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This info is considered Public Domain, so use it as you wish, but not for commercial purposes. It was created to increase the joy and enjoyment of flying gliders/ soaring.
This is meterology for soaring pilots. It is a discussion of how to read the various types of charts and will help you to determine if there is going to be good soaring conditions.
Weather products are updated at various times, depending on the product. The updates vary from every 15 minutes to every 24 hours. They are current through various hyperlinks whcih may be found on the List of weather links page Be Sure to note the date of the image, and to REFRESH, or RELOAD the page - so you aren't looking at an old version of the page that you may have stored in the temporary memory - cache on your computer!
There is a wealth of info now available on the net. I try to keep the links on the List of weather links page up to date, because I essentially use that page as a set of bookmarks. That way I can refer to it from any computer, and others can also use it the same way. If you have other links that are your favorites, please tell us about them so we can all use them.
Satellite Photos:
Not really that helpful in forecasting, but interesting to look
at. Sat photos confirm what you see on the pressure charts, so look
at the sat photos, and then look at the pressure charts, to see how they
match. You will see the jet streams going over the north side of
the highs, and around the south side of the lows. The sat photo with
the pressure chart superimposed on top of it is hlepful for learnig
to see the realtionship.
.
Look at the Northern California ones and you will see mountain wave
- if it is present. The US Navy - Northern California (from
Bay Area north) is the best one for our area (Northern Calif)! It'll show
the wave clouds downwind of the hills. Of course, it is a visual image
and needs daylight. The yellow lines are the terrain at 200 ft MSL (the
yellow dot near Colusa is Sutter Buttes), and the lakes are outlined -
Clear Lake, Bereyessa, Tahoe, etc. There is another visual sat photo
just south of this one where you can see the Byron and Avenal areas.
There are also Enhanced Infrared (IR) Sat photos. Purdue - IR Sat photo shows coldest in red, then yellow, green, grey, and warmest as black. It also has a Sat view of the USA, with depiction of Surface Highs, Lows and Fronts shown, Wind arrows, sky cloud coverage symbols, etc. There is aslo a Purdue - IR of the Eastern Pacific (dark blue is cold, light blue warmer, black is hottest). On the "Details" link from the Purdue link, you will see what the chart symbols represent, how to read them!
Jetstream Charts:
The jet "steers" the movement of the air mass. The low will always
be north of the jet, as the jet passes around the edge of the low in the
shape of a "U". Conversely, it will be in the shape of an inverted "U"
as it wraps around the north side if a high.
First look at "latest available" jet stream chart to see where the jet is located, and to see its strength, then see the forecast to see where it is predicted to be in 24 or 48 hrs, or even in five or six days. The trick is to get an accurate forecast. In the winter months, you have seen the professional forecasters blow it, by saying it is going to be stormy, and it turns out nice, or visa versa. Those kind of errors are a result of inaccurate predictions on where the jet stream was forecast to be, but then it didn't move as forecast. Many people look at a satelitte photo and think the weather is moving in a given direction based on that photo - you can't project where a low will travel by looking at the sat photo, but you can see where the low will be by looking at the jet stream forecast! If the forecast is for the jet to remain in the same location, then the low will not be moving! Realize that the low in the sat photo will not pass through that jet stream.
When you look at the Current "latest available analyses" E.Pacific Chart,
you will see if the upper level pressure patterns are well established.
Compare that to the Forecast "24 Hour" E.Pacific Chart. Comparison
is very helpful to help validate air mass movements that are forecast for
tomorrow. You know the jet is wrapping around the south and east side of
the upper level low!
Text files
Read the forecast by the professional meteorologists, at NOAA, NWS,
etc. You will also find high temps for the day, including seasonal
averages, etc, as well as sunrise/sunset times. I find zone forecasts
and area forecasts more helpful that those for a specific town.
