BarC Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the BarC Observatory clear sky chart from 2005-11-06 to 2024-03-08.

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	11% 5% 2 5% 5% 3 5% 5% 6% 9% 42%	79125
	Night	11% 5% 2 5% 5% 3 4 5% 6% 8% 46%	80413
Transparency	Day	24% 37% 9% 13% 12% 4	78871
	Night	24% 36% 9% 13% 13% 6%	80186
Seeing	Day	12% 5% 17% 26% 26% 14%	13293
	Night	12% 4 18% 29% 27% 9%	23339

By Month

Percentage Cloud Cover SE (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% 5% 2 5% 4 3 5% 4 6% 8% 44%	3156
	11th to 20th	Day	12% 5% 2 5% 3 2 4 3 4 6% 55%	3160
	21st to end	Day	24% 8% 3 5% 3 2 4 4 4 6% 37%	3476
February	1st to 10th	Day	17% 7% 3 6% 5% 3 4 3 4 6% 42%	1900
	11th to 20th	Day	16% 6% 2 5% 4 3 4 4 4 7% 44%	1848
	21st to end	Day	16% 8% 3 6% 7% 3 4 4 6% 7% 37%	1570
March	1st to 10th	Day	13% 6% 2 6% 5% 3 4 4 5% 7% 44%	1870
	11th to 20th	Day	13% 6% 3 5% 5% 3 5% 5% 5% 7% 42%	1957
	21st to end	Day	11% 5% 2 4 4 3 5% 4 5% 8% 47%	2340
April	1st to 10th	Day	9% 5% 2 4 5% 4 5% 5% 6% 10% 45%	2160
	11th to 20th	Day	13% 6% 3 5% 5% 3 4 5% 6% 8% 42%	2134
	21st to end	Day	8% 5% 2 4 4 3 5% 5% 6% 7% 52%	2160
May	1st to 10th	Day	9% 4 2 3 5% 3 4 5% 6% 8% 51%	2160
	11th to 20th	Day	9% 5% 2 5% 5% 4 5% 5% 7% 9% 46%	2124
	21st to end	Day	7% 4 2 4 6% 3 5% 6% 8% 13% 42%	2435
June	1st to 10th	Day	8% 5% 2 6% 5% 4 7% 8% 8% 11% 35%	2340
	11th to 20th	Day	4 3 2 4 5% 4 6% 7% 9% 13% 41%	2340
	21st to end	Day	8% 6% 2 5% 6% 5% 7% 8% 10% 14% 30%	2281
July	1st to 10th	Day	8% 5% 3 7% 7% 5% 9% 9% 8% 13% 26%	2160
	11th to 20th	Day	8% 6% 3 7% 6% 4 8% 9% 10% 12% 28%	2160
	21st to end	Day	5% 5% 3 6% 7% 4 7% 8% 9% 13% 31%	2350
August	1st to 10th	Day	5% 4 2 6% 6% 5% 7% 9% 9% 14% 33%	2148
	11th to 20th	Day	9% 6% 3 5% 6% 5% 8% 8% 10% 11% 28%	2125
	21st to end	Day	9% 5% 2 6% 6% 4 7% 7% 9% 12% 33%	2376
September	1st to 10th	Day	8% 4 2 6% 6% 4 6% 8% 8% 10% 38%	2160
	11th to 20th	Day	9% 5% 2 5% 5% 4 5% 6% 7% 9% 42%	2048
	21st to end	Day	8% 3 2 4 5% 4 5% 5% 7% 8% 49%	1980
October	1st to 10th	Day	5% 2 1 3 4 2 4 5% 6% 8% 58%	1980
	11th to 20th	Day	6% 3 2 4 4 3 5% 4 5% 8% 57%	1836
	21st to end	Day	10% 4 2 4 3 3 4 4 4 6% 57%	1980
November	1st to 10th	Day	9% 4 1 3 4 2 3 4 5% 8% 58%	1835
	11th to 20th	Day	12% 3 2 3 4 3 3 3 4 5% 58%	1710
