Lake Thunderbird
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lake Thunderbird clear sky chart from 2005-11-06 to 2025-11-18.

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% 8% 3 8% 5% 5% 4 4 6% 7% 38%	127103
	Night	13% 9% 3 8% 5% 4 4 3 5% 6% 39%	123522
Transparency	Day	44% 4 12% 24% 14% 2	130610
	Night	45% 3 11% 24% 15% 2	128089
Seeing	Day	17% 9% 28% 26% 16% 5%	17215
	Night	18% 9% 29% 31% 13% 1	27021

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	11% 12% 7% 13% 6% 6% 5% 4 5% 6% 25%	3617
	11th to 20th	Day	10% 10% 5% 12% 7% 6% 6% 5% 6% 5% 28%	3620
	21st to end	Day	15% 15% 8% 8% 6% 5% 4 3 4 6% 27%	4365
February	1st to 10th	Day	15% 14% 4 10% 5% 6% 4 3 4 6% 29%	1800
	11th to 20th	Day	13% 10% 3 9% 7% 5% 5% 4 4 5% 35%	1923
	21st to end	Day	14% 12% 6% 10% 5% 3 4 3 5% 6% 32%	1650
March	1st to 10th	Day	18% 13% 3 6% 3 2 3 2 3 4 42%	4480
	11th to 20th	Day	9% 6% 3 6% 5% 5% 4 4 7% 7% 45%	4836
	21st to end	Day	19% 10% 4 11% 6% 5% 4 4 5% 6% 26%	5889
April	1st to 10th	Day	16% 13% 4 7% 4 4 4 3 6% 7% 33%	2400
	11th to 20th	Day	17% 10% 3 6% 4 4 4 3 5% 7% 37%	2374
	21st to end	Day	20% 8% 3 7% 5% 5% 4 3 5% 5% 35%	2411
May	1st to 10th	Day	15% 7% 3 7% 5% 5% 4 3 6% 9% 35%	5896
	11th to 20th	Day	15% 6% 2 6% 3 4 5% 4 6% 7% 43%	6319
	21st to end	Day	12% 6% 3 7% 5% 4 5% 4 7% 8% 39%	6935
June	1st to 10th	Day	10% 5% 2 5% 4 5% 5% 5% 7% 7% 46%	2800
	11th to 20th	Day	7% 6% 3 6% 5% 5% 6% 4 6% 8% 46%	2800
	21st to end	Day	10% 6% 3 6% 5% 6% 5% 5% 6% 8% 41%	2800
July	1st to 10th	Day	8% 6% 3 7% 5% 5% 5% 4 7% 10% 40%	6360
	11th to 20th	Day	7% 6% 3 7% 5% 5% 5% 4 7% 10% 41%	6360
	21st to end	Day	10% 7% 2 7% 5% 6% 5% 5% 7% 9% 37%	6924
August	1st to 10th	Day	6% 5% 2 6% 5% 5% 5% 4 7% 10% 43%	4350
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 8% 11% 41%	3857
	21st to end	Day	6% 5% 2 5% 4 4 4 4 6% 9% 51%	4312
September	1st to 10th	Day	11% 6% 2 5% 4 4 4 3 5% 7% 49%	2400
	11th to 20th	Day	11% 4 2 4 3 3 3 3 5% 7% 55%	2331
	21st to end	Day	8% 6% 3 6% 4 5% 4 3 5% 8% 47%	2200
October	1st to 10th	Day	11% 7% 2 6% 4 4 4 4 5% 6% 49%	2055
	11th to 20th	Day	15% 8% 2 7% 4 5% 3 3 5% 5% 42%	2000
	21st to end	Day	20% 8% 2 5% 4 4 4 3 5% 6% 40%	2054
November	1st to 10th	Day	11% 8% 2 6% 4 3 3 4 6% 7% 47%	1852
	11th to 20th	Day	16% 10% 4 7% 5% 4 4 3 4 5% 37%	1863
	21st to end	Day	17% 11% 4 8% 5% 4 3 3 4 4 37%	1670
December	1st to 10th	Day	8% 9% 4 8% 4 3 4 3 4 5% 47%	2610
	11th to 20th	Day	13% 15% 7% 13% 6% 6% 4 3 4 6% 22%	3008
	21st to end	Day	25% 21% 7% 13% 5% 3 4 2 3 3 14%	3982
January	1st to 10th	Night	13% 12% 6% 13% 5% 5% 4 3 4 5% 31%	6348
	11th to 20th	Night	12% 15% 5% 12% 9% 5% 5% 4 5% 4 24%	6360
	21st to end	Night	17% 14% 6% 8% 5% 4 4 3 4 4 31%	6610
February	1st to 10th	Night	15% 12% 4 9% 6% 5% 4 3 5% 6% 32%	2600
	11th to 20th	Night	13% 11% 5% 8% 5% 5% 4 3 5% 6% 37%	2362
