Leach Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Leach Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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% 5% 2 5% 3 3 4 3 5% 7% 53%	135919
	Night	8% 5% 2 5% 3 3 3 3 5% 7% 55%	131717
Transparency	Day	31% 4 11% 27% 22% 5%	130690
	Night	30% 4 11% 26% 23% 6%	127953
Seeing	Day	11% 8% 30% 32% 16% 3	17065
	Night	11% 8% 34% 36% 11% 1	26926

By Month

Percentage Cloud Cover SW (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% 8% 2 7% 3 4 4 3 6% 8% 43%	4077
	11th to 20th	Day	6% 9% 3 9% 7% 4 3 3 5% 5% 45%	4080
	21st to end	Day	6% 8% 3 8% 5% 4 4 3 4 5% 50%	4488
February	1st to 10th	Day	8% 10% 3 7% 5% 3 4 3 4 6% 44%	1846
	11th to 20th	Day	7% 6% 3 6% 5% 4 5% 4 5% 7% 47%	1949
	21st to end	Day	7% 7% 2 5% 4 4 3 4 6% 6% 53%	1650
March	1st to 10th	Day	9% 6% 2 5% 3 3 3 2 4 5% 58%	4649
	11th to 20th	Day	9% 6% 2 4 2 2 3 3 4 5% 60%	4980
	21st to end	Day	9% 6% 2 6% 3 4 4 3 5% 7% 50%	5464
April	1st to 10th	Day	11% 5% 2 5% 3 3 3 4 5% 6% 54%	2258
	11th to 20th	Day	10% 7% 2 7% 4 4 4 4 5% 5% 47%	2476
	21st to end	Day	16% 7% 2 5% 3 3 4 4 5% 7% 44%	2600
May	1st to 10th	Day	14% 5% 2 5% 4 3 4 4 5% 6% 49%	5900
	11th to 20th	Day	11% 4 2 4 3 3 3 2 4 6% 58%	5862
	21st to end	Day	20% 5% 2 5% 3 3 4 4 6% 7% 41%	6446
June	1st to 10th	Day	5% 3 2 4 3 3 5% 4 5% 8% 59%	2600
	11th to 20th	Day	6% 4 1 4 4 3 4 3 6% 8% 56%	2600
	21st to end	Day	5% 4 1 4 3 3 4 4 7% 8% 58%	2705
July	1st to 10th	Day	7% 4 2 5% 4 3 4 4 6% 9% 53%	5948
	11th to 20th	Day	7% 4 2 5% 3 4 5% 5% 6% 10% 51%	5900
	21st to end	Day	7% 4 1 4 3 4 4 4 6% 8% 54%	6424
August	1st to 10th	Day	8% 6% 2 5% 4 4 4 3 5% 8% 51%	5801
	11th to 20th	Day	9% 4 1 5% 4 3 4 3 6% 10% 52%	5821
	21st to end	Day	12% 5% 2 5% 4 3 4 4 5% 7% 49%	6199
September	1st to 10th	Day	11% 4 2 4 3 3 3 2 5% 5% 58%	2245
	11th to 20th	Day	9% 4 2 4 2 3 3 4 5% 7% 57%	2200
	21st to end	Day	8% 3 1 5% 3 3 4 3 5% 5% 59%	2200
October	1st to 10th	Day	9% 3 1 3 1 2 2 1 4 6% 69%	3558
	11th to 20th	Day	6% 2 1 3 2 3 3 2 4 7% 68%	3240
	21st to end	Day	10% 4 1 3 2 3 3 2 4 6% 62%	3418
November	1st to 10th	Day	8% 4 1 4 3 2 4 3 5% 6% 62%	1853
	11th to 20th	Day	8% 5% 1 3 3 3 3 2 4 6% 62%	1863
	21st to end	Day	10% 6% 2 3 2 3 3 2 4 6% 60%	1800
December	1st to 10th	Day	5% 5% 1 4 2 2 3 3 4 6% 65%	2940
	11th to 20th	Day	6% 5% 2 5% 3 3 2 2 4 5% 63%	3391
	21st to end	Day	13% 11% 3 9% 4 3 4 3 5% 6% 40%	4488
January	1st to 10th	Night	10% 8% 5% 11% 4 3 4 3 5% 6% 42%	5888
	11th to 20th	Night	5% 6% 3 8% 5% 4 3 3 4 6% 55%	5900
	21st to end	Night	8% 8% 3 7% 4 3 4 3 4 6% 50%	6490
February	1st to 10th	Night	8% 9% 3 6% 5% 4 4 3 5% 6% 46%	2570
	11th to 20th	Night	8% 6% 2 7% 5% 4 5% 4 5% 7% 48%	2338
	21st to end	Night	6% 6% 3 6% 4 3 3 3 3 5% 59%	1980
March	1st to 10th	Night	7% 4 1 3 2 2 2 3 4 5% 66%	5203
