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 2025-05-11.

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% 43%	113799
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	115130
Transparency	Day	38% 5% 12% 22% 18% 5%	117444
	Night	38% 5% 11% 22% 19% 5%	119623
Seeing	Day	19% 11% 27% 24% 14% 5%	15649
	Night	17% 10% 29% 30% 13% 1	25423

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	16% 5% 2 4 3 3 4 3 5% 5% 51%	3946
	11th to 20th	Day	14% 8% 2 5% 4 4 3 2 4 4 50%	4060
	21st to end	Day	15% 7% 2 6% 4 4 4 3 4 5% 45%	4466
February	1st to 10th	Day	13% 8% 3 6% 4 4 5% 4 5% 6% 40%	1800
	11th to 20th	Day	20% 8% 2 5% 4 4 3 3 4 6% 41%	1783
	21st to end	Day	17% 8% 2 6% 4 5% 4 3 5% 6% 38%	1674
March	1st to 10th	Day	17% 6% 2 6% 4 4 4 4 5% 7% 40%	4935
	11th to 20th	Day	10% 5% 2 5% 4 3 4 3 5% 7% 52%	4980
	21st to end	Day	17% 6% 3 6% 4 4 4 4 5% 6% 41%	5465
April	1st to 10th	Day	12% 6% 2 6% 5% 4 4 3 5% 6% 47%	2398
	11th to 20th	Day	13% 6% 2 6% 4 3 4 3 4 7% 48%	2374
	21st to end	Day	15% 7% 2 6% 4 4 4 3 6% 7% 41%	2400
May	1st to 10th	Day	19% 7% 3 7% 5% 4 5% 4 6% 8% 32%	4177
	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	12% 5% 1 5% 4 3 3 3 5% 6% 54%	1942
	11th to 20th	Day	14% 6% 2 4 4 3 3 4 5% 6% 49%	1765
	21st to end	Day	14% 5% 2 5% 3 3 4 3 5% 5% 51%	1600
December	1st to 10th	Day	12% 5% 2 5% 3 3 5% 4 4 6% 51%	2600
	11th to 20th	Day	19% 7% 2 5% 3 3 3 3 4 6% 46%	2990
	21st to end	Day	18% 7% 3 6% 4 3 4 3 5% 6% 40%	3960
January	1st to 10th	Night	14% 6% 2 4 3 3 3 3 4 5% 54%	5981
	11th to 20th	Night	14% 7% 2 4 3 2 3 2 4 4 55%	5900
	21st to end	Night	15% 7% 2 6% 4 3 3 3 4 5% 49%	6490
February	1st to 10th	Night	14% 8% 2 6% 4 5% 5% 4 5% 6% 42%	2600
	11th to 20th	Night	19% 8% 3 6% 3 3 3 3 4 5% 43%	2501
	21st to end	Night	15% 8% 3 5% 4 3 4 4 4 6% 45%	1953
March	1st to 10th	Night	16% 6% 2 5% 3 3 3 3 5% 6% 48%	4918
	11th to 20th	Night	12% 5% 2 4 3 3 3 3 5% 8% 51%	4980
	21st to end	Night	12% 6% 2 6% 4 4 4 3 5% 6% 46%	5467
April	1st to 10th	Night	14% 7% 2 6% 4 4 4 3 5% 6% 45%	2001
	11th to 20th	Night	11% 4 2 6% 4 4 4 4 6% 7% 48%	1975
	21st to end	Night	15% 5% 2 5% 4 3 4 4 6% 7% 45%	2000
May	1st to 10th	Night	20% 9% 3 7% 4 4 4 4 6% 7% 33%	2955
	11th to 20th	Night	24% 8% 3 7% 4 4 4 4 5% 6% 32%	2791
	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 5% 4 3 3 3 5% 5% 53%	2352
	11th to 20th	Night	14% 6% 2 5% 4 3 4 3 5% 6% 49%	2630
	21st to end	Night	12% 4 1 5% 3 4 4 4 5% 6% 51%	2800
December	1st to 10th	Night	10% 5% 2 5% 3 3 3 3 5% 7% 54%	4580
	11th to 20th	Night	20% 7% 2 5% 4 4 4 3 4 5% 42%	5205
	21st to end	Night	17% 6% 2 5% 3 4 4 4 6% 7% 43%	6996

