Lee Farm Observatory
Clear Sky Chart
History

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

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	10% 6% 2 7% 5% 5% 5% 4 6% 8% 41%	114249
	Night	11% 6% 2 6% 4 4 4 4 6% 8% 44%	114704
Transparency	Day	38% 8% 13% 17% 17% 8%	118043
	Night	36% 7% 12% 17% 18% 9%	119036
Seeing	Day	12% 9% 23% 25% 20% 11%	15862
	Night	12% 7% 23% 32% 21% 4	25970

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% 6% 2 5% 3 3 3 3 4 5% 56%	4056
	11th to 20th	Day	18% 7% 2 5% 4 3 4 3 5% 6% 42%	4060
	21st to end	Day	10% 6% 2 7% 4 4 4 3 5% 9% 45%	4664
February	1st to 10th	Day	15% 8% 2 7% 5% 4 4 3 4 6% 42%	2000
	11th to 20th	Day	15% 7% 2 5% 3 4 4 4 5% 6% 46%	1948
	21st to end	Day	14% 8% 2 5% 3 3 3 3 5% 5% 49%	1650
March	1st to 10th	Day	17% 9% 3 6% 4 4 4 3 5% 6% 39%	4888
	11th to 20th	Day	7% 5% 2 5% 4 4 5% 3 6% 8% 53%	4980
	21st to end	Day	13% 5% 2 5% 3 4 4 3 5% 7% 47%	5465
April	1st to 10th	Day	10% 6% 2 5% 4 5% 5% 4 6% 7% 48%	2200
	11th to 20th	Day	10% 6% 2 8% 4 5% 5% 4 5% 6% 46%	2191
	21st to end	Day	6% 3 2 6% 4 4 4 4 6% 8% 52%	2591
May	1st to 10th	Day	8% 5% 2 4 4 4 5% 5% 7% 10% 47%	4212
	11th to 20th	Day	12% 7% 2 8% 4 4 5% 4 7% 9% 37%	4081
	21st to end	Day	5% 4 2 6% 5% 4 6% 5% 8% 11% 45%	4476
June	1st to 10th	Day	10% 7% 2 8% 6% 5% 6% 6% 8% 9% 32%	2470
	11th to 20th	Day	5% 5% 2 7% 7% 8% 8% 6% 8% 10% 33%	2470
	21st to end	Day	8% 5% 2 9% 7% 7% 7% 6% 9% 10% 30%	2470
July	1st to 10th	Day	7% 7% 3 10% 8% 9% 9% 7% 10% 12% 18%	4120
	11th to 20th	Day	5% 4 2 7% 6% 6% 7% 6% 10% 14% 34%	4120
	21st to end	Day	5% 5% 3 10% 8% 8% 9% 7% 10% 13% 22%	4466
August	1st to 10th	Day	14% 6% 2 8% 7% 7% 7% 6% 9% 11% 23%	4104
	11th to 20th	Day	6% 5% 3 7% 5% 6% 7% 5% 9% 11% 37%	3833
	21st to end	Day	10% 7% 3 9% 6% 7% 8% 6% 8% 10% 28%	3966
September	1st to 10th	Day	11% 7% 3 9% 7% 7% 7% 5% 8% 8% 26%	2090
	11th to 20th	Day	12% 8% 3 8% 6% 7% 6% 5% 7% 8% 29%	2090
	21st to end	Day	11% 6% 3 6% 5% 6% 5% 4 5% 8% 41%	2090
October	1st to 10th	Day	12% 6% 2 7% 4 4 5% 3 4 7% 45%	2090
	11th to 20th	Day	8% 5% 2 3 3 4 3 4 5% 6% 59%	1936
	21st to end	Day	8% 4 1 6% 4 4 4 3 5% 6% 54%	2090
November	1st to 10th	Day	10% 5% 1 5% 4 3 4 3 6% 6% 53%	1957
	11th to 20th	Day	10% 5% 2 6% 3 3 3 3 4 5% 55%	1856
	21st to end	Day	12% 6% 2 6% 4 3 4 4 5% 6% 49%	1800
December	1st to 10th	Day	9% 5% 2 5% 4 4 4 3 4 6% 53%	2930
	11th to 20th	Day	13% 8% 2 7% 4 4 3 3 5% 6% 44%	3373
	21st to end	Day	16% 10% 4 7% 3 4 3 3 4 5% 42%	4466
January	1st to 10th	Night	9% 5% 2 5% 3 3 3 3 5% 6% 55%	5889
	11th to 20th	Night	13% 7% 3 7% 4 5% 5% 3 5% 7% 41%	5900
	21st to end	Night	14% 7% 2 6% 4 3 3 4 6% 7% 44%	6288
February	1st to 10th	Night	13% 7% 2 6% 4 4 4 4 6% 6% 44%	2400
	11th to 20th	Night	14% 8% 3 6% 4 3 3 3 5% 5% 47%	2338
	21st to end	Night	12% 7% 2 5% 4 4 3 4 5% 5% 50%	1980
