Lake Matthews
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lake Matthews clear sky chart from 2005-11-06 to 2026-09-14.

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	12% 6% 2 6% 4 4 5% 4 6% 8% 41%	148530
	Night	13% 6% 2 6% 4 4 4 4 6% 8% 43%	142560
Transparency	Day	38% 7% 13% 20% 17% 5%	152306
	Night	37% 6% 12% 19% 19% 7%	146856
Seeing	Day	15% 8% 21% 24% 21% 11%	17387
	Night	15% 8% 23% 30% 22% 4	27350

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	18% 9% 3 6% 4 3 3 3 4 5% 42%	5258
	11th to 20th	Day	10% 7% 2 5% 3 3 2 2 3 5% 58%	5280
	21st to end	Day	24% 9% 3 6% 4 3 3 2 4 5% 37%	5977
February	1st to 10th	Day	15% 7% 3 7% 4 4 4 4 5% 6% 41%	2100
	11th to 20th	Day	22% 7% 2 5% 3 4 4 3 4 6% 41%	2048
	21st to end	Day	19% 6% 3 6% 5% 3 4 4 5% 5% 41%	1730
March	1st to 10th	Day	20% 10% 4 7% 4 4 3 3 5% 6% 35%	4761
	11th to 20th	Day	10% 6% 2 6% 4 4 4 3 5% 7% 50%	5090
	21st to end	Day	16% 6% 2 6% 4 4 4 3 4 5% 46%	5586
April	1st to 10th	Day	12% 5% 2 6% 5% 4 4 4 6% 7% 46%	2341
	11th to 20th	Day	11% 4 2 6% 5% 4 4 3 5% 7% 49%	2494
	21st to end	Day	9% 5% 2 4 4 3 5% 4 6% 9% 49%	2704
May	1st to 10th	Day	14% 7% 3 7% 5% 4 5% 4 6% 9% 37%	7616
	11th to 20th	Day	13% 7% 3 7% 5% 5% 6% 4 7% 9% 34%	7626
	21st to end	Day	16% 8% 3 7% 4 6% 5% 4 6% 7% 32%	8391
June	1st to 10th	Day	9% 6% 2 6% 5% 6% 5% 5% 8% 11% 37%	2730
	11th to 20th	Day	5% 4 1 5% 5% 4 5% 5% 10% 12% 44%	2730
	21st to end	Day	5% 3 2 6% 6% 5% 6% 5% 9% 11% 42%	2730
July	1st to 10th	Day	5% 5% 2 6% 5% 7% 7% 6% 10% 13% 35%	7680
	11th to 20th	Day	6% 5% 2 7% 5% 6% 7% 6% 9% 14% 34%	7680
	21st to end	Day	7% 6% 3 9% 7% 6% 7% 6% 10% 12% 27%	8340
August	1st to 10th	Day	8% 6% 2 7% 6% 6% 7% 6% 10% 12% 29%	6014
	11th to 20th	Day	5% 4 1 5% 3 4 4 4 6% 12% 51%	5627
	21st to end	Day	6% 5% 2 5% 4 5% 5% 4 7% 10% 46%	6057
September	1st to 10th	Day	8% 6% 2 7% 5% 5% 4 5% 7% 9% 41%	2310
	11th to 20th	Day	14% 6% 2 6% 3 4 5% 5% 6% 8% 40%	2233
	21st to end	Day	13% 5% 2 6% 5% 3 3 3 6% 7% 48%	2200
October	1st to 10th	Day	11% 6% 2 4 3 3 4 3 5% 6% 54%	2200
	11th to 20th	Day	10% 4 1 4 3 3 2 3 5% 7% 58%	2075
	21st to end	Day	13% 5% 1 5% 3 3 2 3 5% 5% 55%	2200
November	1st to 10th	Day	12% 5% 3 4 3 2 4 3 4 5% 55%	1894
	11th to 20th	Day	13% 5% 2 4 3 3 3 3 5% 7% 52%	1890
	21st to end	Day	13% 6% 2 4 3 4 4 3 5% 6% 48%	1890
December	1st to 10th	Day	18% 7% 2 5% 3 3 4 3 5% 5% 45%	3020
	11th to 20th	Day	15% 9% 3 7% 4 4 4 3 4 5% 42%	3463
	21st to end	Day	22% 6% 2 5% 3 4 3 3 5% 7% 40%	4565
January	1st to 10th	Night	15% 6% 2 6% 4 4 4 3 5% 7% 46%	7635
	11th to 20th	Night	12% 7% 2 5% 3 3 3 3 4 5% 51%	7680
	21st to end	Night	21% 8% 3 6% 4 3 3 2 4 4 41%	8188
February	1st to 10th	Night	15% 6% 2 5% 4 3 3 4 5% 6% 45%	2520
	11th to 20th	Night	21% 7% 2 6% 3 3 3 3 4 4 42%	2458
	21st to end	Night	20% 6% 2 5% 4 4 3 3 5% 5% 42%	2076
