Ford Lake Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ford Lake Observatory clear sky chart from 2005-11-06 to 2024-01-12.

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% 6% 2 5% 5% 4 5% 5% 6% 8% 42%	74854
	Night	12% 6% 2 5% 5% 3 4 5% 5% 8% 45%	74870
Transparency	Day	39% 7% 11% 19% 18% 5%	74577
	Night	37% 6% 11% 20% 20% 7%	74666
Seeing	Day	15% 7% 20% 23% 21% 13%	13746
	Night	14% 6% 21% 30% 23% 5%	23834

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	16% 8% 3 4 4 2 4 3 5% 6% 45%	1706
	11th to 20th	Day	14% 7% 3 5% 4 3 4 3 4 6% 46%	1629
	21st to end	Day	17% 8% 2 6% 5% 3 3 4 3 6% 44%	1886
February	1st to 10th	Day	16% 7% 2 7% 5% 3 5% 4 6% 7% 39%	1800
	11th to 20th	Day	20% 6% 3 5% 4 3 4 3 3 5% 43%	1748
	21st to end	Day	18% 8% 3 5% 4 3 4 4 5% 6% 39%	1480
March	1st to 10th	Day	13% 6% 3 5% 5% 2 3 4 5% 6% 48%	1844
	11th to 20th	Day	17% 7% 2 6% 5% 3 4 4 5% 7% 39%	1980
	21st to end	Day	15% 6% 2 6% 4 3 4 5% 5% 8% 41%	2178
April	1st to 10th	Day	11% 6% 2 5% 6% 3 4 4 5% 6% 47%	2037
	11th to 20th	Day	11% 6% 3 5% 5% 2 4 4 5% 8% 47%	2134
	21st to end	Day	8% 4 2 4 5% 4 4 4 6% 7% 52%	2280
May	1st to 10th	Day	8% 5% 2 4 5% 3 5% 5% 6% 8% 49%	2340
	11th to 20th	Day	10% 5% 3 5% 7% 4 5% 5% 6% 8% 41%	2301
	21st to end	Day	9% 4 2 5% 6% 4 6% 5% 6% 9% 42%	2531
June	1st to 10th	Day	10% 6% 3 7% 8% 4 6% 6% 7% 9% 34%	2340
	11th to 20th	Day	5% 4 2 5% 6% 4 7% 5% 9% 11% 43%	2340
	21st to end	Day	6% 4 3 5% 7% 5% 7% 7% 9% 10% 37%	2340
July	1st to 10th	Day	9% 5% 3 7% 8% 6% 7% 7% 10% 11% 27%	2340
	11th to 20th	Day	8% 5% 3 6% 7% 5% 8% 7% 8% 12% 31%	2340
	21st to end	Day	5% 4 3 6% 8% 5% 7% 7% 8% 13% 32%	2547
August	1st to 10th	Day	7% 5% 2 6% 8% 6% 7% 9% 9% 13% 28%	2324
	11th to 20th	Day	11% 6% 3 6% 6% 5% 8% 7% 6% 9% 32%	2125
	21st to end	Day	14% 6% 3 7% 7% 4 7% 7% 7% 9% 30%	2376
September	1st to 10th	Day	11% 6% 3 6% 6% 4 5% 5% 6% 8% 39%	1990
	11th to 20th	Day	14% 6% 2 5% 6% 4 5% 6% 5% 7% 41%	1980
	21st to end	Day	13% 6% 3 5% 4 3 4 4 4 6% 48%	1980
October	1st to 10th	Day	13% 5% 1 4 4 2 4 4 4 6% 55%	1980
	11th to 20th	Day	12% 4 2 4 3 2 3 4 5% 7% 55%	1900
	21st to end	Day	12% 4 2 4 4 3 4 3 4 6% 53%	1980
November	1st to 10th	Day	12% 5% 2 5% 4 2 3 3 4 5% 56%	1691
	11th to 20th	Day	13% 5% 2 4 3 2 3 4 5% 7% 53%	1710
	21st to end	Day	12% 6% 2 5% 5% 3 4 4 6% 7% 46%	1710
December	1st to 10th	Day	17% 8% 3 5% 5% 3 5% 5% 5% 6% 39%	1710
	11th to 20th	Day	17% 8% 3 5% 4 3 4 3 4 5% 44%	2153
	21st to end	Day	20% 7% 3 5% 5% 3 4 4 6% 7% 37%	3124
January	1st to 10th	Night	16% 8% 2 6% 5% 3 3 3 4 6% 44%	2459
	11th to 20th	Night	15% 8% 3 4 4 3 4 4 5% 7% 42%	2360
	21st to end	Night	17% 6% 3 5% 4 3 3 4 4 6% 46%	2466
February	1st to 10th	Night	15% 7% 3 6% 6% 3 4 3 5% 7% 43%	2160
	11th to 20th	Night	19% 6% 2 5% 4 2 4 3 4 6% 44%	2098
	21st to end	Night	18% 7% 3 4 5% 3 4 4 4 7% 41%	1776