	21st to end	Night	17% 11% 4 8% 4 4 3 3 4 5% 39%	1980
March	1st to 10th	Night	17% 10% 3 7% 3 3 3 3 4 5% 41%	5373
	11th to 20th	Night	11% 6% 3 6% 4 4 4 4 4 7% 47%	5167
	21st to end	Night	17% 12% 5% 10% 6% 5% 4 3 5% 5% 28%	5038
April	1st to 10th	Night	16% 10% 3 6% 4 3 4 3 6% 6% 39%	2000
	11th to 20th	Night	17% 9% 2 5% 5% 3 3 3 4 6% 42%	1975
	21st to end	Night	17% 8% 2 6% 5% 3 5% 4 5% 8% 37%	1987
May	1st to 10th	Night	19% 8% 3 8% 4 5% 5% 3 5% 7% 34%	3998
	11th to 20th	Night	12% 6% 2 6% 4 4 5% 4 7% 9% 42%	3596
	21st to end	Night	12% 7% 3 7% 4 4 4 3 6% 7% 41%	3942
June	1st to 10th	Night	8% 6% 3 5% 4 4 4 3 5% 7% 51%	1600
	11th to 20th	Night	9% 4 2 7% 4 4 4 3 7% 9% 47%	1600
	21st to end	Night	9% 5% 2 6% 5% 4 5% 5% 6% 9% 46%	1600
July	1st to 10th	Night	9% 5% 2 7% 5% 4 5% 4 7% 9% 43%	3618
	11th to 20th	Night	7% 5% 2 5% 4 4 4 4 6% 9% 50%	3617
	21st to end	Night	10% 6% 3 7% 5% 5% 5% 4 6% 8% 42%	3947
August	1st to 10th	Night	5% 5% 2 5% 4 4 4 3 6% 8% 53%	2794
	11th to 20th	Night	6% 5% 2 6% 4 3 5% 3 6% 8% 51%	3191
	21st to end	Night	6% 4 2 5% 4 3 4 4 6% 8% 53%	3586
September	1st to 10th	Night	12% 4 2 5% 3 2 3 3 4 6% 56%	2000
	11th to 20th	Night	9% 4 1 5% 3 4 4 4 6% 7% 54%	2048
	21st to end	Night	9% 6% 2 5% 4 4 4 5% 6% 8% 48%	2200
October	1st to 10th	Night	10% 5% 2 6% 3 4 3 3 4 6% 54%	2342
	11th to 20th	Night	15% 6% 2 6% 3 3 4 3 4 6% 47%	2400
	21st to end	Night	18% 8% 2 7% 4 4 4 3 5% 6% 39%	2785
November	1st to 10th	Night	11% 6% 2 6% 3 3 3 3 6% 6% 50%	2666
	11th to 20th	Night	14% 9% 3 7% 5% 4 4 3 5% 6% 42%	2692
	21st to end	Night	16% 10% 4 7% 4 4 3 3 4 6% 38%	2725
December	1st to 10th	Night	10% 9% 3 7% 4 4 4 3 5% 6% 45%	4580
	11th to 20th	Night	15% 13% 5% 10% 5% 4 3 3 4 5% 33%	5199
	21st to end	Night	23% 19% 6% 11% 6% 5% 4 3 4 4 16%	6996

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	61% 2 11% 18% 7% 1	3618
	11th to 20th	Day	58% 3 9% 12% 15% 3	3620
	21st to end	Day	60% 3 11% 18% 7%	4365
February	1st to 10th	Day	58% 2 13% 18% 7% 2	1800
	11th to 20th	Day	52% 3 11% 23% 10% 2	1933
	21st to end	Day	54% 3 10% 22% 8% 2	1650
March	1st to 10th	Day	49% 2 8% 25% 13% 3	4480
	11th to 20th	Day	39% 2 10% 33% 15% 1	4836
	21st to end	Day	59% 3 12% 19% 6% 1	5891
April	1st to 10th	Day	51% 3 12% 22% 12%	2400
	11th to 20th	Day	48% 3 10% 24% 13% 3	2374
	21st to end	Day	51% 4 10% 20% 13% 2	2411
May	1st to 10th	Day	47% 3 10% 24% 16% 1	5931
	11th to 20th	Day	41% 4 10% 29% 13% 3	6323
	21st to end	Day	42% 4 11% 25% 15% 3	6949
June	1st to 10th	Day	35% 4 13% 29% 16% 3	2800
	11th to 20th	Day	35% 5% 11% 24% 22% 3	2800
	21st to end	Day	40% 5% 13% 24% 17% 2	2800
July	1st to 10th	Day	39% 4 13% 27% 15% 1	6291
	11th to 20th	Day	37% 6% 14% 27% 15% 1	6285
	21st to end	Day	41% 7% 16% 21% 13% 2	6728
August	1st to 10th	Day	33% 5% 17% 30% 14% 1	4335
	11th to 20th	Day	34% 5% 15% 29% 15% 1	3893
