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 2024-10-25.

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% 5% 2 5% 3 3 4 3 5% 8% 54%	101010
	Night	8% 6% 2 6% 4 4 4 4 6% 8% 48%	98022
Transparency	Day	15% 49% 5% 10% 12% 10%	100771
	Night	19% 50% 5% 9% 10% 7%	97851
Seeing	Day	9% 11% 27% 32% 17% 5%	15409
	Night	11% 6% 16% 33% 28% 6%	24728

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% 7% 3 7% 5% 4 4 3 6% 8% 38%	2740
	11th to 20th	Day	13% 9% 4 10% 6% 6% 5% 4 4 6% 32%	2860
	21st to end	Day	19% 10% 3 9% 6% 5% 4 4 6% 6% 28%	3146
February	1st to 10th	Day	13% 7% 2 6% 5% 4 5% 4 4 6% 44%	1710
	11th to 20th	Day	11% 8% 3 7% 4 4 4 3 6% 9% 41%	1741
	21st to end	Day	10% 6% 3 6% 4 4 4 4 5% 7% 47%	1570
March	1st to 10th	Day	10% 9% 4 10% 6% 6% 7% 5% 7% 8% 29%	3407
	11th to 20th	Day	7% 6% 2 6% 4 4 3 3 5% 8% 51%	3490
	21st to end	Day	10% 7% 2 7% 6% 5% 6% 5% 7% 9% 36%	3872
April	1st to 10th	Day	10% 6% 2 6% 5% 4 4 3 6% 8% 45%	2280
	11th to 20th	Day	8% 5% 2 5% 3 3 3 4 5% 7% 54%	2249
	21st to end	Day	4 5% 1 4 3 3 4 3 6% 10% 57%	2280
May	1st to 10th	Day	5% 3 1 3 2 2 2 2 4 6% 70%	3941
	11th to 20th	Day	3 3 1 3 2 3 2 2 5% 8% 68%	4121
	21st to end	Day	3 7% 2 5% 3 3 3 3 5% 8% 58%	4813
June	1st to 10th	Day	5% 6% 2 4 3 3 4 3 6% 9% 57%	2660
	11th to 20th	Day	1 3 1 3 2 2 3 3 5% 8% 70%	2660
	21st to end	Day	2 4 1 3 2 3 4 3 5% 8% 65%	2660
July	1st to 10th	Day	1 3 1 2 2 2 2 2 3 6% 77%	4395
	11th to 20th	Day	1 2 1 3 2 3 3 3 6% 10% 65%	4110
	21st to end	Day	1 3 1 4 3 3 4 4 5% 12% 60%	4455
August	1st to 10th	Day	1 3 1 4 3 4 5% 5% 7% 13% 51%	4091
	11th to 20th	Day	1 3 1 4 2 2 4 3 5% 10% 66%	3843
	21st to end	Day	1 3 2 4 3 3 3 3 5% 10% 63%	4180
September	1st to 10th	Day	2 4 1 3 3 3 2 3 3 8% 69%	2280
	11th to 20th	Day	3 6% 2 5% 3 3 3 3 6% 10% 55%	2280
	21st to end	Day	3 2 1 3 2 2 3 2 3 8% 71%	2007
October	1st to 10th	Day	3 4 1 4 3 3 2 3 4 7% 67%	1900
	11th to 20th	Day	5% 4 2 4 4 4 4 3 5% 8% 58%	1900
	21st to end	Day	6% 5% 2 5% 3 3 2 3 4 7% 58%	2025
November	1st to 10th	Day	6% 5% 1 5% 3 4 4 3 5% 9% 53%	1842
	11th to 20th	Day	7% 8% 2 7% 5% 5% 5% 4 5% 7% 47%	1710
	21st to end	Day	9% 5% 3 5% 4 3 3 3 5% 8% 52%	1550
December	1st to 10th	Day	10% 6% 4 8% 4 3 5% 4 5% 8% 43%	1520
	11th to 20th	Day	19% 10% 4 8% 6% 4 4 3 6% 6% 30%	1928
	21st to end	Day	15% 10% 3 8% 5% 4 5% 4 6% 8% 32%	2794
January	1st to 10th	Night	13% 8% 3 6% 4 4 4 4 6% 8% 39%	4208
	11th to 20th	Night	16% 10% 4 10% 6% 6% 5% 4 5% 5% 30%	4110
	21st to end	Night	19% 12% 4 9% 5% 5% 4 3 4 6% 29%	4521
February	1st to 10th	Night	12% 7% 2 5% 4 4 4 3 5% 7% 46%	2470
	11th to 20th	Night	10% 7% 2 6% 5% 5% 4 4 5% 8% 43%	2323
	21st to end	Night	10% 6% 2 5% 4 4 4 3 6% 7% 49%	1884
March	1st to 10th	Night	11% 9% 4 9% 6% 5% 5% 4 7% 7% 33%	3464
