Lake Monticello
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lake Monticello clear sky chart from 2005-11-06 to 2025-05-11.

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	14% 6% 2 6% 4 4 4 4 6% 8% 43%	113779
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	115151
Transparency	Day	38% 6% 11% 22% 18% 5%	117429
	Night	37% 5% 10% 22% 19% 6%	119640
Seeing	Day	19% 11% 26% 25% 14% 5%	15663
	Night	17% 8% 28% 31% 14% 1	25520

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	15% 5% 1 4 4 4 4 3 5% 5% 50%	4056
	11th to 20th	Day	13% 7% 2 5% 3 4 4 3 3 5% 52%	4060
	21st to end	Day	15% 7% 2 5% 4 4 5% 3 4 5% 46%	4466
February	1st to 10th	Day	13% 8% 3 6% 4 5% 5% 4 6% 6% 41%	1800
	11th to 20th	Day	20% 6% 2 5% 4 3 3 4 6% 5% 42%	1759
	21st to end	Day	18% 8% 2 6% 5% 4 5% 3 4 7% 39%	1715
March	1st to 10th	Day	16% 6% 2 6% 4 4 5% 4 6% 7% 40%	4935
	11th to 20th	Day	10% 6% 2 5% 4 4 3 3 5% 7% 51%	4980
	21st to end	Day	16% 6% 2 6% 5% 4 5% 3 6% 7% 40%	5465
April	1st to 10th	Day	12% 5% 2 5% 5% 4 4 3 5% 7% 48%	2370
	11th to 20th	Day	13% 5% 2 5% 4 3 4 4 5% 7% 48%	2374
	21st to end	Day	16% 6% 2 6% 5% 4 4 4 4 6% 43%	2407
May	1st to 10th	Day	17% 7% 3 7% 4 5% 5% 4 6% 8% 32%	4212
	11th to 20th	Day	21% 7% 2 6% 4 4 5% 4 6% 8% 32%	4081
	21st to end	Day	10% 4 2 5% 4 4 5% 4 7% 10% 45%	4724
June	1st to 10th	Day	11% 4 2 4 5% 4 5% 5% 7% 9% 44%	2660
	11th to 20th	Day	11% 4 2 6% 5% 5% 4 4 8% 10% 40%	2660
	21st to end	Day	8% 4 2 5% 4 5% 6% 5% 8% 10% 43%	2660
July	1st to 10th	Day	6% 4 1 5% 4 5% 6% 5% 10% 12% 42%	4253
	11th to 20th	Day	10% 6% 2 5% 5% 6% 5% 5% 8% 12% 36%	4120
	21st to end	Day	11% 6% 3 7% 7% 6% 6% 5% 7% 9% 32%	4466
August	1st to 10th	Day	15% 6% 2 7% 5% 5% 6% 5% 7% 11% 32%	4104
	11th to 20th	Day	8% 5% 2 6% 5% 6% 6% 4 8% 10% 40%	3932
	21st to end	Day	9% 4 2 5% 3 4 4 4 6% 10% 49%	4180
September	1st to 10th	Day	12% 6% 2 6% 4 5% 4 3 6% 6% 47%	2280
	11th to 20th	Day	13% 6% 2 6% 3 3 4 4 6% 9% 44%	2218
	21st to end	Day	22% 6% 2 6% 4 4 5% 3 5% 6% 37%	2014
October	1st to 10th	Day	16% 5% 2 6% 3 3 3 3 5% 6% 49%	1900
	11th to 20th	Day	16% 4 1 5% 3 3 3 3 5% 5% 52%	1900
	21st to end	Day	14% 5% 2 5% 3 3 3 2 3 6% 54%	2090
November	1st to 10th	Day	13% 4 1 4 4 3 3 3 5% 6% 54%	1957
	11th to 20th	Day	14% 6% 2 4 4 3 3 3 5% 6% 50%	1785
	21st to end	Day	14% 5% 2 4 4 3 3 4 5% 7% 50%	1600
December	1st to 10th	Day	12% 6% 2 5% 4 4 4 3 5% 6% 50%	2600
	11th to 20th	Day	20% 7% 2 4 2 3 3 2 4 7% 46%	2990
	21st to end	Day	19% 6% 3 7% 4 4 4 3 5% 6% 39%	4006
January	1st to 10th	Night	12% 6% 2 4 3 3 3 3 4 5% 54%	5889
	11th to 20th	Night	13% 7% 2 5% 3 3 3 2 3 5% 55%	5900
	21st to end	Night	15% 6% 2 5% 3 3 3 2 4 4 50%	6490
February	1st to 10th	Night	14% 8% 2 5% 4 4 5% 3 5% 7% 43%	2600
	11th to 20th	Night	21% 7% 2 5% 4 3 3 3 4 6% 43%	2525
	21st to end	Night	15% 7% 2 5% 4 3 4 3 5% 6% 45%	1912