March	1st to 10th	Night	20% 9% 3 5% 3 3 3 3 4 6% 41%	4962
	11th to 20th	Night	8% 5% 1 4 2 3 4 4 7% 9% 52%	4980
	21st to end	Night	12% 6% 2 5% 3 4 4 3 5% 7% 50%	5467
April	1st to 10th	Night	10% 6% 1 5% 4 3 5% 4 6% 8% 49%	2200
	11th to 20th	Night	7% 4 2 6% 3 4 5% 3 6% 6% 52%	2173
	21st to end	Night	5% 4 1 5% 3 3 4 4 5% 8% 56%	1806
May	1st to 10th	Night	6% 5% 2 5% 4 4 5% 4 8% 9% 48%	2903
	11th to 20th	Night	12% 6% 2 7% 5% 4 4 4 6% 7% 42%	2818
	21st to end	Night	6% 4 1 5% 4 4 5% 4 6% 10% 53%	3079
June	1st to 10th	Night	8% 5% 3 6% 7% 5% 6% 6% 8% 13% 33%	1710
	11th to 20th	Night	5% 5% 2 5% 6% 5% 6% 5% 9% 12% 40%	1710
	21st to end	Night	5% 4 2 8% 5% 6% 7% 6% 9% 12% 35%	1710
July	1st to 10th	Night	10% 9% 4 11% 7% 8% 7% 6% 8% 10% 21%	2840
	11th to 20th	Night	4 4 3 7% 6% 7% 6% 5% 10% 16% 32%	2840
	21st to end	Night	6% 6% 3 8% 7% 7% 8% 7% 9% 13% 27%	3087
August	1st to 10th	Night	13% 5% 2 7% 6% 6% 6% 5% 9% 12% 28%	2825
	11th to 20th	Night	5% 4 2 6% 5% 5% 7% 5% 8% 14% 39%	2991
	21st to end	Night	9% 7% 3 8% 5% 4 5% 4 8% 12% 34%	3655
September	1st to 10th	Night	11% 7% 3 8% 6% 5% 6% 5% 9% 10% 30%	2090
	11th to 20th	Night	12% 7% 3 8% 7% 6% 6% 5% 6% 8% 32%	2090
	21st to end	Night	9% 5% 3 6% 4 4 5% 3 7% 9% 45%	2090
October	1st to 10th	Night	10% 5% 2 5% 4 4 4 3 5% 7% 51%	2090
	11th to 20th	Night	9% 5% 2 4 3 3 3 2 4 6% 59%	2242
	21st to end	Night	8% 6% 2 6% 4 3 3 4 4 6% 54%	2508
November	1st to 10th	Night	11% 6% 2 6% 4 3 4 3 5% 6% 51%	2341
	11th to 20th	Night	12% 6% 2 5% 4 3 4 3 4 6% 52%	2540
	21st to end	Night	10% 7% 2 5% 4 4 3 4 5% 8% 48%	2600
December	1st to 10th	Night	10% 5% 2 4 4 3 3 3 5% 6% 55%	4250
	11th to 20th	Night	14% 9% 3 6% 3 4 4 3 4 5% 47%	4822
	21st to end	Night	16% 9% 2 6% 4 4 4 3 5% 6% 42%	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	32% 3 6% 13% 34% 12%	4057
	11th to 20th	Day	42% 4 4 13% 20% 16%	4060
	21st to end	Day	36% 5% 8% 19% 21% 12%	4664
February	1st to 10th	Day	42% 5% 8% 15% 21% 9%	2000
	11th to 20th	Day	38% 5% 7% 15% 24% 11%	1958
	21st to end	Day	38% 4 6% 17% 21% 14%	1650
March	1st to 10th	Day	44% 5% 9% 15% 17% 10%	4888
	11th to 20th	Day	28% 5% 11% 15% 19% 22%	4980
	21st to end	Day	35% 6% 9% 22% 17% 12%	5465
April	1st to 10th	Day	34% 7% 9% 20% 19% 12%	2200
	11th to 20th	Day	37% 5% 8% 16% 22% 11%	2191
	21st to end	Day	29% 5% 10% 25% 22% 8%	2591
May	1st to 10th	Day	29% 9% 14% 26% 17% 5%	4250
	11th to 20th	Day	42% 8% 12% 20% 15% 2	4085
	21st to end	Day	28% 9% 19% 25% 17% 1	4489
June	1st to 10th	Day	42% 11% 16% 18% 12% 1	2470
	11th to 20th	Day	41% 11% 18% 19% 9% 1	2470
	21st to end	Day	44% 13% 20% 16% 7% 1	2470
July	1st to 10th	Day	51% 17% 21% 8% 2	4055
	11th to 20th	Day	35% 17% 32% 13% 2	4051
	21st to end	Day	46% 19% 22% 11% 2	4324
August	1st to 10th	Day	48% 14% 22% 14% 2	4091
	11th to 20th	Day	34% 13% 20% 23% 10%	3872
	21st to end	Day	46% 13% 19% 20% 3 1	3966