March	1st to 10th	Night	12% 6% 2 4 5% 3 4 3 4 7% 49%	2114
	11th to 20th	Night	15% 6% 2 4 5% 3 4 5% 5% 8% 43%	1980
	21st to end	Night	12% 6% 2 4 5% 3 4 5% 6% 8% 46%	2178
April	1st to 10th	Night	10% 5% 3 4 5% 3 4 5% 5% 8% 49%	1937
	11th to 20th	Night	12% 7% 2 3 5% 3 3 4 4 8% 50%	1775
	21st to end	Night	7% 4 3 4 4 3 4 4 5% 8% 54%	1679
May	1st to 10th	Night	8% 3 1 4 5% 3 5% 4 6% 9% 52%	1620
	11th to 20th	Night	9% 5% 3 5% 6% 4 5% 6% 6% 9% 41%	1592
	21st to end	Night	9% 6% 2 5% 5% 4 4 6% 6% 9% 46%	1744
June	1st to 10th	Night	6% 5% 3 6% 5% 4 6% 7% 8% 14% 37%	1620
	11th to 20th	Night	3 3 2 4 6% 3 6% 5% 7% 12% 48%	1620
	21st to end	Night	6% 4 2 6% 7% 3 5% 6% 8% 12% 42%	1620
July	1st to 10th	Night	7% 5% 3 6% 9% 4 7% 6% 8% 11% 35%	1620
	11th to 20th	Night	6% 4 3 6% 6% 4 7% 8% 10% 13% 34%	1620
	21st to end	Night	6% 4 3 5% 6% 4 7% 9% 9% 12% 36%	1771
August	1st to 10th	Night	5% 4 2 8% 8% 5% 6% 7% 8% 11% 37%	1611
	11th to 20th	Night	8% 5% 3 6% 7% 4 6% 7% 6% 10% 38%	1775
	21st to end	Night	11% 7% 3 6% 6% 4 5% 5% 7% 10% 36%	1980
September	1st to 10th	Night	8% 5% 2 6% 5% 4 5% 6% 7% 10% 42%	1952
	11th to 20th	Night	11% 5% 3 5% 5% 2 5% 4 5% 8% 47%	1980
	21st to end	Night	10% 5% 2 5% 5% 3 3 4 5% 7% 50%	1980
October	1st to 10th	Night	10% 4 2 5% 4 2 3 4 5% 5% 56%	1980
	11th to 20th	Night	12% 5% 2 4 3 2 3 3 4 4 57%	2060
	21st to end	Night	11% 5% 2 3 4 2 4 4 4 5% 56%	2376
November	1st to 10th	Night	12% 5% 1 4 3 2 3 3 4 6% 57%	2383
	11th to 20th	Night	13% 5% 2 5% 4 2 3 3 4 6% 54%	2470
	21st to end	Night	13% 5% 2 5% 4 3 4 4 4 7% 48%	2470
December	1st to 10th	Night	16% 6% 2 5% 4 3 4 4 5% 7% 43%	2470
	11th to 20th	Night	17% 7% 2 4 3 2 3 3 4 6% 48%	3042
	21st to end	Night	18% 9% 3 6% 5% 3 5% 5% 6% 6% 34%	4532

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	40% 4 9% 19% 23% 5%	1707
	11th to 20th	Day	41% 3 7% 20% 22% 6%	1629
	21st to end	Day	43% 4 5% 18% 23% 7%	1886
February	1st to 10th	Day	45% 4 8% 17% 20% 5%	1800
	11th to 20th	Day	46% 3 5% 19% 21% 6%	1758
	21st to end	Day	46% 5% 8% 14% 19% 8%	1480
March	1st to 10th	Day	38% 5% 7% 18% 22% 10%	1844
	11th to 20th	Day	44% 6% 11% 15% 17% 6%	1980
	21st to end	Day	42% 5% 10% 22% 18% 4	2178
April	1st to 10th	Day	37% 5% 10% 21% 20% 7%	2037
	11th to 20th	Day	36% 4 7% 20% 24% 9%	2134
	21st to end	Day	30% 5% 11% 24% 26% 3	2280
May	1st to 10th	Day	31% 6% 12% 21% 25% 4	2340
	11th to 20th	Day	39% 8% 16% 21% 14% 2	2305
	21st to end	Day	36% 8% 12% 26% 18% 2	2531
June	1st to 10th	Day	44% 9% 14% 20% 11% 2	2340
	11th to 20th	Day	31% 10% 21% 24% 13% 1	2340
	21st to end	Day	37% 12% 17% 20% 11% 3	2340
July	1st to 10th	Day	44% 12% 19% 16% 8% 1	2275
	11th to 20th	Day	41% 13% 19% 17% 8% 1	2271
	21st to end	Day	38% 14% 19% 21% 7%	2405
August	1st to 10th	Day	41% 12% 21% 22% 4	2311
	11th to 20th	Day	46% 10% 18% 16% 8% 2	2125
	21st to end	Day	47% 9% 16% 15% 9% 2	2376
