Tracy
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tracy clear sky chart from 2005-11-06 to 2024-12-06.

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% 9% 3 8% 5% 4 5% 4 5% 7% 34%	101808
	Night	18% 11% 3 8% 5% 4 4 3 5% 6% 32%	97902
Transparency	Day	48% 7% 14% 21% 10% 1	105270
	Night	52% 6% 12% 19% 10% 1	102528
Seeing	Day	20% 8% 27% 26% 14% 4	14640
	Night	23% 6% 28% 32% 11% 1	23588

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	18% 15% 4 9% 4 5% 4 3 4 5% 29%	2517
	11th to 20th	Day	14% 14% 6% 8% 5% 5% 4 3 5% 6% 30%	2520
	21st to end	Day	13% 15% 6% 11% 6% 5% 5% 4 5% 6% 25%	2772
February	1st to 10th	Day	15% 16% 5% 9% 7% 4 5% 4 3 4 28%	1623
	11th to 20th	Day	14% 13% 4 11% 6% 5% 4 3 6% 5% 28%	1777
	21st to end	Day	17% 15% 4 9% 5% 4 4 4 5% 6% 28%	1570
March	1st to 10th	Day	19% 11% 4 10% 6% 4 5% 4 5% 5% 28%	3116
	11th to 20th	Day	13% 17% 5% 10% 6% 5% 3 3 4 6% 28%	3265
	21st to end	Day	14% 10% 3 10% 5% 4 4 4 5% 5% 37%	4166
April	1st to 10th	Day	21% 11% 3 7% 6% 4 5% 3 5% 5% 31%	2280
	11th to 20th	Day	21% 7% 3 6% 5% 4 4 3 5% 5% 37%	2254
	21st to end	Day	22% 9% 3 8% 4 4 5% 4 4 5% 32%	2470
May	1st to 10th	Day	20% 7% 3 8% 4 5% 4 3 5% 7% 34%	4345
	11th to 20th	Day	16% 7% 3 8% 5% 5% 5% 4 6% 8% 33%	4398
	21st to end	Day	15% 6% 2 6% 5% 5% 5% 4 7% 9% 36%	4837
June	1st to 10th	Day	21% 8% 2 7% 5% 5% 4 4 6% 8% 30%	2660
	11th to 20th	Day	14% 6% 2 7% 4 4 5% 5% 7% 8% 37%	2660
	21st to end	Day	20% 9% 3 8% 5% 4 5% 4 6% 8% 29%	2660
July	1st to 10th	Day	12% 7% 2 7% 5% 5% 6% 5% 7% 9% 36%	4440
	11th to 20th	Day	14% 5% 2 6% 5% 5% 5% 5% 7% 9% 37%	4440
	21st to end	Day	11% 6% 2 5% 4 4 6% 4 5% 8% 44%	4814
August	1st to 10th	Day	13% 6% 2 7% 5% 6% 7% 5% 8% 9% 32%	4423
	11th to 20th	Day	16% 6% 2 7% 5% 4 5% 5% 6% 9% 36%	4044
	21st to end	Day	17% 5% 2 6% 4 4 5% 3 6% 8% 39%	4180
September	1st to 10th	Day	12% 5% 2 7% 4 5% 5% 4 6% 8% 42%	2280
	11th to 20th	Day	15% 5% 2 5% 4 3 3 4 5% 7% 48%	2180
	21st to end	Day	17% 6% 2 6% 4 4 4 4 6% 5% 41%	2090
October	1st to 10th	Day	19% 6% 2 7% 5% 4 4 3 6% 6% 36%	1990
	11th to 20th	Day	19% 9% 3 8% 5% 5% 5% 5% 6% 6% 30%	1825
	21st to end	Day	24% 10% 4 6% 4 4 3 4 4 6% 32%	1881
November	1st to 10th	Day	16% 10% 2 8% 5% 4 5% 4 5% 7% 34%	1761
	11th to 20th	Day	19% 11% 3 8% 4 3 4 4 6% 7% 30%	1732
	21st to end	Day	22% 14% 4 7% 5% 4 5% 3 5% 6% 27%	1600
December	1st to 10th	Day	25% 14% 4 9% 5% 5% 4 3 3 4 24%	1560
	11th to 20th	Day	22% 14% 4 10% 5% 3 4 3 3 4 28%	1906
	21st to end	Day	28% 16% 5% 9% 4 3 3 2 3 4 23%	2772
January	1st to 10th	Night	17% 16% 5% 10% 5% 5% 4 3 4 5% 26%	4429
	11th to 20th	Night	12% 13% 5% 12% 8% 6% 5% 4 5% 5% 26%	4440
	21st to end	Night	14% 16% 4 12% 7% 5% 4 3 5% 5% 24%	4884
February	1st to 10th	Night	15% 14% 5% 12% 7% 4 5% 4 6% 5% 24%	2556
	11th to 20th	Night	14% 12% 5% 10% 6% 6% 5% 4 5% 4 29%	2288
