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 2026-03-18.

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% 5% 5% 4 5% 7% 33%	131206
	Night	17% 11% 4 9% 5% 5% 4 4 5% 6% 30%	130289
Transparency	Day	50% 6% 13% 20% 10% 1	134650
	Night	54% 5% 11% 18% 10% 1	134931
Seeing	Day	20% 9% 27% 26% 14% 4	16176
	Night	22% 6% 28% 32% 11% 1	25232

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	16% 17% 6% 11% 4 4 3 2 4 5% 29%	4661
	11th to 20th	Day	12% 17% 7% 13% 6% 5% 5% 3 4 5% 23%	4650
	21st to end	Day	9% 13% 5% 12% 7% 6% 5% 4 5% 6% 27%	5124
February	1st to 10th	Day	14% 15% 4 10% 7% 4 5% 4 3 4 29%	1795
	11th to 20th	Day	14% 13% 5% 11% 6% 5% 4 3 5% 6% 29%	1970
	21st to end	Day	16% 15% 4 9% 5% 5% 4 3 5% 5% 28%	1730
March	1st to 10th	Day	19% 13% 4 11% 6% 4 4 3 4 5% 26%	4576
	11th to 20th	Day	11% 14% 4 9% 5% 5% 4 3 4 7% 33%	4737
	21st to end	Day	18% 11% 4 10% 5% 4 4 3 4 5% 33%	5970
April	1st to 10th	Day	21% 10% 3 7% 5% 4 5% 3 5% 5% 31%	2400
	11th to 20th	Day	21% 7% 3 6% 5% 4 4 3 5% 5% 38%	2374
	21st to end	Day	22% 9% 3 7% 4 4 5% 4 5% 6% 32%	2600
May	1st to 10th	Day	26% 9% 3 9% 4 5% 4 3 4 6% 27%	6223
	11th to 20th	Day	19% 8% 3 8% 5% 5% 4 3 5% 7% 32%	6318
	21st to end	Day	19% 8% 3 7% 5% 5% 4 4 6% 7% 31%	6949
June	1st to 10th	Day	21% 8% 2 7% 5% 5% 5% 4 6% 8% 31%	2800
	11th to 20th	Day	14% 6% 2 7% 4 4 5% 5% 7% 8% 38%	2800
	21st to end	Day	20% 9% 3 8% 5% 4 5% 4 6% 8% 30%	2800
July	1st to 10th	Day	14% 6% 2 7% 6% 6% 6% 5% 7% 9% 33%	6360
	11th to 20th	Day	12% 5% 2 6% 5% 5% 5% 4 7% 9% 40%	6360
	21st to end	Day	10% 6% 2 5% 4 4 5% 4 6% 9% 45%	6926
August	1st to 10th	Day	13% 6% 2 7% 5% 6% 6% 5% 8% 9% 33%	4563
	11th to 20th	Day	15% 5% 2 7% 5% 4 5% 4 6% 9% 37%	4175
	21st to end	Day	17% 5% 2 6% 4 4 5% 3 5% 8% 39%	4312
September	1st to 10th	Day	12% 5% 2 6% 4 5% 5% 4 6% 8% 42%	2400
	11th to 20th	Day	14% 5% 2 5% 3 3 3 4 5% 6% 49%	2294
	21st to end	Day	17% 6% 2 6% 4 4 4 4 6% 5% 42%	2200
October	1st to 10th	Day	19% 6% 2 6% 5% 4 4 3 6% 6% 38%	2095
	11th to 20th	Day	19% 8% 3 8% 5% 5% 5% 4 6% 6% 31%	1921
	21st to end	Day	24% 10% 3 6% 4 4 3 4 4 6% 31%	1980
November	1st to 10th	Day	16% 11% 2 8% 5% 4 4 4 5% 6% 34%	1851
	11th to 20th	Day	19% 11% 3 8% 4 4 5% 3 6% 7% 30%	1818
	21st to end	Day	22% 14% 4 7% 5% 4 5% 3 4 5% 27%	1680
December	1st to 10th	Day	21% 16% 5% 10% 6% 5% 4 3 3 4 23%	2680
	11th to 20th	Day	19% 13% 4 11% 5% 4 4 4 4 4 29%	3066
	21st to end	Day	24% 13% 4 9% 5% 5% 4 3 5% 5% 23%	4048
January	1st to 10th	Night	14% 17% 7% 13% 5% 4 4 3 4 5% 23%	8234
	11th to 20th	Night	12% 17% 6% 16% 7% 5% 4 4 4 4 19%	8235
	21st to end	Night	10% 13% 4 14% 9% 6% 5% 4 6% 6% 25%	9057
February	1st to 10th	Night	14% 14% 5% 12% 7% 4 5% 4 5% 5% 25%	2824
	11th to 20th	Night	14% 12% 5% 11% 7% 5% 4 4 5% 4 29%	2535
	21st to end	Night	14% 13% 4 10% 6% 4 4 3 5% 5% 31%	2076
