Ester Dome Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ester Dome Observatory clear sky chart from 2005-11-06 to 2024-11-28.

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	12% 12% 4 11% 7% 6% 6% 5% 6% 7% 25%	107173
	Night	7% 13% 5% 12% 7% 6% 6% 4 6% 7% 27%	100964
Transparency	Day	55% 9% 15% 16% 4	106826
	Night	54% 8% 14% 16% 7% 1	100958
Seeing	Day	15% 6% 18% 27% 24% 10%	18777
	Night	13% 4 18% 31% 26% 7%	22894

By Month

Percentage Cloud Cover NW (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	4 10% 5% 13% 6% 4 4 3 4 5% 41%	875
	11th to 20th	Day	3 14% 5% 13% 8% 7% 6% 4 6% 7% 28%	1258
	21st to end	Day	4 8% 4 13% 8% 6% 6% 5% 11% 7% 26%	1713
February	1st to 10th	Day	2 13% 4 10% 7% 4 5% 5% 6% 4 39%	1140
	11th to 20th	Day	3 12% 6% 16% 9% 7% 5% 6% 7% 8% 22%	1396
	21st to end	Day	4 12% 7% 14% 9% 7% 5% 5% 6% 5% 27%	1256
March	1st to 10th	Day	2 17% 9% 20% 7% 6% 4 3 4 4 24%	3043
	11th to 20th	Day	2 11% 6% 13% 8% 6% 6% 4 5% 7% 30%	3234
	21st to end	Day	5% 13% 5% 10% 7% 6% 6% 4 7% 8% 28%	4165
April	1st to 10th	Day	5% 15% 5% 12% 8% 6% 5% 5% 7% 7% 24%	2433
	11th to 20th	Day	4 11% 4 10% 6% 6% 5% 4 5% 6% 39%	2626
	21st to end	Day	4 9% 3 8% 6% 6% 6% 5% 6% 7% 38%	2900
May	1st to 10th	Day	7% 13% 5% 14% 9% 7% 7% 5% 7% 6% 21%	5152
	11th to 20th	Day	5% 9% 4 10% 7% 6% 6% 6% 7% 7% 34%	5505
	21st to end	Day	12% 11% 3 9% 7% 6% 7% 5% 8% 7% 26%	6363
June	1st to 10th	Day	10% 10% 4 10% 7% 8% 7% 6% 7% 8% 24%	3752
	11th to 20th	Day	12% 8% 3 8% 7% 6% 7% 5% 7% 9% 27%	3800
	21st to end	Day	13% 7% 3 8% 6% 6% 6% 6% 7% 9% 28%	3800
July	1st to 10th	Day	21% 12% 4 9% 6% 5% 6% 5% 6% 6% 19%	6330
	11th to 20th	Day	14% 9% 4 10% 7% 6% 6% 6% 7% 8% 23%	5819
	21st to end	Day	21% 10% 3 7% 5% 5% 5% 4 5% 7% 27%	6165
August	1st to 10th	Day	23% 13% 4 9% 5% 5% 5% 4 5% 6% 20%	5052
	11th to 20th	Day	20% 13% 3 11% 6% 6% 5% 4 7% 7% 17%	4812
	21st to end	Day	20% 14% 5% 11% 6% 6% 6% 5% 6% 6% 15%	4884
September	1st to 10th	Day	17% 12% 4 9% 6% 6% 5% 4 7% 6% 23%	2472
	11th to 20th	Day	20% 16% 4 12% 6% 4 5% 3 5% 5% 19%	2257
	21st to end	Day	18% 14% 5% 10% 5% 5% 4 4 4 5% 25%	2047
October	1st to 10th	Day	10% 17% 6% 11% 5% 5% 4 3 4 6% 29%	3010
	11th to 20th	Day	7% 15% 7% 18% 7% 5% 5% 3 5% 5% 24%	2743
	21st to end	Day	10% 15% 7% 17% 8% 6% 5% 3 6% 5% 18%	2453
