Lavell Sky observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lavell Sky observatory clear sky chart from 2005-11-06 to 2026-03-03.

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% 10% 4 9% 5% 5% 5% 4 6% 7% 34%	131596
	Night	12% 10% 4 10% 5% 5% 5% 4 5% 6% 33%	129841
Transparency	Day	50% 4 13% 23% 10% 1	135089
	Night	52% 3 12% 22% 11% 1	134434
Seeing	Day	17% 11% 31% 25% 12% 4	17897
	Night	16% 8% 32% 32% 11% 1	27971

By Month

Percentage Cloud Cover NE (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	7% 12% 6% 18% 8% 6% 5% 3 5% 6% 24%	4124
	11th to 20th	Day	5% 15% 7% 14% 7% 8% 9% 7% 10% 6% 13%	4662
	21st to end	Day	5% 12% 9% 15% 7% 5% 6% 5% 7% 8% 22%	5491
February	1st to 10th	Day	8% 13% 6% 13% 6% 6% 6% 5% 5% 7% 26%	1890
	11th to 20th	Day	7% 11% 4 9% 7% 6% 5% 5% 6% 8% 32%	1844
	21st to end	Day	8% 12% 5% 12% 6% 6% 5% 3 5% 7% 31%	1608
March	1st to 10th	Day	10% 10% 3 8% 4 4 5% 4 5% 8% 38%	4723
	11th to 20th	Day	8% 9% 5% 10% 6% 5% 5% 4 7% 7% 32%	4980
	21st to end	Day	19% 12% 4 9% 4 4 3 3 4 5% 33%	5597
April	1st to 10th	Day	16% 13% 4 8% 6% 6% 5% 3 5% 5% 30%	2400
	11th to 20th	Day	18% 12% 3 9% 5% 4 5% 4 5% 7% 29%	2416
	21st to end	Day	21% 10% 3 7% 5% 4 4 3 5% 5% 32%	2609
May	1st to 10th	Day	16% 8% 2 5% 3 3 3 3 4 6% 47%	6348
	11th to 20th	Day	16% 9% 3 6% 4 4 4 3 6% 6% 40%	6319
	21st to end	Day	14% 8% 3 7% 5% 5% 5% 4 6% 7% 36%	6949
June	1st to 10th	Day	12% 6% 2 6% 5% 4 5% 4 6% 9% 41%	2859
	11th to 20th	Day	16% 7% 3 6% 5% 5% 5% 4 6% 8% 36%	3000
	21st to end	Day	15% 7% 3 7% 5% 5% 4 5% 7% 9% 35%	3000
July	1st to 10th	Day	8% 5% 2 7% 5% 6% 7% 5% 8% 10% 37%	6799
	11th to 20th	Day	13% 5% 2 6% 3 4 5% 4 6% 10% 43%	6546
	21st to end	Day	11% 5% 2 5% 4 5% 5% 4 7% 10% 43%	6926
August	1st to 10th	Day	9% 6% 2 7% 5% 5% 6% 5% 7% 8% 40%	4409
	11th to 20th	Day	14% 5% 1 5% 3 4 4 4 6% 10% 43%	4171
	21st to end	Day	15% 7% 2 7% 4 4 5% 3 6% 7% 40%	4496
September	1st to 10th	Day	11% 7% 2 7% 4 5% 4 4 6% 10% 41%	2400
	11th to 20th	Day	15% 7% 3 7% 4 4 5% 4 4 8% 39%	2400
	21st to end	Day	17% 8% 3 6% 3 4 3 3 5% 6% 41%	2086
October	1st to 10th	Day	17% 9% 3 7% 4 5% 4 4 5% 6% 36%	2000
	11th to 20th	Day	17% 13% 3 7% 4 4 5% 4 5% 7% 30%	2000
	21st to end	Day	19% 15% 5% 8% 5% 4 4 4 4 5% 27%	2091
November	1st to 10th	Day	13% 13% 6% 10% 6% 4 4 4 6% 5% 28%	1677
	11th to 20th	Day	13% 18% 6% 12% 5% 5% 5% 3 4 5% 24%	1680
