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 2026-09-29.

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% 6% 2 6% 4 4 5% 4 6% 8% 42%	148680
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 43%	142696
Transparency	Day	38% 6% 12% 22% 17% 4	152286
	Night	39% 6% 11% 22% 18% 5%	147160
Seeing	Day	18% 11% 26% 25% 14% 5%	17545
	Night	17% 8% 28% 31% 14% 1	27248

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% 6% 2 5% 4 4 4 3 5% 6% 47%	5258
	11th to 20th	Day	12% 7% 2 5% 3 3 3 2 3 4 54%	5280
	21st to end	Day	14% 6% 2 6% 4 5% 5% 3 4 5% 46%	5780
February	1st to 10th	Day	12% 8% 3 6% 4 5% 5% 4 6% 6% 42%	1890
	11th to 20th	Day	20% 6% 2 5% 4 3 3 4 6% 5% 42%	1850
	21st to end	Day	18% 8% 2 6% 5% 4 5% 3 4 7% 38%	1798
March	1st to 10th	Day	16% 6% 2 6% 5% 4 5% 4 5% 7% 40%	5045
	11th to 20th	Day	11% 6% 2 5% 4 4 3 3 5% 7% 51%	5090
	21st to end	Day	16% 6% 2 6% 5% 4 5% 3 6% 7% 40%	5586
April	1st to 10th	Day	12% 5% 2 5% 5% 4 4 3 5% 7% 49%	2489
	11th to 20th	Day	12% 5% 2 5% 4 3 4 4 6% 7% 48%	2494
	21st to end	Day	16% 6% 2 6% 4 4 4 4 4 6% 43%	2527
May	1st to 10th	Day	13% 7% 2 7% 4 5% 5% 4 7% 9% 38%	7616
	11th to 20th	Day	17% 6% 2 6% 4 4 5% 4 6% 8% 38%	7626
	21st to end	Day	20% 5% 2 6% 4 4 4 4 6% 8% 38%	8863
June	1st to 10th	Day	10% 4 2 4 5% 5% 5% 5% 7% 10% 44%	2940
	11th to 20th	Day	11% 4 2 6% 5% 5% 4 4 8% 10% 40%	2940
	21st to end	Day	8% 5% 2 6% 4 5% 6% 5% 8% 10% 43%	2940
July	1st to 10th	Day	7% 5% 2 6% 4 5% 5% 4 8% 11% 43%	7925
	11th to 20th	Day	10% 6% 2 7% 6% 6% 6% 6% 9% 13% 30%	7680
	21st to end	Day	9% 5% 2 7% 5% 5% 6% 5% 8% 11% 37%	8340
August	1st to 10th	Day	14% 7% 3 7% 5% 5% 6% 5% 8% 11% 29%	6014
	11th to 20th	Day	7% 6% 2 7% 6% 6% 7% 5% 8% 11% 35%	5795
	21st to end	Day	7% 4 2 5% 4 4 5% 5% 7% 10% 48%	6080
September	1st to 10th	Day	11% 5% 2 5% 4 5% 4 3 6% 7% 49%	2520
	11th to 20th	Day	13% 6% 2 6% 4 3 4 4 6% 9% 44%	2452
	21st to end	Day	22% 7% 2 6% 4 4 4 4 5% 6% 35%	2206
October	1st to 10th	Day	15% 5% 2 6% 3 3 3 3 5% 6% 49%	2000
	11th to 20th	Day	15% 4 1 5% 4 3 3 3 5% 5% 52%	2000
	21st to end	Day	14% 5% 2 5% 3 3 3 2 3 6% 54%	2200
November	1st to 10th	Day	12% 4 1 4 4 3 4 3 5% 6% 54%	2057
	11th to 20th	Day	13% 6% 2 4 4 3 3 3 5% 6% 51%	1874
	21st to end	Day	15% 5% 2 4 4 3 3 4 5% 7% 50%	1680
December	1st to 10th	Day	12% 6% 2 5% 4 4 4 3 5% 6% 50%	2680
	11th to 20th	Day	19% 7% 2 4 2 3 3 2 4 7% 47%	3070
	21st to end	Day	19% 7% 3 7% 4 4 4 3 5% 6% 39%	4095
January	1st to 10th	Night	12% 6% 2 5% 4 4 4 4 5% 6% 50%	7635
	11th to 20th	Night	13% 7% 2 5% 3 3 3 2 3 5% 55%	7680
	21st to end	Night	14% 5% 2 6% 3 3 3 2 4 5% 51%	8401
February	1st to 10th	Night	14% 8% 2 5% 4 4 5% 3 5% 7% 43%	2730
	11th to 20th	Night	21% 7% 2 6% 4 3 3 3 4 6% 43%	2654
	21st to end	Night	15% 7% 2 6% 4 3 4 3 5% 6% 45%	2005
March	1st to 10th	Night	15% 6% 2 5% 3 4 4 3 5% 6% 46%	5028
