Nick Drew Observatory
Clear Sky Chart
History

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

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% 7% 3 6% 4 4 4 4 6% 8% 42%	101222
	Night	13% 8% 3 7% 4 4 4 3 5% 7% 42%	97904
Transparency	Day	39% 6% 12% 25% 17% 2	104827
	Night	40% 5% 11% 24% 18% 3	102440
Seeing	Day	14% 8% 25% 27% 19% 7%	15641
	Night	16% 8% 28% 32% 15% 2	25446

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% 10% 5% 10% 5% 5% 4 3 6% 6% 31%	2527
	11th to 20th	Day	14% 9% 4 8% 7% 8% 4 4 6% 6% 31%	2636
	21st to end	Day	22% 19% 6% 7% 3 3 3 2 4 4 28%	3135
February	1st to 10th	Day	16% 11% 4 7% 4 4 4 4 5% 6% 35%	1724
	11th to 20th	Day	15% 9% 3 7% 4 5% 5% 4 6% 7% 35%	1848
	21st to end	Day	15% 11% 4 7% 5% 3 3 4 4 6% 38%	1570
March	1st to 10th	Day	19% 12% 3 6% 3 3 3 3 4 4 39%	3120
	11th to 20th	Day	11% 8% 3 5% 3 4 4 4 5% 7% 46%	3446
	21st to end	Day	16% 9% 3 8% 5% 5% 5% 3 6% 7% 33%	4066
April	1st to 10th	Day	12% 7% 2 6% 5% 5% 5% 4 5% 8% 41%	2280
	11th to 20th	Day	16% 7% 3 7% 5% 4 3 3 4 5% 42%	2254
	21st to end	Day	17% 7% 3 7% 4 4 4 3 5% 7% 39%	2280
May	1st to 10th	Day	13% 6% 2 7% 5% 6% 5% 5% 8% 11% 31%	3947
	11th to 20th	Day	16% 8% 3 7% 5% 4 4 3 5% 8% 37%	4291
	21st to end	Day	9% 6% 2 6% 5% 4 5% 5% 6% 8% 45%	4824
June	1st to 10th	Day	8% 5% 2 6% 4 5% 5% 4 6% 9% 47%	2660
	11th to 20th	Day	8% 4 2 5% 4 5% 5% 4 6% 8% 49%	2660
	21st to end	Day	5% 5% 2 6% 5% 4 6% 5% 8% 11% 45%	2660
July	1st to 10th	Day	12% 5% 2 7% 4 5% 5% 5% 7% 9% 40%	4440
	11th to 20th	Day	4 4 2 6% 4 5% 6% 5% 7% 11% 47%	4440
	21st to end	Day	7% 5% 3 6% 6% 5% 6% 5% 8% 10% 40%	4691
August	1st to 10th	Day	5% 4 2 4 4 4 5% 4 6% 10% 51%	4026
	11th to 20th	Day	12% 5% 2 5% 4 5% 5% 5% 6% 10% 40%	3729
	21st to end	Day	5% 5% 1 5% 4 5% 4 4 7% 10% 52%	4180
September	1st to 10th	Day	12% 6% 3 4 4 3 3 2 5% 8% 50%	2280
	11th to 20th	Day	8% 6% 3 6% 4 4 4 3 5% 9% 50%	2228
	21st to end	Day	8% 5% 2 5% 4 4 5% 3 5% 7% 51%	2090
October	1st to 10th	Day	10% 4 1 5% 4 3 4 3 5% 6% 56%	1943
	11th to 20th	Day	13% 5% 2 5% 4 3 4 2 5% 6% 50%	1900
	21st to end	Day	18% 7% 2 6% 4 3 3 3 4 6% 46%	2027
November	1st to 10th	Day	11% 6% 1 4 3 3 4 3 4 7% 54%	1717
	11th to 20th	Day	14% 8% 2 5% 4 3 3 3 4 7% 46%	1710
	21st to end	Day	15% 11% 3 8% 4 4 3 2 4 6% 40%	1672
December	1st to 10th	Day	14% 12% 4 7% 4 4 3 4 5% 7% 37%	1520
	11th to 20th	Day	15% 9% 4 7% 5% 4 5% 3 5% 6% 36%	1918
	21st to end	Day	23% 13% 5% 9% 5% 4 4 3 5% 5% 25%	2783
January	1st to 10th	Night	14% 10% 5% 10% 5% 4 4 3 5% 5% 34%	4428
	11th to 20th	Night	13% 12% 5% 11% 6% 6% 7% 4 3 4 29%	4330
	21st to end	Night	25% 14% 5% 9% 4 3 4 2 4 5% 25%	4532
February	1st to 10th	Night	15% 11% 5% 8% 4 4 4 3 5% 7% 33%	2454
	11th to 20th	Night	14% 7% 3 9% 5% 3 5% 3 5% 5% 41%	2218
	21st to end	Night	17% 9% 3 7% 5% 3 3 3 4 5% 39%	1884
