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 2026-07-31.

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	17% 10% 3 8% 5% 4 4 4 5% 7% 33%	138219
	Night	17% 11% 4 9% 5% 4 4 3 5% 6% 31%	134813
Transparency	Day	49% 8% 12% 20% 10% 1	141712
	Night	52% 8% 12% 18% 10% 1	139409
Seeing	Day	24% 18% 27% 19% 9% 2	17013
	Night	23% 16% 28% 25% 8% 1	25994

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	13% 13% 8% 17% 6% 4 4 3 5% 5% 21%	4661
	11th to 20th	Day	13% 13% 5% 12% 7% 5% 5% 4 6% 6% 23%	4650
	21st to end	Day	14% 13% 4 8% 6% 5% 4 3 5% 5% 32%	5241
February	1st to 10th	Day	14% 14% 5% 10% 8% 5% 5% 4 5% 6% 24%	1890
	11th to 20th	Day	14% 15% 5% 13% 7% 4 4 3 5% 6% 25%	2032
	21st to end	Day	18% 15% 4 10% 7% 4 4 3 4 5% 28%	1730
March	1st to 10th	Day	21% 13% 6% 10% 5% 4 3 2 4 5% 27%	4576
	11th to 20th	Day	16% 14% 4 8% 4 4 3 3 4 5% 35%	4795
	21st to end	Day	21% 10% 3 9% 4 4 4 3 4 5% 31%	6102
April	1st to 10th	Day	20% 13% 4 6% 5% 4 3 3 5% 4 33%	2520
	11th to 20th	Day	17% 10% 3 7% 5% 4 5% 4 5% 5% 35%	2494
	21st to end	Day	21% 9% 3 7% 4 4 4 3 5% 5% 34%	2635
May	1st to 10th	Day	27% 10% 3 7% 4 4 5% 4 6% 7% 24%	7752
	11th to 20th	Day	18% 9% 3 7% 5% 5% 5% 4 6% 7% 33%	8221
	21st to end	Day	21% 8% 2 7% 5% 4 5% 4 6% 8% 30%	9060
June	1st to 10th	Day	16% 7% 2 7% 5% 5% 6% 5% 6% 8% 32%	2940
	11th to 20th	Day	18% 6% 2 6% 4 4 5% 5% 7% 10% 33%	2940
	21st to end	Day	16% 9% 2 6% 5% 5% 4 4 6% 8% 34%	2940
July	1st to 10th	Day	11% 7% 3 7% 5% 5% 5% 5% 7% 9% 35%	6500
	11th to 20th	Day	9% 5% 2 7% 5% 5% 6% 5% 8% 10% 37%	6500
	21st to end	Day	14% 6% 2 6% 5% 4 4 4 6% 7% 41%	7066
August	1st to 10th	Day	15% 7% 2 7% 5% 5% 6% 4 7% 8% 34%	4538
	11th to 20th	Day	13% 7% 2 7% 5% 5% 5% 5% 7% 8% 36%	3883
	21st to end	Day	11% 5% 3 6% 4 5% 5% 4 6% 9% 41%	4312
September	1st to 10th	Day	13% 5% 2 5% 4 3 4 4 6% 7% 47%	2400
	11th to 20th	Day	14% 5% 1 5% 3 4 4 3 5% 8% 48%	2296
	21st to end	Day	17% 6% 2 5% 4 4 4 4 6% 8% 41%	2200
October	1st to 10th	Day	20% 8% 2 7% 5% 4 4 4 4 5% 37%	2105
	11th to 20th	Day	15% 8% 3 6% 4 4 5% 3 5% 8% 38%	1990
	21st to end	Day	23% 10% 3 8% 5% 3 3 3 5% 6% 31%	1980
November	1st to 10th	Day	13% 10% 3 6% 4 4 4 3 5% 7% 41%	1851
	11th to 20th	Day	16% 11% 3 7% 6% 4 5% 3 5% 5% 36%	1890
	21st to end	Day	19% 12% 4 8% 5% 4 4 5% 4 6% 30%	1735
December	1st to 10th	Day	17% 15% 5% 10% 6% 4 5% 3 5% 6% 25%	2680
	11th to 20th	Day	19% 15% 5% 8% 4 4 3 3 4 5% 30%	3066
	21st to end	Day	21% 16% 5% 10% 4 3 4 3 4 5% 25%	4048
January	1st to 10th	Night	14% 13% 8% 16% 5% 4 4 3 5% 5% 23%	8234
	11th to 20th	Night	12% 15% 7% 16% 8% 5% 5% 3 5% 5% 20%	8235
	21st to end	Night	14% 9% 4 11% 7% 6% 5% 4 5% 6% 29%	8937
February	1st to 10th	Night	15% 13% 4 12% 7% 5% 5% 3 5% 5% 26%	2730
	11th to 20th	Night	13% 13% 5% 13% 7% 5% 4 3 5% 5% 27%	2474
	21st to end	Night	16% 15% 4 10% 4 3 6% 4 5% 6% 28%	2076
