Tom's Yard
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tom's Yard clear sky chart from 2005-11-06 to 2025-07-21.

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% 9% 3 8% 5% 5% 5% 4 5% 7% 33%	120277
	Night	16% 9% 4 8% 5% 4 4 4 5% 6% 34%	118831
Transparency	Day	48% 6% 12% 20% 12% 2	123914
	Night	48% 6% 12% 20% 12% 2	123331
Seeing	Day	25% 16% 25% 19% 11% 4	15670
	Night	22% 13% 25% 28% 12% 1	25147

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% 17% 6% 18% 4 3 4 3 3 4 25%	3661
	11th to 20th	Day	12% 18% 10% 16% 8% 5% 4 3 3 4 17%	4060
	21st to end	Day	16% 20% 5% 10% 5% 5% 4 3 4 5% 25%	4466
February	1st to 10th	Day	16% 16% 5% 9% 7% 4 4 3 5% 5% 26%	1800
	11th to 20th	Day	15% 11% 5% 12% 6% 5% 3 3 6% 5% 29%	1753
	21st to end	Day	18% 10% 4 12% 5% 5% 5% 3 5% 4 30%	1629
March	1st to 10th	Day	18% 14% 3 6% 4 4 4 3 4 5% 36%	4930
	11th to 20th	Day	13% 10% 3 8% 5% 5% 4 3 5% 7% 37%	4980
	21st to end	Day	14% 9% 3 7% 5% 5% 4 4 5% 5% 38%	5482
April	1st to 10th	Day	17% 11% 4 7% 6% 4 4 4 5% 6% 32%	2400
	11th to 20th	Day	15% 11% 3 6% 5% 5% 4 4 5% 6% 36%	2374
	21st to end	Day	20% 8% 3 7% 4 4 5% 3 5% 5% 36%	2475
May	1st to 10th	Day	24% 7% 2 7% 5% 5% 5% 4 5% 7% 28%	5862
	11th to 20th	Day	23% 7% 3 7% 5% 5% 5% 3 5% 7% 30%	6174
	21st to end	Day	18% 7% 2 7% 4 5% 6% 4 6% 8% 33%	6934
June	1st to 10th	Day	14% 7% 2 6% 5% 5% 6% 4 6% 8% 37%	2800
	11th to 20th	Day	15% 6% 2 7% 5% 5% 5% 4 7% 9% 35%	2800
	21st to end	Day	11% 5% 2 6% 5% 5% 5% 4 7% 10% 40%	2800
July	1st to 10th	Day	7% 5% 2 7% 6% 6% 6% 5% 7% 10% 40%	4580
	11th to 20th	Day	7% 5% 2 7% 6% 6% 6% 5% 8% 10% 39%	4482
	21st to end	Day	11% 7% 3 8% 5% 5% 5% 4 6% 7% 38%	4466
August	1st to 10th	Day	18% 6% 2 7% 5% 6% 6% 4 7% 9% 31%	4104
	11th to 20th	Day	11% 6% 2 7% 5% 6% 6% 5% 6% 8% 36%	3995
	21st to end	Day	12% 5% 2 7% 5% 4 5% 4 6% 9% 43%	4180
September	1st to 10th	Day	17% 5% 2 6% 4 4 4 3 5% 6% 45%	2280
	11th to 20th	Day	14% 5% 2 5% 3 3 4 3 5% 7% 47%	2256
	21st to end	Day	20% 8% 2 6% 4 4 5% 3 6% 6% 36%	1995
October	1st to 10th	Day	18% 7% 2 7% 5% 4 5% 3 4 6% 40%	1900
	11th to 20th	Day	17% 6% 3 7% 5% 4 5% 4 5% 6% 37%	1900
	21st to end	Day	18% 8% 2 7% 4 4 4 3 5% 6% 38%	2090
November	1st to 10th	Day	11% 7% 3 6% 4 5% 3 3 6% 7% 45%	1859
	11th to 20th	Day	16% 10% 3 7% 5% 5% 4 5% 5% 6% 34%	1660
	21st to end	Day	17% 12% 4 7% 6% 4 3 4 5% 6% 31%	1600
December	1st to 10th	Day	15% 9% 8% 13% 6% 5% 4 4 5% 5% 25%	2600
	11th to 20th	Day	22% 15% 4 7% 4 4 4 3 4 5% 29%	2990
	21st to end	Day	19% 17% 5% 11% 6% 5% 4 4 5% 5% 17%	3960
January	1st to 10th	Night	12% 14% 6% 20% 5% 4 3 3 4 5% 22%	6280
	11th to 20th	Night	13% 14% 9% 15% 6% 4 4 3 4 6% 22%	5900
	21st to end	Night	18% 14% 4 10% 5% 5% 4 3 6% 6% 26%	6490
February	1st to 10th	Night	16% 12% 4 8% 6% 4 5% 4 5% 6% 30%	2600
	11th to 20th	Night	16% 10% 4 10% 5% 5% 5% 4 5% 6% 30%	2533