	21st to end	Night	15% 13% 4 10% 6% 4 4 3 5% 5% 31%	1884
March	1st to 10th	Night	22% 13% 4 8% 4 3 4 3 4 5% 31%	3724
	11th to 20th	Night	17% 18% 4 9% 4 4 4 3 4 4 28%	3688
	21st to end	Night	14% 10% 4 9% 5% 4 4 4 4 5% 36%	3466
April	1st to 10th	Night	24% 9% 3 8% 4 4 4 3 5% 6% 31%	1900
	11th to 20th	Night	21% 8% 3 7% 5% 4 4 4 4 6% 35%	1875
	21st to end	Night	22% 10% 3 6% 4 4 3 4 5% 5% 35%	1710
May	1st to 10th	Night	22% 10% 3 8% 4 5% 5% 3 5% 6% 29%	2624
	11th to 20th	Night	21% 7% 3 7% 5% 5% 4 4 5% 7% 33%	2495
	21st to end	Night	16% 7% 3 7% 5% 5% 5% 4 5% 8% 35%	2738
June	1st to 10th	Night	20% 8% 2 6% 3 4 5% 4 6% 7% 34%	1520
	11th to 20th	Night	18% 6% 2 6% 4 4 5% 4 6% 7% 38%	1520
	21st to end	Night	19% 7% 3 7% 5% 4 6% 4 6% 7% 32%	1520
July	1st to 10th	Night	12% 7% 3 8% 6% 5% 6% 5% 7% 8% 34%	2520
	11th to 20th	Night	16% 5% 2 7% 4 4 5% 4 6% 7% 41%	2520
	21st to end	Night	12% 5% 2 6% 5% 5% 4 5% 6% 7% 45%	2739
August	1st to 10th	Night	15% 7% 4 8% 5% 4 5% 4 6% 7% 36%	2506
	11th to 20th	Night	17% 6% 2 7% 5% 5% 5% 3 5% 7% 38%	2723
	21st to end	Night	18% 6% 3 7% 6% 4 4 4 5% 6% 38%	3476
September	1st to 10th	Night	13% 5% 2 6% 5% 4 4 4 5% 7% 46%	1900
	11th to 20th	Night	15% 5% 2 6% 4 3 3 3 4 7% 49%	2000
	21st to end	Night	17% 6% 2 5% 4 4 4 3 5% 7% 43%	2090
October	1st to 10th	Night	18% 6% 2 7% 5% 3 4 3 5% 6% 40%	2186
	11th to 20th	Night	20% 9% 2 8% 4 4 4 3 5% 5% 35%	2353
	21st to end	Night	23% 9% 3 7% 5% 5% 4 3 4 6% 31%	2717
November	1st to 10th	Night	19% 10% 3 7% 5% 4 5% 4 5% 5% 33%	2537
	11th to 20th	Night	19% 10% 3 8% 5% 4 4 4 5% 5% 33%	2665
	21st to end	Night	24% 14% 3 8% 5% 4 3 3 3 5% 27%	2800
December	1st to 10th	Night	22% 15% 5% 9% 6% 4 5% 4 5% 5% 22%	2737
	11th to 20th	Night	23% 14% 4 8% 5% 3 3 2 4 5% 28%	3288
	21st to end	Night	27% 16% 5% 8% 4 4 3 3 3 4 23%	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	57% 7% 15% 16% 4 1	2518
	11th to 20th	Day	55% 6% 13% 18% 8%	2520
	21st to end	Day	57% 8% 12% 17% 6% 1	2772
February	1st to 10th	Day	59% 4 10% 18% 7% 2	1623
	11th to 20th	Day	56% 6% 12% 15% 10% 1	1786
	21st to end	Day	58% 5% 10% 16% 9% 3	1570
March	1st to 10th	Day	56% 7% 10% 17% 8% 1	3119
	11th to 20th	Day	57% 5% 13% 11% 11% 4	3265
	21st to end	Day	46% 5% 11% 20% 16% 3	4166
April	1st to 10th	Day	54% 5% 9% 18% 11% 2	2280
	11th to 20th	Day	47% 6% 10% 22% 13% 2	2254
	21st to end	Day	54% 5% 10% 17% 12% 3	2470
May	1st to 10th	Day	50% 6% 10% 21% 11% 1	4359
	11th to 20th	Day	45% 10% 18% 16% 9% 1	4402
	21st to end	Day	43% 8% 12% 21% 14% 2	4837
June	1st to 10th	Day	51% 7% 12% 18% 10% 2	2660
	11th to 20th	Day	42% 8% 13% 22% 13% 2	2660
	21st to end	Day	52% 8% 11% 19% 9%	2660
July	1st to 10th	Day	42% 8% 15% 25% 10% 1	4370
	11th to 20th	Day	41% 8% 17% 26% 7%	4365
	21st to end	Day	37% 8% 13% 31% 10%	4661
August	1st to 10th	Day	44% 9% 17% 22% 7%	4409
	11th to 20th	Day	42% 9% 20% 23% 6%	4085
