De Forest
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the De Forest clear sky chart from 2005-11-06 to 2024-01-12.

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	13% 8% 4 7% 6% 3 5% 5% 6% 8% 37%	74782
	Night	14% 9% 4 6% 5% 3 4 5% 5% 7% 39%	74968
Transparency	Day	45% 7% 14% 22% 12% 1	74482
	Night	45% 7% 12% 22% 13% 2	74788
Seeing	Day	16% 10% 29% 26% 14% 5%	14730
	Night	17% 10% 31% 30% 11% 1	24724

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	11% 11% 6% 9% 7% 4 5% 4 6% 7% 30%	1517
	11th to 20th	Day	10% 15% 9% 11% 7% 3 6% 4 5% 8% 22%	1453
	21st to end	Day	11% 13% 7% 13% 7% 4 5% 5% 6% 6% 23%	1652
February	1st to 10th	Day	12% 11% 6% 8% 6% 3 5% 5% 5% 7% 31%	1620
	11th to 20th	Day	13% 10% 3 7% 6% 4 6% 5% 4 7% 36%	1686
	21st to end	Day	12% 12% 4 10% 7% 4 5% 6% 4 6% 30%	1480
March	1st to 10th	Day	13% 9% 5% 6% 5% 3 5% 6% 6% 7% 36%	1800
	11th to 20th	Day	12% 7% 3 6% 6% 2 7% 5% 6% 7% 39%	1906
	21st to end	Day	19% 10% 4 7% 6% 3 5% 5% 4 7% 31%	2348
April	1st to 10th	Day	17% 12% 5% 7% 5% 4 4 4 6% 7% 30%	2160
	11th to 20th	Day	17% 9% 3 6% 7% 3 5% 5% 6% 8% 32%	2134
	21st to end	Day	19% 9% 4 7% 6% 3 4 4 5% 7% 33%	2227
May	1st to 10th	Day	16% 8% 2 7% 6% 4 5% 6% 8% 8% 30%	2420
	11th to 20th	Day	17% 7% 3 6% 6% 2 5% 4 5% 8% 36%	2479
	21st to end	Day	15% 7% 3 5% 5% 3 6% 5% 6% 7% 39%	2725
June	1st to 10th	Day	10% 4 2 5% 5% 3 5% 6% 6% 8% 45%	2520
	11th to 20th	Day	11% 6% 3 5% 5% 3 4 5% 6% 10% 42%	2520
	21st to end	Day	10% 6% 2 5% 5% 3 5% 5% 7% 10% 41%	2520
July	1st to 10th	Day	7% 5% 2 4 4 3 5% 5% 7% 10% 47%	2520
	11th to 20th	Day	7% 4 2 6% 6% 3 5% 6% 7% 10% 45%	2520
	21st to end	Day	7% 4 2 4 5% 3 5% 5% 8% 11% 46%	2743
August	1st to 10th	Day	7% 4 2 5% 5% 3 5% 7% 8% 11% 43%	2486
	11th to 20th	Day	11% 6% 2 6% 6% 3 4 5% 5% 8% 44%	2178
	21st to end	Day	10% 5% 2 5% 4 3 4 5% 6% 8% 46%	2376
September	1st to 10th	Day	13% 5% 2 5% 6% 3 6% 4 5% 7% 43%	2160
	11th to 20th	Day	13% 5% 3 4 3 2 4 4 5% 7% 49%	2101
	21st to end	Day	12% 7% 3 6% 5% 3 4 5% 5% 7% 44%	1980
October	1st to 10th	Day	14% 6% 3 5% 5% 3 4 4 4 6% 45%	1840
	11th to 20th	Day	21% 9% 3 6% 6% 3 4 5% 5% 7% 33%	1800
	21st to end	Day	18% 10% 4 6% 5% 3 3 3 4 5% 38%	1806
November	1st to 10th	Day	11% 8% 4 6% 4 2 5% 4 5% 7% 44%	1672
	11th to 20th	Day	12% 11% 6% 7% 8% 4 4 5% 5% 7% 31%	1692
	21st to end	Day	15% 12% 4 7% 6% 3 4 3 4 6% 36%	1520
December	1st to 10th	Day	13% 13% 5% 7% 7% 4 4 5% 5% 7% 29%	1520
	11th to 20th	Day	12% 13% 5% 10% 7% 3 4 5% 6% 7% 28%	1918
	21st to end	Day	19% 18% 9% 11% 7% 4 5% 4 4 5% 14%	2783
January	1st to 10th	Night	12% 11% 6% 9% 6% 4 5% 5% 5% 5% 35%	2648
	11th to 20th	Night	14% 11% 6% 10% 7% 4 5% 5% 5% 6% 27%	2547
	21st to end	Night	13% 13% 8% 10% 7% 4 5% 5% 5% 6% 26%	2703
February	1st to 10th	Night	12% 13% 5% 8% 7% 3 5% 5% 5% 6% 32%	2340
	11th to 20th	Night	11% 10% 3 7% 6% 3 5% 5% 5% 6% 38%	2161
