Lalen Lake Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lalen Lake Observatory clear sky chart from 2005-11-06 to 2024-02-10.

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	19% 12% 4 8% 6% 4 5% 5% 5% 6% 25%	76486
	Night	16% 14% 5% 8% 7% 4 5% 5% 5% 6% 26%	82486
Transparency	Day	57% 9% 15% 13% 5% 1	76132
	Night	57% 9% 12% 14% 7% 2	82423
Seeing	Day	23% 6% 16% 25% 22% 8%	16850
	Night	23% 6% 17% 27% 22% 5%	21580

By Month

Percentage Cloud Cover NW (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	6% 15% 4 8% 7% 5% 7% 7% 8% 7% 26%	1269
	11th to 20th	Day	12% 13% 5% 6% 5% 3 5% 4 6% 7% 34%	1587
	21st to end	Day	11% 14% 6% 8% 6% 5% 5% 4 5% 6% 30%	2090
February	1st to 10th	Day	11% 16% 6% 7% 6% 2 4 3 4 5% 36%	1282
	11th to 20th	Day	13% 24% 5% 9% 8% 4 5% 4 4 5% 18%	1393
	21st to end	Day	9% 16% 5% 8% 6% 5% 4 4 4 4 36%	1278
March	1st to 10th	Day	11% 16% 5% 7% 5% 3 4 3 3 5% 37%	1767
	11th to 20th	Day	6% 11% 4 7% 6% 3 5% 5% 5% 7% 40%	1855
	21st to end	Day	8% 16% 5% 8% 7% 3 5% 5% 5% 7% 33%	2362
April	1st to 10th	Day	12% 17% 5% 8% 6% 4 6% 6% 6% 6% 25%	2208
	11th to 20th	Day	8% 12% 5% 9% 5% 3 5% 6% 7% 6% 33%	2486
	21st to end	Day	13% 13% 4 8% 8% 4 5% 5% 6% 7% 27%	2556
May	1st to 10th	Day	17% 15% 5% 9% 8% 4 5% 5% 5% 6% 20%	2856
	11th to 20th	Day	15% 12% 4 8% 7% 4 6% 5% 6% 6% 27%	2833
	21st to end	Day	19% 10% 3 7% 6% 4 5% 4 5% 6% 30%	3405
June	1st to 10th	Day	18% 10% 3 8% 7% 4 6% 6% 7% 8% 22%	3240
	11th to 20th	Day	20% 9% 4 8% 8% 4 6% 6% 6% 7% 22%	3240
	21st to end	Day	20% 10% 5% 8% 7% 4 6% 6% 6% 8% 21%	3240
July	1st to 10th	Day	24% 8% 4 7% 6% 4 6% 6% 6% 7% 23%	3240
	11th to 20th	Day	24% 9% 3 8% 7% 4 6% 6% 5% 8% 20%	3186
	21st to end	Day	28% 10% 4 7% 6% 3 5% 5% 5% 6% 21%	3167
August	1st to 10th	Day	32% 9% 4 7% 7% 3 5% 4 5% 6% 18%	2865
	11th to 20th	Day	34% 8% 3 7% 6% 4 5% 5% 5% 6% 17%	2532
	21st to end	Day	31% 8% 3 6% 6% 3 4 5% 6% 7% 21%	2772
September	1st to 10th	Day	34% 11% 3 8% 6% 4 5% 4 4 5% 16%	2233
	11th to 20th	Day	33% 13% 4 9% 6% 3 4 3 4 4 18%	2160
	21st to end	Day	27% 11% 4 8% 6% 4 4 4 4 5% 24%	1916
October	1st to 10th	Day	30% 13% 5% 7% 6% 3 4 5% 5% 5% 18%	1782
	11th to 20th	Day	22% 12% 5% 7% 6% 5% 5% 4 4 6% 24%	1602
	21st to end	Day	17% 13% 5% 8% 5% 3 5% 6% 5% 5% 28%	1540
November	1st to 10th	Day	13% 18% 5% 9% 5% 3 5% 5% 5% 3 28%	1287
	11th to 20th	Day	14% 12% 3 8% 6% 3 4 4 4 4 37%	1140