	21st to end	Day	15% 13% 7% 12% 7% 5% 5% 4 5% 5% 23%	1680
December	1st to 10th	Day	12% 14% 6% 19% 14% 5% 4 3 4 5% 14%	2690
	11th to 20th	Day	10% 19% 7% 12% 5% 6% 5% 4 4 4 23%	3084
	21st to end	Day	19% 20% 6% 19% 7% 5% 3 2 3 4 12%	3642
January	1st to 10th	Night	6% 11% 6% 19% 9% 6% 5% 3 5% 6% 24%	8787
	11th to 20th	Night	6% 12% 6% 14% 10% 8% 9% 8% 7% 4 17%	8236
	21st to end	Night	5% 14% 9% 11% 6% 5% 7% 5% 6% 7% 25%	8721
February	1st to 10th	Night	7% 13% 5% 10% 6% 6% 6% 5% 5% 7% 30%	2730
	11th to 20th	Night	7% 9% 4 9% 7% 6% 6% 4 6% 6% 35%	2663
	21st to end	Night	8% 13% 5% 9% 5% 6% 6% 4 6% 8% 30%	2197
March	1st to 10th	Night	10% 8% 3 8% 5% 4 4 4 5% 7% 43%	5204
	11th to 20th	Night	9% 13% 4 12% 7% 6% 6% 5% 5% 6% 27%	4979
	21st to end	Night	19% 14% 5% 10% 5% 4 3 3 4 4 30%	5333
April	1st to 10th	Night	16% 11% 3 9% 5% 4 4 3 5% 5% 35%	2000
	11th to 20th	Night	16% 12% 4 7% 4 5% 5% 3 5% 6% 34%	1931
	21st to end	Night	20% 10% 4 8% 4 4 3 3 5% 5% 33%	1790
May	1st to 10th	Night	16% 7% 2 5% 3 3 3 3 4 6% 48%	3613
	11th to 20th	Night	15% 6% 2 5% 4 3 4 3 5% 7% 46%	3596
	21st to end	Night	16% 7% 3 6% 4 4 4 4 5% 7% 40%	3949
June	1st to 10th	Night	14% 6% 2 6% 5% 4 4 4 7% 7% 41%	1535
	11th to 20th	Night	12% 5% 2 6% 4 5% 4 4 7% 7% 44%	1400
	21st to end	Night	9% 6% 2 6% 4 4 4 4 7% 9% 45%	1400
July	1st to 10th	Night	7% 4 2 5% 3 4 4 5% 6% 11% 48%	3180
	11th to 20th	Night	11% 4 2 4 3 3 3 3 4 9% 54%	3430
	21st to end	Night	11% 5% 2 6% 4 4 4 4 6% 9% 45%	3948
August	1st to 10th	Night	10% 4 2 6% 4 4 4 4 6% 8% 48%	2749
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 6% 7% 48%	2888
	21st to end	Night	17% 6% 2 6% 3 4 3 4 5% 8% 43%	3394
September	1st to 10th	Night	10% 6% 2 5% 4 4 4 4 6% 7% 48%	2000
	11th to 20th	Night	11% 5% 2 5% 3 4 4 4 4 8% 50%	2000
	21st to end	Night	15% 7% 2 6% 4 3 4 3 5% 6% 43%	2289
October	1st to 10th	Night	14% 8% 3 7% 4 4 4 3 5% 6% 42%	2400
	11th to 20th	Night	16% 12% 3 7% 4 4 4 4 5% 7% 34%	2400
	21st to end	Night	18% 11% 3 10% 5% 5% 6% 4 4 6% 28%	2747
November	1st to 10th	Night	14% 12% 3 9% 5% 4 5% 3 5% 7% 33%	2837
	11th to 20th	Night	14% 13% 6% 10% 5% 5% 5% 3 5% 6% 28%	2940
	21st to end	Night	12% 10% 5% 13% 6% 5% 6% 4 5% 6% 28%	2940
December	1st to 10th	Night	10% 14% 7% 21% 9% 6% 5% 4 5% 5% 14%	4720
	11th to 20th	Night	10% 15% 6% 15% 6% 6% 6% 4 5% 5% 21%	5343
	21st to end	Night	20% 20% 7% 15% 5% 3 4 3 4 4 15%	7572

