MET 10 - Lecture 4

Chapter 3: Air TemperatureDr. Craig ClementsDr. Craig Clements

San Jose State UniversitySan Jose State University

The daily variation in air temperature is controlled by incoming energy (primarily from the sun) and outgoing energy from the earth’s surface.

Where incoming energy exceeds outgoing energy the air temperature rises. Where outgoing energy exceeds incoming energy the air temperature falls

Diurnal Temperature Variation

Incoming energy exceeds outgoing energy (orange shade), the air temperature rises. Where outgoing energy exceeds incoming energy (blue shade), the air temperature falls

5 MET 112 Global Climate MET 112 Global Climate ChangeChange Fig. 3-11, p. 65

On a sunny, calm day, the air near the surface can be substantially warmer than the air a meter or so above the surface.

On a clear, calm night, the air near the surface can be much colder than the air above. The increase in air temperature with increasing height above the surface is called a radiation temperature inversion.

Death Valley, California

Thermal Belts: cold air in valley bottoms

Diurnal temperature distribution above the ground

Temperature sensors are attached to balloons

What controls temperature?

LatitudeLand and water distributionOcean currentsElevation

Average air temperature near sea level in January (°F)

Lines of constant temperature are called isotherms

Average air temperature near sea level in July (°F)

Temperature Measurements

Temperature is measured using a few different types of thermometers.

Typically, the daily temperatures are measured using automated weather stations (AWS).

Sensors include:Electrical resistance thermometers i.e., Thermocouples (two-metals give voltage)

A portable, remote weather station

Thermometer shelter

Why do we need a shelter?

It is impossible to measure temperature accurately in direct sunlight.

What effects does this have on the measured temperatures?

Heating degree-day: is based on the assumption that people will begin to use their furnaces when the mean daily temperature drops below 65 °F.

This is calculated by subtracting the mean temperature for the day from 65 °F.

Cooling degree-day: is based on the fact that people begin to cool their indoor environment as the ambient temperature climbs above 65 °F.

This number is used during warm weather to estimate the energy needed to cool indoor air.

Use of Temperature Data

Use of Temperature Data

Growing degree-day: is an index used by farmers as a guide to planting and determining the approximate dates when a crop will be ready for harvesting.

This is defined as a day on which the mean daily temperature is one degree above the base temperature-- the minimum temperature required for growth of a crop.

For example, on a summer day in Iowa, the mean temperature might be 80 F. On this day, sweet corn would accumulate(80F - 50F(base temp)), or 30 growing degree-days. Theoretically, sweet corn can be harvested when it accumulates a total of 2200 growing degree-days.

Air Temperature and Human Comfort

The human body loses heat by emitting infrared energyConduction and convection also cool and heat the body.

On a cold day, a thin layer of warm air molecules forms close to the skin, protecting it from the surrounding cold air.

Once the wind starts to blow, this insulating air is swept away from the skin and heat is rapidly removed. The faster the wind blows, the greater the heat loss, and the colder we feel.

How cold the wind makes us feel is expressed as a wind-chill index.

Air Temperature and Human Comfort

Air Temperature and Human Comfort

High winds, in below-freezing air, can remove heat from exposed skin so quickly that the skin may actually freeze and discolor. The freezing skin, called frostbite, usually occurs on the body extremities first because they are the greatest distance from the source of body heat.

The rapid loss of body heat may lower the body temperature below its normal level and bring on a condition of hypothermia!

frostbite

A thousand degrees and freezing to death!