San Mateo
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the San Mateo clear sky chart from 2005-11-06 to 2026-08-30.

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% 4 2 5% 4 4 4 3 6% 9% 55%	152330
	Night	7% 6% 2 6% 4 4 5% 4 6% 8% 48%	147428
Transparency	Day	18% 34% 5% 14% 17% 12%	147253
	Night	23% 35% 6% 12% 15% 9%	143495
Seeing	Day	8% 10% 24% 32% 20% 6%	17820
	Night	10% 5% 16% 34% 29% 6%	27114

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	12% 6% 3 6% 4 4 4 4 7% 10% 41%	5119
	11th to 20th	Day	7% 5% 2 6% 4 4 3 3 4 7% 54%	5340
	21st to end	Day	12% 8% 3 8% 5% 4 4 4 7% 7% 37%	5843
February	1st to 10th	Day	13% 7% 2 6% 4 4 5% 4 4 6% 44%	1890
	11th to 20th	Day	11% 8% 4 7% 5% 4 5% 3 6% 9% 38%	1930
	21st to end	Day	10% 6% 3 6% 4 4 4 4 5% 8% 45%	1730
March	1st to 10th	Day	9% 7% 3 8% 6% 6% 6% 5% 8% 8% 33%	5013
	11th to 20th	Day	8% 5% 2 6% 5% 5% 4 4 6% 8% 48%	5110
	21st to end	Day	8% 6% 2 6% 5% 6% 6% 5% 8% 11% 36%	5668
April	1st to 10th	Day	9% 6% 2 6% 5% 4 4 3 6% 8% 46%	2520
	11th to 20th	Day	7% 5% 2 6% 3 4 4 4 5% 7% 53%	2489
	21st to end	Day	4 6% 1 5% 3 3 4 3 6% 10% 55%	2520
May	1st to 10th	Day	3 3 2 5% 3 3 3 3 5% 7% 64%	7315
	11th to 20th	Day	2 2 1 2 1 2 2 2 4 8% 74%	7697
	21st to end	Day	2 4 1 4 3 3 3 3 6% 10% 60%	8992
June	1st to 10th	Day	4 5% 2 4 2 3 4 3 6% 9% 58%	2940
	11th to 20th	Day	1 2 1 2 2 2 3 3 5% 9% 69%	2940
	21st to end	Day	2 4 1 3 3 3 3 3 5% 8% 65%	2940
July	1st to 10th	Day	2 1 3 2 2 3 2 5% 8% 71%	8168
	11th to 20th	Day	2 3 2 4 3 3 3 4 6% 10% 62%	7650
	21st to end	Day	3 2 6% 4 4 5% 4 5% 11% 57%	8304
August	1st to 10th	Day	1 2 1 3 3 3 4 5% 6% 12% 60%	5907
	11th to 20th	Day	2 2 4 3 3 4 3 5% 9% 64%	5659
	21st to end	Day	1 2 1 4 3 3 4 3 6% 11% 62%	6093
September	1st to 10th	Day	2 4 2 3 3 3 2 3 3 8% 68%	2400
	11th to 20th	Day	3 6% 2 6% 4 4 3 3 6% 10% 55%	2400
	21st to end	Day	3 3 1 3 2 2 3 2 3 8% 70%	2112
October	1st to 10th	Day	2 3 1 4 2 2 3 3 4 6% 70%	3280
	11th to 20th	Day	3 3 1 6% 4 4 4 3 5% 7% 59%	3261
	21st to end	Day	4 4 2 5% 4 4 3 3 6% 8% 57%	3558
November	1st to 10th	Day	6% 5% 1 5% 3 4 4 3 5% 9% 54%	2040
	11th to 20th	Day	8% 8% 2 7% 5% 5% 5% 4 5% 7% 45%	1890
	21st to end	Day	10% 5% 3 5% 4 3 3 3 5% 8% 52%	1712
December	1st to 10th	Day	6% 3 2 6% 3 3 4 3 6% 11% 52%	2700
	11th to 20th	Day	15% 8% 3 9% 6% 7% 6% 4 7% 7% 28%	3108
	21st to end	Day	16% 10% 4 9% 6% 5% 5% 4 6% 8% 28%	4092
January	1st to 10th	Night	12% 7% 3 6% 4 4 4 4 6% 9% 40%	7792
	11th to 20th	Night	9% 6% 3 6% 4 4 4 3 5% 7% 49%	7650
	21st to end	Night	11% 10% 3 9% 5% 5% 4 3 5% 7% 37%	8371
February	1st to 10th	Night	12% 7% 3 6% 4 4 4 3 5% 7% 46%	2730
	11th to 20th	Night	10% 7% 3 7% 5% 5% 4 4 5% 8% 41%	2574
	21st to end	Night	10% 6% 3 6% 4 4 4 3 6% 7% 47%	2076
March	1st to 10th	Night	10% 9% 3 8% 6% 5% 6% 5% 6% 8% 34%	5098