	21st to end	Night	16% 11% 4 9% 5% 5% 4 4 6% 5% 32%	2000
March	1st to 10th	Night	19% 11% 3 8% 4 4 4 2 5% 5% 35%	4918
	11th to 20th	Night	12% 9% 3 6% 4 4 4 3 5% 6% 44%	4980
	21st to end	Night	13% 9% 3 8% 5% 5% 4 3 4 5% 40%	5435
April	1st to 10th	Night	19% 11% 3 8% 5% 5% 4 4 4 5% 31%	2000
	11th to 20th	Night	16% 8% 2 7% 5% 4 4 4 5% 8% 38%	1975
	21st to end	Night	17% 6% 2 6% 4 5% 4 4 6% 7% 40%	1940
May	1st to 10th	Night	26% 8% 3 6% 5% 4 4 4 5% 7% 27%	4033
	11th to 20th	Night	26% 7% 2 6% 4 3 4 4 4 7% 33%	3718
	21st to end	Night	19% 8% 2 7% 4 4 4 4 6% 8% 34%	3920
June	1st to 10th	Night	12% 7% 2 7% 4 4 4 3 5% 7% 45%	1600
	11th to 20th	Night	13% 5% 2 8% 4 4 5% 5% 7% 9% 39%	1600
	21st to end	Night	10% 5% 2 5% 5% 5% 5% 4 6% 12% 43%	1600
July	1st to 10th	Night	8% 5% 2 7% 5% 5% 5% 4 7% 9% 42%	2600
	11th to 20th	Night	7% 4 2 6% 5% 6% 6% 4 7% 11% 43%	2693
	21st to end	Night	13% 6% 2 7% 5% 5% 4 3 6% 7% 40%	3087
August	1st to 10th	Night	18% 6% 3 7% 4 4 4 3 6% 9% 36%	2825
	11th to 20th	Night	15% 6% 2 6% 4 4 4 4 6% 7% 41%	2800
	21st to end	Night	14% 5% 2 6% 4 4 4 4 5% 8% 45%	3476
September	1st to 10th	Night	15% 5% 2 6% 5% 4 3 3 5% 5% 46%	1900
	11th to 20th	Night	11% 5% 2 5% 3 4 3 4 4 7% 51%	1920
	21st to end	Night	18% 6% 3 7% 4 4 3 4 5% 7% 40%	2185
October	1st to 10th	Night	17% 6% 2 5% 4 3 4 4 5% 6% 44%	2280
	11th to 20th	Night	17% 5% 2 5% 4 4 4 3 5% 7% 43%	2280
	21st to end	Night	20% 8% 2 5% 3 3 4 3 5% 7% 39%	2508
November	1st to 10th	Night	10% 7% 2 6% 4 4 4 3 5% 6% 51%	2436
	11th to 20th	Night	17% 10% 3 6% 4 5% 4 4 5% 5% 35%	2738
	21st to end	Night	16% 10% 3 7% 5% 4 4 3 6% 6% 35%	2800
December	1st to 10th	Night	14% 10% 8% 9% 5% 4 5% 4 5% 6% 29%	4580
	11th to 20th	Night	21% 12% 4 8% 5% 4 4 4 4 5% 27%	5205
	21st to end	Night	19% 14% 5% 10% 6% 5% 5% 5% 5% 4 22%	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	63% 5% 10% 15% 5% 1	3662
	11th to 20th	Day	71% 4 8% 9% 6% 2	4060
	21st to end	Day	62% 4 10% 15% 8%	4466
February	1st to 10th	Day	59% 5% 12% 16% 7% 1	1800
	11th to 20th	Day	56% 6% 10% 21% 7% 1	1762
	21st to end	Day	56% 5% 12% 18% 7% 3	1629
March	1st to 10th	Day	50% 5% 11% 20% 10% 4	4930
	11th to 20th	Day	46% 5% 14% 23% 8% 3	4980
	21st to end	Day	47% 5% 11% 20% 14% 3	5482
April	1st to 10th	Day	52% 5% 11% 17% 12% 2	2400
	11th to 20th	Day	47% 6% 9% 19% 16% 2	2374
	21st to end	Day	48% 6% 9% 21% 14% 2	2475
May	1st to 10th	Day	54% 6% 12% 15% 10% 2	5897
	11th to 20th	Day	53% 7% 10% 20% 10% 1	6178
	21st to end	Day	46% 8% 12% 20% 12% 2	6949
June	1st to 10th	Day	43% 8% 12% 21% 15% 1	2800
	11th to 20th	Day	42% 10% 13% 19% 14% 2	2800
	21st to end	Day	38% 7% 13% 26% 14% 1	2800
July	1st to 10th	Day	39% 8% 14% 25% 11% 3	4510
	11th to 20th	Day	37% 7% 16% 24% 16% 1	4407
	21st to end	Day	43% 7% 10% 22% 18% 1	4324
August	1st to 10th	Day	48% 8% 14% 21% 9%	4091
	11th to 20th	Day	43% 7% 12% 26% 12% 1	4034
