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Air and forces

Air is impacted by pressure, buoyancy and friction.

Pressure is usually measured in millibar^[1][^2]. Standard pressure at sea level is 1013mb, higher means good weather, whereas a rapid drop means approaching weather system^[3]. Often weather is reported as sea-level equivalent^[4].

Hot air is more buoyant than cold air, so cold air sinks.

Moving air is wind. At low altitude air encounters friction with the ground, so winds are usually stronger at higher altitude^[5].

Pressure decreases with altitude^[6], which results in a drop of temperature as well. This drop in temperature is due to expansion with reduced pressure, which makes air molecules expend energy^[7]. Average drop is 5 degrees C per 1000m (1 degree per 200m). Large differences of pressure result in strong winds.

Most of the weather phenomena happen in the troposphere^[8].

heat flow

There are 3 mechanisms or heat transfer:

Conduction, when 2 bodies are in physical contact - not important for weather^[9], however the heat capacity of oceans is playing a much major role, notably making temperatures of coasts to be more moderate.
Radiation, when an object radiates. This is playing a major role, temperature mostly being a result of the balance of incoming and outgoing radiations. Earth radiates at infrared levels^[10]. White clouds are reflective, so the amount of radiations leaving earth at night is lower, making for hotter nights^[11].
Convection, (transport of heat through the motion of a fluid from one place to another). This happens when masses of air move from one location to another or water (oceanic currents, particularly from equator to poles). The circular loop where hot moves to cool and vice-versa is called a convection cell^[12]. This is what cause sea-breeze in the day and land breeze in the night. The temperature between equator and each pole is also causing the creation of 3 different cells, resulting in 3 different climates.

Coriolis effect

The Coriolis effect is due to a linear motion seen from a rotating reference frame^[14]. At a large scale^[15], air on the equator is "travelling faster" than earth on the lower latitudes, and therefore it wind blowing towards the poles seems to be rotating eastward. Respectively air coming from the lower latitudes and moving towards the center seems to be rotating westward. Therefore creating counterclockwise cells in the north, clockwise in the south.

![[Pasted image 20240610193735.png]]

Global weather systems

Prevailing winds are surface level wind patterns associated with global convection cells.

In temperate zones, dominant winds tend to blow from west to east, precipitations accompany alternating warm/cold front. In tropics, dominant winds are east to west and precipitations occur in afternoon storms. ![[Pasted image 20240610202102.png]]

ITCZ (inter transfer convection zone) is where the equator's cells are meeting, where air raises vertically, with no horizontal movement, and is very quite (known as doldrums). ![[Pasted image 20240610202420.png]]

The cell between equator and the 30th latitude (horse latitude) is called "Hadley cell". Heated air rises from the equator up to the tropopause, and moves towards the pole. Then it gets cold and falls down. Because of Coriolis, in the North: winds at high altitude (northbound) are skewed towards east, then southbound winds at low altitude towards the west. These are called trade winds. Respectively for the south.

The next cell is called "Ferrel cell". The boundary between both cells share the falling wind. In the north: northbound air is travelling low, and skews to the east, then returns to the south, skewing west. This notably impacted sail travel history, as above 30th, winds are dominantly western (eastbound), and dominantly westbound under it.

![[Pasted image 20240610203359.png]]

The last cell is called "Polar cell". Air raises in the boundary between Ferrel Cell and polar cell. As with the Hadley cell, air travels southbound in higher altitudes and northbound in lower altitudes. Due to Coriolis, this creates "Polar easterlies": ![[Pasted image 20240610204043.png]]

While there are global flows, there are also some local patterns appearing^[19].

references

Backyard Meteorology: The Science of Weather

[1]: The SI unit is the Pascal == 1N/sqm. 1 mb == 1 hp == 100 p.

[2]: Sometimes recorded in inches or millimeter of mercury (called a Torr)

[3]: Typical pressures:

Average at sea level = 1013mb
On a clear day = 1030mb
Typically can go down to 980 when bad weather approaching
Rapid drop = approach of a storm
Lowest recorded = 870 during a typhoon in pacific. 900 for continental US.
Highest recorded = 1086mb.

[4]: Using a ratio to bring the average back to 1013mb. This is an easy conversion, pressure drops by 1mb per 10 meters.