March	1st to 10th	Night	19% 10% 4 10% 6% 4 5% 4 5% 5% 27%	5484
	11th to 20th	Night	17% 13% 3 7% 5% 4 4 3 5% 7% 32%	5368
	21st to end	Night	24% 13% 5% 10% 4 3 3 2 3 4 29%	5072
April	1st to 10th	Night	22% 12% 3 7% 4 4 4 3 5% 6% 30%	2100
	11th to 20th	Night	21% 9% 3 7% 4 4 4 3 5% 6% 33%	2075
	21st to end	Night	20% 10% 3 5% 4 4 2 4 5% 6% 37%	1985
May	1st to 10th	Night	29% 10% 3 7% 4 4 4 3 5% 5% 25%	5202
	11th to 20th	Night	18% 9% 3 8% 6% 5% 5% 4 6% 8% 29%	4639
	21st to end	Night	22% 9% 3 7% 5% 5% 5% 4 5% 6% 30%	5114
June	1st to 10th	Night	19% 7% 3 6% 4 4 4 3 5% 7% 38%	1680
	11th to 20th	Night	18% 6% 2 5% 4 5% 4 4 6% 8% 37%	1680
	21st to end	Night	14% 5% 2 6% 4 5% 4 4 6% 9% 40%	1680
July	1st to 10th	Night	10% 7% 3 8% 5% 5% 5% 4 7% 9% 38%	3680
	11th to 20th	Night	13% 8% 3 7% 6% 5% 6% 4 5% 8% 35%	3680
	21st to end	Night	11% 6% 3 6% 4 4 4 4 5% 7% 47%	4006
August	1st to 10th	Night	21% 7% 3 7% 4 4 4 3 5% 8% 34%	2586
	11th to 20th	Night	16% 6% 3 7% 5% 5% 5% 4 6% 6% 38%	3152
	21st to end	Night	12% 6% 2 6% 4 4 4 4 6% 8% 43%	3586
September	1st to 10th	Night	14% 7% 3 6% 4 3 4 3 6% 6% 45%	2000
	11th to 20th	Night	14% 5% 2 4 4 4 3 3 4 7% 52%	2103
	21st to end	Night	15% 6% 2 7% 4 4 4 4 4 6% 43%	2200
October	1st to 10th	Night	20% 7% 3 7% 4 5% 4 4 4 6% 37%	2290
	11th to 20th	Night	15% 8% 2 7% 4 4 5% 3 6% 6% 39%	2408
	21st to end	Night	23% 10% 3 6% 4 5% 4 3 5% 7% 30%	2860
November	1st to 10th	Night	15% 10% 3 7% 4 4 4 4 4 5% 39%	2667
	11th to 20th	Night	16% 10% 4 7% 5% 5% 5% 4 5% 5% 35%	2730
	21st to end	Night	17% 11% 4 8% 5% 4 4 4 5% 6% 32%	2882
December	1st to 10th	Night	18% 16% 6% 11% 6% 4 4 3 5% 6% 21%	4720
	11th to 20th	Night	24% 13% 5% 8% 5% 3 3 2 4 5% 28%	5348
	21st to end	Night	22% 16% 5% 8% 4 4 4 3 4 4 26%	7150

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% 10% 14% 4	4662
	11th to 20th	Day	58% 9% 11% 11% 10% 1	4680
	21st to end	Day	53% 7% 9% 21% 8% 1	5241
February	1st to 10th	Day	59% 8% 11% 15% 7%	1890
	11th to 20th	Day	61% 6% 10% 15% 7% 1	2041
	21st to end	Day	59% 7% 10% 15% 9% 1	1730
March	1st to 10th	Day	59% 6% 10% 13% 8% 3	4579
	11th to 20th	Day	49% 8% 16% 17% 8% 3	4795
	21st to end	Day	53% 8% 7% 13% 17% 3	6102
April	1st to 10th	Day	55% 6% 8% 19% 10% 2	2520
	11th to 20th	Day	49% 7% 10% 18% 14% 1	2494
	21st to end	Day	51% 7% 9% 17% 12% 3	2635
May	1st to 10th	Day	57% 8% 11% 16% 7% 1	7764
	11th to 20th	Day	48% 9% 13% 21% 8% 1	8225
	21st to end	Day	50% 8% 11% 22% 8% 1	9061
June	1st to 10th	Day	46% 9% 14% 19% 10% 1	2940
	11th to 20th	Day	46% 9% 13% 19% 12% 1	2940
	21st to end	Day	47% 9% 14% 20% 10% 1	2940
July	1st to 10th	Day	42% 9% 13% 23% 12% 1	6430
	11th to 20th	Day	38% 11% 16% 24% 12% 1	6425
	21st to end	Day	40% 7% 13% 27% 12%	6868
August	1st to 10th	Day	46% 9% 15% 21% 9%	4524
	11th to 20th	Day	43% 9% 18% 24% 6% 1	3920