March	1st to 10th	Night	15% 6% 2 5% 3 3 4 3 5% 6% 46%	4918
	11th to 20th	Night	10% 5% 2 4 3 3 3 3 5% 8% 54%	4980
	21st to end	Night	13% 6% 2 5% 4 4 4 4 5% 7% 46%	5467
April	1st to 10th	Night	14% 6% 2 6% 4 4 3 3 5% 7% 45%	2028
	11th to 20th	Night	12% 5% 2 5% 4 4 4 3 5% 8% 48%	1975
	21st to end	Night	14% 4 2 4 5% 4 4 4 5% 8% 46%	2000
May	1st to 10th	Night	19% 9% 3 6% 5% 4 4 3 5% 8% 35%	2913
	11th to 20th	Night	23% 7% 2 6% 4 4 4 4 6% 7% 32%	2818
	21st to end	Night	10% 4 2 6% 5% 4 5% 4 6% 9% 43%	2826
June	1st to 10th	Night	10% 5% 2 6% 5% 4 5% 5% 7% 8% 43%	1520
	11th to 20th	Night	10% 5% 2 6% 4 5% 5% 5% 7% 10% 42%	1520
	21st to end	Night	8% 4 1 6% 5% 6% 5% 4 7% 11% 43%	1520
July	1st to 10th	Night	7% 5% 3 7% 4 5% 6% 6% 8% 10% 39%	2701
	11th to 20th	Night	8% 7% 3 8% 6% 6% 6% 6% 8% 11% 34%	2840
	21st to end	Night	10% 8% 3 9% 7% 7% 6% 5% 7% 8% 30%	3087
August	1st to 10th	Night	16% 6% 3 7% 5% 5% 6% 4 7% 10% 32%	2825
	11th to 20th	Night	10% 7% 3 8% 5% 5% 6% 6% 7% 8% 36%	2863
	21st to end	Night	10% 6% 2 8% 4 4 5% 4 6% 8% 42%	3476
September	1st to 10th	Night	14% 5% 2 6% 3 4 4 4 5% 7% 45%	1900
	11th to 20th	Night	13% 6% 2 6% 4 5% 4 4 5% 7% 45%	1958
	21st to end	Night	23% 7% 2 6% 3 3 4 3 4 6% 38%	2164
October	1st to 10th	Night	17% 6% 2 5% 3 3 4 3 5% 7% 46%	2280
	11th to 20th	Night	15% 5% 2 4 4 3 3 3 4 6% 49%	2280
	21st to end	Night	14% 4 1 4 4 4 3 3 5% 6% 53%	2508
November	1st to 10th	Night	12% 6% 2 6% 4 4 3 3 4 5% 53%	2341
	11th to 20th	Night	14% 5% 2 5% 4 4 3 3 5% 6% 49%	2610
	21st to end	Night	13% 4 1 5% 4 4 4 4 5% 5% 51%	2800
December	1st to 10th	Night	10% 5% 2 5% 3 3 4 3 5% 7% 54%	4580
	11th to 20th	Night	20% 7% 2 5% 4 3 4 3 4 5% 42%	5205
	21st to end	Night	17% 6% 2 5% 4 4 4 4 5% 7% 43%	6932

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	35% 4 5% 24% 22% 10%	4057
	11th to 20th	Day	36% 3 6% 19% 23% 13%	4060
	21st to end	Day	41% 4 7% 26% 19% 4	4466
February	1st to 10th	Day	42% 5% 9% 25% 15% 3	1800
	11th to 20th	Day	44% 3 7% 21% 20% 5%	1768
	21st to end	Day	45% 5% 10% 16% 18% 6%	1715
March	1st to 10th	Day	42% 5% 9% 18% 16% 10%	4935
	11th to 20th	Day	33% 4 8% 25% 22% 9%	4980
	21st to end	Day	43% 6% 9% 17% 17% 7%	5465
April	1st to 10th	Day	35% 5% 11% 21% 19% 9%	2370
	11th to 20th	Day	35% 5% 11% 21% 22% 6%	2374
	21st to end	Day	41% 6% 10% 23% 15% 4	2407
May	1st to 10th	Day	47% 9% 13% 18% 13% 1	4250
	11th to 20th	Day	47% 8% 12% 21% 11% 1	4085
	21st to end	Day	32% 9% 14% 22% 20% 2	4738
June	1st to 10th	Day	34% 8% 16% 25% 16% 1	2660
	11th to 20th	Day	36% 9% 15% 22% 15% 2	2660
	21st to end	Day	33% 9% 16% 25% 15% 2	2660
July	1st to 10th	Day	31% 10% 23% 19% 11% 6%	4188
	11th to 20th	Day	37% 7% 19% 26% 9% 1	4051
	21st to end	Day	45% 8% 12% 21% 13% 1	4324
August	1st to 10th	Day	44% 7% 17% 24% 7% 1	4091
	11th to 20th	Day	36% 8% 15% 25% 15% 1	3971
	21st to end	Day	30% 6% 18% 27% 17% 1	4180