	11th to 20th	Night	9% 8% 3 5% 3 4 3 2 5% 6% 51%	3490
	21st to end	Night	11% 10% 4 10% 6% 5% 5% 4 5% 6% 35%	3781
April	1st to 10th	Night	7% 7% 3 6% 5% 3 4 3 7% 8% 46%	1900
	11th to 20th	Night	8% 6% 2 4 4 3 3 3 5% 8% 53%	1879
	21st to end	Night	3 4 1 4 3 3 4 3 5% 9% 58%	1900
May	1st to 10th	Night	4 4 1 4 3 3 2 2 4 6% 67%	3030
	11th to 20th	Night	2 3 2 4 4 4 4 4 6% 9% 57%	2775
	21st to end	Night	3 4 1 5% 3 4 3 3 5% 5% 63%	2762
June	1st to 10th	Night	3 5% 2 6% 4 3 4 3 7% 10% 53%	1520
	11th to 20th	Night	1 2 1 3 3 4 3 3 6% 9% 66%	1520
	21st to end	Night	1 4 2 4 3 3 3 3 5% 8% 63%	1520
July	1st to 10th	Night	1 3 1 3 2 3 3 2 3 7% 73%	2561
	11th to 20th	Night	1 3 2 4 3 4 6% 6% 8% 10% 54%	2860
	21st to end	Night	6% 3 7% 7% 7% 7% 5% 6% 10% 40%	3109
August	1st to 10th	Night	2 4 3 7% 5% 7% 10% 6% 9% 13% 34%	2848
	11th to 20th	Night	4 1 6% 4 4 6% 4 5% 9% 58%	2999
	21st to end	Night	1 3 2 6% 4 4 5% 4 7% 10% 54%	3498
September	1st to 10th	Night	1 2 1 5% 3 3 4 4 6% 10% 62%	1900
	11th to 20th	Night	3 5% 2 5% 4 4 4 3 6% 10% 52%	1900
	21st to end	Night	2 2 1 4 3 3 3 2 3 8% 68%	2166
October	1st to 10th	Night	4 5% 1 4 3 3 3 3 4 7% 62%	2280
	11th to 20th	Night	5% 4 1 5% 4 4 4 4 4 7% 57%	2280
	21st to end	Night	6% 4 2 5% 3 4 4 3 4 7% 58%	2436
November	1st to 10th	Night	7% 5% 2 5% 4 4 3 3 5% 8% 53%	2234
	11th to 20th	Night	8% 7% 3 6% 4 3 4 4 6% 8% 47%	2470
	21st to end	Night	8% 5% 1 4 4 4 4 4 5% 8% 54%	2626
December	1st to 10th	Night	11% 7% 4 7% 4 3 4 4 5% 8% 44%	2660
	11th to 20th	Night	23% 10% 3 7% 5% 3 4 3 6% 7% 28%	3276
	21st to end	Night	15% 8% 2 9% 5% 5% 6% 5% 7% 7% 32%	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	27% 39% 9% 20% 4 2	2740
	11th to 20th	Day	42% 41% 6% 7% 2 2	2860
	21st to end	Day	38% 42% 5% 10% 4 1	3146
February	1st to 10th	Day	18% 65% 4 6% 5% 1	1710
	11th to 20th	Day	16% 65% 3 6% 7% 2	1750
	21st to end	Day	11% 66% 4 11% 6% 3	1570
March	1st to 10th	Day	35% 47% 5% 6% 7% 1	3407
	11th to 20th	Day	13% 42% 7% 20% 8% 9%	3490
	21st to end	Day	26% 48% 9% 13% 3 1	3872
April	1st to 10th	Day	14% 66% 4 8% 7% 1	2280
	11th to 20th	Day	10% 66% 6% 10% 7% 1	2252
	21st to end	Day	8% 66% 7% 9% 9% 1	2280
May	1st to 10th	Day	9% 40% 8% 19% 15% 8%	3941
	11th to 20th	Day	8% 40% 7% 12% 26% 7%	4124
	21st to end	Day	14% 44% 6% 16% 19% 2	4814
June	1st to 10th	Day	8% 65% 4 10% 8% 5%	2660
	11th to 20th	Day	4 65% 3 10% 13% 4	2660
	21st to end	Day	7% 63% 3 4 11% 12%	2660
July	1st to 10th	Day	6% 35% 3 4 5% 48%	4326
	11th to 20th	Day	9% 36% 3 4 9% 39%	4041
	21st to end	Day	13% 34% 2 7% 21% 24%	4315
August	1st to 10th	Day	18% 37% 4 8% 17% 17%	4076
	11th to 20th	Day	10% 36% 3 9% 30% 12%	3882
	21st to end	Day	12% 36% 4 11% 22% 15%	4179
September	1st to 10th	Day	9% 59% 3 6% 13% 11%	2280
	11th to 20th	Day	10% 61% 3 9% 11% 6%	2280