	21st to end	Day	12% 5% 2 5% 5% 3 4 4 5% 7% 47%	1710
December	1st to 10th	Day	13% 7% 3 5% 4 2 5% 3 5% 6% 45%	1710
	11th to 20th	Day	14% 7% 3 7% 5% 3 5% 4 4 7% 41%	2153
	21st to end	Day	17% 7% 3 6% 6% 3 4 4 5% 6% 39%	3293
January	1st to 10th	Night	15% 6% 2 4 5% 2 4 4 5% 7% 47%	3789
	11th to 20th	Night	14% 6% 3 4 4 2 3 3 4 6% 52%	3800
	21st to end	Night	24% 8% 2 3 3 2 3 4 4 6% 41%	4180
February	1st to 10th	Night	15% 7% 3 5% 6% 3 4 4 5% 6% 43%	2280
	11th to 20th	Night	17% 7% 3 5% 4 2 3 3 5% 7% 43%	2218
	21st to end	Night	16% 8% 3 6% 5% 4 5% 4 5% 7% 39%	1884
March	1st to 10th	Night	13% 7% 3 5% 5% 2 4 4 5% 7% 43%	2251
	11th to 20th	Night	13% 6% 3 4 5% 3 5% 6% 5% 9% 42%	2016
	21st to end	Night	12% 4 2 4 4 2 3 5% 5% 7% 52%	2015
April	1st to 10th	Night	12% 7% 2 5% 4 2 4 5% 5% 7% 47%	1800
	11th to 20th	Night	12% 5% 2 4 5% 3 5% 5% 5% 7% 46%	1775
	21st to end	Night	7% 3 2 4 4 2 4 4 6% 9% 57%	1800
May	1st to 10th	Night	10% 4 2 3 4 2 4 4 5% 9% 54%	1800
	11th to 20th	Night	4 4 2 5% 6% 3 6% 7% 7% 10% 46%	1769
	21st to end	Night	5% 4 2 5% 5% 3 5% 6% 8% 12% 47%	1831
June	1st to 10th	Night	8% 5% 2 5% 5% 3 6% 6% 8% 11% 41%	1620
	11th to 20th	Night	3 3 2 4 4 3 6% 7% 9% 14% 44%	1620
	21st to end	Night	7% 4 3 6% 6% 4 5% 6% 9% 12% 38%	1671
July	1st to 10th	Night	6% 6% 3 4 5% 4 6% 7% 8% 11% 39%	1800
	11th to 20th	Night	3 3 2 7% 6% 4 7% 6% 10% 14% 38%	1800
	21st to end	Night	6% 4 3 5% 5% 3 6% 7% 9% 14% 38%	1968
August	1st to 10th	Night	3 4 2 4 3 3 4 6% 8% 14% 50%	1790
	11th to 20th	Night	4 4 3 5% 6% 5% 6% 7% 11% 17% 32%	1776
	21st to end	Night	8% 4 2 6% 6% 3 6% 6% 8% 11% 38%	1980
September	1st to 10th	Night	7% 4 2 6% 6% 4 7% 7% 7% 9% 40%	1800
	11th to 20th	Night	8% 6% 3 5% 4 2 5% 6% 7% 10% 44%	1892
	21st to end	Night	8% 4 2 4 5% 3 4 5% 6% 9% 51%	1980
October	1st to 10th	Night	6% 3 2 5% 4 2 4 4 4 7% 59%	1980
	11th to 20th	Night	7% 3 2 3 3 2 4 3 5% 7% 61%	2118
	21st to end	Night	11% 4 2 4 4 3 3 4 5% 6% 55%	2376
November	1st to 10th	Night	9% 4 2 5% 4 3 4 3 4 6% 57%	2239
	11th to 20th	Night	10% 4 2 4 4 3 4 4 5% 6% 55%	2470
	21st to end	Night	10% 5% 2 6% 5% 4 4 4 5% 7% 48%	2470
December	1st to 10th	Night	14% 6% 3 5% 5% 2 4 3 5% 7% 45%	2470
	11th to 20th	Night	14% 7% 3 5% 5% 3 4 5% 5% 6% 44%	3042
	21st to end	Night	17% 8% 3 6% 6% 3 4 4 5% 5% 38%	4343

