Houghton Lake Observatory Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Houghton Lake Observatory Site clear sky chart from 2005-11-06 to 2025-10-04.

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% 11% 4 9% 6% 5% 5% 4 5% 7% 32%	125754
	Night	13% 13% 5% 10% 5% 5% 4 3 5% 6% 30%	123810
Transparency	Day	51% 6% 11% 20% 12% 1	129266
	Night	52% 6% 11% 19% 10% 1	128372
Seeing	Day	20% 16% 31% 21% 9% 3	16894
	Night	19% 12% 32% 26% 9% 1	26585

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	10% 16% 9% 22% 8% 6% 5% 3 4 5% 14%	3617
	11th to 20th	Day	11% 15% 7% 22% 13% 10% 8% 3 3 3 6%	3620
	21st to end	Day	14% 18% 9% 15% 7% 4 5% 3 4 5% 15%	3982
February	1st to 10th	Day	11% 16% 6% 14% 11% 6% 6% 3 4 5% 18%	1776
	11th to 20th	Day	11% 14% 7% 15% 11% 5% 6% 4 3 4 19%	1923
	21st to end	Day	12% 17% 6% 14% 8% 4 5% 4 5% 4 21%	1650
March	1st to 10th	Day	12% 11% 4 9% 7% 5% 5% 4 5% 6% 32%	4480
	11th to 20th	Day	13% 10% 6% 12% 5% 4 5% 4 6% 8% 28%	4925
	21st to end	Day	22% 11% 7% 11% 5% 4 4 3 4 4 24%	5552
April	1st to 10th	Day	17% 17% 4 9% 5% 4 3 3 4 6% 28%	2400
	11th to 20th	Day	18% 13% 3 7% 4 4 4 3 4 5% 36%	2498
	21st to end	Day	15% 11% 3 9% 5% 4 5% 4 5% 7% 32%	2600
May	1st to 10th	Day	17% 9% 3 7% 5% 4 4 4 5% 7% 36%	5862
	11th to 20th	Day	12% 9% 3 7% 5% 4 5% 4 6% 7% 39%	5896
	21st to end	Day	13% 8% 3 7% 4 4 4 4 5% 7% 41%	6935
June	1st to 10th	Day	12% 6% 3 7% 5% 5% 4 4 6% 8% 40%	2900
	11th to 20th	Day	13% 7% 2 5% 4 5% 5% 4 6% 8% 40%	3000
	21st to end	Day	13% 7% 2 7% 5% 4 6% 3 6% 8% 38%	2880
July	1st to 10th	Day	12% 6% 2 7% 5% 5% 5% 4 6% 8% 39%	6360
	11th to 20th	Day	9% 6% 2 6% 4 4 5% 4 6% 9% 45%	6360
	21st to end	Day	7% 5% 2 6% 4 5% 5% 4 7% 10% 44%	6924
August	1st to 10th	Day	9% 5% 2 7% 6% 5% 6% 5% 8% 9% 38%	4273
	11th to 20th	Day	13% 6% 2 5% 4 5% 5% 4 5% 7% 43%	4174
	21st to end	Day	11% 6% 3 7% 5% 5% 5% 5% 5% 8% 42%	4526
September	1st to 10th	Day	12% 7% 2 6% 5% 5% 5% 4 7% 8% 40%	2340
	11th to 20th	Day	12% 5% 3 6% 4 4 4 3 5% 9% 46%	2200
	21st to end	Day	14% 8% 3 7% 5% 4 5% 4 6% 8% 36%	2200
October	1st to 10th	Day	16% 10% 3 6% 4 4 4 4 4 6% 39%	2114
	11th to 20th	Day	22% 13% 3 10% 6% 5% 4 3 5% 6% 23%	1834
	21st to end	Day	21% 15% 4 10% 5% 5% 5% 4 5% 6% 20%	1881
November	1st to 10th	Day	15% 15% 5% 7% 4 4 5% 4 5% 6% 30%	1762
	11th to 20th	Day	14% 18% 6% 12% 5% 5% 5% 4 5% 6% 21%	1800
	21st to end	Day	20% 18% 8% 11% 6% 4 3 2 4 5% 18%	1696
December	1st to 10th	Day	13% 23% 9% 18% 8% 5% 4 3 4 4 9%	2327
	11th to 20th	Day	14% 20% 11% 15% 8% 5% 4 3 3 3 14%	2625
	21st to end	Day	23% 25% 8% 13% 5% 4 4 2 3 3 10%	3862
January	1st to 10th	Night	11% 14% 7% 22% 8% 6% 4 3 4 4 17%	6348
	11th to 20th	Night	11% 15% 7% 18% 11% 10% 6% 4 4 3 9%	6360
	21st to end	Night	11% 19% 9% 18% 7% 5% 6% 4 5% 5% 12%	6996
February	1st to 10th	Night	10% 15% 6% 13% 10% 8% 5% 4 5% 5% 17%	2622
