Weathertop Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Weathertop Observatory clear sky chart from 2005-11-06 to 2025-03-27.

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	16% 10% 3 8% 5% 4 4 4 5% 7% 33%	108862
	Night	17% 11% 4 9% 5% 4 4 3 5% 6% 31%	110476
Transparency	Day	48% 8% 13% 19% 10% 1	112401
	Night	52% 8% 12% 18% 10% 1	115101
Seeing	Day	23% 18% 27% 20% 9% 3	15232
	Night	23% 16% 28% 25% 8% 1	24413

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	12% 13% 9% 18% 6% 4 4 3 4 4 23%	3597
	11th to 20th	Day	13% 13% 6% 13% 6% 5% 5% 3 5% 6% 24%	3570
	21st to end	Day	16% 14% 5% 8% 5% 5% 4 3 5% 4 30%	4049
February	1st to 10th	Day	15% 14% 5% 10% 8% 5% 5% 4 5% 6% 24%	1800
	11th to 20th	Day	14% 14% 5% 13% 7% 4 4 3 5% 6% 25%	1933
	21st to end	Day	19% 14% 4 10% 7% 4 4 3 4 4 28%	1650
March	1st to 10th	Day	24% 14% 4 9% 5% 4 3 2 4 5% 27%	3216
	11th to 20th	Day	15% 16% 4 8% 5% 4 4 3 5% 6% 31%	3393
	21st to end	Day	20% 11% 3 11% 4 4 4 3 4 4 32%	4238
April	1st to 10th	Day	19% 13% 4 6% 5% 4 3 3 5% 4 34%	2280
	11th to 20th	Day	17% 9% 3 7% 5% 4 5% 4 5% 5% 36%	2254
	21st to end	Day	21% 9% 3 7% 4 4 4 4 5% 5% 33%	2384
May	1st to 10th	Day	25% 10% 3 7% 4 4 5% 4 5% 6% 29%	4165
	11th to 20th	Day	17% 8% 3 7% 6% 5% 5% 4 6% 7% 33%	4398
	21st to end	Day	12% 6% 2 6% 5% 4 4 4 7% 9% 42%	4837
June	1st to 10th	Day	16% 8% 3 7% 5% 5% 5% 5% 6% 8% 32%	2660
	11th to 20th	Day	18% 6% 2 6% 4 4 5% 5% 7% 10% 33%	2660
	21st to end	Day	16% 9% 2 6% 5% 5% 4 4 6% 9% 33%	2660
July	1st to 10th	Day	11% 6% 3 6% 5% 5% 5% 5% 7% 10% 38%	4440
	11th to 20th	Day	10% 5% 2 7% 5% 5% 6% 5% 8% 9% 38%	4440
	21st to end	Day	14% 7% 2 7% 6% 5% 5% 5% 6% 7% 35%	4814
August	1st to 10th	Day	16% 7% 2 7% 5% 5% 6% 4 7% 8% 33%	4399
	11th to 20th	Day	13% 7% 2 7% 5% 5% 5% 5% 7% 8% 35%	3762
	21st to end	Day	12% 5% 3 6% 4 5% 5% 5% 6% 9% 40%	4180
September	1st to 10th	Day	13% 5% 2 5% 4 3 4 4 6% 7% 45%	2280
	11th to 20th	Day	15% 6% 1 5% 3 4 4 3 4 8% 48%	2181
	21st to end	Day	16% 6% 3 5% 4 4 4 3 6% 8% 41%	2090
October	1st to 10th	Day	21% 8% 3 7% 5% 4 4 4 4 5% 35%	2000
	11th to 20th	Day	14% 8% 3 6% 4 4 5% 3 5% 7% 38%	1891
	21st to end	Day	24% 10% 3 8% 5% 3 4 3 5% 6% 30%	1881
November	1st to 10th	Day	13% 10% 3 6% 4 4 4 3 5% 7% 42%	1761
	11th to 20th	Day	16% 10% 3 7% 5% 4 5% 3 5% 5% 37%	1800
	21st to end	Day	19% 11% 4 8% 5% 4 4 5% 5% 6% 30%	1653
December	1st to 10th	Day	17% 15% 5% 10% 6% 4 5% 3 5% 6% 24%	2600
	11th to 20th	Day	19% 15% 5% 8% 4 4 3 3 4 5% 30%	2986
	21st to end	Day	21% 16% 5% 10% 4 3 4 3 4 5% 25%	3960
January	1st to 10th	Night	13% 14% 8% 17% 5% 4 3 3 4 4 24%	6349
	11th to 20th	Night	11% 13% 6% 16% 8% 5% 5% 3 5% 5% 22%	6315
	21st to end	Night	17% 11% 4 11% 6% 5% 5% 4 5% 6% 27%	6904
February	1st to 10th	Night	16% 13% 4 12% 7% 5% 5% 4 5% 5% 25%	2600
	11th to 20th	Night	13% 12% 5% 13% 7% 5% 4 3 5% 5% 26%	2353