March	1st to 10th	Night	20% 11% 2 6% 4 3 4 3 4 4 39%	3733
	11th to 20th	Night	13% 7% 2 4 3 3 3 3 4 7% 51%	3544
	21st to end	Night	17% 9% 3 8% 5% 4 5% 3 5% 6% 35%	3563
April	1st to 10th	Night	12% 7% 2 7% 5% 4 3 3 4 7% 46%	1900
	11th to 20th	Night	14% 7% 3 5% 4 4 5% 3 5% 6% 45%	1875
	21st to end	Night	18% 8% 3 5% 4 4 3 3 4 6% 40%	1900
May	1st to 10th	Night	17% 8% 3 6% 5% 4 4 3 6% 9% 36%	2948
	11th to 20th	Night	13% 7% 2 6% 4 3 4 3 5% 7% 44%	2607
	21st to end	Night	7% 6% 3 7% 5% 5% 6% 4 7% 10% 40%	2743
June	1st to 10th	Night	7% 5% 2 5% 4 4 6% 4 6% 9% 49%	1520
	11th to 20th	Night	6% 4 1 5% 4 3 5% 4 6% 8% 54%	1520
	21st to end	Night	5% 4 2 5% 4 4 5% 4 7% 12% 50%	1520
July	1st to 10th	Night	12% 5% 2 5% 3 4 4 4 6% 10% 46%	2530
	11th to 20th	Night	4 3 1 5% 3 3 4 3 6% 11% 55%	2530
	21st to end	Night	7% 6% 2 7% 5% 5% 5% 5% 8% 10% 39%	2867
August	1st to 10th	Night	4 3 2 4 3 3 4 5% 6% 10% 55%	2897
	11th to 20th	Night	9% 5% 2 6% 4 4 5% 3 6% 7% 49%	3100
	21st to end	Night	5% 3 1 3 3 4 4 4 6% 10% 56%	3476
September	1st to 10th	Night	10% 4 2 4 4 3 4 3 6% 7% 54%	1900
	11th to 20th	Night	7% 5% 2 6% 4 4 5% 4 5% 7% 52%	1933
	21st to end	Night	9% 5% 2 5% 4 4 4 4 6% 6% 52%	2090
October	1st to 10th	Night	9% 4 2 4 3 3 4 3 4 7% 58%	2233
	11th to 20th	Night	14% 7% 2 4 2 2 3 2 4 7% 54%	2280
	21st to end	Night	17% 7% 2 6% 3 3 4 3 4 5% 46%	2570
November	1st to 10th	Night	10% 6% 2 4 3 3 4 3 5% 6% 55%	2483
	11th to 20th	Night	13% 6% 2 5% 4 4 3 3 4 7% 48%	2470
	21st to end	Night	18% 8% 3 5% 4 3 3 3 4 5% 44%	2503
December	1st to 10th	Night	16% 11% 3 7% 4 4 4 4 5% 6% 34%	2660
	11th to 20th	Night	17% 7% 3 7% 4 4 3 2 4 6% 42%	3279
	21st to end	Night	21% 18% 4 10% 5% 4 4 3 4 5% 21%	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	51% 6% 9% 24% 9% 1	2528
	11th to 20th	Day	52% 5% 14% 17% 11%	2636
	21st to end	Day	61% 3 9% 17% 9% 1	3135
February	1st to 10th	Day	49% 5% 12% 23% 10% 1	1724
	11th to 20th	Day	48% 5% 10% 21% 14% 3	1858
	21st to end	Day	48% 4 10% 21% 14% 3	1570
March	1st to 10th	Day	50% 4 7% 22% 16% 3	3120
	11th to 20th	Day	36% 6% 13% 24% 16% 4	3446
	21st to end	Day	47% 7% 15% 23% 7% 1	4066
April	1st to 10th	Day	41% 6% 11% 25% 16% 1	2280
	11th to 20th	Day	45% 4 9% 24% 15% 3	2254
	21st to end	Day	45% 6% 12% 23% 12% 1	2280
May	1st to 10th	Day	43% 9% 15% 24% 8% 1	3982
	11th to 20th	Day	45% 6% 11% 25% 11% 1	4295
	21st to end	Day	36% 7% 11% 23% 21% 1	4837
June	1st to 10th	Day	32% 7% 14% 28% 16% 3	2660
	11th to 20th	Day	29% 8% 12% 24% 23% 3	2660
	21st to end	Day	32% 8% 17% 23% 17% 4	2660
July	1st to 10th	Day	38% 7% 10% 29% 13% 3	4371
	11th to 20th	Day	29% 8% 13% 30% 18% 2	4365
	21st to end	Day	36% 8% 18% 26% 12% 1	4543
August	1st to 10th	Day	27% 7% 13% 37% 15%	4012
	11th to 20th	Day	37% 8% 13% 25% 17%	3765
	21st to end	Day	27% 6% 15% 34% 15% 3	4179
