Ladd Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ladd Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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	18% 7% 2 6% 4 4 4 4 5% 7% 39%	126288
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 40%	123441
Transparency	Day	28% 39% 7% 14% 10% 2	129834
	Night	28% 38% 7% 14% 11% 2	127982
Seeing	Day	24% 18% 32% 18% 6% 2	16350
	Night	23% 15% 33% 23% 6% 1	25210

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% 7% 2 8% 4 4 4 3 5% 6% 47%	3597
	11th to 20th	Day	15% 8% 2 6% 3 3 4 3 4 5% 48%	3570
	21st to end	Day	17% 9% 2 6% 4 3 4 3 4 4 43%	3960
February	1st to 10th	Day	19% 7% 2 5% 4 5% 5% 4 6% 6% 37%	1906
	11th to 20th	Day	14% 7% 2 5% 4 4 5% 4 5% 6% 42%	1948
	21st to end	Day	19% 8% 3 5% 3 4 4 4 5% 7% 38%	1650
March	1st to 10th	Day	22% 11% 3 5% 3 3 3 2 4 6% 38%	4476
	11th to 20th	Day	16% 9% 2 5% 4 4 4 4 5% 7% 42%	4768
	21st to end	Day	24% 9% 2 6% 4 4 4 4 5% 5% 32%	5778
April	1st to 10th	Day	20% 8% 3 6% 4 3 3 3 5% 7% 38%	2400
	11th to 20th	Day	18% 7% 3 5% 3 4 4 3 6% 7% 40%	2374
	21st to end	Day	21% 6% 2 6% 4 4 4 4 4 6% 39%	2600
May	1st to 10th	Day	29% 10% 3 7% 3 3 4 3 4 7% 26%	5886
	11th to 20th	Day	22% 9% 3 7% 5% 4 4 3 4 6% 33%	5901
	21st to end	Day	20% 6% 2 6% 4 4 5% 4 6% 7% 35%	6949
June	1st to 10th	Day	18% 6% 2 6% 4 4 5% 4 7% 9% 34%	2800
	11th to 20th	Day	16% 6% 2 6% 5% 4 5% 4 6% 9% 37%	2800
	21st to end	Day	15% 6% 2 6% 5% 4 5% 4 7% 9% 37%	2800
July	1st to 10th	Day	17% 7% 2 6% 5% 5% 5% 5% 7% 9% 32%	6360
	11th to 20th	Day	12% 7% 2 7% 5% 5% 6% 5% 8% 10% 33%	6360
	21st to end	Day	16% 7% 2 6% 4 4 4 4 5% 7% 40%	6926
August	1st to 10th	Day	17% 8% 3 8% 5% 5% 5% 4 6% 8% 31%	4247
	11th to 20th	Day	12% 6% 2 8% 5% 5% 6% 4 6% 8% 38%	4081
	21st to end	Day	7% 4 2 6% 5% 5% 6% 5% 7% 9% 45%	4312
September	1st to 10th	Day	15% 7% 2 6% 4 4 4 4 6% 6% 45%	2395
	11th to 20th	Day	13% 6% 2 5% 3 3 3 3 4 6% 51%	2200
	21st to end	Day	19% 7% 2 5% 4 4 5% 3 4 6% 42%	2200
October	1st to 10th	Day	22% 6% 1 5% 3 3 3 3 4 5% 44%	2200
	11th to 20th	Day	16% 5% 2 5% 3 3 3 3 5% 6% 48%	1952
	21st to end	Day	18% 5% 2 6% 4 4 3 3 5% 5% 44%	1881
November	1st to 10th	Day	16% 6% 2 5% 4 3 3 3 4 5% 48%	1761
	11th to 20th	Day	18% 6% 2 4 3 3 3 3 4 7% 47%	1800
	21st to end	Day	18% 7% 2 5% 4 3 4 3 4 5% 45%	1800
December	1st to 10th	Day	15% 8% 2 5% 3 3 3 3 4 6% 48%	2704
	11th to 20th	Day	19% 6% 2 6% 3 2 2 3 3 5% 49%	2986
	21st to end	Day	19% 10% 3 7% 4 3 3 2 4 5% 39%	3960
January	1st to 10th	Night	14% 6% 1 5% 3 3 4 3 4 6% 51%	6349
	11th to 20th	Night	14% 10% 3 8% 4 4 3 3 4 4 43%	6315
	21st to end	Night	16% 9% 3 7% 4 4 4 3 5% 5% 38%	6996
February	1st to 10th	Night	17% 8% 3 6% 5% 4 4 3 5% 6% 40%	2488
	11th to 20th	Night	17% 8% 3 6% 5% 3 3 4 5% 5% 41%	2338
	21st to end	Night	17% 9% 3 7% 4 3 4 3 4 4 41%	1980
