Marengo
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Marengo clear sky chart from 2005-11-06 to 2026-07-31.

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	13% 9% 3 8% 5% 4 5% 4 5% 7% 37%	138335
	Night	14% 10% 4 8% 4 4 4 3 5% 6% 38%	134994
Transparency	Day	45% 6% 11% 23% 14% 2	141843
	Night	45% 5% 10% 23% 15% 2	139563
Seeing	Day	16% 11% 30% 25% 13% 5%	18250
	Night	17% 10% 32% 29% 11% 1	28050

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	11% 13% 8% 14% 6% 5% 5% 4 5% 6% 23%	4691
	11th to 20th	Day	8% 12% 6% 13% 4 4 5% 6% 6% 5% 31%	4710
	21st to end	Day	11% 13% 6% 10% 5% 5% 4 3 4 7% 33%	5370
February	1st to 10th	Day	12% 15% 5% 10% 7% 5% 6% 3 5% 6% 26%	1890
	11th to 20th	Day	13% 11% 3 8% 6% 5% 4 3 5% 5% 37%	2046
	21st to end	Day	13% 14% 5% 9% 6% 3 4 3 6% 6% 31%	1730
March	1st to 10th	Day	16% 10% 3 6% 4 3 3 3 4 5% 43%	4580
	11th to 20th	Day	12% 6% 3 7% 4 4 5% 3 5% 8% 43%	4812
	21st to end	Day	21% 13% 6% 11% 6% 5% 4 3 4 5% 22%	6099
April	1st to 10th	Day	18% 13% 4 8% 5% 5% 4 3 4 5% 31%	2520
	11th to 20th	Day	16% 10% 2 7% 6% 5% 5% 4 6% 6% 33%	2494
	21st to end	Day	19% 9% 3 7% 4 5% 5% 4 5% 7% 32%	2614
May	1st to 10th	Day	14% 7% 2 7% 5% 5% 4 4 6% 8% 38%	7616
	11th to 20th	Day	13% 5% 2 6% 5% 4 5% 4 6% 7% 41%	8213
	21st to end	Day	14% 7% 3 6% 5% 5% 5% 4 6% 8% 38%	9047
June	1st to 10th	Day	10% 6% 2 6% 4 5% 4 5% 6% 8% 44%	2940
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 4 7% 10% 44%	2940
	21st to end	Day	10% 7% 3 7% 4 4 5% 5% 6% 9% 40%	2940
July	1st to 10th	Day	11% 6% 3 7% 5% 5% 5% 4 6% 9% 38%	6500
	11th to 20th	Day	9% 4 2 6% 5% 4 5% 4 7% 9% 44%	6500
	21st to end	Day	11% 6% 2 7% 5% 5% 5% 4 6% 9% 41%	7065
August	1st to 10th	Day	11% 5% 2 6% 5% 4 5% 5% 6% 8% 44%	4517
	11th to 20th	Day	12% 5% 2 4 4 4 5% 4 6% 9% 44%	3849
	21st to end	Day	9% 5% 2 5% 4 4 4 4 5% 8% 48%	4312
September	1st to 10th	Day	12% 5% 2 5% 4 4 4 3 6% 7% 48%	2400
	11th to 20th	Day	12% 4 1 4 4 3 4 3 6% 7% 53%	2296
	21st to end	Day	11% 7% 2 6% 4 4 5% 3 6% 7% 45%	2200
October	1st to 10th	Day	12% 7% 2 6% 4 3 4 3 4 7% 48%	2105
	11th to 20th	Day	20% 9% 3 7% 4 4 4 3 6% 7% 35%	2000
	21st to end	Day	20% 9% 3 7% 5% 4 4 2 5% 5% 38%	1987
November	1st to 10th	Day	12% 9% 3 6% 4 4 4 4 5% 6% 44%	1852
	11th to 20th	Day	15% 14% 4 9% 4 4 4 4 5% 6% 32%	1890
	21st to end	Day	17% 13% 3 10% 4 4 3 2 4 5% 34%	1762
December	1st to 10th	Day	9% 9% 5% 9% 5% 4 4 3 5% 6% 41%	2690
	11th to 20th	Day	11% 16% 5% 11% 6% 5% 6% 4 6% 6% 23%	3088
	21st to end	Day	23% 22% 8% 11% 6% 4 4 2 3 5% 12%	4070
January	1st to 10th	Night	11% 14% 6% 13% 6% 4 4 3 4 4 31%	8232
	11th to 20th	Night	10% 13% 7% 15% 6% 6% 5% 5% 6% 4 22%	8280
	21st to end	Night	13% 14% 6% 9% 5% 4 4 3 5% 6% 32%	8841
February	1st to 10th	Night	13% 15% 5% 9% 6% 4 3 3 4 5% 32%	2730
	11th to 20th	Night	13% 10% 3 8% 5% 4 4 3 4 6% 38%	2458