	21st to end	Day	30% 5% 12% 33% 17% 3	4311
September	1st to 10th	Day	34% 3 11% 26% 21% 4	2400
	11th to 20th	Day	30% 2 12% 29% 24% 4	2331
	21st to end	Day	36% 4 8% 29% 21% 2	2200
October	1st to 10th	Day	25% 3 6% 28% 36% 2	3341
	11th to 20th	Day	30% 2 10% 24% 31% 3	3240
	21st to end	Day	41% 2 9% 29% 16% 3	3292
November	1st to 10th	Day	37% 3 15% 27% 15% 3	1852
	11th to 20th	Day	50% 2 11% 23% 12% 1	1863
	21st to end	Day	52% 3 13% 23% 9%	1670
December	1st to 10th	Day	40% 3 13% 34% 9%	2610
	11th to 20th	Day	63% 3 10% 16% 6% 1	3005
	21st to end	Day	77% 3 8% 8% 4	3982
January	1st to 10th	Night	57% 2 11% 21% 8%	6348
	11th to 20th	Night	64% 2 10% 11% 13% 1	6360
	21st to end	Night	58% 2 9% 19% 11% 1	6610
February	1st to 10th	Night	55% 3 11% 19% 12% 1	2600
	11th to 20th	Night	50% 2 10% 24% 12% 1	2363
	21st to end	Night	50% 2 9% 22% 13% 3	1980
March	1st to 10th	Night	48% 2 8% 25% 13% 4	5373
	11th to 20th	Night	38% 3 10% 29% 17% 3	5167
	21st to end	Night	60% 2 10% 20% 7% 1	5038
April	1st to 10th	Night	45% 3 11% 25% 14% 2	2000
	11th to 20th	Night	45% 2 8% 26% 15% 4	1975
	21st to end	Night	46% 3 13% 23% 12% 4	1987
May	1st to 10th	Night	50% 3 10% 19% 15% 3	4025
	11th to 20th	Night	39% 4 14% 26% 14% 4	3597
	21st to end	Night	41% 4 10% 26% 16% 3	3950
June	1st to 10th	Night	33% 4 15% 25% 20% 4	1600
	11th to 20th	Night	34% 3 12% 24% 23% 4	1600
	21st to end	Night	35% 6% 15% 22% 19% 3	1600
July	1st to 10th	Night	38% 5% 12% 26% 17% 2	3580
	11th to 20th	Night	31% 4 14% 28% 21% 1	3572
	21st to end	Night	39% 7% 15% 21% 15% 3	3832
August	1st to 10th	Night	29% 5% 17% 29% 19% 1	2785
	11th to 20th	Night	31% 5% 16% 31% 17% 1	3221
	21st to end	Night	28% 5% 12% 33% 18% 3	3586
September	1st to 10th	Night	31% 3 9% 28% 25% 5%	2000
	11th to 20th	Night	29% 3 10% 29% 25% 4	2048
	21st to end	Night	33% 5% 11% 28% 21% 3	2200
October	1st to 10th	Night	23% 2 7% 31% 33% 3	3836
	11th to 20th	Night	29% 3 8% 23% 33% 4	3899
	21st to end	Night	40% 3 9% 25% 20% 3	4497
November	1st to 10th	Night	35% 3 10% 29% 18% 4	2666
	11th to 20th	Night	45% 2 11% 24% 16% 2	2692
	21st to end	Night	50% 3 10% 24% 13% 1	2725
December	1st to 10th	Night	41% 2 13% 32% 10% 1	4580
	11th to 20th	Night	55% 2 12% 20% 10%	5201
	21st to end	Night	73% 2 9% 12% 4	6996

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	23% 13% 31% 24% 8%	300
	11th to 20th	Day	24% 19% 38% 13% 4 1	300
	21st to end	Day	23% 19% 34% 18% 6%	330
February	1st to 10th	Day	21% 17% 41% 19% 3	300
	11th to 20th	Day	22% 19% 37% 16% 5%	300
	21st to end	Day	25% 13% 47% 11% 4	249
March	1st to 10th	Day	22% 19% 37% 17% 4	300
	11th to 20th	Day	25% 18% 32% 18% 6%	300
	21st to end	Day	28% 14% 34% 16% 6% 1	511
April	1st to 10th	Day	24% 13% 33% 23% 6% 1	500
	11th to 20th	Day	23% 17% 27% 20% 11% 2	492
	21st to end	Day	26% 9% 32% 23% 9% 1	500