March	1st to 10th	Night	20% 8% 3 6% 3 4 4 3 5% 6% 37%	5310
	11th to 20th	Night	10% 5% 2 5% 3 3 3 3 6% 8% 52%	5090
	21st to end	Night	14% 6% 2 5% 4 4 4 3 5% 6% 46%	5588
April	1st to 10th	Night	10% 5% 1 5% 4 4 5% 5% 6% 7% 47%	2295
	11th to 20th	Night	12% 5% 2 4 3 3 3 3 5% 7% 52%	2075
	21st to end	Night	9% 6% 2 5% 4 3 4 3 5% 7% 51%	1916
May	1st to 10th	Night	12% 7% 3 8% 6% 5% 5% 4 6% 8% 36%	5237
	11th to 20th	Night	14% 6% 3 7% 4 5% 4 4 6% 8% 37%	5234
	21st to end	Night	15% 8% 2 7% 5% 5% 5% 5% 6% 8% 34%	5761
June	1st to 10th	Night	7% 5% 2 7% 5% 5% 7% 6% 7% 11% 37%	1890
	11th to 20th	Night	5% 4 1 6% 5% 5% 5% 5% 8% 13% 43%	1890
	21st to end	Night	6% 4 1 5% 4 5% 7% 6% 8% 11% 43%	1890
July	1st to 10th	Night	5% 6% 2 8% 5% 6% 7% 6% 8% 13% 34%	5278
	11th to 20th	Night	6% 5% 2 7% 6% 6% 7% 6% 10% 14% 30%	5277
	21st to end	Night	8% 8% 3 9% 6% 6% 6% 6% 7% 12% 29%	5736
August	1st to 10th	Night	8% 6% 3 9% 6% 6% 7% 6% 10% 12% 28%	4135
	11th to 20th	Night	5% 4 2 6% 4 3 4 4 8% 12% 47%	4414
	21st to end	Night	7% 5% 2 6% 5% 5% 5% 4 6% 9% 46%	5046
September	1st to 10th	Night	8% 5% 2 6% 5% 5% 5% 4 7% 9% 42%	2297
	11th to 20th	Night	12% 7% 2 6% 3 5% 4 5% 7% 8% 43%	2234
	21st to end	Night	12% 6% 2 6% 4 4 4 4 5% 8% 45%	2200
October	1st to 10th	Night	11% 5% 2 4 3 3 4 3 5% 7% 52%	2200
	11th to 20th	Night	9% 4 2 5% 3 3 3 3 3 4 60%	2324
	21st to end	Night	12% 4 1 4 3 2 3 3 4 5% 57%	2640
November	1st to 10th	Night	12% 6% 1 4 3 3 2 3 4 5% 56%	2621
	11th to 20th	Night	14% 5% 2 4 3 3 2 3 4 6% 55%	2730
	21st to end	Night	15% 6% 2 4 3 3 4 4 5% 6% 49%	2730
December	1st to 10th	Night	15% 6% 2 5% 3 3 3 3 4 5% 50%	4380
	11th to 20th	Night	17% 6% 2 5% 3 3 3 2 5% 6% 47%	4952
	21st to end	Night	20% 7% 2 5% 4 4 3 3 4 6% 42%	6633

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	45% 4 7% 20% 18% 5%	5259
	11th to 20th	Day	30% 3 5% 20% 31% 12%	5280
	21st to end	Day	51% 2 5% 15% 18% 9%	5977
February	1st to 10th	Day	43% 5% 8% 19% 20% 4	2100
	11th to 20th	Day	45% 4 7% 19% 20% 6%	2058
	21st to end	Day	44% 5% 7% 15% 20% 8%	1730
March	1st to 10th	Day	51% 4 8% 11% 19% 7%	4761
	11th to 20th	Day	33% 5% 8% 14% 24% 16%	5090
	21st to end	Day	41% 4 7% 22% 21% 4	5586
April	1st to 10th	Day	37% 5% 9% 23% 20% 5%	2341
	11th to 20th	Day	34% 5% 9% 20% 25% 7%	2494
	21st to end	Day	30% 6% 13% 28% 21% 2	2704
May	1st to 10th	Day	43% 7% 14% 21% 14% 1	7651
	11th to 20th	Day	44% 8% 16% 20% 10% 1	7630
	21st to end	Day	48% 9% 11% 18% 14% 1	8405
June	1st to 10th	Day	38% 9% 18% 23% 10% 2	2730
	11th to 20th	Day	28% 11% 23% 24% 13% 1	2730
	21st to end	Day	32% 11% 17% 25% 12% 3	2730
July	1st to 10th	Day	34% 12% 26% 22% 5% 1	7615
	11th to 20th	Day	36% 15% 29% 17% 3 1	7611
	21st to end	Day	43% 13% 19% 19% 6%	8156
August	1st to 10th	Day	41% 12% 19% 23% 5%	6001
	11th to 20th	Day	25% 9% 25% 28% 12% 1	5666
	21st to end	Day	31% 9% 15% 25% 18% 3	6057
