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 2024-01-16.

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	18% 9% 3 6% 6% 3 5% 5% 5% 7% 34%	74789
	Night	19% 10% 4 7% 6% 3 4 4 5% 6% 33%	75010
Transparency	Day	49% 7% 13% 20% 10% 1	74494
	Night	52% 6% 11% 19% 11% 1	74839
Seeing	Day	21% 8% 27% 26% 14% 4	13460
	Night	23% 6% 28% 32% 11% 1	22512

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	20% 16% 5% 8% 6% 3 5% 4 3 4 25%	1517
	11th to 20th	Day	18% 16% 5% 7% 6% 4 5% 3 5% 5% 25%	1480
	21st to end	Day	12% 14% 4 8% 7% 3 5% 5% 6% 7% 29%	1584
February	1st to 10th	Day	16% 16% 5% 8% 7% 3 4 4 4 4 28%	1538
	11th to 20th	Day	16% 13% 4 9% 7% 4 5% 4 5% 6% 29%	1681
	21st to end	Day	17% 14% 6% 8% 6% 3 5% 5% 4 5% 28%	1480
March	1st to 10th	Day	15% 11% 4 8% 7% 4 6% 5% 5% 6% 30%	1797
	11th to 20th	Day	16% 12% 4 6% 6% 4 4 4 5% 6% 33%	1849
	21st to end	Day	18% 10% 3 7% 6% 3 5% 4 5% 5% 35%	2376
April	1st to 10th	Day	21% 9% 4 6% 6% 3 5% 5% 4 6% 31%	2160
	11th to 20th	Day	22% 6% 3 5% 6% 3 3 4 4 6% 38%	2134
	21st to end	Day	22% 9% 3 6% 5% 3 5% 5% 5% 5% 32%	2340
May	1st to 10th	Day	20% 7% 3 6% 5% 3 4 3 4 7% 36%	2467
	11th to 20th	Day	18% 6% 3 6% 6% 3 5% 5% 5% 8% 37%	2478
	21st to end	Day	18% 6% 3 6% 6% 3 5% 5% 6% 8% 35%	2725
June	1st to 10th	Day	21% 7% 3 6% 6% 4 5% 5% 5% 8% 31%	2520
	11th to 20th	Day	15% 6% 2 6% 5% 3 5% 6% 7% 8% 38%	2520
	21st to end	Day	19% 7% 3 7% 6% 3 5% 5% 6% 8% 30%	2520
July	1st to 10th	Day	12% 6% 2 5% 5% 4 6% 6% 7% 10% 38%	2520
	11th to 20th	Day	13% 4 3 5% 6% 4 5% 7% 7% 9% 37%	2520
	21st to end	Day	12% 6% 3 5% 6% 3 6% 7% 7% 10% 36%	2744
August	1st to 10th	Day	12% 6% 3 6% 5% 4 7% 7% 7% 9% 35%	2506
	11th to 20th	Day	10% 5% 2 6% 6% 4 6% 7% 7% 9% 39%	2319
	21st to end	Day	13% 5% 2 5% 5% 3 5% 4 6% 9% 43%	2376
September	1st to 10th	Day	12% 4 2 6% 5% 4 6% 6% 6% 7% 41%	2160
	11th to 20th	Day	15% 5% 2 4 4 3 3 5% 5% 7% 47%	2066
	21st to end	Day	17% 6% 2 5% 5% 3 4 5% 6% 5% 42%	1980
October	1st to 10th	Day	20% 6% 2 6% 6% 3 5% 5% 6% 6% 36%	1886
	11th to 20th	Day	20% 8% 3 7% 7% 5% 5% 5% 6% 6% 29%	1729
	21st to end	Day	25% 10% 4 5% 5% 3 3 4 4 6% 30%	1782
November	1st to 10th	Day	16% 9% 3 7% 5% 3 4 5% 5% 7% 34%	1671
	11th to 20th	Day	20% 10% 4 7% 5% 2 4 4 6% 7% 29%	1646
	21st to end	Day	21% 12% 4 7% 6% 3 5% 5% 4 6% 28%	1520
December	1st to 10th	Day	26% 12% 5% 8% 6% 3 4 4 3 4 25%	1520
	11th to 20th	Day	22% 13% 4 9% 6% 3 3 4 3 5% 28%	1906
	21st to end	Day	28% 15% 6% 8% 5% 2 3 3 3 4 23%	2772
January	1st to 10th	Night	19% 16% 6% 11% 6% 3 4 4 4 6% 22%	2649
	11th to 20th	Night	17% 16% 6% 8% 8% 3 5% 4 4 4 25%	2602
	21st to end	Night	12% 14% 4 8% 8% 4 5% 4 6% 6% 29%	2772
February	1st to 10th	Night	15% 13% 5% 11% 8% 4 5% 5% 5% 5% 24%	2421
