Tracking

Once the balloon is released, you can only watch. If you are using WSPR, it is time to start your tracking software to get the wsprnet.org data and send it to APRS.fi. You can also go to http://lu7aa.org/ and enter the data about your launch. If you are using APRS, it will show up on APRS.fi. Those websites will give you real time data and the track the balloon is following.

Knowing where the balloon will be in a day or two is very interesting. To do that, the HYSPLIT program at https://www.ready.noaa.gov/HYSPLIT_traj.php. The wonderful thing is that this program is written specifically for superpressure balloons. You need to put in the location and altitude and specify how many days prediction you want and how many hours between points. Click run and wait a bit. It will take a little time to generate the map and then you can see what is likely. You can check that against the storms shown on windy.com and have an idea about whether or not you are in trouble.

If you are interested in atmospheric data, the Univerisity of Wyoming has data that is interesting. They have isobar charts for high altitude. They can be found at http://weather.uwyo.edu/upperair/uamap.shtml. They also have a program that was written to predict the path of bursing balloons. That program can be interesting if you want to look at the pressure where the balloon is floating. It prints out the altitude vs. pressure in the area and you can use gps altitude to interpolate the pressure where the balloon is. It will also give you enough information to know the altitude of the tropopause where your balloon is. You may be below on in the tropopause so it could be interesting to study.

You can also use WSPRnet.org to see where your balloon is located. Put the call in and it will show who is receiving and where the balloon is located. It will not give you a track so you can only know the location at that time.

At times the balloon will be very fast and other times very slow. It will depend very much on what the jet stream is doing. Information about the jet stream can be found at https://www.netweather.tv/charts-and-data/jetstream. When in the jet stream, the speed can be over 200 miles per hour and when not in, it can be just a few miles per hour and even go in circles.

There are differences in the seasons. The wind is most convoluted in the summer when the sun puts more energy in the northern hemisphere. In the winter, the jet stream is closer to linear and it can carry the balloon around the world more quickly. Either way it is more interesting but there is some gratification is going around the world.

Superpressure

It is important to understand why these balloons do what they do. The latex balloons will rise until they burst. The balloon gets bigger and bigger with the expansion of the gas until the latex fails. They do not float. The superpressure balloon is not stretchy like latex. There is a little stretch but not much.

The superpressure balloon is give a certain amount of gas so it has the correct lift. The balloon will not be full. As it rises the gas expands and eventually fills the balloon. Since the plastic does not stretch (much) the density is now fixed. The balloon will rise a bit more and reach a level where the balloon overall density equals the outside air and it will stay there. The pressure inside the balloon will be higher than the outside.

There are very good articles detailing the behavior of a superpressure balloon and we will give a simplified explaination to get started. Anybody wanting to dig into the math can find great articles to detail the math.

The gases we use can be considered ideal gases. The particles of an ideal gas have no interactions with each other and are so small they never bump into each other. The nature of an ideal gas is that if you have one mole of particles, they occupy about 22.4 liters on the ground. A mole is a convient number for chemists and is 6.02 x 10^23 particles. It does not matter what the weight of the particles are. For air, the average weight is 29 grams and for hydrogen the weight is 2 grams. A mole of hydrogen will produce a lift of 27 grams. As the balloon rises, the air gets thinner and so does the hydrogen. As long as the balloon is not full, the lift remains the same.

The superpressure is an important number. The amount of free lift that the balloon has will determine what the pressure inside the balloon is at float. If the pressure is too high, it will burst. It will not be too low and the balloon will not launch. Over time, gas will diffuse though the plastic and it will rise slightly. The balloon will remain at float until the free lift goes to zero. This can be months so not much of a worry.

There is worry about the loss of gas waiting to launch a balloon. It is a reality and has to be considered. The SBS-13 balloons have a high quality multi-layer plastic that can hold the gas very well. The clear chinese balloons are less capable of holding the gas. They will lose gas at about 6 times the rate of the SBS-13. The simplest way to deal with this is to fill the balloons the night before when you have no time pressure and just roll up and clip the neck. In the morning, check again and add what you think it needs. Then do the heat seal.

The diffusion of gas at altitude is not an issue. There are too many ways to fail and it won’t be diffusion. At ground level, the pressure of the gas inside is 14.7 psi and the outside pressure of the gas is zero. At altitude the inside pressure is 2.3 and the outside still zero psi so the driving force is about 1/7th of that on the grouond but the temperature is dramatically lower so the diffusion through the palstic is lowered exponentially. The net result is, don’t worry.

