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 2024-11-04.

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% 34%	101292
	Night	16% 9% 3 7% 5% 4 4 4 5% 7% 35%	97667
Transparency	Day	47% 7% 12% 21% 12% 2	104931
	Night	46% 7% 12% 21% 13% 2	102167
Seeing	Day	24% 16% 25% 19% 11% 4	14798
	Night	22% 13% 25% 28% 12% 1	24263

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	15% 17% 6% 10% 5% 4 4 2 3 5% 28%	2553
	11th to 20th	Day	12% 18% 8% 15% 7% 5% 4 3 4 4 21%	2840
	21st to end	Day	19% 19% 6% 11% 6% 6% 4 3 4 4 19%	3124
February	1st to 10th	Day	16% 16% 4 9% 7% 4 4 3 5% 5% 26%	1710
	11th to 20th	Day	14% 11% 5% 12% 6% 5% 3 3 6% 5% 29%	1663
	21st to end	Day	19% 10% 4 11% 5% 4 5% 3 5% 4 30%	1550
March	1st to 10th	Day	22% 13% 3 6% 5% 4 4 3 4 6% 29%	3430
	11th to 20th	Day	13% 11% 4 8% 5% 5% 4 3 6% 6% 35%	3480
	21st to end	Day	16% 10% 3 7% 5% 4 3 3 5% 5% 39%	3832
April	1st to 10th	Day	16% 11% 3 7% 6% 4 4 4 5% 6% 33%	2280
	11th to 20th	Day	15% 11% 3 6% 5% 5% 4 4 5% 6% 36%	2254
	21st to end	Day	21% 9% 3 7% 4 4 5% 3 5% 5% 35%	2351
May	1st to 10th	Day	27% 7% 3 7% 5% 4 4 4 4 7% 29%	4082
	11th to 20th	Day	23% 8% 3 7% 5% 6% 5% 3 5% 6% 30%	4296
	21st to end	Day	12% 6% 2 7% 4 5% 6% 4 6% 9% 39%	4823
June	1st to 10th	Day	14% 6% 2 6% 6% 5% 6% 5% 6% 9% 36%	2660
	11th to 20th	Day	13% 6% 2 7% 5% 5% 5% 4 7% 9% 36%	2660
	21st to end	Day	11% 5% 2 6% 6% 5% 5% 5% 7% 10% 39%	2660
July	1st to 10th	Day	8% 5% 2 7% 6% 6% 6% 5% 7% 10% 40%	4440
	11th to 20th	Day	7% 5% 2 7% 6% 6% 6% 5% 8% 10% 40%	4345
	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 4 6% 7% 45%	1794
	11th to 20th	Day	16% 10% 3 7% 5% 5% 4 5% 5% 6% 34%	1577
	21st to end	Day	18% 11% 4 7% 6% 4 3 4 6% 6% 32%	1520
December	1st to 10th	Day	19% 13% 4 10% 5% 6% 4 5% 5% 6% 25%	1520
	11th to 20th	Day	18% 15% 5% 7% 5% 4 4 3 5% 6% 29%	1910
	21st to end	Day	21% 17% 5% 11% 7% 5% 4 4 4 4 18%	2772
January	1st to 10th	Night	14% 16% 6% 10% 6% 5% 4 3 5% 6% 26%	4388
	11th to 20th	Night	16% 15% 7% 11% 5% 4 4 3 4 5% 24%	4120
	21st to end	Night	23% 15% 5% 12% 5% 4 3 3 5% 5% 22%	4532
February	1st to 10th	Night	16% 12% 4 8% 6% 4 5% 4 5% 6% 30%	2470
	11th to 20th	Night	15% 11% 3 9% 6% 5% 5% 4 5% 7% 30%	2403
	21st to end	Night	16% 11% 4 9% 5% 5% 4 3 6% 4 32%	1903
March	1st to 10th	Night	22% 10% 3 7% 5% 4 4 2 5% 5% 34%	3418
	11th to 20th	Night	12% 11% 4 7% 4 4 4 3 5% 6% 38%	3480
	21st to end	Night	15% 8% 3 7% 4 4 4 2 4 5% 43%	3799
April	1st to 10th	Night	18% 11% 3 8% 5% 5% 4 4 4 5% 32%	1900
	11th to 20th	Night	16% 7% 2 7% 5% 4 4 4 5% 8% 38%	1875
	21st to end	Night	17% 6% 2 7% 4 5% 4 4 6% 7% 40%	1843
May	1st to 10th	Night	27% 9% 3 5% 5% 4 4 4 4 6% 28%	2813
	11th to 20th	Night	26% 8% 2 6% 5% 4 4 4 4 6% 30%	2596
	21st to end	Night	11% 7% 2 7% 4 5% 5% 4 7% 10% 37%	2732
June	1st to 10th	Night	12% 7% 2 6% 4 4 5% 3 6% 7% 45%	1520
	11th to 20th	Night	12% 5% 2 8% 4 5% 5% 5% 7% 9% 39%	1520
	21st to end	Night	10% 5% 2 5% 4 5% 5% 4 6% 12% 43%	1520
July	1st to 10th	Night	8% 5% 2 7% 5% 5% 5% 4 7% 9% 42%	2520
	11th to 20th	Night	6% 4 2 6% 4 6% 6% 4 7% 11% 43%	2610
	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% 50%	2354
	11th to 20th	Night	17% 10% 3 6% 4 4 4 4 5% 5% 36%	2601
	21st to end	Night	16% 10% 3 7% 5% 4 4 3 6% 6% 35%	2660
December	1st to 10th	Night	19% 11% 5% 8% 5% 4 5% 4 5% 6% 27%	2660
	11th to 20th	Night	18% 12% 4 8% 5% 5% 4 4 5% 6% 29%	3285
	21st to end	Night	19% 13% 4 10% 6% 6% 5% 5% 5% 5% 22%	4884

