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 2025-07-06.

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 3 3 2 4 5% 67%	123769
	Night	7% 3 1 4 3 3 3 2 4 5% 64%	123931
Transparency	Day	20% 4 8% 19% 26% 24%	123523
	Night	23% 4 9% 19% 24% 21%	123764
Seeing	Day	7% 6% 17% 28% 30% 14%	16143
	Night	8% 4 18% 39% 28% 4	25550

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	14% 8% 3 7% 5% 5% 5% 4 6% 8% 37%	3657
	11th to 20th	Day	11% 7% 3 8% 5% 4 3 2 3 5% 48%	4100
	21st to end	Day	14% 8% 3 8% 5% 4 4 3 4 7% 39%	4510
February	1st to 10th	Day	16% 6% 3 5% 5% 4 3 4 5% 6% 43%	1800
	11th to 20th	Day	11% 4 3 5% 4 4 4 4 5% 8% 48%	1921
	21st to end	Day	12% 4 2 5% 4 4 5% 3 6% 5% 51%	1650
March	1st to 10th	Day	10% 7% 2 7% 5% 4 6% 5% 7% 8% 39%	4667
	11th to 20th	Day	11% 4 1 4 3 4 4 4 5% 8% 52%	5000
	21st to end	Day	9% 6% 2 6% 4 4 5% 4 6% 8% 46%	5700
April	1st to 10th	Day	9% 5% 2 5% 3 4 3 3 5% 7% 54%	2400
	11th to 20th	Day	7% 3 1 4 3 3 3 3 5% 6% 62%	2369
	21st to end	Day	5% 3 1 3 2 2 3 2 4 6% 68%	2400
May	1st to 10th	Day	3 2 1 2 1 2 2 2 3 4 79%	5440
	11th to 20th	Day	1 1 1 2 1 1 1 1 2 4 84%	6167
	21st to end	Day	1 1 1 1 1 1 1 2 3 87%	6902
June	1st to 10th	Day	2 2 1 2 2 2 2 2 3 6% 75%	2800
	11th to 20th	Day	1 1 1 1 1 1 1 2 4 87%	2800
	21st to end	Day	1 1 1 1 1 1 1 2 2 88%	2800
July	1st to 10th	Day	1 1 1 1 95%	4491
	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	1 1 1 2 2 2 2 2 3 5% 80%	3180
	11th to 20th	Day	2 1 1 2 2 2 1 2 3 4 80%	3161
	21st to end	Day	3 2 1 3 2 3 3 3 5% 6% 68%	3448
November	1st to 10th	Day	5% 3 1 5% 4 4 4 4 5% 8% 57%	1831
	11th to 20th	Day	8% 6% 2 6% 4 3 3 3 6% 7% 53%	1800
	21st to end	Day	11% 7% 2 6% 3 3 3 4 4 7% 50%	1734
December	1st to 10th	Day	8% 4 2 6% 4 5% 5% 4 6% 10% 46%	2620
	11th to 20th	Day	17% 11% 5% 13% 7% 5% 4 4 4 7% 24%	3028
	21st to end	Day	16% 7% 3 9% 6% 6% 6% 5% 6% 9% 27%	4004
January	1st to 10th	Night	10% 5% 2 6% 4 4 5% 3 6% 10% 44%	6284
	11th to 20th	Night	12% 6% 2 6% 4 4 3 2 4 5% 51%	5880
	21st to end	Night	13% 6% 2 7% 4 4 4 3 4 6% 48%	6468
February	1st to 10th	Night	16% 5% 1 5% 3 3 3 3 4 6% 50%	2600
	11th to 20th	Night	11% 4 1 4 4 3 3 3 6% 8% 53%	2365
	21st to end	Night	12% 5% 2 5% 4 2 3 2 5% 6% 55%	1980
March	1st to 10th	Night	12% 6% 2 7% 5% 5% 5% 4 6% 7% 42%	5211
	11th to 20th	Night	11% 5% 2 5% 3 3 3 3 4 5% 55%	5000
	21st to end	Night	14% 6% 2 5% 3 4 4 3 5% 7% 47%	5273
April	1st to 10th	Night	9% 4 2 3 4 3 3 3 5% 7% 57%	2000
	11th to 20th	Night	5% 3 1 3 3 3 3 3 5% 6% 63%	1979
	21st to end	Night	4 3 1 2 3 2 3 3 4 6% 71%	2000
May	1st to 10th	Night	1 1 1 2 1 2 2 2 3 5% 81%	4560
	11th to 20th	Night	1 1 1 1 1 1 1 2 5% 84%	3725
	21st to end	Night	1 1 1 1 1 1 2 2 3 88%	3972
June	1st to 10th	Night	2 2 1 2 3 1 2 2 3 5% 78%	1600
	11th to 20th	Night	1 1 1 1 1 3 3 89%	1600
	21st to end	Night	1 1 1 1 1 2 2 2 89%	1600
July	1st to 10th	Night	1 1 1 1 95%	2580
	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	2 1 1 1 1 2 2 2 3 4 82%	3800
	11th to 20th	Night	2 1 1 2 1 2 2 2 3 4 82%	3777
	21st to end	Night	4 2 1 3 2 2 2 2 4 6% 72%	4121
November	1st to 10th	Night	7% 3 1 3 2 3 2 3 5% 7% 65%	2468
	11th to 20th	Night	7% 3 1 5% 3 3 3 3 5% 7% 60%	2600
	21st to end	Night	10% 4 1 3 3 3 4 3 5% 6% 58%	2665
December	1st to 10th	Night	7% 4 2 4 3 4 5% 4 6% 7% 54%	4560
	11th to 20th	Night	17% 8% 3 8% 5% 5% 5% 4 6% 9% 28%	5176
	21st to end	Night	17% 7% 3 8% 6% 6% 5% 4 6% 7% 30%	6952

