Temperature, Air Pressure, and Winds

  • Energy is the capacity to do work. 

    • The “work” in terms of temperature involves heating and cooling. This energy, in terms of temperature, is called Heat which is transferred in and out of an object needed to change the temperature of that object. Temperature is the measure of the average kinetic energy of atoms or molecules in an object. Also known as mean molecular motion. 

    • When energy is lost, temperature decreases, but when energy is gained, temperature increases.

    • Units of temperature are Fahrenheit in the US, Celsius in metric, and absolute for Kelvin (used for computational methods)

    • Use a thermometer to measure temperature. For many years, we have used a liquid in a glass with alcohol and mercury. 

    • There are also many Cooperative Observer locations to record data all around the country. They have grown in size throughout the years. They are ventilated objects above ground that measure the temperature of the air. It is built to not absorb solar radiation but reflect it. Most of these are located in rural areas as they were originally built to support farms. An observer usually goes out daily in the morning to read the temperature max and min from the day before. They have a long record. 

    • There are also automated weather stations that records the data and stores them without human interaction. They record more variables more frequently. These were established mainly at airports but fewer amount than the cooperative observer stations. They use sensors to track electrical signals and resistance which can track data (high equals warm). There are ASOS and AWOS stations to track data. 

    • On these electronic weather sensors, they track data on wireless data transmitters to send signals in real time which is better than the physical systems and hard drives. Because they are new they don’t build a long-scale climate record. 

    • Isotherms are lines of equal temperature on a map contouring for weather. Gradient is rate of change between these isotherms. Where the isotherms are close together, that implies temperatures are varying at a high rate. Vice versa with when isotherms are farther away. 

    • Change in elevations create changes in temperature. Coast lines also have big variability.

  • Latitude - where are you at on the map. The lower the lattitude the less the variability. Higher = more. The higher the latitude the greater change in sun angles throughout the seasons. 

  • Differential heating of land and water - Places with more water or closer to water lead to less temperature variability. When water is heated, convection distributes heat through a large mass. When land is heated, conduction distributes heat through a small mass. This comes from specific heat which is the amount of energy required to change the temperature of a substance. Takes way more to heat water than a solid material. When solar energy strikes water, evaporation occurs which makes water not available for heating (sensible). 

  • Ocean currents - warm currents transfer warm water while cool currents transfer cold currents. Places next to these warm ocean currents will naturally be warmer. Cold ocean currents also bring drier conditions. 

  • Altitude/Elevation - The earth warms from the bottom up, this is because air density is higher at the bottom. Because the air is less dense and pressure is greater higher up, going up from sea level atmospheric mass is below you making it colder. 

  • Geographic position - landlocked vs on the coast. Land warms and cools faster than water. Land by more land has greater temperature variability. Land by water has less temperature variability. 

  • Cloud cover and albedo - influences the amount of solar radiation reflected. Temperature will be higher on a clear day than a overcast day because of the sun being reflected more by the clouds. The range of temperatures are also bigger on clear days than on overcast days (DTR). Landscapes with lighter colors (like a sheet of snow) have a higher level of albedo. That means more energy is reflected. 


  • Solar Declination is higher in the summertime than in the winter time. 

  • Urban Heat Island - Temperatures are usually higher in urban areas and suburban communities as they have much lower albedos (ex. Asphalt roadways). 

  • Degree Days: Heating and cooling degree days (energy use)

  • Human Thermal Comfort: Apparent temperatures (feels-like). 

  • Heat Index: measures a scale of heat that describes the negative effects of heat with prolonged exposure and/or physical activity From “caution” to “extreme caution” to “danger” to “extreme danger”.

  • Windchill: as the wind gusts get fast with air temperature that gets cold can lead to decreased time to get frostbite. 

  • Pressure changes with altitude. Half of the total atmospheric mass lies below ~5.7km which is ranges from 500-1013mb. From ~11.3km upwards, a quarter of all atmospheric mass is here and less than 10% of the mass of the atmosphere is above 16km. 

  • Pressure changes with temperature. Cold column of air has high pressure at the surface but pressure drops rapidly, making the surface above have low pressure. Warm column of air has low pressure at the surface but when altitude changes (unlike cold), pressure drops slowly and then the surface above becomes high pressure. 

