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 2024-11-08.

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	12% 8% 3 8% 5% 4 5% 4 6% 7% 38%	101496
	Night	13% 9% 3 8% 5% 4 4 4 5% 6% 38%	97691
Transparency	Day	43% 7% 13% 24% 13% 1	105023
	Night	44% 6% 12% 23% 14% 2	102304
Seeing	Day	16% 10% 29% 26% 14% 5%	15926
	Night	17% 10% 31% 30% 11% 1	25774

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% 13% 6% 15% 7% 6% 5% 2 5% 5% 26%	2527
	11th to 20th	Day	9% 12% 5% 13% 7% 5% 6% 8% 7% 9% 21%	2530
	21st to end	Day	16% 15% 8% 15% 4 4 3 3 4 5% 21%	2893
February	1st to 10th	Day	12% 12% 5% 10% 5% 5% 4 4 5% 7% 31%	1710
	11th to 20th	Day	12% 10% 3 8% 5% 5% 5% 4 4 6% 36%	1782
	21st to end	Day	11% 12% 3 11% 6% 5% 5% 4 5% 6% 31%	1570
March	1st to 10th	Day	13% 7% 3 5% 4 4 4 4 5% 7% 45%	3120
	11th to 20th	Day	11% 5% 2 8% 5% 5% 6% 4 6% 7% 43%	3350
	21st to end	Day	21% 12% 3 10% 5% 4 4 3 4 6% 28%	4123
April	1st to 10th	Day	17% 13% 4 7% 5% 4 4 3 6% 7% 30%	2280
	11th to 20th	Day	17% 9% 3 7% 6% 4 5% 4 6% 7% 32%	2254
	21st to end	Day	19% 9% 3 8% 5% 4 4 4 5% 6% 33%	2351
May	1st to 10th	Day	14% 7% 2 7% 5% 4 5% 4 7% 7% 37%	4232
	11th to 20th	Day	13% 7% 2 7% 4 4 4 3 5% 7% 45%	4399
	21st to end	Day	13% 8% 3 6% 4 4 5% 4 6% 7% 39%	4824
June	1st to 10th	Day	10% 5% 2 6% 5% 4 5% 4 6% 8% 45%	2660
	11th to 20th	Day	11% 6% 3 6% 4 4 4 3 6% 10% 42%	2660
	21st to end	Day	11% 6% 2 6% 4 4 5% 4 8% 10% 40%	2660
July	1st to 10th	Day	10% 7% 3 7% 5% 5% 6% 5% 8% 10% 35%	4440
	11th to 20th	Day	4 4 2 5% 4 4 5% 5% 7% 10% 51%	4440
	21st to end	Day	8% 6% 2 6% 5% 5% 5% 4 7% 9% 42%	4813
August	1st to 10th	Day	9% 6% 2 6% 4 5% 5% 4 7% 9% 41%	4374
	11th to 20th	Day	11% 5% 2 6% 4 4 4 4 5% 8% 49%	3821
	21st to end	Day	10% 6% 2 6% 4 4 5% 4 6% 9% 44%	4180
September	1st to 10th	Day	13% 5% 2 6% 5% 4 5% 4 5% 7% 44%	2280
	11th to 20th	Day	12% 5% 2 5% 3 3 4 3 5% 8% 49%	2217
	21st to end	Day	12% 7% 3 7% 4 4 4 4 5% 7% 45%	2090
October	1st to 10th	Day	13% 6% 3 6% 4 4 4 3 5% 6% 47%	1942
	11th to 20th	Day	20% 9% 2 7% 5% 4 4 4 5% 7% 34%	1900
	21st to end	Day	17% 11% 3 7% 4 4 3 3 4 5% 39%	1906
November	1st to 10th	Day	12% 9% 3 6% 4 4 4 3 5% 7% 43%	1735
	11th to 20th	Day	12% 12% 5% 9% 7% 4 5% 4 5% 7% 30%	1692
	21st to end	Day	15% 12% 4 8% 5% 4 4 3 4 6% 36%	1520
December	1st to 10th	Day	13% 14% 5% 8% 7% 5% 4 4 5% 7% 29%	1520
	11th to 20th	Day	12% 14% 4 11% 7% 3 4 4 6% 7% 28%	1918
	21st to end	Day	19% 19% 8% 13% 6% 5% 4 3 4 5% 14%	2783
January	1st to 10th	Night	11% 12% 7% 12% 5% 4 4 3 4 4 31%	4428
	11th to 20th	Night	11% 13% 6% 15% 6% 5% 5% 4 6% 7% 21%	4440
