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 2024-11-03.

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	6% 3 1 4 3 2 3 2 4 5% 68%	101116
	Night	8% 4 1 4 3 3 3 2 4 5% 64%	98101
Transparency	Day	20% 4 8% 18% 24% 27%	100869
	Night	22% 4 9% 19% 22% 23%	97937
Seeing	Day	7% 5% 16% 28% 30% 14%	15307
	Night	8% 3 18% 39% 28% 4	24680

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% 11% 4 8% 5% 4 4 4 5% 6% 31%	2543
	11th to 20th	Day	16% 10% 4 12% 6% 5% 4 3 4 6% 30%	2860
	21st to end	Day	19% 9% 4 9% 5% 4 5% 3 5% 8% 28%	3146
February	1st to 10th	Day	16% 6% 3 5% 5% 4 4 4 5% 6% 43%	1710
	11th to 20th	Day	11% 4 3 5% 4 3 4 4 5% 7% 49%	1822
	21st to end	Day	13% 4 2 5% 4 3 4 2 6% 5% 51%	1570
March	1st to 10th	Day	9% 8% 3 9% 6% 5% 6% 5% 8% 8% 34%	3248
	11th to 20th	Day	8% 4 1 4 3 3 3 3 4 7% 60%	3490
	21st to end	Day	9% 7% 2 6% 4 4 6% 4 6% 7% 43%	3974
April	1st to 10th	Day	10% 6% 2 5% 3 3 3 3 5% 7% 54%	2280
	11th to 20th	Day	7% 3 1 4 3 3 2 3 5% 6% 62%	2249
	21st to end	Day	5% 2 1 3 2 2 3 2 4 6% 69%	2280
May	1st to 10th	Day	4 2 1 2 1 2 2 2 3 4 77%	3800
	11th to 20th	Day	2 1 2 1 1 1 1 2 4 84%	4293
	21st to end	Day	2 1 1 1 1 2 1 2 3 83%	4813
June	1st to 10th	Day	2 2 1 2 2 2 2 2 3 6% 74%	2660
	11th to 20th	Day	1 1 1 1 1 1 1 2 4 86%	2660
	21st to end	Day	1 1 1 1 1 1 1 2 2 87%	2660
July	1st to 10th	Day	1 1 1 1 95%	4420
	11th to 20th	Day	1 1 2 2 2 2 1 2 3 85%	4335
	21st to end	Day	1 1 1 1 1 1 2 2 91%	4455
August	1st to 10th	Day	1 1 2 1 1 1 2 3 4 83%	4074
	11th to 20th	Day	1 1 1 1 1 1 2 3 91%	3724
	21st to end	Day	1 1 1 1 1 1 1 2 2 90%	4180
September	1st to 10th	Day	1 1 1 1 1 1 1 2 91%	2280
	11th to 20th	Day	1 1 1 2 1 1 2 2 3 5% 81%	2280
	21st to end	Day	2 1 1 2 1 1 1 2 4 83%	2061
October	1st to 10th	Day	2 2 1 2 2 2 1 2 3 5% 76%	1900
	11th to 20th	Day	3 2 1 2 1 2 2 2 4 6% 75%	1900
	21st to end	Day	5% 3 1 4 2 2 3 3 4 6% 68%	2090
November	1st to 10th	Day	6% 3 1 5% 4 4 4 4 5% 8% 57%	1759
	11th to 20th	Day	8% 6% 2 6% 4 3 3 3 6% 7% 53%	1710
	21st to end	Day	10% 7% 2 6% 3 3 3 4 4 6% 51%	1648
December	1st to 10th	Day	12% 7% 3 7% 5% 4 3 2 4 8% 44%	1520
	11th to 20th	Day	20% 12% 4 9% 5% 4 3 3 4 7% 28%	1928
	21st to end	Day	13% 8% 3 9% 6% 6% 6% 5% 5% 8% 31%	2794
January	1st to 10th	Night	14% 7% 3 8% 5% 4 4 3 5% 8% 41%	4397
	11th to 20th	Night	17% 9% 3 8% 5% 5% 4 3 5% 6% 34%	4110
	21st to end	Night	18% 8% 3 8% 5% 4 5% 3 5% 7% 35%	4521
February	1st to 10th	Night	16% 5% 1 5% 3 3 3 3 4 6% 51%	2470
	11th to 20th	Night	10% 4 1 4 4 3 3 3 6% 8% 54%	2244
	21st to end	Night	13% 5% 2 5% 4 3 2 2 5% 6% 54%	1884
March	1st to 10th	Night	13% 6% 2 7% 5% 6% 5% 4 6% 6% 39%	3623
	11th to 20th	Night	10% 5% 2 4 3 2 2 2 4 5% 62%	3490
	21st to end	Night	16% 6% 2 5% 4 4 4 3 5% 7% 44%	3677
April	1st to 10th	Night	9% 4 2 3 4 3 3 3 5% 7% 57%	1900
	11th to 20th	Night	5% 3 1 3 3 3 3 3 5% 6% 63%	1879
	21st to end	Night	4 2 1 2 3 2 3 3 4 6% 71%	1900
