The Decker-Grebner-Van Zandt Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Decker-Grebner-Van Zandt Observatory clear sky chart from 2005-11-06 to 2024-11-02.

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% 3 7% 5% 4 5% 4 6% 7% 38%	101365
	Night	13% 9% 3 7% 4 4 4 3 5% 6% 40%	97685
Transparency	Day	44% 6% 12% 23% 13% 2	101153
	Night	44% 5% 12% 23% 14% 2	97569
Seeing	Day	15% 11% 29% 26% 15% 5%	15766
	Night	16% 9% 31% 30% 12% 1	25585

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	12% 13% 8% 14% 6% 5% 4 3 4 6% 24%	2527
	11th to 20th	Day	12% 12% 5% 13% 8% 6% 7% 5% 7% 5% 20%	2530
	21st to end	Day	17% 19% 10% 12% 5% 4 4 2 3 5% 18%	3104
February	1st to 10th	Day	17% 12% 5% 10% 6% 5% 4 3 4 5% 29%	1710
	11th to 20th	Day	13% 10% 3 8% 7% 5% 5% 3 5% 6% 35%	1810
	21st to end	Day	14% 12% 5% 11% 5% 4 4 3 5% 6% 31%	1570
March	1st to 10th	Day	16% 14% 4 7% 4 4 4 3 4 5% 36%	3120
	11th to 20th	Day	9% 8% 2 6% 4 5% 4 4 7% 7% 44%	3433
	21st to end	Day	19% 11% 3 11% 5% 5% 5% 4 4 6% 26%	4085
April	1st to 10th	Day	15% 12% 4 8% 5% 4 3 3 4 7% 36%	2280
	11th to 20th	Day	16% 9% 3 6% 5% 5% 5% 4 4 6% 38%	2254
	21st to end	Day	20% 7% 3 7% 5% 5% 4 3 5% 6% 35%	2280
May	1st to 10th	Day	16% 7% 3 7% 5% 4 5% 4 7% 9% 33%	4079
	11th to 20th	Day	17% 7% 2 6% 4 4 4 4 5% 7% 40%	4399
	21st to end	Day	11% 6% 2 5% 4 4 5% 4 6% 8% 45%	4824
June	1st to 10th	Day	9% 5% 2 6% 5% 4 4 4 6% 9% 46%	2660
	11th to 20th	Day	8% 4 3 6% 4 5% 5% 4 6% 8% 47%	2660
	21st to end	Day	10% 6% 3 6% 5% 5% 5% 5% 7% 9% 40%	2660
July	1st to 10th	Day	10% 6% 3 7% 5% 5% 6% 5% 8% 10% 34%	4440
	11th to 20th	Day	7% 4 2 5% 4 5% 5% 4 7% 9% 49%	4440
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 7% 10% 42%	4813
August	1st to 10th	Day	6% 5% 2 6% 4 5% 5% 4 8% 11% 44%	4145
	11th to 20th	Day	10% 5% 2 6% 5% 4 5% 5% 8% 9% 40%	3759
	21st to end	Day	5% 4 2 6% 4 4 4 4 6% 9% 50%	4180
September	1st to 10th	Day	11% 5% 2 5% 4 5% 4 4 5% 7% 50%	2280
	11th to 20th	Day	10% 4 2 4 3 3 3 4 5% 8% 53%	2232
	21st to end	Day	9% 6% 2 6% 4 4 4 3 6% 8% 48%	2090
October	1st to 10th	Day	12% 6% 2 6% 3 3 4 3 5% 6% 51%	1929
	11th to 20th	Day	15% 9% 2 6% 4 3 4 2 4 6% 45%	1900
	21st to end	Day	19% 9% 3 6% 3 4 4 3 4 6% 39%	1981
November	1st to 10th	Day	10% 8% 2 6% 4 4 3 3 6% 6% 49%	1681
	11th to 20th	Day	17% 10% 3 8% 3 4 4 4 4 8% 36%	1710
	21st to end	Day	16% 12% 4 8% 4 3 4 3 3 5% 38%	1579
December	1st to 10th	Day	13% 12% 5% 7% 6% 5% 5% 3 4 6% 33%	1520
	11th to 20th	Day	14% 14% 3 10% 5% 5% 4 3 5% 7% 30%	1918
	21st to end	Day	24% 18% 8% 12% 4 4 3 2 3 4 16%	2783
January	1st to 10th	Night	14% 13% 5% 12% 5% 4 4 3 4 5% 32%	4428
	11th to 20th	Night	12% 18% 6% 11% 6% 5% 5% 4 7% 4 22%	4440
	21st to end	Night	21% 17% 7% 10% 5% 3 4 2 4 4 24%	4560
February	1st to 10th	Night	15% 13% 4 8% 6% 4 4 3 5% 6% 32%	2470
	11th to 20th	Night	14% 10% 4 8% 5% 5% 4 3 4 6% 38%	2254