	11th to 20th	Night	10% 5% 2 4 3 3 3 3 5% 8% 54%	5090
	21st to end	Night	12% 6% 2 5% 4 4 4 4 5% 7% 46%	5588
April	1st to 10th	Night	14% 6% 2 5% 4 4 3 3 5% 8% 46%	2129
	11th to 20th	Night	12% 5% 2 5% 4 4 4 3 6% 9% 48%	2075
	21st to end	Night	14% 4 2 5% 5% 4 4 4 5% 8% 46%	2100
May	1st to 10th	Night	15% 8% 3 7% 5% 4 5% 3 6% 8% 36%	5247
	11th to 20th	Night	18% 6% 2 5% 4 4 4 4 6% 8% 38%	5234
	21st to end	Night	21% 8% 3 7% 4 4 4 3 5% 7% 35%	5286
June	1st to 10th	Night	9% 5% 2 6% 5% 4 5% 5% 7% 8% 43%	1680
	11th to 20th	Night	10% 5% 2 6% 4 5% 5% 5% 6% 10% 42%	1680
	21st to end	Night	8% 4 1 6% 5% 6% 5% 4 7% 11% 44%	1680
July	1st to 10th	Night	11% 6% 3 7% 5% 5% 5% 5% 7% 9% 37%	5013
	11th to 20th	Night	12% 9% 3 9% 6% 6% 6% 6% 7% 9% 26%	5277
	21st to end	Night	9% 7% 3 8% 6% 6% 6% 5% 7% 8% 33%	5736
August	1st to 10th	Night	16% 8% 3 7% 5% 5% 6% 5% 7% 10% 28%	4135
	11th to 20th	Night	10% 8% 3 9% 6% 6% 6% 5% 8% 9% 31%	4203
	21st to end	Night	9% 7% 3 8% 4 5% 5% 4 6% 8% 41%	5045
September	1st to 10th	Night	13% 5% 2 6% 3 4 4 4 5% 8% 47%	2100
	11th to 20th	Night	13% 6% 2 6% 4 5% 4 4 5% 7% 45%	2164
	21st to end	Night	24% 7% 2 6% 3 3 4 3 4 6% 36%	2369
October	1st to 10th	Night	17% 6% 2 5% 3 3 3 3 5% 7% 47%	2400
	11th to 20th	Night	15% 5% 2 4 4 3 3 4 4 6% 49%	2400
	21st to end	Night	14% 4 1 4 4 4 3 3 5% 5% 53%	2640
November	1st to 10th	Night	12% 6% 2 6% 4 4 3 3 3 5% 53%	2461
	11th to 20th	Night	14% 5% 2 5% 4 4 3 3 5% 6% 49%	2741
	21st to end	Night	12% 4 1 6% 4 4 4 4 5% 6% 51%	2940
December	1st to 10th	Night	10% 5% 3 5% 3 3 4 3 5% 7% 53%	4720
	11th to 20th	Night	20% 7% 2 5% 4 3 4 3 4 5% 42%	5345
	21st to end	Night	17% 6% 2 5% 4 4 4 4 5% 7% 43%	7085

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	38% 3 8% 23% 20% 8%	5259
	11th to 20th	Day	34% 3 5% 18% 25% 15%	5280
	21st to end	Day	41% 3 6% 28% 18% 4	5780
February	1st to 10th	Day	41% 5% 9% 25% 16% 4	1890
	11th to 20th	Day	43% 3 7% 21% 20% 5%	1859
	21st to end	Day	46% 5% 10% 17% 17% 5%	1798
March	1st to 10th	Day	42% 5% 9% 18% 16% 10%	5045
	11th to 20th	Day	33% 4 8% 25% 21% 9%	5090
	21st to end	Day	43% 6% 10% 18% 17% 7%	5586
April	1st to 10th	Day	34% 5% 11% 22% 20% 8%	2489
	11th to 20th	Day	34% 5% 11% 21% 22% 6%	2494
	21st to end	Day	41% 6% 10% 24% 15% 4	2527
May	1st to 10th	Day	41% 8% 13% 22% 16% 1	7651
	11th to 20th	Day	42% 6% 12% 23% 16% 1	7630
	21st to end	Day	43% 7% 11% 18% 18% 3	8878
June	1st to 10th	Day	34% 8% 16% 24% 17% 1	2940
	11th to 20th	Day	36% 9% 16% 23% 15% 2	2940
	21st to end	Day	33% 9% 16% 26% 15% 2	2940
July	1st to 10th	Day	33% 9% 20% 23% 12% 3	7860
	11th to 20th	Day	42% 9% 18% 23% 7% 1	7611
	21st to end	Day	39% 8% 14% 22% 17% 1	8156
August	1st to 10th	Day	46% 8% 20% 20% 5%	6001
	11th to 20th	Day	40% 9% 18% 22% 10% 1	5834
	21st to end	Day	30% 6% 20% 26% 17% 1	6080