	11th to 20th	Night	13% 14% 6% 12% 7% 7% 5% 3 6% 6% 20%	2361
	21st to end	Night	14% 14% 5% 13% 7% 5% 4 3 5% 5% 24%	1980
March	1st to 10th	Night	14% 10% 4 8% 7% 4 4 3 4 6% 35%	5373
	11th to 20th	Night	11% 11% 5% 9% 4 4 4 4 6% 8% 35%	5035
	21st to end	Night	18% 13% 6% 9% 4 3 4 3 4 5% 31%	5408
April	1st to 10th	Night	15% 15% 4 8% 4 4 4 3 5% 6% 32%	2000
	11th to 20th	Night	18% 12% 3 6% 4 3 4 3 4 6% 36%	1849
	21st to end	Night	16% 11% 3 7% 4 3 4 3 5% 5% 38%	1800
May	1st to 10th	Night	14% 8% 3 8% 5% 4 4 4 5% 7% 38%	4033
	11th to 20th	Night	14% 10% 3 6% 5% 4 4 4 5% 7% 37%	3996
	21st to end	Night	18% 6% 2 5% 3 3 4 3 5% 6% 45%	3942
June	1st to 10th	Night	14% 6% 2 5% 5% 4 4 3 6% 8% 42%	1490
	11th to 20th	Night	9% 5% 2 6% 4 5% 5% 4 7% 9% 44%	1400
	21st to end	Night	10% 7% 2 7% 6% 4 5% 3 7% 7% 43%	1511
July	1st to 10th	Night	10% 7% 2 6% 4 4 5% 5% 6% 8% 42%	3618
	11th to 20th	Night	7% 6% 2 5% 3 4 4 3 5% 8% 52%	3617
	21st to end	Night	8% 5% 2 6% 4 3 5% 3 6% 7% 50%	3947
August	1st to 10th	Night	10% 5% 2 5% 4 5% 5% 4 6% 8% 46%	2874
	11th to 20th	Night	13% 6% 1 4 3 3 4 2 5% 8% 50%	2875
	21st to end	Night	9% 6% 2 6% 4 4 4 4 6% 7% 48%	3355
September	1st to 10th	Night	10% 5% 2 6% 3 4 4 4 5% 7% 51%	2040
	11th to 20th	Night	11% 6% 2 4 3 3 4 4 5% 7% 52%	2200
	21st to end	Night	15% 7% 3 8% 4 3 4 4 6% 6% 41%	2200
October	1st to 10th	Night	15% 10% 3 6% 4 4 3 2 4 6% 44%	2133
	11th to 20th	Night	19% 12% 4 9% 4 4 3 3 4 6% 32%	2342
	21st to end	Night	22% 15% 3 8% 4 3 4 3 5% 7% 27%	2717
November	1st to 10th	Night	11% 14% 3 8% 4 4 5% 3 5% 8% 34%	2536
	11th to 20th	Night	14% 16% 5% 11% 6% 5% 5% 4 5% 5% 24%	2600
	21st to end	Night	17% 19% 7% 11% 6% 4 4 3 5% 5% 19%	2700
December	1st to 10th	Night	11% 17% 6% 15% 7% 6% 6% 4 5% 6% 16%	4859
	11th to 20th	Night	12% 24% 7% 15% 7% 5% 5% 3 4 4 14%	5582
	21st to end	Night	23% 24% 7% 12% 5% 4 3 2 3 3 13%	7111

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	72% 8% 8% 9% 3	3618
	11th to 20th	Day	84% 6% 4 5% 2	3620
	21st to end	Day	69% 6% 12% 8% 4	3982
February	1st to 10th	Day	69% 5% 7% 14% 4 1	1776
	11th to 20th	Day	67% 6% 10% 11% 6% 1	1933
	21st to end	Day	64% 5% 10% 16% 4 1	1650
March	1st to 10th	Day	50% 7% 13% 16% 10% 3	4480
	11th to 20th	Day	51% 6% 16% 17% 8% 2	4925
	21st to end	Day	62% 5% 8% 16% 8% 1	5554
April	1st to 10th	Day	58% 5% 10% 16% 11% 1	2400
	11th to 20th	Day	51% 5% 9% 19% 15% 1	2498
	21st to end	Day	51% 6% 12% 16% 13% 2	2600
May	1st to 10th	Day	46% 7% 10% 18% 16% 3	5897
	11th to 20th	Day	42% 8% 11% 23% 15% 2	5900
	21st to end	Day	42% 6% 10% 19% 19% 3	6949
June	1st to 10th	Day	40% 7% 12% 26% 14% 1	2900
	11th to 20th	Day	40% 8% 13% 22% 16% 1	3000
	21st to end	Day	42% 7% 13% 23% 14% 1	2880
July	1st to 10th	Day	40% 5% 12% 25% 16% 1	6291
	11th to 20th	Day	35% 6% 12% 31% 16% 1	6285
	21st to end	Day	34% 6% 14% 29% 16% 1	6728