	11th to 20th	Night	11% 7% 3 6% 4 4 4 3 5% 7% 48%	5110
	21st to end	Night	10% 9% 3 8% 5% 4 4 4 6% 7% 39%	5549
April	1st to 10th	Night	7% 7% 3 6% 5% 3 4 4 7% 8% 46%	2100
	11th to 20th	Night	7% 5% 2 5% 5% 4 3 4 5% 7% 52%	2079
	21st to end	Night	3 4 1 4 4 3 5% 3 5% 9% 57%	2100
May	1st to 10th	Night	4 5% 2 7% 5% 4 4 2 4 5% 57%	5645
	11th to 20th	Night	1 2 1 3 3 4 5% 4 6% 7% 65%	5181
	21st to end	Night	2 4 2 6% 4 4 4 4 6% 6% 59%	5181
June	1st to 10th	Night	3 5% 2 5% 3 4 4 4 7% 10% 53%	1680
	11th to 20th	Night	1 2 1 3 3 4 3 3 6% 9% 66%	1680
	21st to end	Night	1 4 2 4 3 4 3 3 5% 8% 62%	1680
July	1st to 10th	Night	2 1 4 3 4 6% 6% 6% 7% 61%	4775
	11th to 20th	Night	2 3 2 6% 4 4 8% 8% 8% 8% 47%	5340
	21st to end	Night	5% 4 9% 8% 8% 7% 4 5% 7% 41%	5806
August	1st to 10th	Night	2 3 2 5% 4 6% 8% 5% 8% 10% 46%	4125
	11th to 20th	Night	3 1 5% 5% 6% 7% 5% 5% 8% 53%	4419
	21st to end	Night	1 2 2 6% 4 5% 7% 5% 8% 9% 50%	5105
September	1st to 10th	Night	1 3 1 4 3 3 5% 4 6% 10% 60%	2000
	11th to 20th	Night	3 6% 2 6% 4 4 5% 3 6% 10% 52%	2000
	21st to end	Night	2 2 1 5% 3 3 3 2 3 7% 67%	2281
October	1st to 10th	Night	3 3 1 3 3 3 3 3 5% 6% 67%	3920
	11th to 20th	Night	4 4 2 5% 4 5% 6% 5% 5% 7% 53%	3897
	21st to end	Night	5% 4 2 5% 3 3 4 3 5% 7% 59%	4253
November	1st to 10th	Night	7% 5% 2 5% 4 4 4 3 5% 8% 53%	2476
	11th to 20th	Night	8% 7% 3 6% 4 3 4 3 6% 8% 46%	2730
	21st to end	Night	8% 5% 2 5% 4 4 4 4 5% 9% 51%	2903
December	1st to 10th	Night	7% 4 2 5% 4 3 4 4 5% 7% 55%	4700
	11th to 20th	Night	17% 8% 3 8% 6% 5% 6% 4 7% 10% 26%	5316
	21st to end	Night	18% 9% 3 9% 5% 5% 6% 4 6% 6% 29%	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	28% 25% 14% 19% 11% 2	5120
	11th to 20th	Day	25% 23% 4 11% 21% 16%	5340
	21st to end	Day	35% 25% 4 14% 10% 11%	5843
February	1st to 10th	Day	20% 60% 4 7% 6% 3	1890
	11th to 20th	Day	20% 59% 4 7% 7% 2	1939
	21st to end	Day	14% 60% 5% 12% 6% 3	1730
March	1st to 10th	Day	33% 33% 5% 14% 14% 1	5013
	11th to 20th	Day	20% 30% 12% 22% 8% 7%	5110
	21st to end	Day	27% 35% 13% 18% 6% 1	5668
April	1st to 10th	Day	15% 60% 5% 10% 8% 1	2520
	11th to 20th	Day	12% 60% 7% 11% 8% 2	2492
	21st to end	Day	11% 60% 7% 11% 10% 1	2520
May	1st to 10th	Day	17% 23% 6% 21% 23% 10%	7315
	11th to 20th	Day	6% 22% 6% 22% 34% 10%	7700
	21st to end	Day	13% 24% 7% 23% 30% 3	8994
June	1st to 10th	Day	8% 59% 5% 11% 11% 6%	2940
	11th to 20th	Day	5% 59% 3 10% 17% 6%	2940
	21st to end	Day	8% 57% 3 5% 14% 13%	2940
July	1st to 10th	Day	11% 19% 3 10% 21% 37%	8100
	11th to 20th	Day	17% 20% 2 4 14% 43%	7581
	21st to end	Day	19% 19% 4 11% 20% 26%	8119
August	1st to 10th	Day	17% 35% 3 8% 19% 17%	4336
	11th to 20th	Day	12% 34% 3 9% 30% 12%	4130
	21st to end	Day	13% 34% 4 11% 23% 14%	4420
September	1st to 10th	Day	11% 56% 3 7% 13% 11%	2400
	11th to 20th	Day	11% 59% 3 10% 12% 6%	2400