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	58% 6% 12% 18% 4 2	2554
	11th to 20th	Day	66% 4 10% 11% 8% 1	2840
	21st to end	Day	68% 5% 9% 13% 4 1	3124
February	1st to 10th	Day	59% 5% 13% 16% 7% 1	1710
	11th to 20th	Day	55% 6% 10% 21% 7% 1	1672
	21st to end	Day	56% 5% 11% 17% 8% 3	1550
March	1st to 10th	Day	54% 6% 12% 21% 7% 1	3430
	11th to 20th	Day	47% 6% 14% 22% 9% 2	3480
	21st to end	Day	47% 6% 13% 21% 12% 2	3832
April	1st to 10th	Day	51% 5% 11% 18% 12% 2	2280
	11th to 20th	Day	47% 7% 9% 19% 16% 2	2254
	21st to end	Day	49% 6% 9% 21% 13% 2	2351
May	1st to 10th	Day	55% 7% 9% 18% 10% 1	4120
	11th to 20th	Day	54% 8% 12% 17% 8% 1	4300
	21st to end	Day	37% 10% 14% 22% 14% 2	4837
June	1st to 10th	Day	43% 8% 12% 21% 15% 1	2660
	11th to 20th	Day	41% 10% 13% 20% 14% 2	2660
	21st to end	Day	39% 7% 13% 26% 13% 1	2660
July	1st to 10th	Day	39% 8% 15% 25% 11% 3	4370
	11th to 20th	Day	37% 7% 16% 24% 16% 1	4270
	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% 14% 22% 16% 4	1794
	11th to 20th	Day	49% 6% 11% 20% 11% 2	1577
	21st to end	Day	52% 6% 11% 23% 8%	1520
December	1st to 10th	Day	59% 8% 13% 14% 6%	1520
	11th to 20th	Day	56% 5% 12% 21% 5%	1906
	21st to end	Day	67% 6% 12% 11% 4	2772
January	1st to 10th	Night	58% 6% 11% 18% 7% 1	4388
	11th to 20th	Night	61% 7% 12% 14% 6% 1	4120
	21st to end	Night	65% 5% 9% 16% 5% 1	4532
February	1st to 10th	Night	52% 7% 11% 21% 8%	2470
	11th to 20th	Night	53% 6% 11% 18% 12%	2405
	21st to end	Night	53% 6% 10% 18% 11% 2	1903
March	1st to 10th	Night	52% 5% 11% 22% 8% 1	3418
	11th to 20th	Night	43% 9% 12% 26% 9% 1	3480
	21st to end	Night	42% 5% 10% 22% 18% 3	3799
April	1st to 10th	Night	52% 6% 11% 17% 13% 2	1900
	11th to 20th	Night	43% 7% 11% 21% 16% 3	1875
	21st to end	Night	42% 7% 10% 22% 14% 3	1843
May	1st to 10th	Night	55% 7% 7% 17% 11% 2	2840
	11th to 20th	Night	52% 9% 12% 18% 9% 1	2597
	21st to end	Night	37% 11% 16% 22% 11% 4	2739
June	1st to 10th	Night	38% 7% 12% 26% 15% 2	1520
	11th to 20th	Night	37% 10% 13% 23% 14% 2	1520
	21st to end	Night	33% 9% 15% 26% 16% 1	1520
July	1st to 10th	Night	38% 9% 14% 21% 14% 4	2480
	11th to 20th	Night	34% 8% 16% 24% 17% 1	2562
	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% 5% 14% 26% 18% 3	2354
	11th to 20th	Night	48% 5% 10% 21% 14% 3	2601
	21st to end	Night	48% 6% 13% 21% 10% 1	2660
December	1st to 10th	Night	55% 8% 14% 17% 7%	2660
	11th to 20th	Night	55% 6% 12% 20% 8%	3288
	21st to end	Night	61% 7% 13% 15% 3	4884

