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 2026-09-14.

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	9% 6% 2 7% 5% 5% 6% 5% 7% 9% 39%	148542
	Night	10% 6% 2 6% 4 4 5% 4 6% 9% 43%	142539
Transparency	Day	37% 9% 15% 18% 15% 7%	152292
	Night	36% 8% 14% 17% 16% 8%	146848
Seeing	Day	12% 8% 23% 25% 21% 10%	17718
	Night	12% 7% 23% 33% 21% 4	27698

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% 7% 2 5% 4 4 3 3 5% 7% 50%	5258
	11th to 20th	Day	16% 7% 2 5% 4 3 4 4 6% 7% 42%	5280
	21st to end	Day	9% 7% 3 7% 4 4 4 4 6% 9% 43%	6034
February	1st to 10th	Day	14% 7% 2 6% 5% 4 4 3 4 7% 43%	2100
	11th to 20th	Day	14% 7% 2 5% 3 4 4 4 5% 6% 46%	2048
	21st to end	Day	13% 8% 2 5% 3 3 3 3 5% 5% 49%	1730
March	1st to 10th	Day	17% 9% 3 6% 4 4 4 3 5% 6% 40%	4997
	11th to 20th	Day	7% 5% 2 5% 4 4 5% 3 6% 8% 53%	5090
	21st to end	Day	12% 5% 2 5% 3 4 4 3 5% 7% 48%	5586
April	1st to 10th	Day	10% 6% 2 5% 4 5% 5% 4 6% 7% 47%	2310
	11th to 20th	Day	10% 6% 2 8% 4 5% 4 4 5% 6% 48%	2302
	21st to end	Day	6% 3 2 6% 5% 4 4 4 6% 8% 52%	2721
May	1st to 10th	Day	12% 7% 3 7% 4 4 5% 4 6% 9% 38%	7616
	11th to 20th	Day	12% 6% 2 6% 4 4 4 4 7% 10% 42%	7626
	21st to end	Day	5% 4 2 7% 6% 5% 7% 6% 9% 12% 38%	8391
June	1st to 10th	Day	10% 7% 2 8% 6% 5% 6% 6% 9% 9% 32%	2730
	11th to 20th	Day	6% 6% 2 8% 7% 8% 8% 6% 9% 11% 31%	2730
	21st to end	Day	7% 5% 2 8% 7% 7% 7% 6% 8% 11% 31%	2730
July	1st to 10th	Day	5% 6% 3 9% 7% 8% 9% 7% 10% 12% 25%	7680
	11th to 20th	Day	3 3 2 6% 6% 6% 7% 6% 11% 15% 34%	7680
	21st to end	Day	4 4 2 7% 6% 7% 8% 7% 11% 16% 30%	8340
August	1st to 10th	Day	13% 7% 2 8% 8% 7% 8% 6% 9% 12% 20%	6014
	11th to 20th	Day	4 4 2 6% 5% 6% 7% 6% 10% 13% 37%	5649
	21st to end	Day	9% 7% 3 9% 6% 7% 8% 6% 8% 11% 26%	5775
September	1st to 10th	Day	10% 7% 3 9% 7% 7% 7% 5% 8% 9% 27%	2310
	11th to 20th	Day	11% 7% 3 8% 6% 7% 6% 5% 7% 8% 32%	2233
	21st to end	Day	11% 6% 3 7% 5% 5% 5% 4 5% 8% 41%	2200
October	1st to 10th	Day	12% 6% 2 7% 4 5% 5% 4 4 7% 44%	2200
	11th to 20th	Day	8% 5% 2 3 3 3 3 4 5% 6% 59%	2038
	21st to end	Day	8% 4 1 6% 4 4 4 3 4 7% 54%	2200
November	1st to 10th	Day	9% 5% 1 4 4 3 4 3 6% 6% 54%	2057
	11th to 20th	Day	10% 5% 2 6% 3 3 3 3 4 5% 57%	1949
	21st to end	Day	11% 5% 2 6% 4 4 5% 4 5% 6% 49%	1890
December	1st to 10th	Day	10% 5% 2 5% 4 4 4 3 4 6% 52%	3020
	11th to 20th	Day	13% 8% 2 7% 4 4 3 3 5% 6% 45%	3463
	21st to end	Day	16% 10% 4 7% 3 4 3 3 4 5% 42%	4565
January	1st to 10th	Night	9% 7% 2 5% 3 3 4 3 5% 8% 51%	7635
	11th to 20th	Night	12% 6% 3 6% 4 4 5% 3 6% 7% 43%	7680
	21st to end	Night	15% 7% 2 6% 3 3 3 3 5% 7% 44%	8143
February	1st to 10th	Night	13% 7% 2 5% 4 4 4 4 5% 6% 45%	2520
	11th to 20th	Night	14% 8% 2 6% 4 4 3 3 5% 5% 47%	2458
	21st to end	Night	12% 6% 2 5% 4 4 3 4 5% 6% 50%	2076
March	1st to 10th	Night	20% 9% 3 5% 3 3 3 3 4 6% 42%	5073
