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 2024-03-18.

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	7% 4 2 4 3 2 3 3 4 5% 64%	78800
	Night	9% 4 2 3 3 2 3 3 4 6% 61%	81025
Transparency	Day	23% 4 9% 19% 24% 21%	78516
	Night	25% 5% 10% 20% 22% 19%	80830
Seeing	Day	7% 5% 16% 27% 30% 14%	14387
	Night	8% 3 17% 39% 29% 4	23921

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	17% 10% 4 7% 6% 3 5% 4 5% 7% 31%	2543
	11th to 20th	Day	16% 10% 4 10% 8% 4 5% 4 4 6% 30%	2860
	21st to end	Day	19% 9% 4 8% 7% 3 5% 5% 5% 8% 28%	3146
February	1st to 10th	Day	16% 5% 3 5% 5% 3 4 5% 4 6% 44%	1710
	11th to 20th	Day	11% 4 3 4 4 2 5% 5% 5% 8% 49%	1822
	21st to end	Day	13% 4 2 4 5% 3 4 3 5% 5% 51%	1570
March	1st to 10th	Day	11% 7% 3 6% 6% 3 4 5% 7% 9% 40%	1962
	11th to 20th	Day	12% 5% 2 4 4 3 5% 5% 6% 8% 45%	2059
	21st to end	Day	10% 4 2 4 4 3 5% 6% 5% 7% 52%	2263
April	1st to 10th	Day	10% 5% 2 5% 3 2 4 4 5% 7% 53%	2160
	11th to 20th	Day	7% 2 2 4 3 2 3 3 5% 6% 63%	2129
	21st to end	Day	6% 2 1 3 2 2 3 3 4 6% 69%	2160
May	1st to 10th	Day	3 3 1 2 2 2 2 3 4 6% 70%	2160
	11th to 20th	Day	3 2 1 2 2 1 2 3 4 5% 74%	2419
	21st to end	Day	3 2 1 2 2 1 3 3 4 5% 73%	2723
June	1st to 10th	Day	2 2 1 2 2 1 3 3 3 6% 74%	2520
	11th to 20th	Day	1 1 1 1 1 1 2 2 4 86%	2520
	21st to end	Day	1 1 1 1 1 1 1 2 2 2 87%	2520
July	1st to 10th	Day	1 2 2 2 92%	2520
	11th to 20th	Day	1 1 1 2 1 2 2 2 3 86%	2474
	21st to end	Day	1 1 1 1 1 1 2 3 4 85%	2546
August	1st to 10th	Day	1 2 1 2 2 1 1 1 1 3 85%	2324
	11th to 20th	Day	1 1 1 1 1 1 2 4 88%	2122
	21st to end	Day	1 1 1 1 1 1 2 3 89%	2376
September	1st to 10th	Day	1 1 1 1 1 1 1 1 2 90%	2160
	11th to 20th	Day	1 1 1 1 1 1 2 2 3 5% 82%	2160
	21st to end	Day	2 1 1 2 1 2 1 3 4 83%	1954
October	1st to 10th	Day	2 2 1 2 2 2 2 2 3 5% 76%	1800
	11th to 20th	Day	3 2 1 2 2 2 2 3 4 6% 74%	1800
	21st to end	Day	5% 3 1 3 3 2 2 3 5% 5% 68%	1980
November	1st to 10th	Day	5% 3 2 4 5% 3 4 4 5% 8% 57%	1738
	11th to 20th	Day	8% 5% 2 6% 5% 2 3 4 6% 7% 54%	1710
	21st to end	Day	11% 6% 3 5% 4 2 3 4 5% 6% 51%	1648
December	1st to 10th	Day	12% 6% 4 6% 6% 3 4 3 4 7% 44%	1520
	11th to 20th	Day	20% 11% 4 8% 6% 3 4 4 4 7% 28%	1928
	21st to end	Day	14% 7% 3 8% 7% 4 6% 6% 6% 8% 31%	2794
January	1st to 10th	Night	14% 7% 3 7% 6% 3 4 4 5% 8% 41%	4397
	11th to 20th	Night	17% 9% 3 6% 6% 4 5% 4 5% 6% 34%	4110
	21st to end	Night	18% 8% 3 7% 6% 3 5% 4 4 7% 35%	4521
February	1st to 10th	Night	16% 4 2 4 3 3 4 3 3 6% 52%	2470
	11th to 20th	Night	11% 3 2 4 4 2 3 4 6% 8% 54%	2244
	21st to end	Night	13% 5% 2 4 4 2 3 2 5% 6% 55%	1884
March	1st to 10th	Night	13% 6% 2 6% 5% 3 5% 4 6% 6% 45%	2215
	11th to 20th	Night	12% 5% 2 4 4 2 3 4 5% 7% 51%	2067
