Leslie George Marczi Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Leslie George Marczi Observatory clear sky chart from 2005-11-06 to 2025-09-04.

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	15% 10% 4 9% 5% 5% 5% 4 5% 7% 32%	126281
	Night	15% 11% 4 9% 5% 5% 4 4 5% 6% 31%	122403
Transparency	Day	49% 7% 12% 20% 11% 1	129876
	Night	51% 7% 12% 19% 11% 1	126894
Seeing	Day	19% 12% 26% 24% 14% 5%	15544
	Night	21% 10% 27% 28% 13% 1	24935

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	13% 19% 7% 14% 6% 4 4 4 5% 5% 20%	3597
	11th to 20th	Day	13% 22% 7% 18% 8% 6% 5% 3 4 4 9%	3976
	21st to end	Day	19% 17% 7% 14% 8% 7% 5% 3 4 4 13%	4466
February	1st to 10th	Day	18% 18% 4 13% 7% 6% 5% 3 4 5% 16%	1800
	11th to 20th	Day	14% 17% 6% 14% 7% 5% 5% 4 5% 4 19%	1753
	21st to end	Day	20% 15% 4 12% 8% 4 5% 3 6% 5% 20%	1564
March	1st to 10th	Day	17% 11% 5% 11% 6% 4 4 3 4 5% 29%	4932
	11th to 20th	Day	10% 17% 4 8% 5% 5% 5% 3 5% 7% 31%	4980
	21st to end	Day	19% 12% 4 10% 5% 4 4 3 4 5% 31%	5465
April	1st to 10th	Day	18% 15% 4 7% 5% 4 4 3 4 5% 31%	2365
	11th to 20th	Day	18% 10% 3 6% 5% 4 4 3 5% 6% 35%	2418
	21st to end	Day	19% 10% 4 6% 6% 5% 4 3 4 5% 34%	2600
May	1st to 10th	Day	20% 9% 3 8% 5% 5% 5% 3 5% 6% 31%	5888
	11th to 20th	Day	14% 11% 4 9% 5% 5% 5% 4 6% 7% 31%	6226
	21st to end	Day	17% 7% 3 7% 4 4 4 4 5% 7% 38%	6949
June	1st to 10th	Day	14% 7% 3 6% 4 4 5% 4 6% 7% 39%	2800
	11th to 20th	Day	10% 5% 3 7% 6% 5% 5% 4 7% 9% 38%	2800
	21st to end	Day	13% 6% 2 5% 3 4 5% 4 6% 9% 42%	2800
July	1st to 10th	Day	8% 5% 2 6% 4 4 6% 4 7% 10% 43%	6360
	11th to 20th	Day	10% 6% 2 6% 5% 5% 5% 4 6% 10% 41%	6343
	21st to end	Day	8% 5% 2 6% 4 5% 5% 4 7% 8% 46%	6424
August	1st to 10th	Day	16% 6% 2 7% 3 4 4 4 7% 8% 38%	4234
	11th to 20th	Day	12% 6% 3 7% 5% 5% 6% 5% 8% 10% 34%	4174
	21st to end	Day	9% 5% 2 6% 4 4 4 4 6% 10% 45%	4454
September	1st to 10th	Day	13% 7% 2 5% 4 5% 4 4 7% 8% 41%	2316
	11th to 20th	Day	10% 5% 2 5% 5% 4 5% 3 6% 7% 46%	2176
	21st to end	Day	15% 7% 3 8% 5% 5% 5% 5% 7% 8% 33%	2045
October	1st to 10th	Day	19% 8% 2 7% 4 4 4 4 5% 7% 36%	1900
	11th to 20th	Day	19% 10% 3 8% 5% 5% 5% 3 5% 6% 31%	1900
	21st to end	Day	23% 12% 4 9% 5% 5% 5% 3 4 7% 23%	2090
November	1st to 10th	Day	15% 9% 4 8% 6% 5% 5% 4 5% 6% 34%	1740
	11th to 20th	Day	15% 12% 3 8% 6% 5% 5% 3 5% 7% 31%	1600
	21st to end	Day	20% 15% 4 7% 6% 4 4 4 5% 7% 23%	1600
December	1st to 10th	Day	20% 15% 6% 11% 7% 6% 5% 4 6% 5% 15%	2600
	11th to 20th	Day	20% 17% 5% 10% 4 4 4 3 5% 4 22%	2986
	21st to end	Day	24% 20% 6% 12% 7% 6% 4 3 4 4 12%	3960
January	1st to 10th	Night	15% 15% 5% 11% 6% 5% 5% 4 5% 5% 25%	6349
	11th to 20th	Night	11% 18% 7% 15% 7% 6% 5% 4 5% 5% 16%	5905
	21st to end	Night	20% 16% 6% 13% 6% 5% 4 3 4 5% 17%	6490
February	1st to 10th	Night	17% 17% 5% 11% 6% 5% 4 3 5% 4 21%	2600
	11th to 20th	Night	16% 14% 4 12% 6% 5% 5% 4 5% 6% 22%	2533