The Rate of Ascent

The rate at which the balloon rises is useful to know. It allows you to predict when and where the balloon will come to float. If you know when and where you will launch, there are websites that will tell you where you will come to float and which way you will be going.

If you have an SBS balloon, they give the rate in their instructions. If you have a chinese balloon, you need to assume something. You could just use the SBS numbers and be close. If you want to get better data, it can be done.

You can use an app like VidAnalysis or some other to video the release of a balloon. It can be done indoors with a high ceiling and the appropriate weight attached to the balloon. With a video, you can pinpoint the same spot and graph the ascent. The balloon will stabilize within a meter or so and there you have a measurement. This can be done with several weights attached to the balloon to verify how ascent is affected by free lift. A great classroom project.

The rate of ascent of the balloon is essentially constant until the balloon reaches full inflation and then it will slow down until it reaches float. Since the balloon is travelling slowly, the velocity is directly proportional to the free lift. The flow around the balloon is laminar and that is different from airplanes where the drag and velocity are related to the velocity squared. Recall that the lift is constant until the balloon is fully inflated.

Once the balloon is inflated, the velocity will decrease linearly with altitude until it reaches float. This resembles the discharge of a capacitor and the time constant is the distance between full inflation and float divided by the rate of ascent. It takes about three time constants to come to float. This will not matter much since it will be in the order of 5 to 10 minutes but it is comforting to know it is done.

Measuring Free Lift

The free lift is the extra lift that causes the balloon to rise. It is the lift after all assembly and ready to release. This is done when filling the balloon so the weights of the beacon, antenna and harness need to be known. The weight of the balloon itself is not necessary for this step as the gas will be lifting the balloon and it is the extra gas that is being measured. If one balloon is used, the math is simple. If two are used, do the same math but divide by 2 and do that in each balloon.

There are many small scales that can be bought. They are cheap and can be less than $10. There is always concern about the accuracy of the scale and if the scale has changed. It is a good idea to buy a low cost set of laboratory weights to verify the operation of the scale is correct. I use the set sold on Amazon as “American Weigh Scales 14-Piece Small Calibration Weight Kit – Red”. It costs about $13. You can check linearity and accuracy and have some peace of mind about the launch.

The usual method is to put a weight with a clip to hold the balloon. Gas is put into the balloon until it can lift itself and then a bit more. At any point, pause, role up the neck of the balloon just to keep the gas in and clip it to the weight. The scale will read a negative number which is the lift. (If it cannot display negative numbers, don’t tare and just do the math.) Do this in small increments as it is easier to add gas than to remove it.

The room needs to be still. The balloon will wave in breezes and just moving around makes enough to be seen. Allow the air to be still and that will be the free lift. The neck can be rolled up and clipped for a while as there is no pressure in the balloon and the gas will stay where you want it.

Choosing the time and place For A Launch

The time and place to launch your picoballoon is important. Things to consider are location, surface wind, cloud cover, the path the balloon will follow during ascent and the path it will follow one at float altitude. Consider the things that can go wrong. The balloon can drag across the ground, snag a tree or power line, drift into lee turbulence from a bluff, dip into water, get into clouds and get wet, get drawn into an updraft in a cumulus or cumulonimbus cloud from below, float over a growing thunderstorm and so on. With a little thought the list would get much longer. So, plan the time and place…..

Choose an open spot. It can be a field, dock or a beach. If the day is very still, almost any place will work. At this point, you should have a pretty good idea of the float altitude and some idea of the rate of ascent. If it is an SBS-13, they mention the rate of ascent as a function of free lift. I you are using clear chinese balloons, you probably should learn by experiment. For a first guess, if you have to guess, use between 1 and 2 meters per second.

Use the windy.com to see what the surface winds at your chosen location will be. You can look days in advance for planning but look for wind of 5 miles per hour or less. You can probably launch during higher wind speed but the risks of problems increase.

Having chosen the launch location and time, go to to http://predict.habhub.org/ which will predict the trajectory during the rise to float. Input the location, the launch time, and where it asks for burst altitude, put float altitude. Put in your expected rate of ascent and for the descent rate, put in a very small number as the program exects a bursting balloon with a payload returning by parachute. If you put in a very low descent rate, the line it draws will be the direction it will follow at float.