	21st to end	Night	13% 4 2 4 4 2 3 3 4 7% 54%	2091
April	1st to 10th	Night	10% 4 2 3 4 3 3 4 5% 7% 56%	1800
	11th to 20th	Night	6% 3 1 2 3 2 3 4 5% 7% 64%	1779
	21st to end	Night	4 2 1 2 2 2 3 4 4 7% 70%	1800
May	1st to 10th	Night	3 2 1 2 3 2 2 3 3 6% 73%	1800
	11th to 20th	Night	3 2 1 1 1 1 2 2 3 7% 76%	1469
	21st to end	Night	3 1 1 1 1 2 3 3 5% 78%	1552
June	1st to 10th	Night	2 2 1 2 3 1 3 3 3 4 78%	1440
	11th to 20th	Night	1 1 1 1 1 2 3 3 88%	1440
	21st to end	Night	1 1 1 1 1 2 2 2 3 89%	1440
July	1st to 10th	Night	1 1 1 1 1 1 2 2 91%	1440
	11th to 20th	Night	1 1 1 1 1 1 2 2 3 87%	1481
	21st to end	Night	1 1 1 2 1 1 3 2 3 4 81%	1773
August	1st to 10th	Night	1 1 1 1 1 1 2 2 3 86%	1614
	11th to 20th	Night	1 1 1 1 1 2 2 3 4 85%	1780
	21st to end	Night	1 1 1 1 1 1 1 2 2 90%	1980
September	1st to 10th	Night	1 1 1 1 1 2 2 90%	1800
	11th to 20th	Night	2 1 1 1 1 1 1 1 2 3 86%	1800
	21st to end	Night	1 1 2 2 1 1 1 2 3 86%	2002
October	1st to 10th	Night	4 1 1 2 2 1 2 2 3 4 76%	2160
	11th to 20th	Night	3 2 1 2 2 1 2 3 3 4 78%	2160
	21st to end	Night	4 1 1 2 2 1 2 2 3 6% 76%	2376
November	1st to 10th	Night	7% 3 1 3 3 2 2 3 4 7% 65%	2341
	11th to 20th	Night	7% 3 1 4 3 2 3 3 5% 7% 60%	2470
	21st to end	Night	9% 3 2 3 3 2 4 3 4 6% 60%	2531
December	1st to 10th	Night	12% 6% 2 5% 3 2 4 4 5% 6% 50%	2660
	11th to 20th	Night	19% 7% 3 6% 5% 3 5% 5% 5% 7% 34%	3276
	21st to end	Night	17% 6% 3 7% 7% 4 5% 5% 5% 7% 34%	4862

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	48% 9% 15% 21% 6% 1	2543
	11th to 20th	Day	53% 8% 10% 14% 11% 3	2860
	21st to end	Day	52% 9% 11% 17% 10% 1	3146
February	1st to 10th	Day	39% 6% 11% 23% 15% 6%	1710
	11th to 20th	Day	33% 6% 13% 23% 20% 5%	1832
	21st to end	Day	33% 6% 12% 24% 21% 4	1570
March	1st to 10th	Day	38% 7% 12% 23% 16% 4	1962
	11th to 20th	Day	34% 7% 12% 24% 18% 5%	2059
	21st to end	Day	30% 8% 18% 21% 19% 5%	2263
April	1st to 10th	Day	31% 6% 12% 25% 18% 8%	2160
	11th to 20th	Day	22% 5% 13% 28% 25% 8%	2132
	21st to end	Day	18% 3 12% 31% 29% 7%	2160
May	1st to 10th	Day	14% 4 11% 24% 34% 12%	2160
	11th to 20th	Day	13% 4 7% 26% 34% 15%	2422
	21st to end	Day	14% 4 12% 29% 30% 11%	2724
June	1st to 10th	Day	13% 4 8% 23% 33% 19%	2520
	11th to 20th	Day	6% 2 6% 20% 34% 34%	2520
	21st to end	Day	7% 1 2 9% 31% 49%	2520
July	1st to 10th	Day	3 1 2 8% 22% 64%	2451
	11th to 20th	Day	7% 1 3 8% 23% 57%	2399
	21st to end	Day	5% 1 2 8% 28% 56%	2405
August	1st to 10th	Day	9% 1 2 8% 28% 52%	2308
	11th to 20th	Day	4 1 3 9% 33% 50%	2122
	21st to end	Day	4 1 2 10% 35% 49%	2376
September	1st to 10th	Day	4 2 2 8% 27% 57%	2160
	11th to 20th	Day	7% 2 5% 20% 40% 27%	2160
	21st to end	Day	8% 2 4 18% 40% 29%	1954
October	1st to 10th	Day	12% 3 7% 20% 29% 30%	1800