	11th to 20th	Night	8% 5% 1 4 2 3 4 4 7% 9% 53%	5090
	21st to end	Night	12% 5% 2 5% 3 4 4 3 5% 7% 51%	5588
April	1st to 10th	Night	10% 6% 1 5% 4 4 4 4 6% 8% 48%	2310
	11th to 20th	Night	7% 4 2 6% 3 4 5% 3 5% 6% 54%	2283
	21st to end	Night	5% 4 1 5% 3 4 5% 4 5% 8% 55%	1896
May	1st to 10th	Night	9% 8% 3 7% 5% 5% 5% 4 7% 9% 39%	5237
	11th to 20th	Night	12% 6% 2 6% 4 4 4 4 6% 9% 43%	5234
	21st to end	Night	7% 5% 3 8% 5% 5% 6% 5% 7% 11% 38%	5761
June	1st to 10th	Night	9% 6% 3 6% 6% 5% 6% 5% 8% 13% 33%	1890
	11th to 20th	Night	6% 5% 2 6% 6% 5% 6% 6% 9% 12% 38%	1890
	21st to end	Night	5% 4 2 8% 5% 6% 7% 6% 9% 12% 36%	1890
July	1st to 10th	Night	8% 7% 3 10% 6% 6% 7% 6% 8% 12% 26%	5278
	11th to 20th	Night	4 4 2 8% 6% 7% 7% 6% 10% 15% 30%	5277
	21st to end	Night	5% 5% 2 8% 6% 6% 7% 6% 9% 14% 32%	5736
August	1st to 10th	Night	13% 7% 3 8% 7% 6% 6% 6% 9% 12% 24%	4135
	11th to 20th	Night	4 3 2 6% 5% 6% 7% 6% 10% 16% 37%	4391
	21st to end	Night	9% 8% 3 8% 6% 5% 6% 5% 8% 12% 30%	5301
September	1st to 10th	Night	11% 7% 2 8% 6% 5% 6% 5% 8% 10% 31%	2310
	11th to 20th	Night	12% 6% 3 7% 7% 6% 6% 5% 6% 9% 33%	2234
	21st to end	Night	9% 5% 3 6% 4 4 5% 3 7% 9% 44%	2200
October	1st to 10th	Night	10% 5% 3 6% 4 4 4 3 5% 7% 50%	2200
	11th to 20th	Night	9% 5% 2 4 3 3 3 2 4 6% 60%	2360
	21st to end	Night	9% 6% 2 6% 4 3 3 4 4 6% 54%	2640
November	1st to 10th	Night	11% 6% 2 6% 3 3 4 3 5% 6% 52%	2461
	11th to 20th	Night	11% 6% 2 4 4 3 3 3 3 6% 54%	2667
	21st to end	Night	10% 7% 2 5% 4 4 4 4 5% 8% 48%	2730
December	1st to 10th	Night	11% 6% 2 4 4 3 3 3 5% 6% 54%	4380
	11th to 20th	Night	14% 9% 3 6% 3 4 3 3 4 5% 48%	4952
	21st to end	Night	15% 9% 2 6% 4 4 4 3 5% 6% 43%	6633

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	34% 4 8% 15% 29% 9%	5259
	11th to 20th	Day	40% 4 5% 14% 19% 17%	5280
	21st to end	Day	37% 5% 8% 20% 18% 12%	6034
February	1st to 10th	Day	41% 5% 8% 15% 21% 10%	2100
	11th to 20th	Day	38% 5% 7% 16% 24% 10%	2058
	21st to end	Day	37% 4 7% 17% 21% 14%	1730
March	1st to 10th	Day	44% 5% 9% 16% 17% 10%	4997
	11th to 20th	Day	28% 5% 11% 15% 19% 21%	5090
	21st to end	Day	34% 6% 9% 22% 17% 12%	5586
April	1st to 10th	Day	35% 7% 9% 20% 18% 11%	2310
	11th to 20th	Day	36% 5% 8% 16% 24% 11%	2302
	21st to end	Day	29% 5% 10% 25% 22% 8%	2721
May	1st to 10th	Day	40% 8% 13% 21% 15% 3	7651
	11th to 20th	Day	35% 7% 15% 27% 13% 2	7630
	21st to end	Day	34% 12% 21% 18% 14% 1	8405
June	1st to 10th	Day	42% 11% 16% 17% 12% 2	2730
	11th to 20th	Day	42% 12% 19% 18% 9% 1	2730
	21st to end	Day	42% 14% 20% 16% 7% 1	2730
July	1st to 10th	Day	44% 17% 25% 12% 2	7615
	11th to 20th	Day	32% 17% 36% 14% 1	7611
	21st to end	Day	34% 16% 25% 21% 3	8156
August	1st to 10th	Day	49% 15% 22% 12% 2	6001
	11th to 20th	Day	30% 14% 27% 22% 7%	5688
	21st to end	Day	46% 14% 20% 16% 3	5775
