Modesto
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Modesto clear sky chart from 2005-11-06 to 2026-10-29.

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	5% 3 1 4 2 2 3 2 3 5% 69%	157776
	Night	7% 3 1 4 3 3 3 2 4 5% 65%	151847
Transparency	Day	19% 3 7% 18% 27% 26%	152739
	Night	22% 4 8% 19% 24% 23%	147869
Seeing	Day	7% 6% 17% 28% 29% 13%	17919
	Night	8% 4 19% 39% 27% 4	27264

By Month

Percentage Cloud Cover SW (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% 8% 3 7% 5% 5% 5% 4 6% 8% 35%	4754
	11th to 20th	Day	9% 6% 3 8% 6% 5% 4 3 4 6% 47%	5340
	21st to end	Day	12% 8% 4 10% 6% 4 5% 3 5% 8% 37%	5843
February	1st to 10th	Day	16% 6% 3 6% 5% 4 4 4 5% 6% 43%	1890
	11th to 20th	Day	12% 4 3 5% 4 4 5% 4 5% 8% 47%	2020
	21st to end	Day	12% 4 2 5% 5% 4 5% 3 6% 6% 50%	1730
March	1st to 10th	Day	10% 6% 2 7% 5% 4 5% 5% 7% 7% 40%	4770
	11th to 20th	Day	11% 4 1 4 3 3 4 4 5% 8% 53%	5110
	21st to end	Day	9% 6% 2 6% 4 4 5% 4 7% 8% 46%	5826
April	1st to 10th	Day	9% 5% 2 5% 3 4 3 3 5% 7% 54%	2520
	11th to 20th	Day	6% 3 1 4 3 3 3 3 6% 6% 61%	2489
	21st to end	Day	5% 3 1 3 2 3 3 2 4 6% 68%	2520
May	1st to 10th	Day	4 2 1 3 2 2 2 2 3 4 75%	7058
	11th to 20th	Day	1 1 1 1 1 1 1 2 4 86%	8001
	21st to end	Day	1 1 1 1 1 2 1 2 4 85%	8992
June	1st to 10th	Day	2 2 1 2 2 2 2 2 3 6% 75%	2940
	11th to 20th	Day	1 1 1 1 1 1 1 2 4 87%	2940
	21st to end	Day	1 1 1 1 1 1 1 2 2 88%	2940
July	1st to 10th	Day	1 1 1 2 94%	8219
	11th to 20th	Day	2 1 2 1 2 2 1 2 3 84%	8057
	21st to end	Day	1 1 1 1 1 2 3 92%	8304
August	1st to 10th	Day	1 1 1 1 1 1 1 2 3 89%	7530
	11th to 20th	Day	1 1 1 1 1 2 4 88%	7001
	21st to end	Day	2 1 1 1 2 2 2 3 5% 81%	7750
September	1st to 10th	Day	1 1 1 1 1 1 2 3 89%	2520
	11th to 20th	Day	2 1 1 2 1 1 2 2 3 5% 81%	2520
	21st to end	Day	2 1 2 2 1 1 1 2 4 83%	2278
October	1st to 10th	Day	1 1 1 2 2 2 2 2 3 5% 79%	3380
	11th to 20th	Day	2 2 1 2 2 2 1 2 3 4 80%	3361
	21st to end	Day	3 3 1 3 2 3 3 3 5% 7% 67%	3639
November	1st to 10th	Day	5% 3 1 4 4 4 4 3 5% 8% 58%	1924
	11th to 20th	Day	9% 6% 2 7% 4 3 3 3 6% 7% 50%	1890
