MASIL Observatory East
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the MASIL Observatory East clear sky chart from 2005-11-06 to 2026-04-02.

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% 7% 2 6% 4 4 4 4 5% 7% 38%	130917
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 39%	130903
Transparency	Day	44% 5% 11% 22% 15% 2	134437
	Night	44% 5% 11% 21% 16% 3	135470
Seeing	Day	24% 17% 31% 18% 7% 2	16712
	Night	22% 15% 32% 23% 7% 1	25713

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	12% 7% 3 9% 5% 6% 6% 5% 5% 6% 36%	4661
	11th to 20th	Day	15% 8% 3 7% 3 3 3 3 5% 6% 43%	4650
	21st to end	Day	14% 7% 2 5% 4 4 4 3 5% 6% 46%	5124
February	1st to 10th	Day	16% 8% 3 6% 4 5% 6% 4 6% 6% 36%	1922
	11th to 20th	Day	13% 7% 3 7% 4 5% 4 3 5% 8% 40%	2048
	21st to end	Day	16% 11% 3 8% 5% 4 4 3 5% 5% 37%	1730
March	1st to 10th	Day	21% 11% 3 7% 4 5% 4 3 4 5% 33%	4576
	11th to 20th	Day	17% 9% 3 7% 4 4 4 4 5% 6% 37%	4891
	21st to end	Day	25% 9% 3 6% 4 4 4 3 5% 7% 31%	5885
April	1st to 10th	Day	21% 10% 3 6% 5% 4 3 3 4 5% 37%	2412
	11th to 20th	Day	17% 7% 2 6% 5% 5% 5% 3 5% 7% 38%	2404
	21st to end	Day	20% 7% 3 7% 4 5% 4 3 5% 6% 36%	2600
May	1st to 10th	Day	31% 10% 2 7% 4 4 4 3 5% 6% 24%	5886
	11th to 20th	Day	20% 8% 3 9% 5% 4 5% 4 5% 7% 31%	6013
	21st to end	Day	20% 6% 2 6% 4 4 5% 4 6% 8% 35%	6949
June	1st to 10th	Day	20% 7% 2 6% 5% 5% 5% 4 6% 8% 32%	2800
	11th to 20th	Day	17% 6% 2 6% 5% 4 5% 4 6% 8% 37%	2800
	21st to end	Day	18% 7% 3 6% 5% 5% 5% 4 5% 8% 33%	2800
July	1st to 10th	Day	16% 7% 2 7% 5% 5% 6% 5% 6% 9% 32%	6360
	11th to 20th	Day	9% 6% 2 7% 4 5% 5% 5% 7% 10% 39%	6360
	21st to end	Day	14% 6% 2 6% 4 4 4 4 6% 8% 42%	6926
August	1st to 10th	Day	15% 6% 3 7% 5% 6% 5% 5% 7% 8% 34%	4306
	11th to 20th	Day	16% 8% 2 7% 5% 4 4 5% 6% 7% 35%	4156
	21st to end	Day	8% 4 2 6% 4 5% 5% 5% 7% 9% 45%	4312
September	1st to 10th	Day	12% 5% 2 5% 4 4 4 5% 5% 6% 47%	2394
	11th to 20th	Day	14% 5% 2 5% 3 3 4 2 4 6% 52%	2200
	21st to end	Day	16% 6% 2 6% 4 4 4 4 4 6% 44%	2200
October	1st to 10th	Day	22% 6% 2 5% 4 4 4 3 4 5% 42%	2200
	11th to 20th	Day	16% 5% 2 6% 4 4 3 3 6% 6% 45%	1937
	21st to end	Day	22% 7% 2 6% 4 3 4 3 4 6% 39%	1980
November	1st to 10th	Day	15% 6% 3 6% 5% 3 3 3 5% 5% 48%	1851
	11th to 20th	Day	17% 6% 1 5% 3 3 3 3 4 6% 47%	1890
	21st to end	Day	20% 7% 2 5% 3 3 3 3 5% 6% 42%	1890
December	1st to 10th	Day	14% 8% 2 6% 4 3 4 3 5% 7% 43%	2690
	11th to 20th	Day	19% 6% 2 6% 3 4 3 3 4 6% 44%	3066
	21st to end	Day	21% 9% 3 6% 4 3 3 3 5% 5% 37%	4048
January	1st to 10th	Night	13% 8% 3 6% 4 4 4 3 6% 6% 43%	8234
	11th to 20th	Night	14% 10% 3 9% 5% 4 4 3 5% 6% 36%	8235
	21st to end	Night	13% 9% 3 6% 4 5% 5% 4 5% 6% 41%	9057
February	1st to 10th	Night	15% 9% 3 6% 5% 3 5% 4 5% 7% 38%	2694
	11th to 20th	Night	13% 8% 3 6% 5% 4 4 4 7% 7% 40%	2458
