MVAS Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the MVAS Observatory clear sky chart from 2005-11-06 to 2025-12-18.

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	17% 12% 4 9% 5% 4 4 4 5% 6% 28%	127485
	Night	18% 13% 5% 11% 6% 4 4 3 4 5% 26%	123548
Transparency	Day	55% 6% 11% 18% 9% 1	130976
	Night	59% 6% 10% 16% 8% 1	128144
Seeing	Day	26% 17% 27% 18% 9% 3	16085
	Night	26% 16% 26% 24% 8% 1	25262

By Month

Percentage Cloud Cover NE (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	11% 21% 8% 24% 6% 4 3 2 3 4 14%	3597
	11th to 20th	Day	13% 20% 11% 20% 8% 4 3 2 4 3 11%	3570
	21st to end	Day	14% 20% 8% 12% 8% 6% 5% 3 4 4 15%	4298
February	1st to 10th	Day	16% 18% 5% 18% 8% 5% 5% 4 4 4 14%	1800
	11th to 20th	Day	13% 18% 7% 16% 8% 5% 4 3 4 4 17%	1829
	21st to end	Day	18% 18% 6% 15% 7% 4 4 3 3 4 18%	1650
March	1st to 10th	Day	22% 16% 7% 13% 5% 5% 3 3 3 4 21%	4476
	11th to 20th	Day	12% 19% 6% 9% 4 3 3 3 4 5% 31%	4952
	21st to end	Day	18% 12% 6% 11% 5% 4 4 3 4 5% 29%	5802
April	1st to 10th	Day	18% 16% 4 9% 5% 3 4 3 4 5% 29%	2400
	11th to 20th	Day	19% 13% 3 7% 4 4 4 3 5% 6% 32%	2374
	21st to end	Day	22% 13% 3 6% 4 4 3 3 4 5% 31%	2410
May	1st to 10th	Day	32% 11% 3 7% 5% 3 4 3 4 6% 24%	6291
	11th to 20th	Day	17% 10% 4 8% 5% 4 5% 4 5% 8% 29%	6318
	21st to end	Day	22% 10% 3 7% 5% 3 5% 3 6% 8% 29%	6949
June	1st to 10th	Day	18% 8% 3 8% 5% 4 5% 4 6% 8% 31%	2800
	11th to 20th	Day	20% 6% 2 6% 4 5% 5% 4 6% 9% 32%	2800
	21st to end	Day	18% 7% 2 6% 6% 5% 6% 4 7% 8% 31%	2800
July	1st to 10th	Day	11% 6% 3 7% 4 5% 6% 5% 8% 10% 36%	6360
	11th to 20th	Day	10% 6% 2 7% 5% 6% 6% 5% 7% 10% 35%	6360
	21st to end	Day	13% 6% 2 6% 5% 4 5% 4 6% 8% 40%	6926
August	1st to 10th	Day	17% 7% 2 7% 4 5% 5% 4 6% 8% 35%	4376
	11th to 20th	Day	16% 8% 3 8% 5% 5% 6% 4 8% 9% 30%	3988
	21st to end	Day	16% 6% 3 6% 4 4 4 4 6% 8% 39%	4312
September	1st to 10th	Day	15% 6% 2 5% 4 4 5% 4 6% 8% 41%	2400
	11th to 20th	Day	15% 7% 2 6% 4 4 4 2 5% 7% 45%	2378
	21st to end	Day	17% 7% 2 7% 5% 4 6% 5% 5% 7% 35%	2175
October	1st to 10th	Day	20% 8% 3 7% 5% 4 4 3 5% 7% 34%	2000
	11th to 20th	Day	20% 9% 2 6% 5% 5% 5% 4 5% 6% 33%	2000
	21st to end	Day	25% 15% 3 7% 4 5% 4 4 4 6% 24%	2065
November	1st to 10th	Day	16% 14% 3 7% 4 4 5% 4 5% 6% 30%	1851
	11th to 20th	Day	15% 19% 4 11% 6% 4 3 3 4 4 27%	1816
	21st to end	Day	17% 20% 6% 10% 5% 4 4 3 5% 5% 20%	1680
December	1st to 10th	Day	19% 21% 11% 17% 5% 4 4 2 3 4 10%	2680
	11th to 20th	Day	18% 18% 10% 16% 6% 4 3 2 3 3 16%	3042
	21st to end	Day	21% 19% 8% 10% 5% 4 3 3 4 6% 17%	3960
January	1st to 10th	Night	13% 20% 10% 26% 7% 4 3 2 3 3 8%	6349
	11th to 20th	Night	10% 20% 9% 23% 8% 5% 4 3 4 4 11%	6315
	21st to end	Night	14% 17% 6% 17% 8% 6% 5% 3 4 5% 15%	6656
February	1st to 10th	Night	16% 18% 6% 16% 7% 5% 4 3 4 5% 16%	2600