September	1st to 10th	Day	49% 12% 14% 15% 10% 1	2310
	11th to 20th	Day	47% 9% 15% 17% 11% 2	2233
	21st to end	Day	40% 7% 12% 20% 17% 5%	2200
October	1st to 10th	Day	46% 4 7% 11% 16% 16%	3590
	11th to 20th	Day	17% 3 5% 19% 30% 26%	3291
	21st to end	Day	22% 4 6% 17% 36% 15%	3538
November	1st to 10th	Day	30% 5% 10% 17% 23% 14%	2057
	11th to 20th	Day	30% 4 7% 18% 28% 13%	1949
	21st to end	Day	34% 5% 9% 18% 23% 10%	1890
December	1st to 10th	Day	33% 5% 7% 16% 27% 11%	3020
	11th to 20th	Day	39% 5% 8% 19% 21% 7%	3460
	21st to end	Day	46% 4 5% 17% 24% 5%	4565
January	1st to 10th	Night	31% 4 10% 15% 30% 10%	7636
	11th to 20th	Night	39% 3 6% 14% 25% 12%	7680
	21st to end	Night	40% 4 8% 18% 20% 9%	8143
February	1st to 10th	Night	38% 5% 7% 16% 21% 13%	2520
	11th to 20th	Night	39% 5% 8% 16% 18% 13%	2459
	21st to end	Night	35% 5% 9% 20% 18% 14%	2076
March	1st to 10th	Night	44% 5% 9% 16% 16% 11%	5073
	11th to 20th	Night	26% 5% 11% 18% 21% 20%	5090
	21st to end	Night	34% 4 9% 18% 21% 13%	5588
April	1st to 10th	Night	33% 7% 13% 18% 18% 12%	2310
	11th to 20th	Night	30% 5% 8% 20% 24% 12%	2283
	21st to end	Night	27% 6% 11% 27% 20% 9%	1896
May	1st to 10th	Night	39% 10% 15% 20% 11% 5%	5264
	11th to 20th	Night	35% 9% 18% 25% 10% 3	5235
	21st to end	Night	37% 10% 17% 22% 13% 1	5771
June	1st to 10th	Night	39% 12% 19% 17% 12% 2	1890
	11th to 20th	Night	33% 14% 25% 20% 8%	1890
	21st to end	Night	35% 15% 24% 18% 7% 1	1890
July	1st to 10th	Night	45% 15% 26% 11% 2	5235
	11th to 20th	Night	38% 18% 32% 11% 2	5226
	21st to end	Night	37% 16% 25% 20% 2	5606
August	1st to 10th	Night	48% 16% 25% 10% 1	4124
	11th to 20th	Night	28% 16% 30% 21% 5%	4418
	21st to end	Night	44% 13% 23% 17% 3 1	5301
September	1st to 10th	Night	44% 13% 19% 15% 9% 1	2310
	11th to 20th	Night	46% 11% 14% 17% 11% 2	2234
	21st to end	Night	34% 7% 16% 22% 15% 5%	2200
October	1st to 10th	Night	42% 7% 9% 13% 17% 13%	3590
	11th to 20th	Night	17% 3 5% 16% 33% 26%	3833
	21st to end	Night	21% 4 5% 17% 35% 17%	4252
November	1st to 10th	Night	33% 4 7% 20% 21% 14%	2461
	11th to 20th	Night	33% 3 7% 17% 26% 14%	2667
	21st to end	Night	35% 5% 10% 19% 19% 12%	2730
December	1st to 10th	Night	32% 4 9% 17% 24% 14%	4380
	11th to 20th	Night	39% 6% 8% 18% 20% 9%	4954
	21st to end	Night	42% 5% 8% 17% 24% 4	6633

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	15% 20% 47% 15% 3 1	330
	11th to 20th	Day	18% 15% 39% 18% 7% 2	330
	21st to end	Day	23% 23% 40% 12% 2	363
February	1st to 10th	Day	20% 15% 42% 18% 5%	330
	11th to 20th	Day	23% 17% 41% 16% 3	330
	21st to end	Day	18% 18% 42% 17% 5%	273
March	1st to 10th	Day	16% 16% 31% 24% 11%	516
	11th to 20th	Day	15% 15% 35% 21% 11% 2	540
	21st to end	Day	15% 15% 32% 27% 10% 1	594
April	1st to 10th	Day	17% 13% 31% 26% 12% 2	540
	11th to 20th	Day	11% 13% 29% 28% 13% 5%	532
	21st to end	Day	6% 9% 27% 32% 19% 7%	540
