McCauley Lake Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the McCauley Lake Observatory clear sky chart from 2005-11-06 to 2025-05-26.

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	15% 13% 4 9% 6% 5% 4 3 5% 6% 29%	114579
	Night	14% 14% 5% 11% 6% 5% 4 3 5% 6% 27%	114643
Transparency	Day	54% 6% 10% 19% 9% 1	118168
	Night	55% 7% 11% 17% 9% 1	119217
Seeing	Day	23% 18% 32% 18% 7% 2	15283
	Night	22% 16% 32% 22% 8% 1	24303

By Month

Percentage Cloud Cover NE (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% 15% 7% 16% 9% 8% 6% 3 5% 5% 15%	3597
	11th to 20th	Day	10% 18% 6% 17% 9% 7% 6% 4 5% 5% 14%	3570
	21st to end	Day	12% 19% 11% 15% 6% 5% 4 3 4 4 18%	4420
February	1st to 10th	Day	10% 17% 6% 15% 11% 6% 4 3 4 4 20%	1800
	11th to 20th	Day	11% 15% 6% 14% 8% 6% 5% 3 4 5% 23%	1753
	21st to end	Day	14% 17% 5% 11% 7% 5% 5% 4 5% 5% 21%	1544
March	1st to 10th	Day	16% 17% 5% 12% 5% 4 4 3 4 6% 24%	4789
	11th to 20th	Day	10% 14% 5% 12% 6% 5% 5% 4 5% 7% 26%	4980
	21st to end	Day	15% 15% 6% 10% 5% 4 3 3 3 5% 34%	5550
April	1st to 10th	Day	19% 15% 4 9% 4 4 4 3 4 5% 28%	2400
	11th to 20th	Day	21% 12% 3 7% 3 4 4 3 4 4 34%	2409
	21st to end	Day	21% 13% 4 8% 4 4 3 3 5% 4 33%	2600
May	1st to 10th	Day	19% 10% 3 9% 4 5% 4 3 5% 6% 31%	4463
	11th to 20th	Day	16% 10% 3 7% 5% 4 4 3 5% 7% 35%	4538
	21st to end	Day	17% 8% 3 6% 4 4 4 3 5% 6% 39%	4907
June	1st to 10th	Day	21% 9% 3 7% 6% 5% 4 4 5% 7% 29%	2660
	11th to 20th	Day	16% 7% 2 6% 5% 5% 5% 4 6% 8% 36%	2692
	21st to end	Day	18% 7% 3 7% 5% 5% 6% 4 5% 8% 32%	2850
July	1st to 10th	Day	14% 8% 3 7% 5% 5% 5% 5% 7% 9% 32%	4578
	11th to 20th	Day	13% 7% 2 6% 5% 5% 5% 5% 6% 8% 38%	4440
	21st to end	Day	11% 6% 2 6% 5% 5% 4 4 6% 8% 44%	4814
August	1st to 10th	Day	15% 8% 3 8% 6% 5% 6% 5% 6% 8% 31%	4114
	11th to 20th	Day	17% 8% 3 6% 5% 5% 5% 4 6% 8% 33%	4044
	21st to end	Day	10% 6% 2 5% 5% 6% 5% 5% 7% 10% 39%	4309
September	1st to 10th	Day	16% 7% 3 5% 4 5% 4 4 6% 8% 39%	2280
	11th to 20th	Day	14% 8% 3 6% 4 4 4 3 5% 7% 43%	2265
	21st to end	Day	20% 9% 3 7% 5% 4 4 3 5% 6% 33%	1991
October	1st to 10th	Day	18% 11% 3 7% 4 5% 3 4 5% 6% 33%	1900
	11th to 20th	Day	22% 16% 3 9% 6% 5% 3 3 4 5% 24%	1900
	21st to end	Day	26% 17% 5% 9% 5% 4 4 2 3 5% 20%	2043
November	1st to 10th	Day	15% 16% 5% 9% 4 4 4 3 6% 6% 29%	1633
	11th to 20th	Day	15% 18% 6% 11% 7% 5% 4 3 4 5% 22%	1600
	21st to end	Day	19% 20% 8% 13% 6% 5% 4 3 3 4 14%	1600
December	1st to 10th	Day	14% 24% 11% 14% 7% 4 4 3 4 4 11%	2600
	11th to 20th	Day	15% 21% 7% 14% 8% 4 3 2 3 3 19%	2986
	21st to end	Day	18% 21% 8% 13% 7% 3 3 2 3 4 17%	3960
January	1st to 10th	Night	11% 18% 9% 17% 9% 6% 4 3 4 4 14%	6349
	11th to 20th	Night	8% 15% 6% 19% 10% 9% 7% 4 4 5% 13%	6315
	21st to end	Night	10% 17% 9% 17% 6% 7% 6% 3 4 4 16%	6522
February	1st to 10th	Night	11% 17% 6% 13% 8% 6% 4 4 5% 6% 19%	2600