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	41% 8% 18% 20% 10% 3	3657
	11th to 20th	Day	39% 5% 8% 14% 24% 10%	4100
	21st to end	Day	43% 6% 9% 14% 23% 4	4510
February	1st to 10th	Day	40% 6% 11% 22% 15% 6%	1800
	11th to 20th	Day	33% 6% 14% 24% 19% 5%	1931
	21st to end	Day	33% 5% 12% 24% 21% 4	1650
March	1st to 10th	Day	37% 9% 11% 20% 20% 2	4667
	11th to 20th	Day	29% 6% 13% 30% 14% 8%	5000
	21st to end	Day	33% 8% 16% 30% 12% 2	5700
April	1st to 10th	Day	30% 6% 12% 26% 20% 8%	2400
	11th to 20th	Day	22% 5% 13% 28% 24% 8%	2372
	21st to end	Day	18% 4 12% 31% 29% 7%	2400
May	1st to 10th	Day	11% 2 6% 27% 34% 20%	5440
	11th to 20th	Day	8% 2 5% 27% 42% 16%	6170
	21st to end	Day	6% 2 6% 22% 46% 18%	6904
June	1st to 10th	Day	12% 4 8% 24% 34% 19%	2800
	11th to 20th	Day	5% 1 5% 18% 35% 34%	2800
	21st to end	Day	7% 1 1 9% 30% 52%	2800
July	1st to 10th	Day	2 1 1 4 13% 79%	4422
	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	10% 2 4 12% 25% 47%	3180
	11th to 20th	Day	10% 2 6% 25% 38% 20%	3161
	21st to end	Day	17% 2 6% 21% 39% 15%	3448
November	1st to 10th	Day	24% 5% 11% 25% 27% 8%	1831
	11th to 20th	Day	29% 6% 14% 28% 20% 4	1800
	21st to end	Day	33% 5% 11% 25% 21% 6%	1734
December	1st to 10th	Day	33% 5% 12% 30% 17% 4	2620
	11th to 20th	Day	59% 6% 9% 15% 10% 1	3028
	21st to end	Day	48% 11% 17% 15% 8% 1	4004
January	1st to 10th	Night	32% 8% 19% 27% 10% 4	6284
	11th to 20th	Night	35% 5% 10% 17% 24% 8%	5880
	21st to end	Night	37% 5% 10% 14% 25% 8%	6468
February	1st to 10th	Night	35% 4 9% 24% 20% 8%	2600
	11th to 20th	Night	27% 7% 13% 23% 24% 7%	2366
	21st to end	Night	30% 4 10% 27% 22% 6%	1980
March	1st to 10th	Night	37% 8% 12% 18% 21% 4	5211
	11th to 20th	Night	32% 5% 12% 26% 16% 9%	5000
	21st to end	Night	33% 8% 16% 26% 13% 4	5273
April	1st to 10th	Night	26% 6% 13% 25% 22% 9%	2000
	11th to 20th	Night	21% 4 13% 29% 23% 11%	1979
	21st to end	Night	15% 4 13% 29% 31% 8%	2000
May	1st to 10th	Night	9% 2 7% 27% 32% 22%	4557
	11th to 20th	Night	7% 1 5% 26% 41% 20%	3725
	21st to end	Night	5% 1 5% 23% 47% 18%	3972
June	1st to 10th	Night	11% 3 9% 21% 37% 19%	1600
	11th to 20th	Night	4 2 5% 17% 33% 39%	1600
	21st to end	Night	4 2 2 10% 29% 54%	1600
July	1st to 10th	Night	2 1 4 13% 80%	2540
	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	9% 2 4 11% 19% 54%	3800
	11th to 20th	Night	9% 2 5% 26% 33% 26%	3777
	21st to end	Night	16% 2 5% 20% 36% 22%	4121
November	1st to 10th	Night	19% 4 10% 27% 25% 15%	2468
	11th to 20th	Night	23% 5% 11% 26% 28% 7%	2600
	21st to end	Night	27% 3 10% 25% 27% 9%	2665
December	1st to 10th	Night	26% 5% 10% 28% 23% 7%	4560
	11th to 20th	Night	50% 6% 12% 18% 11% 2	5176
	21st to end	Night	49% 8% 15% 18% 7% 2	6952