	21st to end	Night	15% 18% 9% 11% 5% 4 3 3 4 4 23%	4759
February	1st to 10th	Night	11% 13% 4 9% 6% 5% 4 4 5% 5% 33%	2470
	11th to 20th	Night	11% 10% 3 8% 5% 4 4 4 5% 5% 40%	2285
	21st to end	Night	16% 12% 4 8% 5% 3 4 3 4 7% 35%	1884
March	1st to 10th	Night	15% 10% 2 6% 4 4 4 4 4 5% 42%	3733
	11th to 20th	Night	10% 6% 2 5% 4 5% 5% 4 6% 9% 44%	3640
	21st to end	Night	22% 11% 3 10% 4 4 4 3 3 5% 30%	3508
April	1st to 10th	Night	17% 11% 3 6% 5% 4 4 4 4 7% 36%	1900
	11th to 20th	Night	19% 8% 2 6% 5% 4 2 3 5% 6% 38%	1875
	21st to end	Night	17% 8% 2 6% 3 4 4 5% 6% 7% 38%	1829
May	1st to 10th	Night	16% 8% 3 7% 4 4 5% 3 7% 7% 36%	2664
	11th to 20th	Night	13% 5% 2 6% 5% 5% 6% 4 5% 6% 44%	2506
	21st to end	Night	11% 9% 2 7% 5% 4 4 4 5% 8% 41%	2743
June	1st to 10th	Night	7% 5% 3 6% 5% 5% 4 4 6% 9% 46%	1520
	11th to 20th	Night	12% 6% 2 5% 4 3 4 3 5% 9% 47%	1520
	21st to end	Night	8% 7% 2 5% 4 4 3 4 5% 8% 49%	1520
July	1st to 10th	Night	14% 7% 2 5% 4 4 4 3 6% 8% 43%	2530
	11th to 20th	Night	4 3 1 5% 4 5% 4 4 5% 8% 56%	2530
	21st to end	Night	6% 4 2 6% 4 5% 5% 5% 7% 7% 48%	2750
August	1st to 10th	Night	9% 4 2 5% 4 4 4 4 6% 8% 49%	2563
	11th to 20th	Night	9% 6% 2 6% 4 4 4 3 5% 7% 49%	3009
	21st to end	Night	9% 5% 1 6% 4 4 4 3 5% 9% 50%	3476
September	1st to 10th	Night	9% 5% 2 5% 3 3 3 3 5% 7% 55%	1900
	11th to 20th	Night	12% 4 2 5% 3 4 4 4 5% 6% 52%	1943
	21st to end	Night	10% 6% 2 5% 5% 3 4 3 5% 7% 49%	2090
October	1st to 10th	Night	11% 6% 2 6% 3 3 3 3 5% 6% 50%	2235
	11th to 20th	Night	16% 7% 2 6% 5% 3 4 2 5% 7% 42%	2280
	21st to end	Night	18% 8% 3 7% 4 4 4 4 6% 7% 35%	2690
November	1st to 10th	Night	13% 8% 2 6% 5% 4 3 3 5% 6% 44%	2504
	11th to 20th	Night	14% 11% 3 9% 5% 4 5% 4 6% 6% 34%	2484
	21st to end	Night	17% 10% 3 6% 4 4 5% 4 5% 5% 37%	2660
December	1st to 10th	Night	13% 13% 4 9% 5% 5% 4 4 6% 6% 31%	2660
	11th to 20th	Night	15% 14% 6% 9% 6% 4 5% 4 5% 5% 28%	3279
	21st to end	Night	20% 18% 5% 10% 5% 4 4 4 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	59% 7% 13% 15% 6%	2528
	11th to 20th	Day	57% 8% 13% 16% 6%	2530
	21st to end	Day	65% 5% 10% 14% 5%	2893
February	1st to 10th	Day	51% 5% 14% 19% 9% 1	1710
	11th to 20th	Day	47% 7% 13% 22% 10% 2	1791
	21st to end	Day	52% 7% 10% 18% 10% 2	1570
March	1st to 10th	Day	38% 7% 12% 28% 13% 2	3120
	11th to 20th	Day	38% 7% 13% 28% 12% 1	3350
	21st to end	Day	57% 7% 12% 16% 7% 2	4123
April	1st to 10th	Day	53% 7% 10% 19% 10%	2280
	11th to 20th	Day	48% 7% 10% 20% 12% 2	2254
	21st to end	Day	49% 7% 12% 19% 11% 1	2351
May	1st to 10th	Day	42% 8% 13% 21% 15% 1	4270
	11th to 20th	Day	38% 8% 11% 29% 14%	4403