August	1st to 10th	Day	38% 8% 13% 26% 14% 1	4258
	11th to 20th	Day	39% 6% 12% 28% 14% 1	4213
	21st to end	Day	40% 6% 12% 27% 13% 1	4525
September	1st to 10th	Day	39% 8% 13% 24% 14% 2	2340
	11th to 20th	Day	36% 7% 15% 28% 15% 1	2200
	21st to end	Day	44% 7% 14% 25% 11%	2200
October	1st to 10th	Day	34% 5% 8% 32% 18% 3	3491
	11th to 20th	Day	51% 4 9% 22% 14%	3024
	21st to end	Day	61% 6% 11% 17% 6%	3081
November	1st to 10th	Day	52% 8% 12% 19% 9%	1762
	11th to 20th	Day	62% 8% 13% 12% 5%	1800
	21st to end	Day	70% 6% 12% 10% 3	1696
December	1st to 10th	Day	78% 5% 9% 7% 1	2327
	11th to 20th	Day	76% 5% 7% 10% 2	2621
	21st to end	Day	80% 4 5% 7% 4	3862
January	1st to 10th	Night	68% 6% 9% 12% 4 1	6348
	11th to 20th	Night	78% 6% 6% 6% 3	6360
	21st to end	Night	71% 7% 8% 10% 3	6996
February	1st to 10th	Night	65% 6% 9% 13% 6% 1	2622
	11th to 20th	Night	61% 7% 11% 15% 6%	2362
	21st to end	Night	62% 5% 11% 14% 7%	1980
March	1st to 10th	Night	49% 6% 15% 19% 9% 1	5373
	11th to 20th	Night	44% 6% 20% 20% 9% 1	5035
	21st to end	Night	53% 7% 11% 17% 10% 1	5408
April	1st to 10th	Night	52% 5% 9% 20% 13% 1	2000
	11th to 20th	Night	48% 5% 9% 19% 16% 2	1849
	21st to end	Night	46% 6% 11% 19% 16% 2	1800
May	1st to 10th	Night	43% 7% 11% 17% 19% 4	4060
	11th to 20th	Night	45% 7% 9% 25% 14% 1	3997
	21st to end	Night	38% 5% 10% 25% 17% 5%	3950
June	1st to 10th	Night	38% 7% 11% 24% 18% 1	1490
	11th to 20th	Night	34% 8% 15% 24% 18% 1	1400
	21st to end	Night	38% 7% 12% 26% 15% 2	1511
July	1st to 10th	Night	38% 4 13% 26% 14% 4	3580
	11th to 20th	Night	30% 5% 12% 34% 18% 1	3572
	21st to end	Night	32% 6% 15% 29% 17% 1	3832
August	1st to 10th	Night	35% 7% 15% 28% 14% 1	2865
	11th to 20th	Night	32% 5% 16% 32% 14%	2902
	21st to end	Night	34% 6% 14% 27% 17% 1	3355
September	1st to 10th	Night	31% 7% 14% 29% 17% 2	2040
	11th to 20th	Night	30% 6% 15% 34% 15%	2200
	21st to end	Night	42% 7% 13% 28% 9% 1	2200
October	1st to 10th	Night	33% 4 12% 33% 17% 1	3536
	11th to 20th	Night	45% 4 11% 25% 15%	3891
	21st to end	Night	51% 4 12% 26% 7%	4467
November	1st to 10th	Night	47% 7% 13% 22% 10% 1	2536
	11th to 20th	Night	60% 8% 12% 14% 6% 1	2600
	21st to end	Night	67% 6% 14% 11% 2 1	2700
December	1st to 10th	Night	66% 7% 14% 11% 2	4859
	11th to 20th	Night	72% 6% 8% 10% 4	5585
	21st to end	Night	76% 5% 6% 6% 6%	7111

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	25% 19% 37% 13% 5%	300
	11th to 20th	Day	26% 20% 41% 10% 3	300
	21st to end	Day	30% 17% 35% 14% 4 1	330
February	1st to 10th	Day	23% 29% 34% 10% 3 1	300
	11th to 20th	Day	24% 30% 36% 9% 1	300
	21st to end	Day	31% 28% 35% 5% 1	249
March	1st to 10th	Day	30% 26% 35% 8% 1	300
	11th to 20th	Day	25% 20% 34% 16% 5% 1	471
	21st to end	Day	32% 16% 28% 18% 5% 1	550
April	1st to 10th	Day	28% 19% 32% 15% 5% 1	500
	11th to 20th	Day	30% 21% 27% 14% 7% 2	492