	11th to 20th	Night	8% 4 1 3 3 3 4 3 6% 7% 59%	4980
	21st to end	Night	10% 6% 2 5% 4 4 5% 3 5% 7% 49%	5467
April	1st to 10th	Night	10% 6% 1 4 4 3 3 4 5% 7% 52%	2139
	11th to 20th	Night	11% 4 2 5% 4 3 4 3 5% 6% 52%	1870
	21st to end	Night	15% 6% 2 5% 3 3 4 3 5% 6% 48%	1800
May	1st to 10th	Night	16% 5% 1 5% 3 3 3 3 5% 6% 49%	4080
	11th to 20th	Night	9% 3 1 4 3 3 4 3 5% 7% 56%	4053
	21st to end	Night	19% 4 1 4 3 3 4 3 5% 8% 46%	4451
June	1st to 10th	Night	6% 2 1 4 2 3 3 4 6% 9% 60%	1800
	11th to 20th	Night	5% 4 2 4 4 3 4 3 7% 10% 56%	1800
	21st to end	Night	4 3 1 4 4 3 4 4 7% 11% 55%	1670
July	1st to 10th	Night	7% 4 2 5% 4 4 4 3 6% 9% 52%	3982
	11th to 20th	Night	8% 3 2 5% 4 3 5% 4 7% 10% 49%	4080
	21st to end	Night	7% 4 2 4 3 4 4 4 6% 9% 54%	4449
August	1st to 10th	Night	5% 3 1 4 4 4 4 4 7% 12% 52%	4011
	11th to 20th	Night	7% 4 1 6% 4 4 4 4 5% 9% 52%	4028
	21st to end	Night	10% 5% 2 6% 4 4 4 4 5% 7% 50%	4761
September	1st to 10th	Night	8% 2 1 4 3 3 3 2 5% 6% 63%	2155
	11th to 20th	Night	8% 3 1 4 3 3 4 3 6% 7% 58%	2200
	21st to end	Night	7% 4 1 4 3 3 3 3 5% 5% 61%	2200
October	1st to 10th	Night	7% 3 1 2 2 2 2 2 3 4 72%	3590
	11th to 20th	Night	6% 3 1 2 2 2 3 3 4 6% 69%	3896
	21st to end	Night	7% 3 1 3 2 2 3 2 4 7% 66%	4373
November	1st to 10th	Night	7% 4 2 4 3 2 2 2 4 6% 64%	2665
	11th to 20th	Night	8% 4 1 3 3 3 3 2 4 7% 63%	2692
	21st to end	Night	11% 6% 1 4 3 3 3 3 3 5% 59%	2600
December	1st to 10th	Night	4 5% 1 3 3 3 3 2 4 5% 67%	4250
	11th to 20th	Night	5% 4 2 4 3 3 3 3 5% 6% 60%	4816
	21st to end	Night	13% 11% 3 7% 3 3 3 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	38% 5% 13% 25% 17% 3	4078
	11th to 20th	Day	42% 4 9% 23% 19% 3	4080
	21st to end	Day	38% 4 11% 25% 19% 3	4488
February	1st to 10th	Day	40% 6% 15% 22% 12% 5%	1846
	11th to 20th	Day	35% 6% 16% 24% 15% 5%	1959
	21st to end	Day	29% 5% 10% 26% 23% 7%	1650
March	1st to 10th	Day	30% 3 7% 19% 28% 13%	4649
	11th to 20th	Day	28% 3 8% 21% 29% 12%	4980
	21st to end	Day	34% 5% 16% 34% 9% 1	5464
April	1st to 10th	Day	31% 4 12% 24% 22% 5%	2258
	11th to 20th	Day	37% 6% 11% 25% 18% 3	2479
	21st to end	Day	40% 5% 12% 26% 16% 1	2600
May	1st to 10th	Day	36% 5% 11% 26% 17% 5%	5900
	11th to 20th	Day	28% 5% 9% 24% 24% 10%	5866
	21st to end	Day	42% 6% 10% 25% 16% 2	6447
June	1st to 10th	Day	23% 8% 16% 32% 17% 4	2600
	11th to 20th	Day	26% 6% 13% 27% 22% 5%	2600
	21st to end	Day	23% 6% 12% 29% 23% 7%	2705
July	1st to 10th	Day	28% 4 12% 25% 26% 4	5885
	11th to 20th	Day	30% 4 15% 26% 22% 3	5831
	21st to end	Day	28% 4 11% 28% 24% 5%	6240
August	1st to 10th	Day	32% 5% 13% 25% 22% 3	4220
	11th to 20th	Day	32% 5% 17% 25% 18% 3	4210
	21st to end	Day	25% 4 14% 27% 22% 8%	4461
September	1st to 10th	Day	29% 4 10% 24% 23% 9%	2245