	21st to end	Night	16% 12% 4 7% 6% 2 4 4 4 7% 34%	1776
March	1st to 10th	Night	16% 9% 3 6% 6% 3 4 5% 5% 6% 37%	2160
	11th to 20th	Night	13% 6% 3 5% 5% 4 5% 5% 6% 9% 40%	2070
	21st to end	Night	19% 9% 3 7% 5% 3 5% 5% 5% 6% 33%	2008
April	1st to 10th	Night	17% 11% 3 5% 5% 4 5% 4 4 7% 36%	1800
	11th to 20th	Night	21% 8% 2 6% 6% 3 3 4 5% 7% 37%	1775
	21st to end	Night	17% 8% 2 5% 4 3 4 5% 5% 7% 37%	1733
May	1st to 10th	Night	16% 8% 3 6% 5% 3 6% 5% 7% 7% 34%	1536
	11th to 20th	Night	18% 6% 3 6% 5% 4 6% 4 5% 6% 38%	1416
	21st to end	Night	11% 7% 3 5% 5% 3 5% 5% 5% 10% 41%	1551
June	1st to 10th	Night	7% 5% 2 5% 5% 4 5% 5% 5% 9% 47%	1440
	11th to 20th	Night	12% 6% 2 4 4 2 4 4 5% 9% 47%	1440
	21st to end	Night	8% 6% 2 5% 4 3 4 4 5% 9% 51%	1440
July	1st to 10th	Night	6% 3 1 3 4 3 5% 5% 6% 8% 55%	1440
	11th to 20th	Night	6% 4 2 5% 4 3 5% 4 5% 8% 55%	1440
	21st to end	Night	6% 3 2 4 5% 3 4 5% 7% 9% 52%	1575
August	1st to 10th	Night	5% 3 2 5% 6% 3 5% 6% 7% 9% 49%	1450
	11th to 20th	Night	9% 5% 2 5% 4 3 4 4 5% 8% 51%	1721
	21st to end	Night	8% 4 2 5% 5% 2 4 4 5% 8% 53%	1980
September	1st to 10th	Night	10% 4 2 5% 3 3 3 4 6% 7% 53%	1800
	11th to 20th	Night	12% 3 2 4 4 3 4 4 5% 6% 53%	1840
	21st to end	Night	11% 5% 3 5% 5% 3 4 4 5% 7% 48%	1980
October	1st to 10th	Night	12% 6% 2 5% 4 3 3 4 5% 6% 49%	2117
	11th to 20th	Night	17% 7% 2 5% 5% 3 4 4 5% 7% 41%	2160
	21st to end	Night	19% 7% 3 6% 5% 3 4 5% 5% 8% 35%	2548
November	1st to 10th	Night	12% 7% 3 5% 6% 3 4 4 5% 6% 46%	2406
	11th to 20th	Night	14% 10% 4 8% 6% 3 5% 5% 5% 6% 34%	2484
	21st to end	Night	17% 9% 3 5% 5% 3 5% 5% 5% 5% 38%	2660
December	1st to 10th	Night	14% 12% 4 8% 6% 3 5% 5% 5% 6% 31%	2660
	11th to 20th	Night	15% 13% 6% 9% 6% 3 5% 5% 5% 5% 28%	3279
	21st to end	Night	21% 17% 5% 9% 7% 3 4 5% 4 5% 20%	4884

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	52% 7% 14% 17% 9%	1518
	11th to 20th	Day	61% 7% 12% 13% 7%	1453
	21st to end	Day	60% 7% 12% 13% 8%	1652
February	1st to 10th	Day	53% 5% 15% 18% 8% 1	1620
	11th to 20th	Day	48% 7% 13% 20% 10% 2	1695
	21st to end	Day	54% 7% 10% 17% 10% 2	1480
March	1st to 10th	Day	48% 8% 13% 21% 9% 1	1800
	11th to 20th	Day	43% 6% 15% 21% 13% 1	1906
	21st to end	Day	54% 6% 13% 20% 7% 1	2348
April	1st to 10th	Day	53% 7% 11% 19% 10%	2160
	11th to 20th	Day	49% 7% 11% 20% 11% 2	2134
	21st to end	Day	50% 7% 12% 19% 10% 1	2227
May	1st to 10th	Day	49% 8% 15% 21% 7% 1	2420
	11th to 20th	Day	47% 7% 13% 21% 12%	2483
	21st to end	Day	43% 9% 12% 21% 13% 2	2725
June	1st to 10th	Day	35% 8% 15% 25% 15% 1	2520
	11th to 20th	Day	37% 8% 13% 24% 17% 1	2520
	21st to end	Day	37% 9% 16% 23% 14% 1	2520
July	1st to 10th	Day	30% 8% 15% 30% 13% 2	2451
	11th to 20th	Day	32% 10% 16% 25% 16% 2	2445
	21st to end	Day	30% 8% 19% 26% 15% 3	2591
August	1st to 10th	Day	32% 9% 18% 28% 11% 1	2471