September	1st to 10th	Day	41% 9% 14% 16% 15% 5%	1990
	11th to 20th	Day	41% 6% 11% 18% 22% 2	1980
	21st to end	Day	38% 4 9% 21% 22% 7%	1980
October	1st to 10th	Day	31% 4 10% 20% 27% 7%	1980
	11th to 20th	Day	30% 4 7% 25% 25% 8%	1900
	21st to end	Day	33% 3 6% 16% 31% 11%	1980
November	1st to 10th	Day	33% 3 6% 15% 31% 11%	1691
	11th to 20th	Day	32% 4 6% 20% 27% 12%	1710
	21st to end	Day	37% 5% 8% 18% 25% 7%	1710
December	1st to 10th	Day	45% 5% 7% 19% 17% 6%	1710
	11th to 20th	Day	43% 3 5% 16% 24% 8%	2150
	21st to end	Day	45% 5% 8% 18% 21% 3	3124
January	1st to 10th	Night	43% 3 7% 17% 24% 6%	2459
	11th to 20th	Night	42% 3 8% 20% 21% 6%	2360
	21st to end	Night	41% 3 5% 19% 25% 7%	2466
February	1st to 10th	Night	43% 3 7% 19% 19% 9%	2160
	11th to 20th	Night	42% 4 5% 18% 22% 8%	2099
	21st to end	Night	43% 5% 9% 16% 18% 10%	1776
March	1st to 10th	Night	36% 3 8% 19% 20% 13%	2114
	11th to 20th	Night	39% 6% 12% 16% 20% 7%	1980
	21st to end	Night	36% 5% 9% 23% 21% 7%	2178
April	1st to 10th	Night	34% 5% 9% 21% 22% 9%	1937
	11th to 20th	Night	35% 3 6% 22% 23% 10%	1775
	21st to end	Night	29% 5% 11% 27% 22% 7%	1679
May	1st to 10th	Night	29% 6% 12% 24% 24% 5%	1620
	11th to 20th	Night	37% 9% 15% 20% 17% 2	1593
	21st to end	Night	35% 8% 11% 24% 20% 3	1745
June	1st to 10th	Night	36% 11% 18% 21% 14% 1	1620
	11th to 20th	Night	26% 10% 21% 29% 12% 2	1620
	21st to end	Night	33% 9% 20% 21% 14% 3	1620
July	1st to 10th	Night	39% 11% 20% 19% 9% 2	1575
	11th to 20th	Night	35% 12% 26% 16% 9% 2	1566
	21st to end	Night	34% 12% 24% 22% 7% 1	1672
August	1st to 10th	Night	36% 13% 19% 26% 5% 1	1600
	11th to 20th	Night	37% 10% 20% 20% 12% 1	1775
	21st to end	Night	41% 12% 17% 18% 11% 3	1980
September	1st to 10th	Night	34% 10% 15% 19% 17% 5%	1952
	11th to 20th	Night	36% 8% 12% 21% 19% 4	1980
	21st to end	Night	33% 7% 11% 19% 21% 9%	1980
October	1st to 10th	Night	30% 4 10% 21% 26% 9%	1980
	11th to 20th	Night	31% 3 6% 21% 28% 10%	2060
	21st to end	Night	30% 4 5% 17% 30% 14%	2376
November	1st to 10th	Night	30% 3 7% 18% 27% 14%	2383
	11th to 20th	Night	34% 3 5% 18% 27% 14%	2470
	21st to end	Night	37% 4 7% 18% 26% 9%	2470
December	1st to 10th	Night	40% 5% 9% 18% 21% 6%	2470
	11th to 20th	Night	39% 3 6% 15% 28% 7%	3044
	21st to end	Night	50% 3 8% 18% 17% 3	4532

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	21% 19% 38% 20% 2	270
	11th to 20th	Day	20% 19% 33% 18% 8% 2	243
	21st to end	Day	20% 21% 41% 12% 5% 2	264
February	1st to 10th	Day	21% 19% 37% 18% 5%	240
	11th to 20th	Day	32% 15% 34% 17% 2	240
	21st to end	Day	23% 17% 39% 18% 3	198
March	1st to 10th	Day	17% 14% 35% 25% 8% 1	284
	11th to 20th	Day	21% 11% 32% 24% 9% 1	420
	21st to end	Day	18% 13% 32% 26% 10% 1	462
April	1st to 10th	Day	15% 13% 25% 28% 15% 3	420
	11th to 20th	Day	12% 13% 29% 24% 19% 3	412
