Monrovia
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Monrovia clear sky chart from 2005-11-05 to 2023-05-20.

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	4 2 1 2 2 1 3 3 4 5% 67%	70063
	Night	5% 2 1 2 2 1 3 3 4 5% 66%	69178
Transparency	Day	17% 4 7% 16% 26% 26%	69762
	Night	19% 4 8% 16% 24% 26%	68982
Seeing	Day	6% 9% 28% 36% 14% 4	13500
	Night	6% 2 15% 37% 31% 6%	23352

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	10% 3 3 4 2 3 4 4 7% 56%	1423
	11th to 20th	Day	8% 3 1 3 3 1 3 3 4 6% 60%	1621
	21st to end	Day	10% 4 1 4 3 1 2 2 3 4 59%	1927
February	1st to 10th	Day	8% 2 2 3 2 1 2 3 3 5% 64%	1771
	11th to 20th	Day	9% 3 1 3 2 1 2 3 3 6% 61%	1721
	21st to end	Day	8% 5% 1 4 3 2 3 3 3 7% 54%	1471
March	1st to 10th	Day	8% 5% 2 5% 4 2 5% 5% 4 7% 48%	1800
	11th to 20th	Day	7% 3 1 3 3 2 3 3 5% 8% 56%	1813
	21st to end	Day	6% 3 1 3 3 2 4 5% 6% 6% 56%	2352
April	1st to 10th	Day	6% 3 1 3 3 2 3 3 5% 8% 59%	2129
	11th to 20th	Day	3 3 1 2 3 2 3 4 4 6% 63%	2097
	21st to end	Day	4 3 1 2 2 1 3 2 3 5% 67%	2158
May	1st to 10th	Day	5% 3 1 4 3 2 3 4 4 6% 60%	2217
	11th to 20th	Day	3 2 1 4 3 1 4 3 5% 6% 62%	2234
	21st to end	Day	5% 3 1 3 3 2 4 3 4 5% 63%	2332
June	1st to 10th	Day	3 2 1 2 1 4 3 5% 7% 66%	2210
	11th to 20th	Day	1 1 2 1 1 3 2 3 4 77%	2371
	21st to end	Day	1 1 1 2 1 3 2 3 3 78%	2363
July	1st to 10th	Day	1 1 1 2 1 2 3 4 5% 74%	2379
	11th to 20th	Day	1 1 1 1 1 2 2 4 4 77%	2300
	21st to end	Day	1 1 1 1 1 1 3 3 5% 7% 71%	2234
August	1st to 10th	Day	1 1 1 2 2 2 4 82%	1997
	11th to 20th	Day	1 1 2 1 2 5% 83%	1947
	21st to end	Day	2 1 2 3 4 4 78%	2230
September	1st to 10th	Day	1 1 1 1 1 2 3 4 79%	2028
	11th to 20th	Day	1 1 1 1 1 1 2 3 84%	1904
	21st to end	Day	1 1 1 1 1 1 3 4 4 78%	1858
October	1st to 10th	Day	4 1 1 2 2 2 3 2 4 5% 68%	1861
	11th to 20th	Day	3 2 1 1 1 1 3 3 4 75%	1745
	21st to end	Day	4 1 2 2 1 2 2 3 4 73%	1813
November	1st to 10th	Day	3 2 1 3 2 1 2 3 3 5% 70%	1584
	11th to 20th	Day	4 2 1 1 2 1 2 3 4 5% 70%	1612
	21st to end	Day	5% 2 1 2 3 2 3 3 4 6% 63%	1607
December	1st to 10th	Day	7% 3 1 4 3 2 2 4 5% 7% 57%	1620
	11th to 20th	Day	11% 3 1 3 2 1 3 3 5% 9% 53%	1620
	21st to end	Day	10% 4 1 3 2 2 3 3 4 6% 57%	1714
January	1st to 10th	Night	10% 3 1 3 2 2 3 3 4 6% 57%	2491
	11th to 20th	Night	6% 3 1 3 2 1 2 3 4 6% 62%	2313
	21st to end	Night	10% 3 2 3 2 1 2 2 3 4 61%	2347
February	1st to 10th	Night	7% 3 2 3 2 1 2 2 3 4 65%	2145
	11th to 20th	Night	7% 3 2 3 3 1 3 2 3 5% 62%	2044
	21st to end	Night	10% 5% 1 3 3 2 3 3 3 5% 56%	1774
March	1st to 10th	Night	8% 5% 2 3 3 2 4 4 5% 8% 52%	2159
	11th to 20th	Night	8% 3 1 3 3 1 4 4 5% 6% 56%	2090
	21st to end	Night	6% 3 1 3 3 2 4 3 4 6% 58%	1980