March	1st to 10th	Night	22% 14% 4 8% 5% 4 4 3 4 4 28%	5484
	11th to 20th	Night	15% 14% 4 8% 5% 5% 5% 4 6% 6% 30%	5371
	21st to end	Night	16% 11% 4 10% 5% 5% 4 4 5% 5% 31%	4962
April	1st to 10th	Night	24% 9% 3 8% 4 4 4 3 5% 6% 31%	2000
	11th to 20th	Night	21% 8% 3 7% 5% 5% 4 4 4 6% 34%	1975
	21st to end	Night	23% 10% 3 6% 4 4 3 4 5% 5% 35%	1800
May	1st to 10th	Night	26% 11% 4 9% 4 5% 5% 3 5% 6% 24%	3760
	11th to 20th	Night	22% 8% 3 8% 5% 5% 4 3 5% 6% 31%	3575
	21st to end	Night	19% 9% 3 8% 5% 5% 5% 4 5% 7% 30%	3926
June	1st to 10th	Night	21% 8% 2 6% 3 4 5% 4 6% 7% 35%	1600
	11th to 20th	Night	17% 6% 2 6% 4 5% 5% 4 6% 8% 37%	1600
	21st to end	Night	19% 7% 3 7% 5% 4 5% 4 6% 7% 33%	1600
July	1st to 10th	Night	12% 6% 3 7% 6% 5% 6% 5% 7% 9% 34%	3600
	11th to 20th	Night	16% 5% 2 7% 4 5% 5% 4 6% 7% 40%	3600
	21st to end	Night	10% 5% 2 6% 5% 5% 4 5% 7% 8% 44%	3926
August	1st to 10th	Night	15% 7% 4 7% 5% 4 5% 4 6% 7% 37%	2586
	11th to 20th	Night	16% 6% 2 7% 5% 5% 5% 3 5% 7% 39%	2811
	21st to end	Night	18% 6% 3 7% 6% 4 4 4 5% 6% 38%	3586
September	1st to 10th	Night	13% 5% 2 5% 5% 4 4 4 6% 7% 46%	2000
	11th to 20th	Night	14% 5% 2 6% 4 3 3 3 4 7% 50%	2105
	21st to end	Night	17% 6% 2 5% 4 4 4 3 5% 7% 43%	2200
October	1st to 10th	Night	18% 5% 2 7% 5% 3 4 3 5% 6% 42%	2301
	11th to 20th	Night	19% 9% 2 8% 4 4 4 3 5% 6% 36%	2477
	21st to end	Night	23% 9% 3 7% 5% 5% 4 3 5% 6% 31%	2860
November	1st to 10th	Night	19% 10% 3 7% 5% 4 5% 4 5% 5% 33%	2667
	11th to 20th	Night	19% 11% 3 8% 5% 4 4 4 5% 5% 33%	2798
	21st to end	Night	24% 14% 3 8% 5% 4 3 3 3 5% 27%	2940
December	1st to 10th	Night	18% 15% 5% 10% 7% 6% 6% 4 4 4 19%	4720
	11th to 20th	Night	24% 12% 4 9% 5% 4 4 3 5% 5% 27%	5348
	21st to end	Night	24% 15% 5% 9% 4 4 4 3 5% 5% 22%	7150

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% 5% 12% 19% 6%	4662
	11th to 20th	Day	62% 4 8% 14% 9% 1	4680
	21st to end	Day	55% 5% 10% 16% 13% 1	5124
February	1st to 10th	Day	58% 4 10% 19% 7% 2	1795
	11th to 20th	Day	56% 6% 11% 16% 10% 1	1979
	21st to end	Day	57% 4 10% 16% 10% 2	1730
March	1st to 10th	Day	60% 6% 8% 13% 11% 3	4579
	11th to 20th	Day	51% 6% 17% 12% 11% 3	4737
	21st to end	Day	52% 4 9% 17% 16% 2	5970
April	1st to 10th	Day	54% 5% 9% 18% 12% 2	2400
	11th to 20th	Day	48% 6% 10% 22% 13% 2	2374
	21st to end	Day	53% 5% 10% 17% 12% 3	2600
May	1st to 10th	Day	58% 5% 9% 19% 8% 1	6234
	11th to 20th	Day	49% 8% 16% 17% 9% 2	6322
	21st to end	Day	50% 7% 11% 18% 12% 2	6949
June	1st to 10th	Day	51% 7% 12% 18% 11% 2	2800
	11th to 20th	Day	42% 8% 13% 23% 13% 2	2800
	21st to end	Day	51% 8% 11% 19% 10% 1	2800
July	1st to 10th	Day	45% 8% 14% 25% 8%	6290
	11th to 20th	Day	38% 8% 15% 29% 9%	6285
	21st to end	Day	35% 7% 15% 33% 10%	6728
August	1st to 10th	Day	43% 9% 18% 22% 7%	4549
	11th to 20th	Day	42% 9% 20% 23% 6%	4216