	21st to end	Day	12% 9% 26% 27% 19% 6%	420
May	1st to 10th	Day	10% 11% 23% 28% 20% 8%	423
	11th to 20th	Day	11% 2 19% 28% 26% 14%	447
	21st to end	Day	13% 3 11% 26% 28% 19%	485
June	1st to 10th	Day	13% 1 12% 26% 28% 19%	450
	11th to 20th	Day	7% 2 11% 23% 26% 32%	450
	21st to end	Day	9% 2 9% 28% 25% 26%	450
July	1st to 10th	Day	9% 3 5% 15% 37% 31%	445
	11th to 20th	Day	8% 2 3 15% 33% 39%	444
	21st to end	Day	6% 2 5% 17% 36% 35%	488
August	1st to 10th	Day	10% 2 7% 22% 28% 32%	446
	11th to 20th	Day	9% 2 5% 18% 36% 30%	439
	21st to end	Day	16% 2 5% 24% 34% 20%	495
September	1st to 10th	Day	11% 3 8% 22% 33% 23%	450
	11th to 20th	Day	14% 3 7% 26% 32% 18%	450
	21st to end	Day	14% 2 13% 27% 33% 12%	450
October	1st to 10th	Day	18% 4 15% 30% 24% 10%	450
	11th to 20th	Day	16% 6% 18% 27% 23% 10%	450
	21st to end	Day	17% 6% 20% 31% 21% 6%	495
November	1st to 10th	Day	20% 7% 24% 27% 17% 6%	289
	11th to 20th	Day	16% 11% 37% 25% 10% 1	270
	21st to end	Day	9% 14% 35% 28% 12% 2	270
December	1st to 10th	Day	27% 11% 36% 17% 10%	270
	11th to 20th	Day	26% 12% 29% 24% 8% 1	270
	21st to end	Day	29% 11% 36% 16% 7% 1	297
January	1st to 10th	Night	23% 13% 34% 25% 5% 1	760
	11th to 20th	Night	19% 12% 31% 28% 9%	727
	21st to end	Night	21% 10% 38% 26% 5%	792
February	1st to 10th	Night	17% 10% 33% 33% 7%	720
	11th to 20th	Night	22% 10% 37% 27% 3	703
	21st to end	Night	22% 11% 37% 27% 4	592
March	1st to 10th	Night	16% 9% 34% 33% 7%	720
	11th to 20th	Night	17% 9% 34% 27% 12% 1	720
	21st to end	Night	15% 10% 30% 31% 13% 1	792
April	1st to 10th	Night	13% 10% 29% 34% 14% 1	677
	11th to 20th	Night	15% 7% 28% 31% 17% 2	533
	21st to end	Night	10% 6% 25% 35% 22% 2	540
May	1st to 10th	Night	9% 5% 24% 35% 26% 2	540
	11th to 20th	Night	11% 3 17% 35% 29% 5%	531
	21st to end	Night	12% 1 8% 33% 38% 8%	584
June	1st to 10th	Night	7% 2 7% 38% 39% 6%	540
	11th to 20th	Night	4 2 10% 30% 38% 17%	540
	21st to end	Night	6% 1 5% 34% 43% 11%	540
July	1st to 10th	Night	8% 1 4 22% 48% 17%	525
	11th to 20th	Night	7% 1 3 19% 52% 18%	522
	21st to end	Night	5% 2 5% 26% 47% 15%	591
August	1st to 10th	Night	6% 2 3 23% 50% 15%	537
	11th to 20th	Night	10% 2 4 24% 49% 11%	534
	21st to end	Night	14% 4 5% 24% 41% 13%	594
September	1st to 10th	Night	9% 2 5% 23% 47% 13%	692
	11th to 20th	Night	13% 2 6% 29% 43% 7%	720
	21st to end	Night	13% 3 10% 32% 39% 4	720
October	1st to 10th	Night	11% 2 16% 35% 28% 7%	720
	11th to 20th	Night	14% 5% 20% 34% 23% 4	720
	21st to end	Night	14% 5% 23% 35% 21% 3	792
November	1st to 10th	Night	15% 6% 21% 35% 22% 1	740
	11th to 20th	Night	17% 7% 29% 31% 14% 1	760
	21st to end	Night	16% 7% 28% 36% 12% 1	760
December	1st to 10th	Night	17% 10% 31% 30% 11% 1	760
	11th to 20th	Night	22% 6% 31% 31% 10% 1	760
	21st to end	Night	25% 9% 29% 29% 8%	836

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