May	1st to 10th	Night	2 1 1 2 2 2 1 2 3 5% 81%	3180
	11th to 20th	Night	2 1 1 1 1 1 1 2 5% 85%	2597
	21st to end	Night	1 1 1 1 1 1 2 2 3 86%	2762
June	1st to 10th	Night	2 2 1 2 3 2 2 2 3 5% 77%	1520
	11th to 20th	Night	1 1 1 1 1 3 3 88%	1520
	21st to end	Night	1 1 1 1 1 2 2 2 89%	1520
July	1st to 10th	Night	1 1 1 1 95%	2540
	11th to 20th	Night	1 1 1 2 1 1 2 1 2 3 85%	2623
	21st to end	Night	1 1 1 1 1 1 1 2 3 88%	3109
August	1st to 10th	Night	1 1 1 1 1 1 1 2 4 4 82%	2865
	11th to 20th	Night	1 1 1 1 1 2 2 91%	3126
	21st to end	Night	2 1 1 1 1 1 1 1 1 91%	3498
September	1st to 10th	Night	1 1 1 1 1 1 2 2 91%	1900
	11th to 20th	Night	2 2 1 1 1 1 1 1 2 3 85%	1900
	21st to end	Night	1 1 2 2 1 1 1 2 3 86%	2114
October	1st to 10th	Night	4 2 1 2 2 2 2 2 4 4 77%	2280
	11th to 20th	Night	3 1 1 2 2 2 2 2 3 4 78%	2280
	21st to end	Night	4 1 1 2 1 1 2 2 3 6% 75%	2508
November	1st to 10th	Night	7% 3 1 3 2 3 2 3 5% 6% 65%	2365
	11th to 20th	Night	7% 3 1 5% 3 3 3 3 5% 7% 60%	2470
	21st to end	Night	9% 4 2 3 3 3 4 3 5% 6% 60%	2531
December	1st to 10th	Night	12% 6% 2 6% 3 3 4 3 5% 6% 50%	2660
	11th to 20th	Night	19% 7% 3 6% 5% 4 5% 4 6% 7% 34%	3276
	21st to end	Night	17% 7% 3 8% 6% 6% 5% 4 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	39% 12% 14% 17% 16% 2	3248
	11th to 20th	Day	23% 6% 12% 30% 18% 11%	3490
	21st to end	Day	34% 9% 17% 24% 13% 3	3974
April	1st to 10th	Day	30% 6% 12% 26% 19% 8%	2280
	11th to 20th	Day	22% 5% 13% 28% 24% 7%	2252
	21st to end	Day	17% 4 12% 31% 29% 7%	2280
May	1st to 10th	Day	12% 3 7% 28% 31% 20%	3800
	11th to 20th	Day	8% 2 4 25% 43% 19%	4296
	21st to end	Day	8% 2 7% 27% 38% 17%	4814
June	1st to 10th	Day	13% 4 9% 23% 33% 19%	2660
	11th to 20th	Day	6% 1 5% 19% 35% 33%	2660
	21st to end	Day	7% 1 1 9% 30% 51%	2660
July	1st to 10th	Day	2 1 1 4 13% 80%	4351
	11th to 20th	Day	9% 1 2 5% 14% 69%	4260
	21st to end	Day	3 1 1 5% 26% 65%	4315
August	1st to 10th	Day	9% 1 2 5% 26% 57%	4059
	11th to 20th	Day	3 1 2 6% 33% 56%	3760
	21st to end	Day	5% 1 2 7% 33% 52%	4179
September	1st to 10th	Day	4 2 2 8% 27% 57%	2280
	11th to 20th	Day	8% 2 5% 19% 40% 26%	2280
	21st to end	Day	8% 2 3 18% 40% 30%	2061
October	1st to 10th	Day	12% 3 6% 19% 28% 32%	1900
	11th to 20th	Day	12% 3 9% 23% 30% 24%	1900
	21st to end	Day	18% 3 7% 21% 31% 19%	2090
November	1st to 10th	Day	25% 5% 10% 25% 26% 8%	1759
	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	39% 10% 14% 16% 16% 4	3623
	11th to 20th	Night	25% 5% 10% 26% 21% 12%	3490
	21st to end	Night	36% 8% 15% 25% 11% 5%	3677
April	1st to 10th	Night	27% 6% 13% 24% 22% 9%	1900
	11th to 20th	Night	21% 4 12% 29% 23% 11%	1879
	21st to end	Night	14% 5% 13% 28% 32% 9%	1900
May	1st to 10th	Night	9% 3 9% 26% 32% 20%	3180
	11th to 20th	Night	7% 2 4 23% 39% 26%	2597
	21st to end	Night	6% 2 7% 29% 41% 15%	2762
June	1st to 10th	Night	12% 3 9% 21% 36% 19%	1520
	11th to 20th	Night	4 2 6% 18% 34% 37%	1520