	11th to 20th	Day	36% 8% 15% 27% 12% 1	2178
	21st to end	Day	33% 8% 14% 24% 19% 2	2376
September	1st to 10th	Day	38% 8% 12% 24% 17% 2	2160
	11th to 20th	Day	33% 7% 13% 30% 15% 1	2101
	21st to end	Day	39% 6% 12% 30% 12% 1	1980
October	1st to 10th	Day	39% 7% 10% 28% 15% 1	1840
	11th to 20th	Day	50% 5% 13% 22% 10% 1	1800
	21st to end	Day	49% 5% 11% 22% 12% 1	1806
November	1st to 10th	Day	40% 5% 15% 24% 14% 2	1672
	11th to 20th	Day	52% 6% 12% 20% 9% 1	1692
	21st to end	Day	50% 5% 16% 22% 7%	1520
December	1st to 10th	Day	54% 7% 15% 16% 7% 1	1520
	11th to 20th	Day	55% 6% 15% 15% 7% 3	1915
	21st to end	Day	71% 7% 8% 8% 5%	2783
January	1st to 10th	Night	51% 6% 10% 21% 11% 1	2648
	11th to 20th	Night	58% 7% 12% 15% 8%	2547
	21st to end	Night	59% 6% 11% 14% 9% 1	2703
February	1st to 10th	Night	53% 5% 11% 18% 11% 2	2340
	11th to 20th	Night	47% 6% 12% 21% 13% 1	2162
	21st to end	Night	50% 5% 11% 19% 11% 3	1776
March	1st to 10th	Night	48% 5% 15% 20% 9% 3	2160
	11th to 20th	Night	40% 7% 13% 22% 16% 2	2070
	21st to end	Night	52% 6% 10% 20% 11% 1	2008
April	1st to 10th	Night	47% 8% 10% 19% 14% 1	1800
	11th to 20th	Night	48% 5% 8% 20% 16% 3	1775
	21st to end	Night	43% 8% 12% 22% 12% 2	1733
May	1st to 10th	Night	46% 8% 13% 20% 12%	1536
	11th to 20th	Night	44% 8% 13% 21% 13% 1	1417
	21st to end	Night	38% 8% 14% 24% 15% 2	1552
June	1st to 10th	Night	33% 7% 13% 28% 18% 1	1440
	11th to 20th	Night	34% 7% 11% 24% 23% 2	1440
	21st to end	Night	31% 7% 17% 25% 18% 3	1440
July	1st to 10th	Night	25% 7% 15% 31% 19% 3	1400
	11th to 20th	Night	27% 8% 12% 29% 21% 3	1392
	21st to end	Night	25% 9% 16% 29% 16% 5%	1487
August	1st to 10th	Night	29% 9% 17% 27% 16% 3	1441
	11th to 20th	Night	30% 6% 15% 30% 17% 1	1721
	21st to end	Night	29% 7% 14% 27% 20% 3	1980
September	1st to 10th	Night	30% 8% 12% 25% 23% 2	1800
	11th to 20th	Night	32% 6% 11% 32% 18% 2	1840
	21st to end	Night	34% 6% 11% 29% 18% 1	1980
October	1st to 10th	Night	35% 6% 10% 28% 19% 1	2117
	11th to 20th	Night	43% 5% 12% 22% 17% 1	2160
	21st to end	Night	47% 7% 12% 22% 11% 1	2548
November	1st to 10th	Night	38% 6% 11% 26% 16% 2	2406
	11th to 20th	Night	49% 7% 13% 19% 11% 1	2484
	21st to end	Night	47% 6% 14% 23% 9% 1	2660
December	1st to 10th	Night	54% 6% 13% 19% 8% 1	2660
	11th to 20th	Night	57% 6% 11% 17% 7% 1	3281
	21st to end	Night	65% 6% 10% 14% 4	4884

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% 12% 35% 24% 9% 3	270
	11th to 20th	Day	17% 13% 40% 21% 9% 1	245
	21st to end	Day	19% 11% 35% 27% 7% 1	264
February	1st to 10th	Day	15% 15% 42% 24% 3 1	240
	11th to 20th	Day	21% 18% 30% 22% 8%	240
	21st to end	Day	20% 16% 40% 22% 2 1	198
March	1st to 10th	Day	15% 15% 41% 24% 5%	240
	11th to 20th	Day	17% 16% 33% 23% 6% 5%	240
	21st to end	Day	28% 11% 27% 24% 9% 2	434
April	1st to 10th	Day	22% 16% 35% 23% 5%	420
	11th to 20th	Day	23% 17% 26% 20% 11% 3	412
