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 2025-03-27.

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%	108962
	Night	14% 10% 4 8% 4 4 4 3 5% 6% 38%	110579
Transparency	Day	45% 6% 12% 23% 14% 2	112515
	Night	46% 5% 11% 22% 15% 2	115167
Seeing	Day	16% 11% 30% 25% 13% 5%	16279
	Night	17% 10% 32% 29% 11% 1	26319

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	10% 11% 8% 15% 7% 5% 6% 4 5% 5% 24%	3617
	11th to 20th	Day	9% 12% 6% 12% 4 4 4 7% 6% 5% 30%	3620
	21st to end	Day	14% 15% 7% 9% 4 3 3 3 5% 7% 31%	4140
February	1st to 10th	Day	12% 15% 5% 10% 7% 5% 6% 3 5% 6% 26%	1800
	11th to 20th	Day	14% 11% 3 9% 6% 5% 3 3 5% 5% 36%	1946
	21st to end	Day	13% 14% 4 9% 6% 3 4 3 6% 6% 32%	1650
March	1st to 10th	Day	14% 11% 4 7% 5% 4 4 3 4 5% 39%	3220
	11th to 20th	Day	12% 6% 3 7% 5% 4 5% 4 6% 7% 42%	3397
	21st to end	Day	21% 14% 5% 11% 6% 5% 4 4 4 5% 22%	4237
April	1st to 10th	Day	18% 13% 4 8% 5% 4 4 3 4 5% 32%	2280
	11th to 20th	Day	17% 10% 3 6% 6% 5% 5% 4 5% 6% 34%	2254
	21st to end	Day	20% 9% 3 7% 4 5% 5% 4 5% 6% 32%	2365
May	1st to 10th	Day	17% 7% 2 7% 5% 5% 5% 4 6% 7% 36%	4082
	11th to 20th	Day	15% 6% 2 6% 4 4 4 4 5% 7% 43%	4390
	21st to end	Day	12% 6% 2 6% 4 5% 5% 4 7% 9% 40%	4824
June	1st to 10th	Day	9% 6% 2 6% 4 4 4 5% 6% 8% 45%	2660
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 4 7% 10% 45%	2660
	21st to end	Day	11% 7% 3 7% 4 4 5% 5% 6% 9% 39%	2660
July	1st to 10th	Day	12% 7% 3 8% 6% 5% 6% 5% 7% 9% 33%	4440
	11th to 20th	Day	6% 4 2 5% 5% 4 5% 4 7% 9% 49%	4440
	21st to end	Day	9% 6% 3 7% 5% 5% 5% 4 6% 8% 41%	4813
August	1st to 10th	Day	11% 5% 2 6% 5% 4 5% 5% 6% 8% 44%	4379
	11th to 20th	Day	12% 5% 2 4 4 4 5% 5% 6% 9% 45%	3729
	21st to end	Day	9% 5% 2 6% 4 4 4 4 5% 8% 48%	4180
September	1st to 10th	Day	12% 5% 2 5% 4 4 4 3 6% 7% 48%	2280
	11th to 20th	Day	13% 4 1 4 4 3 4 3 6% 7% 52%	2181
	21st to end	Day	12% 7% 2 7% 4 4 5% 3 6% 7% 44%	2090
October	1st to 10th	Day	13% 7% 2 6% 4 3 5% 3 4 7% 47%	2000
	11th to 20th	Day	20% 9% 3 7% 4 4 4 3 6% 7% 35%	1900
	21st to end	Day	21% 9% 3 7% 5% 4 4 2 4 5% 37%	1888
November	1st to 10th	Day	13% 9% 3 6% 4 4 4 4 5% 6% 44%	1762
	11th to 20th	Day	15% 13% 4 9% 5% 4 4 4 5% 6% 31%	1800
	21st to end	Day	17% 13% 3 10% 5% 4 3 2 4 5% 35%	1678
December	1st to 10th	Day	9% 9% 5% 8% 4 4 4 3 5% 7% 42%	2610
	11th to 20th	Day	11% 16% 5% 11% 6% 5% 6% 4 6% 6% 23%	3008
	21st to end	Day	22% 22% 8% 11% 6% 4 4 2 3 5% 12%	3982
January	1st to 10th	Night	11% 13% 6% 14% 6% 4 4 2 4 4 31%	6348
	11th to 20th	Night	13% 14% 7% 14% 5% 5% 4 5% 6% 4 22%	6360
	21st to end	Night	16% 15% 6% 10% 4 4 3 3 4 5% 30%	6835
February	1st to 10th	Night	13% 15% 5% 9% 6% 4 3 3 4 5% 32%	2600
	11th to 20th	Night	13% 11% 3 8% 6% 4 4 3 4 6% 37%	2338
	21st to end	Night	16% 14% 4 7% 4 3 3 3 4 5% 36%	1980