	21st to end	Day	14% 17% 6% 9% 7% 4 5% 4 4 4 27%	955
December	1st to 10th	Day	20% 17% 7% 8% 7% 4 4 3 4 5% 22%	798
	11th to 20th	Day	8% 23% 9% 10% 7% 3 4 5% 4 6% 20%	967
	21st to end	Day	7% 15% 6% 9% 9% 4 6% 6% 6% 6% 26%	1397
January	1st to 10th	Night	7% 14% 5% 9% 8% 4 6% 6% 7% 6% 27%	5686
	11th to 20th	Night	13% 14% 4 8% 6% 3 3 3 4 8% 33%	5322
	21st to end	Night	10% 13% 4 8% 7% 3 5% 4 5% 6% 34%	5577
February	1st to 10th	Night	10% 16% 6% 8% 5% 3 4 3 4 5% 36%	2880
	11th to 20th	Night	17% 21% 6% 10% 7% 4 4 4 4 6% 18%	2454
	21st to end	Night	11% 14% 5% 9% 8% 4 5% 4 5% 4 31%	1978
March	1st to 10th	Night	11% 14% 5% 7% 6% 3 4 4 5% 5% 37%	2193
	11th to 20th	Night	7% 11% 4 8% 5% 3 5% 4 5% 7% 40%	2100
	21st to end	Night	10% 15% 5% 7% 5% 3 4 4 5% 6% 35%	1994
April	1st to 10th	Night	11% 18% 6% 10% 8% 5% 5% 5% 4 4 24%	1751
	11th to 20th	Night	11% 15% 5% 7% 6% 4 5% 4 5% 7% 31%	1423
	21st to end	Night	13% 13% 5% 8% 8% 4 5% 4 4 5% 30%	1404
May	1st to 10th	Night	15% 14% 8% 11% 8% 4 6% 6% 5% 6% 19%	1103
	11th to 20th	Night	17% 11% 5% 7% 7% 3 7% 5% 5% 7% 26%	1062
	21st to end	Night	18% 9% 3 8% 8% 4 6% 5% 4 6% 29%	868
June	1st to 10th	Night	18% 12% 5% 10% 8% 4 5% 5% 6% 5% 22%	720
	11th to 20th	Night	19% 12% 5% 8% 5% 4 6% 3 6% 8% 23%	720
	21st to end	Night	23% 10% 6% 9% 6% 4 4 7% 5% 8% 19%	720
July	1st to 10th	Night	25% 10% 4 7% 7% 4 6% 6% 8% 7% 17%	720
	11th to 20th	Night	31% 11% 3 4 4 3 5% 6% 7% 7% 17%	774
	21st to end	Night	32% 8% 4 9% 6% 2 3 4 5% 6% 20%	1151
August	1st to 10th	Night	31% 9% 4 7% 6% 3 5% 5% 5% 8% 18%	1074
	11th to 20th	Night	37% 9% 3 7% 6% 4 5% 5% 4 5% 16%	1366
	21st to end	Night	34% 9% 3 6% 7% 3 4 5% 4 6% 19%	1584
September	1st to 10th	Night	36% 12% 3 7% 6% 3 5% 3 4 5% 16%	1725
	11th to 20th	Night	36% 12% 3 8% 6% 2 4 4 4 4 18%	1800
	21st to end	Night	29% 10% 3 7% 6% 4 5% 5% 4 5% 22%	2042
October	1st to 10th	Night	33% 12% 4 6% 6% 3 3 3 3 5% 22%	2178
	11th to 20th	Night	22% 12% 5% 7% 6% 4 5% 4 4 6% 25%	2358
	21st to end	Night	19% 13% 5% 8% 6% 3 4 4 5% 6% 25%	2816
November	1st to 10th	Night	15% 18% 5% 8% 6% 3 4 4 5% 5% 26%	2789
	11th to 20th	Night	13% 12% 3 7% 5% 3 4 5% 5% 5% 37%	3040
	21st to end	Night	14% 18% 6% 10% 7% 3 5% 4 4 5% 26%	3225
December	1st to 10th	Night	21% 18% 4 9% 6% 3 4 4 4 5% 20%	3379
	11th to 20th	Night	11% 23% 8% 11% 7% 4 5% 4 5% 5% 19%	4240
	21st to end	Night	8% 16% 6% 10% 8% 4 6% 6% 7% 8% 20%	6270