	21st to end	Night	16% 9% 2 5% 4 4 4 4 5% 6% 40%	2076
March	1st to 10th	Night	23% 9% 2 6% 4 4 4 4 5% 6% 35%	5484
	11th to 20th	Night	16% 10% 3 7% 4 4 5% 3 5% 6% 36%	5288
	21st to end	Night	26% 8% 2 5% 4 3 4 4 6% 6% 32%	5271
April	1st to 10th	Night	19% 9% 2 7% 5% 4 4 3 5% 6% 35%	2011
	11th to 20th	Night	21% 9% 2 5% 4 3 4 3 4 6% 39%	1945
	21st to end	Night	20% 6% 3 6% 4 3 4 3 5% 6% 39%	1800
May	1st to 10th	Night	33% 10% 3 6% 4 4 4 3 5% 6% 23%	4054
	11th to 20th	Night	15% 8% 3 9% 6% 6% 7% 4 7% 7% 29%	3909
	21st to end	Night	20% 8% 4 7% 5% 4 4 4 5% 6% 32%	3926
June	1st to 10th	Night	19% 8% 3 5% 5% 5% 4 3 5% 7% 37%	1600
	11th to 20th	Night	19% 6% 3 6% 4 3 5% 4 5% 9% 37%	1600
	21st to end	Night	18% 7% 2 6% 4 5% 4 4 6% 8% 37%	1600
July	1st to 10th	Night	12% 6% 3 7% 5% 5% 5% 5% 6% 9% 37%	3600
	11th to 20th	Night	11% 7% 2 8% 6% 5% 5% 4 7% 9% 38%	3600
	21st to end	Night	11% 6% 2 5% 4 4 4 4 5% 6% 48%	3926
August	1st to 10th	Night	19% 7% 3 6% 5% 4 5% 3 5% 7% 35%	2809
	11th to 20th	Night	12% 8% 3 8% 5% 5% 6% 4 6% 8% 36%	2888
	21st to end	Night	8% 4 2 5% 3 3 4 4 6% 8% 53%	3586
September	1st to 10th	Night	12% 6% 1 5% 3 3 3 4 5% 6% 50%	2005
	11th to 20th	Night	13% 5% 2 5% 3 3 3 3 5% 7% 52%	2200
	21st to end	Night	16% 7% 3 5% 3 4 4 2 4 7% 47%	2200
October	1st to 10th	Night	20% 7% 2 5% 4 4 3 3 4 6% 42%	2200
	11th to 20th	Night	16% 5% 2 5% 4 4 4 3 6% 6% 45%	2458
	21st to end	Night	21% 7% 3 5% 4 4 3 3 4 6% 39%	2860
November	1st to 10th	Night	15% 6% 2 5% 3 4 4 4 4 5% 47%	2667
	11th to 20th	Night	16% 6% 2 5% 4 4 4 2 4 6% 46%	2730
	21st to end	Night	16% 7% 2 6% 4 4 3 3 5% 7% 43%	2730
December	1st to 10th	Night	15% 10% 3 6% 5% 4 4 3 5% 6% 40%	4704
	11th to 20th	Night	22% 6% 3 5% 4 3 3 3 4 5% 41%	5348
	21st to end	Night	18% 10% 3 7% 4 4 4 3 4 5% 38%	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	46% 5% 10% 25% 14% 1	4662
	11th to 20th	Day	42% 4 10% 21% 19% 3	4680
	21st to end	Day	38% 4 9% 25% 21% 3	5124
February	1st to 10th	Day	47% 6% 12% 21% 13% 2	1922
	11th to 20th	Day	43% 5% 12% 21% 15% 4	2057
	21st to end	Day	49% 4 10% 20% 14% 3	1730
March	1st to 10th	Day	53% 4 8% 14% 11% 9%	4579
	11th to 20th	Day	45% 6% 13% 22% 12% 2	4891
	21st to end	Day	53% 4 6% 16% 16% 7%	5885
April	1st to 10th	Day	50% 4 8% 17% 17% 4	2412
	11th to 20th	Day	46% 5% 10% 19% 17% 3	2404
	21st to end	Day	48% 5% 8% 17% 18% 3	2600
May	1st to 10th	Day	60% 7% 11% 12% 9% 1	5897
	11th to 20th	Day	51% 7% 13% 17% 10% 2	6017
	21st to end	Day	46% 7% 12% 20% 14% 1	6949
June	1st to 10th	Day	49% 7% 11% 21% 12% 1	2800
	11th to 20th	Day	43% 7% 12% 19% 16% 3	2800
	21st to end	Day	47% 8% 11% 20% 13% 1	2800
July	1st to 10th	Day	48% 6% 11% 21% 13% 1	6290
	11th to 20th	Day	38% 7% 15% 25% 14% 1	6285
	21st to end	Day	40% 5% 11% 26% 16% 1	6728
August	1st to 10th	Day	46% 8% 14% 22% 10% 1	4292
	11th to 20th	Day	45% 7% 12% 25% 10% 1	4195
