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 2026-09-14.

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 5% 4 6% 8% 42%	148686
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 44%	142371
Transparency	Day	39% 5% 12% 22% 17% 4	152288
	Night	39% 5% 11% 22% 18% 5%	146839
Seeing	Day	19% 11% 27% 24% 14% 5%	17467
	Night	17% 10% 29% 30% 13% 1	27078

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 5% 3 4 4 3 5% 6% 47%	5122
	11th to 20th	Day	13% 7% 2 5% 3 3 3 2 3 4 53%	5280
	21st to end	Day	14% 7% 2 7% 4 4 5% 3 4 5% 45%	5780
February	1st to 10th	Day	13% 8% 3 6% 4 4 5% 4 5% 6% 42%	1890
	11th to 20th	Day	19% 8% 2 5% 4 3 3 3 4 6% 41%	1875
	21st to end	Day	18% 8% 2 6% 5% 5% 4 3 5% 6% 38%	1755
March	1st to 10th	Day	17% 6% 2 6% 4 4 4 4 5% 7% 40%	5045
	11th to 20th	Day	10% 5% 2 5% 4 3 4 3 5% 7% 51%	5090
	21st to end	Day	17% 6% 2 6% 4 4 4 4 5% 6% 41%	5586
April	1st to 10th	Day	11% 6% 2 5% 5% 3 4 3 5% 7% 48%	2518
	11th to 20th	Day	13% 5% 2 6% 4 3 4 3 5% 7% 48%	2494
	21st to end	Day	15% 7% 2 6% 4 4 4 3 6% 7% 41%	2520
May	1st to 10th	Day	15% 6% 2 6% 5% 4 5% 4 6% 9% 37%	7554
	11th to 20th	Day	18% 5% 2 5% 4 4 4 4 6% 9% 39%	7662
	21st to end	Day	21% 5% 2 5% 4 4 4 3 6% 8% 38%	9045
June	1st to 10th	Day	11% 5% 2 5% 4 5% 5% 5% 7% 9% 44%	2940
	11th to 20th	Day	11% 4 2 6% 5% 4 5% 5% 7% 9% 42%	2940
	21st to end	Day	9% 4 2 5% 5% 4 5% 5% 6% 10% 44%	2940
July	1st to 10th	Day	9% 5% 2 6% 5% 4 5% 4 7% 10% 43%	8187
	11th to 20th	Day	10% 6% 3 6% 5% 6% 6% 6% 9% 13% 30%	7680
	21st to end	Day	9% 5% 2 6% 5% 5% 6% 5% 7% 10% 39%	8340
August	1st to 10th	Day	13% 6% 3 7% 6% 6% 6% 5% 7% 11% 30%	6014
	11th to 20th	Day	9% 5% 3 7% 6% 6% 6% 4 8% 11% 35%	5746
	21st to end	Day	9% 3 1 5% 3 4 5% 4 7% 10% 48%	6080
September	1st to 10th	Day	12% 5% 2 5% 5% 4 4 3 5% 7% 47%	2520
	11th to 20th	Day	13% 5% 2 5% 4 4 5% 4 6% 7% 45%	2415
	21st to end	Day	22% 6% 2 5% 4 4 4 4 5% 6% 36%	2096
October	1st to 10th	Day	14% 6% 2 5% 4 3 4 3 5% 6% 49%	2000
	11th to 20th	Day	16% 4 2 4 4 4 4 3 4 5% 52%	2000
	21st to end	Day	13% 6% 2 5% 3 3 3 3 4 6% 52%	2200
November	1st to 10th	Day	12% 5% 1 5% 4 3 3 3 5% 6% 54%	2041
	11th to 20th	Day	13% 6% 2 4 4 2 3 4 5% 6% 50%	1853
	21st to end	Day	15% 5% 2 5% 3 3 4 3 4 5% 50%	1680
December	1st to 10th	Day	12% 6% 2 5% 3 3 5% 4 4 6% 50%	2680
	11th to 20th	Day	18% 7% 2 5% 3 3 3 3 4 6% 46%	3070
	21st to end	Day	18% 7% 3 6% 4 3 4 3 5% 6% 40%	4048
January	1st to 10th	Night	12% 6% 2 4 3 4 4 4 5% 6% 50%	7753
	11th to 20th	Night	14% 7% 2 5% 3 2 3 2 3 4 55%	7680
	21st to end	Night	14% 6% 2 6% 4 4 3 3 4 5% 50%	8401
February	1st to 10th	Night	13% 8% 2 6% 4 4 4 4 5% 6% 43%	2730
	11th to 20th	Night	19% 8% 3 6% 3 3 3 3 4 5% 43%	2629
	21st to end	Night	15% 8% 3 5% 4 3 4 3 4 6% 45%	2048
March	1st to 10th	Night	16% 6% 2 5% 3 3 3 3 5% 6% 48%	5028