	21st to end	Day	37% 8% 16% 29% 9% 1	4312
September	1st to 10th	Day	36% 9% 13% 28% 14% 1	2400
	11th to 20th	Day	34% 7% 12% 25% 20% 1	2296
	21st to end	Day	40% 7% 13% 25% 13% 2	2200
October	1st to 10th	Day	45% 9% 10% 21% 14% 1	3430
	11th to 20th	Day	42% 6% 14% 23% 14% 1	3218
	21st to end	Day	38% 7% 13% 29% 11% 2	3180
November	1st to 10th	Day	43% 7% 15% 22% 12% 1	1851
	11th to 20th	Day	49% 8% 12% 17% 13% 2	1890
	21st to end	Day	54% 8% 15% 17% 5% 1	1735
December	1st to 10th	Day	60% 8% 13% 15% 4	2680
	11th to 20th	Day	55% 8% 11% 18% 8%	3066
	21st to end	Day	60% 7% 8% 14% 10% 1	4048
January	1st to 10th	Night	62% 8% 12% 12% 6% 1	8234
	11th to 20th	Night	64% 8% 10% 10% 6% 1	8280
	21st to end	Night	53% 7% 11% 14% 12% 2	8937
February	1st to 10th	Night	59% 7% 11% 15% 7% 1	2730
	11th to 20th	Night	58% 7% 12% 14% 8% 1	2476
	21st to end	Night	56% 7% 12% 15% 8% 1	2076
March	1st to 10th	Night	55% 9% 9% 14% 12% 1	5494
	11th to 20th	Night	50% 8% 17% 16% 8% 2	5369
	21st to end	Night	59% 5% 6% 13% 15% 2	5072
April	1st to 10th	Night	55% 6% 11% 17% 10% 1	2100
	11th to 20th	Night	51% 8% 9% 18% 13% 1	2075
	21st to end	Night	48% 7% 10% 16% 15% 3	1985
May	1st to 10th	Night	58% 8% 11% 14% 9% 1	5208
	11th to 20th	Night	50% 9% 12% 20% 8% 1	4640
	21st to end	Night	53% 9% 9% 20% 9% 1	5115
June	1st to 10th	Night	45% 7% 12% 23% 11% 1	1680
	11th to 20th	Night	43% 8% 15% 21% 12%	1680
	21st to end	Night	39% 10% 15% 24% 12% 1	1680
July	1st to 10th	Night	41% 9% 15% 22% 13%	3640
	11th to 20th	Night	45% 10% 11% 20% 14% 1	3632
	21st to end	Night	36% 8% 13% 31% 12% 1	3889
August	1st to 10th	Night	49% 8% 15% 21% 7%	2576
	11th to 20th	Night	43% 10% 18% 21% 8%	3181
	21st to end	Night	35% 9% 16% 30% 10% 1	3586
September	1st to 10th	Night	40% 7% 12% 26% 15% 1	2000
	11th to 20th	Night	34% 6% 12% 29% 19% 1	2103
	21st to end	Night	41% 8% 11% 24% 16% 1	2200
October	1st to 10th	Night	49% 7% 12% 20% 11% 1	3745
	11th to 20th	Night	38% 6% 14% 22% 19% 1	3919
	21st to end	Night	42% 8% 13% 28% 10%	4610
November	1st to 10th	Night	46% 7% 10% 21% 13% 2	2667
	11th to 20th	Night	50% 8% 12% 16% 12% 2	2730
	21st to end	Night	53% 7% 16% 17% 7%	2882
December	1st to 10th	Night	64% 8% 11% 14% 3	4720
	11th to 20th	Night	58% 8% 12% 13% 10%	5348
	21st to end	Night	60% 7% 8% 14% 8% 2	7150

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	30% 25% 28% 15% 3	220
	11th to 20th	Day	34% 25% 28% 10% 2 1	220
	21st to end	Day	25% 25% 34% 10% 5%	263
February	1st to 10th	Day	30% 33% 26% 9% 2	330
	11th to 20th	Day	30% 28% 28% 12% 2	520
	21st to end	Day	29% 28% 27% 14% 2	446
March	1st to 10th	Day	31% 23% 28% 16% 2	540
	11th to 20th	Day	31% 27% 25% 13% 3 1	540
	21st to end	Day	34% 26% 22% 13% 5% 2	594
April	1st to 10th	Day	37% 20% 24% 12% 6%	540
	11th to 20th	Day	32% 26% 23% 12% 6% 1	532
	21st to end	Day	30% 18% 31% 15% 5% 2	540