	11th to 20th	Night	14% 14% 7% 17% 9% 5% 5% 4 4 5% 16%	2455
	21st to end	Night	16% 16% 6% 17% 7% 5% 4 3 4 5% 17%	1980
March	1st to 10th	Night	18% 11% 8% 14% 9% 4 4 3 4 5% 20%	5364
	11th to 20th	Night	18% 17% 6% 8% 4 3 3 3 4 6% 28%	4990
	21st to end	Night	23% 13% 5% 12% 5% 3 3 3 3 5% 25%	5129
April	1st to 10th	Night	21% 16% 4 7% 4 4 4 3 4 6% 27%	2000
	11th to 20th	Night	21% 11% 3 7% 5% 4 4 4 4 5% 32%	1975
	21st to end	Night	21% 11% 2 6% 4 3 4 3 6% 6% 35%	1988
May	1st to 10th	Night	33% 12% 3 7% 4 4 3 3 4 5% 22%	3691
	11th to 20th	Night	14% 12% 4 9% 6% 4 5% 4 5% 6% 31%	3575
	21st to end	Night	23% 10% 3 7% 4 4 4 3 6% 6% 30%	3926
June	1st to 10th	Night	19% 7% 2 7% 4 5% 4 3 5% 7% 36%	1600
	11th to 20th	Night	20% 7% 2 5% 4 3 5% 3 5% 9% 37%	1600
	21st to end	Night	16% 6% 3 6% 5% 4 5% 3 6% 8% 38%	1600
July	1st to 10th	Night	11% 6% 3 8% 6% 6% 5% 5% 7% 9% 36%	3600
	11th to 20th	Night	15% 7% 2 6% 4 4 5% 4 5% 7% 40%	3600
	21st to end	Night	12% 6% 2 6% 4 4 4 3 6% 7% 46%	3926
August	1st to 10th	Night	20% 9% 2 6% 3 4 3 3 5% 6% 38%	2769
	11th to 20th	Night	15% 8% 3 8% 5% 5% 4 3 5% 7% 39%	3026
	21st to end	Night	13% 8% 3 6% 4 4 4 4 6% 7% 42%	3586
September	1st to 10th	Night	14% 7% 2 6% 4 4 4 4 5% 7% 43%	2000
	11th to 20th	Night	16% 6% 2 4 3 3 3 3 5% 6% 48%	2020
	21st to end	Night	18% 8% 2 5% 4 5% 5% 3 5% 7% 38%	2221
October	1st to 10th	Night	22% 7% 2 8% 5% 5% 4 3 5% 6% 35%	2400
	11th to 20th	Night	21% 10% 3 7% 4 4 4 4 5% 6% 32%	2400
	21st to end	Night	27% 13% 3 7% 5% 4 4 3 4 5% 25%	2774
November	1st to 10th	Night	17% 13% 3 9% 5% 4 5% 3 5% 5% 31%	2667
	11th to 20th	Night	18% 16% 4 9% 4 4 4 3 5% 6% 27%	2803
	21st to end	Night	17% 20% 7% 10% 5% 4 4 4 5% 6% 20%	2940
December	1st to 10th	Night	17% 22% 11% 14% 6% 4 5% 3 4 4 11%	4720
	11th to 20th	Night	22% 19% 6% 13% 5% 4 3 3 4 4 17%	5307
	21st to end	Night	22% 19% 8% 11% 6% 4 4 2 3 5% 17%	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	75% 4 8% 10% 3	3598
	11th to 20th	Day	78% 4 7% 7% 4 1	3600
	21st to end	Day	70% 6% 8% 11% 5% 1	4298
February	1st to 10th	Day	73% 6% 7% 11% 3	1800
	11th to 20th	Day	71% 5% 7% 12% 4	1838
	21st to end	Day	71% 3 9% 11% 6%	1650
March	1st to 10th	Day	68% 4 10% 11% 6% 1	4479
	11th to 20th	Day	55% 4 18% 12% 8% 2	4952
	21st to end	Day	58% 4 8% 13% 15% 1	5802
April	1st to 10th	Day	59% 4 8% 19% 9% 2	2400
	11th to 20th	Day	53% 5% 11% 18% 11% 1	2374
	21st to end	Day	55% 5% 8% 18% 11% 2	2410
May	1st to 10th	Day	62% 5% 8% 14% 9% 1	6302
	11th to 20th	Day	52% 8% 11% 21% 8% 1	6322
	21st to end	Day	52% 7% 11% 21% 8%	6949
June	1st to 10th	Day	50% 8% 11% 19% 11% 1	2800
	11th to 20th	Day	48% 7% 14% 22% 9% 1	2800
	21st to end	Day	49% 8% 14% 20% 9%	2800
July	1st to 10th	Day	41% 8% 14% 25% 12%	6290
	11th to 20th	Day	41% 9% 15% 25% 10%	6285
	21st to end	Day	41% 6% 13% 30% 10%	6728
August	1st to 10th	Day	47% 9% 13% 20% 11% 1	4362