	11th to 20th	Night	10% 16% 6% 13% 7% 6% 4 3 5% 5% 24%	2533
	21st to end	Night	11% 17% 5% 12% 7% 6% 5% 4 5% 6% 23%	2085
March	1st to 10th	Night	15% 15% 4 12% 6% 5% 4 4 3 5% 27%	5047
	11th to 20th	Night	9% 13% 5% 10% 6% 5% 6% 5% 7% 8% 26%	4979
	21st to end	Night	15% 13% 4 10% 6% 4 4 3 5% 6% 30%	5381
April	1st to 10th	Night	19% 15% 4 9% 4 3 3 3 5% 5% 30%	2000
	11th to 20th	Night	18% 11% 4 7% 5% 4 3 3 4 5% 36%	1956
	21st to end	Night	17% 11% 5% 8% 5% 5% 4 3 4 6% 31%	1800
May	1st to 10th	Night	18% 11% 3 6% 3 4 4 3 5% 7% 35%	2696
	11th to 20th	Night	17% 9% 3 7% 4 4 4 4 5% 7% 36%	2575
	21st to end	Night	12% 9% 3 8% 5% 4 4 3 5% 7% 41%	2784
June	1st to 10th	Night	16% 9% 3 6% 4 5% 6% 4 5% 8% 34%	1520
	11th to 20th	Night	14% 6% 2 6% 4 4 4 3 7% 8% 43%	1477
	21st to end	Night	16% 8% 3 7% 5% 4 4 5% 6% 6% 37%	1330
July	1st to 10th	Night	11% 8% 3 7% 5% 5% 5% 3 5% 7% 40%	2365
	11th to 20th	Night	14% 8% 2 6% 4 4 4 3 5% 6% 43%	2520
	21st to end	Night	9% 7% 2 5% 4 4 5% 3 6% 8% 48%	2739
August	1st to 10th	Night	11% 6% 2 6% 4 4 4 4 7% 9% 44%	2813
	11th to 20th	Night	16% 6% 3 6% 3 4 5% 3 5% 7% 42%	2785
	21st to end	Night	14% 5% 1 4 3 4 4 4 6% 9% 45%	3314
September	1st to 10th	Night	12% 6% 1 5% 4 4 5% 4 5% 8% 46%	1900
	11th to 20th	Night	14% 8% 2 6% 3 3 3 3 4 6% 49%	1912
	21st to end	Night	17% 9% 3 6% 3 4 4 3 5% 6% 40%	2188
October	1st to 10th	Night	19% 10% 3 6% 4 3 4 3 5% 6% 38%	2280
	11th to 20th	Night	19% 14% 4 7% 5% 5% 4 3 4 6% 28%	2280
	21st to end	Night	23% 17% 4 8% 4 5% 4 3 4 5% 22%	2553
November	1st to 10th	Night	16% 15% 4 8% 5% 4 4 4 4 5% 29%	2661
	11th to 20th	Night	15% 15% 5% 10% 7% 6% 5% 3 4 6% 24%	2800
	21st to end	Night	18% 19% 7% 12% 7% 5% 4 3 4 6% 16%	2800
December	1st to 10th	Night	16% 22% 7% 14% 8% 6% 4 4 4 5% 10%	4580
	11th to 20th	Night	15% 21% 8% 13% 6% 4 3 3 4 4 19%	5208
	21st to end	Night	21% 20% 7% 14% 7% 4 3 2 3 4 14%	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	69% 7% 8% 10% 6%	3598
	11th to 20th	Day	70% 6% 8% 8% 7% 1	3600
	21st to end	Day	70% 4 5% 11% 8% 2	4420
February	1st to 10th	Day	67% 6% 8% 13% 6% 1	1800
	11th to 20th	Day	63% 5% 9% 12% 9% 1	1762
	21st to end	Day	63% 6% 8% 12% 10% 1	1544
March	1st to 10th	Day	61% 5% 10% 9% 14% 2	4792
	11th to 20th	Day	55% 7% 14% 15% 7% 2	4980
	21st to end	Day	54% 4 7% 19% 14% 3	5550
April	1st to 10th	Day	58% 4 8% 18% 10% 3	2400
	11th to 20th	Day	53% 5% 7% 21% 13% 1	2409
	21st to end	Day	54% 5% 7% 17% 13% 3	2600
May	1st to 10th	Day	50% 8% 9% 20% 10% 2	4476
	11th to 20th	Day	46% 7% 12% 24% 10% 1	4542
	21st to end	Day	44% 8% 11% 18% 16% 3	4907
June	1st to 10th	Day	52% 8% 10% 18% 10% 1	2660
	11th to 20th	Day	44% 8% 12% 22% 13% 2	2692
	21st to end	Day	49% 8% 12% 20% 11% 1	2850
July	1st to 10th	Day	44% 8% 14% 23% 10% 1	4508
	11th to 20th	Day	40% 8% 15% 28% 9%	4365
	21st to end	Day	37% 7% 11% 31% 13%	4661