	21st to end	Day	39% 10% 12% 22% 15% 3	4837
June	1st to 10th	Day	35% 8% 15% 25% 15% 2	2660
	11th to 20th	Day	38% 8% 13% 24% 17% 1	2660
	21st to end	Day	38% 9% 16% 23% 13% 2	2660
July	1st to 10th	Day	42% 8% 15% 26% 8% 2	4371
	11th to 20th	Day	26% 8% 17% 31% 16% 1	4365
	21st to end	Day	36% 8% 15% 20% 18% 2	4662
August	1st to 10th	Day	37% 7% 16% 28% 11% 1	4359
	11th to 20th	Day	33% 7% 14% 33% 12% 1	3859
	21st to end	Day	35% 6% 14% 27% 16% 1	4179
September	1st to 10th	Day	37% 8% 12% 25% 17% 1	2280
	11th to 20th	Day	32% 6% 13% 31% 16% 1	2217
	21st to end	Day	38% 6% 12% 30% 13% 1	2090
October	1st to 10th	Day	23% 4 6% 40% 25% 1	3228
	11th to 20th	Day	35% 4 10% 29% 21% 2	3140
	21st to end	Day	43% 4 10% 29% 13% 2	3118
November	1st to 10th	Day	41% 5% 15% 23% 13% 2	1735
	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	54% 7% 11% 20% 8% 1	4428
	11th to 20th	Night	61% 6% 10% 14% 9%	4440
	21st to end	Night	65% 5% 10% 14% 5%	4759
February	1st to 10th	Night	51% 5% 12% 18% 12% 2	2470
	11th to 20th	Night	45% 6% 12% 23% 14% 1	2286
	21st to end	Night	49% 5% 12% 20% 11% 3	1884
March	1st to 10th	Night	44% 5% 13% 24% 8% 5%	3733
	11th to 20th	Night	34% 7% 12% 26% 17% 3	3640
	21st to end	Night	58% 5% 8% 17% 11% 2	3508
April	1st to 10th	Night	47% 8% 10% 20% 14% 1	1900
	11th to 20th	Night	47% 5% 8% 20% 17% 3	1875
	21st to end	Night	43% 8% 12% 23% 12% 2	1829
May	1st to 10th	Night	43% 9% 13% 16% 18% 1	2691
	11th to 20th	Night	37% 11% 12% 25% 16% 1	2507
	21st to end	Night	38% 10% 11% 19% 18% 5%	2751
June	1st to 10th	Night	33% 8% 14% 27% 18% 1	1520
	11th to 20th	Night	34% 7% 11% 24% 22% 2	1520
	21st to end	Night	33% 6% 16% 24% 18% 3	1520
July	1st to 10th	Night	39% 6% 16% 22% 13% 3	2490
	11th to 20th	Night	25% 7% 15% 34% 18% 2	2482
	21st to end	Night	31% 9% 15% 22% 20% 3	2662
August	1st to 10th	Night	32% 7% 15% 25% 19% 2	2554
	11th to 20th	Night	33% 6% 14% 32% 15% 1	3037
	21st to end	Night	31% 7% 14% 26% 19% 3	3476
September	1st to 10th	Night	28% 8% 12% 27% 24% 2	1900
	11th to 20th	Night	32% 6% 11% 31% 18% 1	1943
	21st to end	Night	33% 6% 11% 29% 20% 1	2090
October	1st to 10th	Night	25% 4 7% 37% 27% 1	3729
	11th to 20th	Night	32% 4 11% 29% 22% 2	3779
	21st to end	Night	43% 5% 10% 26% 15% 1	4428
November	1st to 10th	Night	39% 6% 11% 26% 15% 2	2504
	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	16% 14% 40% 21% 8% 1	270
	21st to end	Day	21% 10% 34% 27% 7% 1	297
February	1st to 10th	Day	14% 15% 40% 26% 3 2	270
	11th to 20th	Day	19% 19% 34% 21% 7%	270
	21st to end	Day	19% 16% 41% 22% 3	225
March	1st to 10th	Day	16% 14% 43% 23% 5%	270
