Temperature, Pressure, Winds, Humidity & Precipitation

• The content covers temperature, pressure and winds, and humidity and precipitation.

Weather vs. Climate

• Weather refers to atmospheric conditions in a specific place over a relatively short time.
• Climate refers to atmospheric conditions in a specific place over a considerable period of time, typically 30 to 35 years.

Temperature

• Temperature indicates how hot or cold a particular location is.
• It is determined by the sun's energy that reaches Earth.
• Energy from the sun travels through the atmosphere and is absorbed by the Earth's surface, warming it and heating the atmosphere.
• Temperature is measured using an analogue or digital thermometer and reported in Celsius (C) or Fahrenheit.
• Most countries use Celsius, while only a few use Fahrenheit. (e.g. USA)

Temperature Calculations

• The mean daily temperature is the average temperature of a place on any one day.
  • Mean daily temperature = (Daily maximum temperature + Daily minimum temperature) / 2
• The daily temperature range is the difference between the maximum and minimum temperatures recorded in a day.
  • Daily temperature range = Daily maximum temperature - Daily minimum temperature
• Knowing the daily temperature range helps determine how much temperature varies in a day.
• The mean monthly temperature is the average temperature of a place in a month.
  • Mean monthly temperature = (Sum of mean daily temperatures for the month) / (Number of days in the month)
• Annual temperature range = Maximum monthly temperature – minimum monthly temperature
• Annual temperature range helps determine how much temperature varies across the year.
• Isotherms are lines joining places with the same temperature together.

Factors Affecting Temperature

• These include latitude, altitude, distance from the sea, cloud cover, and microclimatic factors.

Factor 1: Latitude

• Latitudes are imaginary lines that run in an east-west direction across the globe.
• The equator is the latitude that divides the globe into the northern and southern hemispheres.
• Latitudes indicate how far a location is from the equator.
• The spherical shape of the globe results in variations in insolation from the sun.
• The Earth tilts at 23.5° on its axis which causes the sun's rays to strike various parts of the world at different angles of incidence as it orbits the sun.
• The angle of incidence is the angle at which solar radiation strikes the Earth's surface.
• The angle of incidence is lower at higher latitudes, so insolation is less concentrated, resulting in lower temperatures.
• The angle of incidence is high at lower latitudes, so insolation is concentrated on a smaller area, leading to higher temperatures.
• Temperatures are lower at higher latitudes because the sun's rays strike at a lower angle, spreading solar energy over a wider area.
• During summer in the northern hemisphere, the North Pole is completely exposed to the sun's rays due to the Earth's tilt, resulting in months of sunlight.
• Since the angle of incidence is high in the area between the Tropic of Cancer (23.5°N) and the Equator, heat from the sun gets concentrated on a small area.
• Therefore, temperatures are higher at lower latitudes like the tropics, located between 23.5°N and S of the Equator.
• The South Pole is shielded from the sun's rays and experiences months of darkness without sunlight.
• During the months where the northern hemisphere is tilted towards the sun, it receives more sunlight which contributes to higher temperatures (Spring → Summer).

Factor 2: Altitude

• Temperatures are lower at higher altitudes.
• The Earth's surface absorbs shortwave radiation (light) from the sun, then emits longwave radiation (heat).
• The higher the altitude, the further away it is from the Earth's surface, which is the source of longwave radiation.
• Air density and pressure are lower at higher altitudes.
• Lower ability to absorb and radiate heat means lower temperatures.
• Temperature falls by 6.5 deg Celsius for every 1000m increase in altitude, known as the normal lapse rate.

Factor 3: Distance From Sea

• The difference in the rate of heating and cooling between land and sea affects the temperatures of coastal areas.
• Coastal areas tend to have lower summer temperatures, higher winter temperatures, and smaller temperature ranges.
• Land heats up and cools down more quickly than the sea.
• The Maritime effect: Cool winds from the sea lowers the temperature of the coastal areas.
• During winter, land cools down faster than the sea.
• Maritime effect: Warmer winds from the sea warms the temperature of the coastal areas.
• Winds lose their moderating effect as it moves inland.

Factor 4: Cloud Cover

• Clouds are good reflectors of shortwave radiation (sunlight) and good absorbers of longwave radiation (heat).
• Presence of clouds during the day reflects incoming radiation, so less solar radiation gets absorbed by land, resulting in lower daytime temperatures.
• Clouds during the night absorb heat from the ground for higher night temperatures.
• Absence of clouds means daytime results in land absorbing all the solar radiation, so the high day temperatures.
• Heat (longwave radiation) is lost rapidly during the night.

