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 2024-12-02.

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% 4 8% 5% 5% 5% 4 6% 7% 34%	102179
	Night	13% 10% 4 9% 5% 5% 5% 4 5% 7% 34%	97580
Transparency	Day	49% 4 13% 24% 10% 1	105689
	Night	50% 3 13% 23% 10% 1	102157
Seeing	Day	17% 10% 31% 26% 12% 4	16223
	Night	17% 8% 32% 32% 11% 1	26151

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	10% 15% 7% 16% 6% 5% 5% 3 5% 6% 23%	2218
	11th to 20th	Day	5% 12% 5% 13% 10% 11% 10% 6% 7% 6% 15%	2513
	21st to end	Day	8% 14% 12% 18% 8% 5% 6% 4 5% 6% 16%	2952
February	1st to 10th	Day	8% 12% 6% 12% 6% 5% 5% 5% 5% 7% 28%	1710
	11th to 20th	Day	8% 11% 4 10% 7% 5% 5% 5% 6% 8% 31%	1664
	21st to end	Day	9% 12% 5% 11% 6% 6% 5% 3 5% 7% 31%	1459
March	1st to 10th	Day	11% 11% 3 8% 5% 4 5% 3 5% 7% 38%	3273
	11th to 20th	Day	7% 10% 6% 11% 6% 4 4 3 6% 7% 36%	3480
	21st to end	Day	18% 14% 5% 9% 5% 4 4 3 4 5% 30%	3905
April	1st to 10th	Day	16% 13% 4 8% 6% 6% 5% 3 5% 5% 31%	2280
	11th to 20th	Day	18% 11% 3 8% 5% 4 5% 4 6% 7% 29%	2294
	21st to end	Day	21% 10% 3 7% 5% 4 4 4 5% 5% 32%	2478
May	1st to 10th	Day	20% 8% 3 6% 4 4 4 3 4 6% 37%	4428
	11th to 20th	Day	16% 7% 2 6% 4 4 4 4 7% 7% 39%	4399
	21st to end	Day	19% 8% 3 7% 5% 4 4 4 5% 8% 32%	4837
June	1st to 10th	Day	13% 6% 2 6% 5% 4 5% 4 6% 8% 41%	2716
	11th to 20th	Day	16% 6% 3 6% 5% 5% 5% 4 6% 8% 36%	2850
	21st to end	Day	15% 6% 3 7% 5% 5% 4 5% 7% 9% 35%	2850
July	1st to 10th	Day	9% 5% 2 7% 5% 6% 6% 6% 8% 10% 36%	4750
	11th to 20th	Day	8% 4 2 6% 4 4 5% 4 7% 10% 46%	4574
	21st to end	Day	9% 5% 2 6% 4 5% 5% 3 6% 10% 45%	4814
August	1st to 10th	Day	9% 6% 2 7% 5% 5% 6% 5% 7% 8% 40%	4274
	11th to 20th	Day	14% 5% 1 5% 3 4 4 4 6% 10% 43%	4041
	21st to end	Day	16% 7% 2 7% 4 4 4 3 6% 7% 40%	4358
September	1st to 10th	Day	11% 7% 2 7% 4 5% 4 4 6% 10% 42%	2280
	11th to 20th	Day	15% 6% 3 7% 4 4 5% 4 4 8% 40%	2280
	21st to end	Day	17% 8% 3 6% 3 4 3 3 5% 6% 40%	1982
October	1st to 10th	Day	18% 9% 3 7% 4 5% 4 4 5% 6% 35%	1900
	11th to 20th	Day	17% 13% 3 7% 4 4 5% 4 5% 7% 30%	1900
	21st to end	Day	19% 15% 4 8% 5% 4 4 4 4 6% 28%	1987
November	1st to 10th	Day	13% 13% 6% 10% 6% 4 4 4 6% 5% 28%	1596
	11th to 20th	Day	13% 18% 6% 13% 5% 5% 6% 3 4 5% 22%	1600
	21st to end	Day	15% 13% 7% 11% 7% 5% 5% 4 5% 6% 22%	1600
December	1st to 10th	Day	11% 12% 7% 13% 11% 6% 5% 4 7% 6% 17%	1528
	11th to 20th	Day	12% 17% 7% 14% 7% 6% 5% 5% 5% 4 19%	1914
	21st to end	Day	23% 21% 7% 12% 6% 4 3 2 3 4 15%	2495
January	1st to 10th	Night	8% 14% 6% 14% 8% 5% 4 4 6% 6% 25%	4737
	11th to 20th	Night	7% 11% 5% 14% 10% 9% 9% 5% 5% 5% 21%	4440
	21st to end	Night	8% 19% 13% 13% 5% 4 5% 3 4 5% 21%	4713
February	1st to 10th	Night	7% 12% 5% 10% 6% 6% 6% 5% 5% 7% 31%	2470
	11th to 20th	Night	7% 9% 4 9% 7% 6% 6% 4 6% 6% 35%	2403