March	1st to 10th	Night	23% 9% 2 4 3 4 3 3 5% 5% 38%	5364
	11th to 20th	Night	17% 10% 3 6% 4 3 4 3 5% 6% 40%	5191
	21st to end	Night	25% 9% 2 5% 4 4 4 4 5% 4 34%	5151
April	1st to 10th	Night	19% 9% 2 6% 5% 3 5% 3 5% 6% 37%	2000
	11th to 20th	Night	19% 8% 2 6% 4 4 4 4 5% 6% 38%	1975
	21st to end	Night	20% 7% 3 7% 4 3 3 3 4 6% 41%	1800
May	1st to 10th	Night	28% 9% 3 7% 5% 4 4 3 4 7% 26%	4054
	11th to 20th	Night	21% 9% 3 8% 5% 5% 5% 4 6% 7% 28%	4033
	21st to end	Night	21% 7% 3 7% 5% 5% 4 4 6% 7% 32%	3927
June	1st to 10th	Night	16% 7% 3 8% 4 5% 4 4 6% 7% 37%	1600
	11th to 20th	Night	19% 6% 2 6% 5% 4 4 3 5% 8% 38%	1600
	21st to end	Night	16% 7% 2 5% 4 4 4 5% 6% 9% 39%	1600
July	1st to 10th	Night	16% 8% 4 6% 4 4 5% 4 6% 9% 34%	3600
	11th to 20th	Night	15% 10% 3 6% 4 3 5% 5% 6% 9% 35%	3600
	21st to end	Night	14% 7% 2 7% 4 4 4 4 6% 7% 41%	3926
August	1st to 10th	Night	21% 7% 3 7% 5% 4 5% 4 7% 8% 30%	2868
	11th to 20th	Night	9% 7% 3 7% 5% 7% 6% 6% 6% 9% 36%	2961
	21st to end	Night	8% 4 2 4 4 3 3 4 5% 8% 55%	3586
September	1st to 10th	Night	13% 5% 3 6% 5% 4 5% 4 5% 6% 43%	2005
	11th to 20th	Night	15% 5% 1 5% 3 3 3 3 4 6% 52%	2200
	21st to end	Night	19% 9% 2 6% 4 4 4 3 3 6% 41%	2200
October	1st to 10th	Night	19% 7% 3 5% 4 3 4 3 3 7% 43%	2200
	11th to 20th	Night	16% 5% 2 4 4 3 4 3 5% 7% 48%	2402
	21st to end	Night	20% 7% 2 6% 4 3 4 3 4 6% 41%	2717
November	1st to 10th	Night	14% 5% 2 5% 4 4 3 4 5% 5% 49%	2537
	11th to 20th	Night	18% 7% 2 5% 3 3 3 3 4 5% 47%	2600
	21st to end	Night	16% 6% 2 4 3 3 4 4 5% 7% 46%	2600
December	1st to 10th	Night	18% 8% 2 5% 3 3 3 2 4 5% 47%	4474
	11th to 20th	Night	23% 8% 2 5% 3 3 3 3 4 5% 42%	5208
	21st to end	Night	17% 11% 3 7% 4 3 3 2 4 5% 41%	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	27% 27% 4 21% 18% 3	3598
	11th to 20th	Day	27% 27% 6% 17% 15% 7%	3600
	21st to end	Day	35% 29% 6% 13% 15% 2	3960
February	1st to 10th	Day	18% 62% 7% 8% 5% 1	1906
	11th to 20th	Day	19% 61% 2 8% 9% 2	1957
	21st to end	Day	16% 62% 5% 10% 7% 1	1650
March	1st to 10th	Day	38% 29% 7% 8% 8% 10%	4479
	11th to 20th	Day	29% 31% 10% 21% 8% 1	4768
	21st to end	Day	42% 30% 3 11% 9% 5%	5778
April	1st to 10th	Day	21% 62% 3 7% 6% 1	2400
	11th to 20th	Day	16% 63% 3 8% 9% 1	2374
	21st to end	Day	19% 62% 3 8% 7% 2	2600
May	1st to 10th	Day	45% 32% 11% 9% 4	5897
	11th to 20th	Day	41% 31% 11% 11% 6% 1	5905
	21st to end	Day	37% 31% 7% 15% 10%	6949
June	1st to 10th	Day	17% 62% 4 9% 7%	2800
	11th to 20th	Day	16% 62% 5% 10% 6% 1	2800
	21st to end	Day	15% 61% 4 10% 9% 1	2800
July	1st to 10th	Day	36% 30% 9% 15% 10% 1	6290
	11th to 20th	Day	31% 30% 14% 17% 8% 1	6285
	21st to end	Day	32% 27% 8% 20% 13% 1	6728
August	1st to 10th	Day	34% 39% 9% 13% 4 1	4233
	11th to 20th	Day	27% 39% 9% 20% 6%	4120
	21st to end	Day	16% 40% 10% 20% 12% 1	4312