	21st to end	Night	18% 13% 4 11% 8% 6% 5% 3 5% 6% 21%	2063
March	1st to 10th	Night	17% 11% 5% 9% 6% 5% 4 3 4 5% 30%	4908
	11th to 20th	Night	13% 11% 3 7% 4 4 4 4 5% 8% 37%	4979
	21st to end	Night	18% 9% 3 7% 4 4 3 3 4 6% 37%	5467
April	1st to 10th	Night	20% 11% 3 9% 6% 4 3 4 5% 4 31%	2035
	11th to 20th	Night	19% 8% 3 7% 4 5% 4 3 5% 6% 36%	1947
	21st to end	Night	15% 9% 4 6% 4 4 6% 4 5% 6% 36%	1800
May	1st to 10th	Night	21% 12% 4 8% 5% 4 4 3 4 5% 30%	4053
	11th to 20th	Night	11% 8% 3 7% 5% 5% 5% 6% 6% 8% 36%	3652
	21st to end	Night	17% 7% 2 7% 4 4 4 4 6% 7% 39%	3926
June	1st to 10th	Night	11% 7% 3 6% 5% 6% 6% 5% 7% 7% 36%	1600
	11th to 20th	Night	9% 6% 2 6% 6% 5% 4 5% 6% 8% 43%	1600
	21st to end	Night	12% 6% 2 6% 5% 4 5% 4 6% 8% 42%	1600
July	1st to 10th	Night	7% 6% 2 6% 5% 4 4 4 6% 9% 44%	3600
	11th to 20th	Night	8% 6% 2 7% 4 4 4 4 6% 8% 46%	3600
	21st to end	Night	8% 5% 2 6% 4 4 4 4 6% 8% 50%	4428
August	1st to 10th	Night	9% 6% 2 7% 4 4 5% 4 6% 8% 44%	2915
	11th to 20th	Night	13% 7% 3 6% 4 4 5% 3 6% 8% 40%	2875
	21st to end	Night	7% 5% 2 5% 5% 5% 5% 5% 6% 9% 46%	3410
September	1st to 10th	Night	13% 7% 2 6% 4 3 3 3 5% 7% 46%	1931
	11th to 20th	Night	11% 4 2 5% 3 3 3 4 5% 7% 53%	2000
	21st to end	Night	15% 7% 3 8% 6% 5% 5% 4 5% 6% 36%	2134
October	1st to 10th	Night	17% 7% 2 7% 5% 4 4 4 5% 7% 40%	2280
	11th to 20th	Night	20% 10% 3 8% 5% 4 4 3 5% 5% 33%	2280
	21st to end	Night	23% 9% 3 7% 4 4 6% 4 5% 6% 29%	2508
November	1st to 10th	Night	16% 9% 3 7% 4 4 4 3 4 7% 39%	2551
	11th to 20th	Night	15% 11% 4 8% 6% 5% 5% 4 5% 8% 30%	2800
	21st to end	Night	19% 13% 4 8% 6% 5% 5% 4 6% 7% 24%	2800
December	1st to 10th	Night	17% 17% 6% 11% 6% 5% 5% 5% 6% 7% 16%	4580
	11th to 20th	Night	17% 21% 5% 9% 5% 5% 4 3 4 5% 23%	5208
	21st to end	Night	25% 17% 5% 10% 6% 5% 5% 4 5% 4 15%	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	64% 9% 10% 9% 7% 1	3598
	11th to 20th	Day	77% 6% 8% 5% 3 1	4010
	21st to end	Day	74% 6% 10% 6% 4	4466
February	1st to 10th	Day	70% 6% 10% 11% 3 1	1800
	11th to 20th	Day	67% 7% 9% 13% 4	1762
	21st to end	Day	65% 6% 10% 13% 5%	1564
March	1st to 10th	Day	56% 6% 13% 16% 8%	4935
	11th to 20th	Day	50% 7% 17% 16% 8% 2	4980
	21st to end	Day	54% 6% 7% 22% 10% 1	5465
April	1st to 10th	Day	55% 5% 11% 18% 11% 1	2365
	11th to 20th	Day	49% 6% 11% 20% 13% 2	2418
	21st to end	Day	51% 6% 11% 17% 13% 3	2600
May	1st to 10th	Day	52% 7% 11% 16% 10% 5%	5897
	11th to 20th	Day	49% 9% 10% 19% 12% 1	6230
	21st to end	Day	44% 7% 11% 23% 12% 2	6949
June	1st to 10th	Day	41% 8% 13% 21% 15% 2	2800
	11th to 20th	Day	39% 8% 14% 25% 13% 1	2800
	21st to end	Day	37% 8% 13% 26% 15% 1	2800
July	1st to 10th	Day	33% 8% 13% 27% 17% 1	6290
	11th to 20th	Day	36% 8% 13% 28% 15% 1	6268
	21st to end	Day	32% 8% 15% 26% 18% 1	6240
August	1st to 10th	Day	40% 9% 14% 23% 13% 1	4221