	11th to 20th	Night	12% 4 2 4 3 3 3 3 5% 8% 51%	5090
	21st to end	Night	12% 6% 2 6% 4 4 4 3 5% 7% 46%	5588
April	1st to 10th	Night	13% 7% 2 6% 4 4 4 3 5% 6% 46%	2101
	11th to 20th	Night	11% 4 2 6% 4 4 4 4 6% 8% 48%	2075
	21st to end	Night	15% 5% 2 5% 4 3 4 4 6% 7% 45%	2100
May	1st to 10th	Night	17% 7% 3 8% 5% 4 4 4 6% 8% 34%	5343
	11th to 20th	Night	19% 6% 2 6% 4 4 4 4 6% 7% 38%	5179
	21st to end	Night	22% 8% 3 6% 4 4 4 3 5% 7% 34%	5108
June	1st to 10th	Night	10% 6% 2 5% 4 5% 5% 4 6% 9% 45%	1680
	11th to 20th	Night	11% 5% 3 6% 5% 5% 5% 4 7% 8% 41%	1680
	21st to end	Night	8% 4 2 6% 5% 4 6% 5% 8% 11% 42%	1680
July	1st to 10th	Night	12% 6% 2 7% 4 5% 4 4 7% 10% 38%	4752
	11th to 20th	Night	13% 8% 4 8% 6% 6% 6% 5% 7% 9% 28%	5277
	21st to end	Night	10% 6% 3 7% 6% 5% 6% 5% 7% 8% 37%	5736
August	1st to 10th	Night	16% 7% 3 8% 5% 6% 5% 4 7% 9% 30%	4135
	11th to 20th	Night	12% 8% 3 9% 6% 6% 6% 4 6% 9% 32%	4254
	21st to end	Night	11% 6% 2 7% 4 5% 5% 4 6% 9% 42%	5045
September	1st to 10th	Night	13% 5% 2 5% 3 4 4 4 5% 6% 48%	2100
	11th to 20th	Night	12% 7% 3 5% 4 3 4 3 4 8% 47%	2047
	21st to end	Night	23% 7% 3 5% 4 3 4 3 4 6% 38%	2302
October	1st to 10th	Night	18% 6% 2 5% 4 3 3 3 5% 5% 47%	2400
	11th to 20th	Night	16% 5% 2 5% 2 3 3 3 4 5% 50%	2400
	21st to end	Night	13% 4 2 5% 3 3 4 3 5% 6% 51%	2640
November	1st to 10th	Night	12% 6% 2 5% 4 3 4 3 5% 5% 53%	2473
	11th to 20th	Night	14% 6% 2 5% 4 3 4 3 5% 6% 49%	2762
	21st to end	Night	12% 4 1 5% 3 4 4 4 5% 6% 50%	2940
December	1st to 10th	Night	10% 5% 2 5% 3 3 4 3 5% 6% 54%	4720
	11th to 20th	Night	20% 7% 2 5% 4 4 4 3 4 5% 42%	5345
	21st to end	Night	17% 6% 2 5% 3 4 4 4 5% 7% 43%	7150

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	39% 2 8% 23% 20% 7%	5123
	11th to 20th	Day	36% 3 5% 18% 25% 13%	5280
	21st to end	Day	41% 3 6% 28% 17% 4	5780
February	1st to 10th	Day	42% 5% 10% 26% 14% 3	1890
	11th to 20th	Day	46% 4 7% 21% 19% 4	1884
	21st to end	Day	47% 5% 10% 17% 17% 4	1755
March	1st to 10th	Day	43% 4 7% 19% 17% 9%	5045
	11th to 20th	Day	32% 4 8% 24% 23% 8%	5090
	21st to end	Day	44% 4 11% 17% 17% 7%	5586
April	1st to 10th	Day	36% 4 12% 21% 19% 8%	2518
	11th to 20th	Day	35% 4 12% 21% 22% 6%	2494
	21st to end	Day	41% 5% 11% 25% 13% 5%	2520
May	1st to 10th	Day	43% 6% 12% 22% 16% 1	7588
	11th to 20th	Day	42% 5% 12% 25% 15% 1	7666
	21st to end	Day	44% 6% 11% 18% 18% 3	9061
June	1st to 10th	Day	35% 8% 15% 25% 16% 2	2940
	11th to 20th	Day	36% 8% 15% 23% 17% 2	2940
	21st to end	Day	33% 9% 14% 26% 17% 1	2940
July	1st to 10th	Day	37% 6% 20% 23% 12% 3	8118
	11th to 20th	Day	43% 7% 18% 24% 7% 1	7611
	21st to end	Day	40% 5% 14% 21% 19% 1	8156
August	1st to 10th	Day	48% 6% 20% 21% 5%	6001
	11th to 20th	Day	42% 6% 18% 22% 11%	5785
	21st to end	Day	31% 4 20% 27% 17% 1	6080
September	1st to 10th	Day	36% 7% 11% 20% 21% 5%	2520