Factor 5: Microclimatic Factors

• The urban heat island: Urban areas are generally warmer than the surrounding countryside.

Humidity and Precipitation

• Relative humidity refers to the proportion of water vapour present in the air, relative to the maximum amount the air can hold at a particular temperature and is expressed as a percentage.
• Saturation occurs when the air carries the maximum amount of water vapour it can hold at that temperature, indicating relative humidity at 100%.

Factors Affecting Relative Humidity

• Availability of water sources and vegetation increases water vapour

Temperature

• As temperature increases, so does the capacity to hold water vapour.
• Warm air can hold more water vapour due to the spaces between the air molecules.
• Since temperature and water vapour capacity are proportional, as temperature increases, relative humidity decreases if the amount of water vapour in the air remains constant.
• Dewpoint/saturation point is the temperature at which the air can no longer "hold" all of the water vapour, so some of the water vapor condenses into liquid water.

Types of Precipitation

• Convection rain is very common in tropical and temperate regions during summer.
• On a hot day, warm air above the ground gets heated and rises; it cools and condenses as it rises above saturation point (100% relative humidity).
• Water droplets accumulate to form towering cumulonimbus clouds; when water droplets in clouds are too heavy to be suspended, convectional rain falls.
• Convectional rain is often accompanied by lightning and thunder.
• Relief rain forms when moist onshore air rises up the windward slope of a mountain and cools as temperature falls with increasing altitude.
• Further cooling after saturation at dew point causes the water vapour to condense into water droplets around atmospheric particles such as dust and smoke.
• When the water droplets becomes too heavy they fall.
• There is often little or no rain on the leeward side of the mountain, known as the rain shadow.
• This is because the descending air increases in temperature, therefore relative humidity decreases.

Pressure and Winds

• Air pressure refers to the downward force exerted by the weight of air per unit area on the Earth's surface.

Factors that affect Air Pressure

• As altitude increases, air becomes thinner, thus, places at higher altitudes experience lower air pressure.
• As temperature increases, so do the spaces between the air molecules .
• Hence, air pressure decreases as temperature increases.
• Wind speed is measured using a barometer in millibars (mb).
• Places with the same air pressure are joined with an isobar.
• Winds generate from air moving from an area of high pressure to an area of low pressure.
• The greater the difference in air pressure, the stronger the wind.

Types of Winds

• These include local, regional, and global winds.
• Local winds - Sea Breeze : Land heats up faster than the sea; warm air soars to create area of low pressure over land.
• Cool air from area of high sea pressure moves in, creating sea breeze.
• Local winds - Land Breeze : At night, land loses heat faster than the sea.
• Cool air from land moves to sea, creating land breeze.
• Regional winds - Monsoon Winds are seasonal movements of air masses across the Earth, as the Northern and Southern Hemispheres experience different seasons.
• These results from extreme heating and cooling of large landmasses; they deflect as they move across Earth because of the Coriolis effect.
• Winds deflect because of the Coriolis effect.

Coriolis Effect

• Deflection is the change in wind direction.
• Winds deflect towards the right in the Northern hemisphere and left in the Southern hemisphere.
• The coriolis effect is strongest at the poles and weakest at the equator.
• Hence the angle of deflection increases with distance from the equator.

Monsoons

• During southeast and southwest monsoons (June to Sept), over Northern Australia, the cold air contracts, becomes denser and sinks, creating intense high pressure.
• Warm air over Central Asia and Northwest India expands making it less dense and rises, generating intense low pressure.
• Winds blow out from the high-pressure area in Australia toward the low-pressure area in Central Asia as southeast monsoon in the southern hemisphere.
• After crossing the equator, Earth's rotation deflects winds to the left and comes to India/Central Asia as southwest monsoon in the northern hemisphere.
• During northeast/northwest monsoon(Oct to Jan), the land masses in Central Asia and Northwest India lose heat quickly as it is winter, creating intense high pressure.
• Warm air expands in the southern hemisphere= low pressure develops
• Winds blow out from the high- pressure area in Central Asia toward the low-pressure area in Australia.
• Winds arrive to India and Bangladesh as Northeast Monsoon and deflect to the right= Northern hemisphere
• Arrive in Australia as northwest monsoon with Earth’s rotation deflecting the winds left as it cross the equator.