	11th to 20th	Day	41% 9% 16% 24% 9% 1	4213
	21st to end	Day	34% 6% 13% 34% 12% 1	4454
September	1st to 10th	Day	39% 6% 14% 27% 12% 1	2316
	11th to 20th	Day	35% 7% 12% 30% 16% 1	2176
	21st to end	Day	46% 8% 15% 21% 9% 1	2045
October	1st to 10th	Day	36% 7% 16% 26% 15% 1	3160
	11th to 20th	Day	44% 6% 15% 21% 14%	3140
	21st to end	Day	51% 6% 15% 20% 7%	3428
November	1st to 10th	Day	50% 7% 13% 19% 9% 2	1740
	11th to 20th	Day	53% 7% 15% 16% 8% 1	1600
	21st to end	Day	60% 8% 14% 13% 4	1600
December	1st to 10th	Day	68% 8% 14% 9% 1	2600
	11th to 20th	Day	64% 7% 11% 10% 8% 1	2986
	21st to end	Day	75% 6% 10% 7% 2	3960
January	1st to 10th	Night	59% 9% 9% 17% 6%	6349
	11th to 20th	Night	67% 9% 12% 8% 4	5946
	21st to end	Night	69% 7% 8% 8% 8% 1	6490
February	1st to 10th	Night	67% 6% 10% 13% 5%	2600
	11th to 20th	Night	62% 6% 12% 15% 6%	2535
	21st to end	Night	63% 7% 10% 13% 7%	2063
March	1st to 10th	Night	55% 5% 10% 18% 10% 1	4918
	11th to 20th	Night	45% 6% 17% 20% 10% 2	4980
	21st to end	Night	48% 4 10% 20% 16% 2	5467
April	1st to 10th	Night	55% 5% 8% 19% 11% 2	2035
	11th to 20th	Night	49% 6% 9% 19% 15% 2	1947
	21st to end	Night	48% 6% 10% 19% 15% 3	1800
May	1st to 10th	Night	57% 5% 8% 13% 12% 5%	4060
	11th to 20th	Night	43% 9% 14% 21% 11% 1	3653
	21st to end	Night	43% 6% 10% 24% 16% 1	3927
June	1st to 10th	Night	41% 10% 10% 21% 16% 2	1600
	11th to 20th	Night	39% 6% 13% 23% 17% 2	1600
	21st to end	Night	39% 7% 13% 22% 17% 2	1600
July	1st to 10th	Night	35% 6% 12% 23% 22% 2	3560
	11th to 20th	Night	34% 7% 13% 30% 16% 1	3552
	21st to end	Night	32% 7% 14% 24% 23% 2	4297
August	1st to 10th	Night	36% 7% 14% 26% 14% 2	2904
	11th to 20th	Night	41% 7% 14% 27% 11%	2902
	21st to end	Night	31% 8% 14% 32% 14% 1	3410
September	1st to 10th	Night	37% 6% 14% 25% 16% 2	1931
	11th to 20th	Night	30% 7% 11% 32% 19% 1	2000
	21st to end	Night	46% 7% 11% 22% 13% 1	2134
October	1st to 10th	Night	37% 6% 10% 25% 21% 1	3800
	11th to 20th	Night	48% 5% 12% 22% 13%	3779
	21st to end	Night	41% 6% 16% 26% 10% 1	4120
November	1st to 10th	Night	46% 6% 11% 24% 12% 1	2551
	11th to 20th	Night	51% 7% 14% 18% 9% 1	2800
	21st to end	Night	58% 10% 13% 15% 4	2800
December	1st to 10th	Night	66% 9% 14% 9% 2	4580
	11th to 20th	Night	64% 6% 8% 11% 9% 1	5208
	21st to end	Night	69% 8% 11% 9% 3	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	22% 19% 35% 17% 7% 2	200
	11th to 20th	Day	26% 21% 34% 13% 5% 1	288
	21st to end	Day	25% 19% 31% 18% 7% 1	330
February	1st to 10th	Day	26% 18% 37% 16% 3	300
	11th to 20th	Day	24% 25% 33% 15% 2	300
	21st to end	Day	29% 23% 31% 15% 1	288
March	1st to 10th	Day	22% 15% 35% 20% 6% 1	500
	11th to 20th	Day	23% 16% 33% 21% 6% 1	500
	21st to end	Day	28% 11% 29% 23% 7% 1	550
April	1st to 10th	Day	25% 14% 29% 23% 8% 1	500
	11th to 20th	Day	27% 15% 27% 17% 10% 3	492
	21st to end	Day	23% 11% 30% 23% 10% 2	500