[5]: Air and forces

[6]: Pressure:

Air exercises pressure on us: ![[Pasted image 20240610104034.png]]

We usually don't notice because the pressure inside us = outside us. ![[Pasted image 20240610104224.png]]

However we can notice when the pressure changes rapidly. Vacuumed containers crushing is the result of the atmospheric pressure crushing it.

[7]: 202406101757-weather--WEATH_V1_2_Weather_Basics_part_2.txt Air expands, molecules lose energy -> temperature drop. This is the same process as a fridge.

3/4 of air is in troposphere. This is where most of the weather phenomena are happening. Past tropopause, air temperature raises. Height of tropopause varies, 18km at the equator, 9km at the poles. Clouds are rarely seen above the tropopause.

![[Pasted image 20240610105807.png]]

[8]: 202406101802-weather--WEATH_V1_3_Weather_Basics_part_3.txt We use isobars as contours of a mass of air with the same pressure (similar to a topographic map). When lines are far apart, delta of pressure is low. When they're close, difference of pressure is large, and we're expecting strong winds.

![[Pasted image 20240610110511.png]]

[9]: Warm air can contain more water than cold air. When the air is saturated, water condenses into droplets -> clouds, and rain.

When hot, wet air raises, the condensation is releasing more heat, which in turns gets more air to raise. This also creates a vacuum that sucks up more air, and results in thunderstorms. This is called "latent heat of transformation".

![[Pasted image 20240610110732.png]]

[9]: 202406101953-WEATH_V1_heattransfer_part1_rc_sl.txt

heat capacity: amount of heat necessary to change the temperature by 1c. Water can hold much more heat than air.

[10]: You can feel infrared when you open an oven door.

[11]: 202406102004-weather--WEATH_V1_heattransfer_part2_rc_sl.txt An example of how radiations impacts weather: at night, earth is radiating heat. If there are clouds, less heat is radiating, and night is hotter. In the summer, this causes dew.

![[Pasted image 20240610130512.png]]

[12]: 202406102010-WEATH_V1_heattransfer_part3_rc_sl.txt: Convection cells: hot fluid is more buoyant causing it to raise, then cools down and go back down, forming a circular motion.

![[Pasted image 20240610131308.png]]

[13]: 202406102015-weather--WEATH_V1_4_heat_transfer_sea_breezes_wrap_rc_sl.txt See breeze: ~ 10 miles in scale. The returning stream is cool air coming back from the sea. Land heats faster than water, air raises and causes a vacuum. Cold air comes from the sea, causing a vacuum that pulls the hot air, which then cools down, etc. ![[Pasted image 20240610131605.png]]

During sea breeze, clouds go one direction, sea breeze goes the opposite direction: at the border where air starts moving down, vortices can appear in the clouds. ![[Pasted image 20240610131911.png]]

At night the process reverts, because the land cools down fast than the ocean, causing a land breeze. ![[Pasted image 20240610131658.png]]

[13]: When the temperature difference between the hot side and cool side is large enough, multiple cells appear. This creates alternating flows, as the cells go clockwise and counter clockwise:

![[Pasted image 20240610132233.png]]

The temperature difference between the equator and each pole is creating 3 cells: ![[Pasted image 20240610132535.png]]

[14]: 202406110234-weather--WEATH_V1_3_Coriolis_Effect_Demo_rcv2_SL.txt When the ball is thrown in a straight line, from the rotational point of view it looks like it follows a curve. ![[Pasted image 20240610193615.png]]

Earth rotates at different "speeds" depending on latitude. The equator travels at 1000mph, the poles are stationary, each point between them are decreasing in speed.

![[Pasted image 20240610193735.png]]

[15]: Coriolis effect doesn't apply to syphons, but does apply to long-range artillery shells

[16]: 202406110332-weather--WEATH_V1_Global_Weather_Systems_part1_rcv2_sl.txt

[17]: 202406110336-weather--WEATH_V1_Global_Weather_Systems_part2_rcv2_sl.txt Traditionally winds are called by where they're coming from (south wind = wind coming from the south)

[18]: 202406110339-weather--WEATH_V1_Global_Weather_Systems_part3_rcv2_sl.txt Dominant winds:

![[Pasted image 20240610203817.png]]