	21st to end	Day	32% 7% 12% 29% 18% 2	4312
September	1st to 10th	Day	35% 7% 12% 24% 20% 2	2394
	11th to 20th	Day	35% 4 9% 26% 24% 2	2200
	21st to end	Day	40% 5% 9% 22% 21% 2	2200
October	1st to 10th	Day	44% 5% 8% 23% 19% 3	3590
	11th to 20th	Day	33% 4 8% 29% 24% 3	3127
	21st to end	Day	34% 4 8% 31% 21% 3	3180
November	1st to 10th	Day	39% 4 13% 25% 17% 2	1851
	11th to 20th	Day	39% 4 8% 23% 22% 3	1890
	21st to end	Day	44% 5% 15% 22% 13% 1	1890
December	1st to 10th	Day	41% 4 13% 29% 11% 1	2690
	11th to 20th	Day	42% 3 13% 18% 19% 5%	3066
	21st to end	Day	49% 4 8% 19% 14% 5%	4048
January	1st to 10th	Night	40% 4 12% 29% 13% 2	8234
	11th to 20th	Night	49% 4 10% 18% 14% 4	8280
	21st to end	Night	44% 4 8% 20% 19% 5%	9057
February	1st to 10th	Night	44% 6% 12% 20% 15% 2	2694
	11th to 20th	Night	43% 4 12% 21% 16% 3	2460
	21st to end	Night	44% 5% 10% 23% 14% 4	2076
March	1st to 10th	Night	49% 4 9% 16% 15% 7%	5494
	11th to 20th	Night	47% 6% 15% 17% 12% 2	5289
	21st to end	Night	51% 4 6% 12% 21% 6%	5271
April	1st to 10th	Night	48% 5% 10% 17% 16% 3	2011
	11th to 20th	Night	47% 5% 8% 20% 18% 2	1945
	21st to end	Night	46% 6% 10% 18% 17% 4	1800
May	1st to 10th	Night	61% 6% 11% 12% 8% 2	4060
	11th to 20th	Night	51% 8% 15% 16% 9% 1	3910
	21st to end	Night	51% 7% 14% 17% 10% 1	3927
June	1st to 10th	Night	48% 6% 11% 22% 12% 1	1600
	11th to 20th	Night	44% 8% 13% 21% 13% 2	1600
	21st to end	Night	44% 9% 12% 21% 13% 1	1600
July	1st to 10th	Night	43% 6% 12% 24% 14% 1	3560
	11th to 20th	Night	40% 7% 17% 20% 15%	3552
	21st to end	Night	36% 6% 11% 31% 15% 1	3809
August	1st to 10th	Night	48% 8% 13% 19% 11% 1	2799
	11th to 20th	Night	44% 9% 14% 22% 11% 1	2915
	21st to end	Night	28% 6% 15% 28% 22% 1	3586
September	1st to 10th	Night	34% 6% 13% 27% 20% 1	2005
	11th to 20th	Night	32% 6% 10% 25% 26% 2	2200
	21st to end	Night	39% 5% 10% 26% 18% 2	2200
October	1st to 10th	Night	41% 5% 14% 22% 16% 2	3590
	11th to 20th	Night	29% 4 11% 27% 27% 3	4007
	21st to end	Night	36% 4 11% 32% 17% 2	4610
November	1st to 10th	Night	38% 6% 10% 24% 19% 3	2667
	11th to 20th	Night	40% 3 10% 23% 21% 3	2730
	21st to end	Night	42% 4 14% 25% 14% 1	2730
December	1st to 10th	Night	46% 4 9% 29% 11% 1	4704
	11th to 20th	Night	46% 4 13% 14% 21% 2	5348
	21st to end	Night	48% 4 8% 21% 15% 3	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	34% 21% 29% 15% 2	220
	11th to 20th	Day	33% 23% 30% 11% 2 1	220
	21st to end	Day	28% 27% 32% 10% 2	242
February	1st to 10th	Day	31% 23% 33% 11% 2	412
	11th to 20th	Day	32% 26% 30% 10% 2	536
	21st to end	Day	28% 24% 31% 13% 3	446
March	1st to 10th	Day	33% 19% 35% 11% 2	540
	11th to 20th	Day	30% 26% 29% 11% 3 1	540
	21st to end	Day	34% 22% 30% 8% 4 2	594
April	1st to 10th	Day	36% 22% 25% 12% 5% 1	504
	11th to 20th	Day	30% 27% 26% 12% 5% 1	492
	21st to end	Day	27% 18% 34% 14% 5% 1	500