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% 24% 26% 10% 3 1	191
	11th to 20th	Day	40% 22% 29% 7% 1	270
	21st to end	Day	31% 30% 29% 7% 3 1	297
February	1st to 10th	Day	34% 29% 23% 12% 3	270
	11th to 20th	Day	41% 28% 26% 5%	270
	21st to end	Day	38% 21% 25% 14% 1	362
March	1st to 10th	Day	35% 18% 24% 17% 5%	460
	11th to 20th	Day	35% 21% 25% 13% 4 2	460
	21st to end	Day	31% 23% 26% 13% 5% 2	506
April	1st to 10th	Day	33% 22% 23% 16% 4 2	460
	11th to 20th	Day	26% 21% 23% 18% 9% 4	452
	21st to end	Day	31% 21% 26% 14% 7% 2	460
May	1st to 10th	Day	31% 19% 25% 14% 7% 3	463
	11th to 20th	Day	26% 18% 25% 17% 11% 3	487
	21st to end	Day	18% 15% 26% 20% 14% 6%	529
June	1st to 10th	Day	19% 16% 27% 21% 14% 3	490
	11th to 20th	Day	19% 13% 24% 26% 15% 3	490
	21st to end	Day	14% 13% 33% 25% 11% 3	490
July	1st to 10th	Day	15% 8% 26% 25% 19% 8%	485
	11th to 20th	Day	9% 7% 29% 28% 19% 7%	484
	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% 15% 25% 23% 14% 1	422
	11th to 20th	Day	31% 19% 24% 17% 7% 1	237
	21st to end	Day	29% 24% 29% 14% 3 1	180
December	1st to 10th	Day	34% 25% 26% 7% 6% 2	180
	11th to 20th	Day	35% 19% 28% 14% 3	180
	21st to end	Day	32% 23% 24% 16% 5%	198
January	1st to 10th	Night	26% 22% 31% 18% 3	760
	11th to 20th	Night	29% 19% 28% 21% 3	760
	21st to end	Night	25% 21% 29% 22% 2	836
February	1st to 10th	Night	28% 16% 28% 25% 3	760
	11th to 20th	Night	30% 23% 29% 17% 1	743
	21st to end	Night	30% 18% 29% 21% 2	628
March	1st to 10th	Night	26% 18% 30% 22% 4	760
	11th to 20th	Night	29% 19% 26% 20% 6%	760
	21st to end	Night	23% 18% 32% 20% 6% 1	836
April	1st to 10th	Night	27% 18% 30% 21% 5%	760
	11th to 20th	Night	21% 16% 28% 26% 8% 1	751
	21st to end	Night	23% 14% 25% 31% 7%	703
May	1st to 10th	Night	24% 10% 22% 31% 13% 1	570
	11th to 20th	Night	20% 8% 29% 34% 9%	436
	21st to end	Night	13% 8% 23% 36% 19% 2	411
June	1st to 10th	Night	13% 6% 25% 38% 18%	380
	11th to 20th	Night	13% 5% 23% 40% 18% 1	380
	21st to end	Night	10% 4 28% 44% 14%	380
July	1st to 10th	Night	11% 4 17% 43% 22% 3	370
	11th to 20th	Night	7% 4 18% 42% 29% 1	419
	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	755
	11th to 20th	Night	26% 18% 22% 25% 8% 1	760
	21st to end	Night	25% 16% 27% 25% 6% 1	760
December	1st to 10th	Night	29% 17% 28% 20% 7%	760
	11th to 20th	Night	28% 16% 26% 27% 4	760
	21st to end	Night	25% 18% 28% 28% 2	836

Page generated at 2024-11-01 22:47:36. Version ac07

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