	11th to 20th	Day	12% 3 9% 23% 28% 25%	1800
	21st to end	Day	18% 3 7% 22% 30% 20%	1980
November	1st to 10th	Day	25% 5% 10% 24% 26% 9%	1738
	11th to 20th	Day	28% 6% 14% 27% 21% 4	1710
	21st to end	Day	32% 5% 12% 25% 21% 6%	1648
December	1st to 10th	Day	40% 5% 13% 19% 17% 6%	1520
	11th to 20th	Day	55% 7% 9% 15% 12% 2	1928
	21st to end	Day	45% 12% 18% 15% 9% 2	2794
January	1st to 10th	Night	38% 9% 17% 25% 9% 2	4397
	11th to 20th	Night	47% 8% 13% 16% 9% 7%	4110
	21st to end	Night	47% 7% 14% 16% 12% 3	4521
February	1st to 10th	Night	34% 5% 10% 24% 20% 8%	2470
	11th to 20th	Night	26% 7% 13% 23% 25% 7%	2245
	21st to end	Night	30% 5% 10% 27% 23% 6%	1884
March	1st to 10th	Night	38% 7% 13% 19% 18% 6%	2215
	11th to 20th	Night	32% 6% 10% 22% 21% 8%	2067
	21st to end	Night	30% 6% 13% 23% 18% 9%	2091
April	1st to 10th	Night	27% 5% 13% 24% 21% 9%	1800
	11th to 20th	Night	21% 4 12% 28% 23% 11%	1779
	21st to end	Night	14% 5% 13% 28% 32% 9%	1800
May	1st to 10th	Night	14% 4 10% 21% 34% 17%	1800
	11th to 20th	Night	12% 3 7% 29% 33% 17%	1469
	21st to end	Night	10% 3 12% 32% 31% 12%	1552
June	1st to 10th	Night	12% 3 9% 20% 36% 19%	1440
	11th to 20th	Night	4 2 6% 18% 33% 37%	1440
	21st to end	Night	4 2 2 11% 30% 51%	1440
July	1st to 10th	Night	3 1 2 8% 22% 66%	1400
	11th to 20th	Night	6% 1 4 8% 23% 58%	1433
	21st to end	Night	8% 2 3 7% 24% 55%	1674
August	1st to 10th	Night	7% 1 2 12% 26% 52%	1605
	11th to 20th	Night	5% 2 3 9% 29% 52%	1780
	21st to end	Night	4 1 2 9% 32% 52%	1980
September	1st to 10th	Night	4 2 2 6% 26% 59%	1800
	11th to 20th	Night	7% 1 3 19% 38% 31%	1800
	21st to end	Night	7% 1 4 18% 36% 34%	2002
October	1st to 10th	Night	13% 3 7% 18% 26% 33%	2160
	11th to 20th	Night	12% 3 7% 19% 28% 30%	2160
	21st to end	Night	12% 3 7% 20% 33% 26%	2376
November	1st to 10th	Night	20% 4 10% 26% 25% 16%	2341
	11th to 20th	Night	23% 5% 11% 25% 29% 7%	2470
	21st to end	Night	25% 3 10% 25% 27% 9%	2531
December	1st to 10th	Night	33% 6% 12% 23% 17% 9%	2660
	11th to 20th	Night	47% 6% 12% 18% 13% 3	3276
	21st to end	Night	45% 9% 16% 18% 9% 3	4862

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	28% 8% 21% 23% 16% 4	184
	11th to 20th	Day	16% 5% 18% 25% 22% 14%	370
	21st to end	Day	15% 4 20% 24% 22% 14%	407
February	1st to 10th	Day	15% 10% 19% 18% 21% 17%	370
	11th to 20th	Day	14% 11% 23% 22% 19% 12%	442
	21st to end	Day	10% 12% 20% 29% 23% 7%	382
March	1st to 10th	Day	16% 8% 22% 28% 19% 7%	460
	11th to 20th	Day	13% 10% 24% 24% 18% 12%	449
	21st to end	Day	13% 10% 22% 29% 18% 8%	462
April	1st to 10th	Day	15% 11% 24% 24% 18% 8%	420
	11th to 20th	Day	8% 11% 26% 27% 18% 10%	412
	21st to end	Day	5% 8% 24% 32% 19% 12%	420
