Icefog Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Icefog Observatory clear sky chart from 2005-11-06 to 2025-01-07.

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	14% 11% 4 10% 6% 6% 5% 4 6% 7% 26%	108219
	Night	8% 13% 5% 11% 6% 6% 5% 4 6% 6% 29%	109448
Transparency	Day	54% 9% 16% 16% 4	107874
	Night	53% 8% 15% 16% 7% 1	109440
Seeing	Day	15% 6% 16% 27% 26% 11%	18796
	Night	13% 4 16% 32% 28% 7%	23210

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 11% 5% 10% 7% 5% 3 3 3 5% 44%	892
	11th to 20th	Day	4 13% 6% 11% 8% 7% 5% 4 5% 6% 32%	1258
	21st to end	Day	5% 10% 5% 10% 7% 8% 7% 6% 10% 6% 27%	1698
February	1st to 10th	Day	2 13% 6% 10% 5% 4 5% 5% 5% 5% 40%	1140
	11th to 20th	Day	4 13% 5% 16% 6% 7% 6% 5% 6% 8% 24%	1394
	21st to end	Day	4 19% 6% 11% 7% 5% 5% 5% 6% 6% 27%	1256
March	1st to 10th	Day	3 18% 9% 18% 7% 5% 4 3 4 4 25%	3043
	11th to 20th	Day	3 13% 6% 14% 6% 6% 6% 4 6% 7% 29%	3224
	21st to end	Day	6% 13% 5% 10% 6% 5% 5% 4 7% 7% 31%	4160
April	1st to 10th	Day	6% 16% 6% 11% 7% 7% 4 5% 7% 7% 25%	2429
	11th to 20th	Day	5% 10% 4 9% 6% 5% 4 4 5% 7% 40%	2629
	21st to end	Day	6% 8% 3 8% 7% 6% 5% 4 7% 7% 40%	2897
May	1st to 10th	Day	8% 14% 5% 13% 8% 7% 7% 5% 6% 6% 21%	5180
	11th to 20th	Day	5% 8% 3 11% 7% 6% 7% 5% 7% 8% 34%	5480
	21st to end	Day	13% 10% 3 9% 6% 6% 6% 6% 7% 7% 26%	6393
June	1st to 10th	Day	12% 7% 4 10% 7% 7% 6% 5% 7% 7% 26%	3738
	11th to 20th	Day	12% 9% 3 9% 6% 6% 6% 5% 8% 8% 28%	3800
	21st to end	Day	15% 7% 3 7% 7% 5% 6% 4 8% 9% 29%	3800
July	1st to 10th	Day	30% 8% 3 7% 6% 5% 5% 4 6% 6% 19%	6330
	11th to 20th	Day	17% 9% 3 9% 6% 6% 5% 5% 7% 8% 25%	5822
	21st to end	Day	24% 8% 3 7% 5% 4 5% 4 5% 7% 27%	6151
August	1st to 10th	Day	29% 10% 4 8% 5% 5% 4 4 5% 6% 21%	5081
	11th to 20th	Day	25% 11% 3 9% 6% 5% 5% 4 6% 7% 19%	4827
	21st to end	Day	26% 12% 4 10% 5% 5% 5% 4 5% 6% 17%	4884
September	1st to 10th	Day	20% 11% 4 9% 5% 5% 5% 4 6% 7% 24%	2475
	11th to 20th	Day	22% 13% 5% 10% 5% 5% 5% 4 5% 6% 19%	2245
	21st to end	Day	19% 14% 4 10% 6% 4 3 4 5% 4 26%	2064
October	1st to 10th	Day	12% 17% 5% 10% 5% 4 4 3 4 5% 31%	2984
	11th to 20th	Day	9% 18% 8% 13% 7% 6% 4 4 5% 4 22%	2758
	21st to end	Day	13% 18% 6% 14% 7% 5% 5% 3 5% 5% 20%	2439
November	1st to 10th	Day	9% 15% 6% 13% 7% 5% 5% 5% 6% 6% 24%	1211
	11th to 20th	Day	9% 12% 6% 14% 9% 7% 3 3 4 6% 28%	1000
	21st to end	Day	10% 16% 6% 11% 7% 7% 4 4 5% 6% 25%	740
December	1st to 10th	Day	11% 12% 4 7% 5% 5% 5% 3 6% 9% 32%	980
	11th to 20th	Day	4 15% 7% 10% 8% 4 6% 4 7% 4 31%	805
	21st to end	Day	3 11% 5% 12% 6% 5% 6% 5% 6% 7% 34%	1012
January	1st to 10th	Night	4 10% 5% 11% 6% 4 5% 4 5% 6% 41%	6225
	11th to 20th	Night	5% 14% 5% 12% 6% 5% 4 4 6% 7% 32%	5650
	21st to end	Night	3 9% 4 11% 7% 6% 7% 5% 9% 7% 32%	5969
February	1st to 10th	Night	2 12% 6% 13% 6% 4 4 3 5% 7% 40%	3040
	11th to 20th	Night	4 12% 5% 10% 7% 7% 7% 5% 7% 8% 28%	2673