	11th to 20th	Day	18% 17% 34% 21% 6% 4	270
	21st to end	Day	28% 12% 27% 24% 8% 1	477
April	1st to 10th	Day	23% 15% 35% 23% 4	460
	11th to 20th	Day	24% 18% 25% 20% 10% 2	452
	21st to end	Day	23% 13% 27% 24% 10% 2	460
May	1st to 10th	Day	20% 12% 31% 26% 9% 1	548
	11th to 20th	Day	20% 10% 31% 24% 12% 3	675
	21st to end	Day	17% 9% 29% 23% 16% 6%	734
June	1st to 10th	Day	9% 8% 30% 31% 17% 5%	680
	11th to 20th	Day	14% 7% 31% 28% 16% 5%	680
	21st to end	Day	14% 8% 29% 28% 17% 4	680
July	1st to 10th	Day	10% 4 19% 29% 26% 13%	671
	11th to 20th	Day	8% 6% 27% 30% 20% 10%	668
	21st to end	Day	7% 3 25% 35% 24% 6%	739
August	1st to 10th	Day	8% 3 21% 33% 25% 10%	676
	11th to 20th	Day	13% 4 18% 29% 23% 13%	535
	21st to end	Day	9% 7% 22% 30% 21% 10%	539
September	1st to 10th	Day	14% 7% 22% 29% 18% 8%	490
	11th to 20th	Day	12% 4 20% 30% 26% 8%	490
	21st to end	Day	16% 11% 28% 25% 15% 6%	490
October	1st to 10th	Day	18% 11% 26% 27% 15% 3	342
	11th to 20th	Day	23% 16% 33% 18% 9%	300
	21st to end	Day	25% 17% 34% 19% 5% 1	330
November	1st to 10th	Day	15% 16% 37% 23% 7% 2	291
	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	19% 13% 37% 25% 5% 1	950
	21st to end	Night	20% 12% 35% 26% 6% 1	1045
February	1st to 10th	Night	17% 11% 40% 25% 6%	950
	11th to 20th	Night	16% 15% 36% 27% 5%	810
	21st to end	Night	23% 13% 38% 21% 4 1	628
March	1st to 10th	Night	21% 10% 36% 25% 8% 1	760
	11th to 20th	Night	17% 11% 35% 29% 8%	665
	21st to end	Night	23% 10% 30% 29% 8% 1	627
April	1st to 10th	Night	21% 14% 36% 25% 5%	570
	11th to 20th	Night	23% 14% 27% 30% 6%	563
	21st to end	Night	21% 10% 31% 29% 8% 1	570
May	1st to 10th	Night	21% 10% 33% 29% 7% 1	570
	11th to 20th	Night	18% 7% 32% 32% 10%	561
	21st to end	Night	15% 6% 26% 38% 15% 1	617
June	1st to 10th	Night	10% 4 30% 41% 14%	570
	11th to 20th	Night	16% 4 29% 38% 13%	570
	21st to end	Night	10% 5% 26% 42% 16% 1	570
July	1st to 10th	Night	9% 3 14% 40% 31% 3	555
	11th to 20th	Night	7% 4 23% 41% 23% 2	552
	21st to end	Night	5% 2 19% 49% 22% 2	624
August	1st to 10th	Night	6% 3 17% 47% 25% 2	567
	11th to 20th	Night	10% 3 14% 42% 24% 5%	564
	21st to end	Night	10% 2 19% 44% 24% 2	627
September	1st to 10th	Night	11% 4 23% 35% 22% 5%	570
	11th to 20th	Night	12% 3 23% 35% 23% 3	613
	21st to end	Night	14% 7% 26% 34% 18% 1	760
October	1st to 10th	Night	15% 7% 28% 33% 16% 1	760
	11th to 20th	Night	20% 12% 30% 29% 9%	760
	21st to end	Night	22% 13% 30% 27% 7% 1	836
November	1st to 10th	Night	17% 12% 36% 26% 9% 1	771
	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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