	21st to end	Night	9% 14% 5% 8% 5% 5% 6% 4 6% 8% 31%	1994
March	1st to 10th	Night	9% 9% 4 8% 5% 5% 4 3 4 7% 41%	3595
	11th to 20th	Night	9% 10% 3 12% 7% 5% 5% 4 5% 6% 32%	3479
	21st to end	Night	20% 14% 4 10% 5% 4 3 3 4 4 28%	3725
April	1st to 10th	Night	16% 11% 3 9% 5% 5% 4 3 5% 5% 34%	1900
	11th to 20th	Night	16% 11% 4 7% 5% 5% 5% 3 5% 5% 34%	1834
	21st to end	Night	20% 10% 4 8% 4 4 3 3 5% 5% 33%	1701
May	1st to 10th	Night	20% 9% 2 6% 4 3 3 3 5% 7% 38%	2523
	11th to 20th	Night	14% 6% 2 5% 4 4 5% 4 6% 7% 43%	2506
	21st to end	Night	20% 8% 2 6% 5% 5% 4 3 4 6% 36%	2750
June	1st to 10th	Night	13% 6% 2 5% 5% 4 4 4 7% 7% 41%	1458
	11th to 20th	Night	12% 5% 2 5% 4 5% 4 4 7% 7% 45%	1330
	21st to end	Night	8% 6% 2 6% 4 4 4 4 7% 9% 45%	1330
July	1st to 10th	Night	8% 4 2 4 3 5% 4 5% 7% 12% 46%	2220
	11th to 20th	Night	5% 4 2 3 3 3 3 3 5% 9% 60%	2392
	21st to end	Night	7% 4 2 5% 4 3 4 4 7% 9% 53%	2750
August	1st to 10th	Night	9% 4 2 6% 4 4 4 4 6% 8% 48%	2664
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 6% 8% 48%	2798
	21st to end	Night	17% 6% 2 6% 3 3 3 3 5% 8% 43%	3290
September	1st to 10th	Night	10% 6% 2 5% 4 4 4 4 6% 8% 48%	1900
	11th to 20th	Night	10% 5% 2 5% 3 4 4 4 4 8% 50%	1900
	21st to end	Night	16% 7% 2 6% 4 3 4 3 5% 6% 43%	2174
October	1st to 10th	Night	14% 8% 2 7% 4 5% 4 3 5% 6% 41%	2280
	11th to 20th	Night	16% 12% 3 7% 4 4 4 4 5% 7% 34%	2280
	21st to end	Night	19% 11% 3 10% 5% 5% 5% 4 4 6% 28%	2609
November	1st to 10th	Night	14% 12% 3 8% 5% 4 5% 3 5% 7% 34%	2698
	11th to 20th	Night	15% 13% 6% 10% 5% 5% 5% 4 5% 6% 26%	2800
	21st to end	Night	12% 10% 5% 12% 6% 5% 5% 4 5% 7% 28%	2800
December	1st to 10th	Night	10% 13% 6% 15% 7% 5% 6% 5% 6% 7% 20%	2687
	11th to 20th	Night	11% 15% 6% 12% 7% 6% 6% 4 5% 7% 21%	3283
	21st to end	Night	23% 21% 6% 11% 5% 3 4 3 4 5% 16%	5167

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 9% 18% 7%	2219
	11th to 20th	Day	70% 2 7% 17% 4	2513
	21st to end	Day	70% 2 8% 14% 5%	2952
February	1st to 10th	Day	55% 3 10% 19% 11% 2	1710
	11th to 20th	Day	50% 4 12% 20% 11% 1	1673
	21st to end	Day	54% 3 12% 17% 12% 1	1459
March	1st to 10th	Day	48% 2 11% 26% 12% 1	3275
	11th to 20th	Day	49% 3 12% 24% 12%	3480
	21st to end	Day	58% 3 8% 24% 6% 1	3905
April	1st to 10th	Day	56% 2 8% 18% 13% 2	2280
	11th to 20th	Day	54% 4 11% 19% 10% 1	2294
	21st to end	Day	54% 3 8% 23% 11% 1	2478
May	1st to 10th	Day	49% 3 10% 23% 14% 1	4440
	11th to 20th	Day	45% 3 14% 27% 10% 2	4403
	21st to end	Day	52% 3 11% 23% 11% 1	4837
June	1st to 10th	Day	41% 3 16% 29% 10% 1	2716
	11th to 20th	Day	46% 4 14% 23% 13%	2850
	21st to end	Day	46% 4 15% 26% 8%	2850
July	1st to 10th	Day	40% 7% 16% 25% 12%	4676
	11th to 20th	Day	32% 5% 18% 34% 10% 1	4494
	21st to end	Day	36% 4 16% 33% 11% 1	4662
August	1st to 10th	Day	40% 5% 16% 27% 11% 1	4259
