ESCI 211: Ch 03 - Seasonal and Daily Temperatures

Main Ideas

1. Seasons Are Caused Mainly by Earth’s Axial Tilt

• Earth’s axis is tilted about 23.5°. As Earth orbits the Sun, this tilt changes the angle and duration of sunlight received by each hemisphere.
• A hemisphere has summer when it is tilted toward the Sun and winter when it is tilted away. Sunlight concentrated over a smaller area delivers more energy per unit area; sunlight spread over a larger area delivers less.
• The June and December solstices mark the Sun’s most northerly and southerly overhead positions. At the March and September equinoxes, the Sun is overhead at the equator.
• Earth’s orbit is slightly elliptical, but the lecture emphasizes that this has little effect on the seasons compared with axial tilt. The transcript’s numerical details about Earth–Sun distance appear garbled.
• Over the year, the tropics receive an energy surplus and the poles an energy deficit. Atmospheric and oceanic circulation help transfer heat poleward.

2. Local Geography Changes Temperature

• In the Northern Hemisphere, south-facing slopes generally receive more direct sunlight. They tend to be warmer and often drier, with longer growing seasons.
• North-facing slopes are more shaded, so they tend to be cooler and moister. Vegetation may be denser, and snow may last longer.
• Valleys can develop thermal belts at higher elevations: dense, cool air settles near the valley floor while warmer air remains above it.

3. Daily Heating and Cooling

• During the day, sunlight heats the ground first. The air near the surface warms mainly from the ground because air conducts heat poorly.
• The ground can therefore be warmer than the air just above it. On calm days, temperature can change sharply with height; wind mixes the air and makes the temperature profile more uniform.
• The day’s strongest solar input is around noon, but the highest temperature often occurs later—commonly around 4 p.m.—because the surface and nearby air continue gaining heat for a while after peak sunlight.
• At night, the ground loses energy by emitting infrared radiation. This radiational cooling can cool the surface faster than the air above it, producing a temperature inversion, in which temperature increases with height near the ground.
• Long nights allow more cooling. Clouds, humidity, vegetation, soil, wind, and nearby water also affect daily temperature changes.

4. Major Controls on Temperature

• Latitude: Sunlight is more concentrated at lower latitudes and spread over a larger area at higher latitudes.
• Land and water: Land heats and cools quickly because it has a lower specific heat. Water changes temperature more slowly, moderating nearby climates.
• Ocean currents: Warm currents can raise temperatures in nearby regions; cold currents can lower them. The lecture gives England and San Francisco as examples of contrasting current influences.
• Elevation: In the troposphere, temperature generally decreases with altitude.
• These controls also shape patterns shown by isotherms, lines connecting places with the same temperature.

5. Temperature Measurements and Summaries

The lecture describes measuring air temperature about 1.5 m above the ground, with the thermometer shaded and protected from precipitation so it measures the air rather than direct sunlight or wetting.

Common temperature statistics include:

• Daily range: Daily maximum minus daily minimum.
• Mean daily temperature: Average of the daily maximum and minimum.
• Monthly average: Average of the daily mean temperatures during that month.
• Average temperature or normal: A long-term average for a date or period; the lecture refers to a 30-year period.
• Annual temperature range: Average temperature of the warmest month minus that of the coldest month. It is generally larger over land and smaller near water.
• Mean annual temperature: Average of the 12 monthly average temperatures.

Heating and cooling degree days compare temperatures with a 65°F reference point. Each degree below the reference contributes to heating demand; each degree above contributes to cooling demand. These estimates help determine building heating and cooling needs.

Growing degree days track accumulated warmth above a crop’s threshold, helping estimate crop development and suitability for different regions.

6. “Feels Like” Temperature

• Wind chill describes why wind can make people feel colder: it removes the insulating layer of air near the body.
• Heat index accounts for humidity. High humidity slows sweat evaporation, making it harder for the body to cool itself and causing conditions to feel hotter.

7. Temperature Instruments Discussed

• Liquid-in-glass maximum thermometer: A constriction helps retain the maximum reading until the instrument is reset.
• Minimum thermometer: An index records the lowest temperature reached; the lecture describes resetting it by tilting the instrument.
• Electrical-resistance sensors and thermistors: Infer temperature from changes in electrical resistance.
• Thermocouples: Use the electrical signal produced at the junction of two different metals to infer temperature differences.
• Radiometers and infrared sensors: Estimate temperature from emitted infrared radiation.
• Bimetallic thermometers and thermographs: Use the different expansion rates of two metals to move a pointer or pen. A rotating drum can record temperature over time.

Speakers and Sources Featured

• Speaker: The course lecturer, identified in the video context as Timothy Duman. No other speakers are featured.
• Named source or institution: The National Weather Service is mentioned in connection with temperature measurement and the wind-chill and heat-index concepts.
• The lecture also uses diagrams, graphs, and isotherm maps, but the subtitles do not identify their external source.