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 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	12% 6% 2 6% 4 4 5% 4 6% 8% 42%	126200
	Night	12% 6% 2 6% 4 4 4 4 6% 8% 44%	123181
Transparency	Day	38% 6% 11% 20% 18% 6%	129998
	Night	37% 5% 11% 20% 20% 7%	127451
Seeing	Day	15% 8% 20% 24% 21% 13%	16088
	Night	14% 6% 21% 30% 23% 4	26062

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 5% 3 3 3 3 4 4 49%	4056
	11th to 20th	Day	11% 7% 3 4 3 3 3 2 5% 6% 53%	4060
	21st to end	Day	21% 10% 3 6% 3 3 3 3 4 5% 39%	4724
February	1st to 10th	Day	15% 7% 3 7% 4 5% 4 4 6% 6% 39%	2000
	11th to 20th	Day	20% 8% 2 6% 4 3 4 3 3 6% 42%	1948
	21st to end	Day	16% 8% 3 5% 4 4 4 4 5% 6% 41%	1650
March	1st to 10th	Day	20% 10% 3 7% 4 3 4 3 5% 5% 35%	4608
	11th to 20th	Day	9% 6% 2 6% 4 4 4 3 5% 7% 51%	4980
	21st to end	Day	16% 6% 2 7% 4 4 4 3 4 6% 44%	5465
April	1st to 10th	Day	11% 6% 2 6% 5% 4 4 4 5% 6% 47%	2264
	11th to 20th	Day	10% 6% 2 6% 4 3 4 4 5% 7% 49%	2374
	21st to end	Day	8% 4 2 6% 4 5% 4 4 6% 7% 50%	2534
May	1st to 10th	Day	13% 6% 2 7% 5% 5% 5% 5% 6% 9% 38%	5862
	11th to 20th	Day	17% 7% 3 7% 5% 5% 5% 4 7% 9% 31%	5861
	21st to end	Day	10% 5% 2 6% 5% 5% 5% 5% 6% 9% 42%	6433
June	1st to 10th	Day	9% 7% 3 8% 6% 5% 6% 5% 8% 9% 34%	2600
	11th to 20th	Day	4 4 2 5% 6% 5% 5% 5% 9% 12% 43%	2600
	21st to end	Day	6% 5% 2 6% 6% 7% 7% 6% 9% 9% 38%	2600
July	1st to 10th	Day	8% 5% 2 8% 6% 6% 7% 6% 9% 11% 31%	5900
	11th to 20th	Day	5% 4 2 6% 5% 6% 6% 5% 8% 13% 39%	5900
	21st to end	Day	8% 7% 3 8% 7% 7% 7% 6% 8% 11% 26%	6424
August	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 6% 8% 11% 36%	4217
	11th to 20th	Day	7% 4 2 6% 4 5% 5% 4 6% 9% 48%	3849
	21st to end	Day	8% 4 2 6% 4 4 5% 5% 7% 9% 47%	4312
September	1st to 10th	Day	11% 6% 2 7% 6% 4 5% 4 6% 8% 40%	2210
	11th to 20th	Day	14% 6% 2 6% 5% 5% 5% 5% 5% 8% 41%	2200
	21st to end	Day	13% 6% 2 6% 3 4 4 3 4 6% 48%	2200
October	1st to 10th	Day	12% 5% 2 4 3 3 4 3 4 5% 55%	2123
	11th to 20th	Day	11% 4 2 4 2 3 3 3 5% 6% 57%	2005
	21st to end	Day	12% 5% 2 4 3 4 3 3 4 6% 55%	2090
November	1st to 10th	Day	13% 6% 2 6% 4 2 3 3 4 4 53%	1782
	11th to 20th	Day	13% 6% 1 4 3 3 3 3 5% 7% 53%	1800
	21st to end	Day	11% 6% 2 6% 4 4 3 3 6% 7% 47%	1800
December	1st to 10th	Day	16% 7% 2 5% 4 3 3 3 5% 6% 47%	2930
	11th to 20th	Day	16% 8% 2 6% 4 4 4 3 4 5% 43%	3373
	21st to end	Day	21% 7% 3 5% 3 3 4 3 5% 6% 41%	4466
January	1st to 10th	Night	14% 6% 2 6% 3 4 4 3 5% 6% 48%	5889
	11th to 20th	Night	12% 7% 3 5% 4 4 4 3 5% 6% 47%	5900
	21st to end	Night	18% 10% 2 5% 4 4 3 3 4 5% 42%	6214
February	1st to 10th	Night	14% 7% 2 7% 5% 4 4 3 5% 6% 44%	2400
	11th to 20th	Night	20% 6% 2 6% 4 3 3 3 4 6% 43%	2338
	21st to end	Night	16% 7% 2 5% 5% 3 4 3 4 7% 43%	1980
March	1st to 10th	Night	21% 9% 2 6% 4 4 4 3 5% 7% 36%	5243