May	1st to 10th	Day	24% 12% 25% 25% 10% 5%	503
	11th to 20th	Day	16% 10% 24% 27% 18% 6%	527
	21st to end	Day	14% 8% 24% 27% 21% 7%	573
June	1st to 10th	Day	16% 9% 30% 29% 14% 2	530
	11th to 20th	Day	12% 9% 26% 30% 19% 4	530
	21st to end	Day	17% 6% 28% 31% 15% 4	530
July	1st to 10th	Day	8% 5% 19% 31% 28% 8%	525
	11th to 20th	Day	15% 6% 20% 28% 22% 10%	524
	21st to end	Day	10% 5% 24% 33% 21% 7%	576
August	1st to 10th	Day	10% 5% 19% 30% 24% 12%	526
	11th to 20th	Day	14% 5% 17% 29% 24% 12%	519
	21st to end	Day	12% 6% 21% 29% 21% 11%	583
September	1st to 10th	Day	14% 8% 18% 27% 23% 11%	502
	11th to 20th	Day	11% 5% 14% 25% 27% 18%	490
	21st to end	Day	16% 11% 23% 23% 18% 8%	490
October	1st to 10th	Day	21% 9% 25% 19% 18% 8%	490
	11th to 20th	Day	23% 14% 29% 23% 9% 2	490
	21st to end	Day	31% 15% 25% 20% 9%	539
November	1st to 10th	Day	22% 16% 29% 22% 8% 3	329
	11th to 20th	Day	26% 19% 27% 22% 6% 2	200
	21st to end	Day	30% 20% 35% 13% 3	200
December	1st to 10th	Day	25% 20% 33% 16% 5% 1	200
	11th to 20th	Day	29% 21% 31% 15% 5% 1	200
	21st to end	Day	26% 21% 33% 14% 4 1	220
January	1st to 10th	Night	27% 17% 33% 16% 7%	800
	11th to 20th	Night	28% 17% 31% 20% 4	800
	21st to end	Night	24% 11% 34% 24% 6%	880
February	1st to 10th	Night	27% 17% 32% 21% 3	800
	11th to 20th	Night	24% 15% 36% 22% 4	783
	21st to end	Night	27% 12% 35% 23% 3	660
March	1st to 10th	Night	22% 13% 34% 26% 6%	800
	11th to 20th	Night	24% 13% 32% 24% 7%	800
	21st to end	Night	21% 9% 32% 26% 10% 1	880
April	1st to 10th	Night	24% 11% 32% 28% 5%	800
	11th to 20th	Night	22% 9% 31% 29% 8% 2	763
	21st to end	Night	21% 7% 27% 34% 11%	600
May	1st to 10th	Night	21% 5% 25% 33% 14% 3	600
	11th to 20th	Night	15% 4 26% 41% 14% 1	432
	21st to end	Night	15% 4 24% 35% 21% 2	433
June	1st to 10th	Night	13% 5% 29% 37% 16% 1	400
	11th to 20th	Night	12% 5% 24% 41% 18% 1	400
	21st to end	Night	13% 6% 22% 42% 17%	400
July	1st to 10th	Night	7% 3 17% 45% 26% 2	390
	11th to 20th	Night	11% 3 20% 38% 26% 3	388
	21st to end	Night	11% 3 17% 43% 24% 2	657
August	1st to 10th	Night	9% 3 13% 41% 31% 4	597
	11th to 20th	Night	9% 3 15% 32% 37% 3	594
	21st to end	Night	11% 2 16% 41% 27% 4	769
September	1st to 10th	Night	15% 4 18% 34% 24% 5%	773
	11th to 20th	Night	13% 4 15% 30% 30% 7%	760
	21st to end	Night	18% 6% 20% 30% 21% 5%	760
October	1st to 10th	Night	19% 6% 23% 29% 20% 3	760
	11th to 20th	Night	26% 11% 25% 26% 10% 2	760
	21st to end	Night	29% 10% 28% 28% 6%	836
November	1st to 10th	Night	21% 10% 28% 28% 11% 2	780
	11th to 20th	Night	22% 14% 34% 19% 10% 2	800
	21st to end	Night	27% 12% 35% 20% 7%	800
December	1st to 10th	Night	32% 15% 29% 19% 5%	800
	11th to 20th	Night	26% 19% 32% 19% 5%	800
	21st to end	Night	31% 13% 31% 22% 3	880

Page generated at 2025-09-01 01:51:06. Version ac07

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