	21st to end	Day	10% 8% 3 7% 3 3 3 4 4 7% 49%	1820
December	1st to 10th	Day	7% 4 3 7% 4 5% 5% 4 6% 10% 45%	2700
	11th to 20th	Day	16% 11% 5% 13% 7% 6% 4 4 4 7% 23%	3108
	21st to end	Day	16% 7% 3 9% 6% 6% 6% 5% 6% 9% 27%	4092
January	1st to 10th	Night	14% 6% 3 7% 4 4 5% 3 5% 8% 41%	8137
	11th to 20th	Night	9% 5% 2 5% 4 5% 5% 4 5% 6% 49%	7650
	21st to end	Night	11% 7% 3 8% 5% 5% 4 3 4 6% 44%	8371
February	1st to 10th	Night	16% 5% 1 5% 3 3 4 3 4 6% 50%	2730
	11th to 20th	Night	11% 4 1 4 4 3 3 4 6% 8% 52%	2486
	21st to end	Night	12% 5% 2 5% 4 2 3 2 5% 6% 54%	2076
March	1st to 10th	Night	11% 6% 2 7% 5% 5% 5% 4 6% 7% 43%	5328
	11th to 20th	Night	11% 5% 2 5% 3 3 3 3 4 5% 55%	5110
	21st to end	Night	13% 6% 2 5% 3 4 4 3 5% 7% 48%	5389
April	1st to 10th	Night	9% 4 2 3 4 3 3 3 5% 7% 57%	2100
	11th to 20th	Night	5% 3 1 3 4 3 3 4 5% 7% 62%	2079
	21st to end	Night	5% 3 1 2 2 2 3 3 4 6% 70%	2100
May	1st to 10th	Night	2 2 1 2 2 2 2 2 3 5% 77%	5919
	11th to 20th	Night	1 1 1 1 1 1 1 2 4 86%	4848
	21st to end	Night	1 1 1 1 1 1 2 2 3 86%	5181
June	1st to 10th	Night	2 1 1 2 2 2 2 2 3 5% 78%	1680
	11th to 20th	Night	1 1 1 1 2 3 89%	1680
	21st to end	Night	1 1 1 1 1 1 2 3 89%	1680
July	1st to 10th	Night	1 1 1 96%	4740
	11th to 20th	Night	2 2 1 3 1 2 2 2 2 2 82%	4893
	21st to end	Night	1 1 1 1 1 2 3 89%	5806
August	1st to 10th	Night	1 1 1 1 1 1 1 2 2 89%	5292
	11th to 20th	Night	1 1 1 1 1 3 4 86%	5886
	21st to end	Night	2 1 1 1 1 2 2 3 4 83%	6499
September	1st to 10th	Night	1 1 1 1 2 1 2 3 89%	2100
	11th to 20th	Night	2 2 1 1 1 1 1 1 3 3 84%	2100
	21st to end	Night	1 1 2 2 1 1 1 2 4 85%	2336
October	1st to 10th	Night	2 1 1 1 1 2 2 2 3 4 82%	4040
	11th to 20th	Night	2 1 1 2 1 2 2 2 2 4 82%	4017
	21st to end	Night	4 2 1 3 2 2 2 2 4 6% 71%	4349
November	1st to 10th	Night	7% 3 1 3 2 3 2 3 4 7% 66%	2595
	11th to 20th	Night	8% 3 1 5% 4 3 3 3 5% 7% 58%	2730
	21st to end	Night	9% 5% 2 4 3 3 4 3 5% 6% 57%	2798
December	1st to 10th	Night	7% 4 2 5% 3 4 5% 4 6% 7% 54%	4700
	11th to 20th	Night	17% 8% 3 8% 6% 5% 5% 4 6% 9% 28%	5316
	21st to end	Night	17% 8% 3 9% 6% 6% 5% 4 6% 7% 30%	7106