April	1st to 10th	Night	6% 3 2 3 3 2 5% 4 4 6% 57%	1781
	11th to 20th	Night	4 4 1 2 1 1 4 3 4 6% 64%	1750
	21st to end	Night	4 3 1 2 2 1 3 5% 4 6% 63%	1785
May	1st to 10th	Night	4 4 2 4 3 2 5% 3 5% 6% 57%	1720
	11th to 20th	Night	2 5% 2 3 2 1 4 5% 5% 6% 60%	1568
	21st to end	Night	2 3 1 2 2 2 5% 3 3 5% 68%	1597
June	1st to 10th	Night	1 1 1 2 2 5% 4 6% 7% 65%	1530
	11th to 20th	Night	1 1 1 5% 3 3 4 76%	1363
	21st to end	Night	1 1 1 1 3 3 4 5% 75%	1344
July	1st to 10th	Night	2 1 1 3 3 4 5% 76%	1360
	11th to 20th	Night	1 1 2 1 3 3 6% 7% 71%	1376
	21st to end	Night	2 1 1 1 1 3 5% 5% 7% 68%	1778
August	1st to 10th	Night	1 1 2 1 1 2 2 4 5% 76%	1680
	11th to 20th	Night	1 1 1 1 2 4 4 81%	1655
	21st to end	Night	1 1 1 4 6% 4 77%	1856
September	1st to 10th	Night	1 1 2 2 2 3 4 4 76%	1700
	11th to 20th	Night	1 1 1 1 1 3 3 84%	1823
	21st to end	Night	1 1 1 2 2 4 4 80%	1851
October	1st to 10th	Night	2 2 1 2 4 2 3 2 5% 4 68%	1868
	11th to 20th	Night	3 1 2 2 1 3 2 3 4 73%	1951
	21st to end	Night	2 2 2 2 1 3 2 4 4 72%	2287
November	1st to 10th	Night	4 2 2 2 1 2 2 4 5% 69%	2265
	11th to 20th	Night	3 2 1 2 2 1 2 2 4 5% 70%	2328
	21st to end	Night	5% 2 1 2 3 2 3 3 5% 6% 61%	2327
December	1st to 10th	Night	7% 3 1 4 4 2 3 3 5% 7% 57%	2330
	11th to 20th	Night	11% 4 1 4 3 2 3 3 3 6% 55%	2340
	21st to end	Night	9% 3 1 2 2 1 3 3 4 6% 61%	2622

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% 5% 7% 16% 22% 20%	1424
	11th to 20th	Day	25% 4 6% 12% 25% 26%	1621
	21st to end	Day	28% 3 6% 16% 17% 27%	1926
February	1st to 10th	Day	24% 4 5% 13% 24% 29%	1772
	11th to 20th	Day	23% 3 5% 15% 25% 25%	1722
	21st to end	Day	27% 4 9% 16% 23% 18%	1471
March	1st to 10th	Day	30% 5% 9% 16% 16% 20%	1800
	11th to 20th	Day	23% 5% 11% 18% 23% 16%	1830
	21st to end	Day	23% 7% 11% 21% 22% 13%	2348
April	1st to 10th	Day	22% 6% 13% 25% 20% 11%	2127
	11th to 20th	Day	20% 5% 8% 20% 28% 17%	2098
	21st to end	Day	17% 4 10% 23% 30% 13%	2158
May	1st to 10th	Day	21% 7% 11% 19% 27% 13%	2216
	11th to 20th	Day	20% 5% 8% 21% 30% 12%	2242
	21st to end	Day	20% 4 10% 26% 28% 9%	2332
June	1st to 10th	Day	17% 5% 11% 18% 30% 16%	2207
	11th to 20th	Day	13% 3 4 12% 32% 34%	2371
	21st to end	Day	12% 2 4 10% 27% 42%	2365
July	1st to 10th	Day	10% 3 5% 13% 22% 44%	2287
	11th to 20th	Day	11% 4 6% 18% 28% 31%	2211
	21st to end	Day	10% 5% 10% 21% 31% 19%	2098
August	1st to 10th	Day	8% 2 3 13% 31% 39%	1984
	11th to 20th	Day	6% 2 3 14% 36% 36%	1947
	21st to end	Day	8% 3 6% 12% 36% 33%	2231
September	1st to 10th	Day	8% 2 5% 11% 31% 40%	2029
	11th to 20th	Day	7% 2 3 12% 33% 39%	1906
	21st to end	Day	8% 2 5% 16% 32% 34%	1858
October	1st to 10th	Day	17% 4 5% 16% 24% 32%	1861
	11th to 20th	Day	12% 4 5% 14% 27% 35%	1747