The sooner you associate pressure altitudes with each of the pressure levels, the easier it will be to understand the charts. Realize that a pressure chart is just a horizontal slice of the atmosphere, and there is a lot of vertical movement as well as horizontal flow, so it is most helpful if you begin to think in the third dimension. All weather charts are a single cross-section at a given altitude or pressure level. You need to look at the sky vertically - not just in the horizontal plane. Review each of the following in quick sucession, bounce back and forth between them and you'll see what I'm talking about. The wind changes in strength and direction as you go aloft. Note the direction of the pressure lines and the distance between them (wind direction and strength). Combine this with the sounding of wet and dry air temps, and you can almost "see" the clouds forming.
300 mb - 30,000 ft
500 mb - 18,000 ft
700 mb - 10,000 ft
850 mb - 5,000 ft
Surface
Sounding:
The latest sampling of the upper air. Info starts at the surface
and rises to above 50,000 ft. They are done every twelve hours and report
both the temps and winds. They are posted on the web by servers at Ohio
State and Unisys. I have both posted sources on my wxndx.html
page because sometimes one of them isn't working. you can change
the one with a black background to a white background (so you can print
it) just by selecting inverse on the screen - if you have that choice
on the screen you happen to be in.
There is a product produced at the Reno Office of the National Weather Service (a result of Doug Armstrong's efforts, I'm sure), which provides a soaring index (which is another way to say "rate of climb forecasted in feet per minute". Even if you aren't flying in the mountains, take a look at it, and compare it to what you have computed for the lowlands. If there are no drastic air mass movements, you will see some real consistencies within a couple of hundred miles.
See the Tips on Thermal Soaring Adiabat Chart, and other info on Thermals. This will teach you how to compute the height and strength of the thermals, using a simple phone call to Flight Service. Elementary and essential knowledge for all glider pilots.You need to understand the Adiabat chart if you are going to use the Sounding data.
There is The
Soaring Diary which provides you with reports received from various
pilots about their flights, and occasionally will have a weather forecast
in it, but it will only be generalized, since time is of the essence on
a good soaring day, so it is not likely that I will take the time to post
a forecast of good soaring conditions, because if I have the time available
to do anything, I will be driving to the gliderort to take advantage of
the conditions. Check out the Diary to see what's been happening
in this area. It goes back a year or two.
Additional Soaring Weather info is available on the PASCO Home Page. Also, the National Weather Service, San Francisco has a large selection of area weather info. You may go directly to the San Francisco NWS, Aviation Weather Page.
For a last minute check of soaring conditions, I still find it quickest and easiest to either use DUAT or phone the nearest FSS (at 1-800-WX-BRIEF). I request the forecast winds and temps up to 18,000 ft for my area of flight. I then plot the data on my homemade skew T and it provides me with an accurate estimate of strength and height of thermals. Unfortunately, cirrus may come in and we don't get the heat that was forecast, or the instability may be there, but the winds might be wrong. With winds from the North or East here in Northwest California, the upper air heats up too fast, nearly eliminating thermals. If the wind is too strong, the thermals are blown apart. Then again, maybe its forecast to be stable, but the winds might be right for Mountain Wave Lift (increasingly strong as you ascend, from the West).
The review of the soundings is normally a last minute thing. They are actually taken every 12 hours. We're looking for unstable air - convection! Temps can't be forecasted accurately enough during changing conditions, but the latest info is a point to depart from. You'll want to use the temps that were observed that morning. Soundings are done throughtout the USA at 7 AM EST, which makes it 4 AM PST, but don't become availalbe untill about 4 hours later. So, if takes an hour to drive to the gliderport, and you want to be there by 9 AM, It's unlikely the 4 AM soundings will be available before you leave home. Soundings ARE important to the Contest Director, so he can call the right task for the day. But if you've already commited to fly, and are at the airport, you can use them to see what height and strength thermals you can expect, and what the trigger temp will be. At Williams Gliderport, the VSA computer will call DUAT, and do most of the computation for you.
What we need to watch for are changing conditions. I'll often look at the satelitte, and always review pressure charts to see what kind of a flight I should plan on flying. If you see a good day coming, you need to plan for ground crew, early launch, etc, if you are hoping to fly 200 or 300 miles!