	11th to 20th	Night	15% 11% 4 9% 8% 4 6% 5% 5% 4 29%	2164
	21st to end	Night	16% 12% 5% 9% 6% 3 5% 3 4 5% 31%	1776
March	1st to 10th	Night	17% 13% 5% 8% 6% 3 4 4 5% 5% 30%	2150
	11th to 20th	Night	17% 13% 5% 7% 5% 3 4 4 4 5% 32%	2104
	21st to end	Night	18% 10% 4 6% 5% 3 4 5% 4 4 37%	1980
April	1st to 10th	Night	24% 8% 4 7% 5% 3 4 4 5% 6% 30%	1800
	11th to 20th	Night	21% 7% 3 6% 6% 4 4 4 4 6% 36%	1775
	21st to end	Night	23% 9% 4 5% 5% 3 3 5% 4 5% 34%	1620
May	1st to 10th	Night	22% 9% 3 7% 5% 2 4 4 5% 6% 33%	1508
	11th to 20th	Night	20% 7% 3 5% 5% 3 4 5% 4 8% 35%	1415
	21st to end	Night	17% 6% 3 5% 5% 3 5% 5% 5% 7% 39%	1550
June	1st to 10th	Night	21% 7% 3 5% 4 3 5% 5% 6% 7% 34%	1440
	11th to 20th	Night	18% 6% 2 6% 5% 3 5% 5% 5% 8% 37%	1440
	21st to end	Night	18% 6% 3 6% 6% 3 5% 6% 6% 8% 33%	1440
July	1st to 10th	Night	11% 5% 3 5% 5% 3 6% 7% 6% 9% 40%	1440
	11th to 20th	Night	12% 6% 2 7% 5% 4 6% 5% 6% 8% 40%	1440
	21st to end	Night	12% 5% 3 5% 5% 4 5% 7% 7% 8% 39%	1574
August	1st to 10th	Night	13% 5% 3 5% 6% 3 4 6% 7% 9% 40%	1432
	11th to 20th	Night	13% 5% 2 6% 6% 4 5% 5% 6% 8% 41%	1561
	21st to end	Night	15% 5% 2 5% 5% 3 4 6% 6% 6% 43%	1980
September	1st to 10th	Night	13% 5% 2 5% 5% 4 4 5% 5% 7% 45%	1800
	11th to 20th	Night	16% 5% 2 5% 4 2 3 4 4 7% 49%	1894
	21st to end	Night	17% 5% 2 4 4 3 4 4 6% 7% 43%	1980
October	1st to 10th	Night	18% 5% 2 6% 6% 2 4 4 5% 6% 41%	2070
	11th to 20th	Night	21% 9% 3 8% 5% 3 4 4 5% 5% 34%	2229
	21st to end	Night	24% 9% 3 6% 6% 4 4 4 4 6% 30%	2574
November	1st to 10th	Night	19% 10% 3 6% 6% 4 5% 5% 5% 5% 33%	2407
	11th to 20th	Night	20% 9% 3 7% 5% 3 5% 4 5% 5% 33%	2531
	21st to end	Night	24% 13% 4 7% 6% 3 4 4 3 5% 28%	2660
December	1st to 10th	Night	22% 13% 6% 8% 6% 3 5% 5% 4 5% 22%	2660
	11th to 20th	Night	24% 13% 5% 8% 5% 2 3 3 4 5% 28%	3288
	21st to end	Night	27% 16% 5% 7% 5% 3 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	63% 6% 11% 14% 6%	1518
	11th to 20th	Day	62% 4 11% 12% 11%	1480
	21st to end	Day	53% 5% 11% 20% 10% 1	1584
February	1st to 10th	Day	60% 4 10% 18% 7% 2	1538
	11th to 20th	Day	57% 6% 12% 15% 10% 1	1690
	21st to end	Day	58% 5% 9% 16% 9% 2	1480
March	1st to 10th	Day	54% 7% 10% 17% 11% 2	1800
	11th to 20th	Day	51% 5% 12% 15% 15% 2	1849
	21st to end	Day	50% 5% 11% 19% 12% 3	2376
April	1st to 10th	Day	54% 5% 10% 18% 11% 2	2160
	11th to 20th	Day	47% 6% 10% 22% 13% 2	2134
	21st to end	Day	54% 5% 10% 16% 12% 3	2340
May	1st to 10th	Day	50% 4 9% 21% 14% 3	2467
	11th to 20th	Day	46% 6% 11% 19% 16% 3	2482
	21st to end	Day	47% 7% 12% 18% 14% 2	2725
June	1st to 10th	Day	51% 7% 12% 18% 10% 2	2520
	11th to 20th	Day	43% 8% 12% 22% 13% 2	2520
	21st to end	Day	52% 8% 11% 19% 10%	2520
July	1st to 10th	Day	38% 11% 14% 24% 11% 1	2450