August	1st to 10th	Day	48% 8% 12% 23% 8%	4100
	11th to 20th	Day	45% 9% 15% 25% 6% 1	4083
	21st to end	Day	37% 7% 14% 31% 10%	4309
September	1st to 10th	Day	41% 8% 14% 26% 11% 1	2280
	11th to 20th	Day	40% 6% 14% 27% 13%	2265
	21st to end	Day	50% 8% 11% 23% 9%	1991
October	1st to 10th	Day	50% 7% 10% 24% 8% 1	3160
	11th to 20th	Day	55% 5% 9% 19% 10%	3140
	21st to end	Day	63% 6% 11% 15% 4	3345
November	1st to 10th	Day	54% 7% 13% 19% 6% 1	1633
	11th to 20th	Day	63% 7% 10% 12% 7% 1	1600
	21st to end	Day	73% 6% 8% 9% 4 1	1600
December	1st to 10th	Day	77% 6% 8% 8% 2	2600
	11th to 20th	Day	70% 5% 5% 11% 8% 1	2986
	21st to end	Day	72% 4 5% 13% 6%	3960
January	1st to 10th	Night	72% 6% 8% 9% 5%	6349
	11th to 20th	Night	68% 10% 8% 8% 6% 1	6360
	21st to end	Night	68% 7% 6% 8% 9% 1	6522
February	1st to 10th	Night	65% 5% 11% 12% 6% 1	2600
	11th to 20th	Night	61% 5% 9% 16% 8% 1	2535
	21st to end	Night	59% 6% 9% 18% 7% 1	2085
March	1st to 10th	Night	58% 5% 9% 13% 10% 4	5057
	11th to 20th	Night	51% 9% 16% 15% 7% 1	4980
	21st to end	Night	52% 6% 9% 18% 13% 2	5381
April	1st to 10th	Night	57% 4 8% 18% 12% 2	2000
	11th to 20th	Night	50% 5% 8% 23% 14% 1	1956
	21st to end	Night	55% 4 8% 16% 15% 2	1800
May	1st to 10th	Night	45% 9% 12% 23% 10% 2	2702
	11th to 20th	Night	43% 10% 11% 24% 10% 1	2576
	21st to end	Night	40% 9% 12% 23% 15% 2	2785
June	1st to 10th	Night	46% 8% 12% 21% 12% 1	1520
	11th to 20th	Night	38% 7% 13% 26% 15% 1	1477
	21st to end	Night	45% 8% 15% 19% 12%	1330
July	1st to 10th	Night	41% 9% 16% 19% 14% 1	2325
	11th to 20th	Night	39% 6% 13% 31% 10%	2472
	21st to end	Night	34% 7% 13% 31% 15%	2651
August	1st to 10th	Night	36% 7% 16% 26% 13% 1	2803
	11th to 20th	Night	39% 9% 18% 26% 8% 1	2812
	21st to end	Night	35% 7% 15% 30% 10% 2	3314
September	1st to 10th	Night	34% 7% 17% 28% 12% 1	1900
	11th to 20th	Night	38% 5% 14% 31% 13%	1912
	21st to end	Night	44% 7% 13% 26% 10%	2188
October	1st to 10th	Night	40% 5% 13% 27% 14%	3800
	11th to 20th	Night	51% 5% 13% 19% 11%	3779
	21st to end	Night	56% 6% 18% 15% 5%	4201
November	1st to 10th	Night	55% 6% 14% 18% 7%	2661
	11th to 20th	Night	60% 7% 11% 15% 6% 1	2800
	21st to end	Night	69% 7% 9% 11% 3	2800
December	1st to 10th	Night	74% 6% 10% 7% 2	4580
	11th to 20th	Night	68% 5% 6% 11% 9%	5208
	21st to end	Night	75% 4 5% 10% 6%	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	21% 24% 39% 13% 4 1	200
	11th to 20th	Day	28% 16% 38% 16% 2 2	200
	21st to end	Day	25% 22% 33% 17% 4	320
February	1st to 10th	Day	30% 28% 32% 8% 2 1	300
	11th to 20th	Day	28% 31% 34% 6% 1	300
	21st to end	Day	34% 21% 34% 9% 1	308
March	1st to 10th	Day	26% 23% 36% 14% 2	500
	11th to 20th	Day	25% 21% 36% 14% 3 1	500
	21st to end	Day	29% 25% 29% 13% 4	550
April	1st to 10th	Day	28% 22% 32% 13% 5% 1	500
	11th to 20th	Day	35% 22% 21% 13% 6% 2	492
	21st to end	Day	30% 19% 33% 12% 5% 1	500