	11th to 20th	Day	27% 5% 11% 26% 26% 4	2200
	21st to end	Day	27% 5% 9% 27% 26% 6%	2200
October	1st to 10th	Day	21% 2 5% 21% 41% 10%	3558
	11th to 20th	Day	20% 2 9% 34% 26% 9%	3240
	21st to end	Day	26% 2 8% 25% 32% 6%	3418
November	1st to 10th	Day	23% 4 11% 30% 27% 5%	1853
	11th to 20th	Day	25% 3 9% 28% 30% 5%	1863
	21st to end	Day	28% 3 12% 29% 24% 4	1800
December	1st to 10th	Day	23% 3 11% 37% 23% 3	2940
	11th to 20th	Day	26% 3 12% 43% 16% 1	3389
	21st to end	Day	46% 4 9% 28% 12% 1	4488
January	1st to 10th	Night	44% 4 12% 23% 17% 2	5888
	11th to 20th	Night	32% 4 10% 26% 24% 4	5900
	21st to end	Night	37% 4 10% 22% 19% 9%	6490
February	1st to 10th	Night	39% 6% 11% 22% 15% 6%	2570
	11th to 20th	Night	36% 5% 11% 27% 17% 4	2338
	21st to end	Night	31% 3 8% 22% 26% 11%	1980
March	1st to 10th	Night	22% 3 7% 26% 30% 12%	5203
	11th to 20th	Night	25% 4 8% 18% 30% 15%	4980
	21st to end	Night	35% 4 17% 33% 9% 1	5467
April	1st to 10th	Night	30% 5% 13% 22% 24% 4	2139
	11th to 20th	Night	32% 5% 14% 24% 21% 3	1870
	21st to end	Night	37% 6% 11% 25% 19% 3	1800
May	1st to 10th	Night	36% 5% 13% 23% 18% 6%	4080
	11th to 20th	Night	26% 6% 12% 23% 22% 10%	4054
	21st to end	Night	36% 7% 15% 25% 15% 2	4452
June	1st to 10th	Night	19% 8% 18% 32% 19% 5%	1800
	11th to 20th	Night	23% 7% 14% 29% 22% 5%	1800
	21st to end	Night	23% 6% 15% 26% 25% 5%	1670
July	1st to 10th	Night	30% 4 15% 24% 23% 5%	3937
	11th to 20th	Night	31% 4 12% 27% 23% 3	4026
	21st to end	Night	28% 4 12% 28% 24% 4	4320
August	1st to 10th	Night	26% 6% 15% 27% 21% 5%	2916
	11th to 20th	Night	33% 5% 16% 25% 18% 3	2915
	21st to end	Night	24% 4 15% 28% 23% 7%	3430
September	1st to 10th	Night	24% 5% 11% 24% 26% 10%	2155
	11th to 20th	Night	26% 6% 11% 24% 26% 8%	2200
	21st to end	Night	25% 5% 10% 21% 32% 7%	2200
October	1st to 10th	Night	18% 4 5% 18% 47% 9%	3590
	11th to 20th	Night	19% 2 8% 28% 30% 13%	3896
	21st to end	Night	20% 3 8% 27% 36% 6%	4373
November	1st to 10th	Night	22% 3 9% 28% 29% 9%	2665
	11th to 20th	Night	23% 3 9% 29% 29% 7%	2692
	21st to end	Night	29% 3 10% 26% 25% 6%	2600
December	1st to 10th	Night	22% 3 12% 38% 23% 3	4250
	11th to 20th	Night	25% 3 9% 37% 23% 3	4817
	21st to end	Night	44% 4 9% 29% 12% 1	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	9% 10% 38% 28% 13% 1	300
	11th to 20th	Day	15% 14% 35% 26% 9% 1	300
	21st to end	Day	9% 8% 41% 34% 8%	330
February	1st to 10th	Day	13% 13% 43% 25% 5%	300
	11th to 20th	Day	13% 12% 43% 23% 7% 2	300
	21st to end	Day	7% 16% 52% 20% 5%	249
March	1st to 10th	Day	13% 12% 43% 25% 7% 1	300
	11th to 20th	Day	18% 11% 39% 23% 7% 2	300
	21st to end	Day	18% 14% 43% 20% 5% 1	330
April	1st to 10th	Day	14% 16% 39% 24% 6% 1	358
	11th to 20th	Day	13% 16% 42% 24% 5%	597
	21st to end	Day	20% 15% 36% 20% 7% 2	700