September	1st to 10th	Day	50% 11% 14% 14% 9% 1	2090
	11th to 20th	Day	50% 9% 14% 15% 10% 2	2090
	21st to end	Day	40% 7% 12% 19% 17% 5%	2090
October	1st to 10th	Day	46% 4 6% 10% 16% 17%	3480
	11th to 20th	Day	17% 3 5% 19% 30% 26%	3189
	21st to end	Day	21% 4 6% 17% 36% 15%	3428
November	1st to 10th	Day	30% 5% 10% 18% 23% 14%	1957
	11th to 20th	Day	32% 4 7% 17% 26% 13%	1856
	21st to end	Day	34% 5% 9% 18% 24% 10%	1800
December	1st to 10th	Day	32% 5% 7% 17% 28% 12%	2930
	11th to 20th	Day	40% 5% 8% 19% 21% 7%	3370
	21st to end	Day	46% 4 5% 17% 24% 4	4466
January	1st to 10th	Night	31% 3 7% 12% 35% 13%	5889
	11th to 20th	Night	42% 4 6% 15% 25% 8%	5900
	21st to end	Night	39% 5% 9% 17% 21% 10%	6288
February	1st to 10th	Night	39% 5% 7% 16% 20% 12%	2400
	11th to 20th	Night	39% 5% 8% 16% 19% 14%	2339
	21st to end	Night	35% 5% 8% 20% 18% 14%	1980
March	1st to 10th	Night	45% 5% 8% 16% 16% 11%	4962
	11th to 20th	Night	26% 5% 11% 18% 21% 20%	4980
	21st to end	Night	34% 4 9% 18% 22% 13%	5467
April	1st to 10th	Night	32% 7% 13% 19% 18% 13%	2200
	11th to 20th	Night	31% 6% 8% 20% 23% 12%	2173
	21st to end	Night	26% 6% 12% 28% 20% 9%	1806
May	1st to 10th	Night	29% 11% 15% 24% 14% 6%	2930
	11th to 20th	Night	37% 10% 14% 24% 12% 3	2819
	21st to end	Night	25% 8% 14% 31% 20% 2	3087
June	1st to 10th	Night	38% 13% 19% 18% 11% 2	1710
	11th to 20th	Night	31% 13% 24% 21% 9%	1710
	21st to end	Night	35% 15% 25% 17% 7% 2	1710
July	1st to 10th	Night	53% 15% 20% 9% 2	2795
	11th to 20th	Night	35% 19% 31% 12% 3	2786
	21st to end	Night	44% 19% 24% 11% 2	2988
August	1st to 10th	Night	43% 14% 29% 12% 2	2814
	11th to 20th	Night	30% 14% 25% 23% 7%	3018
	21st to end	Night	39% 12% 26% 20% 3 1	3655
September	1st to 10th	Night	45% 12% 19% 14% 8% 1	2090
	11th to 20th	Night	48% 11% 14% 16% 10% 1	2090
	21st to end	Night	34% 7% 16% 22% 15% 6%	2090
October	1st to 10th	Night	42% 6% 9% 13% 17% 13%	3480
	11th to 20th	Night	17% 3 5% 17% 33% 26%	3715
	21st to end	Night	21% 4 5% 18% 35% 16%	4120
November	1st to 10th	Night	34% 4 8% 20% 20% 14%	2341
	11th to 20th	Night	34% 4 7% 16% 25% 13%	2540
	21st to end	Night	35% 5% 10% 19% 20% 12%	2600
December	1st to 10th	Night	31% 4 9% 17% 25% 15%	4250
	11th to 20th	Night	40% 6% 8% 18% 20% 9%	4824
	21st to end	Night	43% 5% 8% 16% 24% 4	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	14% 22% 46% 15% 3 1	300
	11th to 20th	Day	17% 15% 39% 20% 7% 2	300
	21st to end	Day	22% 24% 41% 11% 2	330
February	1st to 10th	Day	20% 14% 42% 18% 5%	300
	11th to 20th	Day	24% 17% 40% 16% 4	300
	21st to end	Day	15% 19% 44% 17% 5%	249
March	1st to 10th	Day	18% 17% 32% 22% 10%	477
	11th to 20th	Day	15% 15% 36% 21% 11% 2	500
	21st to end	Day	16% 16% 32% 26% 9% 1	550
April	1st to 10th	Day	17% 14% 31% 26% 11% 2	500
	11th to 20th	Day	11% 14% 30% 28% 13% 4	492