	21st to end	Night	5% 15% 8% 12% 8% 6% 5% 4 5% 7% 26%	2198
March	1st to 10th	Night	3 18% 8% 17% 6% 6% 4 3 3 4 28%	3810
	11th to 20th	Night	6% 11% 5% 14% 8% 6% 6% 4 5% 7% 29%	3734
	21st to end	Night	7% 12% 3 8% 4 6% 5% 4 7% 9% 35%	3469
April	1st to 10th	Night	6% 15% 5% 11% 6% 6% 6% 5% 6% 6% 27%	1748
	11th to 20th	Night	5% 8% 3 9% 7% 6% 5% 5% 6% 6% 38%	1500
	21st to end	Night	6% 8% 3 7% 6% 6% 6% 4 7% 7% 40%	1280
May	1st to 10th	Night	7% 12% 4 11% 9% 8% 8% 6% 7% 8% 20%	1786
	11th to 20th	Night	6% 7% 3 8% 6% 6% 6% 6% 8% 8% 35%	1418
	21st to end	Night	16% 11% 4 9% 6% 5% 5% 4 5% 7% 28%	1193
June	1st to 10th	Night	8% 9% 3 8% 9% 5% 7% 5% 10% 10% 24%	441
	11th to 20th	Night	12% 9% 3 9% 7% 7% 5% 4 6% 12% 25%	380
	21st to end	Night	13% 8% 1 9% 5% 4 8% 6% 9% 9% 27%	380
July	1st to 10th	Night	28% 12% 4 8% 6% 4 5% 5% 5% 6% 17%	640
	11th to 20th	Night	19% 10% 4 7% 6% 5% 7% 5% 6% 6% 27%	1147
	21st to end	Night	22% 9% 3 7% 5% 5% 4 5% 5% 7% 27%	1411
August	1st to 10th	Night	26% 9% 4 9% 6% 5% 5% 3 6% 7% 18%	1857
	11th to 20th	Night	32% 12% 3 7% 5% 5% 4 4 4 5% 18%	2009
	21st to end	Night	25% 12% 4 9% 6% 6% 5% 4 5% 5% 18%	2783
September	1st to 10th	Night	17% 11% 4 10% 5% 7% 6% 4 6% 7% 26%	1704
	11th to 20th	Night	20% 13% 4 10% 6% 6% 5% 3 6% 6% 21%	1935
	21st to end	Night	19% 14% 4 8% 5% 5% 4 4 5% 5% 25%	2114
October	1st to 10th	Night	10% 15% 4 9% 5% 6% 6% 4 6% 7% 28%	3982
	11th to 20th	Night	10% 16% 6% 11% 7% 6% 5% 4 5% 6% 23%	4156
	21st to end	Night	10% 19% 7% 15% 8% 5% 3 3 4 5% 22%	5120
November	1st to 10th	Night	9% 17% 5% 12% 6% 5% 5% 4 5% 4 26%	3087
	11th to 20th	Night	8% 14% 7% 14% 8% 5% 4 4 5% 5% 26%	3400
	21st to end	Night	8% 16% 5% 13% 7% 5% 5% 4 6% 5% 25%	3637
December	1st to 10th	Night	7% 13% 5% 11% 7% 6% 5% 4 6% 7% 30%	6210
	11th to 20th	Night	5% 15% 7% 12% 6% 6% 5% 4 5% 6% 30%	7396
	21st to end	Night	3 12% 6% 11% 7% 6% 6% 5% 6% 7% 31%	9966

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	45% 6% 11% 23% 14% 2	892
	11th to 20th	Day	48% 9% 18% 15% 7% 4	1270
	21st to end	Day	50% 7% 11% 13% 13% 5%	1698
February	1st to 10th	Day	44% 6% 7% 19% 19% 5%	1140
	11th to 20th	Day	55% 8% 11% 16% 8%	1401
	21st to end	Day	55% 7% 10% 16% 10% 1	1256
March	1st to 10th	Day	61% 6% 9% 11% 12% 1	3043
	11th to 20th	Day	49% 11% 13% 18% 8% 1	3224
	21st to end	Day	47% 10% 19% 14% 8% 1	4160
April	1st to 10th	Day	54% 7% 14% 17% 6% 1	2429
	11th to 20th	Day	42% 8% 16% 25% 8% 2	2629
	21st to end	Day	39% 8% 15% 28% 9%	2897
May	1st to 10th	Day	57% 10% 14% 15% 4	5180
	11th to 20th	Day	40% 13% 16% 26% 6%	5483
	21st to end	Day	49% 12% 16% 18% 5%	6394
June	1st to 10th	Day	51% 11% 18% 17% 3	3738
	11th to 20th	Day	49% 11% 18% 19% 3	3800
	21st to end	Day	48% 11% 21% 17% 3	3800
July	1st to 10th	Day	64% 9% 17% 8% 1	6231
	11th to 20th	Day	53% 10% 17% 20% 1	5718
	21st to end	Day	54% 10% 18% 14% 4	5957