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% 27% 5% 11% 19% 10%	3157
	11th to 20th	Day	21% 26% 5% 15% 25% 8%	3160
	21st to end	Day	40% 24% 3 14% 14% 6%	3476
February	1st to 10th	Day	28% 34% 7% 10% 14% 6%	1900
	11th to 20th	Day	28% 37% 5% 9% 12% 9%	1858
	21st to end	Day	31% 36% 5% 12% 11% 6%	1570
March	1st to 10th	Day	24% 35% 6% 17% 14% 4	1870
	11th to 20th	Day	24% 39% 8% 10% 13% 6%	1957
	21st to end	Day	22% 38% 10% 15% 13% 3	2340
April	1st to 10th	Day	21% 38% 12% 14% 9% 7%	2160
	11th to 20th	Day	24% 36% 7% 14% 13% 5%	2134
	21st to end	Day	18% 37% 9% 16% 16% 4	2160
May	1st to 10th	Day	22% 38% 9% 18% 11% 2	2160
	11th to 20th	Day	20% 41% 13% 15% 11% 1	2128
	21st to end	Day	18% 39% 12% 20% 9% 1	2435
June	1st to 10th	Day	24% 39% 14% 18% 5% 1	2340
	11th to 20th	Day	16% 40% 14% 21% 8% 1	2340
	21st to end	Day	24% 41% 14% 13% 8%	2281
July	1st to 10th	Day	33% 44% 14% 8% 2	2100
	11th to 20th	Day	26% 45% 15% 11% 2 1	2097
	21st to end	Day	25% 44% 19% 9% 2	2219
August	1st to 10th	Day	21% 42% 18% 14% 5%	2136
	11th to 20th	Day	29% 41% 15% 11% 4	2125
	21st to end	Day	27% 42% 12% 13% 5% 1	2376
September	1st to 10th	Day	24% 40% 12% 13% 9% 2	2160
	11th to 20th	Day	26% 40% 10% 11% 8% 4	2048
	21st to end	Day	19% 39% 9% 13% 13% 7%	1980
October	1st to 10th	Day	16% 37% 6% 14% 15% 12%	1980
	11th to 20th	Day	17% 36% 6% 15% 17% 9%	1836
	21st to end	Day	17% 38% 4 13% 18% 9%	1980
November	1st to 10th	Day	11% 41% 5% 13% 20% 9%	1835
	11th to 20th	Day	16% 40% 5% 12% 21% 7%	1710
	21st to end	Day	23% 40% 5% 12% 14% 6%	1710
December	1st to 10th	Day	24% 41% 6% 13% 11% 5%	1710
	11th to 20th	Day	31% 33% 5% 9% 17% 5%	2150
	21st to end	Day	30% 26% 6% 14% 20% 3	3293
January	1st to 10th	Night	28% 26% 5% 11% 21% 9%	3789
	11th to 20th	Night	26% 25% 4 14% 26% 6%	3800
	21st to end	Night	38% 23% 4 12% 16% 7%	4180
February	1st to 10th	Night	28% 35% 4 11% 15% 7%	2280
	11th to 20th	Night	30% 35% 4 8% 12% 10%	2219
	21st to end	Night	29% 35% 7% 13% 8% 8%	1884
March	1st to 10th	Night	25% 35% 6% 12% 16% 7%	2251
	11th to 20th	Night	22% 39% 10% 11% 12% 6%	2016
	21st to end	Night	20% 37% 10% 13% 15% 5%	2015
April	1st to 10th	Night	23% 37% 9% 14% 11% 6%	1800
	11th to 20th	Night	23% 35% 6% 16% 13% 6%	1775
	21st to end	Night	14% 37% 7% 19% 17% 5%	1800
May	1st to 10th	Night	21% 37% 10% 19% 13% 2	1800
	11th to 20th	Night	21% 41% 11% 14% 12% 2	1770
	21st to end	Night	18% 39% 12% 20% 9% 1	1832
June	1st to 10th	Night	22% 40% 14% 18% 6% 1	1620
	11th to 20th	Night	16% 40% 18% 18% 8%	1620
	21st to end	Night	22% 40% 17% 14% 6%	1671
July	1st to 10th	Night	28% 42% 18% 9% 2	1750
	11th to 20th	Night	20% 43% 21% 12% 3 1	1740
	21st to end	Night	23% 43% 20% 11% 2	1858
August	1st to 10th	Night	13% 41% 22% 18% 5%	1778
	11th to 20th	Night	22% 43% 19% 13% 3	1776
	21st to end	Night	26% 41% 14% 12% 4 2	1980
September	1st to 10th	Night	24% 41% 12% 11% 9% 3	1800
	11th to 20th	Night	25% 40% 11% 11% 9% 5%	1892
	21st to end	Night	19% 38% 10% 13% 13% 7%	1980
October	1st to 10th	Night	19% 36% 4 12% 15% 14%	1980
	11th to 20th	Night	18% 37% 6% 12% 18% 10%	2118
	21st to end	Night	18% 38% 4 11% 19% 11%	2376
November	1st to 10th	Night	14% 39% 3 13% 20% 11%	2239
	11th to 20th	Night	18% 40% 4 11% 19% 8%	2470
	21st to end	Night	24% 39% 4 13% 13% 7%	2470
December	1st to 10th	Night	22% 40% 7% 12% 15% 4	2470
	11th to 20th	Night	27% 34% 6% 10% 14% 8%	3044
	21st to end	Night	33% 26% 6% 14% 16% 5%	4343