	21st to end	Night	16% 14% 4 10% 4 3 6% 4 5% 6% 28%	1980
March	1st to 10th	Night	22% 11% 3 10% 5% 4 5% 4 5% 5% 26%	3844
	11th to 20th	Night	15% 15% 4 8% 5% 4 4 3 5% 6% 30%	3783
	21st to end	Night	20% 14% 4 10% 4 4 3 2 4 4 31%	3533
April	1st to 10th	Night	20% 12% 2 7% 4 4 5% 3 5% 7% 31%	1900
	11th to 20th	Night	21% 9% 3 7% 5% 4 4 3 5% 6% 33%	1875
	21st to end	Night	21% 10% 3 6% 4 3 3 4 5% 6% 36%	1796
May	1st to 10th	Night	27% 9% 2 7% 4 4 4 3 5% 5% 29%	2804
	11th to 20th	Night	17% 9% 3 9% 7% 5% 5% 5% 6% 5% 29%	2495
	21st to end	Night	13% 7% 2 6% 6% 5% 5% 4 5% 8% 38%	2738
June	1st to 10th	Night	20% 7% 3 6% 4 5% 4 3 5% 7% 36%	1520
	11th to 20th	Night	18% 5% 2 5% 4 5% 4 4 6% 8% 39%	1520
	21st to end	Night	14% 5% 2 6% 5% 5% 4 4 6% 9% 40%	1520
July	1st to 10th	Night	12% 7% 3 8% 6% 5% 5% 4 7% 8% 36%	2520
	11th to 20th	Night	14% 8% 3 7% 5% 4 5% 4 5% 8% 37%	2520
	21st to end	Night	14% 7% 3 7% 5% 5% 4 5% 5% 7% 39%	2739
August	1st to 10th	Night	22% 8% 3 7% 4 4 4 3 5% 8% 33%	2506
	11th to 20th	Night	16% 6% 3 7% 5% 5% 5% 4 6% 6% 37%	3053
	21st to end	Night	12% 6% 3 6% 4 4 4 4 6% 9% 42%	3476
September	1st to 10th	Night	14% 7% 3 6% 4 3 4 3 6% 6% 44%	1900
	11th to 20th	Night	15% 5% 2 4 4 3 3 3 4 7% 51%	1998
	21st to end	Night	15% 6% 2 6% 4 4 4 4 4 6% 44%	2090
October	1st to 10th	Night	21% 7% 3 7% 4 5% 4 4 4 6% 35%	2175
	11th to 20th	Night	15% 8% 2 7% 4 4 5% 3 6% 6% 39%	2287
	21st to end	Night	24% 10% 3 6% 4 5% 4 3 5% 7% 30%	2717
November	1st to 10th	Night	14% 10% 3 7% 4 4 5% 4 4 5% 40%	2537
	11th to 20th	Night	17% 9% 3 7% 5% 5% 5% 4 5% 5% 36%	2600
	21st to end	Night	17% 11% 4 8% 5% 4 4 3 5% 6% 32%	2745
December	1st to 10th	Night	18% 16% 6% 11% 6% 4 4 3 5% 6% 21%	4580
	11th to 20th	Night	25% 13% 4 8% 4 3 3 2 4 4 28%	5208
	21st to end	Night	22% 16% 5% 8% 4 4 4 3 4 4 26%	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	64% 8% 9% 16% 4	3598
	11th to 20th	Day	58% 9% 11% 11% 9% 2	3600
	21st to end	Day	56% 8% 9% 17% 9% 1	4049
February	1st to 10th	Day	60% 8% 11% 14% 6% 1	1800
	11th to 20th	Day	61% 6% 10% 15% 8% 1	1942
	21st to end	Day	59% 6% 10% 15% 9% 1	1650
March	1st to 10th	Day	60% 6% 12% 13% 8% 1	3219
	11th to 20th	Day	51% 10% 16% 13% 8% 2	3393
	21st to end	Day	53% 7% 7% 14% 17% 2	4238
April	1st to 10th	Day	54% 6% 8% 20% 10% 2	2280
	11th to 20th	Day	48% 8% 11% 18% 14% 1	2254
	21st to end	Day	52% 7% 9% 17% 12% 3	2384
May	1st to 10th	Day	55% 8% 11% 17% 9% 1	4179
	11th to 20th	Day	47% 10% 14% 19% 8% 1	4402
	21st to end	Day	37% 10% 14% 25% 13% 1	4837
June	1st to 10th	Day	47% 9% 15% 18% 10% 1	2660
	11th to 20th	Day	45% 9% 14% 19% 12% 1	2660
	21st to end	Day	48% 10% 13% 19% 10% 1	2660
July	1st to 10th	Day	40% 10% 13% 24% 13% 1	4370
	11th to 20th	Day	39% 10% 16% 25% 10%	4365
	21st to end	Day	44% 9% 16% 20% 11%	4661
August	1st to 10th	Day	46% 9% 15% 21% 8%	4385
	11th to 20th	Day	43% 9% 18% 24% 6% 1	3799