August	1st to 10th	Day	62% 8% 19% 9% 1	5061
	11th to 20th	Day	61% 11% 20% 8%	4875
	21st to end	Day	65% 9% 17% 8%	4883
September	1st to 10th	Day	57% 11% 20% 11% 1	2475
	11th to 20th	Day	63% 10% 16% 11%	2245
	21st to end	Day	61% 8% 13% 13% 3	2064
October	1st to 10th	Day	56% 6% 12% 21% 5%	2984
	11th to 20th	Day	64% 9% 12% 14% 2	2758
	21st to end	Day	65% 8% 12% 13% 2	2439
November	1st to 10th	Day	58% 9% 17% 12% 4	1211
	11th to 20th	Day	59% 8% 13% 17% 4	1000
	21st to end	Day	58% 8% 12% 16% 6%	740
December	1st to 10th	Day	47% 7% 20% 20% 5% 1	980
	11th to 20th	Day	52% 6% 19% 14% 3 6%	807
	21st to end	Day	44% 11% 18% 14% 12% 1	1012
January	1st to 10th	Night	41% 8% 17% 24% 8% 2	6225
	11th to 20th	Night	49% 8% 18% 15% 9% 1	5700
	21st to end	Night	45% 7% 10% 15% 16% 7%	5969
February	1st to 10th	Night	46% 4 9% 16% 21% 4	3040
	11th to 20th	Night	49% 9% 13% 20% 9%	2677
	21st to end	Night	57% 7% 12% 14% 9% 1	2198
March	1st to 10th	Night	60% 4 7% 15% 12% 2	3810
	11th to 20th	Night	52% 9% 14% 16% 8% 1	3736
	21st to end	Night	41% 11% 24% 13% 10% 2	3469
April	1st to 10th	Night	52% 9% 14% 18% 6% 1	1748
	11th to 20th	Night	42% 9% 19% 21% 7% 1	1500
	21st to end	Night	39% 7% 16% 29% 10%	1280
May	1st to 10th	Night	54% 15% 12% 16% 2	1786
	11th to 20th	Night	39% 12% 18% 27% 5%	1418
	21st to end	Night	51% 11% 11% 18% 8%	1193
June	1st to 10th	Night	48% 12% 18% 18% 3	441
	11th to 20th	Night	52% 11% 15% 18% 4	380
	21st to end	Night	49% 12% 22% 16% 1	380
July	1st to 10th	Night	67% 10% 14% 7% 2	630
	11th to 20th	Night	56% 10% 15% 19%	1128
	21st to end	Night	54% 9% 17% 16% 4	1366
August	1st to 10th	Night	63% 8% 19% 8% 2	1852
	11th to 20th	Night	67% 8% 17% 7%	2027
	21st to end	Night	66% 8% 16% 9% 1	2783
September	1st to 10th	Night	56% 10% 21% 12% 2	1704
	11th to 20th	Night	63% 9% 14% 13% 1	1935
	21st to end	Night	59% 10% 13% 15% 3	2114
October	1st to 10th	Night	55% 7% 12% 21% 5%	3982
	11th to 20th	Night	60% 10% 16% 12% 2	4156
	21st to end	Night	66% 6% 11% 15% 2	5120
November	1st to 10th	Night	58% 8% 14% 15% 3 1	3087
	11th to 20th	Night	60% 8% 14% 15% 3	3400
	21st to end	Night	57% 9% 12% 17% 6%	3637
December	1st to 10th	Night	50% 7% 18% 18% 6% 1	6210
	11th to 20th	Night	53% 7% 17% 14% 5% 4	7393
	21st to end	Night	48% 8% 18% 13% 13% 1	9966

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	6% 3 16% 45% 24% 6%	96
	11th to 20th	Day	7% 1 14% 38% 36% 4	90
	21st to end	Day	8% 5% 12% 37% 24% 14%	197
February	1st to 10th	Day	6% 6% 14% 30% 25% 19%	280
	11th to 20th	Day	8% 6% 21% 29% 25% 11%	338
	21st to end	Day	9% 7% 19% 35% 22% 7%	307
March	1st to 10th	Day	6% 3 21% 36% 25% 9%	370
	11th to 20th	Day	8% 5% 21% 30% 24% 11%	370
	21st to end	Day	9% 6% 20% 29% 27% 8%	611
April	1st to 10th	Day	10% 9% 22% 30% 23% 6%	621
	11th to 20th	Day	8% 5% 23% 28% 28% 8%	640
	21st to end	Day	7% 4 14% 32% 33% 10%	650
May	1st to 10th	Day	10% 5% 20% 31% 26% 8%	653