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	47% 16% 13% 13% 9% 2	1269
	11th to 20th	Day	44% 12% 10% 20% 11% 3	1602
	21st to end	Day	52% 7% 8% 18% 10% 4	2090
February	1st to 10th	Day	50% 6% 10% 17% 13% 4	1282
	11th to 20th	Day	66% 8% 8% 11% 7% 1	1400
	21st to end	Day	52% 6% 10% 20% 11% 1	1278
March	1st to 10th	Day	51% 5% 10% 16% 13% 5%	1767
	11th to 20th	Day	40% 7% 10% 23% 16% 3	1855
	21st to end	Day	50% 8% 16% 13% 11% 3	2362
April	1st to 10th	Day	57% 7% 15% 15% 6% 1	2208
	11th to 20th	Day	47% 8% 15% 21% 9%	2486
	21st to end	Day	53% 8% 13% 17% 8% 1	2556
May	1st to 10th	Day	62% 7% 16% 12% 3	2856
	11th to 20th	Day	54% 7% 15% 18% 6%	2836
	21st to end	Day	52% 6% 17% 20% 4	3406
June	1st to 10th	Day	55% 11% 20% 12% 2	3240
	11th to 20th	Day	56% 11% 19% 10% 3	3240
	21st to end	Day	58% 12% 23% 8%	3240
July	1st to 10th	Day	57% 13% 19% 10% 1	3151
	11th to 20th	Day	61% 11% 23% 5%	3087
	21st to end	Day	60% 12% 21% 8%	2991
August	1st to 10th	Day	65% 10% 19% 6%	2846
	11th to 20th	Day	65% 10% 16% 7% 2	2532
	21st to end	Day	62% 12% 14% 9% 2	2772
September	1st to 10th	Day	69% 9% 12% 8% 2	2233
	11th to 20th	Day	69% 7% 14% 9% 1	2160
	21st to end	Day	62% 7% 15% 15% 2	1916
October	1st to 10th	Day	66% 7% 14% 11% 2	1782
	11th to 20th	Day	59% 8% 15% 13% 3 1	1602
	21st to end	Day	53% 9% 15% 14% 8% 1	1540
November	1st to 10th	Day	57% 8% 12% 14% 7% 1	1287
	11th to 20th	Day	49% 7% 12% 23% 9% 1	1140
	21st to end	Day	60% 6% 9% 12% 11% 2	955
December	1st to 10th	Day	64% 8% 11% 10% 8%	798
	11th to 20th	Day	63% 9% 12% 12% 3 1	970
	21st to end	Day	51% 12% 13% 9% 13% 2	1397
January	1st to 10th	Night	51% 14% 12% 12% 8% 2	5686
	11th to 20th	Night	47% 12% 10% 19% 9% 2	5370
	21st to end	Night	47% 8% 8% 18% 14% 5%	5577
February	1st to 10th	Night	50% 6% 7% 19% 13% 5%	2880
	11th to 20th	Night	66% 6% 10% 10% 7% 1	2458
	21st to end	Night	54% 7% 10% 16% 11% 1	1978
March	1st to 10th	Night	49% 6% 11% 16% 13% 6%	2193
	11th to 20th	Night	44% 6% 8% 19% 20% 3	2102
	21st to end	Night	48% 7% 16% 14% 11% 3	1994
April	1st to 10th	Night	62% 7% 12% 14% 5% 1	1751
	11th to 20th	Night	51% 8% 14% 20% 7%	1423
	21st to end	Night	55% 5% 15% 16% 8%	1404
May	1st to 10th	Night	65% 7% 13% 13% 2	1103
	11th to 20th	Night	56% 8% 13% 18% 6%	1063
	21st to end	Night	52% 8% 17% 20% 3	868
June	1st to 10th	Night	62% 11% 15% 12% 1	720
	11th to 20th	Night	58% 13% 18% 8% 3	720
	21st to end	Night	63% 13% 18% 5%	720
July	1st to 10th	Night	64% 9% 17% 9%	700
	11th to 20th	Night	65% 12% 17% 6%	750
	21st to end	Night	65% 10% 19% 6%	1087
August	1st to 10th	Night	64% 12% 20% 5%	1068
	11th to 20th	Night	71% 9% 11% 8% 1	1366
	21st to end	Night	66% 9% 15% 8% 2	1584
September	1st to 10th	Night	72% 9% 10% 8% 2	1725
	11th to 20th	Night	69% 7% 12% 10% 1	1800
	21st to end	Night	63% 7% 14% 13% 3	2042
October	1st to 10th	Night	67% 6% 12% 12% 3	2178
	11th to 20th	Night	59% 8% 16% 10% 6% 1	2358
	21st to end	Night	59% 7% 12% 14% 8%	2816
November	1st to 10th	Night	58% 8% 11% 15% 6% 1	2789
	11th to 20th	Night	48% 7% 11% 25% 10%	3040
	21st to end	Night	60% 7% 9% 11% 10% 2	3225
December	1st to 10th	Night	65% 8% 9% 10% 8%	3379
	11th to 20th	Night	65% 8% 9% 13% 4	4236
	21st to end	Night	55% 12% 14% 10% 7% 2	6270