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% 7% 20% 25% 16% 4	205
	11th to 20th	Day	15% 5% 20% 25% 23% 13%	400
	21st to end	Day	14% 4 21% 25% 22% 14%	440
February	1st to 10th	Day	17% 11% 20% 18% 19% 16%	400
	11th to 20th	Day	13% 11% 24% 22% 18% 11%	481
	21st to end	Day	9% 12% 20% 29% 23% 7%	414
March	1st to 10th	Day	15% 9% 21% 29% 19% 6%	500
	11th to 20th	Day	14% 10% 24% 24% 18% 11%	500
	21st to end	Day	14% 11% 22% 28% 18% 7%	550
April	1st to 10th	Day	13% 11% 24% 25% 18% 9%	500
	11th to 20th	Day	7% 9% 27% 27% 20% 10%	492
	21st to end	Day	4 8% 23% 32% 20% 12%	500
May	1st to 10th	Day	5% 9% 27% 27% 22% 10%	503
	11th to 20th	Day	5% 6% 21% 29% 28% 11%	528
	21st to end	Day	5% 6% 16% 24% 34% 15%	573
June	1st to 10th	Day	2 5% 16% 30% 34% 12%	530
	11th to 20th	Day	1 5% 21% 36% 29% 8%	530
	21st to end	Day	2 4 13% 31% 37% 14%	530
July	1st to 10th	Day	1 1 8% 31% 43% 17%	507
	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% 25% 31% 19%	539
November	1st to 10th	Day	7% 5% 18% 21% 27% 23%	367
	11th to 20th	Day	7% 7% 17% 25% 27% 16%	300
	21st to end	Day	12% 3 15% 31% 30% 9%	266
December	1st to 10th	Day	15% 4 24% 21% 25% 13%	200
	11th to 20th	Day	13% 8% 21% 23% 22% 14%	200
	21st to end	Day	15% 8% 29% 19% 23% 7%	220
January	1st to 10th	Night	16% 5% 16% 34% 25% 5%	800
	11th to 20th	Night	13% 5% 13% 35% 29% 6%	800
	21st to end	Night	15% 3 15% 35% 28% 4	880
February	1st to 10th	Night	16% 5% 17% 34% 24% 4	800
	11th to 20th	Night	11% 6% 21% 32% 27% 4	784
	21st to end	Night	13% 5% 21% 40% 18% 3	660
March	1st to 10th	Night	14% 6% 20% 37% 21% 2	800
	11th to 20th	Night	13% 7% 20% 33% 24% 3	800
	21st to end	Night	13% 6% 24% 35% 20% 2	880
April	1st to 10th	Night	10% 6% 25% 36% 22% 1	800
	11th to 20th	Night	6% 7% 29% 35% 21% 3	791
	21st to end	Night	5% 4 28% 37% 23% 3	800
May	1st to 10th	Night	2 4 30% 41% 20% 3	800
	11th to 20th	Night	4 3 24% 43% 25% 1	451
	21st to end	Night	3 4 26% 44% 22% 1	433
June	1st to 10th	Night	3 3 26% 47% 20% 2	400
	11th to 20th	Night	1 2 27% 47% 23% 1	400
	21st to end	Night	1 2 19% 50% 28% 1	400
July	1st to 10th	Night	13% 55% 32%	380
	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% 3 14% 33% 35% 8%	780
	11th to 20th	Night	8% 2 15% 38% 32% 5%	800
	21st to end	Night	10% 3 14% 37% 30% 5%	800
December	1st to 10th	Night	13% 4 14% 38% 26% 6%	800
	11th to 20th	Night	19% 3 14% 36% 25% 3	800
	21st to end	Night	17% 6% 19% 35% 21% 2	880

Page generated at 2025-07-02 22:30:16. Version ac07

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