March	1st to 10th	Night	18% 10% 3 5% 4 4 3 3 4 6% 41%	3853
	11th to 20th	Night	9% 7% 2 7% 4 5% 4 4 5% 7% 47%	3810
	21st to end	Night	23% 12% 6% 10% 4 4 3 3 4 4 28%	3532
April	1st to 10th	Night	17% 10% 4 6% 4 4 4 4 4 5% 39%	1900
	11th to 20th	Night	19% 9% 4 6% 4 4 3 3 4 5% 38%	1875
	21st to end	Night	18% 8% 3 6% 4 3 4 3 5% 6% 38%	1815
May	1st to 10th	Night	17% 7% 3 6% 4 5% 5% 4 6% 7% 37%	2813
	11th to 20th	Night	14% 6% 2 6% 4 4 5% 5% 7% 8% 41%	2511
	21st to end	Night	12% 8% 2 7% 4 5% 4 3 6% 7% 43%	2743
June	1st to 10th	Night	10% 5% 1 6% 4 4 5% 4 7% 8% 47%	1520
	11th to 20th	Night	9% 5% 2 5% 3 4 4 4 6% 8% 48%	1520
	21st to end	Night	10% 5% 2 5% 6% 4 5% 4 6% 7% 44%	1520
July	1st to 10th	Night	16% 6% 2 6% 5% 5% 5% 4 6% 8% 39%	2530
	11th to 20th	Night	6% 3 1 5% 4 4 4 4 7% 8% 53%	2530
	21st to end	Night	8% 4 2 6% 5% 5% 5% 4 8% 9% 45%	2750
August	1st to 10th	Night	7% 3 2 6% 4 4 4 4 7% 9% 51%	2558
	11th to 20th	Night	11% 5% 1 6% 3 3 3 3 6% 8% 50%	3100
	21st to end	Night	9% 4 2 4 4 4 4 4 5% 8% 53%	3476
September	1st to 10th	Night	10% 5% 2 5% 4 3 4 3 4 6% 56%	1900
	11th to 20th	Night	11% 3 1 5% 4 3 4 3 4 7% 54%	1978
	21st to end	Night	12% 5% 2 5% 5% 4 4 3 6% 7% 48%	2090
October	1st to 10th	Night	11% 7% 2 6% 4 3 3 3 4 6% 51%	2175
	11th to 20th	Night	16% 8% 2 6% 4 3 3 3 4 5% 46%	2280
	21st to end	Night	20% 9% 3 6% 3 4 4 3 5% 6% 38%	2708
November	1st to 10th	Night	12% 9% 3 6% 4 3 3 3 5% 5% 48%	2536
	11th to 20th	Night	15% 12% 3 8% 4 3 5% 3 5% 6% 36%	2600
	21st to end	Night	15% 12% 4 8% 4 4 3 3 5% 5% 35%	2720
December	1st to 10th	Night	11% 9% 3 8% 4 4 4 4 5% 6% 42%	4580
	11th to 20th	Night	17% 17% 4 8% 5% 4 4 3 4 5% 29%	5199
	21st to end	Night	23% 18% 7% 10% 5% 4 4 4 4 4 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	62% 4 11% 17% 7%	3618
	11th to 20th	Day	53% 4 12% 11% 17% 4	3620
	21st to end	Day	54% 4 13% 19% 10%	4140
February	1st to 10th	Day	58% 5% 12% 14% 10% 1	1800
	11th to 20th	Day	50% 4 10% 24% 9% 2	1956
	21st to end	Day	53% 6% 9% 19% 11% 1	1650
March	1st to 10th	Day	47% 6% 10% 23% 11% 3	3220
	11th to 20th	Day	40% 6% 12% 31% 11% 1	3397
	21st to end	Day	64% 7% 11% 13% 4 1	4237
April	1st to 10th	Day	55% 5% 10% 20% 10%	2280
	11th to 20th	Day	50% 5% 9% 22% 12% 3	2254
	21st to end	Day	51% 6% 13% 16% 13% 1	2365
May	1st to 10th	Day	46% 7% 11% 22% 14% 1	4120
	11th to 20th	Day	40% 6% 11% 28% 14%	4394
	21st to end	Day	38% 7% 12% 21% 17% 5%	4837
June	1st to 10th	Day	35% 7% 12% 27% 17% 2	2660
	11th to 20th	Day	34% 7% 13% 23% 21% 3	2660
	21st to end	Day	40% 9% 12% 23% 14% 2	2660
July	1st to 10th	Day	45% 8% 11% 23% 11% 2	4371
	11th to 20th	Day	30% 7% 16% 28% 18% 1	4365
	21st to end	Day	39% 8% 14% 17% 19% 3	4662
August	1st to 10th	Day	36% 8% 15% 28% 12% 1	4364
	11th to 20th	Day	34% 6% 13% 32% 15% 1	3765
	21st to end	Day	34% 6% 12% 28% 18% 2	4179
