Tomstown
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tomstown clear sky chart from 2005-11-06 to 2025-09-19.

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	13% 7% 3 7% 5% 5% 5% 4 6% 8% 37%	126331
	Night	13% 7% 3 7% 5% 5% 5% 4 6% 7% 39%	122713
Transparency	Day	26% 40% 8% 15% 9% 2	129926
	Night	26% 40% 8% 15% 10% 2	127187
Seeing	Day	23% 17% 27% 20% 10% 3	16005
	Night	19% 16% 27% 26% 11% 1	25461

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	13% 8% 3 12% 6% 5% 5% 4 4 5% 35%	3666
	11th to 20th	Day	12% 11% 5% 9% 5% 6% 5% 3 5% 6% 34%	4060
	21st to end	Day	14% 10% 3 7% 6% 5% 5% 4 5% 6% 36%	4466
February	1st to 10th	Day	14% 11% 3 8% 6% 5% 4 4 5% 6% 35%	1800
	11th to 20th	Day	15% 9% 3 8% 7% 5% 4 3 5% 6% 35%	1770
	21st to end	Day	17% 11% 3 7% 4 4 4 3 5% 5% 36%	1650
March	1st to 10th	Day	17% 7% 2 7% 4 4 4 3 5% 6% 40%	4886
	11th to 20th	Day	10% 7% 3 6% 5% 4 5% 4 6% 7% 44%	4980
	21st to end	Day	14% 8% 2 8% 5% 4 5% 4 6% 6% 38%	5528
April	1st to 10th	Day	15% 9% 3 7% 4 4 5% 4 6% 7% 35%	2400
	11th to 20th	Day	14% 7% 3 7% 5% 4 3 4 5% 6% 41%	2374
	21st to end	Day	17% 7% 2 7% 4 4 5% 4 5% 6% 39%	2415
May	1st to 10th	Day	22% 7% 3 7% 5% 4 5% 5% 6% 9% 28%	5862
	11th to 20th	Day	21% 8% 3 7% 4 5% 5% 4 7% 8% 29%	6175
	21st to end	Day	15% 6% 2 7% 5% 5% 5% 5% 7% 9% 34%	6934
June	1st to 10th	Day	12% 6% 2 5% 5% 5% 5% 4 7% 10% 39%	2800
	11th to 20th	Day	13% 6% 3 6% 5% 5% 5% 5% 6% 10% 37%	2800
	21st to end	Day	10% 5% 2 5% 4 5% 5% 4 6% 9% 44%	2800
July	1st to 10th	Day	5% 5% 2 6% 5% 5% 6% 5% 8% 11% 42%	6360
	11th to 20th	Day	7% 6% 3 8% 6% 6% 7% 6% 9% 11% 32%	6206
	21st to end	Day	8% 7% 3 7% 5% 5% 5% 4 7% 10% 39%	6424
August	1st to 10th	Day	14% 8% 2 7% 5% 5% 5% 5% 6% 9% 34%	4234
	11th to 20th	Day	10% 6% 2 7% 5% 5% 5% 4 7% 9% 38%	4057
	21st to end	Day	10% 6% 2 5% 4 4 5% 4 6% 7% 46%	4312
September	1st to 10th	Day	15% 5% 2 5% 4 4 4 3 5% 5% 47%	2400
	11th to 20th	Day	12% 5% 2 5% 4 4 4 3 6% 8% 47%	2376
	21st to end	Day	19% 8% 3 6% 4 4 4 3 6% 6% 37%	1986
October	1st to 10th	Day	15% 5% 2 6% 5% 4 5% 3 5% 6% 45%	1900
	11th to 20th	Day	15% 6% 2 5% 4 4 3 4 5% 7% 45%	1900
	21st to end	Day	14% 7% 2 6% 4 3 3 4 5% 7% 45%	2090
November	1st to 10th	Day	10% 6% 3 5% 3 3 4 3 6% 7% 49%	1852
	11th to 20th	Day	14% 6% 2 6% 5% 4 4 3 5% 6% 43%	1718
	21st to end	Day	14% 8% 2 6% 6% 4 4 4 5% 7% 39%	1600
December	1st to 10th	Day	13% 6% 4 8% 5% 5% 4 5% 7% 9% 35%	2600
	11th to 20th	Day	19% 11% 3 7% 4 4 4 4 4 6% 35%	2990
	21st to end	Day	15% 10% 4 9% 5% 5% 6% 5% 6% 8% 28%	3960
January	1st to 10th	Night	10% 9% 3 9% 6% 6% 5% 4 6% 6% 35%	6275
	11th to 20th	Night	12% 8% 3 9% 5% 5% 5% 4 4 5% 38%	5900
	21st to end	Night	15% 8% 3 7% 4 4 4 3 5% 7% 41%	6490
February	1st to 10th	Night	15% 8% 3 8% 4 4 4 4 6% 7% 36%	2600
	11th to 20th	Night	17% 9% 3 7% 4 4 5% 4 5% 7% 35%	2514
