Meteorology Exam 1 - chap 3

Calculate the cooling degree-day for Grand Forks on a day with a minimum temperature of 71˚F and maximum temperature of 95˚F.

You must first calculate the mean (average) daily temperature. In this example, with a high of 95˚F and low 71˚F the average temperature is 83˚F. Then subtract the base temperature (65˚F) from the mean daily temperature (83˚F) and you get a cooling degree-day of: 83˚F - 65˚F = 18.

Calculate the heating degree-day for Grand Forks on a day with a minimum temperature of -26˚F and maximum temperature of -10˚F.

You must first calculate the mean (average) daily temperature. With a high of -10˚F and low of -26˚F, the average temperature for the day would be -18˚F. Then subtract the mean daily temperature (-18˚F) from the base temperature of (65˚F) and you get a heating degree-day of: 65˚F - (-18˚F) = 83.

Define terms used to describe the Daily, Monthly, and Yearly temperature values. 1. Daily or Diurnal Range of Temperature 2. Mean or Average Daily Temperature 3. Mean or Average Monthly Temperature 4. Annual Range of Temperature 5. Mean or Average Annual Temperature

1. Daily or Diurnal Range of Temperature The difference between the daily maximum and minimum temperature 2. Mean or Average Daily Temperature The average of the highest and lowest temperature for a 24-hour period 3. Mean or Average Monthly Temperature The average of the mean daily temperatures for that month (GFK: Jan 6.4°F, July 68.6°F 4. Annual Range of Temperature The difference between the average monthly temperature of the warmest and coldest months GFK: 68.6 - 6.4 = 62.2°F 5. Mean or Average Annual Temperature The average temperature for the entire year which represents the average of the twelve average monthly temperatures GFK = 40.0°F

Name three methods that farmers can use to protect their crops against the cold. Explain the physical principle behind each method.

1. Orchard Heaters - They circulate the air by setting up convection currents 2. Wind Machines - They mix cooler surface air with warmer air above 3. Coat with Ice - Sprinklers emit a fine spray of water which freezes to the vegetation. The release of latent heat when the water changes to ice keeps the ice at ≈ 32˚F, protecting the vegetation beneath the ice.

How would you construct an instrument shelter that holds thermometers to measure atmospheric temperature?

1. Paint it white 2. Face it to the north 3. Put slots in the side for free air flow 4. Place it 5.0 to 5.5 feet above the ground 5. Make sure it is located above a grassy surface and away from buildings and surface concrete and/or pavement

Identify and name the major lines of latitude starting north and working south.

90˚ N North Pole 66½˚ N Arctic Circle 23½˚ N Tropic of Cancer 0˚ Equator 23½˚ S Tropic of Capricorn 66½˚ S Antarctic Circle 90˚ S South Pole

What is a 'sensible temperature'

A wind-chill temperature and heat index temperature are both considered sensible temperatures. These are temperatures that the body feels like in contrast to the actual temperature of the environment as measured with a thermometer. In extreme cases, very low wind-chill and high heat index temperatures can be life threatening.

What nighttime weather conditions would create the greatest temperature gradient from the surface to areas just a few feet above the surface?

Clear, calm conditions. As night progresses, the ground and the air in contact with it continue to cool more rapidly than the air a few meters higher. This is due to the ground and air above the ground radiating infrared energy, a process called radiational cooling. Radiational cooling is most efficient under clear skies and calm winds. An increase in cloud cover and/or windy conditions will decrease the effects of radiational cooling.

During the Northern Hemisphere's summer, the daylight hours in the far northern latitudes are longer than those in the mid-latitudes, but the temperatures are not warmer farther north. Why?

Even though the far northern latitudes are receiving more hours of sunlight in the summer, the sun angle is still considerably low. It is true that the northern latitudes are receiving more solar radiation at the top of the atmosphere, but the sunlight penetrating through the northern atmosphere will be partially scattered by fine dust and air molecules, reflected by clouds, and absorbed by atmospheric gases. These factors combined, reduce the amount of solar radiation reaching the ground. The solar radiation that does reach the ground, is partially reflected by ice and snow, is used to melt ice, snow and frozen ground and heat the lower atmosphere.

The ground and air near the ground will cool quicker than the air above by radiating infrared radiation (radiation cooling). The warmer temperatures aloft do transfer some heat downward but the process is slow due to the air's poor thermal conductivity. Therefore, by late night or early morning, the coldest air is found next to the ground, with slightly warmer air above (radiational inversion).

Grand Forks will experience its coldest nights and strongest radiational inversions during the winter months when, skies are clear, winds are calm, air is dry, and snow is on the ground.

Why does Grand Forks, ND have much colder temperatures than Seattle, WA in January even though they are both located at roughly the same latitude (48˚ N)?