September	1st to 10th	Day	17% 58% 4 12% 9%	2395
	11th to 20th	Day	17% 57% 4 10% 11% 2	2200
	21st to end	Day	21% 57% 4 9% 8% 1	2200
October	1st to 10th	Day	23% 37% 6% 15% 15% 3	3590
	11th to 20th	Day	17% 36% 3 17% 23% 3	3168
	21st to end	Day	16% 37% 9% 20% 16% 2	3081
November	1st to 10th	Day	18% 57% 6% 9% 9% 1	1761
	11th to 20th	Day	16% 57% 4 8% 13% 3	1800
	21st to end	Day	16% 57% 4 14% 6% 2	1800
December	1st to 10th	Day	24% 36% 9% 21% 10%	2704
	11th to 20th	Day	27% 32% 9% 11% 15% 6%	2986
	21st to end	Day	37% 27% 5% 16% 11% 4	3960
January	1st to 10th	Night	23% 27% 6% 28% 14% 2	6349
	11th to 20th	Night	33% 27% 8% 16% 12% 5%	6360
	21st to end	Night	38% 29% 6% 11% 12% 4	6996
February	1st to 10th	Night	18% 62% 5% 8% 5% 1	2488
	11th to 20th	Night	18% 61% 3 6% 11% 2	2340
	21st to end	Night	18% 61% 3 8% 7% 2	1980
March	1st to 10th	Night	37% 30% 6% 10% 11% 6%	5374
	11th to 20th	Night	32% 30% 12% 14% 9% 2	5192
	21st to end	Night	41% 30% 3 7% 14% 5%	5151
April	1st to 10th	Night	22% 62% 3 7% 5% 2	2000
	11th to 20th	Night	17% 63% 5% 6% 8% 1	1975
	21st to end	Night	19% 62% 3 8% 7% 1	1800
May	1st to 10th	Night	43% 33% 10% 9% 4 1	4060
	11th to 20th	Night	42% 32% 10% 10% 4 1	4034
	21st to end	Night	39% 31% 9% 14% 6%	3928
June	1st to 10th	Night	17% 62% 4 9% 8%	1600
	11th to 20th	Night	17% 62% 4 10% 6%	1600
	21st to end	Night	14% 61% 5% 12% 7% 1	1600
July	1st to 10th	Night	36% 29% 8% 16% 11%	3560
	11th to 20th	Night	35% 29% 11% 16% 7% 1	3552
	21st to end	Night	32% 27% 8% 21% 12% 1	3809
August	1st to 10th	Night	37% 39% 7% 10% 6% 1	2858
	11th to 20th	Night	28% 41% 8% 18% 5%	2988
	21st to end	Night	14% 38% 12% 20% 17%	3586
September	1st to 10th	Night	17% 58% 4 13% 7% 1	2005
	11th to 20th	Night	16% 57% 3 11% 11% 1	2200
	21st to end	Night	23% 57% 3 9% 7%	2200
October	1st to 10th	Night	24% 38% 7% 17% 11% 2	3590
	11th to 20th	Night	12% 36% 4 18% 27% 3	3925
	21st to end	Night	18% 37% 7% 21% 15% 2	4467
November	1st to 10th	Night	18% 57% 5% 10% 9% 2	2537
	11th to 20th	Night	17% 56% 4 9% 12% 3	2600
	21st to end	Night	16% 57% 5% 14% 6% 2	2600
December	1st to 10th	Night	27% 35% 7% 20% 10%	4474
	11th to 20th	Night	33% 31% 9% 9% 16% 2	5208
	21st to end	Night	35% 28% 5% 15% 14% 3	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	31% 26% 27% 14% 3	200
	11th to 20th	Day	29% 29% 34% 7% 3	200
	21st to end	Day	27% 25% 34% 11% 1 1	220
February	1st to 10th	Day	33% 24% 31% 11% 1	434
	11th to 20th	Day	33% 29% 30% 6% 2	496
	21st to end	Day	30% 23% 33% 11% 3	414
March	1st to 10th	Day	33% 21% 32% 12% 2	500
	11th to 20th	Day	30% 26% 29% 11% 3 1	500
	21st to end	Day	31% 24% 31% 8% 4 2	550
April	1st to 10th	Day	35% 23% 26% 11% 5% 1	500
	11th to 20th	Day	31% 25% 29% 12% 3 1	492
	21st to end	Day	29% 20% 30% 17% 2 2	500