	21st to end	Day	37% 6% 12% 28% 15% 2	4180
September	1st to 10th	Day	40% 7% 10% 22% 18% 3	2280
	11th to 20th	Day	35% 6% 10% 27% 19% 3	2256
	21st to end	Day	47% 7% 11% 17% 17% 2	1995
October	1st to 10th	Day	39% 5% 9% 25% 18% 3	3160
	11th to 20th	Day	40% 7% 10% 23% 17% 3	3140
	21st to end	Day	31% 4 15% 28% 21% 1	3428
November	1st to 10th	Day	38% 6% 15% 21% 17% 4	1859
	11th to 20th	Day	48% 6% 11% 20% 12% 2	1660
	21st to end	Day	53% 6% 11% 22% 8%	1600
December	1st to 10th	Day	60% 7% 13% 15% 6% 1	2600
	11th to 20th	Day	58% 4 12% 20% 5%	2986
	21st to end	Day	67% 6% 12% 9% 5% 1	3960
January	1st to 10th	Night	64% 5% 9% 15% 7% 1	6280
	11th to 20th	Night	63% 6% 10% 12% 7% 2	5900
	21st to end	Night	57% 5% 10% 17% 9% 1	6490
February	1st to 10th	Night	53% 6% 11% 22% 8%	2600
	11th to 20th	Night	53% 6% 11% 18% 11% 1	2535
	21st to end	Night	53% 6% 10% 19% 11% 2	2000
March	1st to 10th	Night	51% 5% 11% 21% 10% 2	4918
	11th to 20th	Night	38% 7% 15% 27% 9% 4	4980
	21st to end	Night	46% 5% 10% 19% 19% 2	5435
April	1st to 10th	Night	53% 6% 10% 17% 13% 2	2000
	11th to 20th	Night	43% 7% 11% 21% 16% 3	1975
	21st to end	Night	42% 7% 10% 23% 15% 3	1940
May	1st to 10th	Night	56% 7% 9% 14% 11% 3	4060
	11th to 20th	Night	50% 7% 11% 22% 9% 1	3719
	21st to end	Night	46% 8% 12% 21% 11% 3	3927
June	1st to 10th	Night	39% 7% 12% 26% 15% 2	1600
	11th to 20th	Night	39% 10% 13% 22% 14% 2	1600
	21st to end	Night	34% 8% 15% 26% 17% 1	1600
July	1st to 10th	Night	38% 9% 14% 21% 15% 4	2560
	11th to 20th	Night	35% 8% 17% 23% 17% 1	2645
	21st to end	Night	41% 8% 13% 16% 21% 2	2988
August	1st to 10th	Night	44% 8% 15% 23% 9%	2814
	11th to 20th	Night	41% 7% 11% 27% 14% 1	2827
	21st to end	Night	37% 7% 12% 31% 12% 2	3476
September	1st to 10th	Night	39% 5% 11% 22% 19% 3	1900
	11th to 20th	Night	32% 6% 10% 29% 21% 2	1920
	21st to end	Night	44% 6% 10% 21% 17% 2	2185
October	1st to 10th	Night	38% 7% 10% 24% 20% 1	3800
	11th to 20th	Night	32% 6% 11% 26% 22% 3	3779
	21st to end	Night	33% 3 12% 29% 21% 2	4120
November	1st to 10th	Night	35% 4 14% 25% 19% 3	2436
	11th to 20th	Night	48% 5% 10% 20% 14% 3	2738
	21st to end	Night	49% 6% 13% 21% 10% 1	2800
December	1st to 10th	Night	55% 5% 16% 19% 5%	4580
	11th to 20th	Night	58% 5% 13% 17% 6%	5208
	21st to end	Night	63% 6% 11% 14% 7%	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	37% 26% 24% 9% 3	213
	11th to 20th	Day	41% 22% 29% 7% 1	300
	21st to end	Day	31% 31% 28% 7% 3 1	330
February	1st to 10th	Day	35% 29% 22% 11% 3	300
	11th to 20th	Day	43% 28% 24% 5%	300
	21st to end	Day	36% 23% 26% 14% 1	393
March	1st to 10th	Day	37% 18% 24% 17% 4	500
	11th to 20th	Day	35% 20% 25% 14% 4 1	500
	21st to end	Day	31% 23% 27% 13% 5% 2	550
April	1st to 10th	Day	35% 21% 24% 15% 4 2	500
	11th to 20th	Day	27% 22% 22% 17% 8% 4	492
	21st to end	Day	30% 21% 26% 14% 7% 2	500