	21st to end	Day	23% 12% 27% 24% 11% 2	420
May	1st to 10th	Day	20% 12% 32% 27% 9% 1	503
	11th to 20th	Day	22% 10% 31% 23% 11% 3	625
	21st to end	Day	17% 9% 28% 23% 16% 7%	679
June	1st to 10th	Day	8% 7% 30% 33% 18% 5%	630
	11th to 20th	Day	14% 7% 29% 28% 17% 6%	630
	21st to end	Day	13% 7% 29% 29% 18% 5%	630
July	1st to 10th	Day	10% 3 18% 29% 27% 14%	621
	11th to 20th	Day	8% 6% 27% 30% 20% 9%	618
	21st to end	Day	7% 3 26% 35% 23% 6%	684
August	1st to 10th	Day	9% 3 20% 34% 25% 10%	626
	11th to 20th	Day	13% 4 17% 29% 23% 14%	492
	21st to end	Day	9% 8% 23% 31% 20% 11%	495
September	1st to 10th	Day	16% 7% 21% 30% 18% 8%	450
	11th to 20th	Day	13% 5% 20% 29% 24% 8%	450
	21st to end	Day	16% 11% 28% 24% 16% 6%	450
October	1st to 10th	Day	19% 11% 24% 27% 15% 4	310
	11th to 20th	Day	24% 18% 33% 17% 7%	270
	21st to end	Day	26% 16% 33% 19% 5% 1	297
November	1st to 10th	Day	13% 17% 38% 24% 7% 2	270
	11th to 20th	Day	18% 17% 35% 22% 7%	270
	21st to end	Day	22% 12% 41% 19% 5%	270
December	1st to 10th	Day	17% 16% 42% 19% 6% 1	270
	11th to 20th	Day	18% 10% 39% 22% 10% 1	270
	21st to end	Day	25% 17% 40% 15% 3	297
January	1st to 10th	Night	16% 16% 36% 23% 8% 1	950
	11th to 20th	Night	20% 12% 36% 26% 5% 1	910
	21st to end	Night	20% 12% 36% 26% 6% 1	990
February	1st to 10th	Night	18% 11% 41% 25% 5%	900
	11th to 20th	Night	17% 15% 36% 27% 5%	766
	21st to end	Night	23% 14% 37% 21% 4 1	592
March	1st to 10th	Night	21% 10% 35% 26% 8% 1	720
	11th to 20th	Night	17% 10% 35% 29% 8%	630
	21st to end	Night	22% 10% 30% 29% 8% 1	594
April	1st to 10th	Night	20% 14% 36% 25% 5%	540
	11th to 20th	Night	24% 13% 26% 32% 7%	533
	21st to end	Night	21% 9% 31% 29% 8% 1	540
May	1st to 10th	Night	21% 9% 33% 29% 8% 1	540
	11th to 20th	Night	19% 7% 33% 31% 10%	531
	21st to end	Night	15% 5% 26% 38% 15% 1	584
June	1st to 10th	Night	11% 4 29% 42% 15%	540
	11th to 20th	Night	16% 4 28% 38% 14%	540
	21st to end	Night	9% 4 26% 43% 16% 1	540
July	1st to 10th	Night	8% 3 14% 40% 31% 3	525
	11th to 20th	Night	7% 3 24% 42% 23% 2	522
	21st to end	Night	6% 2 20% 49% 22% 2	591
August	1st to 10th	Night	5% 3 17% 47% 26% 2	537
	11th to 20th	Night	10% 3 14% 42% 25% 5%	534
	21st to end	Night	10% 2 19% 44% 23% 2	594
September	1st to 10th	Night	11% 5% 23% 34% 21% 5%	540
	11th to 20th	Night	13% 3 24% 34% 22% 3	580
	21st to end	Night	14% 7% 26% 33% 19% 1	720
October	1st to 10th	Night	16% 7% 27% 32% 17% 1	720
	11th to 20th	Night	21% 12% 29% 29% 8%	720
	21st to end	Night	23% 13% 29% 27% 7% 1	792
November	1st to 10th	Night	15% 11% 37% 26% 9% 1	740
	11th to 20th	Night	20% 15% 38% 22% 5%	774
	21st to end	Night	21% 15% 33% 23% 7%	950
December	1st to 10th	Night	21% 14% 38% 21% 6%	950
	11th to 20th	Night	22% 13% 36% 23% 6% 1	950
	21st to end	Night	28% 13% 35% 21% 3	1045

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