Mid-ocean surface temperatures change relatively little from summer to winter compared to the much larger annual temperature changes over the middle of continents. This is due to the difference in specific heat values between water (1.0 Cal/gr˚C) and land (0.3 Cal/gr˚C). Water will heat and cool slower than land due to its larger heat capacity and therefore moderate the air temperatures in Seattle since it is located near the coast, versus Grand Forks, located in the middle of the North American continent.

Identify the major dates that correspond to the change of astronomical seasons in the Northern Hemisphere.

Sept 22nd Autumn Equinox 0˚ Equator Dec 21st Winter Solstice 23½˚S Tropic of Capricorn Mar 20th Vernal Equinox 0˚ Equator June 21st Summer Solstice 23½˚N Tropic of Cancer

If it is winter and January in New York City, what is the season in Sydney, Australia?

Since Sydney, Australia (33½˚ S Lat) is in the Southern Hemisphere, it would be summer but the month of the year is still January.

Explain why the warmest time of the day is usually in the afternoon, even though the sun's rays are most direct at solar noon

Solar noon depicts the time when incoming solar radiation is the greatest. But there continues to be a surplus of incoming solar radiation versus outgoing infrared radiation until late afternoon. Therefore, maximum temperatures usually coincide with this time reach is typically 2-4 hours later than the time of peak incoming solar radiation.

What daytime weather conditions in the summer would create the greatest temperature gradient from the surface to an area just a few feet above the surface?

Sunny, calm conditions. The sun heats the ground which in turn heats the air in contact with it by conduction. Because air is such a poor conductor of heat, there exists a thermal boundary separating the hot surface air from the slightly cooler air above. On windy days, however, turbulent eddies are able to mix hot surface air with the cooler air above reducing the temperature gradient from the surface upward.

What are the main factors that determine seasonal temperature variations?

The 23.5˚ tilt in the earth's axis combined with the earth's elliptical orbit around the sun are the main factors that determine seasonal temperature variations. The angle at which a sun beam strikes the earth and the length of daylight hours are directly related to the tilt of the earth's axis and the location that the earth is at in its orbit around the sun.

In the Northern Hemisphere, why are summers warmer than winters, even though the earth is actually closer to the sun in January?

The earth is 3.1 million miles closer to the sun in January than in July but due to the 23.5˚ tilt in the earth's axis the sun angle is at its lowest point in the Northern Hemisphere during the end of December and first part of January. The sunlight beam that strikes at an angle is spread across a greater surface area, and is therefore a less intense heat source than a beam striking the earth surface directly. Another important factor is that the daylight hours are shorter in the Northern Hemisphere during the winter months.

Explain how radiational cooling at night produces a radiation temperature inversion.

The ground and air near the ground will cool quicker than the air above by radiating infrared radiation (radiation cooling). The warmer temperatures aloft do transfer some heat downward but the process is slow due to the air's poor thermal conductivity. Therefore, by late night or early morning, the coldest air is found next to the ground, with slightly warmer air above (radiational inversion).

Describe each of the controls of temperature. •Time of Year •Latitude •Land vs Water Distribution •Ocean Currents •Elevation

Time of Year - determines the location of the earth in its orbit around the sun which determines what season you are in Latitude - determines the amount of incoming solar radiation, slant angle of sun and duration of daylight hours, at a given latitude Land vs Water distribution - areas with more water (Southern Hemisphere) will have smaller seasonal temperature variations. Areas located in the middle of continents (Grand Forks) will have a much greater seasonal temperature variation than areas along the coast Ocean Currents - warm vs cold currents will affect coastal temperatures Elevation - higher elevations are colder due to decreasing temperatures with height but they also experience smaller temperature variations.

The earth is closet to the sun in January when the Southern Hemisphere is experiencing their summer season. So why are the Southern Hemisphere summers are not warmer than Northern Hemisphere summers?

t is true that the sun is closet to the earth during the period when the Southern Hemisphere is experience summer (January 4th) but the larger amount of water in the Southern Hemisphere (81%) versus the Northern Hemisphere (61%) causes summer temperatures to be cooler in the Southern Hemisphere. But the water's larger heat capacity will also cause the Southern Hemisphere winters to be warmer than the Northern Hemisphere winters, with the exception of the Antarctic due to its large snow and ice pack.

Explain why the daily range of temperature is normally greater (a) in dry regions than in humid regions and (b) on clear days than on cloudy days.

•Dry regions have clearer skies and less water vapor in the atmosphere versus a humid region, therefore, incoming solar radiation is greater during the day in dry regions because the sun's energy is not reflected by clouds (higher temps) and radiational cooling is greater during the overnight hours in a dry regions because less water vapor is available to re-radiate outgoing infrared radiation back to the surface (lower temps). •Clear days maximize daytime incoming solar radiation and nighttime radiational cooling (maximum daily temp range) while cloud cover will decrease daytime incoming solar radiation and hamper radiational cooling (minimal daily temp range).