	21st to end	Day	37% 9% 16% 28% 9% 1	4180
September	1st to 10th	Day	36% 9% 13% 27% 14% 1	2280
	11th to 20th	Day	36% 7% 12% 24% 20% 1	2181
	21st to end	Day	40% 7% 13% 25% 12% 2	2090
October	1st to 10th	Day	46% 9% 10% 21% 13% 1	3325
	11th to 20th	Day	42% 6% 14% 24% 14% 1	3119
	21st to end	Day	38% 7% 13% 28% 11% 2	3081
November	1st to 10th	Day	43% 7% 15% 22% 12% 1	1761
	11th to 20th	Day	48% 8% 12% 17% 13% 2	1800
	21st to end	Day	54% 7% 16% 17% 5% 1	1653
December	1st to 10th	Day	60% 8% 13% 14% 4	2600
	11th to 20th	Day	55% 8% 12% 17% 8%	2986
	21st to end	Day	60% 7% 8% 14% 11% 1	3960
January	1st to 10th	Night	62% 8% 11% 12% 6% 1	6349
	11th to 20th	Night	62% 9% 11% 11% 6% 1	6360
	21st to end	Night	57% 7% 11% 12% 10% 2	6904
February	1st to 10th	Night	60% 8% 11% 14% 6% 1	2600
	11th to 20th	Night	58% 8% 12% 14% 8% 1	2355
	21st to end	Night	56% 7% 13% 15% 8% 1	1980
March	1st to 10th	Night	57% 9% 9% 16% 8% 1	3854
	11th to 20th	Night	53% 9% 12% 18% 7% 1	3784
	21st to end	Night	56% 6% 7% 13% 16% 3	3533
April	1st to 10th	Night	53% 6% 12% 18% 10% 1	1900
	11th to 20th	Night	50% 8% 10% 18% 13% 1	1875
	21st to end	Night	49% 7% 10% 16% 14% 4	1796
May	1st to 10th	Night	55% 8% 11% 15% 10% 1	2810
	11th to 20th	Night	52% 11% 12% 16% 8% 1	2496
	21st to end	Night	41% 11% 12% 19% 15% 1	2739
June	1st to 10th	Night	46% 8% 13% 23% 10% 1	1520
	11th to 20th	Night	42% 8% 16% 22% 12%	1520
	21st to end	Night	40% 11% 14% 23% 12% 1	1520
July	1st to 10th	Night	43% 9% 14% 19% 14% 1	2480
	11th to 20th	Night	43% 10% 13% 23% 12%	2472
	21st to end	Night	43% 9% 11% 26% 10% 1	2651
August	1st to 10th	Night	49% 8% 15% 21% 7%	2496
	11th to 20th	Night	44% 10% 18% 20% 8%	3082
	21st to end	Night	34% 9% 16% 30% 10%	3476
September	1st to 10th	Night	41% 7% 12% 26% 14% 1	1900
	11th to 20th	Night	34% 6% 12% 28% 19% 1	1998
	21st to end	Night	41% 8% 11% 24% 15% 1	2090
October	1st to 10th	Night	50% 7% 12% 20% 10% 1	3630
	11th to 20th	Night	38% 7% 14% 22% 19% 1	3798
	21st to end	Night	42% 8% 13% 28% 9%	4467
November	1st to 10th	Night	45% 8% 11% 21% 14% 2	2537
	11th to 20th	Night	49% 8% 12% 16% 12% 2	2600
	21st to end	Night	53% 7% 16% 17% 7%	2745
December	1st to 10th	Night	64% 8% 11% 14% 3	4580
	11th to 20th	Night	58% 8% 12% 13% 10%	5208
	21st to end	Night	60% 7% 8% 13% 8% 2	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	29% 25% 28% 15% 4	200
	11th to 20th	Day	33% 25% 29% 11% 2 1	200
	21st to end	Day	25% 25% 34% 10% 5%	239
February	1st to 10th	Day	31% 35% 26% 6% 1	300
	11th to 20th	Day	30% 30% 28% 11% 2	481
	21st to end	Day	29% 26% 28% 15% 2	414
March	1st to 10th	Day	31% 23% 27% 17% 3	500
	11th to 20th	Day	29% 28% 26% 13% 3 1	500
	21st to end	Day	32% 25% 22% 14% 5% 2	530
April	1st to 10th	Day	36% 19% 25% 13% 7%	460
	11th to 20th	Day	30% 25% 23% 13% 7% 1	452
	21st to end	Day	30% 17% 30% 16% 4 2	460