	21st to end	Day	14% 4 4 10% 21% 44%	1812
November	1st to 10th	Day	15% 3 6% 16% 26% 30%	1584
	11th to 20th	Day	16% 3 6% 15% 19% 38%	1612
	21st to end	Day	21% 4 8% 11% 22% 31%	1611
December	1st to 10th	Day	25% 6% 7% 16% 17% 26%	1620
	11th to 20th	Day	27% 5% 10% 17% 21% 17%	1620
	21st to end	Day	27% 4 7% 17% 21% 21%	1714
January	1st to 10th	Night	27% 4 7% 14% 24% 20%	2492
	11th to 20th	Night	23% 4 7% 14% 21% 28%	2317
	21st to end	Night	26% 4 7% 14% 18% 28%	2350
February	1st to 10th	Night	24% 3 7% 14% 22% 28%	2145
	11th to 20th	Night	22% 4 7% 16% 23% 25%	2054
	21st to end	Night	28% 5% 9% 13% 21% 21%	1774
March	1st to 10th	Night	30% 5% 9% 18% 18% 17%	2160
	11th to 20th	Night	24% 6% 10% 16% 23% 17%	2102
	21st to end	Night	25% 5% 10% 22% 22% 13%	1980
April	1st to 10th	Night	26% 6% 10% 23% 18% 14%	1781
	11th to 20th	Night	18% 7% 9% 22% 24% 17%	1750
	21st to end	Night	20% 6% 11% 22% 29% 10%	1782
May	1st to 10th	Night	23% 9% 9% 20% 23% 13%	1723
	11th to 20th	Night	22% 8% 9% 24% 23% 11%	1569
	21st to end	Night	17% 4 11% 27% 29% 9%	1597
June	1st to 10th	Night	15% 6% 13% 19% 26% 17%	1522
	11th to 20th	Night	11% 4 8% 14% 32% 28%	1362
	21st to end	Night	12% 3 6% 12% 26% 39%	1344
July	1st to 10th	Night	11% 3 4 13% 24% 42%	1312
	11th to 20th	Night	11% 4 9% 19% 25% 29%	1320
	21st to end	Night	13% 5% 10% 23% 27% 19%	1671
August	1st to 10th	Night	10% 3 5% 11% 34% 34%	1670
	11th to 20th	Night	6% 3 6% 15% 35% 32%	1658
	21st to end	Night	8% 4 7% 13% 34% 31%	1860
September	1st to 10th	Night	12% 2 6% 10% 27% 40%	1700
	11th to 20th	Night	7% 3 4 11% 34% 39%	1822
	21st to end	Night	10% 2 5% 15% 32% 33%	1851
October	1st to 10th	Night	19% 4 6% 15% 23% 31%	1869
	11th to 20th	Night	14% 4 5% 15% 26% 33%	1952
	21st to end	Night	16% 4 3 9% 22% 43%	2285
November	1st to 10th	Night	17% 4 6% 15% 24% 31%	2265
	11th to 20th	Night	16% 3 7% 14% 19% 38%	2328
	21st to end	Night	23% 4 8% 16% 16% 31%	2328
December	1st to 10th	Night	26% 5% 8% 13% 17% 28%	2330
	11th to 20th	Night	30% 4 8% 17% 23% 15%	2340
	21st to end	Night	24% 5% 8% 15% 25% 21%	2617

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	16% 5% 28% 35% 12% 2	160
	11th to 20th	Day	11% 12% 30% 31% 12% 1	311
	21st to end	Day	10% 16% 29% 31% 10% 1	454
February	1st to 10th	Day	11% 11% 31% 30% 11% 3	415
	11th to 20th	Day	12% 16% 31% 31% 7%	412
	21st to end	Day	11% 20% 31% 28% 8%	345
March	1st to 10th	Day	13% 19% 35% 20% 10%	420
	11th to 20th	Day	13% 14% 33% 27% 9% 1	418
	21st to end	Day	11% 20% 35% 25% 7%	460
April	1st to 10th	Day	8% 16% 32% 26% 13% 2	414
	11th to 20th	Day	5% 20% 39% 24% 6% 2	403
	21st to end	Day	4 17% 33% 29% 12% 2	420
May	1st to 10th	Day	9% 15% 35% 26% 9% 2	421