The rate of ascent for slow balloons like we send up is pretty much constant all the way up. It is also proportional to lift so, double the lift, double the speed. It does not act like an airplane where drag is proportional to the square of speed. It is too slow and the flow is laminar. Once the balloon is fully inflated it will slow and reach float altitude. The approach to float has math like the discharge of a capacitor. The time constant is the distance between full inflation and float divided by the rate of rise. There is no need to worry about the approach to float but it will be in the last 15 30to minutes of the ascent, depending on your free lift.

The above program does not take that into account but don’t worry, it is trivial.

At this point, you have a fairly good idea of where and when the balloon will begin float. It is time to run HYSPLIT.

https://www.ready.noaa.gov/hypub-bin/trajsrc.pl?trjtype=4

Put in the altitude and time. Select how far in the future for the prediction you want to see and it will tell you where the balloon will go. This program is specifically written for superpressure balloons so it is perfect. Just what the doctor ordered. It will take a little time to sort out the inputs as there is a lot to select and it too much for me to detail here.

After you know where the balloon is going to go, switch back to windy.com. Look at the cloud tops information. What you want to understand is if you will be floating into a storm or a cloud. A storm can tear things apart and a cloud will probably get it wet so it will descend. Remember it has no net lift at that point so every extra gram is a problem. There is nothing you can do about these things but evaluating before launch might save the balloon for later.

Assemble the balloon and payload. Half the dipole is above and half below for wspr. Just a 2 meter whip is below for the aprs trackers. Make sure the line from the balloon to an aprs tracker is long enough that the beacon and balloon are unlikely to bump in turbulence.

So, you have assumed a location and time. If you are using 2 meters and aprs, the balloon is easily launched by one or two people. Release the balloon upwind from the beacon and release the beacon when directly below the balloon. You don’t want it to swing down and drag on the ground. You might not want to launch in a wind faster than you are willing to run!

If you are using a tracker on 20 meters and using wspr, the antenna is significantly longer. I suggest three people but it can be done with less. Line up with the wind, release the balloon, keep the payload below the balloon until the line is straight and overhead and release. The third person keeps the line from tangling in weeds and so on.

Now the balloon is floating into the sky and all you can do is watch. Make sure you get video to share as it is easy to get engrossed in the launch and get no video. That is a good job for spectators and they are usually eager to be there.

Now watching the progress is the fun part. If you have a 2 meter aprs beacon, it will show automatically on APRS.fi. Once it is over about 10,000 feet, it will be loaded to https://tracker.habhub.org/#!mt=roadmap&mz=11&qm=1_day&f=AD1L-11&q=!RS_*;

For APRS.fi, put in the balloon call sign to see where it is. For the habhub tracker, the column on the left lets you select the balloon you wish to see and clicking on it will zoom to it’s location. Both sites give some telemetry coming from the beacon. Using APRS will give good tracking over North America and Europe. It might be picked up in China and Japan and again as it comes back to North America. There is lots of nail biteing while you wait.

If you choose to use a wspr beacon, the coverage is wonderful and you will almost always know where the balloon is. Night and very high latitudes will stop transmission but it still is just great. Usually the signal is sent on 20 meters and can go as far as 8,000 miles. You will know where it is over the ocean and over the land. Some nations make it illegal to transmit in their airspace and the beacons stop sending in that space. That would be primarily England and North Korea. There may be GPS jamming over some conflict areas in the middle east. The problem with wspr is the signal does not get picked up by aprs.fi. It is picked up by the wspr network. You can run a python script to go to wsprnet.org and download the data, convert the format and upload it to aprs.fi. That script is on Github. You can also ask for the tracking to be done by http://lu7aa.com.ar/wspr.asp?other=lu7aa and the map on his site will give you a great display.

It is also interesting to see what the jet stream is doing and where your balloon is relative to that. The jet stream can be seeh here: https://www.netweather.tv/charts-and-data/jetstream . If you would like to see what the air pressure at high altitude to see if the balloon is at the altitude/pressure you expected, go to http://weather.uwyo.edu/upperair/uamap.shtml . If you would like to study what pressure the balloon floats at, http://weather.uwyo.edu/upperair/balloon_traj.html, written for tracking balloon ascents and descents will give pressure and altitude information that can be used to interpolate the pressure from the gps altitude.