	21st to end	Night	16% 11% 4 8% 4 3 3 3 4 5% 39%	1884
March	1st to 10th	Night	18% 12% 3 5% 4 4 4 3 3 5% 40%	3733
	11th to 20th	Night	11% 8% 3 5% 3 3 3 3 4 6% 49%	3557
	21st to end	Night	21% 11% 5% 10% 5% 4 5% 3 5% 5% 27%	3544
April	1st to 10th	Night	14% 10% 2 7% 4 4 3 3 6% 7% 40%	1900
	11th to 20th	Night	16% 9% 3 5% 4 3 4 3 5% 5% 43%	1875
	21st to end	Night	18% 7% 3 6% 5% 4 3 3 6% 7% 38%	1900
May	1st to 10th	Night	18% 8% 3 8% 5% 5% 4 4 6% 7% 33%	2797
	11th to 20th	Night	14% 6% 2 6% 5% 5% 5% 4 6% 8% 40%	2506
	21st to end	Night	8% 6% 2 6% 4 4 4 5% 6% 8% 46%	2743
June	1st to 10th	Night	9% 5% 2 5% 4 4 4 4 6% 8% 51%	1520
	11th to 20th	Night	7% 6% 1 6% 5% 4 5% 3 5% 10% 48%	1520
	21st to end	Night	9% 6% 2 7% 4 4 5% 4 5% 8% 46%	1520
July	1st to 10th	Night	11% 6% 3 6% 4 4 5% 5% 6% 9% 40%	2530
	11th to 20th	Night	4 3 2 4 3 4 3 4 6% 8% 59%	2530
	21st to end	Night	6% 6% 2 6% 5% 5% 5% 5% 7% 9% 46%	2750
August	1st to 10th	Night	5% 4 2 3 4 4 5% 4 7% 9% 54%	2792
	11th to 20th	Night	6% 4 2 5% 4 4 4 4 6% 8% 53%	3070
	21st to end	Night	5% 4 1 5% 4 4 4 3 6% 9% 53%	3476
September	1st to 10th	Night	12% 4 2 5% 3 2 3 3 4 5% 56%	1900
	11th to 20th	Night	9% 5% 1 4 4 3 4 3 6% 7% 54%	1928
	21st to end	Night	9% 4 2 6% 4 4 5% 4 6% 8% 48%	2090
October	1st to 10th	Night	10% 5% 2 5% 4 4 3 3 4 6% 54%	2249
	11th to 20th	Night	13% 6% 2 6% 3 2 4 3 5% 6% 50%	2280
	21st to end	Night	18% 8% 3 6% 5% 4 3 3 5% 6% 40%	2617
November	1st to 10th	Night	11% 7% 2 5% 3 3 3 3 5% 7% 52%	2431
	11th to 20th	Night	13% 8% 3 7% 5% 4 4 3 5% 7% 42%	2470
	21st to end	Night	17% 10% 4 7% 3 3 3 4 4 5% 39%	2598
December	1st to 10th	Night	15% 14% 3 8% 5% 3 4 3 5% 6% 36%	2660
	11th to 20th	Night	15% 12% 4 9% 5% 4 3 3 5% 6% 36%	3279
	21st to end	Night	23% 18% 7% 11% 6% 4 4 2 4 4 17%	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	61% 5% 11% 18% 6%	2528
	11th to 20th	Day	64% 5% 10% 13% 8%	2530
	21st to end	Day	70% 4 9% 12% 5%	3104
February	1st to 10th	Day	58% 4 13% 17% 7% 2	1710
	11th to 20th	Day	50% 5% 12% 21% 10% 1	1820
	21st to end	Day	53% 5% 10% 22% 8% 2	1570
March	1st to 10th	Day	51% 5% 11% 19% 13% 2	3120
	11th to 20th	Day	36% 6% 14% 27% 16% 1	3433
	21st to end	Day	57% 7% 15% 18% 3 1	4085
April	1st to 10th	Day	49% 5% 11% 23% 12%	2280
	11th to 20th	Day	47% 5% 9% 24% 12% 3	2254
	21st to end	Day	50% 6% 10% 18% 14% 2	2280
May	1st to 10th	Day	44% 8% 14% 22% 12%	4117
	11th to 20th	Day	44% 6% 11% 30% 9%	4403
	21st to end	Day	35% 8% 10% 22% 22% 2	4837
June	1st to 10th	Day	34% 7% 14% 27% 15% 3	2660
	11th to 20th	Day	34% 7% 11% 23% 22% 3	2660
	21st to end	Day	38% 8% 14% 22% 15% 3	2660
July	1st to 10th	Day	42% 7% 14% 23% 12% 2	4371
	11th to 20th	Day	29% 7% 17% 28% 18% 1	4365
	21st to end	Day	36% 9% 18% 19% 16% 2	4662
August	1st to 10th	Day	32% 7% 18% 30% 12% 1	4130
	11th to 20th	Day	37% 7% 15% 25% 16% 1	3795