If you want a close up view of Northern or Southern California from a Satelitte, then take a look at the US Navy products listed above. The Northern one goes from Calif/ Oregon border South to a line from Bay area to Walker Lake, NV. The Southern one goes from Bay Area south in the Valley. The West coast one, goes all the way to the equator, so stop the downloading after about 40 % complete. If you can't go out to fly that day, or need to drive to the gliderport and launch after seeing the sat photos, it'll all be different by the time you get into the air, but at least you'll see what you missed.
For a broader look at Calif, and Oregon, you can see many of the other sat photos listed above, but these are rarely helpful in the summer months to glider pilots, and even during the spring and fall they are of little use, unless you look at the composite ones which have the pressure lines on them. I recommend you look at the pressure charts first, try to guess where the clouds might be found, and then look at the Satellite Photos. Sometimes you can visualize the cu in the unstable air. On the Sat photo, look for a lot of speckled clouds approaching, popcorn shaped cu's, of chicken wire mesh of clouds- without a solid cloud cover. These indicate good soaring - if it doesn't become stable before it reaches our location. However, you do need to do further research. Is there a High pressure area building? What will the winds be doing? Is the air forecast to be unstable? How hot is it supposed to get?
There are lots of products available. Look at the Current Pressure Chart, one of which is called Surface and 18,000 Foot (500 mb) Chart from Intellicast. This is a good starting point. Read the title of the chart! The title tells you what you are looking at! This one sometimes depicts temps and pressures, or other times leaves the temps off and depicts the atmospheric pressure at both the surface and 18,000 ft levels on the same chart. The Intelllicast product called the 24 Hour forecast which I discuss below has both the surface and 18,000 ft pressure lines. Allow me to explain the chart. The lines of constant pressure are isobars. Each solid lines represents a 500 mb pressure line, and the number associated with each one is the number of meters that line is above mean sea level (since there are 0.3048 meters per ft, 18,000 x .3048 = 5486 meters= 18,000 ft). If the values on the lines are increasing, you are going from a low to a high (it is telling you that to be at 18,000 ft pressure you will be higher above the earth. The dashed lines lines are isobars on the surface, adjusted for MSL. On a Standard Day 29.92 inches of mercury is about 1013 millibars (mb), since 500 is about half of 1,013, the 500 mb chart is 1/2 half of the earth's atmosphere. That's why 500 mb (18,000 ft) is such a good pressure level to look at when forecasting.
If the pressure gradient is high, the lines will be close together. The wind blows parallel to the lines at the 500 mb level. At the surface level, when the terrain is flat, friction causes the wind to be canted about 30 degrees (slightly in towards a Low and slightly out - away from a High). If you see the isobars going North and then bend East, and continue South and around to form a circle, you're looking at High! If the lines are going South, and then curve to the East and continue around to form a circle, they are portraying, and are going around a Low! A Trof of Low pressure is when the isobars start to curve upward, but don't form a circle. Rather, they straighten out, or bend to the right. Conversely, a Ridge of High pressure is displayed by the lines curving downward, but again not forming a circle, but straghtening or reversing.
If the 500 mb lines are spaced apart about the same distance as the California/ Oregon border is wide, then the wind will be about 30 kts. If they are spaced apart about the same distance as the California/ Mexico border, they will be about 50 kts (greater pressure gradient), and you already know the direction, as the wind flows clockwise around a High, and counter-clockwise around a Low. So looking at the chart you immediately know wind strength AND direction. Now look for changes in the wind direction. You know whenever there is a frontal passage there is a shift in the wind -- right? So look for the isobars reversing direction from curving up, TO curving down. That's the front! Compare the surface to the 18,000 ft level. If the pressure patterns are not the same, you should note that an upper level Low may produce instability, or an upper level High may dampen out instability occuring at or near the surface. Let's look at Forecasts, because, .... since it will take you a couple of hours to launch, the current data is already history!