September	1st to 10th	Day	37% 7% 12% 18% 22% 5%	2280
	11th to 20th	Day	37% 6% 8% 26% 20% 4	2218
	21st to end	Day	47% 5% 9% 18% 17% 3	2014
October	1st to 10th	Day	34% 4 7% 19% 30% 5%	3160
	11th to 20th	Day	25% 3 7% 22% 31% 11%	3140
	21st to end	Day	21% 2 9% 30% 25% 12%	3428
November	1st to 10th	Day	32% 4 8% 23% 28% 5%	1957
	11th to 20th	Day	34% 4 10% 21% 22% 8%	1785
	21st to end	Day	35% 4 9% 24% 24% 4	1600
December	1st to 10th	Day	35% 5% 9% 25% 18% 9%	2600
	11th to 20th	Day	40% 3 13% 23% 16% 5%	2986
	21st to end	Day	45% 4 11% 23% 16% 1	4006
January	1st to 10th	Night	33% 3 5% 21% 27% 11%	5889
	11th to 20th	Night	34% 3 5% 20% 27% 11%	5900
	21st to end	Night	39% 3 6% 26% 21% 6%	6490
February	1st to 10th	Night	40% 5% 11% 24% 17% 3	2600
	11th to 20th	Night	44% 3 8% 19% 19% 7%	2527
	21st to end	Night	40% 4 9% 21% 18% 7%	1912
March	1st to 10th	Night	38% 5% 6% 22% 21% 9%	4918
	11th to 20th	Night	29% 4 8% 22% 28% 9%	4980
	21st to end	Night	38% 5% 8% 21% 22% 6%	5467
April	1st to 10th	Night	39% 6% 9% 17% 22% 7%	2028
	11th to 20th	Night	33% 6% 13% 20% 22% 6%	1975
	21st to end	Night	37% 6% 12% 23% 17% 6%	2000
May	1st to 10th	Night	47% 7% 14% 19% 11% 3	2940
	11th to 20th	Night	48% 8% 12% 22% 9% 1	2819
	21st to end	Night	35% 9% 15% 25% 15% 2	2833
June	1st to 10th	Night	34% 8% 12% 28% 16% 1	1520
	11th to 20th	Night	36% 10% 17% 21% 15% 2	1520
	21st to end	Night	33% 9% 15% 22% 18% 3	1520
July	1st to 10th	Night	37% 11% 23% 14% 12% 3	2656
	11th to 20th	Night	41% 10% 20% 21% 7% 1	2786
	21st to end	Night	48% 9% 14% 18% 10%	2988
August	1st to 10th	Night	47% 8% 17% 22% 6% 1	2814
	11th to 20th	Night	41% 9% 15% 22% 12% 1	2890
	21st to end	Night	37% 7% 14% 29% 12% 2	3476
September	1st to 10th	Night	38% 7% 12% 20% 16% 7%	1900
	11th to 20th	Night	37% 7% 10% 23% 21% 3	1958
	21st to end	Night	46% 5% 9% 20% 16% 3	2164
October	1st to 10th	Night	41% 4 9% 18% 21% 6%	3800
	11th to 20th	Night	26% 4 7% 21% 31% 11%	3779
	21st to end	Night	21% 3 7% 31% 27% 11%	4120
November	1st to 10th	Night	35% 4 8% 21% 26% 6%	2341
	11th to 20th	Night	37% 3 8% 20% 24% 8%	2610
	21st to end	Night	34% 4 9% 24% 23% 6%	2800
December	1st to 10th	Night	31% 4 9% 27% 24% 5%	4580
	11th to 20th	Night	45% 3 11% 18% 16% 7%	5208
	21st to end	Night	40% 5% 11% 26% 16% 1	6932

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	24% 22% 31% 17% 5%	300
	11th to 20th	Day	25% 30% 29% 13% 3	300
	21st to end	Day	25% 23% 36% 13% 3	330
February	1st to 10th	Day	24% 21% 39% 11% 4	300
	11th to 20th	Day	41% 23% 26% 9% 2	306
	21st to end	Day	27% 17% 31% 21% 4	414
March	1st to 10th	Day	23% 16% 34% 20% 6%	500
	11th to 20th	Day	23% 19% 34% 17% 7%	500
	21st to end	Day	24% 16% 35% 19% 5% 1	550
April	1st to 10th	Day	23% 20% 30% 18% 8% 1	500
	11th to 20th	Day	19% 15% 30% 21% 9% 5%	492
	21st to end	Day	23% 11% 34% 21% 9% 2	500