  • Pressure changes with moisture content. Water vapor contains less molecular mass than nitrogen or oxygen. This causes humid air masses to displace heavier nitrogen and oxygen molecules, reducing the density of the air. 

  • Pressure changes by airflow aloft. Convergence of air aloft leads to greater surface pressure while divergence leads to lower pressure

  • Pressure Gradient Force (PGF). Weak gradient forces lead to weak winds while strong forces lead to strong winds. This example is when isobars are straight.

  • When isobars are curved, PGF is more sporadic, pointing in various directions. 

  • Coriolis Force (CF). A rotating earth causes a deflection of winds. Larger deflection and higher rotation changes at higher latitudes (stronger CF), less deflection and lower rotation changes at lower latitiudes (less CF). Wind deflected to right in northern hemisphere, wind deflected to left in southern hemisphere. Affects wind direction not wind speed. Instead, affected by how fast the wind is blowing: more wind = more CF.

  • PGF generates wind, CF modifies it.

  • Friction controls wind as it acts to slow the wind. Greatest influence within the boundary layer (bottom 1km of the atmosphere), negligible above this level. Friction reduces the CF, which is proportional to the wind speed.

  • Upper winds (500mb) = Geostophic winds (PGF = CF)

  • As PGF goes from high pressure to low pressure and towards northern latitude, the wind starts to blow perpendicular to the PGF and CF.

  • High pressure system diverges clockwise and the low pressure system converges counterclockwise.

  • Latent Heat - refers to the energy needed to change the state of water w/o changing its temperature or quantity (only abosorbed or released when water changes phase)

  • Sensible Heat - energy needed to change the temperature of a substance w/o changing its state or phase (radiation, convection, etc.)

  • From Solid to gas = Sublimation (1 gram of ice requires 680 calories to subliminate)

  • From Gas to Solid = Deposition

  • Solid to liquid = melting (80 calories for 1 gram), liquid to solid = freezing (releases energy)

  • Liquid to gas = Evaporation (600 calories for 1 gram), Gas to Liquid = Condensation

  • Barometer - Measures air pressure

  • Lines of equal pressure - Isobars

  • Tight isobars = strong winds and steep PGF

  • When lower atmosphere warms, pressure aloft increases adn the 500 mb height rises

  • Anenmoetor measures wind speed

  • Change in molecular speed → fast to slow, thus releasing heat into the environment and warm the surrounding air

  • Humidity - Water Vapor in the Air

  • Absolute Humidity - the mass of water vapor in a given volume of air. We calculate this by the mass of water vapor divided by the volume of air. When volume changes, humidity changes too.

  • Mixing Ratio - the mass of water vapor in a unit of air compared to the remaining mass of dry air. Calculated by mass of water vapor divided by mass of dry air. Its not affected by changes in pressure or temperature becuase its measured in units of mass.

  • Specific Humidity - when comparing the mass of water vapor to the total mass of air

  • If a size of a parcel increases, absolute humidity drops.

  • Each molecule in an air mass exerts some sort of pressure on us

  • Vapor Pressure - Pressure exterted by water vapor. \

  • Saturation water vapor - Pressure exerted by water vapor when air is saturated.

  • Clausius-Clapeyron Relationship - Saturation is temperature dependent. At higher temperatures, it takes more water vapor to saturate in the air. Therefore, warmer air can hold more water vapor than colder air.

  • Relative humidity - Can track how close the air is to saturation, irrespective of the actual quantity of water vapor in the air. Ratio of the air’s acutal water vapor content to the amount of water vapor required for saturation at a gvien temp. and pressure.

  • Moisture content affects relative humidity as when more grams of water vapor are added, relative humidity rises. Temperature decrease means saturation mixing ratio decreases.

  • Dew Point - temperature at which water vapor begins to condense or air reaches saturation. Closer the dew point is to temperature, the closer to saturation.

  • Temperature is the primary factor to how much water vapor the air can control

  • When relative humidity reaches 100%, the air becomes saturated

  • Relative humidity drops after sunrise