	21st to end	Night	16% 13% 4 8% 4 3 3 3 4 5% 36%	2076
March	1st to 10th	Night	17% 10% 3 6% 4 4 3 3 4 5% 43%	5493
	11th to 20th	Night	10% 7% 2 6% 4 5% 4 3 5% 7% 48%	5408
	21st to end	Night	21% 12% 5% 9% 5% 5% 3 3 4 5% 28%	5072
April	1st to 10th	Night	18% 10% 4 6% 4 4 5% 4 4 5% 37%	2100
	11th to 20th	Night	19% 9% 4 6% 4 4 3 3 4 6% 38%	2075
	21st to end	Night	17% 8% 3 7% 4 3 4 3 5% 7% 38%	2006
May	1st to 10th	Night	15% 6% 3 6% 4 4 4 4 6% 8% 41%	5237
	11th to 20th	Night	10% 5% 2 6% 4 3 5% 4 6% 8% 45%	4675
	21st to end	Night	14% 7% 2 7% 5% 5% 4 3 6% 7% 40%	5139
June	1st to 10th	Night	10% 5% 1 5% 4 4 5% 4 6% 8% 47%	1680
	11th to 20th	Night	10% 5% 2 5% 3 4 5% 4 6% 8% 47%	1680
	21st to end	Night	10% 5% 3 6% 6% 4 5% 4 7% 8% 44%	1680
July	1st to 10th	Night	13% 5% 2 6% 5% 5% 5% 3 5% 8% 42%	3698
	11th to 20th	Night	8% 4 2 6% 4 4 4 3 6% 8% 51%	3697
	21st to end	Night	11% 4 2 6% 4 4 4 4 7% 8% 46%	4027
August	1st to 10th	Night	7% 3 2 6% 4 4 4 4 7% 9% 51%	2640
	11th to 20th	Night	12% 5% 1 6% 3 3 3 3 6% 8% 50%	3200
	21st to end	Night	9% 4 2 4 3 4 4 4 5% 8% 53%	3586
September	1st to 10th	Night	9% 5% 2 5% 4 3 4 3 4 6% 55%	2000
	11th to 20th	Night	11% 3 1 5% 4 3 4 3 4 7% 55%	2082
	21st to end	Night	11% 5% 2 5% 4 4 4 3 6% 7% 48%	2200
October	1st to 10th	Night	11% 6% 2 5% 4 3 3 3 4 6% 52%	2290
	11th to 20th	Night	16% 8% 2 6% 4 3 3 3 5% 5% 45%	2400
	21st to end	Night	19% 9% 3 6% 3 4 4 3 5% 6% 38%	2851
November	1st to 10th	Night	13% 9% 3 6% 4 3 3 3 5% 5% 47%	2666
	11th to 20th	Night	15% 12% 3 8% 4 3 5% 3 5% 6% 36%	2730
	21st to end	Night	15% 13% 4 8% 4 4 3 3 5% 5% 35%	2856
December	1st to 10th	Night	11% 9% 3 8% 4 4 4 4 5% 6% 41%	4720
	11th to 20th	Night	17% 17% 4 9% 5% 4 4 3 4 5% 29%	5339
	21st to end	Night	24% 18% 7% 10% 5% 4 4 3 4 4 17%	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	63% 3 13% 15% 6%	4692
	11th to 20th	Day	53% 3 10% 16% 14% 3	4710
	21st to end	Day	53% 4 12% 22% 9%	5370
February	1st to 10th	Day	58% 5% 12% 14% 9% 1	1890
	11th to 20th	Day	50% 4 10% 24% 9% 2	2056
	21st to end	Day	53% 6% 9% 20% 10% 1	1730
March	1st to 10th	Day	45% 5% 10% 26% 11% 4	4580
	11th to 20th	Day	40% 5% 12% 32% 11% 1	4812
	21st to end	Day	64% 6% 9% 13% 7% 1	6101
April	1st to 10th	Day	55% 5% 9% 20% 9%	2520
	11th to 20th	Day	50% 5% 10% 21% 11% 2	2494
	21st to end	Day	51% 6% 13% 18% 12% 1	2614
May	1st to 10th	Day	43% 6% 9% 25% 17% 1	7651
	11th to 20th	Day	41% 7% 10% 25% 17% 1	8217
	21st to end	Day	42% 6% 10% 22% 15% 5%	9061
June	1st to 10th	Day	36% 7% 12% 27% 17% 2	2940
	11th to 20th	Day	34% 7% 13% 24% 20% 2	2940
	21st to end	Day	39% 9% 12% 24% 14% 2	2940
July	1st to 10th	Day	41% 7% 11% 26% 13% 1	6431
	11th to 20th	Day	36% 6% 14% 27% 16% 1	6425
	21st to end	Day	39% 7% 15% 21% 16% 2	6869
August	1st to 10th	Day	36% 8% 15% 29% 12% 1	4502
	11th to 20th	Day	34% 6% 13% 31% 15% 1	3885