May	1st to 10th	Day	30% 19% 27% 13% 7% 3	503
	11th to 20th	Day	27% 18% 25% 17% 11% 3	527
	21st to end	Day	21% 14% 26% 19% 13% 6%	573
June	1st to 10th	Day	19% 16% 27% 20% 14% 4	530
	11th to 20th	Day	22% 12% 25% 25% 14% 3	530
	21st to end	Day	14% 13% 32% 26% 12% 4	530
July	1st to 10th	Day	14% 7% 26% 26% 19% 8%	525
	11th to 20th	Day	10% 7% 28% 28% 19% 8%	524
	21st to end	Day	13% 7% 29% 27% 18% 5%	532
August	1st to 10th	Day	14% 7% 25% 26% 19% 8%	486
	11th to 20th	Day	14% 7% 27% 26% 18% 9%	479
	21st to end	Day	14% 8% 25% 23% 18% 11%	539
September	1st to 10th	Day	21% 8% 19% 24% 21% 7%	490
	11th to 20th	Day	15% 7% 19% 28% 21% 10%	490
	21st to end	Day	28% 13% 24% 17% 13% 5%	490
October	1st to 10th	Day	24% 9% 21% 25% 16% 4	490
	11th to 20th	Day	23% 17% 25% 23% 10% 2	490
	21st to end	Day	24% 18% 28% 21% 8% 2	539
November	1st to 10th	Day	21% 16% 26% 22% 13% 1	445
	11th to 20th	Day	30% 19% 25% 19% 7% 1	260
	21st to end	Day	33% 23% 28% 14% 3 1	200
December	1st to 10th	Day	33% 27% 27% 8% 5% 2	200
	11th to 20th	Day	35% 19% 29% 15% 3	200
	21st to end	Day	30% 24% 25% 17% 5%	220
January	1st to 10th	Night	27% 23% 30% 17% 3	800
	11th to 20th	Night	29% 19% 28% 20% 3	800
	21st to end	Night	25% 21% 31% 22% 2	880
February	1st to 10th	Night	29% 16% 28% 25% 2	800
	11th to 20th	Night	31% 22% 28% 18% 1	783
	21st to end	Night	29% 18% 30% 21% 2	660
March	1st to 10th	Night	26% 18% 30% 21% 4	800
	11th to 20th	Night	28% 18% 27% 21% 6%	800
	21st to end	Night	23% 18% 32% 20% 6% 1	880
April	1st to 10th	Night	28% 17% 29% 21% 5%	800
	11th to 20th	Night	22% 16% 28% 26% 7% 1	791
	21st to end	Night	23% 13% 25% 32% 7%	740
May	1st to 10th	Night	24% 10% 22% 31% 13% 1	600
	11th to 20th	Night	21% 8% 28% 34% 9%	459
	21st to end	Night	15% 8% 23% 35% 18% 2	433
June	1st to 10th	Night	13% 6% 25% 39% 18%	400
	11th to 20th	Night	15% 5% 23% 40% 17% 1	400
	21st to end	Night	10% 4 27% 43% 16% 1	400
July	1st to 10th	Night	11% 4 17% 43% 23% 3	390
	11th to 20th	Night	7% 4 17% 42% 29% 1	442
	21st to end	Night	10% 3 19% 42% 24% 2	624
August	1st to 10th	Night	10% 2 14% 40% 32% 2	567
	11th to 20th	Night	13% 4 16% 36% 28% 4	566
	21st to end	Night	14% 3 18% 33% 28% 3	836
September	1st to 10th	Night	17% 4 18% 33% 25% 3	760
	11th to 20th	Night	14% 6% 18% 27% 31% 4	760
	21st to end	Night	21% 8% 21% 26% 20% 3	760
October	1st to 10th	Night	20% 7% 22% 27% 21% 3	760
	11th to 20th	Night	20% 13% 26% 28% 12% 1	760
	21st to end	Night	25% 11% 25% 29% 8% 1	836
November	1st to 10th	Night	15% 11% 28% 34% 11% 1	780
	11th to 20th	Night	27% 17% 22% 25% 8% 1	800
	21st to end	Night	27% 16% 27% 24% 6% 1	800
December	1st to 10th	Night	29% 17% 28% 20% 6%	800
	11th to 20th	Night	28% 16% 25% 27% 4	800
	21st to end	Night	25% 17% 27% 28% 3	880

Page generated at 2025-07-18 01:10:02. Version ac07

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