	11th to 20th	Day	48% 9% 17% 20% 6%	4027
	21st to end	Day	42% 7% 14% 30% 6% 1	4312
September	1st to 10th	Day	41% 7% 12% 28% 12% 1	2400
	11th to 20th	Day	40% 5% 11% 27% 17% 1	2378
	21st to end	Day	48% 7% 11% 22% 11% 1	2175
October	1st to 10th	Day	46% 7% 10% 27% 9% 1	3260
	11th to 20th	Day	46% 6% 14% 21% 14%	3240
	21st to end	Day	48% 6% 14% 23% 9% 1	3316
November	1st to 10th	Day	53% 6% 11% 18% 10% 1	1851
	11th to 20th	Day	60% 6% 10% 15% 8% 2	1816
	21st to end	Day	65% 7% 14% 10% 4	1680
December	1st to 10th	Day	80% 5% 7% 7% 2	2680
	11th to 20th	Day	71% 6% 9% 9% 4	3042
	21st to end	Day	69% 4 8% 11% 6% 1	3960
January	1st to 10th	Night	81% 5% 7% 5% 2	6349
	11th to 20th	Night	76% 5% 7% 7% 5%	6360
	21st to end	Night	70% 5% 9% 7% 8% 1	6656
February	1st to 10th	Night	71% 5% 8% 12% 5%	2600
	11th to 20th	Night	71% 5% 9% 10% 5%	2457
	21st to end	Night	70% 5% 10% 10% 6%	1980
March	1st to 10th	Night	65% 6% 11% 11% 7%	5374
	11th to 20th	Night	58% 4 16% 13% 5% 3	4991
	21st to end	Night	62% 4 6% 17% 9% 1	5129
April	1st to 10th	Night	59% 6% 10% 17% 7% 2	2000
	11th to 20th	Night	53% 6% 11% 17% 13% 1	1975
	21st to end	Night	50% 5% 11% 18% 13% 3	1988
May	1st to 10th	Night	65% 5% 8% 11% 10% 1	3697
	11th to 20th	Night	51% 8% 12% 20% 9% 1	3576
	21st to end	Night	54% 7% 10% 17% 11% 1	3927
June	1st to 10th	Night	47% 6% 11% 22% 13% 1	1600
	11th to 20th	Night	42% 9% 15% 21% 12% 1	1600
	21st to end	Night	42% 8% 13% 25% 11% 1	1600
July	1st to 10th	Night	42% 7% 16% 21% 13% 1	3560
	11th to 20th	Night	42% 7% 13% 23% 14% 1	3552
	21st to end	Night	37% 6% 12% 29% 15% 1	3809
August	1st to 10th	Night	46% 7% 13% 24% 9%	2759
	11th to 20th	Night	45% 7% 14% 26% 8%	3053
	21st to end	Night	40% 7% 13% 31% 9%	3586
September	1st to 10th	Night	40% 7% 13% 26% 12% 1	2000
	11th to 20th	Night	36% 6% 12% 27% 18%	2020
	21st to end	Night	46% 7% 10% 25% 11% 1	2221
October	1st to 10th	Night	51% 8% 11% 21% 9% 1	3920
	11th to 20th	Night	47% 6% 13% 17% 14% 2	3899
	21st to end	Night	51% 5% 13% 23% 7%	4473
November	1st to 10th	Night	54% 6% 10% 16% 11% 2	2667
	11th to 20th	Night	58% 5% 10% 17% 7% 2	2803
	21st to end	Night	64% 6% 12% 13% 4	2940
December	1st to 10th	Night	76% 6% 9% 8% 1	4720
	11th to 20th	Night	70% 5% 7% 11% 6%	5307
	21st to end	Night	70% 6% 7% 8% 7% 2	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	33% 21% 29% 15% 3	200
	11th to 20th	Day	41% 26% 25% 8% 1 1	200
	21st to end	Day	30% 28% 30% 10% 2	293
February	1st to 10th	Day	43% 27% 23% 7%	300
	11th to 20th	Day	36% 32% 25% 7% 1	376
	21st to end	Day	38% 22% 28% 10% 1	414
March	1st to 10th	Day	32% 24% 27% 15% 2	500
	11th to 20th	Day	33% 25% 25% 13% 3	500
	21st to end	Day	36% 22% 25% 12% 4 1	550
April	1st to 10th	Day	37% 25% 21% 12% 5%	500
	11th to 20th	Day	35% 22% 22% 11% 7% 2	492
	21st to end	Day	35% 19% 25% 14% 6% 1	500