May	1st to 10th	Day	20% 15% 32% 20% 10% 3	503
	11th to 20th	Day	22% 9% 28% 25% 11% 5%	487
	21st to end	Day	17% 4 24% 31% 19% 5%	529
June	1st to 10th	Day	12% 7% 24% 31% 21% 5%	490
	11th to 20th	Day	12% 6% 25% 30% 17% 10%	490
	21st to end	Day	10% 4 22% 37% 21% 6%	490
July	1st to 10th	Day	11% 2 17% 32% 28% 10%	485
	11th to 20th	Day	8% 3 13% 36% 30% 11%	484
	21st to end	Day	11% 4 20% 32% 23% 11%	532
August	1st to 10th	Day	14% 3 18% 27% 25% 13%	486
	11th to 20th	Day	12% 2 19% 30% 25% 12%	479
	21st to end	Day	11% 4 15% 30% 24% 16%	539
September	1st to 10th	Day	13% 6% 15% 27% 23% 17%	490
	11th to 20th	Day	16% 3 14% 35% 23% 10%	490
	21st to end	Day	31% 4 23% 26% 11% 5%	490
October	1st to 10th	Day	19% 4 23% 29% 17% 7%	490
	11th to 20th	Day	15% 11% 24% 25% 21% 4	490
	21st to end	Day	16% 10% 34% 27% 10% 3	539
November	1st to 10th	Day	15% 9% 27% 33% 11% 4	495
	11th to 20th	Day	17% 14% 32% 24% 10% 2	363
	21st to end	Day	21% 16% 32% 22% 8% 2	200
December	1st to 10th	Day	23% 17% 40% 14% 6% 2	200
	11th to 20th	Day	26% 14% 30% 23% 7% 2	200
	21st to end	Day	29% 17% 28% 17% 8% 1	230
January	1st to 10th	Night	18% 14% 40% 24% 4	800
	11th to 20th	Night	20% 13% 37% 26% 4	800
	21st to end	Night	18% 14% 41% 24% 3	880
February	1st to 10th	Night	18% 12% 38% 29% 3	800
	11th to 20th	Night	26% 14% 39% 20% 1	783
	21st to end	Night	22% 15% 38% 23% 2	660
March	1st to 10th	Night	18% 15% 37% 26% 4	800
	11th to 20th	Night	20% 15% 35% 23% 5% 1	800
	21st to end	Night	19% 13% 37% 26% 5%	880
April	1st to 10th	Night	17% 16% 39% 22% 5%	800
	11th to 20th	Night	14% 13% 36% 29% 8%	791
	21st to end	Night	16% 8% 33% 33% 9%	800
May	1st to 10th	Night	19% 8% 30% 30% 11%	610
	11th to 20th	Night	21% 3 23% 35% 17% 1	563
	21st to end	Night	16% 1 18% 39% 23% 3	467
June	1st to 10th	Night	9% 2 19% 42% 27% 1	380
	11th to 20th	Night	12% 3 15% 42% 26% 1	380
	21st to end	Night	8% 1 21% 48% 21% 2	380
July	1st to 10th	Night	11% 2 15% 36% 31% 4	468
	11th to 20th	Night	7% 2 11% 39% 36% 5%	552
	21st to end	Night	8% 2 16% 42% 28% 4	624
August	1st to 10th	Night	14% 2 10% 40% 32% 3	567
	11th to 20th	Night	15% 2 10% 38% 32% 3	603
	21st to end	Night	11% 2 14% 35% 34% 4	836
September	1st to 10th	Night	16% 1 13% 33% 31% 6%	760
	11th to 20th	Night	14% 3 16% 33% 31% 4	760
	21st to end	Night	25% 4 18% 31% 20% 2	760
October	1st to 10th	Night	20% 5% 17% 31% 23% 5%	760
	11th to 20th	Night	18% 7% 25% 33% 16% 1	760
	21st to end	Night	15% 7% 29% 35% 13% 1	836
November	1st to 10th	Night	14% 7% 28% 36% 13% 2	780
	11th to 20th	Night	17% 10% 33% 31% 9%	800
	21st to end	Night	14% 10% 34% 33% 10% 1	800
December	1st to 10th	Night	20% 9% 37% 26% 7%	800
	11th to 20th	Night	18% 9% 32% 36% 5%	800
	21st to end	Night	21% 10% 30% 36% 4	880

Page generated at 2025-05-08 09:33:59. Version ac07

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