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	44% 7% 17% 22% 8% 2	4755
	11th to 20th	Day	37% 4 7% 15% 27% 10%	5340
	21st to end	Day	45% 5% 9% 17% 20% 4	5843
February	1st to 10th	Day	40% 6% 11% 22% 16% 6%	1890
	11th to 20th	Day	34% 6% 14% 23% 18% 5%	2030
	21st to end	Day	33% 6% 12% 24% 20% 4	1730
March	1st to 10th	Day	37% 9% 11% 21% 21% 2	4770
	11th to 20th	Day	29% 6% 13% 30% 15% 8%	5110
	21st to end	Day	32% 8% 15% 30% 12% 2	5826
April	1st to 10th	Day	29% 6% 11% 26% 20% 8%	2520
	11th to 20th	Day	22% 5% 13% 28% 24% 7%	2492
	21st to end	Day	18% 4 12% 31% 29% 7%	2520
May	1st to 10th	Day	14% 2 7% 24% 36% 17%	7058
	11th to 20th	Day	7% 1 4 22% 46% 20%	8004
	21st to end	Day	7% 2 6% 22% 47% 17%	8994
June	1st to 10th	Day	12% 4 8% 23% 34% 20%	2940
	11th to 20th	Day	5% 1 5% 18% 36% 35%	2940
	21st to end	Day	7% 1 1 9% 31% 51%	2940
July	1st to 10th	Day	2 1 4 20% 72%	8151
	11th to 20th	Day	10% 1 1 5% 11% 71%	7982
	21st to end	Day	3 1 1 11% 28% 56%	8119
August	1st to 10th	Day	7% 2 4 29% 59%	5959
	11th to 20th	Day	5% 1 2 18% 33% 41%	5517
	21st to end	Day	10% 1 2 12% 37% 39%	6077
September	1st to 10th	Day	4 2 3 11% 28% 53%	2520
	11th to 20th	Day	8% 2 5% 19% 41% 25%	2520
	21st to end	Day	8% 1 4 18% 38% 29%	2278
October	1st to 10th	Day	10% 2 4 12% 25% 46%	3380
	11th to 20th	Day	11% 2 6% 24% 37% 21%	3361
	21st to end	Day	18% 3 6% 21% 38% 14%	3639
November	1st to 10th	Day	24% 5% 11% 26% 26% 8%	1924
	11th to 20th	Day	31% 6% 13% 28% 19% 4	1890
	21st to end	Day	34% 5% 11% 24% 20% 5%	1820
December	1st to 10th	Day	34% 4 12% 29% 17% 4	2700
	11th to 20th	Day	59% 6% 9% 15% 10% 1	3108
	21st to end	Day	49% 11% 17% 15% 8% 1	4092
January	1st to 10th	Night	38% 7% 17% 26% 9% 3	8137
	11th to 20th	Night	33% 4 10% 17% 27% 9%	7650
	21st to end	Night	41% 5% 9% 18% 21% 6%	8371
February	1st to 10th	Night	35% 4 9% 24% 20% 8%	2730
	11th to 20th	Night	29% 6% 12% 23% 23% 6%	2487
	21st to end	Night	30% 5% 10% 27% 21% 6%	2076
March	1st to 10th	Night	37% 7% 12% 18% 21% 4	5328
	11th to 20th	Night	31% 5% 12% 26% 17% 9%	5110
	21st to end	Night	33% 7% 15% 26% 14% 4	5389
April	1st to 10th	Night	26% 6% 13% 24% 23% 9%	2100
	11th to 20th	Night	21% 4 13% 28% 23% 10%	2079
	21st to end	Night	16% 4 12% 29% 31% 8%	2100
May	1st to 10th	Night	12% 2 9% 23% 32% 21%	5917
	11th to 20th	Night	7% 1 4 21% 42% 26%	4848
	21st to end	Night	6% 1 5% 24% 45% 18%	5182
June	1st to 10th	Night	11% 3 9% 20% 36% 21%	1680
	11th to 20th	Night	4 2 5% 16% 34% 39%	1680
	21st to end	Night	4 2 2 10% 29% 54%	1680
July	1st to 10th	Night	2 1 4 19% 74%	4700
	11th to 20th	Night	11% 1 2 4 12% 70%	4845
	21st to end	Night	4 1 2 11% 26% 56%	5679
August	1st to 10th	Night	6% 1 2 5% 23% 64%	4186
	11th to 20th	Night	6% 1 3 16% 29% 44%	4636
	21st to end	Night	9% 1 2 12% 34% 43%	5092
September	1st to 10th	Night	5% 2 3 8% 27% 56%	2100
	11th to 20th	Night	8% 1 4 19% 39% 30%	2100
	21st to end	Night	7% 1 4 18% 35% 34%	2336
October	1st to 10th	Night	9% 2 4 12% 20% 54%	4040
	11th to 20th	Night	9% 2 5% 25% 33% 26%	4017
	21st to end	Night	16% 2 5% 20% 36% 20%	4349
November	1st to 10th	Night	19% 4 10% 27% 26% 15%	2595
	11th to 20th	Night	25% 5% 11% 25% 27% 7%	2730
	21st to end	Night	28% 3 10% 25% 26% 8%	2798
December	1st to 10th	Night	28% 5% 10% 28% 23% 7%	4700
	11th to 20th	Night	50% 6% 12% 18% 11% 2	5316
	21st to end	Night	49% 8% 15% 18% 7% 2	7106