May	1st to 10th	Day	33% 17% 29% 12% 6% 2	463
	11th to 20th	Day	20% 17% 33% 21% 6% 2	487
	21st to end	Day	18% 15% 27% 26% 10% 4	529
June	1st to 10th	Day	18% 16% 29% 25% 11% 2	490
	11th to 20th	Day	19% 15% 30% 24% 11% 2	490
	21st to end	Day	20% 17% 29% 22% 11% 2	490
July	1st to 10th	Day	14% 10% 30% 27% 16% 3	485
	11th to 20th	Day	12% 6% 35% 32% 13% 2	484
	21st to end	Day	14% 9% 36% 26% 12% 2	532
August	1st to 10th	Day	13% 12% 25% 29% 17% 5%	486
	11th to 20th	Day	17% 8% 27% 29% 13% 7%	479
	21st to end	Day	14% 12% 25% 28% 16% 6%	539
September	1st to 10th	Day	18% 11% 21% 25% 17% 8%	490
	11th to 20th	Day	16% 13% 22% 20% 19% 11%	490
	21st to end	Day	20% 12% 24% 22% 16% 6%	490
October	1st to 10th	Day	25% 14% 23% 20% 13% 5%	490
	11th to 20th	Day	20% 19% 27% 21% 10% 2	485
	21st to end	Day	30% 16% 27% 21% 6% 1	429
November	1st to 10th	Day	20% 23% 25% 18% 13% 1	395
	11th to 20th	Day	26% 25% 24% 19% 5% 1	400
	21st to end	Day	32% 23% 26% 15% 4	253
December	1st to 10th	Day	31% 23% 32% 11% 4 1	200
	11th to 20th	Day	38% 20% 27% 14% 2	200
	21st to end	Day	31% 24% 26% 15% 4	220
January	1st to 10th	Night	24% 25% 30% 18% 2	800
	11th to 20th	Night	26% 23% 28% 20% 2	800
	21st to end	Night	22% 21% 35% 20% 3	880
February	1st to 10th	Night	26% 21% 35% 18% 1	800
	11th to 20th	Night	24% 24% 34% 17% 2	783
	21st to end	Night	27% 23% 31% 17% 3	660
March	1st to 10th	Night	27% 23% 30% 19% 2	800
	11th to 20th	Night	26% 24% 28% 19% 3	800
	21st to end	Night	28% 21% 26% 19% 7%	863
April	1st to 10th	Night	29% 18% 29% 22% 3	760
	11th to 20th	Night	26% 18% 29% 21% 6%	751
	21st to end	Night	27% 17% 28% 23% 6%	656
May	1st to 10th	Night	27% 16% 26% 24% 7% 1	553
	11th to 20th	Night	18% 16% 30% 30% 6% 1	374
	21st to end	Night	13% 10% 27% 37% 11% 1	411
June	1st to 10th	Night	22% 6% 26% 35% 11% 1	380
	11th to 20th	Night	19% 7% 23% 39% 11%	380
	21st to end	Night	15% 6% 26% 42% 11% 1	380
July	1st to 10th	Night	13% 5% 23% 44% 15%	370
	11th to 20th	Night	15% 5% 20% 44% 15% 1	368
	21st to end	Night	14% 4 23% 45% 13% 1	416
August	1st to 10th	Night	13% 4 22% 42% 18% 1	378
	11th to 20th	Night	17% 3 21% 37% 19% 3	718
	21st to end	Night	15% 6% 22% 36% 20% 1	836
September	1st to 10th	Night	17% 6% 26% 33% 17% 1	760
	11th to 20th	Night	18% 7% 20% 29% 21% 5%	760
	21st to end	Night	18% 9% 24% 32% 15% 2	760
October	1st to 10th	Night	26% 10% 21% 28% 13% 2	760
	11th to 20th	Night	20% 11% 29% 30% 11%	760
	21st to end	Night	29% 14% 28% 23% 5%	836
November	1st to 10th	Night	21% 18% 28% 26% 7%	780
	11th to 20th	Night	22% 19% 31% 20% 6% 1	800
	21st to end	Night	24% 19% 31% 21% 5% 1	800
December	1st to 10th	Night	29% 22% 30% 15% 4	800
	11th to 20th	Night	27% 22% 28% 21% 3	800
	21st to end	Night	29% 22% 29% 18% 3	880

Page generated at 2025-03-24 09:04:31. Version ac07

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