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 2024-04-03.

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% 7% 3 5% 5% 3 5% 5% 5% 7% 41%	78886
	Night	14% 8% 3 6% 5% 3 4 4 5% 7% 42%	81291
Transparency	Day	41% 6% 12% 24% 15% 2	78591
	Night	42% 5% 10% 23% 16% 3	81107
Seeing	Day	14% 8% 24% 26% 19% 7%	14721
	Night	16% 8% 28% 32% 14% 2	24743

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	13% 10% 6% 9% 6% 4 5% 4 6% 7% 31%	2527
	11th to 20th	Day	14% 9% 4 7% 8% 7% 5% 4 6% 6% 31%	2636
	21st to end	Day	22% 18% 6% 6% 3 2 3 3 4 4 28%	3135
February	1st to 10th	Day	16% 10% 5% 7% 4 3 4 5% 5% 6% 35%	1724
	11th to 20th	Day	15% 8% 4 5% 5% 3 5% 5% 6% 7% 36%	1848
	21st to end	Day	15% 10% 4 6% 5% 2 4 4 4 6% 38%	1570
March	1st to 10th	Day	16% 8% 3 6% 5% 3 4 5% 5% 5% 41%	1900
	11th to 20th	Day	16% 9% 4 5% 5% 3 5% 5% 5% 6% 36%	2069
	21st to end	Day	20% 10% 5% 7% 6% 4 5% 4 5% 6% 28%	2448
April	1st to 10th	Day	13% 6% 3 6% 5% 3 5% 5% 5% 8% 42%	2184
	11th to 20th	Day	17% 7% 3 6% 6% 3 4 4 4 5% 41%	2134
	21st to end	Day	17% 7% 3 6% 5% 3 4 4 5% 7% 39%	2160
May	1st to 10th	Day	17% 5% 3 5% 6% 4 5% 5% 7% 8% 35%	2258
	11th to 20th	Day	15% 6% 3 5% 6% 3 4 5% 5% 8% 39%	2413
	21st to end	Day	9% 6% 3 6% 6% 4 6% 7% 7% 9% 38%	2725
June	1st to 10th	Day	7% 5% 2 5% 5% 4 5% 5% 6% 10% 47%	2520
	11th to 20th	Day	8% 4 2 5% 5% 3 5% 6% 6% 8% 48%	2520
	21st to end	Day	5% 4 2 5% 6% 3 6% 7% 7% 11% 44%	2520
July	1st to 10th	Day	8% 3 2 5% 5% 4 6% 7% 8% 10% 41%	2520
	11th to 20th	Day	4 3 2 5% 4 3 5% 6% 6% 10% 51%	2520
	21st to end	Day	8% 4 2 4 5% 4 5% 5% 6% 9% 48%	2674
August	1st to 10th	Day	7% 5% 2 5% 6% 4 6% 6% 7% 10% 42%	2292
	11th to 20th	Day	9% 5% 2 6% 5% 3 7% 7% 7% 10% 40%	2125
	21st to end	Day	7% 5% 2 4 4 3 4 5% 6% 9% 50%	2376
September	1st to 10th	Day	13% 5% 3 4 4 2 4 3 5% 8% 49%	2160
	11th to 20th	Day	8% 5% 3 5% 4 3 4 4 5% 9% 50%	2111
	21st to end	Day	8% 4 2 4 4 3 5% 4 6% 7% 53%	1980
October	1st to 10th	Day	11% 4 2 4 5% 2 4 4 4 6% 54%	1841
	11th to 20th	Day	13% 5% 2 5% 5% 3 3 3 5% 6% 49%	1800
	21st to end	Day	18% 7% 2 5% 5% 2 3 3 4 5% 45%	1921
November	1st to 10th	Day	10% 5% 2 3 4 2 4 3 4 7% 54%	1672
	11th to 20th	Day	14% 7% 2 5% 5% 2 3 4 4 7% 46%	1710
	21st to end	Day	15% 10% 4 7% 5% 3 3 3 4 6% 41%	1672
December	1st to 10th	Day	14% 11% 4 6% 5% 3 4 5% 5% 7% 37%	1520
	11th to 20th	Day	15% 9% 4 6% 5% 3 5% 4 4 7% 36%	1918
	21st to end	Day	23% 13% 5% 7% 6% 3 4 3 5% 5% 25%	2783
January	1st to 10th	Night	15% 9% 6% 9% 6% 3 4 4 5% 5% 34%	4428
	11th to 20th	Night	13% 11% 5% 10% 7% 4 7% 5% 3 4 29%	4330
	21st to end	Night	25% 14% 6% 8% 5% 2 4 3 4 5% 25%	4532
February	1st to 10th	Night	15% 10% 6% 8% 5% 3 5% 4 5% 7% 33%	2454
	11th to 20th	Night	14% 7% 3 8% 5% 2 5% 4 5% 6% 41%	2218
	21st to end	Night	17% 9% 3 6% 6% 3 4 4 4 5% 40%	1884
March	1st to 10th	Night	16% 9% 3 6% 5% 2 5% 4 4 4 42%	2280
	11th to 20th	Night	18% 8% 4 6% 4 3 3 4 5% 7% 39%	2128
	21st to end	Night	20% 9% 3 6% 5% 3 4 4 5% 7% 33%	2148
April	1st to 10th	Night	12% 7% 2 6% 5% 3 3 4 4 7% 46%	1820