	11th to 20th	Day	9% 10% 31% 33% 12% 2	442
	21st to end	Day	9% 11% 36% 28% 11% 1	425
June	1st to 10th	Day	2 8% 29% 40% 14% 4	410
	11th to 20th	Day	4 5% 27% 40% 14% 6%	410
	21st to end	Day	2 2 26% 45% 13% 8%	410
July	1st to 10th	Day	2 18% 49% 22% 6%	402
	11th to 20th	Day	1 12% 48% 24% 12%	399
	21st to end	Day	1 15% 49% 21% 11%	440
August	1st to 10th	Day	2 1 19% 50% 18% 8%	397
	11th to 20th	Day	1 1 14% 49% 24% 9%	383
	21st to end	Day	20% 49% 23% 5%	450
September	1st to 10th	Day	2 1 28% 44% 17% 5%	409
	11th to 20th	Day	2 2 30% 43% 16% 4	409
	21st to end	Day	2 6% 28% 39% 16% 6%	408
October	1st to 10th	Day	4 10% 30% 35% 16% 1	409
	11th to 20th	Day	3 9% 30% 39% 15% 1	408
	21st to end	Day	4 8% 26% 35% 19% 5%	402
November	1st to 10th	Day	5% 6% 20% 42% 21% 3	240
	11th to 20th	Day	5% 4 22% 40% 22% 4	240
	21st to end	Day	5% 7% 30% 36% 15% 5%	240
December	1st to 10th	Day	11% 4 25% 42% 13% 2	240
	11th to 20th	Day	5% 5% 29% 38% 17% 3	240
	21st to end	Day	17% 10% 27% 32% 10% 1	234
January	1st to 10th	Night	11% 2 21% 40% 20% 3	713
	11th to 20th	Night	9% 2 21% 42% 21% 2	717
	21st to end	Night	12% 4 23% 40% 17% 2	782
February	1st to 10th	Night	9% 3 20% 40% 21% 3	710
	11th to 20th	Night	8% 5% 23% 37% 21% 3	687
	21st to end	Night	12% 5% 23% 38% 18% 2	591
March	1st to 10th	Night	7% 6% 24% 39% 19% 1	719
	11th to 20th	Night	9% 5% 18% 36% 24% 5%	712
	21st to end	Night	7% 4 23% 41% 20% 3	792
April	1st to 10th	Night	8% 4 16% 45% 23% 2	713
	11th to 20th	Night	5% 6% 19% 39% 25% 2	701
	21st to end	Night	3 5% 20% 38% 25% 5%	712
May	1st to 10th	Night	6% 4 21% 39% 23% 4	646
	11th to 20th	Night	5% 3 16% 39% 31% 3	523
	21st to end	Night	4 2 21% 39% 28% 3	536
June	1st to 10th	Night	2 1 16% 39% 37% 2	506
	11th to 20th	Night	7% 37% 47% 5%	340
	21st to end	Night	2 3 35% 51% 5%	329
July	1st to 10th	Night	2 3 14% 68% 10%	328
	11th to 20th	Night	1 3 19% 62% 12%	366
	21st to end	Night	2 1 2 28% 51% 13%	686
August	1st to 10th	Night	2 4 29% 48% 13%	662
	11th to 20th	Night	3 22% 54% 18%	661
	21st to end	Night	1 5% 23% 52% 16%	744
September	1st to 10th	Night	2 8% 26% 47% 15%	680
	11th to 20th	Night	1 6% 34% 46% 10%	676
	21st to end	Night	1 9% 35% 43% 9%	667
October	1st to 10th	Night	4 1 11% 41% 38% 2	679
	11th to 20th	Night	4 2 13% 39% 33% 7%	674
	21st to end	Night	4 1 15% 36% 30% 11%	747
November	1st to 10th	Night	5% 1 14% 41% 29% 7%	697
	11th to 20th	Night	5% 2 13% 43% 28% 6%	717
	21st to end	Night	6% 1 19% 42% 23% 4	717
December	1st to 10th	Night	8% 2 20% 42% 22% 3	715
	11th to 20th	Night	12% 5% 20% 40% 19% 2	720
	21st to end	Night	10% 5% 25% 38% 16% 3	787

Page generated at 2023-05-22 03:21:05. Version ac06

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