September	1st to 10th	Day	36% 8% 17% 18% 17% 3	2310
	11th to 20th	Day	38% 9% 12% 19% 21% 1	2233
	21st to end	Day	35% 5% 14% 19% 21% 6%	2200
October	1st to 10th	Day	26% 3 7% 13% 39% 12%	3590
	11th to 20th	Day	19% 3 6% 16% 29% 28%	3354
	21st to end	Day	26% 3 6% 22% 31% 12%	3538
November	1st to 10th	Day	32% 4 8% 15% 30% 10%	1894
	11th to 20th	Day	31% 5% 7% 20% 27% 10%	1890
	21st to end	Day	36% 6% 7% 19% 26% 6%	1890
December	1st to 10th	Day	41% 4 5% 18% 26% 7%	3020
	11th to 20th	Day	44% 4 8% 17% 20% 7%	3460
	21st to end	Day	42% 5% 10% 24% 17% 2	4565
January	1st to 10th	Night	37% 5% 9% 22% 23% 5%	7636
	11th to 20th	Night	36% 3 5% 19% 28% 10%	7680
	21st to end	Night	47% 2 5% 15% 23% 8%	8188
February	1st to 10th	Night	40% 4 8% 18% 21% 8%	2520
	11th to 20th	Night	45% 4 6% 18% 20% 8%	2459
	21st to end	Night	44% 4 7% 16% 18% 11%	2076
March	1st to 10th	Night	47% 4 9% 14% 19% 7%	5310
	11th to 20th	Night	29% 5% 8% 15% 26% 18%	5090
	21st to end	Night	38% 4 7% 21% 21% 9%	5588
April	1st to 10th	Night	33% 6% 11% 19% 22% 8%	2295
	11th to 20th	Night	31% 3 8% 22% 25% 10%	2075
	21st to end	Night	32% 6% 11% 25% 21% 5%	1916
May	1st to 10th	Night	43% 8% 15% 17% 14% 3	5264
	11th to 20th	Night	43% 7% 15% 23% 12% 1	5235
	21st to end	Night	46% 9% 12% 18% 14% 2	5771
June	1st to 10th	Night	37% 11% 16% 21% 12% 2	1890
	11th to 20th	Night	30% 10% 21% 26% 13% 1	1890
	21st to end	Night	30% 12% 21% 20% 13% 3	1890
July	1st to 10th	Night	37% 13% 25% 21% 4 1	5235
	11th to 20th	Night	38% 16% 28% 14% 4 1	5226
	21st to end	Night	45% 12% 21% 15% 6% 2	5606
August	1st to 10th	Night	43% 13% 17% 22% 5%	4124
	11th to 20th	Night	27% 9% 24% 26% 13% 1	4441
	21st to end	Night	33% 8% 14% 23% 21% 2	5046
September	1st to 10th	Night	34% 10% 16% 18% 17% 4	2297
	11th to 20th	Night	37% 10% 11% 21% 17% 3	2234
	21st to end	Night	36% 7% 12% 17% 20% 7%	2200
October	1st to 10th	Night	27% 4 10% 12% 36% 11%	3590
	11th to 20th	Night	18% 3 4 17% 31% 27%	3784
	21st to end	Night	23% 3 4 22% 30% 18%	4252
November	1st to 10th	Night	32% 4 8% 18% 27% 12%	2621
	11th to 20th	Night	32% 3 6% 18% 29% 12%	2730
	21st to end	Night	35% 4 8% 19% 25% 8%	2730
December	1st to 10th	Night	37% 4 6% 18% 28% 8%	4380
	11th to 20th	Night	39% 4 11% 16% 24% 7%	4954
	21st to end	Night	43% 4 7% 27% 17% 2	6633

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% 38% 18% 2	330
	11th to 20th	Day	18% 28% 27% 18% 7% 2	330
	21st to end	Day	23% 20% 36% 16% 4 1	363
February	1st to 10th	Day	19% 17% 35% 24% 5%	330
	11th to 20th	Day	35% 18% 29% 16% 2	330
	21st to end	Day	24% 20% 34% 18% 4	273
March	1st to 10th	Day	22% 14% 29% 26% 10% 1	407
	11th to 20th	Day	19% 16% 32% 25% 7% 1	540
	21st to end	Day	19% 13% 30% 27% 9% 1	594
April	1st to 10th	Day	16% 14% 28% 26% 14% 2	540
	11th to 20th	Day	11% 12% 30% 27% 18% 2	532
	21st to end	Day	14% 9% 22% 29% 21% 5%	540