September	1st to 10th	Day	35% 7% 12% 25% 18% 3	2280
	11th to 20th	Day	30% 5% 11% 31% 21% 2	2181
	21st to end	Day	39% 5% 10% 27% 19% 1	2090
October	1st to 10th	Day	25% 4 7% 35% 28% 1	3325
	11th to 20th	Day	33% 4 11% 26% 24% 1	3140
	21st to end	Day	44% 3 8% 28% 13% 4	3088
November	1st to 10th	Day	40% 6% 14% 23% 15% 2	1762
	11th to 20th	Day	54% 5% 11% 21% 9% 1	1800
	21st to end	Day	53% 5% 11% 23% 8%	1678
December	1st to 10th	Day	42% 5% 18% 27% 7% 1	2610
	11th to 20th	Day	60% 5% 11% 16% 7% 1	3005
	21st to end	Day	77% 3 6% 9% 5%	3982
January	1st to 10th	Night	57% 3 9% 20% 11%	6348
	11th to 20th	Night	63% 4 9% 9% 14% 1	6360
	21st to end	Night	57% 4 12% 15% 11% 1	6835
February	1st to 10th	Night	55% 3 10% 17% 13% 2	2600
	11th to 20th	Night	49% 4 11% 22% 14% 1	2339
	21st to end	Night	50% 4 11% 20% 11% 4	1980
March	1st to 10th	Night	45% 5% 12% 23% 10% 4	3853
	11th to 20th	Night	36% 5% 11% 28% 17% 2	3810
	21st to end	Night	60% 5% 8% 18% 8% 1	3532
April	1st to 10th	Night	46% 5% 10% 22% 14% 2	1900
	11th to 20th	Night	47% 5% 7% 20% 17% 4	1875
	21st to end	Night	45% 7% 11% 23% 12% 3	1815
May	1st to 10th	Night	44% 8% 12% 19% 15% 3	2840
	11th to 20th	Night	38% 9% 14% 27% 11% 1	2512
	21st to end	Night	37% 7% 11% 19% 21% 4	2751
June	1st to 10th	Night	33% 8% 13% 25% 19% 3	1520
	11th to 20th	Night	33% 7% 10% 25% 23% 3	1520
	21st to end	Night	38% 7% 14% 21% 18% 2	1520
July	1st to 10th	Night	44% 6% 14% 20% 13% 3	2490
	11th to 20th	Night	27% 6% 14% 33% 18% 1	2482
	21st to end	Night	33% 9% 14% 19% 22% 3	2662
August	1st to 10th	Night	29% 8% 13% 30% 18% 2	2549
	11th to 20th	Night	31% 7% 13% 31% 18%	3130
	21st to end	Night	29% 6% 13% 31% 18% 3	3476
September	1st to 10th	Night	31% 5% 9% 27% 25% 3	1900
	11th to 20th	Night	30% 6% 7% 33% 22% 2	1978
	21st to end	Night	35% 5% 11% 29% 18% 2	2090
October	1st to 10th	Night	23% 4 7% 37% 28% 2	3630
	11th to 20th	Night	36% 3 7% 25% 26% 3	3779
	21st to end	Night	42% 4 8% 25% 18% 3	4458
November	1st to 10th	Night	38% 5% 9% 26% 18% 3	2536
	11th to 20th	Night	49% 5% 11% 22% 12% 1	2600
	21st to end	Night	51% 4 11% 22% 11% 1	2720
December	1st to 10th	Night	42% 5% 14% 27% 10% 2	4580
	11th to 20th	Night	58% 4 11% 16% 10% 1	5201
	21st to end	Night	71% 4 7% 11% 6%	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	17% 15% 32% 29% 6% 1	300
	11th to 20th	Day	18% 16% 42% 20% 2 1	300
	21st to end	Day	19% 15% 39% 21% 5% 1	330
February	1st to 10th	Day	17% 19% 41% 19% 4	300
	11th to 20th	Day	20% 21% 36% 19% 3	300
	21st to end	Day	23% 20% 38% 16% 2	249
March	1st to 10th	Day	18% 21% 40% 18% 3	300
	11th to 20th	Day	24% 19% 36% 14% 7% 1	302
	21st to end	Day	26% 15% 31% 19% 6% 2	530
April	1st to 10th	Day	25% 14% 33% 23% 4	460
	11th to 20th	Day	22% 20% 27% 19% 9% 2	452
	21st to end	Day	23% 15% 30% 21% 9% 2	460
May	1st to 10th	Day	18% 15% 31% 24% 9% 2	463