	21st to end	Day	42% 7% 15% 20% 15% 1	4180
September	1st to 10th	Day	37% 8% 16% 27% 12% 1	2280
	11th to 20th	Day	35% 7% 14% 30% 13% 1	2180
	21st to end	Day	42% 7% 14% 25% 11%	2090
October	1st to 10th	Day	46% 5% 15% 26% 7%	3302
	11th to 20th	Day	45% 7% 17% 24% 6%	3015
	21st to end	Day	44% 5% 20% 22% 9% 1	3081
November	1st to 10th	Day	49% 6% 14% 21% 9% 1	1761
	11th to 20th	Day	52% 6% 12% 18% 11%	1732
	21st to end	Day	58% 6% 14% 15% 5% 1	1600
December	1st to 10th	Day	64% 5% 11% 13% 6%	1560
	11th to 20th	Day	60% 4 9% 20% 7%	1906
	21st to end	Day	67% 5% 12% 10% 7%	2772
January	1st to 10th	Night	58% 7% 16% 13% 5%	4429
	11th to 20th	Night	58% 6% 13% 17% 5% 1	4440
	21st to end	Night	61% 7% 12% 13% 6% 1	4884
February	1st to 10th	Night	61% 5% 10% 16% 8% 1	2556
	11th to 20th	Night	57% 6% 11% 16% 10% 1	2290
	21st to end	Night	55% 5% 9% 17% 13% 2	1884
March	1st to 10th	Night	55% 6% 11% 16% 8% 4	3734
	11th to 20th	Night	59% 4 9% 12% 12% 3	3689
	21st to end	Night	49% 4 10% 21% 13% 3	3466
April	1st to 10th	Night	54% 5% 9% 16% 14% 2	1900
	11th to 20th	Night	49% 6% 8% 19% 15% 3	1875
	21st to end	Night	52% 5% 7% 18% 15% 3	1710
May	1st to 10th	Night	53% 7% 11% 16% 9% 3	2630
	11th to 20th	Night	47% 10% 13% 17% 11% 2	2496
	21st to end	Night	45% 7% 12% 18% 16% 2	2739
June	1st to 10th	Night	47% 6% 11% 21% 13% 1	1520
	11th to 20th	Night	45% 7% 11% 20% 13% 4	1520
	21st to end	Night	49% 8% 10% 18% 14% 1	1520
July	1st to 10th	Night	45% 7% 17% 19% 11% 1	2480
	11th to 20th	Night	40% 7% 18% 26% 8%	2472
	21st to end	Night	37% 6% 20% 25% 12%	2651
August	1st to 10th	Night	47% 7% 13% 25% 8%	2496
	11th to 20th	Night	45% 7% 17% 23% 7%	2747
	21st to end	Night	46% 8% 11% 21% 13% 1	3476
September	1st to 10th	Night	37% 7% 15% 25% 14% 1	1900
	11th to 20th	Night	36% 5% 14% 32% 12% 1	2000
	21st to end	Night	40% 8% 10% 28% 14%	2090
October	1st to 10th	Night	47% 5% 14% 24% 10%	3654
	11th to 20th	Night	44% 6% 13% 29% 8%	3902
	21st to end	Night	47% 5% 16% 19% 12% 2	4467
November	1st to 10th	Night	52% 6% 13% 20% 8% 1	2537
	11th to 20th	Night	53% 5% 12% 18% 11% 1	2665
	21st to end	Night	60% 5% 12% 17% 7%	2800
December	1st to 10th	Night	64% 6% 13% 13% 5%	2737
	11th to 20th	Night	60% 5% 9% 18% 7% 1	3288
	21st to end	Night	65% 5% 9% 16% 5% 1	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	26% 8% 29% 26% 11%	180
	11th to 20th	Day	24% 13% 27% 28% 7% 1	180
	21st to end	Day	17% 9% 28% 32% 12% 3	198
February	1st to 10th	Day	29% 14% 31% 23% 3	229
	11th to 20th	Day	21% 9% 36% 28% 7%	385
	21st to end	Day	24% 9% 34% 27% 6%	382
March	1st to 10th	Day	22% 8% 30% 31% 8% 1	460
	11th to 20th	Day	19% 12% 34% 27% 8% 1	460
	21st to end	Day	25% 8% 30% 22% 11% 3	506
April	1st to 10th	Day	25% 12% 24% 27% 11% 1	460
	11th to 20th	Day	25% 14% 29% 20% 10% 2	452
	21st to end	Day	28% 9% 30% 23% 9% 2	460