	21st to end	Day	30% 6% 13% 30% 17% 3	4179
September	1st to 10th	Day	35% 5% 10% 25% 20% 5%	2280
	11th to 20th	Day	29% 6% 11% 26% 24% 4	2232
	21st to end	Day	34% 6% 10% 27% 21% 2	2090
October	1st to 10th	Day	34% 5% 10% 27% 22% 2	1929
	11th to 20th	Day	43% 4 9% 23% 20% 2	1900
	21st to end	Day	46% 5% 11% 23% 12% 3	1981
November	1st to 10th	Day	35% 5% 15% 25% 17% 3	1681
	11th to 20th	Day	48% 4 11% 23% 13% 1	1710
	21st to end	Day	49% 5% 12% 23% 10%	1579
December	1st to 10th	Day	52% 5% 13% 21% 9% 1	1520
	11th to 20th	Day	53% 5% 10% 24% 7% 1	1915
	21st to end	Day	72% 4 8% 10% 5%	2783
January	1st to 10th	Night	54% 5% 12% 20% 8%	4428
	11th to 20th	Night	62% 3 11% 15% 8% 1	4440
	21st to end	Night	65% 3 10% 16% 6% 1	4560
February	1st to 10th	Night	54% 4 11% 18% 11% 1	2470
	11th to 20th	Night	47% 4 12% 24% 11% 2	2255
	21st to end	Night	48% 3 9% 24% 13% 3	1884
March	1st to 10th	Night	47% 7% 9% 24% 12% 1	3733
	11th to 20th	Night	35% 5% 9% 29% 18% 4	3557
	21st to end	Night	57% 7% 12% 18% 6% 1	3544
April	1st to 10th	Night	43% 5% 11% 25% 14% 2	1900
	11th to 20th	Night	42% 5% 8% 25% 16% 4	1875
	21st to end	Night	46% 5% 12% 20% 14% 3	1900
May	1st to 10th	Night	48% 7% 12% 19% 12% 1	2824
	11th to 20th	Night	41% 8% 14% 26% 11%	2507
	21st to end	Night	33% 7% 13% 22% 22% 3	2751
June	1st to 10th	Night	31% 6% 14% 25% 19% 5%	1520
	11th to 20th	Night	31% 6% 13% 23% 22% 5%	1520
	21st to end	Night	36% 8% 12% 21% 19% 3	1520
July	1st to 10th	Night	40% 6% 13% 25% 13% 2	2490
	11th to 20th	Night	23% 5% 13% 31% 25% 2	2482
	21st to end	Night	31% 9% 16% 21% 18% 4	2662
August	1st to 10th	Night	24% 7% 18% 31% 18% 2	2783
	11th to 20th	Night	29% 6% 15% 31% 17% 1	3100
	21st to end	Night	26% 7% 14% 33% 16% 4	3476
September	1st to 10th	Night	29% 5% 10% 26% 24% 5%	1900
	11th to 20th	Night	28% 5% 11% 27% 23% 5%	1928
	21st to end	Night	33% 6% 11% 25% 21% 4	2090
October	1st to 10th	Night	32% 5% 10% 24% 25% 4	2249
	11th to 20th	Night	35% 5% 9% 26% 22% 3	2280
	21st to end	Night	45% 5% 10% 21% 17% 2	2617
November	1st to 10th	Night	32% 6% 12% 26% 19% 4	2431
	11th to 20th	Night	42% 4 12% 24% 16% 2	2470
	21st to end	Night	47% 4 10% 25% 13% 1	2598
December	1st to 10th	Night	49% 5% 13% 23% 9% 1	2660
	11th to 20th	Night	51% 4 11% 25% 8% 1	3281
	21st to end	Night	72% 4 7% 12% 5%	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	21% 19% 29% 26% 6%	270
	11th to 20th	Day	24% 20% 38% 15% 3	270
	21st to end	Day	21% 20% 34% 20% 6%	297
February	1st to 10th	Day	23% 17% 40% 16% 3	270
	11th to 20th	Day	23% 18% 33% 19% 6% 1	270
	21st to end	Day	21% 15% 48% 15% 1	225
March	1st to 10th	Day	23% 24% 34% 16% 2	270
	11th to 20th	Day	23% 21% 35% 14% 7%	270
	21st to end	Day	29% 15% 33% 17% 4 1	452
April	1st to 10th	Day	23% 13% 35% 24% 5% 1	460
	11th to 20th	Day	19% 17% 34% 17% 12% 2	452
	21st to end	Day	21% 15% 36% 19% 8% 1	460