November	1st to 10th	Day	9% 17% 5% 14% 7% 5% 5% 4 7% 6% 22%	1212
	11th to 20th	Day	7% 10% 6% 15% 8% 8% 5% 4 5% 5% 28%	1000
	21st to end	Day	8% 17% 6% 14% 6% 6% 7% 5% 6% 4 22%	739
December	1st to 10th	Day	8% 17% 6% 10% 8% 5% 6% 3 5% 8% 24%	570
	11th to 20th	Day	2 16% 8% 16% 8% 6% 6% 5% 5% 6% 21%	493
	21st to end	Day	2 12% 4 13% 7% 6% 8% 6% 8% 8% 26%	704
January	1st to 10th	Night	3 10% 5% 11% 7% 5% 5% 4 5% 7% 39%	6109
	11th to 20th	Night	4 12% 5% 14% 7% 6% 5% 4 7% 7% 29%	5650
	21st to end	Night	3 8% 4 11% 7% 6% 6% 6% 10% 7% 31%	5956
February	1st to 10th	Night	3 12% 6% 12% 7% 4 4 4 5% 6% 39%	3040
	11th to 20th	Night	3 11% 5% 12% 8% 7% 6% 5% 7% 8% 29%	2670
	21st to end	Night	4 14% 7% 14% 8% 7% 5% 4 6% 7% 26%	2198
March	1st to 10th	Night	3 15% 9% 17% 7% 6% 4 3 4 4 28%	3811
	11th to 20th	Night	5% 11% 4 13% 9% 7% 7% 4 6% 7% 28%	3721
	21st to end	Night	6% 11% 4 11% 6% 5% 6% 5% 7% 9% 31%	3466
April	1st to 10th	Night	4 15% 6% 11% 8% 7% 6% 5% 6% 6% 27%	1745
	11th to 20th	Night	4 10% 3 9% 7% 6% 6% 5% 7% 8% 36%	1503
	21st to end	Night	6% 8% 4 8% 5% 5% 6% 5% 6% 7% 40%	1277
May	1st to 10th	Night	8% 9% 4 10% 9% 9% 8% 6% 8% 9% 20%	1815
	11th to 20th	Night	5% 9% 3 8% 5% 6% 6% 6% 8% 9% 35%	1393
	21st to end	Night	15% 11% 3 8% 6% 5% 6% 4 6% 6% 30%	1223
June	1st to 10th	Night	9% 10% 3 12% 8% 6% 5% 6% 9% 8% 24%	426
	11th to 20th	Night	12% 10% 3 11% 6% 3 6% 6% 6% 8% 28%	380
	21st to end	Night	12% 7% 1 10% 9% 5% 6% 7% 8% 9% 27%	380
July	1st to 10th	Night	18% 16% 5% 13% 6% 4 3 4 4 8% 19%	640
	11th to 20th	Night	14% 10% 3 10% 6% 5% 5% 5% 6% 7% 29%	1149
	21st to end	Night	17% 11% 3 8% 5% 5% 5% 4 7% 7% 27%	1397
August	1st to 10th	Night	20% 13% 5% 10% 6% 6% 5% 3 6% 7% 18%	1885
	11th to 20th	Night	26% 16% 4 9% 4 4 5% 3 4 6% 18%	2013
	21st to end	Night	19% 13% 4 11% 6% 6% 6% 4 5% 7% 20%	2783
September	1st to 10th	Night	14% 11% 4 10% 6% 5% 5% 5% 6% 7% 25%	1705
	11th to 20th	Night	17% 14% 5% 10% 7% 6% 5% 4 6% 6% 20%	1923
	21st to end	Night	17% 13% 5% 10% 6% 6% 6% 4 5% 5% 24%	2131
October	1st to 10th	Night	9% 15% 5% 9% 5% 6% 6% 5% 6% 6% 28%	3955
	11th to 20th	Night	9% 14% 5% 12% 7% 7% 6% 4 6% 7% 24%	4183
	21st to end	Night	8% 16% 8% 17% 9% 6% 5% 3 4 5% 19%	5106
November	1st to 10th	Night	7% 18% 5% 13% 6% 5% 6% 4 5% 7% 24%	3086
	11th to 20th	Night	8% 13% 6% 15% 8% 6% 5% 3 5% 6% 26%	3400