May	1st to 10th	Day	33% 17% 29% 12% 7% 2	503
	11th to 20th	Day	24% 15% 30% 20% 9% 2	527
	21st to end	Day	20% 13% 32% 21% 11% 3	573
June	1st to 10th	Day	21% 17% 27% 24% 9% 2	530
	11th to 20th	Day	22% 14% 27% 25% 11% 2	530
	21st to end	Day	20% 16% 34% 21% 8% 2	530
July	1st to 10th	Day	13% 10% 31% 28% 13% 5%	525
	11th to 20th	Day	13% 10% 28% 28% 16% 4	524
	21st to end	Day	15% 10% 35% 26% 11% 3	576
August	1st to 10th	Day	14% 11% 22% 29% 18% 7%	526
	11th to 20th	Day	18% 9% 22% 28% 17% 6%	519
	21st to end	Day	15% 12% 29% 25% 13% 6%	583
September	1st to 10th	Day	18% 10% 23% 25% 16% 8%	530
	11th to 20th	Day	14% 11% 23% 22% 18% 11%	530
	21st to end	Day	20% 14% 27% 23% 13% 4	530
October	1st to 10th	Day	24% 13% 24% 19% 15% 4	530
	11th to 20th	Day	26% 20% 26% 17% 9% 1	530
	21st to end	Day	35% 17% 25% 17% 6%	509
November	1st to 10th	Day	32% 20% 21% 18% 8%	425
	11th to 20th	Day	31% 25% 24% 16% 4	356
	21st to end	Day	45% 17% 27% 11%	220
December	1st to 10th	Day	38% 19% 31% 10% 2	220
	11th to 20th	Day	37% 26% 28% 8% 1	214
	21st to end	Day	36% 22% 29% 11% 1	220
January	1st to 10th	Night	31% 26% 28% 14% 2	800
	11th to 20th	Night	32% 22% 29% 14% 2 1	800
	21st to end	Night	23% 25% 33% 17% 2	880
February	1st to 10th	Night	31% 22% 29% 16% 1	800
	11th to 20th	Night	28% 25% 31% 13% 2	783
	21st to end	Night	30% 22% 30% 15% 2	660
March	1st to 10th	Night	29% 20% 29% 20% 2	800
	11th to 20th	Night	30% 22% 30% 18% 2	800
	21st to end	Night	30% 18% 25% 20% 6%	880
April	1st to 10th	Night	29% 18% 30% 21% 2	800
	11th to 20th	Night	28% 18% 27% 19% 7%	791
	21st to end	Night	28% 13% 28% 24% 6%	788
May	1st to 10th	Night	30% 10% 25% 28% 7%	445
	11th to 20th	Night	20% 8% 32% 31% 8%	394
	21st to end	Night	18% 6% 28% 37% 11%	433
June	1st to 10th	Night	18% 9% 23% 39% 10% 1	400
	11th to 20th	Night	21% 8% 21% 38% 12%	400
	21st to end	Night	18% 6% 25% 41% 10% 1	400
July	1st to 10th	Night	11% 5% 23% 43% 17% 1	390
	11th to 20th	Night	12% 5% 19% 46% 16% 1	388
	21st to end	Night	15% 4 19% 46% 14% 1	438
August	1st to 10th	Night	15% 5% 21% 38% 20% 1	503
	11th to 20th	Night	16% 6% 18% 34% 23% 3	680
	21st to end	Night	16% 7% 19% 38% 19% 1	880
September	1st to 10th	Night	17% 7% 22% 34% 19% 2	800
	11th to 20th	Night	18% 7% 17% 32% 24% 3	800
	21st to end	Night	20% 10% 23% 31% 14% 2	800
October	1st to 10th	Night	26% 10% 21% 25% 16% 2	800
	11th to 20th	Night	26% 14% 26% 28% 6%	800
	21st to end	Night	34% 15% 26% 21% 3	880
November	1st to 10th	Night	26% 16% 28% 23% 7%	820
	11th to 20th	Night	28% 21% 27% 17% 6% 1	840
	21st to end	Night	32% 20% 28% 17% 4	840
December	1st to 10th	Night	33% 23% 26% 15% 3	840
	11th to 20th	Night	35% 21% 27% 14% 2	829
	21st to end	Night	33% 25% 26% 15% 1	880

Page generated at 2025-12-15 01:13:55. Version ac07

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