	21st to end	Night	16% 11% 3 7% 5% 4 5% 3 6% 6% 35%	1980
March	1st to 10th	Night	16% 7% 2 6% 4 4 4 3 5% 6% 43%	4963
	11th to 20th	Night	9% 6% 3 7% 4 4 4 4 5% 7% 48%	4980
	21st to end	Night	11% 7% 3 7% 5% 5% 4 4 5% 6% 44%	5404
April	1st to 10th	Night	16% 9% 3 8% 7% 5% 4 4 5% 5% 34%	2000
	11th to 20th	Night	12% 8% 2 7% 5% 4 4 3 7% 8% 41%	1975
	21st to end	Night	15% 6% 3 5% 4 4 4 4 7% 6% 41%	2000
May	1st to 10th	Night	23% 8% 4 8% 5% 5% 4 4 6% 7% 27%	4034
	11th to 20th	Night	20% 10% 3 7% 4 3 4 3 6% 8% 31%	3715
	21st to end	Night	16% 7% 3 8% 4 5% 4 3 7% 8% 34%	3920
June	1st to 10th	Night	11% 6% 2 5% 4 3 4 5% 6% 8% 46%	1600
	11th to 20th	Night	13% 6% 3 7% 4 5% 5% 4 7% 9% 39%	1600
	21st to end	Night	8% 6% 2 6% 5% 5% 5% 4 6% 10% 44%	1600
July	1st to 10th	Night	7% 5% 3 7% 5% 5% 6% 5% 6% 10% 41%	3598
	11th to 20th	Night	7% 7% 4 10% 6% 6% 6% 5% 8% 10% 31%	3733
	21st to end	Night	9% 6% 3 6% 4 5% 5% 4 6% 9% 44%	4427
August	1st to 10th	Night	13% 8% 3 8% 5% 5% 5% 4 6% 9% 35%	2915
	11th to 20th	Night	9% 6% 3 8% 5% 5% 5% 5% 6% 8% 41%	2958
	21st to end	Night	11% 6% 2 6% 5% 4 4 3 5% 9% 46%	3586
September	1st to 10th	Night	13% 5% 2 5% 4 4 3 4 5% 6% 48%	2000
	11th to 20th	Night	10% 6% 2 6% 3 3 4 2 5% 7% 51%	1981
	21st to end	Night	18% 7% 3 5% 3 4 4 3 5% 6% 41%	2190
October	1st to 10th	Night	17% 6% 3 6% 4 4 3 3 5% 6% 44%	2280
	11th to 20th	Night	12% 5% 2 5% 4 4 4 3 4 5% 49%	2280
	21st to end	Night	16% 5% 2 5% 4 4 4 4 5% 7% 45%	2508
November	1st to 10th	Night	9% 6% 2 5% 4 4 4 4 5% 6% 52%	2446
	11th to 20th	Night	13% 8% 2 5% 5% 4 5% 3 5% 7% 43%	2680
	21st to end	Night	13% 6% 2 7% 5% 5% 4 4 6% 8% 40%	2800
December	1st to 10th	Night	9% 6% 3 8% 5% 5% 5% 4 5% 8% 41%	4580
	11th to 20th	Night	19% 7% 3 6% 5% 5% 5% 4 6% 6% 34%	5205
	21st to end	Night	16% 9% 3 8% 6% 6% 6% 5% 6% 7% 29%	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	35% 28% 8% 18% 10% 2	3667
	11th to 20th	Day	35% 29% 6% 14% 13% 4	4060
	21st to end	Day	33% 31% 8% 16% 10% 1	4466
February	1st to 10th	Day	21% 62% 6% 8% 4	1800
	11th to 20th	Day	20% 61% 3 9% 6% 1	1779
	21st to end	Day	18% 62% 4 9% 6% 1	1650
March	1st to 10th	Day	34% 30% 6% 15% 9% 5%	4886
	11th to 20th	Day	24% 32% 8% 22% 9% 4	4980
	21st to end	Day	30% 31% 8% 14% 14% 3	5528
April	1st to 10th	Day	22% 62% 3 6% 6% 1	2400
	11th to 20th	Day	15% 63% 5% 9% 6% 1	2374
	21st to end	Day	17% 62% 4 10% 7%	2415
May	1st to 10th	Day	38% 32% 9% 13% 6% 1	5897
	11th to 20th	Day	38% 31% 8% 18% 5%	6179
	21st to end	Day	31% 33% 9% 15% 11% 1	6949
June	1st to 10th	Day	17% 63% 4 10% 6%	2800
	11th to 20th	Day	16% 64% 5% 9% 7%	2800
	21st to end	Day	14% 62% 5% 11% 7%	2800
July	1st to 10th	Day	26% 32% 10% 18% 11% 2	6290
	11th to 20th	Day	32% 33% 14% 16% 6%	6131
	21st to end	Day	27% 29% 8% 20% 16%	6240
August	1st to 10th	Day	31% 39% 9% 16% 5%	4221
	11th to 20th	Day	24% 40% 8% 21% 7%	4096