September	1st to 10th	Day	35% 7% 13% 19% 22% 4	2520
	11th to 20th	Day	37% 6% 8% 26% 20% 4	2452
	21st to end	Day	49% 5% 9% 17% 16% 3	2206
October	1st to 10th	Day	34% 4 7% 19% 31% 5%	3260
	11th to 20th	Day	25% 3 7% 22% 31% 11%	3240
	21st to end	Day	22% 2 9% 29% 25% 12%	3538
November	1st to 10th	Day	31% 4 9% 23% 28% 5%	2057
	11th to 20th	Day	34% 4 10% 23% 22% 8%	1874
	21st to end	Day	35% 4 9% 24% 23% 4	1680
December	1st to 10th	Day	35% 5% 9% 25% 18% 8%	2680
	11th to 20th	Day	39% 3 13% 24% 16% 5%	3066
	21st to end	Day	45% 4 11% 22% 16% 1	4095
January	1st to 10th	Night	35% 3 9% 22% 23% 8%	7636
	11th to 20th	Night	34% 3 5% 19% 27% 12%	7680
	21st to end	Night	37% 3 6% 27% 21% 7%	8401
February	1st to 10th	Night	40% 5% 11% 24% 18% 3	2730
	11th to 20th	Night	44% 3 8% 20% 19% 7%	2656
	21st to end	Night	41% 4 9% 21% 18% 7%	2005
March	1st to 10th	Night	38% 5% 6% 22% 20% 9%	5028
	11th to 20th	Night	29% 4 8% 22% 27% 9%	5090
	21st to end	Night	38% 5% 9% 21% 21% 6%	5588
April	1st to 10th	Night	38% 6% 10% 17% 22% 6%	2129
	11th to 20th	Night	33% 6% 13% 21% 22% 6%	2075
	21st to end	Night	36% 6% 11% 24% 17% 5%	2100
May	1st to 10th	Night	45% 7% 13% 21% 12% 2	5274
	11th to 20th	Night	42% 6% 13% 25% 13% 1	5235
	21st to end	Night	49% 6% 11% 18% 13% 3	5293
June	1st to 10th	Night	34% 8% 13% 26% 18% 1	1680
	11th to 20th	Night	36% 9% 17% 21% 15% 2	1680
	21st to end	Night	33% 9% 14% 23% 18% 3	1680
July	1st to 10th	Night	41% 11% 17% 17% 12% 2	4970
	11th to 20th	Night	52% 10% 14% 16% 6% 1	5226
	21st to end	Night	45% 8% 14% 18% 15%	5606
August	1st to 10th	Night	49% 9% 18% 19% 5%	4124
	11th to 20th	Night	45% 9% 18% 20% 8%	4230
	21st to end	Night	38% 7% 15% 27% 12% 1	5045
September	1st to 10th	Night	36% 7% 13% 22% 16% 7%	2100
	11th to 20th	Night	38% 7% 10% 23% 20% 2	2164
	21st to end	Night	49% 5% 10% 18% 15% 3	2369
October	1st to 10th	Night	41% 4 9% 18% 22% 6%	3920
	11th to 20th	Night	26% 4 7% 21% 31% 11%	3899
	21st to end	Night	22% 3 7% 30% 28% 10%	4252
November	1st to 10th	Night	35% 4 8% 21% 26% 6%	2461
	11th to 20th	Night	37% 3 8% 20% 24% 8%	2741
	21st to end	Night	35% 4 9% 24% 23% 6%	2940
December	1st to 10th	Night	31% 4 9% 27% 24% 4	4720
	11th to 20th	Night	44% 3 11% 18% 16% 7%	5348
	21st to end	Night	40% 5% 11% 26% 16% 2	7085

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% 32% 18% 5%	330
	11th to 20th	Day	25% 32% 28% 12% 3	330
	21st to end	Day	25% 24% 35% 13% 3	363
February	1st to 10th	Day	24% 21% 39% 12% 5%	330
	11th to 20th	Day	40% 22% 26% 10% 2	337
	21st to end	Day	28% 16% 32% 20% 3	446
March	1st to 10th	Day	23% 16% 34% 20% 6% 1	540
	11th to 20th	Day	24% 19% 33% 17% 7%	540
	21st to end	Day	23% 17% 35% 19% 5% 1	594
April	1st to 10th	Day	22% 19% 31% 19% 8% 1	540
	11th to 20th	Day	19% 14% 32% 21% 9% 4	532
	21st to end	Day	23% 11% 34% 22% 9% 2	540