	11th to 20th	Night	15% 6% 3 4 4 3 5% 4 5% 7% 44%	1775
	21st to end	Night	18% 8% 3 5% 5% 3 3 5% 4 6% 41%	1800
May	1st to 10th	Night	15% 7% 3 6% 5% 3 4 4 5% 9% 41%	1702
	11th to 20th	Night	12% 8% 4 5% 6% 3 4 4 5% 7% 44%	1478
	21st to end	Night	7% 6% 3 7% 5% 4 6% 6% 7% 10% 39%	1551
June	1st to 10th	Night	7% 5% 2 4 4 2 6% 5% 5% 9% 50%	1440
	11th to 20th	Night	6% 4 1 5% 4 2 5% 5% 6% 8% 54%	1440
	21st to end	Night	5% 3 2 4 5% 3 4 5% 6% 12% 51%	1440
July	1st to 10th	Night	8% 3 1 4 4 4 5% 5% 8% 9% 49%	1440
	11th to 20th	Night	5% 3 2 4 5% 3 5% 4 5% 11% 54%	1440
	21st to end	Night	6% 5% 2 5% 5% 3 4 5% 7% 8% 51%	1639
August	1st to 10th	Night	6% 4 2 3 5% 3 5% 6% 6% 10% 50%	1646
	11th to 20th	Night	6% 4 2 6% 6% 3 5% 4 6% 7% 50%	1776
	21st to end	Night	6% 3 1 4 4 3 4 5% 6% 8% 56%	1980
September	1st to 10th	Night	11% 4 2 4 4 2 4 4 5% 7% 53%	1800
	11th to 20th	Night	7% 4 2 5% 5% 3 4 5% 5% 7% 53%	1831
	21st to end	Night	8% 4 1 4 4 3 4 4 5% 7% 55%	1980
October	1st to 10th	Night	9% 4 2 4 4 3 4 5% 5% 7% 56%	2115
	11th to 20th	Night	15% 6% 2 3 3 2 3 4 4 7% 52%	2160
	21st to end	Night	18% 7% 3 5% 3 2 3 4 4 5% 45%	2434
November	1st to 10th	Night	9% 6% 2 3 4 2 4 4 4 6% 55%	2406
	11th to 20th	Night	14% 6% 2 5% 4 3 3 4 4 7% 49%	2470
	21st to end	Night	18% 7% 3 5% 4 2 4 4 4 6% 44%	2503
December	1st to 10th	Night	16% 10% 3 7% 5% 3 5% 5% 5% 6% 34%	2660
	11th to 20th	Night	17% 6% 3 6% 5% 3 3 3 4 6% 42%	3279
	21st to end	Night	21% 18% 4 9% 6% 3 4 5% 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	45% 5% 9% 25% 14% 2	1900
	11th to 20th	Day	48% 6% 11% 20% 12% 2	2069
	21st to end	Day	56% 5% 12% 17% 8% 1	2448
April	1st to 10th	Day	40% 6% 12% 26% 16% 1	2184
	11th to 20th	Day	46% 4 9% 24% 13% 3	2134
	21st to end	Day	45% 6% 12% 23% 12% 1	2160
May	1st to 10th	Day	44% 8% 15% 24% 9% 1	2258
	11th to 20th	Day	42% 7% 13% 23% 14% 1	2417
	21st to end	Day	40% 8% 15% 21% 13% 2	2725
June	1st to 10th	Day	31% 8% 15% 28% 16% 3	2520
	11th to 20th	Day	31% 9% 13% 23% 22% 3	2520
	21st to end	Day	32% 8% 17% 22% 17% 5%	2520
July	1st to 10th	Day	34% 9% 14% 26% 13% 4	2451
	11th to 20th	Day	26% 8% 15% 28% 19% 3	2445
	21st to end	Day	32% 8% 13% 28% 17% 2	2525
August	1st to 10th	Day	34% 10% 13% 27% 16% 1	2278
	11th to 20th	Day	34% 10% 14% 26% 15% 1	2125
	21st to end	Day	30% 7% 14% 24% 19% 6%	2376
September	1st to 10th	Day	34% 5% 11% 27% 19% 4	2160
	11th to 20th	Day	32% 5% 9% 26% 25% 3	2111
	21st to end	Day	29% 6% 9% 28% 25% 3	1980
October	1st to 10th	Day	31% 5% 10% 25% 25% 3	1841
	11th to 20th	Day	36% 4 9% 24% 23% 4	1800
	21st to end	Day	41% 4 9% 27% 17% 3	1921
November	1st to 10th	Day	31% 4 12% 28% 20% 5%	1672
	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	45% 5% 9% 23% 16% 2	2280
	11th to 20th	Night	46% 6% 9% 22% 14% 3	2128
	21st to end	Night	51% 5% 13% 21% 10% 1	2148
April	1st to 10th	Night	39% 5% 10% 27% 17% 3	1820
	11th to 20th	Night	39% 5% 9% 23% 19% 4	1775
	21st to end	Night	45% 6% 8% 22% 16% 3	1800
May	1st to 10th	Night	42% 6% 12% 23% 15% 2	1702
	11th to 20th	Night	40% 7% 12% 24% 16% 2	1479