	11th to 20th	Day	37% 5% 22% 28% 8%	4080
	21st to end	Day	43% 5% 17% 26% 9% 1	4357
September	1st to 10th	Day	39% 3 17% 31% 10%	2280
	11th to 20th	Day	43% 3 19% 27% 7%	2280
	21st to end	Day	45% 4 14% 29% 7%	1982
October	1st to 10th	Day	41% 2 15% 30% 11%	3160
	11th to 20th	Day	46% 4 12% 23% 14%	3140
	21st to end	Day	56% 3 14% 20% 6%	3251
November	1st to 10th	Day	57% 4 10% 19% 9% 1	1596
	11th to 20th	Day	65% 3 11% 14% 6% 1	1600
	21st to end	Day	64% 3 12% 15% 5% 1	1600
December	1st to 10th	Day	67% 4 11% 14% 3	1528
	11th to 20th	Day	68% 3 8% 16% 4 1	1915
	21st to end	Day	77% 2 3 11% 7%	2495
January	1st to 10th	Night	61% 2 11% 19% 7%	4737
	11th to 20th	Night	66% 3 10% 15% 5%	4440
	21st to end	Night	68% 2 9% 15% 5%	4713
February	1st to 10th	Night	52% 4 10% 20% 12% 1	2470
	11th to 20th	Night	49% 4 11% 21% 14% 2	2404
	21st to end	Night	52% 3 10% 22% 11% 1	1994
March	1st to 10th	Night	44% 3 14% 20% 17% 2	3606
	11th to 20th	Night	52% 3 12% 23% 10% 1	3480
	21st to end	Night	61% 2 9% 16% 10% 1	3725
April	1st to 10th	Night	53% 3 8% 19% 15% 1	1900
	11th to 20th	Night	52% 3 8% 24% 13% 1	1834
	21st to end	Night	54% 2 6% 25% 12% 1	1701
May	1st to 10th	Night	48% 3 11% 26% 12%	2530
	11th to 20th	Night	40% 2 12% 26% 15% 4	2507
	21st to end	Night	51% 3 11% 22% 11% 1	2751
June	1st to 10th	Night	40% 4 17% 26% 12% 1	1458
	11th to 20th	Night	38% 4 14% 26% 17% 2	1330
	21st to end	Night	35% 5% 18% 30% 11% 1	1330
July	1st to 10th	Night	30% 7% 21% 29% 13%	2185
	11th to 20th	Night	23% 4 18% 42% 12% 1	2349
	21st to end	Night	26% 6% 18% 37% 12% 1	2662
August	1st to 10th	Night	34% 5% 18% 26% 15% 2	2655
	11th to 20th	Night	35% 5% 18% 34% 7% 1	2825
	21st to end	Night	40% 5% 20% 25% 10%	3290
September	1st to 10th	Night	35% 3 17% 32% 12% 1	1900
	11th to 20th	Night	33% 5% 21% 31% 9%	1900
	21st to end	Night	43% 3 14% 31% 9%	2174
October	1st to 10th	Night	32% 2 11% 37% 16% 2	3800
	11th to 20th	Night	46% 2 12% 26% 14%	3779
	21st to end	Night	57% 3 12% 20% 8%	4295
November	1st to 10th	Night	51% 3 13% 24% 8% 1	2698
	11th to 20th	Night	60% 3 14% 16% 7%	2800
	21st to end	Night	57% 4 16% 16% 6% 1	2800
December	1st to 10th	Night	64% 3 12% 17% 3	2687
	11th to 20th	Night	64% 3 10% 17% 5% 1	3281
	21st to end	Night	74% 2 6% 12% 6%	5167

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	15% 11% 31% 30% 12% 1	270
	11th to 20th	Day	16% 9% 38% 29% 8% 1	270
	21st to end	Day	11% 8% 36% 29% 14% 1	297
February	1st to 10th	Day	13% 12% 46% 20% 8% 1	270
	11th to 20th	Day	16% 17% 37% 20% 7% 4	270
	21st to end	Day	17% 11% 42% 24% 4 1	225
March	1st to 10th	Day	13% 16% 38% 21% 11% 1	270
	11th to 20th	Day	14% 9% 41% 24% 8% 4	270
	21st to end	Day	23% 12% 33% 21% 9% 2	350
April	1st to 10th	Day	24% 14% 31% 23% 8% 1	460
	11th to 20th	Day	25% 11% 28% 25% 10% 1	492
	21st to end	Day	25% 10% 30% 23% 11% 1	650