May	1st to 10th	Day	23% 10% 32% 23% 11% 1	463
	11th to 20th	Day	17% 8% 28% 27% 16% 4	487
	21st to end	Day	18% 8% 33% 23% 15% 2	529
June	1st to 10th	Day	22% 9% 25% 30% 11% 3	490
	11th to 20th	Day	15% 10% 25% 28% 18% 3	490
	21st to end	Day	18% 9% 26% 27% 14% 5%	490
July	1st to 10th	Day	13% 4 23% 30% 22% 7%	485
	11th to 20th	Day	14% 5% 23% 36% 18% 4	484
	21st to end	Day	13% 5% 20% 33% 24% 5%	532
August	1st to 10th	Day	15% 3 21% 31% 24% 6%	486
	11th to 20th	Day	12% 5% 19% 30% 22% 11%	479
	21st to end	Day	14% 7% 25% 22% 21% 11%	539
September	1st to 10th	Day	13% 6% 26% 28% 18% 9%	490
	11th to 20th	Day	14% 7% 16% 30% 22% 11%	490
	21st to end	Day	18% 8% 23% 24% 20% 7%	490
October	1st to 10th	Day	21% 9% 24% 25% 17% 4	490
	11th to 20th	Day	22% 8% 30% 25% 12% 3	450
	21st to end	Day	29% 10% 27% 22% 11%	429
November	1st to 10th	Day	23% 11% 28% 28% 10% 1	395
	11th to 20th	Day	26% 9% 32% 22% 10% 1	332
	21st to end	Day	32% 9% 33% 13% 14% 1	200
December	1st to 10th	Day	32% 7% 24% 21% 12% 4	190
	11th to 20th	Day	23% 13% 32% 25% 5% 1	180
	21st to end	Day	34% 12% 27% 19% 7% 1	198
January	1st to 10th	Night	27% 8% 29% 30% 6%	760
	11th to 20th	Night	27% 8% 30% 28% 8%	760
	21st to end	Night	19% 5% 30% 37% 8% 1	836
February	1st to 10th	Night	22% 6% 35% 31% 7% 1	760
	11th to 20th	Night	20% 7% 36% 31% 6% 1	743
	21st to end	Night	22% 6% 34% 34% 5%	628
March	1st to 10th	Night	25% 7% 29% 33% 5%	760
	11th to 20th	Night	23% 9% 32% 29% 7% 1	760
	21st to end	Night	22% 6% 32% 30% 8% 1	836
April	1st to 10th	Night	28% 8% 28% 29% 7%	760
	11th to 20th	Night	25% 8% 33% 27% 6% 1	751
	21st to end	Night	28% 8% 31% 25% 8%	570
May	1st to 10th	Night	28% 6% 30% 30% 6% 1	451
	11th to 20th	Night	22% 5% 29% 35% 8% 1	374
	21st to end	Night	20% 6% 34% 33% 7%	411
June	1st to 10th	Night	23% 3 27% 36% 10% 1	380
	11th to 20th	Night	19% 3 26% 36% 15% 1	380
	21st to end	Night	23% 2 21% 37% 15% 1	380
July	1st to 10th	Night	11% 3 25% 45% 16% 1	370
	11th to 20th	Night	13% 4 19% 47% 16% 1	368
	21st to end	Night	14% 2 20% 45% 17% 1	416
August	1st to 10th	Night	15% 2 15% 44% 23% 2	378
	11th to 20th	Night	14% 2 19% 39% 23% 3	525
	21st to end	Night	18% 3 21% 34% 22% 2	836
September	1st to 10th	Night	15% 3 23% 40% 17% 1	760
	11th to 20th	Night	17% 4 19% 35% 24% 1	760
	21st to end	Night	18% 5% 25% 31% 18% 2	760
October	1st to 10th	Night	21% 4 26% 35% 12% 1	760
	11th to 20th	Night	25% 6% 27% 30% 11% 1	760
	21st to end	Night	27% 8% 28% 30% 7%	836
November	1st to 10th	Night	23% 9% 29% 28% 10% 1	780
	11th to 20th	Night	24% 8% 32% 26% 9% 1	800
	21st to end	Night	31% 7% 30% 25% 6% 1	800
December	1st to 10th	Night	28% 8% 27% 28% 9%	783
	11th to 20th	Night	25% 9% 31% 27% 7% 1	760
	21st to end	Night	33% 9% 28% 23% 8%	836

Page generated at 2024-12-03 06:45:13. Version ac07

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