	21st to end	Day	26% 16% 28% 21% 7% 2	500
May	1st to 10th	Day	24% 21% 30% 17% 5% 3	503
	11th to 20th	Day	19% 14% 31% 26% 7% 4	536
	21st to end	Day	17% 14% 32% 23% 11% 3	789
June	1st to 10th	Day	15% 16% 33% 25% 8% 3	730
	11th to 20th	Day	16% 15% 34% 22% 10% 3	730
	21st to end	Day	16% 13% 33% 26% 10% 2	730
July	1st to 10th	Day	8% 9% 32% 29% 16% 6%	721
	11th to 20th	Day	11% 10% 29% 30% 15% 5%	718
	21st to end	Day	9% 7% 32% 31% 14% 5%	794
August	1st to 10th	Day	9% 6% 26% 31% 18% 10%	553
	11th to 20th	Day	11% 7% 24% 31% 19% 9%	519
	21st to end	Day	16% 8% 24% 28% 17% 6%	583
September	1st to 10th	Day	16% 11% 26% 24% 16% 8%	530
	11th to 20th	Day	9% 9% 23% 27% 21% 10%	530
	21st to end	Day	19% 12% 22% 27% 15% 5%	530
October	1st to 10th	Day	25% 14% 26% 20% 14% 2	502
	11th to 20th	Day	29% 17% 35% 17% 3	424
	21st to end	Day	31% 24% 31% 9% 4 1	330
November	1st to 10th	Day	26% 20% 35% 14% 4 1	300
	11th to 20th	Day	28% 21% 36% 13% 2	300
	21st to end	Day	26% 27% 33% 13% 2	247
December	1st to 10th	Day	33% 21% 32% 8% 7% 1	200
	11th to 20th	Day	31% 24% 33% 11% 2	200
	21st to end	Day	31% 28% 30% 8% 3 1	303
January	1st to 10th	Night	21% 18% 36% 19% 6%	1000
	11th to 20th	Night	24% 17% 34% 20% 5% 1	1000
	21st to end	Night	19% 13% 40% 22% 5%	1100
February	1st to 10th	Night	19% 18% 38% 21% 4	822
	11th to 20th	Night	23% 20% 32% 21% 4	783
	21st to end	Night	23% 20% 36% 17% 3	660
March	1st to 10th	Night	20% 16% 36% 21% 7% 1	800
	11th to 20th	Night	21% 18% 34% 21% 6%	800
	21st to end	Night	23% 15% 34% 23% 5% 1	859
April	1st to 10th	Night	21% 16% 36% 23% 5%	600
	11th to 20th	Night	23% 15% 31% 25% 6%	593
	21st to end	Night	22% 10% 32% 30% 6% 1	600
May	1st to 10th	Night	15% 11% 36% 30% 8%	600
	11th to 20th	Night	17% 9% 32% 31% 11% 1	591
	21st to end	Night	15% 8% 30% 31% 15% 1	650
June	1st to 10th	Night	15% 8% 32% 34% 10%	490
	11th to 20th	Night	11% 7% 33% 38% 11%	400
	21st to end	Night	12% 6% 31% 34% 15% 1	511
July	1st to 10th	Night	8% 3 22% 45% 21% 2	585
	11th to 20th	Night	8% 4 25% 40% 21% 3	582
	21st to end	Night	11% 3 25% 40% 20% 1	657
August	1st to 10th	Night	10% 2 23% 41% 23% 2	597
	11th to 20th	Night	9% 5% 20% 36% 27% 3	594
	21st to end	Night	12% 5% 22% 39% 20% 1	660
September	1st to 10th	Night	11% 6% 25% 31% 22% 4	640
	11th to 20th	Night	12% 6% 22% 35% 22% 4	800
	21st to end	Night	18% 8% 27% 30% 15% 3	800
October	1st to 10th	Night	19% 11% 27% 29% 14% 1	773
	11th to 20th	Night	24% 14% 30% 25% 6% 1	760
	21st to end	Night	32% 14% 28% 22% 5%	836
November	1st to 10th	Night	20% 13% 37% 22% 8% 1	780
	11th to 20th	Night	24% 17% 36% 19% 3	800
	21st to end	Night	26% 15% 37% 19% 3	800
December	1st to 10th	Night	24% 17% 34% 20% 4	962
	11th to 20th	Night	25% 18% 38% 15% 4 1	1000
	21st to end	Night	30% 14% 34% 20% 2	1100

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