May	1st to 10th	Day	13% 9% 28% 27% 19% 6%	543
	11th to 20th	Day	14% 4 20% 26% 23% 13%	567
	21st to end	Day	17% 4 15% 27% 24% 14%	617
June	1st to 10th	Day	13% 3 14% 25% 30% 16%	570
	11th to 20th	Day	7% 2 14% 27% 24% 26%	570
	21st to end	Day	8% 3 9% 27% 32% 21%	570
July	1st to 10th	Day	8% 2 5% 21% 31% 33%	565
	11th to 20th	Day	7% 1 7% 18% 34% 33%	564
	21st to end	Day	6% 2 7% 21% 40% 24%	620
August	1st to 10th	Day	14% 2 12% 22% 27% 23%	566
	11th to 20th	Day	11% 1 8% 19% 37% 24%	559
	21st to end	Day	14% 2 8% 23% 29% 24%	627
September	1st to 10th	Day	9% 3 11% 25% 32% 19%	570
	11th to 20th	Day	15% 4 10% 21% 35% 15%	542
	21st to end	Day	15% 3 14% 24% 33% 11%	530
October	1st to 10th	Day	14% 4 19% 32% 22% 10%	530
	11th to 20th	Day	10% 8% 24% 25% 25% 8%	530
	21st to end	Day	17% 7% 22% 28% 22% 4	583
November	1st to 10th	Day	18% 8% 26% 25% 19% 5%	372
	11th to 20th	Day	14% 12% 35% 27% 9% 2	330
	21st to end	Day	11% 17% 33% 26% 11% 2	330
December	1st to 10th	Day	28% 15% 33% 17% 5% 1	330
	11th to 20th	Day	22% 14% 30% 25% 8% 1	330
	21st to end	Day	26% 16% 29% 21% 6% 2	363
January	1st to 10th	Night	24% 15% 33% 23% 5%	840
	11th to 20th	Night	19% 15% 33% 25% 8%	840
	21st to end	Night	20% 15% 37% 25% 3	924
February	1st to 10th	Night	18% 13% 32% 31% 6%	840
	11th to 20th	Night	23% 15% 34% 25% 3	823
	21st to end	Night	22% 14% 34% 26% 3	692
March	1st to 10th	Night	17% 12% 32% 33% 6%	840
	11th to 20th	Night	15% 11% 35% 28% 9%	840
	21st to end	Night	17% 11% 29% 33% 10%	924
April	1st to 10th	Night	12% 12% 30% 32% 13% 1	825
	11th to 20th	Night	14% 8% 28% 34% 15% 1	623
	21st to end	Night	11% 7% 24% 36% 21% 1	630
May	1st to 10th	Night	11% 7% 25% 37% 20% 1	630
	11th to 20th	Night	11% 5% 22% 34% 24% 5%	621
	21st to end	Night	14% 1 12% 33% 34% 6%	683
June	1st to 10th	Night	9% 2 10% 39% 34% 7%	630
	11th to 20th	Night	6% 2 13% 30% 36% 12%	630
	21st to end	Night	7% 1 8% 31% 42% 11%	630
July	1st to 10th	Night	9% 2 6% 22% 45% 15%	615
	11th to 20th	Night	6% 2 8% 17% 51% 16%	612
	21st to end	Night	8% 2 6% 25% 50% 10%	690
August	1st to 10th	Night	10% 3 6% 28% 42% 11%	627
	11th to 20th	Night	11% 1 7% 26% 44% 11%	624
	21st to end	Night	14% 2 8% 27% 40% 9%	693
September	1st to 10th	Night	9% 2 9% 28% 42% 10%	827
	11th to 20th	Night	14% 2 9% 27% 42% 5%	813
	21st to end	Night	14% 3 12% 33% 35% 3	800
October	1st to 10th	Night	13% 3 18% 34% 27% 7%	800
	11th to 20th	Night	11% 5% 24% 34% 22% 3	800
	21st to end	Night	15% 6% 22% 34% 20% 3	880
November	1st to 10th	Night	15% 8% 21% 33% 22% 1	820
	11th to 20th	Night	17% 8% 31% 29% 15% 1	840
	21st to end	Night	17% 9% 30% 30% 14% 1	840
December	1st to 10th	Night	19% 11% 32% 29% 9%	840
	11th to 20th	Night	21% 10% 30% 29% 9% 1	840
	21st to end	Night	24% 12% 28% 29% 7%	924

Page generated at 2026-09-11 04:11:51. Version ac07

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