	21st to end	Day	7% 60% 3 9% 15% 6%	2007
October	1st to 10th	Day	5% 59% 2 7% 12% 15%	1900
	11th to 20th	Day	11% 61% 4 6% 10% 9%	1900
	21st to end	Day	11% 61% 3 7% 8% 9%	2025
November	1st to 10th	Day	8% 59% 4 13% 12% 5%	1842
	11th to 20th	Day	15% 61% 4 11% 7% 2	1710
	21st to end	Day	14% 61% 5% 7% 11% 3	1550
December	1st to 10th	Day	19% 62% 4 7% 4 4	1520
	11th to 20th	Day	31% 50% 3 5% 8% 3	1928
	21st to end	Day	31% 40% 6% 13% 9% 1	2794
January	1st to 10th	Night	25% 41% 13% 15% 4 2	4208
	11th to 20th	Night	41% 42% 8% 5% 3 2	4110
	21st to end	Night	41% 42% 4 8% 4 2	4521
February	1st to 10th	Night	17% 65% 4 6% 5% 3	2470
	11th to 20th	Night	15% 65% 4 6% 7% 3	2325
	21st to end	Night	10% 65% 5% 13% 5% 2	1884
March	1st to 10th	Night	33% 46% 7% 6% 7% 1	3464
	11th to 20th	Night	18% 41% 6% 13% 13% 8%	3490
	21st to end	Night	33% 45% 8% 10% 4 1	3781
April	1st to 10th	Night	13% 66% 5% 8% 7% 2	1900
	11th to 20th	Night	11% 67% 4 11% 6% 2	1879
	21st to end	Night	8% 68% 6% 10% 7% 2	1900
May	1st to 10th	Night	13% 41% 7% 15% 14% 9%	3030
	11th to 20th	Night	15% 43% 10% 6% 17% 10%	2776
	21st to end	Night	14% 43% 5% 18% 19% 1	2762
June	1st to 10th	Night	10% 66% 3 8% 8% 5%	1520
	11th to 20th	Night	6% 66% 5% 10% 11% 3	1520
	21st to end	Night	8% 63% 2 4 10% 13%	1520
July	1st to 10th	Night	8% 35% 3 3 7% 45%	2521
	11th to 20th	Night	14% 35% 4 4 11% 33%	2806
	21st to end	Night	22% 35% 2 5% 15% 20%	3011
August	1st to 10th	Night	28% 37% 3 11% 15% 6%	2839
	11th to 20th	Night	14% 37% 3 5% 32% 9%	3026
	21st to end	Night	15% 38% 5% 10% 20% 12%	3498
September	1st to 10th	Night	9% 61% 4 5% 12% 9%	1900
	11th to 20th	Night	10% 60% 3 12% 10% 5%	1900
	21st to end	Night	8% 60% 4 9% 11% 8%	2166
October	1st to 10th	Night	6% 60% 2 5% 12% 16%	2280
	11th to 20th	Night	11% 60% 4 7% 8% 9%	2280
	21st to end	Night	11% 61% 4 8% 7% 9%	2436
November	1st to 10th	Night	10% 59% 4 12% 9% 6%	2234
	11th to 20th	Night	15% 61% 5% 11% 6% 3	2470
	21st to end	Night	14% 61% 5% 8% 9% 4	2626
December	1st to 10th	Night	19% 62% 5% 7% 4 4	2660
	11th to 20th	Night	31% 50% 4 5% 8% 3	3276
	21st to end	Night	28% 42% 9% 12% 7% 2	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	27% 10% 26% 21% 15% 1	303
	11th to 20th	Day	16% 11% 24% 31% 14% 3	370
	21st to end	Day	14% 11% 26% 29% 17% 3	407
February	1st to 10th	Day	15% 15% 25% 25% 18% 2	370
	11th to 20th	Day	16% 17% 31% 25% 11% 1	401
	21st to end	Day	13% 16% 30% 27% 13% 2	382
March	1st to 10th	Day	19% 19% 31% 20% 9% 2	460
	11th to 20th	Day	17% 16% 28% 22% 14% 4	460
	21st to end	Day	13% 20% 33% 25% 7% 2	506
April	1st to 10th	Day	14% 18% 32% 24% 10% 2	460
	11th to 20th	Day	12% 19% 33% 24% 8% 3	452
	21st to end	Day	6% 20% 32% 28% 10% 5%	460
May	1st to 10th	Day	8% 17% 34% 28% 11% 2	463
	11th to 20th	Day	8% 15% 35% 29% 11% 2	488