	11th to 20th	Day	40% 10% 16% 25% 10%	2445
	21st to end	Day	42% 11% 16% 24% 8%	2591
August	1st to 10th	Day	43% 11% 16% 24% 6%	2492
	11th to 20th	Day	37% 12% 17% 24% 10%	2319
	21st to end	Day	36% 9% 17% 24% 13%	2376
September	1st to 10th	Day	38% 8% 16% 25% 12% 1	2160
	11th to 20th	Day	35% 8% 13% 29% 14% 1	2066
	21st to end	Day	42% 8% 14% 25% 11%	1980
October	1st to 10th	Day	48% 5% 13% 23% 10%	1886
	11th to 20th	Day	54% 8% 14% 18% 5%	1729
	21st to end	Day	56% 5% 13% 17% 8% 1	1782
November	1st to 10th	Day	49% 6% 15% 21% 9% 1	1671
	11th to 20th	Day	53% 6% 13% 17% 11%	1646
	21st to end	Day	57% 6% 15% 15% 6% 1	1520
December	1st to 10th	Day	64% 5% 12% 14% 6%	1520
	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	64% 5% 10% 15% 6%	2649
	11th to 20th	Night	63% 5% 10% 13% 8% 1	2602
	21st to end	Night	55% 5% 10% 17% 11% 1	2772
February	1st to 10th	Night	61% 4 10% 16% 8% 1	2421
	11th to 20th	Night	57% 6% 10% 17% 10% 1	2166
	21st to end	Night	55% 4 9% 17% 13% 2	1776
March	1st to 10th	Night	57% 5% 10% 16% 11% 2	2160
	11th to 20th	Night	55% 4 10% 14% 13% 4	2105
	21st to end	Night	50% 4 9% 20% 14% 4	1980
April	1st to 10th	Night	54% 5% 9% 16% 14% 2	1800
	11th to 20th	Night	48% 6% 8% 20% 16% 3	1775
	21st to end	Night	53% 5% 7% 18% 15% 3	1620
May	1st to 10th	Night	52% 4 9% 15% 15% 5%	1508
	11th to 20th	Night	48% 7% 8% 16% 17% 4	1416
	21st to end	Night	44% 7% 11% 19% 16% 3	1551
June	1st to 10th	Night	48% 6% 12% 21% 13% 1	1440
	11th to 20th	Night	45% 7% 10% 20% 13% 4	1440
	21st to end	Night	48% 9% 10% 19% 14%	1440
July	1st to 10th	Night	37% 9% 15% 23% 14% 1	1400
	11th to 20th	Night	40% 8% 13% 27% 11% 1	1392
	21st to end	Night	39% 8% 19% 22% 12%	1486
August	1st to 10th	Night	39% 9% 16% 26% 10%	1422
	11th to 20th	Night	39% 8% 16% 25% 12%	1561
	21st to end	Night	39% 8% 13% 27% 12% 1	1980
September	1st to 10th	Night	38% 7% 15% 25% 13% 1	1800
	11th to 20th	Night	37% 5% 14% 32% 11% 1	1894
	21st to end	Night	39% 8% 10% 28% 15%	1980
October	1st to 10th	Night	45% 6% 11% 24% 14% 1	2070
	11th to 20th	Night	50% 7% 13% 22% 8%	2229
	21st to end	Night	56% 6% 12% 17% 10%	2574
November	1st to 10th	Night	51% 6% 13% 20% 8% 1	2407
	11th to 20th	Night	53% 5% 13% 17% 10% 1	2531
	21st to end	Night	59% 5% 12% 17% 7%	2660
December	1st to 10th	Night	64% 6% 13% 13% 5%	2660
	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	25% 11% 28% 28% 7% 1	170
	21st to end	Day	18% 7% 31% 30% 12% 3	176
February	1st to 10th	Day	32% 14% 32% 20% 2	204
	11th to 20th	Day	23% 8% 35% 28% 6%	349
	21st to end	Day	24% 9% 32% 28% 6%	346
March	1st to 10th	Day	22% 7% 31% 32% 8% 1	420
	11th to 20th	Day	19% 12% 32% 27% 9% 1	420
	21st to end	Day	26% 7% 28% 23% 12% 3	462
April	1st to 10th	Day	25% 10% 24% 28% 11% 1	420
	11th to 20th	Day	25% 13% 29% 20% 11% 3	412