May	1st to 10th	Day	36% 19% 28% 10% 5% 1	503
	11th to 20th	Day	22% 18% 39% 15% 4 2	527
	21st to end	Day	20% 16% 35% 23% 5% 1	573
June	1st to 10th	Day	21% 14% 38% 18% 8% 1	530
	11th to 20th	Day	19% 18% 34% 23% 5% 2	530
	21st to end	Day	18% 16% 37% 19% 7% 2	530
July	1st to 10th	Day	17% 8% 36% 26% 10% 3	525
	11th to 20th	Day	15% 10% 31% 28% 12% 3	524
	21st to end	Day	15% 9% 36% 30% 8% 2	576
August	1st to 10th	Day	13% 10% 30% 29% 14% 3	526
	11th to 20th	Day	17% 9% 28% 28% 13% 4	519
	21st to end	Day	11% 12% 34% 26% 12% 5%	583
September	1st to 10th	Day	17% 11% 29% 23% 14% 6%	530
	11th to 20th	Day	18% 12% 28% 20% 13% 9%	530
	21st to end	Day	20% 12% 31% 22% 8% 6%	530
October	1st to 10th	Day	26% 15% 23% 22% 10% 4	530
	11th to 20th	Day	23% 14% 30% 21% 10% 2	461
	21st to end	Day	30% 14% 31% 18% 6%	462
November	1st to 10th	Day	22% 21% 29% 19% 7% 2	425
	11th to 20th	Day	30% 20% 28% 16% 5%	430
	21st to end	Day	33% 20% 28% 13% 5%	430
December	1st to 10th	Day	22% 27% 31% 15% 5%	230
	11th to 20th	Day	35% 22% 30% 11% 1	220
	21st to end	Day	31% 21% 28% 14% 5%	242
January	1st to 10th	Night	23% 23% 34% 17% 3	840
	11th to 20th	Night	29% 19% 32% 18% 3	840
	21st to end	Night	22% 23% 36% 16% 3	924
February	1st to 10th	Night	24% 18% 38% 19% 1	840
	11th to 20th	Night	24% 20% 38% 16% 2	823
	21st to end	Night	24% 19% 35% 18% 3	692
March	1st to 10th	Night	27% 20% 36% 16% 3	840
	11th to 20th	Night	24% 22% 33% 16% 5%	840
	21st to end	Night	27% 20% 32% 17% 3	924
April	1st to 10th	Night	27% 18% 34% 19% 3	805
	11th to 20th	Night	26% 20% 33% 17% 5%	761
	21st to end	Night	24% 17% 35% 21% 3	600
May	1st to 10th	Night	27% 15% 32% 22% 5%	600
	11th to 20th	Night	20% 16% 34% 25% 5% 1	546
	21st to end	Night	20% 9% 33% 31% 6%	433
June	1st to 10th	Night	21% 9% 33% 29% 8% 1	400
	11th to 20th	Night	21% 9% 31% 31% 8% 1	400
	21st to end	Night	19% 7% 34% 34% 7%	400
July	1st to 10th	Night	15% 6% 27% 41% 10% 1	390
	11th to 20th	Night	13% 5% 25% 44% 13% 1	388
	21st to end	Night	15% 4 29% 40% 12%	438
August	1st to 10th	Night	13% 4 27% 42% 14% 1	530
	11th to 20th	Night	13% 7% 30% 34% 14% 1	595
	21st to end	Night	13% 5% 29% 35% 16% 2	880
September	1st to 10th	Night	15% 7% 33% 30% 15% 1	800
	11th to 20th	Night	15% 8% 28% 29% 18% 2	800
	21st to end	Night	18% 9% 29% 29% 14% 2	800
October	1st to 10th	Night	24% 10% 27% 26% 12% 1	800
	11th to 20th	Night	19% 13% 31% 26% 10% 1	800
	21st to end	Night	26% 16% 29% 23% 6%	880
November	1st to 10th	Night	20% 15% 34% 24% 7% 1	820
	11th to 20th	Night	23% 18% 33% 20% 6% 1	840
	21st to end	Night	23% 20% 33% 19% 5%	840
December	1st to 10th	Night	24% 17% 37% 18% 3	840
	11th to 20th	Night	25% 21% 30% 20% 3	840
	21st to end	Night	27% 18% 34% 20% 2	924

Page generated at 2026-03-30 02:18:43. Version ac07

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