September	1st to 10th	Day	34% 5% 10% 26% 21% 5%	2280
	11th to 20th	Day	33% 5% 9% 25% 26% 3	2228
	21st to end	Day	31% 6% 9% 27% 24% 3	2090
October	1st to 10th	Day	18% 3 6% 20% 45% 8%	3229
	11th to 20th	Day	24% 3 7% 21% 38% 7%	3140
	21st to end	Day	33% 3 7% 27% 27% 3	3317
November	1st to 10th	Day	32% 4 12% 28% 20% 5%	1717
	11th to 20th	Day	40% 4 10% 24% 18% 5%	1710
	21st to end	Day	46% 4 9% 26% 14%	1672
December	1st to 10th	Day	47% 6% 11% 24% 12%	1520
	11th to 20th	Day	49% 4 9% 24% 12% 3	1915
	21st to end	Day	61% 5% 10% 15% 8% 2	2783
January	1st to 10th	Night	51% 5% 11% 23% 9% 1	4428
	11th to 20th	Night	58% 4 9% 19% 9% 1	4330
	21st to end	Night	63% 4 8% 17% 7% 1	4532
February	1st to 10th	Night	52% 5% 9% 21% 12% 1	2454
	11th to 20th	Night	45% 4 11% 21% 17% 2	2219
	21st to end	Night	47% 4 11% 21% 15% 3	1884
March	1st to 10th	Night	47% 4 9% 23% 14% 2	3733
	11th to 20th	Night	34% 5% 11% 24% 21% 4	3544
	21st to end	Night	48% 7% 11% 26% 8% 1	3563
April	1st to 10th	Night	39% 4 10% 27% 17% 3	1900
	11th to 20th	Night	39% 5% 9% 24% 19% 4	1875
	21st to end	Night	45% 6% 8% 22% 16% 3	1900
May	1st to 10th	Night	44% 8% 16% 19% 12% 1	2978
	11th to 20th	Night	38% 7% 13% 28% 13% 1	2608
	21st to end	Night	36% 8% 14% 23% 17% 2	2751
June	1st to 10th	Night	30% 8% 15% 25% 19% 4	1520
	11th to 20th	Night	26% 6% 11% 27% 26% 4	1520
	21st to end	Night	26% 7% 16% 27% 18% 7%	1520
July	1st to 10th	Night	34% 5% 11% 30% 16% 3	2490
	11th to 20th	Night	23% 6% 11% 38% 20% 3	2482
	21st to end	Night	37% 7% 18% 23% 13% 1	2776
August	1st to 10th	Night	23% 7% 15% 36% 17% 1	2887
	11th to 20th	Night	35% 6% 11% 25% 23% 1	3130
	21st to end	Night	22% 6% 14% 35% 19% 3	3476
September	1st to 10th	Night	29% 5% 8% 27% 23% 7%	1900
	11th to 20th	Night	31% 5% 9% 27% 23% 5%	1933
	21st to end	Night	31% 6% 8% 22% 28% 5%	2090
October	1st to 10th	Night	17% 3 6% 19% 47% 8%	3727
	11th to 20th	Night	23% 2 7% 22% 39% 7%	3779
	21st to end	Night	32% 4 8% 25% 25% 6%	4230
November	1st to 10th	Night	31% 5% 11% 24% 23% 6%	2483
	11th to 20th	Night	37% 4 9% 25% 21% 4	2470
	21st to end	Night	43% 4 10% 25% 18% 1	2503
December	1st to 10th	Night	49% 5% 11% 22% 12% 1	2660
	11th to 20th	Night	44% 4 11% 25% 15% 1	3281
	21st to end	Night	65% 5% 9% 13% 7% 1	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% 17% 34% 20% 8%	270
	11th to 20th	Day	28% 16% 34% 19% 3	270
	21st to end	Day	20% 15% 42% 17% 6%	297
February	1st to 10th	Day	27% 10% 40% 18% 5%	270
	11th to 20th	Day	24% 18% 31% 22% 4 1	270
	21st to end	Day	22% 17% 40% 19% 1 1	225
March	1st to 10th	Day	19% 16% 42% 20% 4	270
	11th to 20th	Day	25% 16% 37% 15% 7%	270
	21st to end	Day	26% 15% 32% 20% 6%	446
April	1st to 10th	Day	15% 13% 36% 24% 10% 2	460
	11th to 20th	Day	19% 14% 29% 24% 12% 2	452
	21st to end	Day	20% 12% 35% 21% 11% 1	460