	21st to end	Day	42% 7% 15% 21% 14% 1	4312
September	1st to 10th	Day	36% 8% 16% 28% 12% 1	2400
	11th to 20th	Day	34% 7% 14% 30% 14% 1	2294
	21st to end	Day	42% 7% 14% 25% 12%	2200
October	1st to 10th	Day	45% 5% 15% 26% 8%	3407
	11th to 20th	Day	45% 7% 17% 24% 7%	3111
	21st to end	Day	44% 5% 20% 22% 9% 1	3180
November	1st to 10th	Day	50% 6% 14% 21% 9% 1	1851
	11th to 20th	Day	53% 6% 12% 18% 11%	1818
	21st to end	Day	59% 6% 14% 15% 6% 1	1680
December	1st to 10th	Day	66% 4 9% 15% 5%	2680
	11th to 20th	Day	58% 5% 8% 19% 11%	3066
	21st to end	Day	62% 5% 11% 14% 8%	4048
January	1st to 10th	Night	64% 5% 11% 14% 6% 1	8234
	11th to 20th	Night	67% 4 9% 14% 5%	8280
	21st to end	Night	59% 5% 9% 14% 13%	9057
February	1st to 10th	Night	59% 5% 10% 16% 9% 1	2824
	11th to 20th	Night	57% 5% 10% 17% 10% 1	2537
	21st to end	Night	55% 5% 9% 18% 12% 2	2076
March	1st to 10th	Night	60% 5% 8% 12% 11% 4	5494
	11th to 20th	Night	53% 5% 13% 14% 12% 2	5372
	21st to end	Night	54% 4 8% 21% 11% 2	4962
April	1st to 10th	Night	53% 5% 9% 17% 15% 2	2000
	11th to 20th	Night	50% 6% 8% 18% 15% 3	1975
	21st to end	Night	51% 5% 7% 19% 16% 3	1800
May	1st to 10th	Night	60% 6% 10% 15% 8% 2	3766
	11th to 20th	Night	52% 8% 11% 17% 10% 1	3576
	21st to end	Night	52% 6% 11% 16% 14% 1	3927
June	1st to 10th	Night	47% 6% 11% 21% 14% 1	1600
	11th to 20th	Night	45% 7% 11% 20% 14% 4	1600
	21st to end	Night	48% 8% 10% 19% 15% 1	1600
July	1st to 10th	Night	44% 7% 19% 21% 8% 1	3560
	11th to 20th	Night	41% 6% 16% 28% 8%	3552
	21st to end	Night	36% 7% 18% 28% 11%	3809
August	1st to 10th	Night	45% 7% 13% 26% 9%	2576
	11th to 20th	Night	44% 7% 16% 24% 8%	2835
	21st to end	Night	46% 7% 11% 22% 13% 1	3586
September	1st to 10th	Night	37% 7% 15% 26% 14% 1	2000
	11th to 20th	Night	36% 5% 14% 33% 12% 1	2105
	21st to end	Night	40% 7% 10% 28% 15%	2200
October	1st to 10th	Night	46% 5% 14% 24% 11%	3769
	11th to 20th	Night	44% 6% 13% 29% 8%	4026
	21st to end	Night	47% 5% 15% 20% 12% 2	4610
November	1st to 10th	Night	52% 6% 13% 21% 8% 1	2667
	11th to 20th	Night	53% 5% 12% 18% 10% 1	2798
	21st to end	Night	60% 4 12% 16% 7%	2940
December	1st to 10th	Night	67% 5% 12% 12% 4	4720
	11th to 20th	Night	60% 5% 8% 16% 11% 1	5348
	21st to end	Night	63% 4 8% 15% 9%	7150

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	25% 9% 31% 24% 11%	220
	11th to 20th	Day	24% 15% 27% 25% 6% 1	220
	21st to end	Day	16% 10% 30% 31% 10% 3	242
February	1st to 10th	Day	25% 13% 32% 24% 7%	281
	11th to 20th	Day	21% 10% 35% 27% 7%	458
	21st to end	Day	23% 9% 34% 28% 5%	446
March	1st to 10th	Day	21% 10% 31% 29% 7% 1	540
	11th to 20th	Day	20% 11% 33% 27% 7% 1	528
	21st to end	Day	26% 9% 30% 22% 11% 2	550
April	1st to 10th	Day	25% 12% 24% 26% 11% 1	500
	11th to 20th	Day	24% 15% 29% 20% 10% 2	492
	21st to end	Day	28% 9% 30% 22% 9% 2	500