May	1st to 10th	Day	33% 23% 29% 11% 3	503
	11th to 20th	Day	25% 18% 39% 13% 3 1	527
	21st to end	Day	21% 16% 35% 21% 5% 2	573
June	1st to 10th	Day	23% 14% 34% 21% 8% 1	530
	11th to 20th	Day	17% 19% 37% 21% 5% 1	530
	21st to end	Day	19% 13% 38% 22% 7% 1	530
July	1st to 10th	Day	19% 11% 31% 26% 11% 2	525
	11th to 20th	Day	17% 9% 32% 29% 12% 2	524
	21st to end	Day	14% 11% 39% 26% 9% 1	576
August	1st to 10th	Day	13% 11% 34% 30% 10% 3	526
	11th to 20th	Day	14% 9% 34% 29% 11% 3	519
	21st to end	Day	12% 11% 37% 23% 13% 5%	583
September	1st to 10th	Day	19% 12% 29% 23% 13% 4	530
	11th to 20th	Day	17% 12% 27% 25% 12% 7%	530
	21st to end	Day	24% 11% 34% 20% 7% 4	530
October	1st to 10th	Day	27% 12% 27% 19% 9% 5%	530
	11th to 20th	Day	21% 17% 33% 17% 10% 2	469
	21st to end	Day	25% 18% 30% 17% 7% 2	429
November	1st to 10th	Day	22% 16% 33% 17% 9% 2	395
	11th to 20th	Day	27% 21% 32% 15% 6%	400
	21st to end	Day	27% 22% 30% 15% 6% 1	400
December	1st to 10th	Day	24% 28% 31% 12% 5%	265
	11th to 20th	Day	34% 25% 33% 7% 2 1	200
	21st to end	Day	25% 25% 32% 14% 2 2	220
January	1st to 10th	Night	23% 20% 36% 17% 3	800
	11th to 20th	Night	30% 19% 34% 15% 2	800
	21st to end	Night	22% 22% 35% 17% 3	880
February	1st to 10th	Night	25% 19% 36% 19% 1	800
	11th to 20th	Night	28% 22% 36% 13% 1	783
	21st to end	Night	25% 22% 33% 17% 3	660
March	1st to 10th	Night	26% 19% 35% 17% 3	800
	11th to 20th	Night	22% 22% 36% 17% 3	800
	21st to end	Night	28% 19% 34% 16% 3	880
April	1st to 10th	Night	26% 20% 34% 17% 3	800
	11th to 20th	Night	24% 20% 34% 18% 4	791
	21st to end	Night	25% 18% 38% 17% 3	600
May	1st to 10th	Night	30% 15% 32% 20% 4	600
	11th to 20th	Night	22% 16% 38% 22% 3	592
	21st to end	Night	22% 8% 35% 29% 6%	434
June	1st to 10th	Night	17% 11% 34% 30% 9%	400
	11th to 20th	Night	21% 7% 37% 29% 6%	400
	21st to end	Night	19% 5% 36% 35% 5% 1	400
July	1st to 10th	Night	16% 4 32% 38% 9%	390
	11th to 20th	Night	16% 4 33% 37% 10% 1	388
	21st to end	Night	16% 4 32% 40% 8%	438
August	1st to 10th	Night	15% 5% 34% 37% 10%	563
	11th to 20th	Night	13% 5% 30% 39% 12% 1	642
	21st to end	Night	14% 6% 26% 36% 15% 2	880
September	1st to 10th	Night	17% 6% 30% 35% 11% 1	800
	11th to 20th	Night	17% 7% 28% 29% 16% 3	800
	21st to end	Night	25% 7% 27% 28% 11% 2	800
October	1st to 10th	Night	24% 11% 27% 25% 12% 2	800
	11th to 20th	Night	20% 11% 35% 22% 11% 1	793
	21st to end	Night	24% 15% 33% 21% 7%	836
November	1st to 10th	Night	18% 15% 33% 24% 9% 1	780
	11th to 20th	Night	23% 18% 35% 18% 6%	800
	21st to end	Night	23% 20% 32% 21% 4 1	800
December	1st to 10th	Night	26% 17% 35% 17% 5%	800
	11th to 20th	Night	28% 18% 32% 20% 2	800
	21st to end	Night	26% 19% 34% 18% 3	880

Page generated at 2025-10-16 01:50:28. Version ac07

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