	11th to 20th	Day	37% 5% 9% 24% 20% 3	2415
	21st to end	Day	47% 6% 10% 18% 15% 3	2096
October	1st to 10th	Day	36% 3 7% 19% 30% 5%	3260
	11th to 20th	Day	27% 2 6% 23% 30% 11%	3240
	21st to end	Day	22% 2 10% 28% 24% 13%	3538
November	1st to 10th	Day	33% 4 9% 24% 26% 5%	2041
	11th to 20th	Day	34% 5% 10% 22% 22% 7%	1853
	21st to end	Day	36% 5% 9% 25% 22% 4	1680
December	1st to 10th	Day	36% 3 9% 26% 18% 8%	2680
	11th to 20th	Day	40% 3 13% 25% 15% 4	3066
	21st to end	Day	44% 4 12% 23% 15% 1	4048
January	1st to 10th	Night	36% 2 10% 23% 23% 6%	7754
	11th to 20th	Night	35% 2 5% 20% 26% 11%	7680
	21st to end	Night	38% 2 6% 28% 20% 5%	8401
February	1st to 10th	Night	41% 5% 10% 24% 17% 3	2730
	11th to 20th	Night	44% 4 7% 20% 18% 6%	2631
	21st to end	Night	41% 4 10% 20% 17% 7%	2048
March	1st to 10th	Night	38% 4 8% 22% 20% 8%	5028
	11th to 20th	Night	31% 4 9% 22% 27% 8%	5090
	21st to end	Night	38% 4 10% 21% 21% 7%	5588
April	1st to 10th	Night	38% 5% 11% 18% 23% 6%	2101
	11th to 20th	Night	33% 7% 10% 23% 21% 6%	2075
	21st to end	Night	37% 5% 12% 26% 15% 5%	2100
May	1st to 10th	Night	47% 4 14% 21% 10% 3	5370
	11th to 20th	Night	44% 6% 12% 25% 13% 1	5180
	21st to end	Night	50% 5% 11% 18% 13% 3	5115
June	1st to 10th	Night	35% 8% 14% 25% 17% 1	1680
	11th to 20th	Night	38% 8% 16% 24% 14% 1	1680
	21st to end	Night	34% 9% 15% 20% 19% 3	1680
July	1st to 10th	Night	42% 7% 18% 19% 12% 2	4713
	11th to 20th	Night	52% 7% 16% 17% 7% 1	5226
	21st to end	Night	44% 5% 15% 19% 17%	5606
August	1st to 10th	Night	50% 7% 17% 19% 6%	4124
	11th to 20th	Night	48% 6% 17% 19% 10%	4281
	21st to end	Night	39% 5% 14% 26% 15% 1	5045
September	1st to 10th	Night	36% 7% 13% 22% 16% 6%	2100
	11th to 20th	Night	36% 6% 10% 24% 21% 3	2047
	21st to end	Night	48% 5% 10% 18% 16% 3	2302
October	1st to 10th	Night	41% 3 9% 18% 21% 6%	3920
	11th to 20th	Night	26% 3 8% 22% 31% 11%	3899
	21st to end	Night	22% 3 7% 30% 27% 10%	4252
November	1st to 10th	Night	34% 5% 9% 22% 25% 6%	2473
	11th to 20th	Night	37% 4 8% 20% 24% 7%	2762
	21st to end	Night	34% 5% 9% 24% 23% 5%	2940
December	1st to 10th	Night	31% 3 10% 28% 23% 4	4720
	11th to 20th	Night	45% 3 10% 19% 16% 6%	5348
	21st to end	Night	40% 4 13% 25% 16% 1	7150

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% 24% 30% 17% 4	305
	11th to 20th	Day	25% 29% 29% 13% 3	330
	21st to end	Day	25% 24% 36% 11% 4	363
February	1st to 10th	Day	25% 21% 36% 13% 5%	330
	11th to 20th	Day	36% 22% 30% 9% 2	362
	21st to end	Day	30% 17% 31% 19% 3	446
March	1st to 10th	Day	24% 18% 34% 19% 5% 1	540
	11th to 20th	Day	24% 21% 32% 16% 6% 1	540
	21st to end	Day	25% 17% 35% 18% 5% 1	594
April	1st to 10th	Day	24% 21% 29% 20% 6% 1	540
	11th to 20th	Day	20% 16% 31% 21% 9% 3	532
	21st to end	Day	22% 11% 34% 22% 10% 1	540