May	1st to 10th	Day	15% 10% 31% 30% 12% 1	703
	11th to 20th	Day	15% 11% 29% 25% 16% 3	725
	21st to end	Day	16% 8% 31% 30% 13% 2	789
June	1st to 10th	Day	5% 5% 26% 35% 23% 8%	730
	11th to 20th	Day	5% 5% 32% 37% 18% 3	730
	21st to end	Day	5% 5% 34% 38% 16% 2	730
July	1st to 10th	Day	6% 4 21% 34% 28% 6%	721
	11th to 20th	Day	5% 2 16% 43% 28% 7%	718
	21st to end	Day	7% 2 13% 43% 29% 6%	794
August	1st to 10th	Day	9% 4 14% 43% 27% 4	726
	11th to 20th	Day	7% 2 17% 41% 28% 5%	715
	21st to end	Day	8% 3 18% 38% 27% 6%	681
September	1st to 10th	Day	10% 4 19% 39% 25% 3	375
	11th to 20th	Day	5% 5% 24% 41% 20% 6%	330
	21st to end	Day	11% 5% 25% 30% 21% 7%	330
October	1st to 10th	Day	18% 9% 23% 25% 21% 4	330
	11th to 20th	Day	7% 10% 41% 26% 13% 2	330
	21st to end	Day	16% 15% 32% 25% 10% 2	363
November	1st to 10th	Day	14% 13% 35% 28% 8% 2	330
	11th to 20th	Day	11% 13% 36% 26% 13% 2	321
	21st to end	Day	17% 10% 34% 26% 10% 2	300
December	1st to 10th	Day	8% 8% 40% 28% 12% 3	300
	11th to 20th	Day	11% 13% 38% 29% 9% 1	300
	21st to end	Day	18% 12% 37% 25% 7%	330
January	1st to 10th	Night	10% 8% 37% 38% 7%	1000
	11th to 20th	Night	10% 9% 38% 35% 7% 1	1000
	21st to end	Night	9% 9% 40% 36% 5%	1100
February	1st to 10th	Night	12% 8% 39% 35% 5%	970
	11th to 20th	Night	11% 12% 41% 32% 5%	784
	21st to end	Night	8% 13% 43% 30% 5%	660
March	1st to 10th	Night	11% 13% 39% 31% 5%	733
	11th to 20th	Night	17% 9% 35% 31% 9%	600
	21st to end	Night	19% 13% 40% 23% 5% 1	660
April	1st to 10th	Night	13% 12% 42% 28% 4	600
	11th to 20th	Night	14% 15% 40% 26% 5%	593
	21st to end	Night	17% 12% 35% 29% 7% 1	600
May	1st to 10th	Night	16% 9% 35% 33% 7%	600
	11th to 20th	Night	13% 9% 31% 37% 9% 1	591
	21st to end	Night	12% 9% 32% 33% 12% 2	650
June	1st to 10th	Night	7% 5% 29% 42% 17% 1	600
	11th to 20th	Night	5% 5% 37% 37% 15%	600
	21st to end	Night	4 5% 40% 39% 11% 1	600
July	1st to 10th	Night	6% 4 23% 44% 22% 1	585
	11th to 20th	Night	4 1 26% 47% 20% 1	582
	21st to end	Night	6% 3 18% 47% 25% 1	657
August	1st to 10th	Night	6% 2 19% 53% 20%	597
	11th to 20th	Night	9% 2 18% 52% 19% 1	594
	21st to end	Night	6% 2 20% 47% 23% 2	660
September	1st to 10th	Night	8% 3 24% 47% 18% 2	600
	11th to 20th	Night	9% 3 33% 37% 17% 1	600
	21st to end	Night	10% 4 30% 41% 15% 1	600
October	1st to 10th	Night	13% 7% 29% 36% 15% 1	600
	11th to 20th	Night	11% 9% 37% 34% 9%	797
	21st to end	Night	14% 10% 35% 29% 11% 1	952
November	1st to 10th	Night	9% 9% 34% 38% 10%	1025
	11th to 20th	Night	10% 10% 40% 33% 7%	1036
	21st to end	Night	13% 10% 31% 35% 11% 1	1000
December	1st to 10th	Night	9% 8% 36% 38% 8%	1000
	11th to 20th	Night	13% 11% 36% 34% 7%	1000
	21st to end	Night	16% 9% 33% 35% 6%	1100

Page generated at 2025-11-14 23:55:55. Version ac07

Copyright © 2025 Attilla Danko.