Considerations for generating your own hydrdogen

It is not uncommon, at first, to consider producing your own hydrogen. This has been done successfully by one individual and his balloon circumnavigated the world once. In general the effort is rather big and is just generally a poor idea. If you intend to produce hydrogen with wet chemistry, make sure you know enough about chemistry and the kinetics of a reaction to control what you are doing. If you follow some of the Youtube videos where wads of aluminum foil or soda cans are used, realize that you have started a reaction over which you have no control. If you cannot stop a chemical reaction, you should not start it. If you use some of the gas generators being demonstrated on Youtube, make sure you know what you are buying or building. Some of them produce an explosive mixture of hydrogen and oxygen and things will not go well.

Producing hydrogen can be done with electrochemistry or chemical reaction. Either way, the gas you produce will be saturated with water and will need to be dried for the balloons to function properly. Remember that the balloons will be at temperatures of -50F or so and all the water vapor will be ice. Not a good thing to have in your balloon.

If you choose to produce the gas with electricity, do the math and realize that you are going to need about 5 moles of gas to function and each mole will require two Faraday Constant of electrons to product. That is 96485.3 coulombs each. A coulomb is the number of electrons in one ampere in one second. This is a pretty hefty power supply and will take a lot of time. It is, however, something you have total control over. You can probably use a low cost electrode and pvc to do these things. Seems like a lot of work for something you can buy at the local welding shop.

To just ballpark the prolem, an SBS-13 balloon will require about 10 grams of hydrogen. That is 5 moles. Each mole will require 2 Faraday Constants of charge so 10 Faraday Constants. Each Faraday constant is roughly 95,000 coulombs so 475,000 coulombs. A coulomb is the charge carried by one ampere in one second. A 100 amp supply would require 4,750 seconds or roughly 1.3 days. You will not have 100% efficiency so it could easily be twice that. This is not a trivial task.

You can choose to produce the gas with a chemical reaction. There are a variety of videos doing such a thing on Youtube. I see things like a wad of aluminum foil and muriatic acid. They toss the aluminum foil in the acid and step back. Once the reaction begins to go, it increases rapidly. That is a good reason to step back. Notice that the temperature and surface area involved in the chemical reaction is not controlled. There is a huge surface area in a wad of aluminum foil and when the reaction begins, the temperature is rising quickly. The high surface and temperature make this dangerous. I believe the Youtuber did get garbage bags to lift off. In reality you will need to also dry the gas and carefully measure and control the lift of the balloon. Things will not go well. Don’t do that. Soda cans will be problematic because they are coated and very thin. They won’t work well either.

If you insist on producing hydrogen with chemistry. Consider the aluminum water reaction. It requires a small amount of sodium hydroxide to keep the surface active as aluminum forms an oxide coat which protects it from the outside world. If you use thick pieces of aluminum, like aluminum angle from a hardware store, the surface area will be stable. If you have a heating mantle and the glassware to do this and if you have a condenser to provide reflux, you can produce hydrogen with reasonable possibility of control. And don’t forget you now need a cooling water supply for the condenser, The gas will still need to be dried for the balloon. You can see the problems here. A heating mantle with temperature control, glassware for the reaction vessel, a reflux condenser to contain the water evaporated by the heat of the reaction, a water supply, a drying column to finish the water removal, This is such a poor idea compared to buying a cylinder you should not do it.

General Description of the preparation of a balloon

These tend not to be the electronics but from not understanding the balloons and how they work. So, the first part of this presentation is to build awareness of what the mistakes might be. If you release a balloon and you had an error is how you prepared and planned the launch, the balloon will probably be lost.

If you recall your high school chemistry, you studied the gas laws and promptly forgot them. They are important. The gases we use, hydrogen or helium, and the atmosphere are basically ideal gases. So lets talk about them without using any math.

An ideal gas means the atoms and molecules are moving around as if they are the only ones there. They are not attracted or repelled by their neighbors. The kinetic energy (speed) determines their temperature. The number of them in a volume determines their pressure.

A convient amount of particles for chemists is the mole. (A mole of electrons is 1 Farady Constant of charge.) At sea level on a regular day, that many particles occupies 22.4 liters, no matter which gas it is. That is a cube on the order of 11 inches per edge. If it is a mole of air, it weighs about 29 grams. If it is a mole of hydrogen, it weighs about 2 grams. That much hydrogen in a balloon displaces that much air so the bouyancy is 27 grams. As the balloon rises, the temperature and pressure change and they expand the same and so the lift is the same. That means the lift on the ground will be the lift until the balloon bursts or reaches float.