	21st to end	Night	38% 9% 13% 21% 15% 4	1552
June	1st to 10th	Night	30% 8% 14% 26% 20% 3	1440
	11th to 20th	Night	27% 6% 11% 26% 25% 4	1440
	21st to end	Night	25% 8% 16% 28% 17% 7%	1440
July	1st to 10th	Night	29% 8% 14% 30% 15% 5%	1400
	11th to 20th	Night	24% 7% 15% 31% 19% 4	1392
	21st to end	Night	27% 6% 15% 29% 21% 2	1548
August	1st to 10th	Night	27% 9% 14% 31% 17% 2	1636
	11th to 20th	Night	31% 7% 11% 27% 22% 2	1776
	21st to end	Night	24% 7% 12% 26% 24% 5%	1980
September	1st to 10th	Night	30% 6% 9% 28% 21% 7%	1800
	11th to 20th	Night	30% 5% 9% 28% 23% 5%	1831
	21st to end	Night	29% 6% 9% 23% 29% 5%	1980
October	1st to 10th	Night	28% 5% 10% 25% 25% 6%	2115
	11th to 20th	Night	33% 3 8% 28% 22% 7%	2160
	21st to end	Night	41% 5% 8% 21% 20% 4	2434
November	1st to 10th	Night	31% 5% 11% 24% 23% 6%	2406
	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% 31% 20% 6%	446
April	1st to 10th	Day	15% 13% 36% 24% 11% 2	428
	11th to 20th	Day	19% 13% 29% 24% 12% 2	412
	21st to end	Day	21% 11% 35% 21% 12% 1	420
May	1st to 10th	Day	17% 10% 31% 29% 10% 3	423
	11th to 20th	Day	17% 7% 26% 28% 17% 5%	559
	21st to end	Day	11% 5% 25% 32% 19% 9%	679
June	1st to 10th	Day	10% 5% 17% 30% 27% 11%	630
	11th to 20th	Day	8% 2 19% 35% 26% 10%	630
	21st to end	Day	5% 6% 23% 34% 23% 9%	630
July	1st to 10th	Day	10% 2 12% 24% 30% 21%	621
	11th to 20th	Day	4 4 12% 35% 32% 13%	618
	21st to end	Day	9% 2 14% 32% 31% 13%	684
August	1st to 10th	Day	9% 2 11% 34% 31% 13%	590
	11th to 20th	Day	10% 3 11% 29% 32% 14%	439
	21st to end	Day	8% 2 10% 27% 36% 17%	495
September	1st to 10th	Day	13% 4 12% 24% 32% 14%	450
	11th to 20th	Day	9% 3 11% 27% 34% 16%	450
	21st to end	Day	8% 9% 19% 26% 25% 13%	450
October	1st to 10th	Day	11% 5% 23% 37% 18% 6%	311
	11th to 20th	Day	13% 12% 39% 24% 11% 1	270
	21st to end	Day	30% 14% 29% 20% 7%	297
November	1st to 10th	Day	13% 6% 33% 28% 14% 6%	270
	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% 10% 37% 30% 7%	546
	11th to 20th	Night	17% 8% 34% 28% 12% 1	533
	21st to end	Night	22% 7% 35% 27% 8% 1	540
May	1st to 10th	Night	18% 6% 30% 34% 11% 1	540
	11th to 20th	Night	16% 4 24% 38% 17% 2	531
	21st to end	Night	9% 3 22% 41% 22% 3	584
June	1st to 10th	Night	9% 2 18% 42% 29%	540
	11th to 20th	Night	8% 1 17% 47% 24% 3	540
	21st to end	Night	7% 1 20% 40% 30% 2	540
July	1st to 10th	Night	10% 1 6% 34% 41% 7%	525
	11th to 20th	Night	5% 2 10% 43% 36% 5%	522
	21st to end	Night	8% 1 11% 39% 37% 3	591
August	1st to 10th	Night	9% 1 7% 45% 34% 4	537
	11th to 20th	Night	8% 1 9% 40% 36% 6%	534
	21st to end	Night	7% 1 10% 37% 39% 6%	594
September	1st to 10th	Night	12% 2 10% 39% 30% 7%	540
	11th to 20th	Night	8% 1 13% 38% 35% 5%	571
	21st to end	Night	10% 5% 21% 33% 27% 5%	720
October	1st to 10th	Night	10% 3 25% 34% 24% 4	720
	11th to 20th	Night	17% 9% 27% 32% 13% 1	720
	21st to end	Night	21% 10% 31% 31% 7% 1	792
November	1st to 10th	Night	13% 8% 34% 33% 11% 1	740
	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

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