May	1st to 10th	Day	22% 14% 29% 24% 8% 3	653
	11th to 20th	Day	19% 12% 28% 26% 12% 4	675
	21st to end	Day	20% 8% 29% 28% 12% 3	734
June	1st to 10th	Day	14% 9% 31% 29% 12% 5%	680
	11th to 20th	Day	17% 12% 35% 23% 10% 3	680
	21st to end	Day	16% 8% 33% 22% 14% 6%	680
July	1st to 10th	Day	9% 6% 24% 30% 19% 11%	671
	11th to 20th	Day	11% 8% 28% 27% 19% 7%	668
	21st to end	Day	9% 5% 27% 30% 21% 7%	739
August	1st to 10th	Day	10% 7% 24% 33% 20% 6%	676
	11th to 20th	Day	16% 5% 21% 32% 21% 6%	665
	21st to end	Day	17% 8% 25% 28% 19% 4	652
September	1st to 10th	Day	10% 8% 31% 30% 19% 2	490
	11th to 20th	Day	17% 7% 31% 26% 16% 4	490
	21st to end	Day	22% 12% 32% 19% 11% 5%	306
October	1st to 10th	Day	21% 14% 31% 18% 11% 5%	300
	11th to 20th	Day	19% 16% 39% 22% 3 1	300
	21st to end	Day	28% 17% 34% 16% 5% 1	330
November	1st to 10th	Day	18% 14% 35% 25% 8%	300
	11th to 20th	Day	24% 16% 37% 19% 4	300
	21st to end	Day	22% 15% 33% 24% 6%	300
December	1st to 10th	Day	16% 14% 37% 24% 8% 1	273
	11th to 20th	Day	20% 18% 33% 25% 5%	270
	21st to end	Day	24% 12% 34% 23% 4 2	297
January	1st to 10th	Night	14% 9% 34% 31% 11% 1	950
	11th to 20th	Night	18% 7% 37% 29% 9% 1	950
	21st to end	Night	16% 9% 35% 31% 9%	1045
February	1st to 10th	Night	13% 8% 36% 32% 10% 1	950
	11th to 20th	Night	13% 10% 38% 28% 9% 2	929
	21st to end	Night	16% 7% 36% 30% 9% 2	738
March	1st to 10th	Night	15% 10% 35% 29% 10%	685
	11th to 20th	Night	16% 8% 39% 29% 8% 1	570
	21st to end	Night	20% 8% 29% 29% 12% 2	627
April	1st to 10th	Night	21% 8% 35% 29% 6% 1	570
	11th to 20th	Night	20% 8% 34% 29% 8% 1	563
	21st to end	Night	24% 8% 27% 30% 10% 1	570
May	1st to 10th	Night	19% 9% 33% 31% 7% 1	570
	11th to 20th	Night	18% 6% 32% 32% 11% 1	561
	21st to end	Night	18% 6% 28% 35% 13%	617
June	1st to 10th	Night	15% 5% 27% 39% 12% 1	570
	11th to 20th	Night	14% 4 32% 41% 9%	570
	21st to end	Night	12% 5% 25% 39% 18% 1	570
July	1st to 10th	Night	6% 3 23% 44% 23% 1	555
	11th to 20th	Night	9% 3 27% 38% 22% 1	552
	21st to end	Night	7% 2 23% 46% 21% 2	624
August	1st to 10th	Night	8% 2 23% 44% 22% 1	567
	11th to 20th	Night	12% 3 22% 38% 22% 3	564
	21st to end	Night	14% 6% 22% 39% 18% 2	627
September	1st to 10th	Night	12% 4 23% 44% 16% 2	570
	11th to 20th	Night	13% 4 29% 35% 18% 1	570
	21st to end	Night	18% 9% 31% 28% 13% 2	664
October	1st to 10th	Night	19% 8% 31% 29% 12% 1	760
	11th to 20th	Night	21% 10% 33% 28% 7% 1	760
	21st to end	Night	23% 11% 34% 26% 6%	836
November	1st to 10th	Night	19% 8% 34% 31% 8% 1	942
	11th to 20th	Night	21% 10% 33% 27% 8% 1	1000
	21st to end	Night	17% 13% 37% 27% 6%	1000
December	1st to 10th	Night	17% 9% 34% 29% 9% 1	960
	11th to 20th	Night	21% 8% 36% 25% 9% 1	950
	21st to end	Night	24% 8% 34% 26% 7% 1	1045

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