May	1st to 10th	Day	17% 10% 31% 29% 11% 3	463
	11th to 20th	Day	17% 7% 26% 27% 17% 5%	606
	21st to end	Day	11% 5% 27% 31% 18% 9%	734
June	1st to 10th	Day	10% 6% 19% 30% 25% 10%	680
	11th to 20th	Day	8% 2 19% 35% 26% 10%	680
	21st to end	Day	5% 6% 25% 34% 22% 8%	680
July	1st to 10th	Day	11% 3 13% 26% 29% 19%	671
	11th to 20th	Day	4 4 12% 37% 31% 13%	668
	21st to end	Day	9% 2 14% 32% 31% 12%	739
August	1st to 10th	Day	8% 2 12% 34% 31% 13%	637
	11th to 20th	Day	10% 4 13% 30% 30% 13%	479
	21st to end	Day	8% 2 10% 27% 37% 17%	539
September	1st to 10th	Day	12% 4 11% 25% 32% 15%	490
	11th to 20th	Day	10% 3 10% 27% 35% 16%	490
	21st to end	Day	9% 9% 19% 26% 24% 12%	490
October	1st to 10th	Day	10% 4 22% 36% 19% 9%	343
	11th to 20th	Day	12% 12% 37% 24% 12% 3	300
	21st to end	Day	28% 14% 30% 21% 7%	330
November	1st to 10th	Day	15% 6% 32% 28% 13% 6%	285
	11th to 20th	Day	17% 18% 33% 20% 11% 2	270
	21st to end	Day	24% 13% 26% 27% 9% 1	270
December	1st to 10th	Day	14% 13% 41% 20% 6% 5%	270
	11th to 20th	Day	21% 14% 33% 24% 6%	270
	21st to end	Day	26% 16% 37% 14% 7%	297
January	1st to 10th	Night	18% 14% 37% 27% 5%	950
	11th to 20th	Night	21% 14% 37% 23% 5%	950
	21st to end	Night	21% 12% 38% 26% 2	1045
February	1st to 10th	Night	20% 14% 35% 27% 2	934
	11th to 20th	Night	19% 15% 39% 25% 2	743
	21st to end	Night	23% 14% 38% 24% 2	628
March	1st to 10th	Night	22% 12% 38% 25% 4	760
	11th to 20th	Night	22% 10% 34% 28% 6%	608
	21st to end	Night	23% 11% 31% 28% 6%	627
April	1st to 10th	Night	16% 12% 37% 29% 6%	570
	11th to 20th	Night	17% 9% 34% 28% 11% 1	563
	21st to end	Night	22% 7% 35% 27% 8% 1	570
May	1st to 10th	Night	18% 6% 31% 34% 11% 1	570
	11th to 20th	Night	16% 4 24% 37% 17% 2	561
	21st to end	Night	8% 3 23% 41% 21% 3	617
June	1st to 10th	Night	8% 2 18% 43% 28%	570
	11th to 20th	Night	8% 1 17% 48% 24% 3	570
	21st to end	Night	7% 1 19% 41% 29% 2	570
July	1st to 10th	Night	11% 1 7% 35% 39% 7%	555
	11th to 20th	Night	5% 2 10% 43% 36% 4	552
	21st to end	Night	8% 1 11% 40% 37% 3	624
August	1st to 10th	Night	9% 1 8% 44% 34% 4	567
	11th to 20th	Night	8% 2 9% 41% 35% 5%	564
	21st to end	Night	7% 1 10% 37% 40% 6%	627
September	1st to 10th	Night	11% 2 10% 38% 31% 7%	570
	11th to 20th	Night	9% 1 12% 38% 35% 4	603
	21st to end	Night	10% 5% 21% 33% 26% 5%	760
October	1st to 10th	Night	9% 3 24% 36% 24% 4	760
	11th to 20th	Night	16% 9% 27% 32% 15% 1	760
	21st to end	Night	20% 10% 32% 31% 6%	836
November	1st to 10th	Night	13% 8% 34% 33% 11% 1	764
	11th to 20th	Night	17% 11% 34% 29% 9%	760
	21st to end	Night	23% 10% 33% 27% 7%	793
December	1st to 10th	Night	21% 13% 33% 28% 5% 1	950
	11th to 20th	Night	23% 8% 32% 30% 6%	950
	21st to end	Night	27% 14% 33% 23% 3	1045

Page generated at 2024-11-03 12:18:59. Version ac07

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