	21st to end	Night	8% 15% 6% 14% 7% 6% 5% 5% 6% 6% 23%	3574
December	1st to 10th	Night	7% 16% 6% 12% 7% 6% 5% 4 6% 7% 23%	3610
	11th to 20th	Night	4 17% 8% 16% 8% 7% 6% 4 5% 5% 18%	4698
	21st to end	Night	3 11% 5% 13% 8% 7% 6% 5% 7% 8% 27%	6963

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	46% 6% 13% 20% 15% 1	875
	11th to 20th	Day	51% 8% 15% 15% 6% 4	1270
	21st to end	Day	50% 5% 9% 16% 14% 7%	1713
February	1st to 10th	Day	43% 8% 7% 19% 19% 4	1140
	11th to 20th	Day	56% 9% 11% 15% 8%	1402
	21st to end	Day	56% 7% 10% 14% 11% 1	1256
March	1st to 10th	Day	64% 5% 7% 11% 12% 1	3043
	11th to 20th	Day	50% 9% 11% 19% 9% 1	3234
	21st to end	Day	50% 10% 18% 13% 8% 1	4165
April	1st to 10th	Day	55% 7% 13% 17% 7% 1	2433
	11th to 20th	Day	44% 7% 15% 25% 8% 2	2626
	21st to end	Day	41% 7% 14% 29% 9%	2900
May	1st to 10th	Day	58% 10% 10% 18% 4	5152
	11th to 20th	Day	42% 10% 14% 29% 5%	5508
	21st to end	Day	49% 11% 14% 20% 6%	6364
June	1st to 10th	Day	53% 10% 15% 18% 3	3752
	11th to 20th	Day	50% 11% 16% 19% 3	3800
	21st to end	Day	49% 12% 20% 18% 2	3800
July	1st to 10th	Day	62% 11% 17% 9% 1	6231
	11th to 20th	Day	53% 11% 18% 18% 1	5715
	21st to end	Day	55% 10% 18% 14% 4	5970
August	1st to 10th	Day	63% 8% 20% 8% 1	5032
	11th to 20th	Day	61% 10% 21% 8%	4860
	21st to end	Day	65% 10% 16% 8% 1	4883
September	1st to 10th	Day	58% 11% 19% 11% 1	2472
	11th to 20th	Day	64% 9% 15% 11%	2257
	21st to end	Day	61% 8% 14% 14% 3	2047
October	1st to 10th	Day	57% 6% 12% 20% 5%	3010
	11th to 20th	Day	61% 8% 13% 16% 2	2743
	21st to end	Day	66% 8% 12% 11% 3	2453
November	1st to 10th	Day	61% 8% 15% 12% 4	1212
	11th to 20th	Day	58% 8% 13% 19% 3	1000
	21st to end	Day	61% 9% 11% 14% 6%	739
December	1st to 10th	Day	59% 9% 12% 13% 5% 2	570
	11th to 20th	Day	62% 6% 12% 16% 4	495
	21st to end	Day	48% 8% 14% 18% 11% 1	704
January	1st to 10th	Night	43% 7% 16% 26% 7% 2	6109
	11th to 20th	Night	50% 7% 18% 15% 9% 1	5700
	21st to end	Night	45% 5% 9% 16% 18% 7%	5956
February	1st to 10th	Night	46% 4 8% 17% 20% 4	3040
	11th to 20th	Night	50% 7% 13% 20% 10% 1	2675
	21st to end	Night	56% 6% 12% 15% 9% 1	2198
March	1st to 10th	Night	60% 5% 6% 15% 13% 2	3811
	11th to 20th	Night	52% 7% 13% 18% 8% 1	3723
	21st to end	Night	43% 11% 21% 14% 10% 1	3466
April	1st to 10th	Night	55% 7% 13% 18% 7% 1	1745