May	1st to 10th	Day	35% 18% 28% 12% 6% 2	543
	11th to 20th	Day	22% 18% 32% 20% 6% 2	567
	21st to end	Day	21% 16% 27% 24% 9% 3	617
June	1st to 10th	Day	18% 15% 29% 25% 11% 2	570
	11th to 20th	Day	20% 16% 30% 22% 10% 1	570
	21st to end	Day	19% 17% 30% 22% 11% 1	570
July	1st to 10th	Day	13% 11% 30% 28% 15% 3	565
	11th to 20th	Day	12% 8% 34% 30% 13% 2	564
	21st to end	Day	16% 10% 34% 25% 13% 3	616
August	1st to 10th	Day	12% 11% 25% 29% 18% 5%	526
	11th to 20th	Day	16% 7% 27% 29% 14% 7%	519
	21st to end	Day	14% 12% 26% 27% 16% 6%	583
September	1st to 10th	Day	17% 11% 22% 25% 17% 8%	530
	11th to 20th	Day	15% 12% 22% 22% 19% 11%	530
	21st to end	Day	20% 12% 25% 22% 16% 6%	530
October	1st to 10th	Day	24% 14% 22% 20% 14% 6%	530
	11th to 20th	Day	22% 18% 28% 21% 10% 2	524
	21st to end	Day	29% 15% 28% 21% 6% 1	462
November	1st to 10th	Day	22% 24% 24% 18% 12%	425
	11th to 20th	Day	27% 25% 23% 19% 5% 1	430
	21st to end	Day	33% 24% 25% 14% 3	275
December	1st to 10th	Day	30% 23% 32% 10% 4	220
	11th to 20th	Day	39% 19% 28% 13% 2	220
	21st to end	Day	33% 23% 26% 15% 3	242
January	1st to 10th	Night	24% 25% 31% 18% 2	840
	11th to 20th	Night	28% 23% 28% 19% 2	840
	21st to end	Night	22% 21% 35% 20% 3	924
February	1st to 10th	Night	25% 20% 35% 19% 1	840
	11th to 20th	Night	23% 23% 35% 17% 2	823
	21st to end	Night	26% 23% 32% 16% 3	692
March	1st to 10th	Night	27% 23% 30% 18% 2	840
	11th to 20th	Night	27% 24% 27% 19% 3	840
	21st to end	Night	29% 21% 26% 18% 6%	924
April	1st to 10th	Night	30% 19% 28% 20% 3	840
	11th to 20th	Night	27% 18% 28% 20% 6%	831
	21st to end	Night	27% 16% 30% 22% 5%	725
May	1st to 10th	Night	27% 16% 27% 23% 6%	611
	11th to 20th	Night	18% 16% 30% 29% 6%	414
	21st to end	Night	15% 10% 27% 36% 10% 1	455
June	1st to 10th	Night	21% 7% 26% 35% 10% 1	420
	11th to 20th	Night	20% 8% 25% 36% 10%	420
	21st to end	Night	14% 7% 26% 42% 11%	420
July	1st to 10th	Night	13% 4 24% 44% 14%	410
	11th to 20th	Night	14% 5% 22% 42% 15% 1	408
	21st to end	Night	14% 4 24% 45% 13% 1	458
August	1st to 10th	Night	13% 4 21% 43% 18% 1	398
	11th to 20th	Night	16% 3 21% 36% 20% 3	757
	21st to end	Night	15% 6% 22% 36% 19% 1	880
September	1st to 10th	Night	17% 6% 25% 33% 17% 1	800
	11th to 20th	Night	17% 7% 19% 31% 22% 5%	800
	21st to end	Night	18% 9% 24% 33% 15% 2	800
October	1st to 10th	Night	25% 10% 21% 29% 14% 2	800
	11th to 20th	Night	20% 11% 29% 30% 10%	800
	21st to end	Night	29% 14% 28% 24% 5%	880
November	1st to 10th	Night	23% 17% 28% 25% 7%	820
	11th to 20th	Night	23% 20% 30% 20% 6% 1	840
	21st to end	Night	24% 19% 30% 21% 5%	840
December	1st to 10th	Night	29% 22% 30% 15% 4	840
	11th to 20th	Night	28% 21% 28% 20% 2	840
	21st to end	Night	30% 22% 28% 18% 3	924

Page generated at 2026-07-28 03:25:23. Version ac07

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