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% 3 5% 25% 22% 9%	3947
	11th to 20th	Day	38% 3 6% 19% 22% 12%	4060
	21st to end	Day	42% 4 7% 25% 18% 4	4466
February	1st to 10th	Day	43% 5% 10% 26% 13% 3	1800
	11th to 20th	Day	45% 4 7% 21% 19% 4	1792
	21st to end	Day	46% 5% 10% 16% 18% 5%	1674
March	1st to 10th	Day	43% 4 7% 19% 17% 9%	4935
	11th to 20th	Day	32% 4 8% 24% 23% 8%	4980
	21st to end	Day	44% 4 11% 17% 17% 7%	5465
April	1st to 10th	Day	37% 4 11% 21% 19% 8%	2398
	11th to 20th	Day	36% 5% 12% 20% 22% 6%	2374
	21st to end	Day	41% 6% 11% 24% 14% 5%	2400
May	1st to 10th	Day	48% 9% 12% 19% 11% 1	4214
	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	36% 3 7% 19% 30% 5%	3160
	11th to 20th	Day	26% 3 6% 23% 31% 11%	3140
	21st to end	Day	22% 2 10% 29% 24% 13%	3428
November	1st to 10th	Day	33% 4 8% 24% 26% 5%	1942
	11th to 20th	Day	34% 5% 10% 21% 22% 7%	1765
	21st to end	Day	36% 5% 9% 24% 23% 4	1600
December	1st to 10th	Day	36% 3 9% 26% 18% 8%	2600
	11th to 20th	Day	40% 3 13% 25% 15% 4	2986
	21st to end	Day	44% 4 12% 23% 15% 1	3960
January	1st to 10th	Night	33% 3 7% 22% 27% 8%	5981
	11th to 20th	Night	34% 3 6% 22% 25% 10%	5900
	21st to end	Night	40% 3 6% 27% 20% 4	6490
February	1st to 10th	Night	42% 5% 11% 24% 16% 3	2600
	11th to 20th	Night	44% 4 7% 20% 19% 7%	2503
	21st to end	Night	41% 4 10% 20% 18% 8%	1953
March	1st to 10th	Night	38% 4 8% 22% 20% 8%	4918
	11th to 20th	Night	31% 4 9% 21% 27% 8%	4980
	21st to end	Night	38% 4 9% 21% 21% 7%	5467
April	1st to 10th	Night	39% 5% 10% 17% 23% 6%	2001
	11th to 20th	Night	34% 7% 10% 22% 21% 6%	1975
	21st to end	Night	37% 6% 12% 25% 15% 6%	2000
May	1st to 10th	Night	49% 7% 14% 18% 10% 2	2982
	11th to 20th	Night	49% 9% 11% 20% 9% 1	2792
	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	42% 3 9% 18% 21% 7%	3800
	11th to 20th	Night	25% 3 8% 22% 31% 11%	3779
	21st to end	Night	21% 3 7% 31% 27% 11%	4120
November	1st to 10th	Night	33% 5% 9% 23% 24% 6%	2352
	11th to 20th	Night	37% 4 8% 20% 24% 7%	2630
	21st to end	Night	34% 5% 9% 24% 23% 5%	2800
December	1st to 10th	Night	31% 3 10% 28% 24% 4	4580
	11th to 20th	Night	45% 3 10% 19% 16% 6%	5208
	21st to end	Night	40% 4 13% 25% 17% 1	6996

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	26% 25% 29% 16% 4	277
	11th to 20th	Day	25% 28% 30% 14% 3	300
	21st to end	Day	26% 22% 37% 11% 4	330
February	1st to 10th	Day	25% 21% 37% 13% 3	300
	11th to 20th	Day	37% 22% 30% 10% 1	330
	21st to end	Day	30% 17% 30% 19% 3	414
March	1st to 10th	Day	24% 18% 34% 19% 4 1	500
	11th to 20th	Day	23% 21% 33% 16% 6% 1	500
	21st to end	Day	25% 17% 34% 18% 5% 1	550
April	1st to 10th	Day	25% 21% 28% 19% 6% 1	500
	11th to 20th	Day	20% 16% 30% 21% 9% 3	492
	21st to end	Day	22% 11% 35% 21% 10% 1	500
May	1st to 10th	Day	21% 13% 34% 19% 10% 3	503
	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	17% 9% 32% 26% 13% 3	488
	11th to 20th	Day	20% 10% 35% 23% 9% 2	365
	21st to end	Day	21% 17% 33% 22% 8% 1	200
December	1st to 10th	Day	22% 21% 34% 17% 7% 1	200
	11th to 20th	Day	24% 14% 33% 21% 9% 1	200
	21st to end	Day	27% 15% 32% 18% 6%	220
January	1st to 10th	Night	21% 16% 41% 19% 3	800
	11th to 20th	Night	21% 15% 34% 27% 4	800
	21st to end	Night	19% 16% 38% 25% 2	880
February	1st to 10th	Night	20% 14% 37% 26% 3	800
	11th to 20th	Night	26% 17% 37% 18% 2	783
	21st to end	Night	21% 17% 38% 22% 2	660
March	1st to 10th	Night	19% 18% 35% 26% 3	800
	11th to 20th	Night	22% 16% 36% 20% 6%	800
	21st to end	Night	18% 16% 36% 25% 5%	880
April	1st to 10th	Night	18% 18% 37% 22% 5%	800
	11th to 20th	Night	14% 14% 37% 26% 9%	791
	21st to end	Night	17% 10% 35% 30% 7%	800
May	1st to 10th	Night	21% 8% 34% 26% 12%	639
	11th to 20th	Night	19% 5% 28% 34% 14% 1	549
	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% 31% 13% 2	780
	11th to 20th	Night	18% 11% 34% 29% 8%	800
	21st to end	Night	14% 11% 35% 29% 10% 1	800
December	1st to 10th	Night	20% 11% 37% 23% 8%	800
	11th to 20th	Night	20% 10% 35% 30% 6%	800
	21st to end	Night	21% 12% 30% 33% 5%	880

Page generated at 2025-05-08 09:34:01. Version ac07

Copyright © 2025 Attilla Danko.