	21st to end	Day	20% 38% 7% 20% 14%	4312
September	1st to 10th	Day	16% 58% 6% 10% 9% 1	2400
	11th to 20th	Day	16% 58% 5% 12% 8% 1	2376
	21st to end	Day	20% 61% 4 9% 6% 1	1986
October	1st to 10th	Day	22% 37% 4 14% 18% 4	3160
	11th to 20th	Day	14% 38% 8% 19% 12% 8%	3140
	21st to end	Day	12% 37% 9% 21% 20% 2	3428
November	1st to 10th	Day	18% 56% 6% 9% 8% 3	1852
	11th to 20th	Day	14% 57% 5% 12% 11% 1	1718
	21st to end	Day	17% 57% 7% 13% 5% 1	1600
December	1st to 10th	Day	26% 40% 12% 17% 3 2	2600
	11th to 20th	Day	36% 32% 9% 17% 5% 1	2986
	21st to end	Day	38% 31% 12% 11% 8% 1	3960
January	1st to 10th	Night	31% 30% 8% 18% 11% 2	6275
	11th to 20th	Night	31% 28% 10% 16% 10% 5%	5900
	21st to end	Night	31% 30% 8% 18% 11% 2	6490
February	1st to 10th	Night	17% 63% 7% 10% 3	2600
	11th to 20th	Night	19% 61% 3 9% 8% 1	2516
	21st to end	Night	17% 62% 4 8% 6% 2	1980
March	1st to 10th	Night	31% 30% 6% 19% 7% 6%	4963
	11th to 20th	Night	22% 32% 9% 24% 9% 4	4980
	21st to end	Night	27% 31% 8% 15% 17% 2	5404
April	1st to 10th	Night	19% 63% 3 9% 5% 1	2000
	11th to 20th	Night	15% 64% 5% 9% 6% 1	1975
	21st to end	Night	16% 62% 5% 10% 7% 1	2000
May	1st to 10th	Night	43% 32% 6% 10% 7% 2	4061
	11th to 20th	Night	36% 32% 8% 19% 4	3716
	21st to end	Night	35% 32% 8% 18% 5% 1	3927
June	1st to 10th	Night	16% 62% 4 10% 7%	1600
	11th to 20th	Night	16% 63% 4 10% 6%	1600
	21st to end	Night	14% 62% 6% 10% 7%	1600
July	1st to 10th	Night	28% 31% 12% 14% 12% 3	3560
	11th to 20th	Night	36% 32% 11% 13% 8%	3688
	21st to end	Night	28% 29% 8% 18% 17% 1	4297
August	1st to 10th	Night	32% 39% 10% 14% 4 1	2904
	11th to 20th	Night	23% 40% 8% 21% 8%	2985
	21st to end	Night	20% 38% 8% 22% 12%	3586
September	1st to 10th	Night	15% 58% 7% 11% 8% 1	2000
	11th to 20th	Night	16% 58% 5% 11% 9% 1	1981
	21st to end	Night	20% 61% 4 9% 6%	2190
October	1st to 10th	Night	23% 39% 7% 15% 16% 1	3800
	11th to 20th	Night	13% 38% 6% 17% 19% 7%	3779
	21st to end	Night	12% 37% 8% 21% 18% 4	4120
November	1st to 10th	Night	16% 57% 6% 11% 8% 2	2446
	11th to 20th	Night	17% 57% 3 11% 11% 2	2680
	21st to end	Night	17% 57% 6% 12% 6% 1	2800
December	1st to 10th	Night	24% 38% 11% 21% 5%	4580
	11th to 20th	Night	33% 33% 9% 15% 9% 1	5208
	21st to end	Night	38% 31% 9% 12% 11%	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	36% 27% 27% 7% 1 1	215
	11th to 20th	Day	40% 26% 28% 5% 1	300
	21st to end	Day	29% 35% 27% 7% 2	330
February	1st to 10th	Day	34% 32% 27% 5% 2	300
	11th to 20th	Day	45% 27% 25% 3 1	317
	21st to end	Day	34% 28% 23% 13% 1	414
March	1st to 10th	Day	32% 24% 27% 14% 4	500
	11th to 20th	Day	31% 26% 26% 12% 4 1	500
	21st to end	Day	32% 24% 28% 12% 3 1	550
April	1st to 10th	Day	31% 24% 28% 12% 4 1	500
	11th to 20th	Day	24% 23% 28% 14% 9% 2	492
	21st to end	Day	28% 20% 31% 13% 6% 2	500