May	1st to 10th	Day	20% 14% 36% 18% 9% 3	543
	11th to 20th	Day	22% 11% 31% 22% 10% 3	567
	21st to end	Day	23% 6% 25% 25% 18% 3	617
June	1st to 10th	Day	12% 4 30% 31% 17% 6%	570
	11th to 20th	Day	14% 8% 24% 30% 17% 7%	570
	21st to end	Day	12% 4 26% 33% 19% 6%	570
July	1st to 10th	Day	12% 2 17% 33% 25% 10%	565
	11th to 20th	Day	7% 3 16% 36% 27% 11%	564
	21st to end	Day	12% 3 19% 31% 24% 10%	620
August	1st to 10th	Day	13% 5% 21% 30% 22% 9%	566
	11th to 20th	Day	12% 4 18% 31% 26% 10%	559
	21st to end	Day	10% 3 21% 29% 23% 13%	627
September	1st to 10th	Day	14% 4 19% 29% 20% 14%	570
	11th to 20th	Day	17% 4 15% 31% 23% 10%	542
	21st to end	Day	29% 7% 24% 26% 10% 5%	530
October	1st to 10th	Day	20% 7% 21% 26% 18% 8%	530
	11th to 20th	Day	18% 13% 25% 25% 17% 2	530
	21st to end	Day	17% 13% 32% 25% 9% 3	583
November	1st to 10th	Day	16% 9% 34% 26% 12% 2	527
	11th to 20th	Day	20% 11% 35% 23% 8% 2	393
	21st to end	Day	22% 16% 32% 22% 8%	220
December	1st to 10th	Day	23% 20% 32% 18% 6%	220
	11th to 20th	Day	23% 15% 32% 21% 9%	220
	21st to end	Day	27% 15% 34% 17% 6%	242
January	1st to 10th	Night	20% 16% 41% 20% 3	840
	11th to 20th	Night	21% 15% 35% 26% 3	840
	21st to end	Night	19% 15% 38% 25% 2	924
February	1st to 10th	Night	20% 14% 37% 26% 3	840
	11th to 20th	Night	26% 17% 37% 18% 2	823
	21st to end	Night	21% 18% 37% 22% 2	692
March	1st to 10th	Night	19% 17% 34% 26% 3	840
	11th to 20th	Night	22% 16% 36% 21% 6%	840
	21st to end	Night	18% 16% 37% 24% 4	924
April	1st to 10th	Night	18% 17% 37% 23% 5%	840
	11th to 20th	Night	14% 13% 38% 26% 9%	831
	21st to end	Night	17% 10% 34% 31% 7%	840
May	1st to 10th	Night	20% 8% 35% 24% 11%	671
	11th to 20th	Night	18% 5% 29% 33% 13% 1	605
	21st to end	Night	17% 2 20% 38% 20% 2	455
June	1st to 10th	Night	11% 2 23% 41% 23%	420
	11th to 20th	Night	12% 5% 19% 43% 20% 2	420
	21st to end	Night	9% 4 23% 42% 21% 1	420
July	1st to 10th	Night	13% 3 15% 37% 29% 4	431
	11th to 20th	Night	7% 3 11% 39% 33% 6%	612
	21st to end	Night	8% 2 18% 40% 28% 4	690
August	1st to 10th	Night	13% 1 15% 38% 30% 4	627
	11th to 20th	Night	15% 2 12% 38% 29% 3	692
	21st to end	Night	12% 3 15% 35% 30% 5%	924
September	1st to 10th	Night	15% 2 13% 38% 28% 5%	840
	11th to 20th	Night	14% 3 15% 36% 28% 4	813
	21st to end	Night	26% 5% 19% 30% 19% 1	800
October	1st to 10th	Night	20% 5% 19% 30% 22% 4	800
	11th to 20th	Night	19% 8% 26% 30% 16% 1	800
	21st to end	Night	16% 10% 30% 31% 12% 1	880
November	1st to 10th	Night	14% 9% 31% 31% 13% 1	820
	11th to 20th	Night	19% 11% 34% 29% 7%	840
	21st to end	Night	14% 11% 35% 29% 10% 1	840
December	1st to 10th	Night	20% 11% 37% 24% 8%	840
	11th to 20th	Night	20% 10% 35% 29% 6%	840
	21st to end	Night	20% 12% 30% 33% 5%	924

Page generated at 2026-09-11 04:11:54. Version ac07

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