Pressure and Wind
Our earth has a diameter of 24,901 miles (40,075 km) and rotates at 1,037 miles per hour, at the equator, for one complete rotation per 24 hours. There are four spheres of the atmosphere:
Troposphere: where we live and it varies in thickness from about 20,000 feet up to approximately 50,000 ft. Depending where on the earth the trop is measured, deepest at the equator (warm air) to shallowest at the poles (cold air, which is more dense). All of our weather is within this sphere.
Stratosphere: extending from the Troposphere up to 50 km’s (160,000ft). On occasion some weather may extend into the lower layer of this sphere, usually large thunderstorms can punch up into these altitudes.
The Mesosphere and Thermosphere have little effect on our weather.
What Impacts Wind Patterns?
The rotation of the Earth and it is slanted at 23.5 degrees and orbits in an elliptical pattern around the sun.
The composition of earth is made up of 71% water (mostly oceans) and there is uneven heating and cooling of the surface.
Land masses made up of valleys and mountains; creates and controls much our weather.
Pressure systems and winds move about the planet and gives us wind and weather. The winds blow in Zonal Circulations. However, in most of the contiguous United States, our winds blow as Westerlies. This is why most of our weather comes from the west (northwest through southwest). However, in Florida the trade winds are predominately from the easterly.
As aircraft (A/C) operators we must have standards that we can use for understanding how our A/C will perform. Let’s start with the INTERNATIONAL STANDARD ATMOSPHERE (ISA). The ISA starts at sea level and extends up into the atmosphere:
A column of air weighs 14.7 lbs per square inch, defined as 29.92 Hg (inches of mercury), and is at a temperature of 15 degrees Celsius (59 degrees Fahrenheit)
Pressure is also measured in millibars(mb), ISA is 1013.2 mb at Mean Sea Level (msl).
A 1,000 mb weather chart is used for msl and a 500 mb chart is for 18,000 ft. Half of our atmosphere is within this altitude. Our atmosphere is only about 35,000 ft thick, which is the end of the Troposphere. The Tropopause separates these spheres and that is where we find the Jet Stream (also called the Trop), a tube of fast moving air from west to east. The Jet Stream is most pronounced in the cold winter months and slow to often nonexistent during the doldrums of summer.
Increasing altitude causes air pressure, air temperature and dew point to decrease at predetermined rates. These are known as Lapse Rates. The Adiabatic Lapse Rate occurs when air ascends and expands causing heat energy to be lost, which cools the air; but conversely, increasing descending air temps. Dry air lapses at 5.5 degrees per 1,000 ft increase in altitude and wet air lapses at 3 degrees per 1,000 ft. (approximately). When the dry air lapse rate cools to match the wet lapse rate (due point), the relative humidity of the air is said to be Saturated. This saturation creates clouds and fog. Pressure Altimeters lapse at 1 inch per 1,000 ft, 29.92 ISA msl to 24.92 at 5,000 ft.
Altimetry: Before takeoff you set your altimeter to the airport elevation, which is the airport pressure corrected to sea level msl. As you fly along, you must continually reset your altimeter that you get from ATC or airports along the route. As a rule of thumb: If you do not reset your altimeter enroute, you will be lower or higher that you think, and we use this slogan, to give us information:
From Low (Press) to High- Look to the sky (you are higher than you think).
From High to Low- Look out below (you are lower than you think). Not good!
Isobar maps: What do they tell us? On a map, the areas of equal pressure are connected with lines. The rate of change in pressure is called a Pressure Gradient. The stronger the gradient the stronger the winds. What we see here is a “tightly wrapped low pressure system”. Remember that a Low is Cyclonic and Anti-Clockwise, so the winds are flowing into the center of the low. Quite bumpy in here!
A few more winds to think about:
Sea Breeze: Occurs along a coastline and during daylight hours. This is more pronounced in the summer or tropics. During the day, air over the land warms faster than the air over water due to sunshine (Diurnal Heating). The air over land rises and is replaced by air from the water. This sets up a circulation and does not present a problem for pilots.
Land Breeze: This air is the opposite of the Sea Breeze and occurs at night. The air over the land cools, becomes denser, and sinks (Nocturnal Cooling). The air over the water rises and sets up a reverse flow. Also, not a problem for us pilots.
It is said that wind Blows in Gusts and Lulls.
Fly Safely Know the Weather Plan Ahead File Flight Plan
Until next time,
Captain Will Rondeau
References
Weather Elements, a Text in Elementary Meteorology. Blair/Fite.
FAA Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25B.
Chapters 12&13.
The National Weather Service: Origin of Wind. Weather.gov/Jetstream/wind.