	21st to end	Day	6% 9% 26% 32% 19% 7%	500
May	1st to 10th	Day	6% 11% 26% 30% 21% 6%	503
	11th to 20th	Day	8% 5% 23% 31% 21% 11%	487
	21st to end	Day	10% 3 15% 28% 29% 15%	529
June	1st to 10th	Day	11% 2 13% 28% 30% 15%	490
	11th to 20th	Day	8% 3 15% 26% 27% 21%	490
	21st to end	Day	9% 2 10% 27% 31% 20%	490
July	1st to 10th	Day	6% 3 5% 20% 37% 29%	485
	11th to 20th	Day	6% 2 5% 18% 37% 32%	484
	21st to end	Day	8% 2 5% 22% 39% 23%	532
August	1st to 10th	Day	14% 2 8% 22% 31% 23%	486
	11th to 20th	Day	7% 2 6% 20% 36% 29%	479
	21st to end	Day	11% 3 9% 23% 31% 23%	539
September	1st to 10th	Day	14% 5% 10% 23% 26% 22%	490
	11th to 20th	Day	14% 3 13% 32% 27% 11%	490
	21st to end	Day	14% 3 12% 29% 29% 14%	490
October	1st to 10th	Day	14% 6% 27% 27% 20% 6%	490
	11th to 20th	Day	10% 4 26% 33% 20% 7%	490
	21st to end	Day	9% 8% 26% 33% 21% 3	539
November	1st to 10th	Day	17% 7% 27% 28% 17% 4	495
	11th to 20th	Day	16% 8% 42% 22% 8% 4	356
	21st to end	Day	11% 15% 37% 26% 8% 1	300
December	1st to 10th	Day	14% 17% 36% 21% 11% 1	300
	11th to 20th	Day	20% 13% 39% 21% 7%	300
	21st to end	Day	20% 10% 35% 28% 6% 2	330
January	1st to 10th	Night	15% 17% 35% 27% 6%	800
	11th to 20th	Night	14% 11% 29% 34% 10% 1	800
	21st to end	Night	19% 16% 36% 26% 4	880
February	1st to 10th	Night	17% 12% 31% 31% 8%	800
	11th to 20th	Night	17% 15% 31% 31% 5%	783
	21st to end	Night	14% 13% 35% 34% 3	660
March	1st to 10th	Night	15% 12% 31% 34% 8%	800
	11th to 20th	Night	15% 11% 33% 30% 11% 1	800
	21st to end	Night	10% 13% 35% 32% 10%	880
April	1st to 10th	Night	13% 10% 31% 35% 11%	800
	11th to 20th	Night	10% 8% 33% 36% 12%	791
	21st to end	Night	5% 7% 28% 39% 20% 1	600
May	1st to 10th	Night	7% 5% 27% 41% 20% 1	600
	11th to 20th	Night	8% 4 21% 38% 24% 5%	563
	21st to end	Night	8% 3 11% 38% 36% 5%	617
June	1st to 10th	Night	10% 2 11% 40% 32% 6%	570
	11th to 20th	Night	5% 2 12% 35% 32% 14%	570
	21st to end	Night	7% 2 8% 36% 40% 8%	570
July	1st to 10th	Night	7% 1 7% 29% 39% 17%	555
	11th to 20th	Night	7% 1 4 22% 46% 20%	552
	21st to end	Night	6% 1 4 29% 47% 13%	624
August	1st to 10th	Night	9% 1 6% 29% 44% 11%	567
	11th to 20th	Night	7% 1 4 23% 49% 16%	564
	21st to end	Night	12% 3 11% 25% 36% 12%	728
September	1st to 10th	Night	12% 2 9% 28% 38% 11%	760
	11th to 20th	Night	13% 3 14% 35% 31% 5%	760
	21st to end	Night	13% 2 14% 33% 34% 4	760
October	1st to 10th	Night	13% 4 21% 36% 24% 3	760
	11th to 20th	Night	12% 5% 23% 33% 24% 3	760
	21st to end	Night	10% 6% 24% 38% 21% 1	836
November	1st to 10th	Night	16% 6% 24% 34% 19% 1	780
	11th to 20th	Night	15% 10% 27% 34% 13% 2	800
	21st to end	Night	13% 9% 31% 31% 16% 1	800
December	1st to 10th	Night	15% 9% 31% 32% 13% 1	800
	11th to 20th	Night	18% 9% 30% 30% 13%	800
	21st to end	Night	19% 11% 27% 32% 11%	880

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