	21st to end	Day	8% 57% 3 10% 15% 7%	2112
October	1st to 10th	Day	8% 35% 2 7% 18% 31%	3280
	11th to 20th	Day	16% 37% 5% 16% 18% 9%	3261
	21st to end	Day	16% 36% 6% 18% 17% 8%	3558
November	1st to 10th	Day	9% 54% 5% 14% 13% 5%	2040
	11th to 20th	Day	18% 55% 4 12% 7% 2	1890
	21st to end	Day	16% 56% 5% 9% 12% 3	1712
December	1st to 10th	Day	15% 36% 4 22% 19% 3	2700
	11th to 20th	Day	37% 35% 6% 9% 11% 3	3108
	21st to end	Day	40% 29% 8% 14% 8% 1	4092
January	1st to 10th	Night	31% 26% 15% 17% 10% 2	7792
	11th to 20th	Night	27% 24% 5% 11% 21% 12%	7650
	21st to end	Night	37% 25% 4 12% 14% 8%	8371
February	1st to 10th	Night	19% 60% 4 6% 7% 4	2730
	11th to 20th	Night	19% 60% 5% 8% 7% 3	2576
	21st to end	Night	13% 59% 6% 13% 6% 2	2076
March	1st to 10th	Night	35% 33% 5% 15% 11% 1	5098
	11th to 20th	Night	25% 29% 10% 17% 12% 6%	5110
	21st to end	Night	32% 33% 11% 16% 8% 1	5549
April	1st to 10th	Night	14% 61% 6% 10% 8% 1	2100
	11th to 20th	Night	14% 61% 5% 11% 7% 2	2079
	21st to end	Night	11% 61% 6% 12% 8% 1	2100
May	1st to 10th	Night	24% 23% 6% 18% 22% 7%	5643
	11th to 20th	Night	13% 24% 8% 18% 27% 11%	5182
	21st to end	Night	19% 24% 7% 19% 30% 1	5182
June	1st to 10th	Night	11% 60% 3 10% 11% 5%	1680
	11th to 20th	Night	7% 59% 4 10% 15% 6%	1680
	21st to end	Night	9% 57% 2 6% 11% 14%	1680
July	1st to 10th	Night	14% 19% 2 11% 20% 34%	4735
	11th to 20th	Night	23% 19% 2 4 19% 33%	5286
	21st to end	Night	32% 19% 3 6% 18% 22%	5679
August	1st to 10th	Night	27% 35% 4 12% 16% 7%	3019
	11th to 20th	Night	16% 35% 3 6% 31% 10%	3218
	21st to end	Night	16% 36% 5% 10% 20% 12%	3698
September	1st to 10th	Night	10% 59% 5% 5% 13% 9%	2000
	11th to 20th	Night	11% 57% 4 12% 10% 5%	2000
	21st to end	Night	10% 57% 4 10% 11% 8%	2281
October	1st to 10th	Night	8% 35% 1 5% 17% 33%	3920
	11th to 20th	Night	18% 36% 7% 13% 13% 11%	3897
	21st to end	Night	16% 36% 5% 15% 17% 11%	4253
November	1st to 10th	Night	12% 54% 4 14% 11% 6%	2476
	11th to 20th	Night	18% 55% 6% 12% 7% 2	2730
	21st to end	Night	16% 56% 5% 10% 9% 4	2903
December	1st to 10th	Night	16% 38% 4 21% 16% 5%	4700
	11th to 20th	Night	36% 35% 6% 10% 10% 3	5316
	21st to end	Night	40% 30% 10% 13% 6% 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	25% 9% 25% 23% 16% 3	357
	11th to 20th	Day	14% 10% 21% 29% 18% 8%	430
	21st to end	Day	13% 10% 23% 29% 20% 5%	473
February	1st to 10th	Day	15% 14% 22% 25% 20% 4	430
	11th to 20th	Day	16% 15% 29% 27% 12% 1	470
	21st to end	Day	12% 14% 26% 30% 17% 2	446
March	1st to 10th	Day	17% 17% 30% 22% 11% 3	540
	11th to 20th	Day	16% 14% 26% 21% 18% 6%	540
	21st to end	Day	12% 17% 30% 26% 11% 4	594
April	1st to 10th	Day	13% 15% 30% 24% 14% 4	540
	11th to 20th	Day	11% 17% 30% 26% 12% 4	532
	21st to end	Day	7% 18% 28% 29% 13% 6%	540
May	1st to 10th	Day	7% 15% 31% 29% 14% 4	543