May	1st to 10th	Day	5% 9% 26% 27% 22% 11%	423
	11th to 20th	Day	6% 6% 21% 29% 28% 10%	448
	21st to end	Day	6% 6% 16% 23% 34% 16%	485
June	1st to 10th	Day	3 6% 18% 29% 31% 13%	450
	11th to 20th	Day	1 5% 19% 34% 32% 8%	450
	21st to end	Day	1 4 12% 31% 37% 15%	450
July	1st to 10th	Day	7% 31% 41% 20%	446
	11th to 20th	Day	1 4 27% 51% 17%	444
	21st to end	Day	5% 30% 44% 21%	487
August	1st to 10th	Day	3 1 10% 31% 39% 16%	447
	11th to 20th	Day	1 9% 33% 38% 19%	437
	21st to end	Day	5% 30% 46% 18%	495
September	1st to 10th	Day	1 2 8% 31% 42% 17%	450
	11th to 20th	Day	2 2 13% 33% 34% 16%	450
	21st to end	Day	2 2 13% 28% 38% 17%	450
October	1st to 10th	Day	3 5% 17% 22% 35% 18%	450
	11th to 20th	Day	5% 4 14% 25% 29% 23%	450
	21st to end	Day	5% 5% 14% 23% 32% 20%	495
November	1st to 10th	Day	7% 4 17% 21% 29% 22%	334
	11th to 20th	Day	6% 5% 16% 24% 30% 18%	270
	21st to end	Day	10% 2 14% 32% 32% 10%	240
December	1st to 10th	Day	16% 4 24% 21% 25% 11%	180
	11th to 20th	Day	13% 8% 19% 24% 21% 14%	180
	21st to end	Day	14% 7% 27% 19% 26% 8%	198
January	1st to 10th	Night	17% 4 16% 33% 26% 5%	760
	11th to 20th	Night	14% 5% 12% 34% 29% 6%	760
	21st to end	Night	16% 3 15% 34% 27% 5%	836
February	1st to 10th	Night	15% 4 16% 35% 25% 5%	760
	11th to 20th	Night	10% 6% 20% 32% 28% 4	744
	21st to end	Night	14% 5% 21% 39% 18% 3	628
March	1st to 10th	Night	14% 6% 19% 37% 21% 2	760
	11th to 20th	Night	12% 7% 20% 33% 24% 3	752
	21st to end	Night	13% 6% 24% 35% 20% 1	792
April	1st to 10th	Night	11% 6% 23% 36% 22% 1	720
	11th to 20th	Night	6% 7% 29% 33% 21% 3	711
	21st to end	Night	5% 4 28% 36% 23% 4	720
May	1st to 10th	Night	3 4 28% 41% 21% 3	720
	11th to 20th	Night	4 2 22% 43% 26% 1	407
	21st to end	Night	3 4 24% 44% 24% 1	389
June	1st to 10th	Night	3 3 26% 45% 21% 2	360
	11th to 20th	Night	1 3 23% 48% 25% 1	360
	21st to end	Night	1 1 16% 50% 30% 1	360
July	1st to 10th	Night	11% 54% 34%	350
	11th to 20th	Night	1 10% 58% 30% 1	389
	21st to end	Night	1 1 10% 42% 42% 4	591
August	1st to 10th	Night	1 1 9% 46% 38% 4	540
	11th to 20th	Night	16% 48% 33% 3	712
	21st to end	Night	1 10% 50% 37% 2	792
September	1st to 10th	Night	1 11% 49% 36% 3	720
	11th to 20th	Night	2 2 15% 43% 33% 4	720
	21st to end	Night	2 1 15% 38% 38% 6%	720
October	1st to 10th	Night	5% 3 16% 36% 34% 7%	720
	11th to 20th	Night	3 3 16% 34% 38% 7%	720
	21st to end	Night	4 4 13% 34% 34% 12%	792
November	1st to 10th	Night	7% 2 14% 34% 35% 8%	740
	11th to 20th	Night	8% 2 14% 38% 33% 5%	760
	21st to end	Night	9% 3 13% 39% 31% 5%	760
December	1st to 10th	Night	14% 4 14% 38% 25% 5%	760
	11th to 20th	Night	19% 3 14% 36% 25% 3	760
	21st to end	Night	17% 6% 18% 35% 22% 2	836

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