May	1st to 10th	Day	18% 10% 34% 25% 11% 2	511
	11th to 20th	Day	23% 9% 27% 25% 13% 3	725
	21st to end	Day	17% 8% 28% 24% 15% 8%	789
June	1st to 10th	Day	10% 5% 28% 30% 21% 6%	730
	11th to 20th	Day	9% 4 27% 34% 21% 4	730
	21st to end	Day	12% 7% 29% 31% 15% 6%	730
July	1st to 10th	Day	11% 2 17% 31% 25% 13%	721
	11th to 20th	Day	10% 3 22% 31% 24% 9%	718
	21st to end	Day	9% 3 18% 37% 26% 7%	794
August	1st to 10th	Day	7% 4 19% 34% 25% 12%	726
	11th to 20th	Day	9% 4 17% 29% 28% 13%	527
	21st to end	Day	7% 2 19% 32% 31% 10%	583
September	1st to 10th	Day	13% 5% 16% 30% 28% 7%	530
	11th to 20th	Day	9% 2 13% 29% 33% 14%	530
	21st to end	Day	11% 7% 27% 23% 23% 8%	530
October	1st to 10th	Day	14% 8% 25% 28% 20% 5%	385
	11th to 20th	Day	16% 18% 34% 22% 8% 1	330
	21st to end	Day	32% 11% 30% 22% 6% 1	363
November	1st to 10th	Day	15% 16% 37% 20% 8% 4	330
	11th to 20th	Day	23% 16% 37% 15% 9% 1	321
	21st to end	Day	25% 10% 36% 25% 4	300
December	1st to 10th	Day	17% 15% 42% 19% 6% 2	300
	11th to 20th	Day	20% 14% 37% 22% 6% 1	300
	21st to end	Day	30% 16% 35% 16% 3	330
January	1st to 10th	Night	19% 13% 38% 24% 6% 1	1000
	11th to 20th	Night	21% 13% 37% 23% 5%	1000
	21st to end	Night	21% 12% 35% 27% 5%	1100
February	1st to 10th	Night	24% 11% 38% 23% 4	1000
	11th to 20th	Night	18% 14% 36% 27% 5%	807
	21st to end	Night	23% 12% 39% 22% 4	660
March	1st to 10th	Night	21% 13% 35% 26% 5%	800
	11th to 20th	Night	19% 10% 34% 30% 7%	683
	21st to end	Night	25% 11% 34% 24% 6%	660
April	1st to 10th	Night	22% 12% 35% 26% 6%	600
	11th to 20th	Night	23% 10% 31% 28% 7%	593
	21st to end	Night	20% 11% 30% 31% 7%	600
May	1st to 10th	Night	21% 6% 31% 32% 9% 1	600
	11th to 20th	Night	19% 5% 30% 34% 13% 1	591
	21st to end	Night	14% 4 28% 36% 17% 1	650
June	1st to 10th	Night	10% 4 22% 47% 17% 1	600
	11th to 20th	Night	10% 4 25% 44% 17% 1	600
	21st to end	Night	10% 4 25% 40% 20% 1	600
July	1st to 10th	Night	8% 2 12% 42% 31% 5%	585
	11th to 20th	Night	7% 2 16% 44% 26% 4	582
	21st to end	Night	5% 1 15% 49% 28% 2	657
August	1st to 10th	Night	6% 2 15% 42% 30% 5%	597
	11th to 20th	Night	7% 2 10% 43% 32% 6%	594
	21st to end	Night	9% 1 13% 40% 33% 3	660
September	1st to 10th	Night	13% 4 17% 35% 29% 3	600
	11th to 20th	Night	11% 3 17% 33% 32% 4	648
	21st to end	Night	10% 7% 24% 32% 25% 2	800
October	1st to 10th	Night	12% 5% 26% 34% 21% 2	800
	11th to 20th	Night	20% 11% 29% 26% 13% 1	800
	21st to end	Night	24% 10% 33% 25% 7%	880
November	1st to 10th	Night	16% 10% 37% 27% 9% 1	820
	11th to 20th	Night	19% 14% 34% 26% 5%	829
	21st to end	Night	22% 12% 34% 25% 6%	925
December	1st to 10th	Night	23% 11% 36% 25% 6%	1000
	11th to 20th	Night	23% 11% 32% 28% 6%	1000
	21st to end	Night	30% 13% 32% 22% 3	1100

Page generated at 2025-11-14 23:55:58. Version ac07

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