May	1st to 10th	Day	7% 11% 28% 29% 20% 6%	543
	11th to 20th	Day	9% 5% 24% 31% 21% 10%	567
	21st to end	Day	10% 4 16% 28% 29% 13%	617
June	1st to 10th	Day	11% 4 14% 29% 29% 13%	570
	11th to 20th	Day	8% 4 16% 25% 28% 19%	570
	21st to end	Day	8% 2 9% 27% 32% 21%	570
July	1st to 10th	Day	6% 2 5% 22% 38% 27%	565
	11th to 20th	Day	5% 2 4 20% 39% 29%	564
	21st to end	Day	7% 2 6% 24% 39% 22%	620
August	1st to 10th	Day	14% 2 7% 22% 32% 22%	566
	11th to 20th	Day	6% 3 5% 22% 37% 27%	559
	21st to end	Day	11% 4 9% 24% 30% 21%	627
September	1st to 10th	Day	13% 5% 13% 23% 26% 20%	570
	11th to 20th	Day	13% 3 14% 31% 27% 12%	542
	21st to end	Day	14% 3 12% 28% 29% 14%	530
October	1st to 10th	Day	14% 6% 28% 27% 18% 6%	530
	11th to 20th	Day	9% 5% 25% 33% 21% 7%	530
	21st to end	Day	9% 9% 27% 32% 20% 3	583
November	1st to 10th	Day	16% 7% 27% 28% 18% 3	535
	11th to 20th	Day	14% 8% 42% 23% 8% 3	389
	21st to end	Day	11% 16% 38% 25% 8% 1	330
December	1st to 10th	Day	17% 17% 35% 19% 10% 1	330
	11th to 20th	Day	18% 15% 38% 20% 8%	330
	21st to end	Day	19% 10% 34% 28% 6% 2	363
January	1st to 10th	Night	15% 16% 35% 28% 6%	840
	11th to 20th	Night	15% 11% 30% 34% 10% 1	840
	21st to end	Night	19% 16% 36% 26% 4	924
February	1st to 10th	Night	17% 12% 32% 31% 8%	840
	11th to 20th	Night	18% 15% 31% 31% 5%	823
	21st to end	Night	15% 14% 35% 32% 3	692
March	1st to 10th	Night	14% 12% 30% 35% 9%	840
	11th to 20th	Night	15% 10% 33% 30% 11% 1	840
	21st to end	Night	10% 12% 34% 32% 11%	924
April	1st to 10th	Night	12% 10% 32% 35% 11%	840
	11th to 20th	Night	10% 8% 33% 36% 13%	831
	21st to end	Night	6% 7% 28% 39% 20% 1	630
May	1st to 10th	Night	8% 5% 27% 40% 19% 1	630
	11th to 20th	Night	8% 5% 21% 40% 23% 4	621
	21st to end	Night	8% 3 12% 37% 35% 5%	683
June	1st to 10th	Night	11% 2 11% 40% 30% 5%	630
	11th to 20th	Night	6% 2 12% 34% 33% 13%	630
	21st to end	Night	6% 1 8% 35% 41% 9%	630
July	1st to 10th	Night	7% 1 7% 29% 41% 15%	615
	11th to 20th	Night	7% 1 4 22% 48% 19%	612
	21st to end	Night	6% 1 5% 28% 47% 13%	690
August	1st to 10th	Night	11% 2 6% 29% 42% 11%	627
	11th to 20th	Night	7% 1 4 22% 52% 14%	624
	21st to end	Night	13% 4 11% 26% 35% 12%	805
September	1st to 10th	Night	12% 3 10% 29% 37% 10%	840
	11th to 20th	Night	13% 3 13% 36% 31% 5%	813
	21st to end	Night	13% 2 14% 33% 34% 4	800
October	1st to 10th	Night	13% 4 22% 35% 24% 2	800
	11th to 20th	Night	11% 5% 23% 33% 25% 3	800
	21st to end	Night	11% 6% 24% 38% 20% 1	880
November	1st to 10th	Night	15% 7% 23% 35% 19% 1	820
	11th to 20th	Night	14% 9% 27% 35% 13% 1	840
	21st to end	Night	13% 9% 32% 31% 15% 1	840
December	1st to 10th	Night	16% 9% 30% 32% 12% 1	840
	11th to 20th	Night	17% 9% 29% 31% 13%	840
	21st to end	Night	18% 11% 27% 33% 11%	924

Page generated at 2026-09-11 04:11:52. Version ac07

Copyright © 2026 Attilla Danko.