The 24 Hour Forecast Pressure Charts. Two charts are combined on this product. Although this is very similar to the chart above ("Current Analysis"), this one is FORECAST data. Again, the dashed lines are surface pressures (12, 16, etc, represent 1012 millibars, 1016 millibars, etc) (1013 equates to 29.92 altimeter). The solid lines represent the height in meters of the 500 mb pressure. If the lines are close together, the pressure gradient is higher, and the wind will be stronger. Is there a surfaced based High? Does it extend upwards to 18,000 ft? The upper level High over California often blocks any Low pressures from invading this area, thus we get 6 months of sunshine each year. But good soaring conditions still occur.
You need to look at the direction of the curve of the lines. If the line is curving clockwise it is and is going around a High, it is merely an indication of the pressure gradient. But, if it begins to reverse into a counterclockwise curve, then that is a pressure change!! At the least it is a TROF of Low Pressure and will cause instability. there will probably be clouds as the pressure change / Front passes overhead. A frontal passage is always what we are looking for. Be it ever so slight, it only takes a slight change to produce the instability needed for us to have a great soaring day. Theses periods of slight instability are meaningless to the general public, and even to power pilots. But to us, they're important. Unfortunately, you have to do your own forecasting - so keep reading.
In order to see trends, I suggest you look at the Intellicast 48 and the Intellicast 72 hr forecasts. These will tell you what the experts think about how the jet, and the pressure patterns will change over the next 2 to 3 days. As a novice, why guess? Look at these long range guesses by the professionals, but take them with a grain of salt, because it is a dynamic environment. Meterologists give it their best shot, but sometimes there are more variables than they can possibly adjust for.
You need to know how hot it's going to be during the day! I find the local TV report to be the most accurate source for forecasting temps. There are some web sites that provide the info, but they've been changing. See the intro at the top of this page for the latest list. You must know the estimated max forecast temps before you examine the upper air temps (the soundings). Decide realistically what the maximum temp will be for the airport at your point of departure, at the time you plan to launch. You will enter the soundings with this temp.
The latest measurement of the upper air are The Soundings (they are each listed at the top). Look at the Medford, Oregon, and the Reno, Nv, and the Oakland ones, all for a given day. You'll see there is little difference in temps at 14,000 ft (600 mb) between any of them! See the purdue explaination of the charts. As a quickie, I'll just say- the graphic provides the temperature at all levels in the upper air up to over 30,000 feet. The colors of each line vary with which chart you are looking at. The line furthest to the right side, the right zig-zag line is the temperature, using a dry bulb, ie the regular temp, not considering moisture in the air. This dry temp is the one you are most interested in! The zig-zag line to the left is the wet bulb temp. It is the temp recorded with a thermometer that has a wet cloth around the temp bulb. The third line, a relatively non-zigzag line, is the standard day temps - just a set of numbers that are used for relative difference. I don't use this line. Note that the distance between the two zig-zag lines indicates the amount of moisture, or saturation of the air. Where they are close together, you will have clouds form, with very little convection (vertical mixing). The chart also shows the winds from the surface upwards (a wind arrow from the left is a West wind, one from the right is an East wind, etc.). Both DUAT and FSS will provide data for Red Bluff and Sacramento. This is achieved through interpolation of the soundings from the three nearest observation stations. The FSS/DUAT data is limited, since it only provides dry temps and winds at 3, 6, 9, and 12,000 ft, etc, but it serves the purpose of keeping you focused on generalities. Don't get bogged down in the fine details - it'll probably all be different than forecasted by the time you get into the air anyway! Besides, you don't know what it is near your gliderport below 4,000 feet. What you really need are local soundings. What are the temps from the surface up to to about 4,000 above the airport! That't where you are going to get off tow!