May	1st to 10th	Day	28% 23% 28% 12% 7% 3	503
	11th to 20th	Day	25% 19% 28% 17% 9% 2	527
	21st to end	Day	20% 12% 24% 25% 14% 5%	573
June	1st to 10th	Day	16% 17% 28% 19% 16% 4	530
	11th to 20th	Day	20% 12% 29% 25% 12% 3	530
	21st to end	Day	13% 13% 31% 25% 15% 3	530
July	1st to 10th	Day	11% 6% 27% 29% 18% 8%	525
	11th to 20th	Day	8% 7% 26% 30% 22% 7%	524
	21st to end	Day	14% 7% 26% 32% 16% 6%	576
August	1st to 10th	Day	12% 6% 23% 31% 19% 8%	526
	11th to 20th	Day	13% 9% 21% 31% 18% 8%	519
	21st to end	Day	11% 8% 25% 31% 16% 9%	583
September	1st to 10th	Day	19% 9% 19% 28% 17% 8%	530
	11th to 20th	Day	14% 7% 21% 33% 18% 8%	522
	21st to end	Day	29% 13% 23% 20% 11% 4	490
October	1st to 10th	Day	20% 10% 26% 24% 14% 4	490
	11th to 20th	Day	20% 19% 26% 23% 10% 2	490
	21st to end	Day	19% 18% 37% 18% 7% 1	539
November	1st to 10th	Day	18% 19% 29% 23% 9% 2	442
	11th to 20th	Day	25% 23% 29% 17% 6%	318
	21st to end	Day	31% 24% 32% 10% 4	200
December	1st to 10th	Day	30% 30% 29% 7% 4 1	200
	11th to 20th	Day	33% 25% 29% 12% 3	200
	21st to end	Day	25% 29% 29% 15% 2	220
January	1st to 10th	Night	21% 30% 32% 15% 3	800
	11th to 20th	Night	24% 22% 31% 20% 2	800
	21st to end	Night	20% 27% 35% 17% 1	880
February	1st to 10th	Night	22% 23% 33% 21% 2	800
	11th to 20th	Night	28% 28% 30% 13% 1	783
	21st to end	Night	26% 25% 31% 16% 2	660
March	1st to 10th	Night	23% 24% 32% 18% 3	800
	11th to 20th	Night	27% 21% 28% 18% 5% 1	800
	21st to end	Night	22% 21% 36% 15% 5% 1	880
April	1st to 10th	Night	24% 24% 30% 18% 4	800
	11th to 20th	Night	18% 20% 31% 25% 7%	791
	21st to end	Night	21% 18% 30% 23% 8% 1	800
May	1st to 10th	Night	21% 14% 26% 27% 11% 1	601
	11th to 20th	Night	21% 10% 28% 31% 11%	456
	21st to end	Night	15% 8% 26% 31% 18% 2	433
June	1st to 10th	Night	13% 4 29% 32% 22% 1	400
	11th to 20th	Night	17% 6% 22% 39% 16% 1	400
	21st to end	Night	9% 6% 23% 45% 17% 1	400
July	1st to 10th	Night	13% 4 17% 38% 26% 3	390
	11th to 20th	Night	9% 5% 16% 40% 28% 2	446
	21st to end	Night	10% 3 18% 40% 26% 3	657
August	1st to 10th	Night	12% 3 12% 42% 28% 2	597
	11th to 20th	Night	11% 6% 12% 41% 26% 3	638
	21st to end	Night	13% 4 16% 39% 24% 4	880
September	1st to 10th	Night	16% 4 22% 32% 23% 3	800
	11th to 20th	Night	12% 6% 18% 33% 26% 4	793
	21st to end	Night	23% 10% 19% 28% 19% 3	760
October	1st to 10th	Night	21% 10% 18% 28% 20% 3	760
	11th to 20th	Night	17% 15% 27% 29% 11% 1	760
	21st to end	Night	20% 15% 28% 28% 8% 1	836
November	1st to 10th	Night	14% 15% 31% 29% 10% 1	780
	11th to 20th	Night	19% 22% 30% 22% 6% 1	800
	21st to end	Night	20% 22% 30% 22% 6% 1	800
December	1st to 10th	Night	22% 23% 30% 18% 6%	800
	11th to 20th	Night	21% 22% 32% 23% 3	800
	21st to end	Night	20% 20% 32% 25% 2	880

Page generated at 2025-09-16 01:49:10. Version ac07

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