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 7% 16% 10%	4125
	11th to 20th	Day	68% 2 6% 17% 7%	4693
	21st to end	Day	60% 2 10% 20% 8%	5491
February	1st to 10th	Day	57% 3 10% 19% 10% 1	1890
	11th to 20th	Day	49% 4 12% 21% 12% 1	1853
	21st to end	Day	54% 3 12% 18% 11% 1	1608
March	1st to 10th	Day	46% 2 13% 23% 11% 4	4725
	11th to 20th	Day	50% 3 12% 22% 12% 1	4980
	21st to end	Day	56% 2 7% 22% 12% 1	5597
April	1st to 10th	Day	56% 2 8% 18% 13% 2	2400
	11th to 20th	Day	54% 4 11% 19% 10% 1	2416
	21st to end	Day	54% 3 8% 22% 11% 1	2609
May	1st to 10th	Day	41% 3 11% 26% 18% 1	6357
	11th to 20th	Day	45% 3 13% 23% 13% 3	6323
	21st to end	Day	46% 4 8% 25% 15% 1	6949
June	1st to 10th	Day	41% 3 17% 29% 10% 1	2859
	11th to 20th	Day	46% 4 14% 23% 12%	3000
	21st to end	Day	46% 5% 15% 26% 9% 1	3000
July	1st to 10th	Day	39% 8% 17% 27% 9%	6726
	11th to 20th	Day	37% 5% 19% 31% 8%	6466
	21st to end	Day	37% 5% 16% 31% 10%	6728
August	1st to 10th	Day	40% 5% 16% 27% 11% 1	4394
	11th to 20th	Day	37% 5% 22% 28% 8%	4210
	21st to end	Day	43% 5% 17% 26% 9% 1	4495
September	1st to 10th	Day	40% 4 17% 30% 9%	2400
	11th to 20th	Day	44% 3 19% 26% 7%	2400
	21st to end	Day	45% 4 14% 30% 8%	2086
October	1st to 10th	Day	41% 3 15% 30% 11% 1	3260
	11th to 20th	Day	47% 4 12% 23% 14%	3240
	21st to end	Day	57% 3 14% 20% 6%	3355
November	1st to 10th	Day	58% 3 10% 19% 9% 1	1677
	11th to 20th	Day	64% 3 11% 15% 6% 1	1680
	21st to end	Day	64% 3 12% 15% 5% 1	1680
December	1st to 10th	Day	74% 3 7% 13% 2	2690
	11th to 20th	Day	65% 3 6% 15% 11% 1	3085
	21st to end	Day	79% 2 4 10% 5%	3642
January	1st to 10th	Night	63% 2 10% 14% 11%	8787
	11th to 20th	Night	69% 2 7% 14% 7%	8280
	21st to end	Night	59% 3 9% 18% 11%	8721
February	1st to 10th	Night	54% 4 10% 19% 12% 1	2730
	11th to 20th	Night	50% 4 10% 21% 15% 1	2664
	21st to end	Night	53% 3 10% 22% 10% 1	2197
March	1st to 10th	Night	42% 3 15% 18% 18% 4	5215
	11th to 20th	Night	57% 4 10% 19% 10% 1	4980
	21st to end	Night	59% 2 8% 17% 12% 1	5333
April	1st to 10th	Night	53% 3 7% 20% 15% 1	2000
	11th to 20th	Night	52% 3 8% 23% 12% 1	1931
	21st to end	Night	54% 2 6% 25% 12% 1	1790
May	1st to 10th	Night	39% 3 12% 29% 17%	3620
	11th to 20th	Night	40% 2 12% 23% 19% 4	3597
	21st to end	Night	45% 3 10% 28% 12% 1	3950
June	1st to 10th	Night	40% 4 17% 26% 11% 2	1535
	11th to 20th	Night	38% 4 14% 26% 16% 2	1400
	21st to end	Night	37% 5% 18% 29% 11% 1	1400
July	1st to 10th	Night	30% 6% 20% 31% 12%	3145
	11th to 20th	Night	30% 3 20% 37% 9% 1	3387
	21st to end	Night	35% 6% 17% 30% 11% 1	3832
August	1st to 10th	Night	34% 5% 18% 26% 15% 2	2740
	11th to 20th	Night	35% 5% 18% 35% 7% 1	2915
	21st to end	Night	40% 5% 20% 25% 10%	3394
September	1st to 10th	Night	35% 3 17% 31% 12% 1	2000
	11th to 20th	Night	34% 5% 21% 30% 9%	2000
	21st to end	Night	42% 3 13% 32% 10%	2289
October	1st to 10th	Night	32% 2 11% 37% 16% 2	3920
	11th to 20th	Night	46% 2 12% 26% 14%	3899
	21st to end	Night	57% 3 12% 21% 8%	4433
November	1st to 10th	Night	51% 3 12% 24% 8% 1	2837
	11th to 20th	Night	59% 3 14% 17% 7% 1	2940
	21st to end	Night	57% 4 16% 16% 6% 1	2940
December	1st to 10th	Night	73% 3 8% 13% 2	4720
	11th to 20th	Night	65% 3 8% 15% 9%	5341
	21st to end	Night	75% 2 5% 13% 5%	7572