Lets go over the two types of balloons

Burster: This balloon has a stretchy latex balloon and as the balloon rises, the gas expands causing the balloon to stretch. It will continue to stretch until the latex fails. This balloon can carry payloads of 5 pounds or more so complex packages can be launched. The flight lasts a few hours and reaches altitudes over 100,000 feet. If you are lucky, you can find it and use it again.

Superpressure: This balloon has a plastic envelope. When launched, the envelope is not filled and as the balloon rises, the gas expands and eventually fills the balloon. Once full, the plastic cannot expand and so it’s density the lift decreases until it reaches float. It now has a pressure greater than the air, this is superpressure. This type of balloon can stay aloft for months, if you are both good at it and lucky. It carries a payload of around 15 grams and is solar powered. The are very rarely recovered but a few have been. It can circumnavigate the world. They float in the range of 40,000 feet. They are generally smaller, and will travel with the winds.

Most flights use two sources of balloons. Regular party balloons will not do much for you. The volume and the weight is too low to perform well. The commonly used ones are the SBS-13 made by Scientific Balloon Solutions and the clear 36” balloons made in China through AliExpress.

The SBS-13 balloons cost in the range of $165, are high quality and will float a little higher than the clear Chinese balloons. Follow the instructions given by them and you are in pretty good shape. The volume is around .5 cubic meters and they weigh about 105 grams. The instructions tell how to fill, seal and assemble the flight. Experience points to a “free lift” of about 6.5 grams as a sweet spot.

The Chinese balloons cost around $1.50 each in lots of 10, two are required for a normal payload and the quality control is poor so you must do some prestretching and testing for pinholes, film quality, and weak seals. They stretch to a volume of around .17 cubic meters and weigh about 35 grams. Because of the weight to volume ratio of these, they float a little lower but high enough to circumavigate the earth multiple times.

Qualatex also makes a 36” balloons that works but is less commonly flown.

They both SBS and clear chinese balloons comfortably carry around 15 grams of payload.

It is very common to fail with either of these choices. The best and lightest electronics are useless if the balloons fails so don’t overlook this part of the hobby. The common failure mode can be from too much gas, leaks causing gas loss, insufficient altitude to clear weather, floating into clouds during ascent, a launch that never clears the power lines and so on.

Clear chinese balloons are the most cost effective but require the most skill.

The first step is that of doing the prestrech and pressure testing. The balloons can burst between .5 psi to maybe .65 psi. They need to be pressurized to at least .5 psi to pretretch the balloon. They need to be held at pressure for a while to allow pinholes to appear if they are going to and to allow the plastic to stabilize so it does not contract much.

The equipment needed is an aquarium air pump, tubing, and fittings, a pressure gauge and a water column of some sort for pressure regulation.

(insert photo of test setup)

The aquarium pump provides all the air pressure needed, the balloons can be taped to either the tubing or a stiff tubing of some sort. The pressure regulation is done by a water column and adjusting the depth of the tube in the water. A leak is noticed when the bubbles stop and closing lines off one at a time will find it rather easily.

To prepare for launch, remove the air. One of the air pumps can be turned into a cheap vacuum pump and used or you can just roll up the plastic or put it under a blanket to slowly press the air out.

To prepare for a launch you need a hydrogen supply. The simplest way is to just get a cylinder from a welding supply store and buy a hydrogen regulator. A few have generated their own hydrogen. I am a chemical engineer and I looked into that. You should never start a chemical reaction you cannot stop and it is not a simple as you would think from the idiots on YouTube. Doing it well is expensive and difficult. I have only seen that done once but you can buy a hydrogen generator and do it with electrolysis.

Next you need a small scale to measure in grams. Put hydrogen into the balloon until is begins to rise. At this point, the gas has carried the bag and the extra lift will carry the payload and provide the extra lift called “free lift” The free lift is the critical number. Too little and the balloon will rise slowly and be vulnerable to gusts, moisture, etc. Too much and it will burst. Experience will teach you how much free lift but something like 5 grams per balloon seems to be good. So, zero the scale with a weight and a clip as the tare. Clip the balloon to the scale and read the lift as a negative number. The room should be free of drafts. When you reach your chosen amount, roll the neck and clip it.