May	1st to 10th	Day	19% 13% 33% 26% 6% 3	482
	11th to 20th	Day	20% 11% 29% 25% 12% 3	675
	21st to end	Day	14% 8% 30% 25% 15% 7%	734
June	1st to 10th	Day	8% 7% 26% 33% 20% 6%	680
	11th to 20th	Day	6% 6% 30% 33% 22% 4	680
	21st to end	Day	11% 9% 31% 30% 15% 4	680
July	1st to 10th	Day	10% 4 18% 32% 22% 14%	671
	11th to 20th	Day	9% 3 22% 34% 23% 9%	668
	21st to end	Day	7% 3 22% 37% 26% 6%	739
August	1st to 10th	Day	6% 3 21% 37% 24% 9%	676
	11th to 20th	Day	11% 5% 20% 27% 24% 14%	497
	21st to end	Day	6% 4 17% 31% 32% 10%	539
September	1st to 10th	Day	14% 5% 17% 31% 28% 5%	490
	11th to 20th	Day	9% 2 19% 32% 27% 10%	490
	21st to end	Day	9% 11% 26% 25% 26% 3	490
October	1st to 10th	Day	15% 8% 27% 28% 18% 4	329
	11th to 20th	Day	17% 21% 33% 21% 8%	300
	21st to end	Day	31% 14% 30% 20% 5% 1	330
November	1st to 10th	Day	10% 12% 44% 20% 11% 4	273
	11th to 20th	Day	19% 19% 33% 20% 8% 1	270
	21st to end	Day	21% 14% 39% 23% 2	270
December	1st to 10th	Day	14% 19% 40% 19% 5% 3	270
	11th to 20th	Day	16% 15% 39% 24% 6%	270
	21st to end	Day	22% 23% 39% 14% 2	297
January	1st to 10th	Night	16% 16% 40% 21% 6%	950
	11th to 20th	Night	20% 14% 40% 21% 5%	950
	21st to end	Night	21% 13% 36% 26% 3	1045
February	1st to 10th	Night	22% 12% 38% 24% 4	950
	11th to 20th	Night	18% 15% 36% 27% 3	779
	21st to end	Night	22% 17% 39% 18% 3	628
March	1st to 10th	Night	21% 13% 39% 22% 4	760
	11th to 20th	Night	18% 11% 33% 31% 6%	621
	21st to end	Night	24% 12% 37% 22% 6%	627
April	1st to 10th	Night	19% 14% 35% 26% 5%	570
	11th to 20th	Night	20% 12% 33% 26% 9%	563
	21st to end	Night	20% 10% 33% 29% 6%	570
May	1st to 10th	Night	16% 7% 39% 31% 6% 1	570
	11th to 20th	Night	15% 6% 31% 37% 10% 1	561
	21st to end	Night	12% 4 29% 36% 17% 2	617
June	1st to 10th	Night	10% 4 26% 43% 16% 1	570
	11th to 20th	Night	11% 3 27% 40% 18% 1	570
	21st to end	Night	9% 4 30% 40% 17% 1	570
July	1st to 10th	Night	8% 3 15% 38% 33% 4	555
	11th to 20th	Night	6% 1 21% 43% 26% 3	552
	21st to end	Night	6% 1 14% 46% 31% 1	624
August	1st to 10th	Night	6% 2 16% 43% 28% 5%	567
	11th to 20th	Night	6% 2 15% 44% 28% 5%	564
	21st to end	Night	6% 2 16% 39% 35% 2	627
September	1st to 10th	Night	12% 3 18% 37% 27% 3	570
	11th to 20th	Night	11% 2 18% 36% 29% 4	598
	21st to end	Night	11% 7% 25% 34% 22% 2	760
October	1st to 10th	Night	12% 5% 28% 34% 19% 2	760
	11th to 20th	Night	16% 13% 31% 28% 11% 1	760
	21st to end	Night	23% 11% 35% 23% 7%	836
November	1st to 10th	Night	15% 10% 40% 26% 9% 1	748
	11th to 20th	Night	17% 14% 37% 26% 6%	760
	21st to end	Night	21% 11% 38% 24% 5%	888
December	1st to 10th	Night	21% 13% 37% 24% 5%	950
	11th to 20th	Night	21% 12% 35% 27% 5%	950
	21st to end	Night	26% 15% 35% 22% 2	1045

Page generated at 2024-10-30 12:19:21. Version ac07

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