	21st to end	Day	29% 8% 28% 23% 10% 2	420
May	1st to 10th	Day	23% 10% 32% 23% 11% 1	423
	11th to 20th	Day	18% 8% 29% 26% 15% 4	447
	21st to end	Day	19% 8% 34% 23% 15% 2	485
June	1st to 10th	Day	22% 8% 26% 30% 10% 3	450
	11th to 20th	Day	16% 11% 24% 27% 19% 4	450
	21st to end	Day	17% 8% 26% 28% 16% 5%	450
July	1st to 10th	Day	13% 4 22% 31% 24% 7%	445
	11th to 20th	Day	14% 5% 22% 36% 19% 4	444
	21st to end	Day	14% 6% 19% 32% 24% 5%	488
August	1st to 10th	Day	15% 3 21% 31% 25% 6%	446
	11th to 20th	Day	12% 5% 19% 31% 21% 11%	439
	21st to end	Day	13% 7% 26% 22% 20% 12%	495
September	1st to 10th	Day	13% 6% 26% 27% 19% 10%	450
	11th to 20th	Day	15% 7% 18% 31% 20% 10%	450
	21st to end	Day	18% 7% 24% 24% 19% 7%	450
October	1st to 10th	Day	22% 9% 24% 24% 17% 4	450
	11th to 20th	Day	23% 7% 29% 25% 12% 3	414
	21st to end	Day	31% 9% 26% 22% 11% 1	396
November	1st to 10th	Day	22% 10% 27% 29% 10% 1	365
	11th to 20th	Day	28% 8% 31% 22% 10% 1	306
	21st to end	Day	29% 9% 33% 13% 14% 1	180
December	1st to 10th	Day	32% 8% 24% 21% 12% 4	180
	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% 29% 27% 8%	744
	21st to end	Night	20% 5% 30% 37% 8% 1	792
February	1st to 10th	Night	22% 6% 35% 30% 7% 1	720
	11th to 20th	Night	20% 7% 35% 31% 6% 1	703
	21st to end	Night	22% 6% 34% 34% 5%	592
March	1st to 10th	Night	25% 7% 29% 33% 5%	720
	11th to 20th	Night	24% 8% 32% 28% 7% 1	720
	21st to end	Night	23% 6% 31% 31% 9% 1	792
April	1st to 10th	Night	28% 7% 28% 29% 8%	720
	11th to 20th	Night	25% 7% 33% 28% 6% 1	711
	21st to end	Night	29% 8% 30% 25% 8%	540
May	1st to 10th	Night	28% 6% 29% 30% 6% 1	428
	11th to 20th	Night	23% 5% 29% 34% 7% 1	354
	21st to end	Night	20% 5% 34% 34% 7%	389
June	1st to 10th	Night	24% 3 28% 35% 10% 1	360
	11th to 20th	Night	19% 3 26% 36% 16% 1	360
	21st to end	Night	22% 2 22% 38% 16% 1	360
July	1st to 10th	Night	11% 3 25% 44% 16% 1	350
	11th to 20th	Night	12% 4 19% 47% 17% 1	348
	21st to end	Night	14% 2 20% 45% 16% 1	394
August	1st to 10th	Night	15% 1 15% 44% 23% 2	358
	11th to 20th	Night	14% 2 20% 40% 22% 2	497
	21st to end	Night	18% 2 21% 34% 23% 2	792
September	1st to 10th	Night	15% 3 23% 40% 17% 1	720
	11th to 20th	Night	18% 4 20% 35% 22% 1	720
	21st to end	Night	18% 5% 26% 31% 17% 2	720
October	1st to 10th	Night	21% 4 27% 35% 12% 1	720
	11th to 20th	Night	25% 7% 27% 30% 11% 1	720
	21st to end	Night	28% 7% 28% 30% 7%	792
November	1st to 10th	Night	23% 9% 29% 28% 10% 1	740
	11th to 20th	Night	24% 8% 32% 25% 9% 1	760
	21st to end	Night	30% 7% 30% 26% 7% 1	760
December	1st to 10th	Night	28% 8% 27% 27% 9%	760
	11th to 20th	Night	25% 9% 31% 27% 7% 1	760
	21st to end	Night	33% 9% 28% 23% 8%	836

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