	11th to 20th	Night	11% 5% 1 5% 3 3 4 4 5% 7% 52%	4980
	21st to end	Night	14% 5% 2 6% 5% 5% 4 4 5% 6% 46%	5467
April	1st to 10th	Night	11% 5% 3 5% 5% 4 4 4 5% 7% 48%	2151
	11th to 20th	Night	11% 7% 2 4 4 4 3 3 5% 8% 51%	1975
	21st to end	Night	7% 5% 2 5% 3 4 4 4 5% 7% 52%	1865
May	1st to 10th	Night	9% 6% 2 7% 5% 5% 5% 4 7% 11% 38%	4033
	11th to 20th	Night	18% 6% 2 7% 5% 5% 5% 4 6% 9% 32%	4032
	21st to end	Night	10% 5% 1 5% 4 4 4 4 7% 11% 45%	4421
June	1st to 10th	Night	6% 5% 3 7% 5% 5% 6% 5% 8% 13% 37%	1800
	11th to 20th	Night	4 4 2 5% 5% 5% 6% 4 9% 12% 46%	1800
	21st to end	Night	6% 4 2 7% 5% 5% 5% 5% 8% 12% 42%	1800
July	1st to 10th	Night	9% 5% 2 8% 7% 6% 6% 5% 8% 11% 32%	4058
	11th to 20th	Night	5% 4 2 6% 4 5% 6% 6% 10% 13% 37%	4057
	21st to end	Night	9% 6% 3 9% 6% 6% 7% 6% 8% 11% 28%	4427
August	1st to 10th	Night	7% 4 2 8% 6% 6% 6% 5% 8% 13% 35%	2915
	11th to 20th	Night	7% 4 2 6% 4 4 5% 4 6% 9% 50%	3199
	21st to end	Night	8% 5% 2 5% 4 4 4 3 6% 9% 52%	3586
September	1st to 10th	Night	8% 5% 2 6% 5% 5% 4 5% 8% 10% 42%	2170
	11th to 20th	Night	11% 6% 3 6% 4 3 4 3 6% 8% 47%	2200
	21st to end	Night	11% 5% 2 6% 4 4 3 4 5% 7% 48%	2200
October	1st to 10th	Night	10% 5% 2 5% 4 3 4 3 5% 6% 55%	2124
	11th to 20th	Night	12% 5% 1 4 2 3 3 3 4 4 59%	2175
	21st to end	Night	10% 5% 2 4 3 3 4 3 4 5% 57%	2508
November	1st to 10th	Night	13% 5% 1 4 3 3 3 3 4 6% 55%	2512
	11th to 20th	Night	14% 5% 2 5% 3 3 3 3 4 6% 53%	2600
	21st to end	Night	13% 6% 2 5% 4 3 4 3 4 6% 49%	2600
December	1st to 10th	Night	14% 6% 2 5% 3 3 3 2 4 6% 52%	4250
	11th to 20th	Night	17% 6% 2 5% 3 3 3 3 5% 7% 47%	4822
	21st to end	Night	20% 7% 2 6% 3 4 4 3 5% 5% 41%	6490

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	38% 3 5% 21% 25% 7%	4057
	11th to 20th	Day	33% 3 4 22% 29% 9%	4060
	21st to end	Day	48% 3 5% 15% 21% 8%	4724
February	1st to 10th	Day	44% 4 8% 18% 21% 6%	2000
	11th to 20th	Day	47% 2 5% 19% 21% 6%	1958
	21st to end	Day	44% 4 8% 15% 21% 8%	1650
March	1st to 10th	Day	50% 4 7% 11% 21% 6%	4608
	11th to 20th	Day	33% 4 7% 15% 23% 18%	4980
	21st to end	Day	42% 3 6% 20% 21% 7%	5465
April	1st to 10th	Day	37% 5% 10% 22% 20% 7%	2264
	11th to 20th	Day	34% 4 8% 20% 25% 9%	2374
	21st to end	Day	31% 5% 11% 24% 25% 3	2534
May	1st to 10th	Day	41% 7% 13% 21% 16% 2	5897
	11th to 20th	Day	47% 7% 17% 20% 8% 1	5865
	21st to end	Day	37% 7% 12% 24% 18% 1	6447
June	1st to 10th	Day	43% 9% 14% 20% 11% 2	2600
	11th to 20th	Day	30% 10% 21% 23% 15% 1	2600
	21st to end	Day	36% 12% 17% 20% 12% 3	2600
July	1st to 10th	Day	41% 11% 20% 22% 5% 1	5835
	11th to 20th	Day	33% 12% 29% 21% 4 1	5831
	21st to end	Day	46% 13% 18% 19% 3	6240
August	1st to 10th	Day	38% 8% 16% 30% 8%	4204
	11th to 20th	Day	32% 7% 15% 31% 14% 2	3885
	21st to end	Day	31% 8% 16% 24% 17% 3	4311
September	1st to 10th	Day	40% 8% 14% 17% 16% 5%	2210