	21st to end	Day	34% 6% 11% 29% 19% 2	4311
September	1st to 10th	Day	34% 7% 12% 27% 17% 3	2400
	11th to 20th	Day	30% 5% 11% 31% 20% 2	2296
	21st to end	Day	38% 5% 10% 27% 19% 1	2200
October	1st to 10th	Day	25% 4 7% 35% 28% 1	3430
	11th to 20th	Day	33% 4 11% 26% 24% 1	3240
	21st to end	Day	44% 3 8% 28% 13% 4	3187
November	1st to 10th	Day	40% 6% 14% 24% 15% 2	1852
	11th to 20th	Day	53% 4 11% 21% 9% 1	1890
	21st to end	Day	54% 5% 11% 23% 8%	1762
December	1st to 10th	Day	44% 4 18% 26% 7% 1	2690
	11th to 20th	Day	60% 5% 11% 17% 7% 1	3085
	21st to end	Day	77% 3 6% 9% 5%	4070
January	1st to 10th	Night	57% 3 11% 19% 9%	8233
	11th to 20th	Night	63% 4 9% 11% 12% 1	8280
	21st to end	Night	54% 4 11% 21% 11%	8841
February	1st to 10th	Night	55% 3 10% 18% 12% 1	2730
	11th to 20th	Night	48% 4 11% 23% 14% 1	2459
	21st to end	Night	50% 4 11% 21% 10% 4	2076
March	1st to 10th	Night	45% 4 12% 23% 11% 5%	5493
	11th to 20th	Night	36% 4 11% 31% 14% 3	5408
	21st to end	Night	59% 5% 8% 18% 10% 1	5072
April	1st to 10th	Night	48% 5% 10% 22% 14% 2	2100
	11th to 20th	Night	47% 5% 7% 21% 16% 4	2075
	21st to end	Night	44% 6% 11% 24% 12% 2	2006
May	1st to 10th	Night	41% 6% 10% 22% 18% 2	5264
	11th to 20th	Night	35% 8% 14% 24% 16% 3	4676
	21st to end	Night	42% 6% 9% 21% 18% 5%	5149
June	1st to 10th	Night	33% 7% 13% 25% 20% 3	1680
	11th to 20th	Night	34% 7% 10% 24% 23% 3	1680
	21st to end	Night	38% 7% 14% 22% 18% 2	1680
July	1st to 10th	Night	41% 5% 13% 25% 14% 2	3660
	11th to 20th	Night	31% 6% 12% 30% 20% 1	3652
	21st to end	Night	35% 8% 13% 24% 18% 2	3912
August	1st to 10th	Night	29% 8% 13% 30% 18% 2	2631
	11th to 20th	Night	31% 7% 13% 31% 17%	3230
	21st to end	Night	29% 6% 13% 31% 18% 3	3586
September	1st to 10th	Night	31% 5% 9% 28% 24% 3	2000
	11th to 20th	Night	30% 5% 7% 33% 23% 2	2082
	21st to end	Night	35% 5% 11% 29% 19% 2	2200
October	1st to 10th	Night	24% 4 7% 36% 28% 2	3745
	11th to 20th	Night	36% 3 7% 25% 26% 3	3899
	21st to end	Night	42% 4 8% 25% 18% 3	4601
November	1st to 10th	Night	39% 5% 9% 26% 18% 3	2666
	11th to 20th	Night	48% 5% 11% 22% 12% 1	2730
	21st to end	Night	51% 4 11% 22% 11% 1	2856
December	1st to 10th	Night	43% 5% 14% 27% 10% 2	4720
	11th to 20th	Night	58% 4 11% 16% 10% 1	5341
	21st to end	Night	70% 4 8% 11% 6%	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	18% 14% 33% 28% 5% 1	330
	11th to 20th	Day	18% 18% 42% 19% 2 1	330
	21st to end	Day	18% 16% 40% 20% 4 1	363
February	1st to 10th	Day	16% 19% 41% 20% 5%	330
	11th to 20th	Day	20% 21% 36% 18% 4	330
	21st to end	Day	24% 20% 38% 16% 2	273
March	1st to 10th	Day	20% 19% 41% 17% 3	330
	11th to 20th	Day	25% 18% 37% 13% 6% 1	332
	21st to end	Day	25% 16% 34% 18% 6% 2	594
April	1st to 10th	Day	26% 15% 34% 22% 4	540
	11th to 20th	Day	23% 20% 27% 19% 8% 2	532
	21st to end	Day	22% 14% 32% 21% 9% 2	540