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	14% 12% 33% 29% 11% 1	330
	11th to 20th	Day	15% 11% 37% 29% 7% 1	330
	21st to end	Day	12% 9% 39% 27% 12% 1	363
February	1st to 10th	Day	15% 15% 45% 17% 7% 1	330
	11th to 20th	Day	15% 16% 39% 20% 8% 3	330
	21st to end	Day	18% 11% 44% 22% 4 1	273
March	1st to 10th	Day	14% 17% 39% 19% 10% 1	306
	11th to 20th	Day	16% 10% 41% 22% 7% 3	300
	21st to end	Day	23% 13% 34% 20% 9% 2	386
April	1st to 10th	Day	25% 14% 31% 22% 8% 1	500
	11th to 20th	Day	25% 12% 29% 24% 9% 1	534
	21st to end	Day	25% 11% 30% 23% 11% 1	700
May	1st to 10th	Day	22% 13% 29% 24% 9% 3	703
	11th to 20th	Day	20% 12% 28% 25% 12% 3	725
	21st to end	Day	19% 8% 30% 28% 13% 3	789
June	1st to 10th	Day	13% 9% 32% 29% 12% 5%	730
	11th to 20th	Day	16% 12% 36% 23% 9% 3	730
	21st to end	Day	16% 9% 34% 22% 14% 6%	730
July	1st to 10th	Day	9% 6% 25% 30% 19% 11%	721
	11th to 20th	Day	12% 9% 27% 27% 18% 7%	718
	21st to end	Day	9% 6% 27% 30% 20% 7%	794
August	1st to 10th	Day	10% 7% 24% 32% 20% 6%	726
	11th to 20th	Day	16% 5% 22% 31% 20% 6%	715
	21st to end	Day	15% 7% 26% 28% 20% 4	702
September	1st to 10th	Day	10% 8% 30% 30% 19% 3	530
	11th to 20th	Day	17% 8% 30% 27% 15% 4	530
	21st to end	Day	20% 11% 31% 20% 12% 6%	336
October	1st to 10th	Day	20% 14% 32% 19% 10% 5%	330
	11th to 20th	Day	20% 18% 37% 21% 3 1	330
	21st to end	Day	27% 17% 33% 16% 5% 1	363
November	1st to 10th	Day	20% 15% 34% 23% 8% 1	330
	11th to 20th	Day	24% 16% 35% 18% 5% 1	330
	21st to end	Day	22% 15% 33% 25% 5%	330
December	1st to 10th	Day	15% 15% 39% 23% 7% 1	330
	11th to 20th	Day	20% 19% 33% 24% 4	330
	21st to end	Day	24% 12% 36% 22% 4 2	363
January	1st to 10th	Night	13% 10% 34% 32% 10% 1	1050
	11th to 20th	Night	18% 8% 37% 29% 8% 1	1050
	21st to end	Night	16% 9% 35% 30% 8%	1155
February	1st to 10th	Night	14% 9% 36% 31% 9% 1	1050
	11th to 20th	Night	13% 10% 38% 28% 9% 2	1029
	21st to end	Night	15% 9% 35% 30% 9% 2	813
March	1st to 10th	Night	15% 10% 36% 29% 10%	730
	11th to 20th	Night	16% 9% 39% 28% 8% 1	600
	21st to end	Night	20% 8% 30% 29% 11% 2	660
April	1st to 10th	Night	22% 8% 34% 29% 6% 1	600
	11th to 20th	Night	20% 9% 34% 29% 8% 1	593
	21st to end	Night	25% 8% 27% 30% 10% 1	600
May	1st to 10th	Night	19% 9% 33% 31% 8% 1	600
	11th to 20th	Night	18% 6% 32% 31% 11% 1	591
	21st to end	Night	18% 6% 27% 36% 13%	650
June	1st to 10th	Night	15% 6% 27% 39% 13% 1	600
	11th to 20th	Night	14% 5% 33% 41% 9%	600
	21st to end	Night	12% 6% 26% 39% 17% 1	600
July	1st to 10th	Night	6% 3 23% 45% 22% 1	585
	11th to 20th	Night	10% 4 27% 38% 21% 1	582
	21st to end	Night	7% 2 23% 47% 20% 2	657
August	1st to 10th	Night	8% 3 23% 43% 22% 1	597
	11th to 20th	Night	13% 3 21% 38% 22% 3	594
	21st to end	Night	13% 6% 22% 39% 18% 2	660
September	1st to 10th	Night	12% 4 23% 43% 16% 2	600
	11th to 20th	Night	14% 4 28% 36% 18% 1	600
	21st to end	Night	17% 8% 31% 29% 13% 2	699
October	1st to 10th	Night	18% 9% 32% 29% 12% 1	800
	11th to 20th	Night	21% 11% 33% 28% 7% 1	800
	21st to end	Night	23% 11% 34% 26% 5%	880
November	1st to 10th	Night	19% 9% 34% 30% 7% 1	991
	11th to 20th	Night	21% 11% 32% 28% 8% 1	1050
	21st to end	Night	16% 13% 37% 27% 6%	1050
December	1st to 10th	Night	17% 10% 35% 29% 9% 1	1050
	11th to 20th	Night	21% 10% 36% 25% 8% 1	1050
	21st to end	Night	24% 8% 34% 25% 7% 1	1155

Page generated at 2026-02-28 00:40:24. Version ac07

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