May	1st to 10th	Day	24% 10% 30% 24% 11% 1	503
	11th to 20th	Day	18% 8% 28% 26% 16% 4	527
	21st to end	Day	19% 9% 32% 23% 14% 3	573
June	1st to 10th	Day	22% 8% 26% 29% 11% 3	530
	11th to 20th	Day	15% 11% 26% 27% 18% 3	530
	21st to end	Day	18% 9% 28% 27% 13% 5%	530
July	1st to 10th	Day	14% 4 25% 29% 22% 6%	525
	11th to 20th	Day	13% 6% 24% 36% 17% 4	524
	21st to end	Day	13% 5% 22% 33% 22% 5%	576
August	1st to 10th	Day	14% 3 21% 33% 24% 6%	526
	11th to 20th	Day	12% 5% 18% 32% 22% 11%	519
	21st to end	Day	15% 7% 25% 22% 21% 11%	583
September	1st to 10th	Day	12% 6% 26% 27% 18% 10%	530
	11th to 20th	Day	13% 6% 16% 30% 23% 11%	530
	21st to end	Day	18% 8% 23% 24% 20% 6%	530
October	1st to 10th	Day	20% 9% 25% 25% 17% 4	530
	11th to 20th	Day	21% 8% 30% 25% 13% 3	486
	21st to end	Day	30% 9% 27% 23% 11%	462
November	1st to 10th	Day	23% 12% 29% 27% 9% 1	425
	11th to 20th	Day	26% 9% 31% 23% 10% 1	358
	21st to end	Day	32% 9% 32% 13% 13%	220
December	1st to 10th	Day	29% 8% 27% 20% 11% 4	220
	11th to 20th	Day	23% 13% 33% 25% 5% 1	220
	21st to end	Day	31% 12% 28% 20% 8% 2	242
January	1st to 10th	Night	26% 8% 29% 31% 6%	840
	11th to 20th	Night	26% 9% 29% 27% 8%	840
	21st to end	Night	18% 6% 30% 37% 8% 1	924
February	1st to 10th	Night	21% 6% 34% 30% 8% 1	840
	11th to 20th	Night	20% 7% 36% 30% 6% 1	823
	21st to end	Night	21% 6% 33% 34% 5%	692
March	1st to 10th	Night	25% 8% 31% 31% 5%	840
	11th to 20th	Night	24% 9% 33% 28% 6%	829
	21st to end	Night	23% 7% 32% 30% 8% 1	880
April	1st to 10th	Night	28% 8% 29% 29% 7%	800
	11th to 20th	Night	25% 8% 32% 28% 6% 1	791
	21st to end	Night	28% 8% 31% 25% 7%	600
May	1st to 10th	Night	28% 7% 30% 30% 5% 1	475
	11th to 20th	Night	23% 5% 28% 35% 8% 1	394
	21st to end	Night	21% 6% 33% 33% 7%	433
June	1st to 10th	Night	23% 3 28% 36% 10% 1	400
	11th to 20th	Night	18% 4 27% 37% 15% 1	400
	21st to end	Night	23% 2 22% 37% 15% 1	400
July	1st to 10th	Night	11% 3 25% 45% 16% 1	390
	11th to 20th	Night	13% 5% 19% 47% 16% 1	388
	21st to end	Night	14% 3 20% 46% 16% 1	438
August	1st to 10th	Night	14% 2 15% 44% 24% 2	398
	11th to 20th	Night	14% 2 18% 39% 24% 3	553
	21st to end	Night	19% 3 21% 34% 22% 2	880
September	1st to 10th	Night	15% 4 23% 40% 17% 1	800
	11th to 20th	Night	16% 4 19% 35% 25% 2	800
	21st to end	Night	18% 5% 25% 33% 18% 2	800
October	1st to 10th	Night	20% 4 27% 35% 13% 1	800
	11th to 20th	Night	24% 7% 27% 31% 11% 1	800
	21st to end	Night	27% 8% 28% 30% 7%	880
November	1st to 10th	Night	24% 9% 29% 28% 9% 1	820
	11th to 20th	Night	24% 8% 32% 26% 9% 1	840
	21st to end	Night	30% 7% 31% 25% 6% 1	840
December	1st to 10th	Night	27% 8% 28% 28% 8%	840
	11th to 20th	Night	25% 10% 31% 26% 7% 1	840
	21st to end	Night	32% 9% 27% 23% 8%	924

Page generated at 2026-03-15 02:23:57. Version ac07

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