May	1st to 10th	Day	19% 16% 33% 20% 9% 3	543
	11th to 20th	Day	21% 9% 29% 25% 11% 4	567
	21st to end	Day	20% 6% 25% 28% 17% 4	617
June	1st to 10th	Day	11% 6% 26% 30% 20% 6%	570
	11th to 20th	Day	13% 6% 26% 29% 17% 8%	570
	21st to end	Day	10% 4 22% 36% 22% 6%	570
July	1st to 10th	Day	10% 2 18% 32% 29% 10%	565
	11th to 20th	Day	7% 3 13% 36% 29% 11%	564
	21st to end	Day	10% 4 19% 32% 23% 11%	620
August	1st to 10th	Day	15% 4 18% 28% 24% 11%	566
	11th to 20th	Day	11% 2 19% 32% 25% 11%	559
	21st to end	Day	10% 4 17% 32% 24% 14%	627
September	1st to 10th	Day	12% 5% 17% 28% 23% 15%	570
	11th to 20th	Day	16% 3 14% 34% 24% 9%	570
	21st to end	Day	31% 5% 23% 25% 11% 5%	562
October	1st to 10th	Day	18% 5% 23% 29% 17% 8%	530
	11th to 20th	Day	16% 12% 24% 25% 19% 3	530
	21st to end	Day	18% 12% 32% 26% 9% 3	583
November	1st to 10th	Day	14% 10% 29% 33% 10% 4	535
	11th to 20th	Day	18% 15% 32% 23% 9% 2	392
	21st to end	Day	22% 17% 31% 20% 8% 1	220
December	1st to 10th	Day	24% 15% 40% 14% 6% 1	220
	11th to 20th	Day	25% 15% 30% 23% 6% 2	220
	21st to end	Day	28% 19% 28% 16% 7% 1	253
January	1st to 10th	Night	18% 14% 40% 25% 3	840
	11th to 20th	Night	20% 14% 37% 25% 4	840
	21st to end	Night	18% 14% 41% 24% 2	924
February	1st to 10th	Night	18% 12% 39% 29% 3	840
	11th to 20th	Night	26% 15% 39% 20% 1	823
	21st to end	Night	21% 16% 38% 23% 2	692
March	1st to 10th	Night	18% 15% 37% 26% 4	840
	11th to 20th	Night	20% 15% 35% 23% 5% 1	840
	21st to end	Night	19% 13% 38% 25% 5%	924
April	1st to 10th	Night	16% 16% 39% 23% 5%	840
	11th to 20th	Night	13% 13% 37% 28% 8%	831
	21st to end	Night	16% 8% 33% 34% 9%	840
May	1st to 10th	Night	18% 9% 32% 30% 11%	640
	11th to 20th	Night	20% 4 25% 34% 16% 1	621
	21st to end	Night	19% 2 19% 37% 21% 3	517
June	1st to 10th	Night	9% 2 21% 41% 26% 1	420
	11th to 20th	Night	13% 4 16% 42% 25% 1	420
	21st to end	Night	8% 1 20% 47% 22% 2	420
July	1st to 10th	Night	12% 3 14% 35% 32% 4	519
	11th to 20th	Night	7% 3 11% 39% 36% 5%	612
	21st to end	Night	8% 2 15% 42% 28% 4	690
August	1st to 10th	Night	14% 3 11% 39% 31% 3	627
	11th to 20th	Night	14% 3 10% 38% 33% 3	667
	21st to end	Night	11% 2 15% 37% 31% 4	924
September	1st to 10th	Night	15% 1 13% 34% 31% 5%	840
	11th to 20th	Night	14% 3 15% 34% 30% 4	840
	21st to end	Night	26% 4 18% 31% 19% 2	833
October	1st to 10th	Night	20% 5% 17% 30% 24% 5%	800
	11th to 20th	Night	18% 8% 25% 33% 16% 1	800
	21st to end	Night	16% 8% 29% 34% 13% 1	880
November	1st to 10th	Night	14% 8% 28% 36% 13% 2	820
	11th to 20th	Night	17% 10% 34% 31% 8%	840
	21st to end	Night	13% 10% 34% 33% 9% 1	840
December	1st to 10th	Night	20% 10% 37% 26% 7%	840
	11th to 20th	Night	18% 10% 32% 35% 5%	840
	21st to end	Night	21% 10% 31% 35% 4	924

Page generated at 2026-09-26 04:18:37. Version ac07

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