	21st to end	Night	4 2 2 10% 28% 54%	1520
July	1st to 10th	Night	2 1 4 12% 81%	2500
	11th to 20th	Night	8% 1 3 5% 14% 69%	2575
	21st to end	Night	5% 1 2 6% 22% 64%	3011
August	1st to 10th	Night	8% 1 2 7% 22% 60%	2856
	11th to 20th	Night	3 1 2 5% 27% 62%	3156
	21st to end	Night	5% 2 8% 30% 54%	3498
September	1st to 10th	Night	4 1 2 6% 26% 60%	1900
	11th to 20th	Night	7% 1 4 19% 38% 30%	1900
	21st to end	Night	7% 1 3 17% 36% 35%	2114
October	1st to 10th	Night	13% 3 7% 17% 25% 35%	2280
	11th to 20th	Night	11% 3 7% 20% 29% 30%	2280
	21st to end	Night	13% 3 6% 20% 33% 25%	2508
November	1st to 10th	Night	20% 4 10% 26% 24% 16%	2365
	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	12% 10% 23% 25% 18% 12%	460
	21st to end	Day	14% 10% 22% 30% 17% 7%	506
April	1st to 10th	Day	14% 11% 24% 24% 18% 9%	460
	11th to 20th	Day	8% 10% 25% 27% 19% 10%	452
	21st to end	Day	5% 8% 24% 32% 20% 12%	460
May	1st to 10th	Day	5% 9% 28% 27% 22% 10%	463
	11th to 20th	Day	5% 6% 19% 28% 30% 11%	488
	21st to end	Day	5% 6% 15% 23% 34% 16%	529
June	1st to 10th	Day	3 6% 17% 30% 32% 12%	490
	11th to 20th	Day	1 5% 19% 36% 31% 8%	490
	21st to end	Day	1 3 13% 32% 37% 14%	490
July	1st to 10th	Day	1 8% 30% 43% 18%	486
	11th to 20th	Day	1 5% 29% 50% 15%	484
	21st to end	Day	5% 30% 44% 20%	531
August	1st to 10th	Day	3 1 9% 32% 40% 15%	487
	11th to 20th	Day	1 1 11% 34% 36% 17%	477
	21st to end	Day	1 6% 30% 46% 17%	539
September	1st to 10th	Day	1 1 9% 32% 41% 16%	490
	11th to 20th	Day	2 2 14% 34% 33% 15%	490
	21st to end	Day	2 2 13% 29% 37% 16%	490
October	1st to 10th	Day	3 5% 16% 23% 36% 19%	490
	11th to 20th	Day	5% 5% 14% 25% 29% 22%	490
	21st to end	Day	4 6% 15% 24% 31% 19%	539
November	1st to 10th	Day	7% 5% 17% 21% 29% 22%	343
	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% 25% 3	760
	21st to end	Night	13% 6% 24% 36% 20% 1	836
April	1st to 10th	Night	11% 6% 23% 36% 23% 1	760
	11th to 20th	Night	6% 7% 29% 33% 22% 3	751
	21st to end	Night	4 4 29% 36% 23% 4	760
May	1st to 10th	Night	3 4 29% 42% 20% 3	760
	11th to 20th	Night	4 2 22% 45% 26% 1	429
	21st to end	Night	3 4 24% 45% 23% 1	411
June	1st to 10th	Night	3 2 26% 47% 20% 2	380
	11th to 20th	Night	1 2 25% 47% 24% 1	380
	21st to end	Night	1 1 18% 50% 29% 1	380
July	1st to 10th	Night	12% 55% 32%	370
	11th to 20th	Night	1 11% 57% 29% 1	412
	21st to end	Night	1 1 11% 42% 41% 3	624
August	1st to 10th	Night	1 1 10% 47% 37% 4	571
	11th to 20th	Night	1 18% 48% 31% 2	752
	21st to end	Night	1 1 11% 49% 36% 2	836
September	1st to 10th	Night	1 1 13% 49% 35% 2	760
	11th to 20th	Night	3 2 16% 43% 32% 4	760
	21st to end	Night	2 1 16% 38% 38% 6%	760
October	1st to 10th	Night	4 3 16% 36% 33% 7%	760
	11th to 20th	Night	3 3 17% 34% 37% 6%	760
	21st to end	Night	4 4 13% 34% 34% 11%	836
November	1st to 10th	Night	8% 2 14% 33% 35% 8%	748
	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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