	11th to 20th	Day	41% 6% 10% 19% 21% 2	2200
	21st to end	Day	37% 4 10% 21% 22% 6%	2200
October	1st to 10th	Day	26% 3 6% 14% 37% 14%	3513
	11th to 20th	Day	18% 2 4 18% 30% 28%	3310
	21st to end	Day	25% 2 5% 20% 35% 12%	3428
November	1st to 10th	Day	35% 4 6% 15% 30% 11%	1782
	11th to 20th	Day	32% 4 6% 20% 27% 12%	1800
	21st to end	Day	36% 5% 8% 18% 26% 7%	1800
December	1st to 10th	Day	39% 4 6% 19% 26% 7%	2930
	11th to 20th	Day	44% 4 7% 18% 21% 6%	3370
	21st to end	Day	43% 4 7% 23% 21% 2	4466
January	1st to 10th	Night	37% 3 6% 20% 28% 6%	5889
	11th to 20th	Night	37% 4 5% 19% 27% 8%	5900
	21st to end	Night	44% 3 5% 14% 28% 6%	6214
February	1st to 10th	Night	42% 3 7% 19% 19% 10%	2400
	11th to 20th	Night	44% 3 5% 18% 21% 9%	2339
	21st to end	Night	42% 4 8% 16% 18% 12%	1980
March	1st to 10th	Night	48% 4 9% 14% 16% 9%	5243
	11th to 20th	Night	32% 4 8% 14% 23% 19%	4980
	21st to end	Night	39% 3 6% 20% 22% 10%	5467
April	1st to 10th	Night	35% 5% 9% 21% 21% 9%	2151
	11th to 20th	Night	34% 3 6% 22% 24% 10%	1975
	21st to end	Night	30% 5% 11% 26% 22% 6%	1865
May	1st to 10th	Night	37% 10% 16% 22% 14% 2	4060
	11th to 20th	Night	47% 8% 14% 21% 9% 1	4033
	21st to end	Night	32% 7% 15% 24% 19% 2	4429
June	1st to 10th	Night	36% 10% 17% 20% 15% 1	1800
	11th to 20th	Night	27% 10% 20% 28% 13% 2	1800
	21st to end	Night	33% 9% 20% 21% 15% 3	1800
July	1st to 10th	Night	43% 10% 21% 20% 5% 1	4015
	11th to 20th	Night	32% 13% 32% 18% 4 1	4006
	21st to end	Night	47% 12% 21% 16% 3	4297
August	1st to 10th	Night	38% 10% 16% 29% 7%	2904
	11th to 20th	Night	30% 7% 16% 31% 14% 1	3229
	21st to end	Night	29% 8% 15% 23% 22% 3	3586
September	1st to 10th	Night	35% 10% 15% 19% 16% 5%	2170
	11th to 20th	Night	36% 8% 12% 22% 19% 4	2200
	21st to end	Night	34% 7% 12% 19% 21% 8%	2200
October	1st to 10th	Night	27% 4 7% 14% 32% 15%	3514
	11th to 20th	Night	18% 2 3 16% 30% 31%	3609
	21st to end	Night	22% 3 4 22% 32% 18%	4120
November	1st to 10th	Night	32% 3 8% 18% 26% 13%	2512
	11th to 20th	Night	34% 3 5% 17% 28% 13%	2600
	21st to end	Night	36% 4 7% 18% 27% 9%	2600
December	1st to 10th	Night	35% 4 6% 19% 28% 9%	4250
	11th to 20th	Night	39% 4 10% 16% 24% 7%	4824
	21st to end	Night	45% 3 6% 24% 20% 2	6490

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	22% 20% 39% 17% 2	300
	11th to 20th	Day	20% 20% 34% 17% 7% 2	300
	21st to end	Day	21% 19% 41% 13% 5% 1	330
February	1st to 10th	Day	18% 17% 39% 20% 5%	300
	11th to 20th	Day	34% 15% 32% 16% 2	300
	21st to end	Day	22% 18% 38% 19% 4	249
March	1st to 10th	Day	20% 18% 31% 23% 7% 1	350
	11th to 20th	Day	20% 12% 34% 25% 9% 1	500
	21st to end	Day	19% 13% 33% 25% 9% 1	550
April	1st to 10th	Day	16% 14% 26% 28% 13% 2	500
	11th to 20th	Day	11% 12% 30% 26% 17% 3	492
	21st to end	Day	12% 8% 25% 29% 20% 6%	500
