
June	1st to 10th	Night	11% 2 22% 42% 23% 1	380
	11th to 20th	Night	11% 4 18% 44% 21% 2	380
	21st to end	Night	8% 3 23% 43% 21% 2	380
July	1st to 10th	Night	12% 3 15% 37% 29% 4	389
	11th to 20th	Night	7% 2 12% 39% 34% 6%	552
	21st to end	Night	8% 1 19% 39% 29% 4	624
August	1st to 10th	Night	12% 1 14% 38% 31% 4	567
	11th to 20th	Night	14% 3 13% 38% 29% 4	626
	21st to end	Night	12% 3 14% 34% 32% 6%	836
September	1st to 10th	Night	16% 2 13% 36% 29% 5%	760
	11th to 20th	Night	14% 4 15% 35% 28% 4	760
	21st to end	Night	26% 5% 19% 29% 20% 1	760
October	1st to 10th	Night	21% 5% 19% 30% 22% 3	760
	11th to 20th	Night	18% 8% 26% 31% 16% 1	760
	21st to end	Night	16% 9% 30% 31% 12% 1	836
November	1st to 10th	Night	14% 9% 31% 30% 14% 2	743
	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 2024-10-29 11:20:52. Version ac07

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