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	17% 10% 34% 28% 9% 1	270
	11th to 20th	Day	17% 10% 33% 26% 13% 1	270
	21st to end	Day	20% 10% 39% 25% 6%	297
February	1st to 10th	Day	21% 13% 29% 26% 9% 1	270
	11th to 20th	Day	25% 8% 36% 20% 11% 1	270
	21st to end	Day	14% 14% 49% 20% 3	225
March	1st to 10th	Day	15% 11% 35% 33% 7%	261
	11th to 20th	Day	14% 11% 41% 23% 10% 1	240
	21st to end	Day	16% 8% 27% 32% 14% 2	426
April	1st to 10th	Day	13% 7% 31% 34% 14% 2	420
	11th to 20th	Day	18% 11% 28% 26% 15% 3	412
	21st to end	Day	10% 6% 21% 37% 22% 4	420
May	1st to 10th	Day	12% 4 24% 34% 21% 5%	423
	11th to 20th	Day	10% 2 13% 37% 26% 12%	447
	21st to end	Day	8% 2 9% 31% 37% 13%	485
June	1st to 10th	Day	9% 2 8% 30% 32% 19%	450
	11th to 20th	Day	7% 2 8% 26% 38% 19%	450
	21st to end	Day	10% 2 5% 27% 34% 23%	450
July	1st to 10th	Day	9% 2 3 17% 37% 33%	445
	11th to 20th	Day	11% 2 3 12% 37% 35%	444
	21st to end	Day	7% 1 4 18% 41% 28%	488
August	1st to 10th	Day	5% 2 2 14% 42% 35%	446
	11th to 20th	Day	9% 2 3 15% 35% 37%	439
	21st to end	Day	8% 2 6% 17% 38% 28%	495
September	1st to 10th	Day	8% 1 4 19% 42% 26%	450
	11th to 20th	Day	12% 3 7% 21% 38% 19%	450
	21st to end	Day	9% 2 8% 29% 35% 18%	450
October	1st to 10th	Day	6% 2 14% 36% 31% 11%	450
	11th to 20th	Day	7% 2 17% 31% 30% 14%	306
	21st to end	Day	15% 5% 19% 31% 22% 8%	297
November	1st to 10th	Day	13% 2 18% 33% 29% 5%	270
	11th to 20th	Day	17% 5% 24% 32% 17% 5%	270
	21st to end	Day	16% 4 31% 28% 18% 3	270
December	1st to 10th	Day	16% 6% 27% 34% 14% 3	270
	11th to 20th	Day	22% 8% 30% 30% 10%	270
	21st to end	Day	22% 9% 28% 24% 14% 2	297
January	1st to 10th	Night	19% 8% 30% 33% 10%	760
	11th to 20th	Night	14% 7% 31% 35% 12% 1	760
	21st to end	Night	22% 7% 32% 32% 6%	836
February	1st to 10th	Night	18% 7% 32% 34% 9%	760
	11th to 20th	Night	22% 8% 30% 28% 11%	743
	21st to end	Night	18% 10% 33% 32% 7%	628
March	1st to 10th	Night	16% 8% 30% 34% 12% 1	751
	11th to 20th	Night	15% 5% 27% 33% 19% 2	576
	21st to end	Night	14% 5% 25% 36% 19% 1	594
April	1st to 10th	Night	13% 6% 26% 35% 18% 1	540
	11th to 20th	Night	14% 6% 29% 28% 21% 2	533
	21st to end	Night	9% 3 20% 40% 26% 2	540
May	1st to 10th	Night	11% 2 16% 38% 30% 4	540
	11th to 20th	Night	6% 2 13% 37% 35% 7%	531
	21st to end	Night	5% 2 7% 31% 43% 13%	584
June	1st to 10th	Night	10% 1 4 26% 45% 14%	540
	11th to 20th	Night	3 2 4 21% 51% 18%	540
	21st to end	Night	7% 2 4 20% 44% 24%	540
July	1st to 10th	Night	7% 2 2 12% 50% 27%	525
	11th to 20th	Night	3 1 2 7% 45% 42%	522
	21st to end	Night	5% 1 2 13% 48% 30%	591
August	1st to 10th	Night	3 1 2 10% 46% 39%	537
	11th to 20th	Night	6% 3 9% 47% 35%	534
	21st to end	Night	9% 2 4 14% 43% 28%	594
September	1st to 10th	Night	8% 1 5% 16% 43% 28%	540
	11th to 20th	Night	10% 2 6% 22% 47% 14%	632
	21st to end	Night	9% 1 7% 31% 43% 10%	720
October	1st to 10th	Night	7% 2 12% 37% 36% 6%	720
	11th to 20th	Night	8% 4 12% 35% 32% 8%	720
	21st to end	Night	13% 4 17% 35% 28% 4	792
November	1st to 10th	Night	11% 4 14% 39% 26% 5%	740
	11th to 20th	Night	13% 5% 24% 35% 21% 2	760
	21st to end	Night	12% 6% 25% 35% 20% 2	760
December	1st to 10th	Night	15% 7% 27% 30% 19% 2	760
	11th to 20th	Night	16% 7% 26% 34% 15% 2	760
	21st to end	Night	25% 7% 27% 29% 11%	836

Page generated at 2024-03-05 06:24:04. Version ac07

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