Microbursts

[donnlass]

I have read in Pilot's Weather by Brian Cosgrove that on entering a microburst, the increased headwind increases airspeed and the tailwind reduces airspeed.

Am not too clear on that. Normally, say if you are on a bike and you ride into a headwind it makes for some hard pedalling while the wind behind you can make life a lot easier by pushing you along.

So how does that work?:confused::confused:

Thanks

[reportyourlevel]

Airspeed is not the same as groundspeed. On your bike you're thinking of groundspeed, i.e. the speed of the vehicle over the ground. So a wind from in front slows you down and a wind from behind speeds you up in terms of how much ground you cover in any set time.

In the plane situation you are talking about airspeed, i.e. the speed of the vehicle through the air mass. If the plane is moving in still air the groundspeed will be the same as the airspeed. If there is a wind from in front, the speed of the plane through the air mass will increase. If the wind is from the rear the airspeed will decrease. (Think of how long it takes a molecule of air to travel the length of the plane in each case.)

[Teevee]

Airspeed is the speed of the air over the wing, you're thinking of groundspeed. Using your bike analogy, if you pedal faster into the wind, the harder the force of the wind in your face because relatively the wind speed into your face increases because of your speed INTO the wind. When the wind speed is from your back, the wind speed relative to you decreases because of your speed away from the direction of the wind.

... I think!

[One Outsider]

If you are on your bike on a day with no wind and are keeping a speed of 10 mph then the airspeed is 10 mph and your groundspeed is also 10 mph.

If the wind then picks up and blows with a speed of 5 mph in the opposite direction of what you are going, you will then have an airspeed of 15 mph and a groundspeed of 10 mph. Should you have a 5 mph tailwind your airspeed would be 5 mph and your groundspeed 10 mph. As increased airspeed equals increased drag you are correct in saying that to maintain groundspeed more energy is required with a headwind.

As aircraft are kept in the air by lift and lift is generated by airspeed, it is airspeed that is used in aircraft as the primary speed parameter and not groundspeed as with cars and bikes. So when encountering a microbust the airspeed indicated in the aircraft will initially increase due to the increase in headwind, but the groundspeed will not change (much) due to the kinetic energy/inertia of the aircraft. Where you to maintain this new increased airspeed, power would need to be increased but were you to want to come back to your previous airspeed, power would have to be reduced. As you pass through the microburst and the headwind becomes a tailwind your airspeed is decreased and so is your lift. Where you to have kept the increased airspeed caused by the headwind you will probably now still have sufficient airspeed and lift to keep flying.