You must know how much gas to seal inside the balloon. That is where the gas laws and lots of math comes in. Fortunately, the whole job has been written as a spreadsheet so you only need to know the inputs and outputs. The spreadsheet has the name float1g. The most current version can be obtained from (england website)

(show float1g and mention the inputs and outputs.)

Hydrogen will diffuse through the plastic film. In a day, a chinese balloon will lose in the range of .5 grams of lift. You can seal the balloon once it is filled or wait to just before launch and adjust the gas prior to sealing. Gas diffusion at float is not a problem as the pressure and temperature are much lower. The SBS balloons have something like 1/6 the diffusion loss so they can be sealed upon filling for launch the next morning. Sealing is done with a simple band sealer. Two or three seals are a good idea.

Once sealed, the balloons can be placed in a garbage bag for transport, wrapped in a sheet or towel and put in the back of your car. The whole package is assembled at the launch location. It is itmportant to have low wind and to not launch into clouds.

Altitude, superpressure and Volume Calculations

It is important to know the volume of a balloon in order to run the calculations in the spreadsheet float1g. That spreadsheet does all the math for you and you need to know the payload weight, the balloon weight, the free lift and the volume of the balloon.

The weight of the balloon and the payload are pretty simple, just weigh them with the scale you bought for free lift. The volume of the balloon is another. It can be simply assumed to be what others have found and things will be close enough. The chinese balloons are .17 cubic meters each and the SBS-13 according the SBS is .5 cubic meters. You can just use those numbers.

For the chinese balloons, you might want to measure things as you stretch the balloon. That way you can be sure you know what is going on. The measurements can be simple. Tape a metric stick to the floor, put a box on one end at zero and tape it to the floor to stay put. Place the balloon next to the box, use another box to hold the sides agains the cardboard on both sides and then read the meter stick. Do this for the distance across the seam and across the faces. A good approximation is to us the formula for an oblate spheroid.

Vol= 4Pi*a^2*b

Where a is the radius between the seams and b is the radius across the faces.

There is a formula for mylar balloons that is an interesting formula and the math was quite a task to put together. It has the restriction that the film is inelastic which ours is not so it will not work.

If you really want to get into the weeds and verify how good the formula is, inflate a balloon to the pressure you will tolerate. Take a photo so the seam forms a straight line. Print the photo as large as you can. Make sure you measure the balloon. Draw a line across the seam, find the center, divide the line into convient segments and, knowing the scale, find the dimensions of each line. Since the balloon has radial symmetry, calculate the volume of your annular rings, add them up and compare it to the volume of the oblate spheroid. I think they will be pretty close.

The math used to calculate the float altitude and superpressure is in the spreadsheet float1g. The complete expaination can be found at https://ukhas.org.uk/projects:splat. It includes a massive table of atmospheric parameters for a standard atmosphere. We never have a standard atmosphere but it is the best we can do and does a very good job.

On the use of hydrogen and helium

There are a few surprising things about hydrogen and helium and a few obvious ones. This assumes you will use a cylinder from a welding supply store. The hose for filling can be anything. I use the acetylene line and torch head of an old welding set as it has valves and is handy. The threads on the tank and regulator for hydrogen are left handed as any combustible gas is. That makes the acetylene line a good match.

Purchase a regulator for the cylinder. It is not sensible to use the valve on the cylinder as the pressure is very high and you will have no control.

The helium available from party balloon stores is commonly a diluted mixture and will not work well. Make sure you know if you are getting pure helium. Do not mix hydrogen and helium in one balloon unless you know if the helium is pure. If it is diluted, it is most likely with air which is free. If that is the case, you may be mixing hydrogen with oxygen and that will not end well.

The behavior of these two gasses surprises most of us. We expect gases to cool when expanded and we have a very high pressure gas. This effect is the Joule Thompson effect and applies to gases being released through a valve. For hydrogen or helium, it is basically zero. There will be no change in temperature when filling so no need to wait for the balloon to come to room temperature. It will already be there.

When the balloon is ascending, a different thing happens. The gas expands but cools as the expansion takes some of the energy out of the gas. The cooling is the adiabatic lapse rate. The lapse rates of hydrogen and helium during the ascent of the balloon are greater than the lapse rate of the atmosphere. The balloon will get cooler faster than the air around it. This is not important in a clear sky but it does matter if the balloon were to go into a cloud. It will be cooler than the cloud and there will be condensation that will increase the weight of the balloon. Given that water droplets will stick to the balloon and condensation will also occur, things can go very wrong. Try to have a clear sky.