Before we calculate the height of the thermal, if you are interested in knowing the cloud base, here is a rule you can use:
Now, back to computing the thermal height. If the graphics pf the upper air sounding is not available, or if you are plotting the temps on your homemade skew T, as I do, then you can get the latest soundings from the nearst Flight Service Station by phoning 1-800-WX-BRIEF. They'll give you the latest temps for 6, 9, 12,000 and 18,000 ft. (Be sure to use the Thermal Soaring Adiabat Chart, and see info on Thermals I've created this chart for glider pilots to use up to 18,000 ft (500 mb). Plot the temps - especially if you get the tow plane to give you the temps aloft.
While looking at the upper air soundings graphic, or sounding plot, as it is simply called, follow along with this explaination to see how I read and use this chart:
There are two somewhat vertical lines zig-zaging up the chart. The one on the left is the wet bulb temperature. The one on the right is the dry bulb temperature. This dry bulb temp is the one you want to plot! The temps are at various pressure levels in millibars - as shown on the left vertical scale. Next to each pressure level- starting at 1000 mb, write in the following altitude values: SL, 3, 6, 10, 14, 18. Now you know that the horizontal 800 mb line is the same as 6,000 ft altitude, etc. - those altitudes are close enough for our purposes.
The temperature scale is along the bottom - horizontal axis. The temp lines are the straight one that go from the bottom left to the top right. Since we think about surface temps in F not in celsisus, you need to get some values in mind for the conversion. You already know the following: 0 c = 32 f, and 15 c = 59 f (a standard day), some other easy to remember values are: 10c=50f, 28c=82f, 35c=95f, 40c=104f.
The dry adiabat lines don't have values, but these lines are the ones you need to parallel, as you start with a temp at a given altitude to see how high the thermal is going to rise. The Dry Adiabat lines go from the bottom right to the upper left. To estimate the thermal height, you merely start with a given temp at a given height, parallel the dry adiabat lines as you go up and to the left until you intercept the dry bulb temperature plot. When you hit that line, that's as high as you will climb in that thermal.
An example for you to use and to follow:
Plot these three points: 20c/1000mb, 25c/900mb, minus10c/500mb. That's the dry bulb temps for this example.
Fcst temp on sfc is 104, that's 40c at 1,000mb. Follow the dry adiabat line up and to the left, and you see the thermal will go to 750 mb-- that's about 7 or 8,000 ft. That'd be a good day!
Using the same example three points for a sounding, suppose the forecast high was for 98 F at Marysville, near Williams. Realizing that the sounding plot is above San Francisco Bay, and there is a bigger inversion in the valley, I imagine that second point is really somewhere to the right of 25c/900mb. I don't bother to plot a guess of the inversion, but I do enter the chart at my best guess for the temp at 3,000 ft. For this example, I would guess about six degrees less than the max. forecasted surface temp. for the day. Thus, 98 F minus 6 = 92. That's about 33 C. So, I start at 33c/900mb, parallel the adiabat lines and see the thermals on the hills West of Williams will go to 700 mb or 10,000 ft. -- a typical summer day at Williams.
You will see several indexes recorded on the margin of the graphic. I personally find them near to useless. These generally indicate the stability of the air. The Air is unstable before you get to the following values, but you are cautioned to watch for the following threshold values for any potential thunderstorm development.
LI= Lifted Index....Lower than +2 is more convection, -4 watch for tornadoes.
SI= Showalter Index...Lower than +3 is more convection, -6 watch for tornadoes.
TT= Total Totals Index...Higher than +40 is more convective, +55 watch for tornadoes.
KI= K Index...Higher than +15 is more convective, +40 watch for tornadoes.
SW= Sever Weather Threat (SWEAT) Index...Higher than +300 is more convective, +500 watch for tornadoes.
The following pressures are approximate altitudes. The actual conversion for each altitude, using Standard Day temp, is also shown:
If would like to have a more detailed explaination of reading and using the adiabat charts, then go to the Thermal Soaring Adiabat Chart, and other info on Thermals.
Learn to forecast! Spending too much time on the current weather depiction is akin to the TV weather reports. They spend most of the time telling you what has already happened - but what you really want to put your time into is forecasting what will happen next. Remember, the objective is to be able to see the good soaring days coming - and be ready!
Good luck, and soar safely!
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