May	1st to 10th	Day	24% 20% 30% 18% 7% 2	503
	11th to 20th	Day	21% 19% 31% 20% 8% 2	527
	21st to end	Day	20% 19% 30% 20% 9% 2	549
June	1st to 10th	Day	21% 14% 38% 20% 6% 1	490
	11th to 20th	Day	18% 15% 35% 20% 10% 2	522
	21st to end	Day	20% 15% 34% 21% 7% 1	680
July	1st to 10th	Day	15% 10% 37% 29% 8% 2	571
	11th to 20th	Day	13% 14% 36% 25% 11% 2	484
	21st to end	Day	12% 12% 33% 30% 10% 2	532
August	1st to 10th	Day	15% 11% 29% 29% 10% 5%	486
	11th to 20th	Day	15% 9% 26% 30% 14% 5%	479
	21st to end	Day	16% 12% 33% 24% 11% 4	539
September	1st to 10th	Day	17% 12% 33% 21% 12% 5%	490
	11th to 20th	Day	17% 9% 26% 23% 17% 8%	490
	21st to end	Day	22% 17% 24% 21% 12% 4	490
October	1st to 10th	Day	24% 19% 26% 18% 10% 3	490
	11th to 20th	Day	29% 26% 26% 14% 4 1	490
	21st to end	Day	35% 19% 28% 14% 3	514
November	1st to 10th	Day	30% 22% 32% 12% 4	267
	11th to 20th	Day	31% 20% 34% 12% 3 1	200
	21st to end	Day	30% 22% 40% 6% 3 1	200
December	1st to 10th	Day	27% 23% 34% 16% 1	200
	11th to 20th	Day	33% 26% 29% 10% 1 2	200
	21st to end	Day	30% 23% 26% 16% 3 1	220
January	1st to 10th	Night	23% 22% 36% 16% 3 1	800
	11th to 20th	Night	25% 19% 32% 18% 5% 1	800
	21st to end	Night	17% 17% 36% 22% 6% 1	880
February	1st to 10th	Night	20% 22% 39% 15% 3	800
	11th to 20th	Night	20% 21% 39% 16% 3	783
	21st to end	Night	22% 23% 33% 20% 2	660
March	1st to 10th	Night	21% 23% 35% 18% 3	800
	11th to 20th	Night	20% 20% 37% 17% 5% 1	800
	21st to end	Night	20% 18% 35% 21% 6%	880
April	1st to 10th	Night	27% 17% 33% 19% 4	800
	11th to 20th	Night	24% 20% 28% 18% 9%	772
	21st to end	Night	22% 15% 30% 22% 11%	600
May	1st to 10th	Night	22% 10% 28% 29% 9% 1	463
	11th to 20th	Night	21% 12% 32% 27% 8%	394
	21st to end	Night	15% 12% 35% 27% 10%	423
June	1st to 10th	Night	18% 9% 37% 29% 6% 1	380
	11th to 20th	Night	18% 10% 30% 30% 11%	380
	21st to end	Night	20% 10% 28% 31% 10% 1	380
July	1st to 10th	Night	11% 7% 32% 35% 14% 1	370
	11th to 20th	Night	15% 7% 30% 32% 15% 1	368
	21st to end	Night	9% 7% 37% 33% 12% 1	416
August	1st to 10th	Night	12% 5% 27% 35% 19% 2	555
	11th to 20th	Night	11% 5% 25% 36% 21% 3	564
	21st to end	Night	15% 8% 29% 30% 15% 2	739
September	1st to 10th	Night	14% 9% 30% 29% 16% 2	760
	11th to 20th	Night	17% 9% 27% 25% 18% 4	760
	21st to end	Night	21% 11% 23% 25% 17% 2	760
October	1st to 10th	Night	25% 14% 28% 22% 10% 2	760
	11th to 20th	Night	27% 18% 31% 16% 7% 1	760
	21st to end	Night	31% 18% 29% 18% 4	836
November	1st to 10th	Night	27% 17% 31% 19% 5% 1	780
	11th to 20th	Night	24% 23% 31% 15% 7%	800
	21st to end	Night	32% 18% 29% 18% 3 1	800
December	1st to 10th	Night	29% 21% 34% 11% 5%	800
	11th to 20th	Night	25% 22% 33% 17% 3 1	800
	21st to end	Night	33% 19% 29% 15% 3 1	880

Page generated at 2025-05-23 09:20:57. Version ac07

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