	11th to 20th	Day	7% 7% 20% 31% 27% 8%	760
	21st to end	Day	14% 8% 16% 30% 24% 8%	1050
June	1st to 10th	Day	11% 5% 18% 26% 27% 14%	1060
	11th to 20th	Day	13% 4 14% 23% 32% 15%	1060
	21st to end	Day	14% 3 10% 25% 33% 15%	1060
July	1st to 10th	Day	16% 4 11% 25% 34% 10%	1041
	11th to 20th	Day	18% 5% 14% 25% 24% 13%	919
	21st to end	Day	19% 4 12% 24% 25% 17%	928
August	1st to 10th	Day	27% 4 17% 21% 22% 9%	676
	11th to 20th	Day	23% 7% 16% 25% 22% 6%	665
	21st to end	Day	27% 7% 14% 22% 21% 9%	748
September	1st to 10th	Day	23% 7% 14% 20% 24% 11%	665
	11th to 20th	Day	28% 9% 15% 23% 16% 9%	545
	21st to end	Day	25% 7% 13% 22% 19% 14%	390
October	1st to 10th	Day	21% 9% 19% 21% 19% 11%	390
	11th to 20th	Day	16% 6% 19% 19% 27% 13%	390
	21st to end	Day	16% 6% 13% 21% 29% 15%	429
November	1st to 10th	Day	16% 6% 12% 27% 27% 12%	287
	11th to 20th	Day	11% 4 13% 26% 25% 21%	100
	21st to end	Day	16% 2 10% 32% 35% 5%	100
December	1st to 10th	Day	21% 4 18% 32% 19% 6%	100
	11th to 20th	Day	8% 7% 24% 32% 22% 7%	100
	21st to end	Day	6% 9% 17% 34% 27% 6%	110
January	1st to 10th	Night	7% 2 15% 36% 28% 11%	1145
	11th to 20th	Night	10% 3 17% 39% 24% 6%	940
	21st to end	Night	8% 3 17% 40% 26% 7%	992
February	1st to 10th	Night	5% 5% 19% 33% 28% 10%	850
	11th to 20th	Night	7% 4 19% 40% 25% 5%	834
	21st to end	Night	8% 6% 17% 39% 26% 3	703
March	1st to 10th	Night	8% 3 19% 39% 27% 3	679
	11th to 20th	Night	9% 3 16% 38% 29% 5%	642
	21st to end	Night	10% 4 19% 34% 25% 8%	627
April	1st to 10th	Night	9% 5% 19% 37% 26% 4	570
	11th to 20th	Night	8% 3 18% 38% 29% 4	563
	21st to end	Night	8% 2 12% 33% 39% 6%	520
May	1st to 10th	Night	10% 3 16% 30% 36% 6%	318
	11th to 20th	Night	6% 4 18% 37% 33% 2	187
	21st to end	Night	16% 8% 8% 33% 29% 6%	206
June	1st to 10th	Night	11% 3 15% 26% 37% 8%	190
	11th to 20th	Night	12% 4 13% 29% 32% 9%	190
	21st to end	Night	14% 2 5% 28% 43% 8%	190
July	1st to 10th	Night	12% 4 12% 25% 40% 6%	185
	11th to 20th	Night	20% 4 12% 27% 30% 8%	184
	21st to end	Night	20% 2 10% 27% 34% 7%	208
August	1st to 10th	Night	28% 5% 8% 22% 31% 5%	354
	11th to 20th	Night	25% 4 12% 25% 28% 6%	448
	21st to end	Night	29% 5% 8% 21% 26% 11%	627
September	1st to 10th	Night	22% 4 12% 22% 30% 9%	570
	11th to 20th	Night	24% 4 14% 27% 28% 4	570
	21st to end	Night	24% 4 17% 23% 27% 5%	583
October	1st to 10th	Night	20% 7% 18% 27% 25% 4	774
	11th to 20th	Night	15% 4 16% 29% 26% 10%	860
	21st to end	Night	13% 5% 12% 34% 29% 7%	946
November	1st to 10th	Night	17% 5% 16% 32% 24% 7%	880
	11th to 20th	Night	14% 3 13% 29% 28% 13%	900
	21st to end	Night	14% 3 16% 31% 27% 10%	1055
December	1st to 10th	Night	13% 5% 17% 34% 23% 8%	1200
	11th to 20th	Night	11% 4 19% 30% 27% 8%	1200
	21st to end	Night	9% 5% 14% 34% 28% 10%	1320

Page generated at 2025-01-03 23:23:59. Version ac07

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