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	27% 9% 20% 23% 16% 4	226
	11th to 20th	Day	14% 5% 18% 23% 25% 14%	430
	21st to end	Day	14% 4 21% 26% 22% 13%	473
February	1st to 10th	Day	16% 11% 19% 19% 20% 16%	430
	11th to 20th	Day	14% 12% 24% 22% 17% 10%	520
	21st to end	Day	9% 11% 21% 30% 22% 7%	446
March	1st to 10th	Day	14% 10% 22% 29% 19% 6%	540
	11th to 20th	Day	13% 9% 23% 25% 19% 11%	540
	21st to end	Day	13% 10% 22% 29% 18% 7%	594
April	1st to 10th	Day	12% 11% 24% 25% 19% 9%	540
	11th to 20th	Day	7% 10% 27% 27% 19% 11%	532
	21st to end	Day	5% 8% 24% 31% 20% 12%	540
May	1st to 10th	Day	5% 8% 27% 29% 23% 9%	543
	11th to 20th	Day	5% 7% 20% 30% 27% 10%	568
	21st to end	Day	5% 5% 16% 25% 34% 15%	617
June	1st to 10th	Day	2 5% 17% 31% 33% 11%	570
	11th to 20th	Day	1 5% 19% 38% 30% 7%	570
	21st to end	Day	1 4 14% 31% 37% 13%	570
July	1st to 10th	Day	1 1 11% 32% 40% 16%	566
	11th to 20th	Day	1 6% 32% 48% 13%	564
	21st to end	Day	7% 32% 43% 17%	619
August	1st to 10th	Day	3 1 10% 31% 42% 15%	567
	11th to 20th	Day	1 1 10% 35% 37% 16%	557
	21st to end	Day	1 1 7% 33% 44% 15%	627
September	1st to 10th	Day	1 2 10% 34% 39% 14%	570
	11th to 20th	Day	2 3 15% 33% 33% 14%	570
	21st to end	Day	2 2 15% 30% 36% 15%	570
October	1st to 10th	Day	3 5% 15% 24% 35% 18%	570
	11th to 20th	Day	5% 5% 15% 26% 27% 21%	570
	21st to end	Day	5% 6% 15% 25% 30% 19%	616
November	1st to 10th	Day	7% 4 18% 21% 27% 24%	400
	11th to 20th	Day	11% 8% 18% 24% 25% 15%	330
	21st to end	Day	12% 3 14% 30% 31% 10%	292
December	1st to 10th	Day	14% 3 23% 21% 27% 12%	220
	11th to 20th	Day	13% 8% 21% 22% 21% 15%	220
	21st to end	Day	17% 8% 28% 19% 22% 6%	242
January	1st to 10th	Night	17% 6% 16% 33% 24% 4	840
	11th to 20th	Night	12% 4 12% 35% 31% 6%	840
	21st to end	Night	15% 3 15% 35% 27% 4	924
February	1st to 10th	Night	15% 5% 17% 35% 24% 4	840
	11th to 20th	Night	11% 7% 21% 31% 26% 3	824
	21st to end	Night	13% 5% 22% 40% 18% 2	692
March	1st to 10th	Night	13% 7% 20% 38% 21% 2	840
	11th to 20th	Night	12% 7% 20% 33% 25% 3	840
	21st to end	Night	12% 6% 25% 36% 19% 2	924
April	1st to 10th	Night	10% 6% 25% 35% 22% 1	840
	11th to 20th	Night	6% 7% 29% 35% 21% 3	831
	21st to end	Night	5% 4 29% 37% 22% 3	840
May	1st to 10th	Night	3 4 30% 41% 19% 3	840
	11th to 20th	Night	4 4 24% 43% 24% 1	474
	21st to end	Night	3 4 26% 45% 21% 1	455
June	1st to 10th	Night	3 3 27% 47% 19% 1	420
	11th to 20th	Night	2 28% 46% 23% 1	420
	21st to end	Night	1 2 20% 50% 26% 1	420
July	1st to 10th	Night	17% 53% 29%	410
	11th to 20th	Night	2 13% 56% 27% 1	457
	21st to end	Night	1 1 13% 44% 38% 3	690
August	1st to 10th	Night	1 11% 48% 36% 4	631
	11th to 20th	Night	1 18% 49% 29% 2	832
	21st to end	Night	1 1 13% 49% 34% 2	924
September	1st to 10th	Night	1 1 16% 48% 33% 2	840
	11th to 20th	Night	3 3 17% 43% 31% 4	840
	21st to end	Night	2 2 18% 38% 36% 5%	840
October	1st to 10th	Night	4 3 16% 37% 34% 6%	840
	11th to 20th	Night	4 3 18% 35% 35% 6%	840
	21st to end	Night	4 4 14% 34% 33% 11%	912
November	1st to 10th	Night	7% 3 15% 32% 35% 9%	820
	11th to 20th	Night	9% 3 15% 38% 31% 5%	840
	21st to end	Night	10% 3 14% 37% 31% 5%	840
December	1st to 10th	Night	12% 3 14% 37% 27% 6%	840
	11th to 20th	Night	18% 3 14% 35% 26% 3	840
	21st to end	Night	17% 7% 19% 35% 20% 2	924

Page generated at 2026-10-26 01:37:52. Version ac07

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