	11th to 20th	Night	42% 10% 18% 22% 7% 1	1503
	21st to end	Night	41% 6% 13% 29% 10%	1277
May	1st to 10th	Night	52% 13% 12% 20% 2 1	1815
	11th to 20th	Night	38% 11% 16% 31% 4	1393
	21st to end	Night	49% 10% 11% 22% 8%	1223
June	1st to 10th	Night	50% 15% 15% 18% 2	426
	11th to 20th	Night	52% 9% 14% 20% 4	380
	21st to end	Night	50% 12% 20% 18% 1	380
July	1st to 10th	Night	65% 9% 15% 9% 2	630
	11th to 20th	Night	53% 9% 17% 18% 4	1130
	21st to end	Night	54% 9% 17% 15% 6%	1353
August	1st to 10th	Night	64% 9% 19% 7% 1	1880
	11th to 20th	Night	67% 8% 18% 8%	2031
	21st to end	Night	62% 9% 18% 10% 1	2783
September	1st to 10th	Night	54% 11% 20% 13% 2	1705
	11th to 20th	Night	63% 9% 14% 13% 1	1923
	21st to end	Night	60% 9% 12% 15% 3	2131
October	1st to 10th	Night	54% 7% 13% 21% 4	3955
	11th to 20th	Night	58% 8% 18% 15% 2	4183
	21st to end	Night	67% 7% 11% 13% 2	5106
November	1st to 10th	Night	60% 7% 13% 16% 4	3086
	11th to 20th	Night	60% 6% 15% 16% 3	3400
	21st to end	Night	60% 8% 10% 17% 6%	3574
December	1st to 10th	Night	58% 8% 13% 14% 6% 1	3610
	11th to 20th	Night	66% 7% 10% 13% 5%	4695
	21st to end	Night	51% 8% 12% 15% 13% 1	6963

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	7% 6% 22% 37% 22% 7%	90
	11th to 20th	Day	8% 4 19% 30% 32% 7%	90
	21st to end	Day	8% 4 13% 37% 27% 11%	213
February	1st to 10th	Day	7% 7% 15% 29% 25% 18%	280
	11th to 20th	Day	9% 7% 25% 29% 21% 9%	332
	21st to end	Day	9% 10% 26% 29% 20% 7%	307
March	1st to 10th	Day	7% 7% 25% 36% 19% 6%	370
	11th to 20th	Day	8% 8% 24% 29% 24% 7%	381
	21st to end	Day	9% 6% 25% 30% 23% 7%	616
April	1st to 10th	Day	11% 9% 25% 29% 20% 5%	614
	11th to 20th	Day	8% 7% 25% 31% 23% 6%	640
	21st to end	Day	7% 5% 15% 34% 32% 8%	650
May	1st to 10th	Day	10% 8% 21% 31% 23% 7%	653
	11th to 20th	Day	9% 7% 25% 29% 22% 8%	772
	21st to end	Day	15% 6% 19% 30% 24% 7%	1033
June	1st to 10th	Day	13% 3 19% 27% 26% 12%	1060
	11th to 20th	Day	12% 4 15% 24% 32% 12%	1060
	21st to end	Day	14% 3 12% 25% 31% 15%	1060
July	1st to 10th	Day	16% 4 14% 26% 31% 9%	1041
	11th to 20th	Day	19% 4 15% 26% 24% 12%	901
	21st to end	Day	18% 4 14% 23% 26% 15%	942
August	1st to 10th	Day	26% 6% 16% 23% 20% 9%	676
	11th to 20th	Day	23% 6% 17% 24% 23% 6%	665
	21st to end	Day	23% 8% 17% 21% 21% 10%	748