May	1st to 10th	Day	11% 11% 24% 28% 18% 7%	503
	11th to 20th	Day	13% 3 21% 28% 23% 12%	527
	21st to end	Day	13% 4 15% 27% 25% 16%	573
June	1st to 10th	Day	12% 2 13% 28% 27% 18%	530
	11th to 20th	Day	6% 2 12% 25% 27% 28%	530
	21st to end	Day	8% 2 8% 27% 29% 25%	530
July	1st to 10th	Day	9% 3 5% 16% 37% 30%	525
	11th to 20th	Day	7% 2 3 14% 37% 38%	524
	21st to end	Day	6% 2 4 18% 39% 32%	576
August	1st to 10th	Day	13% 2 8% 23% 27% 28%	526
	11th to 20th	Day	8% 2 6% 19% 36% 29%	519
	21st to end	Day	15% 2 6% 25% 33% 20%	583
September	1st to 10th	Day	11% 4 10% 25% 30% 21%	530
	11th to 20th	Day	15% 3 7% 25% 32% 18%	530
	21st to end	Day	14% 2 14% 28% 32% 11%	530
October	1st to 10th	Day	17% 4 16% 31% 24% 9%	502
	11th to 20th	Day	14% 6% 19% 29% 22% 9%	490
	21st to end	Day	16% 6% 20% 31% 22% 6%	539
November	1st to 10th	Day	21% 7% 23% 27% 17% 5%	320
	11th to 20th	Day	16% 11% 37% 25% 9% 2	300
	21st to end	Day	9% 16% 34% 28% 12% 2	300
December	1st to 10th	Day	26% 11% 38% 16% 9%	300
	11th to 20th	Day	25% 12% 31% 24% 8% 1	300
	21st to end	Day	29% 11% 34% 18% 7% 1	330
January	1st to 10th	Night	22% 14% 34% 25% 5% 1	800
	11th to 20th	Night	19% 13% 32% 27% 8%	800
	21st to end	Night	21% 10% 39% 25% 4	880
February	1st to 10th	Night	16% 10% 33% 35% 7%	800
	11th to 20th	Night	24% 11% 36% 27% 3	783
	21st to end	Night	20% 12% 37% 27% 3	660
March	1st to 10th	Night	18% 9% 33% 33% 7%	800
	11th to 20th	Night	16% 10% 35% 28% 11% 1	800
	21st to end	Night	16% 10% 32% 31% 12%	880
April	1st to 10th	Night	13% 11% 30% 33% 13% 1	751
	11th to 20th	Night	15% 7% 28% 32% 16% 1	593
	21st to end	Night	11% 6% 24% 36% 21% 2	600
May	1st to 10th	Night	9% 5% 23% 37% 24% 2	600
	11th to 20th	Night	12% 3 18% 35% 27% 5%	591
	21st to end	Night	12% 2 10% 34% 35% 7%	650
June	1st to 10th	Night	7% 2 8% 38% 39% 6%	600
	11th to 20th	Night	4 3 10% 31% 37% 16%	600
	21st to end	Night	6% 1 5% 33% 45% 10%	600
July	1st to 10th	Night	8% 2 4 22% 48% 16%	585
	11th to 20th	Night	7% 2 3 18% 53% 18%	582
	21st to end	Night	5% 1 4 26% 48% 15%	657
August	1st to 10th	Night	8% 3 3 24% 48% 14%	597
	11th to 20th	Night	10% 2 5% 26% 47% 11%	594
	21st to end	Night	13% 3 5% 26% 40% 12%	660
September	1st to 10th	Night	9% 2 6% 27% 44% 12%	770
	11th to 20th	Night	14% 2 6% 29% 43% 6%	800
	21st to end	Night	13% 3 10% 32% 38% 4	800
October	1st to 10th	Night	10% 2 16% 37% 28% 7%	773
	11th to 20th	Night	13% 5% 20% 34% 23% 4	760
	21st to end	Night	13% 4 22% 36% 22% 3	836
November	1st to 10th	Night	16% 6% 21% 35% 22% 1	780
	11th to 20th	Night	18% 7% 28% 31% 15% 1	800
	21st to end	Night	15% 8% 28% 36% 12% 1	800
December	1st to 10th	Night	17% 10% 32% 30% 10% 1	800
	11th to 20th	Night	22% 6% 31% 31% 10% 1	800
	21st to end	Night	25% 9% 29% 30% 8%	880

Page generated at 2025-10-01 01:39:08. Version ac07

Copyright © 2025 Attilla Danko.