	11th to 20th	Day	7% 13% 32% 31% 12% 4	568
	21st to end	Day	7% 13% 29% 30% 15% 5%	617
June	1st to 10th	Day	5% 10% 30% 30% 18% 6%	570
	11th to 20th	Day	2 9% 29% 37% 17% 5%	570
	21st to end	Day	2 6% 27% 35% 22% 7%	570
July	1st to 10th	Day	4 3 22% 45% 20% 7%	566
	11th to 20th	Day	2 1 18% 46% 26% 6%	564
	21st to end	Day	3 1 21% 45% 23% 7%	619
August	1st to 10th	Day	4 3 22% 39% 20% 12%	567
	11th to 20th	Day	4 2 21% 42% 25% 7%	557
	21st to end	Day	3 3 18% 42% 26% 9%	620
September	1st to 10th	Day	4 4 15% 43% 25% 9%	530
	11th to 20th	Day	5% 9% 23% 34% 21% 8%	530
	21st to end	Day	4 8% 20% 32% 27% 9%	530
October	1st to 10th	Day	3 7% 25% 30% 23% 12%	530
	11th to 20th	Day	6% 9% 21% 29% 26% 10%	530
	21st to end	Day	9% 7% 20% 30% 24% 10%	583
November	1st to 10th	Day	8% 9% 20% 25% 27% 10%	516
	11th to 20th	Day	10% 8% 17% 33% 24% 7%	330
	21st to end	Day	11% 6% 20% 31% 26% 6%	236
December	1st to 10th	Day	11% 14% 16% 21% 26% 11%	220
	11th to 20th	Day	14% 12% 20% 26% 20% 9%	220
	21st to end	Day	21% 10% 27% 28% 14% 1	242
January	1st to 10th	Night	19% 6% 17% 32% 22% 5%	840
	11th to 20th	Night	14% 6% 14% 33% 27% 6%	840
	21st to end	Night	17% 5% 13% 36% 24% 4	924
February	1st to 10th	Night	17% 7% 16% 34% 24% 3	840
	11th to 20th	Night	13% 9% 19% 34% 20% 4	824
	21st to end	Night	12% 10% 20% 35% 20% 3	692
March	1st to 10th	Night	16% 9% 23% 29% 20% 3	840
	11th to 20th	Night	16% 8% 17% 31% 24% 4	840
	21st to end	Night	15% 10% 21% 33% 19% 2	924
April	1st to 10th	Night	11% 8% 23% 34% 20% 3	840
	11th to 20th	Night	11% 11% 22% 31% 21% 5%	831
	21st to end	Night	8% 7% 24% 34% 22% 5%	840
May	1st to 10th	Night	6% 5% 21% 38% 25% 4	746
	11th to 20th	Night	8% 4 17% 37% 30% 4	585
	21st to end	Night	8% 8% 21% 36% 22% 5%	455
June	1st to 10th	Night	5% 5% 18% 40% 28% 3	420
	11th to 20th	Night	1 3 17% 47% 30% 3	420
	21st to end	Night	4 3 13% 37% 38% 5%	420
July	1st to 10th	Night	3 2 12% 34% 44% 5%	419
	11th to 20th	Night	3 1 7% 36% 42% 11%	612
	21st to end	Night	5% 2 9% 35% 41% 9%	690
August	1st to 10th	Night	8% 3 11% 31% 37% 10%	627
	11th to 20th	Night	6% 2 10% 39% 37% 7%	745
	21st to end	Night	3 1 11% 36% 39% 9%	916
September	1st to 10th	Night	4 1 11% 33% 40% 11%	800
	11th to 20th	Night	8% 3 14% 35% 33% 8%	800
	21st to end	Night	3 2 13% 31% 39% 11%	800
October	1st to 10th	Night	7% 2 12% 34% 35% 10%	800
	11th to 20th	Night	7% 4 12% 29% 37% 10%	800
	21st to end	Night	7% 4 12% 30% 34% 12%	880
November	1st to 10th	Night	9% 3 16% 27% 35% 10%	820
	11th to 20th	Night	12% 4 12% 37% 29% 6%	840
	21st to end	Night	10% 4 12% 38% 29% 5%	840
December	1st to 10th	Night	15% 6% 14% 34% 27% 5%	840
	11th to 20th	Night	23% 6% 15% 31% 23% 4	840
	21st to end	Night	21% 9% 20% 31% 18% 2	924

Page generated at 2026-08-27 01:07:16. Version ac07

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