May	1st to 10th	Day	17% 16% 32% 24% 8% 3	543
	11th to 20th	Day	20% 13% 31% 22% 11% 3	766
	21st to end	Day	16% 9% 30% 24% 15% 5%	844
June	1st to 10th	Day	10% 8% 32% 29% 15% 5%	780
	11th to 20th	Day	11% 8% 34% 29% 14% 4	780
	21st to end	Day	11% 9% 30% 32% 14% 4	780
July	1st to 10th	Day	8% 4 19% 33% 24% 12%	771
	11th to 20th	Day	8% 5% 25% 33% 20% 8%	768
	21st to end	Day	7% 4 25% 39% 18% 7%	845
August	1st to 10th	Day	8% 3 21% 33% 23% 11%	716
	11th to 20th	Day	10% 5% 19% 27% 23% 16%	519
	21st to end	Day	8% 4 19% 31% 26% 11%	583
September	1st to 10th	Day	12% 7% 23% 27% 22% 8%	530
	11th to 20th	Day	8% 3 19% 32% 25% 12%	530
	21st to end	Day	12% 11% 27% 25% 18% 7%	530
October	1st to 10th	Day	12% 9% 31% 25% 17% 6%	435
	11th to 20th	Day	21% 18% 32% 21% 8%	330
	21st to end	Day	28% 13% 33% 20% 5% 1	363
November	1st to 10th	Day	18% 16% 37% 18% 8% 2	330
	11th to 20th	Day	23% 19% 35% 16% 6% 1	330
	21st to end	Day	24% 13% 40% 18% 4	330
December	1st to 10th	Day	15% 20% 43% 15% 6% 1	330
	11th to 20th	Day	16% 17% 39% 21% 7%	330
	21st to end	Day	25% 20% 39% 13% 3	363
January	1st to 10th	Night	17% 16% 38% 23% 5%	1050
	11th to 20th	Night	19% 16% 40% 20% 4 1	1050
	21st to end	Night	19% 13% 38% 25% 5%	1155
February	1st to 10th	Night	21% 12% 40% 23% 5%	1050
	11th to 20th	Night	18% 15% 37% 26% 4	823
	21st to end	Night	23% 13% 40% 21% 3	692
March	1st to 10th	Night	20% 13% 37% 23% 6%	840
	11th to 20th	Night	16% 12% 38% 27% 7%	782
	21st to end	Night	22% 12% 34% 25% 6% 1	693
April	1st to 10th	Night	22% 14% 36% 23% 5%	630
	11th to 20th	Night	23% 13% 31% 26% 7%	623
	21st to end	Night	21% 9% 32% 30% 7%	630
May	1st to 10th	Night	17% 9% 37% 30% 7%	630
	11th to 20th	Night	17% 8% 33% 31% 11%	621
	21st to end	Night	13% 8% 27% 36% 16%	683
June	1st to 10th	Night	10% 6% 26% 43% 15%	630
	11th to 20th	Night	12% 5% 33% 40% 11%	630
	21st to end	Night	8% 5% 30% 39% 18% 1	630
July	1st to 10th	Night	8% 2 18% 40% 29% 3	615
	11th to 20th	Night	8% 3 23% 38% 26% 2	612
	21st to end	Night	8% 3 20% 45% 24% 1	687
August	1st to 10th	Night	5% 1 15% 49% 26% 4	597
	11th to 20th	Night	9% 3 13% 41% 29% 4	594
	21st to end	Night	8% 2 17% 41% 29% 3	660
September	1st to 10th	Night	9% 5% 19% 37% 25% 5%	600
	11th to 20th	Night	11% 4 19% 34% 28% 4	682
	21st to end	Night	12% 9% 25% 33% 20% 2	800
October	1st to 10th	Night	14% 6% 27% 36% 17% 1	800
	11th to 20th	Night	21% 12% 32% 25% 9% 1	800
	21st to end	Night	23% 12% 33% 24% 8%	880
November	1st to 10th	Night	18% 10% 38% 24% 9%	820
	11th to 20th	Night	21% 17% 34% 23% 6%	840
	21st to end	Night	21% 14% 37% 23% 5%	966
December	1st to 10th	Night	21% 13% 39% 22% 4	1050
	11th to 20th	Night	22% 13% 37% 22% 4 1	1050
	21st to end	Night	29% 13% 36% 19% 3	1155

Page generated at 2026-07-28 02:25:20. Version ac07

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