September	1st to 10th	Day	21% 7% 17% 21% 23% 10%	655
	11th to 20th	Day	26% 9% 18% 25% 14% 8%	557
	21st to end	Day	25% 5% 19% 18% 19% 14%	390
October	1st to 10th	Day	20% 10% 22% 20% 20% 8%	390
	11th to 20th	Day	16% 6% 20% 24% 23% 12%	390
	21st to end	Day	15% 7% 14% 25% 26% 13%	429
November	1st to 10th	Day	16% 8% 15% 28% 26% 8%	296
	11th to 20th	Day	10% 3 18% 34% 20% 15%	100
	21st to end	Day	15% 6% 13% 31% 28% 6%	97
December	1st to 10th	Day	20% 6% 19% 30% 18% 8%	90
	11th to 20th	Day	9% 8% 28% 28% 22% 6%	90
	21st to end	Day	5% 13% 27% 29% 19% 6%	99
January	1st to 10th	Night	8% 2 19% 34% 28% 10%	1083
	11th to 20th	Night	10% 4 18% 41% 22% 5%	940
	21st to end	Night	8% 4 18% 41% 23% 6%	998
February	1st to 10th	Night	6% 5% 22% 32% 27% 9%	850
	11th to 20th	Night	7% 5% 22% 38% 24% 5%	834
	21st to end	Night	8% 7% 23% 35% 24% 2	703
March	1st to 10th	Night	8% 5% 22% 37% 25% 3	665
	11th to 20th	Night	9% 4 18% 38% 25% 6%	648
	21st to end	Night	11% 4 19% 35% 23% 9%	627
April	1st to 10th	Night	10% 7% 21% 35% 24% 3	570
	11th to 20th	Night	9% 4 18% 39% 25% 5%	563
	21st to end	Night	9% 2 15% 28% 39% 7%	505
May	1st to 10th	Night	10% 3 13% 26% 41% 7%	334
	11th to 20th	Night	7% 6% 21% 38% 27% 1	187
	21st to end	Night	15% 7% 12% 33% 28% 6%	206
June	1st to 10th	Night	10% 3 22% 23% 33% 9%	190
	11th to 20th	Night	13% 5% 15% 25% 34% 8%	190
	21st to end	Night	14% 2 11% 27% 38% 8%	190
July	1st to 10th	Night	14% 5% 16% 26% 34% 6%	185
	11th to 20th	Night	22% 3 14% 23% 32% 5%	184
	21st to end	Night	17% 3 14% 26% 33% 7%	208
August	1st to 10th	Night	26% 2 12% 26% 28% 5%	369
	11th to 20th	Night	26% 5% 12% 21% 29% 6%	436
	21st to end	Night	25% 4 10% 22% 29% 11%	627
September	1st to 10th	Night	20% 4 15% 21% 31% 9%	570
	11th to 20th	Night	21% 5% 15% 27% 27% 4	570
	21st to end	Night	22% 4 17% 24% 28% 5%	593
October	1st to 10th	Night	23% 6% 20% 22% 25% 4	760
	11th to 20th	Night	15% 4 18% 28% 26% 9%	860
	21st to end	Night	14% 4 16% 31% 28% 7%	946
November	1st to 10th	Night	16% 5% 19% 31% 25% 4	880
	11th to 20th	Night	15% 4 17% 28% 26% 11%	900
	21st to end	Night	14% 4 16% 30% 26% 10%	1020
December	1st to 10th	Night	15% 7% 18% 33% 20% 8%	1130
	11th to 20th	Night	12% 6% 20% 29% 28% 6%	1130
	21st to end	Night	9% 5% 19% 33% 24% 9%	1243

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