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	9% 9% 19% 26% 27% 11%	90
	11th to 20th	Day	22% 9% 13% 25% 20% 11%	253
	21st to end	Day	23% 7% 16% 22% 23% 9%	308
February	1st to 10th	Day	14% 8% 16% 32% 15% 14%	279
	11th to 20th	Day	28% 7% 18% 26% 15% 6%	333
	21st to end	Day	18% 7% 26% 28% 13% 9%	280
March	1st to 10th	Day	19% 4 21% 30% 16% 9%	340
	11th to 20th	Day	13% 6% 21% 29% 21% 10%	395
	21st to end	Day	14% 7% 18% 28% 26% 7%	572
April	1st to 10th	Day	15% 9% 23% 27% 20% 5%	532
	11th to 20th	Day	14% 7% 14% 29% 26% 9%	590
	21st to end	Day	19% 7% 17% 26% 24% 7%	600
May	1st to 10th	Day	21% 10% 19% 24% 22% 4	603
	11th to 20th	Day	19% 8% 16% 28% 22% 7%	625
	21st to end	Day	23% 8% 17% 29% 19% 5%	789
June	1st to 10th	Day	19% 5% 15% 27% 23% 11%	810
	11th to 20th	Day	20% 5% 16% 28% 23% 9%	810
	21st to end	Day	20% 6% 14% 25% 26% 9%	810
July	1st to 10th	Day	21% 5% 13% 27% 26% 8%	796
	11th to 20th	Day	22% 4 12% 30% 25% 6%	792
	21st to end	Day	29% 3 11% 24% 25% 9%	714
August	1st to 10th	Day	29% 5% 15% 24% 20% 7%	626
	11th to 20th	Day	29% 7% 16% 21% 20% 7%	615
	21st to end	Day	32% 6% 12% 19% 23% 8%	693
September	1st to 10th	Day	32% 7% 14% 21% 21% 6%	561
	11th to 20th	Day	39% 5% 11% 17% 20% 8%	540
	21st to end	Day	29% 5% 16% 19% 20% 10%	365
October	1st to 10th	Day	34% 5% 17% 23% 15% 6%	360
	11th to 20th	Day	25% 8% 12% 26% 20% 9%	360
	21st to end	Day	24% 4 14% 26% 20% 11%	396
November	1st to 10th	Day	23% 8% 19% 18% 19% 12%	359
	11th to 20th	Day	18% 5% 15% 28% 23% 12%	280
	21st to end	Day	23% 6% 16% 22% 25% 7%	95
December	1st to 10th	Day	24% 9% 11% 28% 24% 3	90
	11th to 20th	Day	22% 7% 23% 29% 11% 8%	90
	21st to end	Day	22% 7% 23% 22% 21% 4	99
January	1st to 10th	Night	14% 6% 18% 32% 25% 6%	940
	11th to 20th	Night	21% 8% 18% 31% 18% 5%	920
	21st to end	Night	22% 6% 18% 31% 19% 4	935
February	1st to 10th	Night	15% 8% 20% 34% 18% 4	847
	11th to 20th	Night	27% 7% 22% 26% 17% 1	784
	21st to end	Night	18% 6% 23% 34% 15% 5%	564
March	1st to 10th	Night	16% 8% 21% 31% 21% 3	620
	11th to 20th	Night	14% 6% 21% 33% 22% 5%	620
	21st to end	Night	14% 8% 17% 27% 28% 5%	600
April	1st to 10th	Night	19% 7% 21% 24% 23% 6%	540
	11th to 20th	Night	18% 5% 14% 26% 31% 6%	533
	21st to end	Night	17% 4 16% 31% 29% 4	540
May	1st to 10th	Night	18% 6% 15% 28% 29% 3	383
	11th to 20th	Night	20% 3 15% 33% 27% 2	354
	21st to end	Night	19% 5% 15% 34% 26%	204
June	1st to 10th	Night	22% 7% 12% 24% 33% 2	180
	11th to 20th	Night	19% 2 16% 33% 28% 2	180
	21st to end	Night	19% 8% 11% 33% 26% 3	180
July	1st to 10th	Night	31% 5% 9% 23% 30% 3	175
	11th to 20th	Night	34% 4 6% 24% 30% 3	228
	21st to end	Night	35% 6% 6% 19% 29% 5%	394
August	1st to 10th	Night	33% 3 11% 27% 22% 4	358
	11th to 20th	Night	40% 4 14% 20% 19% 3	480
	21st to end	Night	36% 4 10% 20% 24% 5%	594
September	1st to 10th	Night	39% 6% 10% 23% 18% 5%	540
	11th to 20th	Night	40% 5% 6% 18% 24% 6%	540
	21st to end	Night	33% 5% 16% 17% 22% 6%	573
October	1st to 10th	Night	36% 6% 13% 23% 18% 4	648
	11th to 20th	Night	25% 5% 15% 25% 24% 6%	810
	21st to end	Night	25% 5% 16% 26% 24% 3	891
November	1st to 10th	Night	23% 7% 20% 27% 18% 5%	830
	11th to 20th	Night	17% 7% 19% 31% 21% 4	850
	21st to end	Night	25% 7% 20% 24% 20% 5%	850
December	1st to 10th	Night	28% 7% 20% 25% 16% 4	921
	11th to 20th	Night	16% 7% 22% 31% 17% 6%	940
	21st to end	Night	20% 8% 23% 26% 17% 6%	1034

Page generated at 2024-02-07 02:11:59. Version ac07

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