	21st to end	Day	8% 15% 32% 29% 12% 3	529
June	1st to 10th	Day	6% 12% 34% 32% 13% 3	490
	11th to 20th	Day	3 10% 31% 38% 13% 4	490
	21st to end	Day	3 7% 29% 38% 19% 5%	490
July	1st to 10th	Day	5% 4 23% 46% 17% 5%	486
	11th to 20th	Day	1 1 20% 50% 24% 4	484
	21st to end	Day	3 1 23% 46% 20% 7%	531
August	1st to 10th	Day	5% 4 24% 43% 18% 6%	487
	11th to 20th	Day	3 2 24% 45% 21% 5%	477
	21st to end	Day	3 3 20% 44% 23% 6%	539
September	1st to 10th	Day	3 4 16% 44% 24% 9%	490
	11th to 20th	Day	5% 10% 25% 36% 18% 7%	490
	21st to end	Day	3 8% 21% 32% 27% 9%	490
October	1st to 10th	Day	3 7% 26% 30% 22% 11%	490
	11th to 20th	Day	6% 9% 21% 29% 26% 9%	490
	21st to end	Day	9% 8% 23% 29% 23% 8%	514
November	1st to 10th	Day	8% 11% 22% 26% 26% 7%	438
	11th to 20th	Day	7% 9% 17% 33% 26% 8%	270
	21st to end	Day	9% 7% 24% 31% 27% 2	194
December	1st to 10th	Day	13% 17% 20% 23% 21% 6%	180
	11th to 20th	Day	14% 14% 21% 29% 18% 4	180
	21st to end	Day	19% 10% 30% 26% 14% 1	198
January	1st to 10th	Night	19% 6% 16% 31% 22% 5%	760
	11th to 20th	Night	15% 7% 15% 32% 25% 6%	760
	21st to end	Night	18% 6% 15% 34% 24% 4	836
February	1st to 10th	Night	17% 7% 16% 33% 23% 4	760
	11th to 20th	Night	13% 10% 19% 33% 21% 5%	744
	21st to end	Night	13% 11% 21% 33% 19% 3	628
March	1st to 10th	Night	17% 9% 23% 27% 21% 3	760
	11th to 20th	Night	17% 9% 17% 31% 23% 4	760
	21st to end	Night	15% 10% 22% 32% 17% 2	836
April	1st to 10th	Night	12% 9% 24% 34% 19% 3	760
	11th to 20th	Night	12% 12% 22% 30% 19% 5%	751
	21st to end	Night	7% 8% 25% 33% 22% 5%	760
May	1st to 10th	Night	6% 6% 23% 38% 23% 5%	675
	11th to 20th	Night	8% 4 17% 36% 30% 4	529
	21st to end	Night	8% 9% 22% 37% 21% 4	411
June	1st to 10th	Night	6% 5% 20% 41% 26% 3	380
	11th to 20th	Night	1 3 18% 47% 28% 2	380
	21st to end	Night	4 3 14% 37% 38% 4	380
July	1st to 10th	Night	4 2 11% 33% 45% 5%	378
	11th to 20th	Night	3 1 7% 38% 41% 10%	552
	21st to end	Night	5% 2 9% 34% 41% 9%	624
August	1st to 10th	Night	8% 4 12% 33% 36% 8%	567
	11th to 20th	Night	5% 2 11% 40% 35% 7%	673
	21st to end	Night	3 1 12% 36% 38% 9%	836
September	1st to 10th	Night	3 1 11% 33% 41% 11%	760
	11th to 20th	Night	8% 3 14% 35% 33% 7%	760
	21st to end	Night	3 3 13% 30% 40% 11%	760
October	1st to 10th	Night	7% 2 13% 33% 35% 10%	760
	11th to 20th	Night	7% 4 12% 29% 38% 10%	760
	21st to end	Night	8% 4 12% 30% 34% 12%	812
November	1st to 10th	Night	9% 4 16% 28% 34% 9%	740
	11th to 20th	Night	11% 3 12% 35% 32% 7%	760
	21st to end	Night	11% 4 13% 38% 28% 6%	760
December	1st to 10th	Night	16% 7% 15% 33% 24% 5%	760
	11th to 20th	Night	23% 6% 16% 31% 20% 3	760
	21st to end	Night	20% 9% 21% 29% 19% 2	836

Page generated at 2024-10-22 17:21:04. Version ac07

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