	11th to 20th	Day	21% 11% 30% 24% 11% 3	666
	21st to end	Day	16% 9% 30% 24% 15% 6%	734
June	1st to 10th	Day	10% 8% 31% 30% 15% 6%	680
	11th to 20th	Day	11% 7% 32% 30% 15% 4	680
	21st to end	Day	12% 9% 29% 32% 15% 4	680
July	1st to 10th	Day	8% 4 19% 32% 23% 13%	671
	11th to 20th	Day	8% 5% 26% 34% 19% 9%	668
	21st to end	Day	7% 4 24% 39% 19% 8%	739
August	1st to 10th	Day	9% 3 21% 33% 23% 11%	667
	11th to 20th	Day	9% 6% 19% 28% 23% 15%	479
	21st to end	Day	8% 4 18% 31% 27% 11%	539
September	1st to 10th	Day	13% 7% 23% 28% 22% 7%	490
	11th to 20th	Day	9% 3 20% 32% 24% 12%	490
	21st to end	Day	13% 12% 27% 24% 17% 7%	490
October	1st to 10th	Day	13% 10% 30% 25% 17% 5%	400
	11th to 20th	Day	22% 19% 32% 20% 7%	300
	21st to end	Day	29% 14% 32% 19% 5% 1	330
November	1st to 10th	Day	17% 14% 38% 19% 9% 2	300
	11th to 20th	Day	22% 18% 37% 16% 6% 1	300
	21st to end	Day	25% 12% 40% 19% 4	300
December	1st to 10th	Day	16% 20% 43% 15% 6% 1	300
	11th to 20th	Day	17% 16% 38% 21% 7%	300
	21st to end	Day	24% 20% 39% 14% 3	330
January	1st to 10th	Night	17% 16% 38% 23% 5%	1000
	11th to 20th	Night	20% 15% 39% 21% 5% 1	1000
	21st to end	Night	19% 12% 38% 25% 5%	1100
February	1st to 10th	Night	21% 12% 40% 23% 4	1000
	11th to 20th	Night	19% 15% 37% 25% 4	783
	21st to end	Night	23% 13% 39% 21% 3	660
March	1st to 10th	Night	21% 13% 36% 24% 6%	800
	11th to 20th	Night	15% 12% 38% 28% 7%	745
	21st to end	Night	22% 11% 33% 26% 6% 1	647
April	1st to 10th	Night	21% 13% 36% 25% 5%	570
	11th to 20th	Night	23% 13% 31% 26% 7%	563
	21st to end	Night	21% 9% 33% 29% 8% 1	570
May	1st to 10th	Night	17% 9% 36% 29% 7% 1	570
	11th to 20th	Night	18% 7% 32% 32% 11%	561
	21st to end	Night	13% 7% 27% 36% 17%	617
June	1st to 10th	Night	9% 6% 27% 42% 16% 1	570
	11th to 20th	Night	12% 4 33% 40% 12% 1	570
	21st to end	Night	8% 5% 30% 38% 17% 1	570
July	1st to 10th	Night	9% 2 17% 39% 30% 3	555
	11th to 20th	Night	8% 3 24% 38% 25% 2	552
	21st to end	Night	7% 2 20% 45% 26% 1	624
August	1st to 10th	Night	5% 1 15% 49% 25% 4	567
	11th to 20th	Night	9% 2 13% 41% 30% 4	564
	21st to end	Night	8% 2 18% 40% 29% 3	627
September	1st to 10th	Night	10% 5% 19% 38% 25% 4	570
	11th to 20th	Night	11% 4 20% 34% 27% 4	648
	21st to end	Night	12% 9% 25% 33% 19% 2	760
October	1st to 10th	Night	14% 6% 27% 34% 17% 1	760
	11th to 20th	Night	22% 13% 31% 24% 10% 1	760
	21st to end	Night	24% 12% 34% 23% 8%	836
November	1st to 10th	Night	18% 10% 38% 24% 10% 1	780
	11th to 20th	Night	21% 17% 34% 23% 6%	800
	21st to end	Night	20% 14% 38% 23% 6%	920
December	1st to 10th	Night	21% 13% 39% 22% 4	1000
	11th to 20th	Night	23% 12% 37% 23% 5% 1	1000
	21st to end	Night	29% 13% 36% 20% 3	1100

Page generated at 2025-03-24 08:04:29. Version ac07

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